On the Wake Flow Characteristics Over an Articulated Lorry Model with/without AC-DBD Plasma Actuation

9125 words (37 pages) Full Dissertation in Full Dissertations

06/06/19 Full Dissertations Reference this

Disclaimer: This work has been submitted by a student. This is not an example of the work produced by our Dissertation Writing Service. You can view samples of our professional work here.

Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Abstract: An experimental study has been conducted to investigate the effects of linear AC-DBD plasma actuation in affecting the flow characteristics along the wake region of a scale articulated lorry model. Two-component particle image velocimetry measurement was used to resolve the time-averaged velocity, turbulence and vorticity fields along the centreline of the wake region downstream of the articulated lorry model. Results show that implementation of linear AC-DBD plasma actuation at the trailer rear end could neither reduce the size of the wake vortex nor the flow velocity in x-direction along the wake region. It was observed that higher levels of turbulent kinetic energy and z-vorticity fluctuation appeared in those trailers with linear AC-DBD plasma actuation implemented. It is deduced that increase in the turbulent kinetic energy and z-vorticity fluctuation along the wake region could affect the drag level that encountered by an articulated lorry.

Keywords: Heavy vehicle aerodynamics; plasma actuation; tractor-trailer; wind tunnel test

1. Introduction

Development of low-cost and effective flow control devices for achieving drag reduction and therefore, fuel saving in tractor-trailers or articulated lorries is currently an active research topic in both the academia and industrial sector. The European Commission has established a project called <<Aerodynamics and Flexible Trucks>> under the Horizon 2020 framework which aims to achieve 5 to 10% of fuel saving in articulated lorries through aerodynamic efficiency improvement [1]. Altaf et al. [2] estimated that for a lorry in a long-haul journey, up to 65% of fuel consumption is used to overcome the aerodynamic drag that encountered. Bradley [3] concluded that for a lorry weighting 36 tonnes and travelling at 105 km/h, aerodynamic drag accounted for approximately 21% of the total drag acting on the vehicle. In terms of aerodynamic drag reduction and fuel cost relation, Bradley [3] anticipated that up to 4% reduction in fuel consumption could be achieved if the aerodynamic drag that acting on a heavy vehicle during high speed operations is reduced by 20%. Similarly, Hsu and Davis [4] suggest that an annual fuel cost saving of US$10,000 could be achieved if the aerodynamic drag encountered by a heavy vehicle is reduced by 40%.

Broadly speaking, four major drag sources could be identified in an articulated lorry. These drag sources are the front stagnation region, the tractor-trailer gap region, the under-floor flow and the wake region downstream of the trailer rear end [5,6]. Since the present study focuses on the flow characteristics along the wake region of a square-back tractor-trailer with and without linear Alternate-Current Di-electric Barrier Discharge (AC-DBD) plasma actuation implemented; the following literature review is going to focus on current flow control devices that had been developed to achieve flow control in the wake region. Nevertheless, details about existing flow control strategies and devices that had been developed to achieve drag reduction in other regions of a square-back tractor-trailer vehicle could be found in the review paper written by Choi et al. [5].

Fundamentally, at the rear end of a square-back tractor-trailer vehicle the occurrence of flow separation leads to the formation of a large wake region downstream of the trailer rear end. The presence of this wake region significantly reduces the base pressure and hence, increases drag. It is estimated that this wake region contributes approximately 25% of the aerodynamic drag encountered by an articulated lorry [7]. Therefore, effective flow control devices are required to be developed in order to either reduce the size of the wake region or delay the occurrence of flow separation downstream of the trailer rear end to achieve drag reduction.

One of the flow control devices that could be used to effectively achieve flow separation control in the wake region of articulated lorries is the boat tails or base cavities that installed at the rear end of a square-back trailer [5,8-12]. These devices aim to delay the occurrence of flow separation in order to reduce drag. Croll et al. [12] concluded that boat tails could lead to maximum of 8% base drag reduction although Yi [10] claimed that potentially 42% of base drag reduction could be achieved using boat tails. Other than boat tails, Altaf et al. [2] concluded that 6 to 11% of base drag reduction could be achieved by installing flaps at the trailer rear end. Although promising result in terms of base drag reduction is observed using boat tails, base cavities or flaps; the operational and legal constraints induced by using these flow control devices limit their applicability in actual situations.

The effect of vortex generators in achieving base drag reduction and flow separation control on square-back trailers was also investigated in several research [6,13-15]. Lo and Kontis [6] suggested that installing vane-type vortex generators near the front on the roof of a square-back trailer could reduce the size of the wake vortex. Mugnaini et al. [13] found that approximately 1% of fuel saving could be achieved by installing vane-type vortex generators around the rear end of an articulated lorry with a square-back trailer. In contrast, Patten et al. [14] concluded that no noticeable fuel saving could be achieved by installing vortex generators around the rear end of a square-back tractor-trailer vehicle. Leuschen and Cooper [15] found that the fuel consumption of a square-back articulated lorry with vortex generators installed near the rear end of its trailer is at least 1% higher than that encountered by the baseline vehicle. In terms of active flow control strategies, base bleeding or jet blowing is another method that can be effectively achieved base drag reduction in the trailer of square-back articulated lorries [16-20]. In general, base bleeding could reduce the base drag by approximately 10% [20]. However, this flow control method required the use of heavy and bulky blowing pumps that restricted its applicability in practice.

Recently, boundary layer and flow control using AC-DBD (Alternate Current Dielectric Barrier Discharge) plasma actuation becomes an active research topic in academia [21-34]. The operational principle of a linear single dielectric barrier discharge (SDBD) plasma actuator can be described as follows. A linear SDBD plasma actuator consists of an embedded ground electrode and an exposed electrode. These two electrodes are separated by a dielectric material. A high voltage, usually in the range between 10 and 40 kV AC (peak-to-peak), is applied to the exposed electrode. The presence of the dielectric material hinders the electrons to reach the ground electrode. As a result, the air that adjacent to the exposed electrode is ionised and forming plasma. This effect generates an electromotive wind and hence, an airflow is induced along the surface of the plasma actuator which can be used for the purpose of flow control. It was found that linear AC-DBD plasma actuation could effectively achieve flow separation control in an inclined flat plate [25], airfoils [26-30], circular cylinders [31,32] and bluff bodies [33]. Very recently, Roy et al. [34] investigated the effects of AC-DBD plasma actuation in achieving flow separation control and drag reduction using a 1:60 scale square-back articulated lorry model. Interestingly, the authors in [34] found that using continuous linear AC-DBD plasma actuation increases drag by 5.1%. Since the authors did not illustrate the flow characteristics downstream of the trailer rear end; the reasons that lead to drag increase when using linear AC-DBD plasma actuation at the rear end of an articulated lorry model remain unclear. In contrast, the authors in [34] concluded that 14.8% of drag reduction is achieved when continuous serpentine AC-DBD plasma actuation was implemented at the rear end of the same articulated lorry model.

Although considerably promising flow control effects provided by linear AC-DBD plasma actuation were shown in airfoils [26-30] and circular cylinders [31,32]; contradictory outcomes were observed when using the same flow control method in a square-back articulated lorry model [34]. In addition, the flow physics downstream of the trailer rear end with and without implementation of linear AC-DBD plasma actuation has not yet been investigated. The present experimental study aims to investigate the flow characteristics along the wake region of a 1:20 scale articulated lorry model with and without applying linear AC-SDBD plasma actuation. This study could provide an insight about the potential of utilising AC-DBD plasma actuation in achieving base drag reduction in articulated lorries as well as other square-back road vehicles. The data provided could also be used for the purpose of numerical scheme validations.

2. Experimental Setup

2.1 Tractor-trailer Model

A 1:20 scale generic articulated lorry model shown in Figure 1 was used in the present experimental study. Similar scale articulated models also employed by Lo and Kontis [6,35], Taubert and Wygnanski [36] and Ortega et al. [37]. It is generally agreed that the flow features that shown over the articulated lorry model with this scale are comparable to those shown in the actual vehicles.

 

Figure 1. Schematic of the articulate lorry model. Note: all units are in mm.

The tractor model was designed based on some actual tractors that commonly found in the United Kingdom. A cap deflector with two side extenders were installed immediately behind the tractor model to simulate the articulated lorry operating in motorways. The trailer model was a scale-down version of an actual square-back trailer that could be commonly found within the United Kingdom. It should be noted that the normalised gap length (

G/A) between the tractor and trailer models (excluding the side extenders) was set to

G/A= 0.17 while G and A are the gap length between the tractor and trailer and the model frontal area, respectively.

2.2 AC-DBD Plasma Actuator

A linear Single Dielectric-Barrier Discharge (SDBD) plasma actuator was flush mounted around the perimeter at the rear end of the trailer model as shown in Figure 1. A schematic shown in Figure 2 illustrates the construction of the linear SDBD plasma actuator used in the present study. The overall length of the actuator is 375 mm and its width is 25 mm. Copper tape with thickness of 0.06 mm was used in both the exposed power and embedded ground electrodes of the plasma actuator. The exposed and embedded electrodes have the width of 8mm and 20 mm, respectively. The two electrodes were separated by three layers of Kapton tape with total thickness of 0.065 mm which served as the dielectric material of the plasma actuator. The exposed power electrode was placed 17 mm upstream of the rear end of the trailer model. It should be noted that the power and ground electrodes in the plasma actuator used was overlapped by 3 mm.

Figure 2. Schematic of the linear SDBD plasma actuator.

The SDBD plasma actuator was driven by a set of Minipuls 4 high voltage generator which was powered by a Voltcraft VSP2410 variable Direct-Current (DC) power supply unit. The high voltage generator used is capable to generate voltage up to 40 kV peak-to-peak and its operational frequency is ranging from 5 to 20 kHz. It should be noted that the same high voltage generator also used in Erfani et al. [38,39]. In the present study, the Voltcraft VSP2410 variable Direct-Current (DC) power supply unit was operated in the constant voltage mode and the input voltage was set to 20 V DC. A range of plasma actuation frequency between 7 and 11 kHz was used and the corresponding peak-to-peak voltage output from the high voltage generator was ranging from 14.9 to 18.6 kV.

Voltage and current signals outputted from the high voltage plasma generator were monitored and measured using an Agilent DSO1014A digital storage oscilloscope. The input bandwidth, maximum sampling rate and data writing speed are 100 MHz, 1GSa/s and 10 kpts, respectively. A sample voltage and current waveform is shown in Figure 3 captured with the input voltage and driving frequency set to 20 V DC and 8 kHz, respectively. The peak-to-peak voltage output captured at different driving frequency being studied is shown in Figure 4.

Figure 3. Voltage and magnified current output waveform at 18 kV peak-to-peak and 8 kHz.

Figure 4. Peak-to-peak voltage output at different actuation frequencies.

2.3Wind Tunnel Setup

All wind tunnel tests in this study were conducted using the de Haviland wind tunnel of The University of Glasgow. A schematic of the experimental set up employed is shown in Figure 5. It should be noted that similar experimental setup also employed by Lo and Kontis [6]. The wind tunnel has a test section with dimensions of 4 m (length) x 2.7 m (width) x 2.1 m (height). Optical access to the test section is achieved via two glass-made side and two Perplex top windows. The freestream velocity (U) was set to U = 30 ± 2 ms-1 so that the corresponding flow Reynolds number with respect to the height of the tractor model (ReH) is ReH = 3.6 x 105. Although the flow Reynolds number employed is an order of magnitude lower than that experienced by an actual articulated lorry; Gurlek et al. [40] and Krajnovic and Davidson [41] concluded that the flow features and pressure drag that encountered by a scale model are comparable to those acting on an actual vehicle when ReH > 2.1 x 105.

 

Figure 5. Schematic of the setup of the wind tunnel tests. Note: all units are in mm.

The articulated lorry model was mounted on an elevated floor with dimensions of 2 m (length) x 0.8 m (width) in order to avoid the thick boundary layer that developed on the floor of the wind tunnel test section. The beginning of the model was situated 300 mm downstream of the sharp leading-edge of the elevated floor. The boundary layer thickness based on 99% freestream velocity (δ99) measured at that location without the presence of the articulated lorry model is δ99 = 8.6 mm and the corresponding displacement thickness (δ*) is δ* = 1.3 mm which is less than 10% of the model ground clearance. Therefore, the effect induced by the ground clearance in affecting the flow pattern over the articulated lorry model is negligible according to Hucho and Sovran [42] and Kim et al. [43]. It should be noted that the blockage ratio caused by both the model and the elevated floor to the wind tunnel test section is 3.52% [6,35].

2.4 Two-component Particle Image Velocimetry Measurements

Time-averaged velocity and vorticity information was measured along the centreline of the articulated lorry model using two-component Particle Image Velocimetry (PIV) technique. Laser illumination was provided by a pair of Litron LPY742-100, Nd:YAG Q-switched lasers with laser wavelength, pulse energy, pulse width and repetition rate of 532 nm, 100 mJ, 5 ns and 200 Hz, respectively. The laser light sheet with thickness of 2 mm illuminated along the centreline of the elevated floor. Olive oil particles with typical diameter of 1 µm generated by a PIV Tech aerosol generator was used as the seeder particles. Scattered light signals reflected from the seeder particles were captured by a Phantom v341 high-speed camera. The maximum resolution and the corresponding viewing area of the camera were 2560 pixels x 1600 pixels and 478 mm (length) x 294 mm (height), respectively. The time delay between the two laser pulses (∆t), calculated according to the freestream velocity and the interrogation window size used, was set to ∆t = 20 µs. The time-averaged vector fields were constructed by averaging 2400 pairs of raw images captured. Recorded images were processed by cross-correlation algorithm using the software Davis 8.2. The raw images were first processed through two passes of cross-correlation using 32 pixels x 32 pixels interrogation windows. Then the interrogation windows were refined to 16 pixels x 16 pixels and another three passes of cross-correlation were conducted. This approach improves the spatial accuracy of the vectors that resolved [44-46]. The uncertainty of the PIV measurements is approximately 3.6% [6,35].

3. Results and Discussion

3.1. Wind-off Performance of the AC-DBD Plasma Actuator

Before evaluating the effects of linear AC-DBD plasma actuation in affecting the flow characteristics along the wake region downstream of the articulated lorry model; it is necessary to characterised the wall jet that generated by the plasma actuator at various actuation frequencies during wind-off condition. The velocity contours in x- and y-directions downstream of the rear end of the trailer model during wind-off condition are shown in Figures 6 and 7, respectively. The corresponding velocity profiles captured at various normalised x-locations (x/H) downstream of the rear end of the trailer model are shown in Figure 8 and 9 for x- and y-velocity, respectively. From Figure 6, it can be seen that maximum wall jet velocity in x-direction appears immediately downstream of the trailer rear end in all cases being studied. The wall jet velocity in x-direction decreases progressively with increasing the distance from the rear end of the trailer model. Qualitatively, it can be seen that the wall jet velocity immediately downstream of the trailer rear end increases with the actuation frequency from 7 kHz and reaching the peak at 9 kHz. Above 9 kHz, the wall jet velocity decreases progressively with increasing plasma actuation frequency.

Figure 6. Wall jet velocity in x-direction generated by the plasma actuator at various actuation frequencies. (a) 7 kHz, (b) 8 kHz, (c) 9 kHz, (d) 10 kHz and (e) 11 kHz.

Figure 7. Wall jet velocity in y-direction generated by the plasma actuator at various actuation frequencies. (a) 7 kHz, (b) 8 kHz, (c) 9 kHz, (d) 10 kHz and (e) 11 kHz.

Quantitatively, from Figure 8, it can be seen that the wall jet velocity in x-direction is the highest at all normalised x-locations being studied when the actuation frequency is at 9 kHz. At this frequency, the peak wall jet velocity occurred at x/H = 0.2 at which the x-velocity of the wall jet is about 1.98 ms-1. The second highest wall jet velocity in x-direction occurs when the actuation frequency is at 8 kHz. At x/H = 0.2 the x-velocity of the wall jet generated is approximately 1.92 ms-1. In contrast, the wall jet generated at 7 kHz actuation frequency has the lowest velocity in x-direction at all normalised x-locations being studied.

Figure 8. Wall jet x-velocity at various actuation frequencies measured at different normalised x-locations downstream of the trailer rear end. x/H = (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1.0 and (f) 1.2.

Negative wall jet velocity in y-direction (i.e. downward flow movement) could be observed immediately above the location where the plasma actuator situated in all actuation frequencies being studied (Figure 7). This downwards movement of airflow is induced by the effect of the plasma actuation. The lowest y-velocity appears when the actuation frequency is at 8 kHz followed by 9 kHz. This is because the peak-to-peak voltage outputs at these two actuation frequencies are considerably high. A region that shows positive y-velocity (i.e. upward flow movement) appears in the region between 0<x/H<1.5 downstream of the trailer rear end in all cases being studied which could be observed from Figure 7. This indicated that the wall jet generated due to plasma actuation induced flow circulation downstream of the trailer rear end. This could also be confirmed by observing the normalised x-velocity contours in Figure 6 that negative normalised x-velocity occurs at the same region in all cases being studied. However, unlike the x-velocity, there is no general relation between the y-velocity of the wall jet and plasma actuation frequency could be observed from Figure 9.

Figure 9. Wall jet y-velocity at various actuation frequencies measured at different normalised x-locations downstream of the trailer rear end. x/H = (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1.0 and (f) 1.2.

3.2. Flow Characteristics over the Lorry Model with and without Linear AC-DBD Plasma Actuation

3.2.1. Time-averaged Velocity Field

After considering the wall jet generated by the linear AC-DBD plasma actuator at various actuation frequencies during wind-off condition; the actual effects of linear AC-DBD plasma actuation in affecting the time-averaged velocity field along the wake region downstream of the trailer model is considered. Figure 10 displays the streamtraces with and without linear AC-DBD plasma actuation implemented downstream of the rear end of the trailer model. It should be noted that the normalised velocity magnitude contour (|U|/U) is the background contour in Figure 10.

Figure 10. Streamtraces and normalised velocity magnitude contour downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

Figure 10a shows the streamwise flow pattern downstream of the rear end of the trailer model without the use of linear AC-DBD plasma actuation (i.e. the baseline case). A large recirculating vortex appears immediately downstream of the trailer rear end with its core located at x/H = 0.3 and y/H = -0.25. The length and height of this recirculating vortex are 0.8H and 1.4H, respectively. It should be noted that H is the height of the trailer model which is 125 mm. Similar flow pattern in the wake region also observed in [6,35] using a scale articulated lorry model and in [40] using a generic bus model. The formation of this recirculating vortex is due to the coil-up of the lower shear layer emanated from the underbody of the trailer model [35,40]. The streamwise flow pattern downstream of the rear end of the trailer model with linear AC-DBD plasma actuation implemented at 7 kHz is presented in Figure 10b. At this actuation frequency, the streamwise flow pattern along the wake region of the trailer model remains similar to that shown in the baseline trailer. In fact, the length (Lvortex) and height (Hvortex) of the wake vortex in this case are Lvortex = 0.81H and Hvortex = 1.4H which are similar to the size of the wake vortex that presented in the baseline trailer. In addition, the location of the vortex core in the present case is same as that occurred in the baseline trailer which is located at x/H = 0.3 and y/H = -0.25. This implies that using linear AC-DBD plasma actuation at 7 kHz actuation frequency could not exert any observable effects in changing the size of the wake vortex. However, one interesting phenomenon could be observed in the upper shear layer from Figure 10b. Compared to the baseline trailer (Fig. 10a), it can be seen that the upper shear layer in the present case with 7 kHz AC-DBD plasma actuation implemented is deflected slightly more downwards than that shown in the baseline trailer from the normalised x-location (x/H) equals to 0.5 onwards. This implies that although linear AC-DBD plasma actuation at 7 kHz actuation frequency could not reduce the size of the wake vortex; it might reduce the overall size of the wake region.

Similar but more obvious effects could be observed when the actuation frequency is increased to 8 kHz (Fig. 10c) and 9 kHz (Fig. 10d). In these two actuation frequencies, the length and height of the wake vortex remain comparable to those appeared in the baseline trailer. However, at the normalised x-location between 0.5<x/H<1.75, it can be seen clearly that the upper shear layer is deflected more downwards when linear AC-DBD plasma actuation is operated at 8 kHz (Fig. 10c) and 9 kHz (Fig. 10d) compared to that shown in the baseline trailer (Fig. 10a). In fact, the shear layer is deflected slightly more downward when the actuation frequency is at 8 kHz than 9 kHz. The stronger downward deflection of the upper shear layer at 8 kHz is deduced to be due to the highest peak-to-peak voltage output at this actuation frequency amongst all other actuation frequencies being studied. Nevertheless, the streamtraces shown in Figures 10c and d further indicated that using linear AC-DBD plasma actuation at the rear end of a square-back trailer might be able to reduce the overall size of the wake region but shows no observable effects in altering the size of the wake vortex. The streamwise flow pattern becomes similar, in terms of both the size of the wake vortex and the upper shear layer deflection, to those shown in the baseline trailer when the actuation frequency is increased to 10 (Fig. 10e) and 11 kHz (Fig. 10f). This is deduced that at these two actuation frequencies, the peak-to-peak voltage output is relatively low so that the use of plasma actuation could only marginally alter the flow characteristics along the wake region downstream of the trailer rear end.

Figure 11 displays the normalised x- velocity contour along the wake region of the trailer model with and without linear AC-DBD plasma actuation being implemented.

Figure 11. Normalised x-velocity contour downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

From Figure 11, it can be seen that the normalised x-velocity contour along the wake region is very similar in all cases being studied regardless whether linear AC-DBD plasma actuation is implemented. In fact, the flow characteristics along the wake region could be quantified by considering the x-velocity profiles measured at various normalised x-locations downstream of the trailer rear end shown in Figure 12. From Figure 12, it is clear that at all normalised x-locations being studied, the normalised x-velocity profiles in the region between -0.8<y/H<0.5 (i.e. the location at which the wake vortex presents) are very similar. In addition, reverse flow occurs in the region between 0<x/H<1.1 and -0.8<y/H<0 downstream of the rear end of the trailer model in all cases being studied. The data shown in Figure 12 concluded that the use of linear AC-DBD plasma actuation at the rear end of a square-back trailer model could not exert any observable effects in changing the flow velocity in x-direction along the wake region downstream of the trailer rear end.

Figure 12. Normalised x-velocity profiles measured at various normalised x-locations along the wake region. x/H = (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, (e) 0.9 and (f) 1.1.

In contrast, some interesting phenomena could be observed when considering the normalised y-velocity contours shown in Figures 13. From Figure 13, it can be seen that a region that shows considerably high levels of positive y-velocity (i.e. strong upward flow movement) appears between 0.25<x/H<1.5 and -1.25<y/H<-0.25. The length and height of this region in the baseline trailer are 1.2H and 1.1H, respectively (Fig. 13a). However, when linear AC-DBD plasma actuation is implemented and the actuation frequency is at 7, 8, 9 and 11 kHz (Fig. 13b, c, d and f); the size of this region is slightly smaller than that presented in the baseline trailer. This indicated that with these plasma actuation frequencies, the strong upward flow movement in the wake vortex is confined to a smaller area. Surprisingly, the size of this region is the largest when the actuation frequency is at 10 kHz (Fig. 13e) amongst all cases being studied. The reason that leads to the occurrence of this phenomenon is unclear. Further research is required to be conducted in order to explain the occurrence of this phenomenon.

Figure 13. Normalised y-velocity contour downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

Figure 14 shows the normalised y-velocity profiles measured at various normalised x-locations between 0.1<x/H<1.1 along the wake region of the trailer model. At x/H=0.1 (Fig. 14a), it can be seen that the normalised y-velocity profiles in the region between -0.8<y/H<0 (i.e. the beginning of the wake vortex) are very similar in all cases being studied. This indicated that using linear AC-DBD plasma actuation could only exert limited effects in changing the flow velocity in y-direction at this location. When the normalised x-location is moved slightly downstream to x/H=0.3 (Fig. 14b), the normalised y-velocity in the region between 0.2<y/H<0.8 (i.e. middle of the wake vortex) is slightly lowered than that shown in the baseline trailer when the actuation frequency is at 7 kHz. In contrast, slightly higher normalised y-velocity could be observed at the same region when the actuation frequency is at 8 kHz compared to that shown in the baseline trailer.

Figure 14. Normalised x-velocity profiles measured at various normalised x-locations along the wake region. x/H = (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, (e) 0.9 and (f) 1.1.

Changes start to appear when the normalised x-location is moved further downstream to x/H=0.5 (Fig. 14c). At this location, in the region between 0.2<y/H<0.8, the trailer with 7 kHz AC-DBD plasma actuation implemented shows the lowest normalised y-velocity amongst all cases being studied. For other plasma actuation frequencies being studied, similar levels of normalised y-velocity are observed over the same region and their magnitudes are higher than that shown in the baseline trailer. This implied that at x/H=0.5, with the exception that when the actuation frequency is at 7 kHz, stronger upward flow is induced in the wake region in those trailers with linear AC-DBD plasma actuation being implemented compared to that presented in the baseline trailer.

This situation changes when the normalised x-location is moved further downstream to x/H=0.7 (Fig. 14d) and 0.9 (Fig. 14e). At these two locations, lower levels of normalised y-velocity are observed in the region between -0.4<y/H<-1 in all trailers with linear AC-DBD plasma actuation being implemented compared to that of the baseline trailer. This suggests that using linear AC-DBD plasma actuation at the rear end of a square-back trailer, regardless of the actuation frequency used, could reduce the upward flow movement in the wake region at these two locations. Finally, at the normalised x-location x/H= 1.1 (Fig. 14f), the normalised y-velocity profiles become very similar in all cases being studied. This implies that no observable effects in altering the flow velocity in y-direction could be observed by implementing linear AC-DBD plasma actuation at this location within the wake region.

3.2.2. Flow Steadiness and Turbulence

Other than the velocity field, flow fluctuation and turbulence level induced by using linear AC-DBD plasma actuation have also been investigated. Figures 15 and 16 show the normalised root-mean-square (rms) velocity in x- and y-directions along the wake region of the trailer model. From the normalised rms x-velocity contours, it can be seen from the baseline trailer (Fig. 15a) that relatively strong rms x-velocity appears along the upper shear layer (i.e. the region between 0<x/H<1.75 and -0.25<y/H<0.25) and its levels remain relatively low in the region between 0<x/H<0.8 and 0<y/H<-1.5 (i.e. the location at which the wake vortex located). Returning to the upper shear layer, it can be seen from Figure 15b-f that the levels of normalised rms x-velocity increase when linear AC-DBD plasma actuation is implemented. In fact, the levels of normalised rms x-velocity in the upper shear layer increase progressively with the plasma actuation frequency from 7 to 9 kHz (Fig. 15b-d) at which the highest levels of normalised rms x-velocity are observed. The rms x-velocity levels drop progressively with increasing plasma actuation frequency to 10 and 11 kHz (Fig. 15e and f). The results shown in Figure 15 suggested that using linear AC-DBD plasma actuation at the rear end of a square-back trailer model increases velocity fluctuation in x-direction along the upper shear layer. In addition, it also shows that the actuation frequency plays a role in determining the levels of velocity fluctuation in x-direction along the upper shear layer.

In contrast, the implementation of linear AC-DBD plasma actuation at the trailer rear end seems able to reduce the velocity fluctuation in y-direction as shown in Figure 16. Again, relatively high levels of rms y-velocity is shown along the upper shear layer downstream of the rear end of the trailer model (Fig. 16a). However, when linear AC-DBD plasma actuation is implemented, it could be seen from Figures 16b-f that the levels of normalised rms y-velocity along the upper shear layer decrease progressively with the actuation frequency from 7 to 11 kHz. This indicated that using linear AC-DBD plasma actuation could reduce flow fluctuation in y-direction and its effects depend on the actuation frequency used.

Figure 15. Normalised rms x-velocity contour downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

Figure 16. Normalised rms y-velocity contour downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

Flow turbulence downstream of the rear end of the trailer model is presented in the form of turbulent kinetic energy (tke). Figure 17 shows the normalised turbulent kinetic energy contour downstream of the rear end of the trailer model with and without linear AC-DBD plasma actuation being implemented. In general, from Figure 17, it can be seen that considerably high levels of turbulent kinetic energy occur along the upper shear layer because of the occurrence of flow separation immediately downstream of the trailer rear end. The turbulent kinetic energy levels remain considerably low in the region at which the wake vortex is presented (i.e. between 0<x/H<0.8 and -1.5<y/H<0). From Figure 17, it is clear that using linear AC-DBD plasma actuation at the trailer rear end changes the turbulent kinetic energy levels along the upper shear layer. In the baseline trailer (Fig. 17a), moderate levels of turbulent kinetic energy occur along the upper shear layer in the region between -0.1<y/H<0.1.

Figure 17. Normalised turbulent kinetic energy downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

When linear AC-DBD plasma actuation is implemented, at 7 kHz (Fig. 17b), higher levels of turbulent intensity could be observed along the upper shear layer compared to that shown in the baseline trailer. The levels of turbulent kinetic energy increase further with increasing the actuation frequency to 8 (Fig. 17c) and 9 kHz (Fig. 17d). Further increasing the actuation frequency to above 9 kHz reduces the turbulent kinetic energy levels along the upper shear which can be seen from Figures 17e and f for the plasma actuation frequencies of 10 and 11 kHz, respectively. One interesting point should be noted here is that although using linear AC-DBD plasma actuation generally increases the turbulent kinetic energy levels along the upper shear layer; low levels of turbulent kinetic energy were shown in the region at which the wake vortex is presented (i.e. between 0<x/H<0.8 and -1.5<y/H<0) in all cases being studied. In fact, the turbulent kinetic energy levels along the wake region could be further quantified using the normalised turbulent kinetic energy profiles measured at various locations along the wake region shown in Figure 18.

Figure 18. Normalised turbulent kinetic energy profiles measured at various normalised x-locations along the wake region. x/H = (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, (e) 0.9 and (f) 1.1.

From Figure 18, it can be seen that in the region between -1.5<y/H<-0.2 (i.e. below the upper shear layer), very similar turbulent kinetic energy profiles are shown at the locations x/H=0.1, 0.3, 0.5 and 0.7 in all cases being studied. However, stronger fluctuation of turbulent kinetic energy levels in the wake vortex (i.e. the region between 0<x/H<0.8 and -1.2<y/H<-0.2) is observed in those trailers with linear AC-DBD plasma actuation being implemented. This could be confirmed by observing Figures 18a, c and d at the locations x/H=0.1, 0.5 and 0.7, respectively. This might be due to more kinetic energy is being transferred from the upper shear layer to the wake vortex that presents immediately downstream of the rear end of the trailer model. Further downstream of the trailer rear end, at the normalised x-locations x/H=0.9 (Fig.18e) and 1.1 (Fig. 18f), the fluctuation of the turbulent kinetic energy levels is even stronger in the region between -1.5<y/H<0.4 (i.e. below the upper shear layer) in those trailers with linear AC-DBD plasma actuation being implemented. This further suggests that more kinetic energy being transferred from the upper shear layer to generate eddies along the wake region when using linear AC-DBD plasma actuation at the trailer rear end. In addition, from Figure 18, it can be confirmed that, compared to the baseline trailer, using linear AC-DBD plasma actuation generally increases the turbulent kinetic energy levels along the upper shear layer. This can be confirmed by observing the normalised turbulent kinetic energy levels at y/H=0 (i.e. the middle of the upper shear layer) at the locations between 0.3<x/H<1.1 (Fig. 18b-f).

3.2.3. Flow vorticity

The normalised z-vorticity contour downstream of the rear end of the trailer model with and without linear AC-DBD plasma actuation being implemented is provided in Figure 19. In the baseline trailer (Fig. 19a), it can be seen that negative z-vorticity appears in the upper shear layer while positive z-vorticity presents in the lower shear layer. In addition, the coil-up of the lower shear layer that leads to the formation of the wake vortex could be seen clearly from Figure 19a. Considerably high levels of positive z-vorticity appear in the wake vortex. In contrast, relatively low levels of z-vorticity are shown downstream of the wake vortex of the baseline trailer. For those trailers with linear AC-DBD plasma actuation being implemented (Fig. 19b-f), the z-vorticity contours remain similar to that shown in the baseline trailer (Fig. 19a) regardless of the actuation frequency used. This implies that no observable effects in changing the z-vorticity levels along the wake region could be provided by using linear AC-DBD plasma actuation at the rear end of a square-back trailer.

Figure 20 shows the normalised z-vorticity profiles measured at various normalised locations downstream of the trailer rear end. From Figure 20, it can be seen that compared to the baseline trailer, using linear AC-DBD plasma actuation at the rear end of a square-back trailer shows no obvious effects in changing the z-vorticity levels along the wake region below the upper shear layer (i.e. the region between -1<y/H<-0.3) at all locations being studied (Fig. 20a-f). However, in general, stronger vorticity fluctuation downstream of the trailer rear end could be observed in those trailers with linear AC-DBD plasma actuation being implemented. Interestingly, the only exception appears when the actuation frequency is at 11 kHz. At this actuation frequency, the z-vorticity profiles become similar to those shown in the baseline trailer at all normalised x-locations being studied.

Figure 19. Normalised z-vorticity downstream of the trailer rear end at various plasma actuation frequencies. (a) Baseline, (b) 7 kHz, (c) 8 kHz, (d) 9 kHz, (e) 10 kHz and (f) 11 kHz.

Figure 20. Normalised z-vorticity profiles at various normalised x-locations along the wake region. x/H= (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, (e) 0.9 and (f) 1.1.

3.2.4. General Discussion of Results

In section 3.2.1 thru to 3.2.3, the effects of using linear AC-DBD plasma actuation at the trailer rear end in altering the flow characteristics along the wake region have been discussed. This section aims to summarise the results that already presented and provide a general discussion about could flow control be achieved by implementing linear AC-DBD plasma actuation at the rear end of a square-back trailer? From the data shown previously about the velocity field downstream of the trailer rear end, it can be seen that using linear AC-DBD plasma actuation at the rear end of a square-back trailer model shows no observable effects in affecting the size and shape of the wake vortex downstream of the trailer rear end. In addition, linear AC-DBD plasma actuation also shows no obvious effects in changing the flow velocity in x-direction along the wake region although it does affect the flow velocity in y-direction. In contrast, it is clear that compared to the baseline trailer, implementation of linear AC-DBD plasma actuation at the trailer rear end generally increases the turbulent kinetic energy levels along the upper shear layer. It also increases fluctuation in turbulent kinetic energy and z-vorticity along the wake region. The remaining questions here are (i) would the drag encountered by the articulated lorry model increase or decrease due to higher levels of turbulent kinetic energy being generated along the upper shear layer? (ii) Would increase in the levels of vorticity and turbulent kinetic energy fluctuation along the wake region induced by using linear AC-DBD plasma actuation affect the drag encountered by an articulated lorry? (iii) Would the increase in the drag level encountered by a scale articulated lorry model with linear AC-DBD plasma actuation being implemented mentioned in Roy et al. [34] is caused by increasing in the turbulent kinetic energy levels along the upper shear layer as well as stronger fluctuation in vorticity along the wake region?

There seems no information regarding the relations between turbulent kinetic energy levels and drag in a flow field could be found in literature. Morton [47] proposed a mathematical relation which aims to relate aerodynamic drag with the ratio between flow kinetic energy (K.E.) and vorticity (ω) of two-dimensional bluff bodies in turbulent flow. The author in [47] proposed that, for a given Reynolds number, the drag encountered by a two-dimensional bluff body in turbulent flow is related to the kinetic energy-to-vorticity ratio of the flow. This means that increasing turbulent kinetic energy generation as well as the levels of turbulent kinetic energy and vorticity fluctuation induced by using linear AC-DBD plasma actuation could potentially affect the drag level that encountered by an articulated lorry.

In fact, a recent publication by Agrwal et al. [48] also mentioned that vorticity fluctuation along the wake region could affect the drag level encountered by a two-dimensional prism. Unfortunately, the force balance that installed in the wind tunnel used in the present study is not suitable for conducting aerodynamic force measurements to small scale models. As a result, no drag data could be provided in the present study. Further investigations are required to be conducted in order to fully understand how the kinetic energy-to-vorticity ratio of the flow affects the drag level encountered by a three-dimensional bluff body in a fully turbulent flow. Nevertheless, the publication by Morton [47] and Agrwal et al. [48] provided some preliminary insights that might explain why drag is increased when using linear AC-DBD plasma actuation at the rear end of a square-back articulated lorry model shown in Roy et al. [34].

4. Limitations of Study and Future Research

There are three major limitations presented in the current study. Firstly, the optics used for conducting PIV measurements are mounted on a rail that situated along the centreline on the roof of the wind tunnel test section. As a result, currently only PIV data collected along the centreline of the tractor-trailer model could be presented. However, as the flow over an articulated lorry is fully three-dimensional, it is possible that different conclusions could be obtained if PIV data collected from other planes are considered. As a result, it is highly recommended that PIV measurements in other planes in both streamwise and spanwise directions to be conducted in order to fully resolve the flow characteristics along the wake region of a tractor-trailer vehicle with and without linear AC-DBD plasma actuation being implemented. Moreover, only time-averaged PIV data was presented in this study due to the repetition rate of the laser used in the PIV measurements is insufficient to provide time-resolved PIV data. Since the flow over a tractor-trailer is highly unsteady; it is more appropriate to study the flow characteristics using instantaneous PIV data [5]. It is recommended that time-resolve PIV measurements should be conducted in the future in order to resolve the time-dependent flow physics along the wake region of an articulated lorry with and without linear AC-DBD plasma actuation being implemented. Finally, no drag data could be provided in the present study. This is due to the sting balance available in the de Haviland wind tunnel is designed for aerodynamic force measurements of large-scale aircraft models. Its resolution is insufficient to resolve the drag force that acting on the articulated lorry model used due to its relatively small size.

5. Conclusions

An experimental study has been conducted to investigate the time-average streamwise flow characteristics along a 1:20 scale articulated lorry model with and without implementation of linear AC-DBD plasma actuation at the rear end of the trailer. Two-component particle image velocimetry measurements were conducted to obtain the time-averaged flow velocity, turbulence and vorticity information along the centreline downstream of the trailer rear end. Results about the time-average velocity field show that using linear AC-DBD plasma actuation at the trailer rear end could neither alter the size nor the flow velocity in x-direction along the wake region of the trailer model. However, it did show that the flow velocity in y-direction along the wake region could be altered by the effects of linear AC-DBD plasma actuation.

Turbulent kinetic energy was used to assess the flow turbulence levels along the wake region of the trailer model with and without linear AC-DBD plasma actuation being implemented. Results show that higher levels of turbulent kinetic energy were generated in those trailers with linear AC-DBD plasma actuation being implemented at their rear end. It was observed that the highest levels of turbulent kinetic energy appeared along the upper shear layer when the actuation frequency is at 9 kHz. In addition, it was concluded that using linear AC-DBD plasma actuation at the rear end of a square-back trailer model also increased the levels of turbulent kinetic energy and flow vorticity fluctuation along the wake region downstream of the trailer rear end. It is unclear that whether the drag encountered by an articulated lorry model would be affected by the increase in the fluctuation of the turbulent kinetic energy and flow vorticity along the wake region as induced by using linear AC-DBD plasma actuation at the trailer rear end. Further research is required to be conducted in this area in order to understand the role of kinetic energy-to-vorticity ratio in affecting the drag level of three-dimensional objects in turbulent flow. Finally, limitations of the present experimental study have been discussed and future research directions have been proposed.

References

  1. European Commission research and innovation: <<Aerodynamics and Flexible Trucks>> webpage. https://ec.europa.eu/research/participants/portal/desktop/en/opportunities/h2020/topics/gv-09-2017.html

(Date of Visit: 23rd January, 2017).

  1. Altaf, A.; Omar, A.A.; Asrar, W. Passive drag reduction of square back road vehicles. J. Wind Eng. Ind. Aerodyn. 2014, 134, 30-43.
  2. Bradley R.: Technology Roadmap for the 21st Century Truck Program. Tech. Rep. 21 CT-001, United States Department of Energy, Washington DC, United States, 2000.
  3. Hsu, F.-H.; Davis, R.L. Drag reduction of tractor-trailers using optimized add-on devices. J. Fluid Eng. 2010, 132, 0845041-0845046.
  4. Choi, H.; Lee, J.; Park, H. Aerodynamics of Heavy Vehicles. Annu. Rev. Fluid Mech. 2014, 46, 441-468.
  5. Lo, K.H.; Kontis, K. Flow characteristics over a tractor-trailer model with and without vane-type vortex generator installed. J. Wind Eng. Ind. Aerodyn. 2016. 159, 110-122.
  6. Wood, R.M. A discussion of a heavy truck advanced aerodynamic trailer system. Presented at the International Forum of Road Transport Technology (IFRTT) 9th International Symposium on Heavy Vehicle Weights and Dimensions, Pennsylvania State University, State Colleague, Pennsylvania, United States, June 18-22, 2006.
  7. Balkanyi, S.R.; Bernal, L.P., Khalighi, B. Analysis of the near wake of bluff bodies in ground proximity. ASME Pap. 2002, 2002-32347.
  8. Verzicco, R.; Fatica, M.; Iaccarino, G.; Moin, P.; Khalighi, B. Large eddy simulation of a road vehicle with drag-reduction devices. AIAA Journal 2002, 40, 2447-2455.
  9. Yi, W. Drag reduction of a three-dimensional car model using passive control device. PhD Thesis, Seoul National University, Korea, 2007.
  10. Peterson, R.L. Drag reduction obtained by the addition of a boattail to a box shaped vehicle. . Technical report of National Aeronautics and Space Administration, 1981, NASA-CR-163113. NASA, Washington, DC, United States.
  11. Croll, R.H.; Gutierrez, W.T.; Hassan, B.; Suazo, J.E.; Riggins, A.J. Experimental investigation of the ground transportation system (GTS) project for heavy vehicle drag reduction. SAE Technical Paper 1996, paper no. 960907.
  12. Mugnaini, C.M. Aerodynamic drag reduction of a tractor-trailer using vortex generators: a computational fluid dynamic study. Master of Science Thesis, California State University, Sacramento, United States, 2015.
  13. Patten, J.; McAuliffe, B.; Mayda, W.; Tanguay, B. Review of aerodynamic drag reduction devices for heavy trucks and buses. Technical report of National Research Council Canada, Center for Surface Transportation Technology, 2012, CSTT-HVC-TR-205, Ottawa, Canada.
  14. Leuschen, J.; Cooper, K.R. Full-scale wind tunnel tests of production and prototype, second-generation aerodynamic drag-reducing devices for tractor-trailers. SAE Technical Paper 2006, 2006-01-3456.
  15. Englar, R.J. Advanced aerodynamic devices to improve the performance, economics, handling and safety of heavy vehicles. SAE Technical Paper 2001, 2001-01-2072.
  16. Littlewood, R.P.; Passmore, M.A. Aerodynamic drag reduction of a simplified squareback vehicle using steady blowing. Exp. Fluids 2012, 53, 519-529.
  17. Howell, J.; Sheppard, A.; Blakemore, A. Aerodynamic drag reduction for a simple bluff body using base bleed. SAE Technical Paper 2003, 2003-01-0995.
  18. Amitay. M.; Menicovich, D.; Gallardo, D. Enhanced fuel efficiency on tractor-trailers using synthetic jet-based active flow control. Proceeding SPIE 9801, Industrial and Commercial Applications of Smart Structures Technologies 2016, 980102.
  19. Gillieron, P.; Kourta, A. Aerodynamic drag reduction by vertical splitter plates. Exp. Fluids. 2010, 48, 1-16.
  20. Font, G.I. Boundary layer control with atmospheric plasma discharges. AIAA Journal 2006, 44, 1572–1578.
  21. Hale, C.; Erfani, R.; Kontis, K. Multiple encapsulated electrode plasma actuators to influence the induced velocity: Further configurations. 40th Fluid Dynamics Conference and Exhibit, Chicago, AIAA-2010-5106, 2010.
  22. Jacob, J.; Rivir, R.B.; Carter, C.; Estevadeordal, J. Boundary layer flow control using ac discharge plasma actuators. 2nd AIAA Flow Control Meeting, Portland, AIAA-2004-2128, 2004.
  23. Ruisi, R.; Zare-Behtash, H.; Kontis, K.; Erfani, E. Active flow control over a backward-facing step using plasma actuation. Acta Astronautica 2016, 126, 354-363.
  24. Moreau, E. Airflow control by non-thermal plasma actuators. Journal of Physics D: Applied Physics 2007, 40, 605-636.
  25. Roth, J.R. Aerodynamic flow acceleration using paraelectric and peristaltic electrohydrodynamic effects of a One Atmosphere Uniform Glow Discharge Plasma. Physics of Plasma 2003, 10 (5), 2117.
  26. Corke, T. Plasma flow control optimized airfoil. 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, AIAA-2006-1208, 2006.
  27. He, C.; Patel, M.P.; Corke, T.C. Plasma flaps and slats: an application of weakly ionized plasma actuators, Journal of Aircraft 2009, 46, 864–873.
  28. Post, M.L.; Corke, T.C. Separation control using plasma actuators: dynamic stall vortex control on oscillating airfoil. AIAA Journal 2006, 44, 3125–3135.
  29. Ramakumar, K.; Jacob, J.D. Flow control and lift enhancement using plasma actuators. 35th AIAA Fluid Dynamics Conference and Exhibit, Toronto, AIAA 2005-4635, 2005.
  30. Thomas, F.O.; Kozlov, A.; Corke, T.C. Plasma actuators for cylinder flow control and noise reduction. AIAA Journal 2008, 46, 1921–1931.
  31. Luo, X. Plasma based jet actuators for flow control. PhD Thesis, University of Southampton, United Kingdom, 2012.
  32. Do, H.; Kim, W.; Mungal, M.G.; Cappelli, M.A. Bluff body flow separation control using surface dielectric barrier discharges. 45th AIAA Aerospace Sciences Meeting and Exhibit, Reno, AIAA-2007-939, 2007.
  33. Roy, S.; Zhao, P.; DasGupta, A.; Soni, J. Dielectric barrier discharge actuator for vehicle drag reduction at highway speeds. AIP Advances 2016, 6, 025322.
  34. Lo, K.H.; Kontis, K. Flow around and articulated lorry model. Experimental Thermal and Fluid Science 2017. 82, 58-74.
  35. Taubert, L.; Wygnanski, I. Preliminary experiments applying active flow control to a 1/24th scale model of a semi-trailer truck. Lecture Notes in Applied and Computational Mechanics 2009. 41, 105-113.
  36. Ortega, J.; Salari, K.; Storms, B. Investigation of tractor base bleeding for heavy vehicle aerodynamic drag reduction. In proceeding of the International Conference on Aerodynamics of Heavy Vehicles II: Trucks, Buses and Trains, 26-31 August, 2007, Lake Tahoe, California, United States.
  37. Erfani, R.; Zare-Behtash, H.; Kontis, K. Influence of shock wave propagation on dielectric barrier discharge plasma actuator performance. Journal of Physics D: Applied Physics 2012, 45, 225201.
  38. Erfani, R.; Zare-Behtash, H.; Hale, C.; Kontis, K. Development of DBD plasma actuator: The double encapsulated electrode. Acta Astronautica 2015, 109, 132-143.
  39. Gurlek, C.; Sahin, B.; Ozkan, G.M. PIV studies around a bus model. Experimental Thermal and Fluid Science 2012. 38, 115-126.
  40. Krajnovic, S.; Davidson, L. Numerical study of the flow around a bus-shaped body. ASME J. Fluids Eng. 2003. 125, 500-509.
  41. Hucho, W.-H.; Sovran G. Aerodynamics of road vehicles. Annu. Rev. Fluid Mech. 1993. 25, 485-537.
  42. Kim, D.; Lee, H.; Yi, W.; Choi, H. A bio-inspired device for drag reduction on a three-dimensional model vehicle. Bioinspir. Biomim. 2016. 11, 026004.
  43. Erdem, E.; Kontis, K. Numerical and experimental investigation of transverse injection flows. Shock Waves 2010. 20 (2), 103-118.
  44. Lo, K.H.; Kontis, K. Flow characteristics of various three-dimensional rounded contour bumps in a Mach 1.3 freestream. Experimental Thermal and Fluid Science (in press, doi: 10.1016/j.expthermflusci.2016.08.027).
  45. Lo, K.H.; Zare-Behtash, H.; Kontis, K. Control of flow separation on a contour bump by jets in a Mach 1.9 freestream: an experimental study. Acta Astronautica 2016. 126, 229-242.
  46. Morton, T.S. A correlation between drag and an integral property of the wake. Journal of Scientific and Mathematical Research 2007, 1, 2-20.
  47. Agrwal, N.; Dutta, S.;Gandhi, B.K. Experimental investigation of flow field behind triangular prisms at intermediate Reynolds number with different apex angles. Experimental Thermal and Fluid Science 2016. 72, 97-111.

Cite This Work

To export a reference to this article please select a referencing stye below:

Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.

Related Services

View all

DMCA / Removal Request

If you are the original writer of this essay and no longer wish to have the essay published on the UK Essays website then please:

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams Prices from
£124

Undergraduate 2:2 • 1000 words • 7 day delivery

Order now

Delivered on-time or your money back

Rated 4.2 out of 5 by
Reviews.co.uk Logo (29 Reviews)