Shock/Boundary Layer Interaction is unwanted phenomenon that occurs during the high speed fluid flow. Recently developed novel flow control device called micro-ramp (part of micro-vortex generator) shown potential in controlling this problem. Thus, this research aims to study the capability of micro-ramps in controlling SBLI under hypersonic condition. Hypersonic is chosen because most of the previous experiments are conducted under supersonic condition.Â In this experiment, several models of micro-ramps will be placed under the hypersonic flow (Mach 5) and oblique shock waves will be generated. The flow characteristic recorded from Schlieren photograph will then be investigated.
Aim and Background
Hypersonic aircraft relies heavily on their air-breathing propulsion system. However, the existences of Shock/Boundary Layer Interaction (SBLI) during high speed airflow critically lower the performance of the aircraft engine.
Shock/Boundary Layer Interaction (SBLI) is a natural phenomenon that is frequently a defining feature in high speed aerodynamic flow fields. The interactions can be found in practical situations, ranging from transonic aircraft wings to hypersonic vehicles and engines. When shock wave interacts with boundary layer flow, like the figure below, diverse types of flow will occur such as the flow separation, unsteadiness, vertical flow, pressure waves etc.
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Picture credit to H.D. Kim & T.Setoguchi, "Shock Induced Boundary Layer Separation", 8th International Symposium on Experimental & Computational Aerothermodynamics of Internal Flow, Lyon, France, July-2007
The effect of SBLI on external flow is that it increases the aerodynamic drag, which relatively reduces the lift. It will also induce aerodynamic heating and increase instabilities such as inlet buzz and buffeting. Meanwhile, for internal flow, it can cause total pressure loss, unsteadiness and loss of flow control performance.
There are several factors that influence this unwanted phenomenon. The factors include the flow Mach number, Reynolds Number, Characteristic of Boundary layer Flow (Laminar/Turbulent, Boundary layer thickness) and flow geometry (pressure gradient). As Mach number increases, the minimum of wall shear stress is decreased and it will reach zero at a point where a tiny separation bubble is formed. The more Mach number increases, the more the bubble separation grows. SBLI reduce the quality of flow field by triggering large-scale separation, making the flow unsteady and distorted due to total pressure loss, and more worst make an engine unable to start. Most of the interactions are caused by oblique shock waves, but the end interaction is normally due to a normal shock.
The traditional way to control the SBLI's effects is by a bleed technique, which removes low-momentum flow from the boundary layer using suction through a porous surface on the inlet wall. Although this technique reduces the characteristic effects on the boundary layer, bleed systems are heavy and complex, decrease mass flow to the engine and introduce additional drag. As a result, this bleed technique gives a very low efficiency. Therefore, many different types of vortex generators (VGs) have been proposed and investigated as possible replacements to bleed systems.
Micro-ramp, a part of micro-vortex generators is the novel type of flow control device that can gives similar control benefits as the bleed technique but without bleed penalty. It has the ability to alter the near-wall structure of compressible turbulent boundary layers to provide increased mixing of high speed fluid which improves the boundary layer health when subjected to flow disturbance. Since their size is small, micro-ramp are embedded in the boundary layer which provide reduced drag compared to the traditional vortex generators while they are cost-effective and do not require a power source.
The initial idea on using micro-ramp as SBLI control device was proposed by Prof. Babinsky *. They conducted experimental investigations at Mach 2.5 based on the geometries of micro-ramp suggested by Anderson * in their numerical optimization studies. From the experiment, they observed that micro-ramp produces a counter-rotating streamwise vortices flowing downstream and this vortices helped to suppress the SBLI's effect. This motion transports the low-momentum flow at the wall surface to the outer regions of the boundary layer and simultaneously brings the high-momentum flow from the outer regions towards the surface of the wall. As a result, more healthier and robust boundary layer was formed, which is less prone to flow separation.
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Later, the flow characteristic in the downstream region of the micro-ramp was produced in detailed by Li & Liu *[3,4], in their numerical investigation. It was then identified that a chain of vortex ring structures originated from the apex of the micro-ramp due to Kelvin-Helmhotz instability, and these structures propagate further downstream and interact with the impinging shock wave, eventually distorting the structure of the shock wave hence reducing its strength.
Until now, most studies of micro-vortex generators in SBLI have been performed in simple flow-fields, such as the flat plate normal or oblique SBLI and also in supersonic condition. Thus, this research will focuses on hypersonic condition because there's lack of research and knowledge of MVG under this condition. It is important to explore the ability of micro-ramp in controlling SBLI effects under hypersonic condition and later improve the separated boundary layer caused by the incident shock.
The outcome of this research is important to improve the current understanding and knowledge on the efficiency of micro-ramps in hypersonic condition. For this experiment, we will observe the behaviour of the flow over and downstream the micro-ramps and its capability in controlling SBLI effect in a flow speed of Mach 5.
Significant of Research
This research is significant because it can show us the ability of micro-ramps in controlling Shock/Boundary Layer Interaction. As mentioned before, the SBLI will make the flow field become unsteady and can lower the aircraft engine performance. It is necessary for engineers to minimize the effects of SBLI so that aircraft flying in hypersonic condition do not have degraded performance. This is very important especially for military/ jet fighter aircraft and outer space rocket which usually flying in hypersonic.
For this experiment, we need to have the following technology/system to fulfil our experiment expectation. The required system/technology is as follow:
High Supersonic Wind Tunnel
High Supersonic Wind Tunnel is a wind tunnel that can produce hypersonic speeds (Mach number 5 to 15). These types of tunnel must run intermittently with very high pressure ratios when initializing. Since there is temperature drop during expanding flow, the air inside have a possibilities to become liquefied. Thus, pre-heating must be applied and the nozzle may require cooling. Shock tube can be use to produce high pressure and temperature ratios.
(Definition of HSST, sources from: Wikipedia, Hypersonic wind tunnel, http://en.wikipedia.org/wiki/Hypersonic_wind_tunnel)
Figure : High Supersonic Wind Tunnel (HSST)
Micro-ramps are parts of micro vortex generators, which is an aerodynamic surface with small vane that intended to produce vortex. Vortex generators can improve the efficiencies of wings and control surfaces by delaying the flow separation and aerodynamic stalling.
In this experiment, two types of micro ramps model will be investigated. These two models can be differentiated by their height percentage from boundary layer thickness. The first model is MR70 (the height is 70% from boundary layer thickness) and the next one is MR30 (30% from boundary thickness). Boundary layer thickness is defined as the distance between the walls to the point where the velocity of the flow becomes free stream velocity. This boundary layer thickness can be identified from preliminary experiment by using very fast Schlieren images. The boundary layer obtained for this experiment is approximately around 5.8 mm. Later, we designed the micro-ramps based on the dimension proposed by Anderson, with the ratio given by w/h = 5.86 and c/h = 7.2. The micro ramps model dimensions will be stated in the table below:
Picture credit Mohd R.Saad, Hossein Zare-Behtash, Azam Che-Idris and Konstantinos Kontis, "Micro-Ramps for Hypersonic Flow Control", Micromachines
Toepler's z-type Schlieren system
Schlieren picture is used to visual the flow of fluids of varying density around the experimented object. The concept behind this system is that the light from a single collimated source shining on a target object. Later, the collimated light beam will be distorted due to the variations in refractive index of density gradient existed in the fluid. This distortion creates a spatial variation in the intensity of the light, which can be visualized directly with a shadowgraph system.
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This system is provided in many laboratory, for example in Aero-Physics Laboratory in University of Manchester, which consist of a continuous light source with a focusing lens and wide slit, 2 parabolic silver coated mirrors, a knife-edge and macro lens used for focusing purposed.
Full-size image (20 K)
Picture taken from: ScienceDirect, Optics & Laser Technology, "Temperature measurement of air convection using Schlieren system", http://www.sciencedirect.com/science/article/pii/S0030399208001357
IR Thermography can be used to detect radiation in the infrared range of the electromagnetic spectrum and display the images of that radiation. From Black Body Radiation Law, which stated that all objects above absolute zero will emit infrared radiation, it is possible for us to see one's environment with or without visible illumination by using thermography. Since radiation increases when temperature increases, we can see the variation in temperature using thermography.
For this experiment, we will be using FLIR Thermacam SC 3000 Cooled System. With the temperature range detection of -250K to 1,730K and sensitivity of Â±2%, this camera is very suitable for any experiment under hypersonic condition. The recording frequency is 50Hz and added with Cooling system allows it to be cooled to 70K within 6 minutes or less.
ThermaCAMÂ® SC 3000 Infrared Camera
FLIR ThermaCAM SC 3000 Infrared Camera, "http://www.iard.co.il/cubi/Cameras_brief.html"
Since most of the fluids are transparent, we can use flow visualization method to make the invisible flow become visible. For this experiment, we will be using surface flow visualization. This method reveals the streamlines of flow in the limit as a solid surface is approached. Coloured oil flows are used to visualize the flow. The oil that we will be using will be the standard 40W treated with a fluorescent dye or pigment. As the air flows over the model, the oil is carried downstream in a long streak. Surface oil flows will indicate the boundary of a flow separation since the oil cannot penetrate the separation boundary. Treated the surface oil with napthalin can help determine the transition point on the model as oil downstream of the transition point will be swept away.
For this experiment, it is necessary to generate oblique shock waves to observe the effects of micro-ramps in controlling SBLI. Thus, shock generator will be used to produce oblique shock waves on the model.
Firstly, the condition of HSST environment needs to be set up so that it has the flow speed of Mach 5 (with a Reynolds Number of 1,320,000). The operational stagnation pressure and temperature will be set to 650 kPa and 375K respectively. The assumption we made here is that the fluctuation of freestream Reynolds number is very small. Therefore, we can treat the flow as the continuous medium.
Next, the micro ramps models will be mounted on top of aluminium alloy flat plate with this dimension: 300mm long, 50 mm wide and 5 mm thick. From the Toepler's z-type Shlieren system, a Photron APX-RS high speed video camera will be used for high-speed recording at 10,000 fps.
Meanwhile, the thermacam for our infra-red thermography will be mounted on top of the test section at a certain angle from vertical plane, and it is connected to a PC for image storage.
For the oil surface flow visualization, the oil is applied during the wind tunnel is stopped. After the tunnel is operated to its desired condition, we let it run until the surface oil flow streaks are properly established. Then the tunnel will be stopped and someone needs to quickly take the picture of the streaks. The thickness of the oil applied must be correct so that the generated streaks length produced will be meaningful. In order to increase the contrast between plate and the fluorescent oil, the flat plate and the micro-ramps will be painted with black colour. Shock generators will be placed on top of the plate to generate oblique shock waves on the micro-ramps models.
For two different models of micro-ramps and one baseline model (without a micro-ramp), the Schlieren pictures, thermal image and surface flow visualization pictures for each of them will be collected at the end of the experiment. The results obtained will then be analyzed and discussed.
Model set-up, etc
(Material cost & manufacturing process)
(Provided in most of the lab)
Fluorescent oil, napthaline
Based on the proposed timeline and cost, we can see that this experiment is quite feasible. The estimated duration for this experiment is about a week, which is reasonable. The one day on preparation is needed to get the general idea on how the experiment will be conducted. Then, more time should be spend on setting up the laboratory and tools required for the experiment. Micro-ramp can be manufactured by using mechanical machine (guided by an expert). This is important stage as we don't want our model to have any surface error, which can affect the outcome of the experiment. Preliminary experiment is important so that we can have some expected result for our real experiment. Two experiments are enough to get the average results.
In term of cost, some of the materials have been provided by the laboratory. For thermaCAM, the actual cost for buying a new one is quite expensive. Therefore, renting it will be a better choice.
From the analysis of the previous researches, we can have some expectation on what would be observed at the end of the experiment. First, there will be a visible strong shock wave which can be seen originating from micro-ramp leading edge and a weaker shock wave at the trailing edge of the model. Since the exposure time in normal schlieren is too long, the incoming boundary layer can only been seen in high-speed schlieren photograph.
Next, the shock structures between two models are not too distinct but thicker shock line can be observed only at the trailing edge of thicker model (in this experiment, it is on MR 70). A bigger model also will be expected to deflect more flow due to its larger surface area, which requires stronger shock.
From the surface visualization, we expect to see some heavy accumulation of oil at the micro-ramp's leading edge, which indicates the flow separation. At the downstream of the model, there will be a trail of primary vortices which can be determined by an area that is not covered with oil. It is because vortices motions prevent the accumulation of oil on the surface.
Chronology of surface oil-flow experiment (Picture taken from Mohd R.Saad, Hossein Zare-Behtash, Azam Che-Idris and Konstantinos Kontis, "Micro-Ramps for Hypersonic Flow Control", Micromachines)
Another expected observation is that the secondary vortices will appear bigger near the centreline and at the downstream of the model will have a larger wake area.
The infra-red thermal imaging of the surface temperature for both models also can investigate the behaviour of the primary and secondary vortices.
Impact of research
The result of the experiment can help improve our knowledge about the relationship and ability of the micro-ramp as a flow control device in controlling SBLI's effects under hypersonic condition. Practical use of micro-ramp on hypersonic aircraft or engine can greatly reduce the aerodynamic loss, increase the overall performance and minimise the energy loss. It is also environmental-friendly device as it is just a simple mechanical device without any use of energy/chemical. Since the energy loss is minimise, less fuel consumption will be needed to operate the aircraft thus reduce the operational cost.
Mohd R.Saad, Hossein Zare-Behtash, Azam Che-Idris and Konstantinos Kontis, "Micro-Ramps for Hypersonic Flow Control", Micromachines, Published on 26th April 2012
Sang Lee, Large Eddy Simulation of Shock Boundary Layer Interaction Control Using Micro-Vortex Generators, University of Illinois, 2009
Qin Li and Chaoqun Liu , Implicit LES for Supersonic Microramp Vortex Generator: New Discoveries and New Mechanisms, Modelling and Simulation in Engineering Volume 2011 (2011), Article ID 934982, 15 pages doi:10.1155/2011/934982, Accepted 12th January 2011, distributed under the Creative Commons Attribution License.
Holger Babinsky, AFRL-AFOSR-UK-TR-2011-0014 Shock Boundary Layer Interaction Flow Control with Micro Vortex Generators, May 2011, University of Cambridge Engineering Department
Lapsa, Andrew P, Experimental Study of Passive Ramps for Control of Shock-Boundary Layer Interactions, Issue Date 2009
Micro-ramp Flow Control for Oblique Shock Interactions: Comparisons of Computational and Experimental Data, Stefanie M.Hirt (NASA Glenn Research Center), David B. Reich (University of Florida), Michael B. O'Connor (University of Notre Dame).