Methods Of Evaluation Of Vehicle Aerodynamics
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Part a) Different methods of evaluation of vehicle aerodynamics:
In recent years aerodynamics of vehicle has gained a lots of attention because of the major use of negative lift (down force) principle which improves the performance of vehicles. Thus evaluating vehicle aerodynamics has become an important part of car engineering and it is no longer only limited to vehicle's initial designing. Several method and tools are used in this process such as wind tunnel testing, computational fluid aerodynamics or track testing. Each of these tools has their special need for e.g.:- at initial design stage wind tunnel can be used and after car is built it can be tested on the track. In the following paragraphs i discuss three mostly used methods i.e. wind tunnel testing, CFD software and track testing.
Wind tunnel testing:
In 1870's first wind tunnel was designed but until 1960's it was just meant to use for aeronautical purposes but once this method brought to use for automotive purposes, within no time it became an essential part of car developments and provided good environment for aerodynamic evaluation.
In the wind tunnel testing method a geometrical shapes or models is mounted in the test section of wind tunnel and air is blown over or sucked through a duct by a fan or number of fans. To make measurements more accurate closely spaces vertical and horizontal air vane are used which smoothes the turbulent flow before it reaches the model. Usually, measurements are taken from a balance on which model is mounted and various visualisation techniques such as smoke and tufts are used to understand the affect of some geometric features on aerodynamic performance.
Wind tunnel helps to acquire lots of data such as:
Aerodynamic forces drag, lift, side force
Variation of these forces and movements with yaw
Vehicle cooling drag
Affect of aero features
Surface pressure distribution
There are different types of wind tunnels which are as follows:
LOW SPEED WIND TUNNEL also named as SUBSONIC WIND TUNNEL are of open return type or return flow in which air is moved with a propulsion system made up of a huge axial fan which helps in increasing dynamic pressure and thus helps to overcome viscous loss. In this type mach number is kept very low and speed is maintained up to 400 km/h
http://en.wikipedia.org/wiki/Subsonic_and_transonic_wind_tunnel#Subsonic_tun)Staffordshire University logo
HIGH SPEED OR TRANSONIC WIND TUNNEL is designed to reach the speed close to the speed of sound. Mach and Reynolds number both are important here due to viscous or in viscid interactions. As compared to subsonic wind tunnel, much large scale facilities and pressurized wind tunnels are used.
SUPERSONIC WIND TUNNEL generates supersonic speed. In supersonic wind tunnel, high pressure ratio is required and to avoid occurrence of liquefaction or condensation drying or pre heating facility is required.
HYPERSONIC WIND TUNNEL produces hypersonic flow field in test section. It runs with very high pressure ratio and like supersonic; hypersonic also requires pre heating facility. As compared to other types of wind tunnels, hypersonic wind tunnel has more challenges in its designing.
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WIND TUNNEL SCALE:
Model scale wind tunnel testing (MWT) plays an important role in aerodynamic development of motorsport vehicles, training and research though now it has been outmoded by CFD.
It is an ideal way to determine the affects of various features and body styles on aerodynamics of vehicles. In model scale testing, model lie between 30%,60%nand they can be be produced by various techniques such as fibre glass, stylist clay models, milled foam and rapid prototype models or a combination of all of these.
Model scale testing doesn't needs high running cost as sophisticated test methods such as fitting a moving ground plane doesn't need high expense. In all, if styling method can be utilized than this testing method is a cheaper method to develop vehicle aerodynamics.Staffordshire University logo
In spite of all, model scale testing has its own limitation such as:
Ensuring geometric similarity: some special features can be difficult to model at small model scale.
Reynolds number effects: Reynolds number (Re) of the flow around vehicles reduces with the reduction in scale which then requires an appropriate increase in the airflow.
FULL SCALE WIND TUNNEL TESTING (FSWT):-
FSWT has totally taken over MWT as it many problems faced during MWT are overcome by using full scale testing:
It eliminates the effect caused due to Reynolds number.
It totally avoids the cost of production of models.
It helps to determine the influence of small geometric changes.
Aerodynamics: Radical LMP2 in MIRA's Full Scale Wind Tunnel
COMPUTATIONAL FLUID AERODYNAMICS (CFD):
It is a phenomenon or software used to improve the car aerodynamics. The software provides benefits from number crunching capability of computers used to solve the problems of fluid flow, heat transfer and related processes like chemical reactions with the use of simulations of fluid flow. The fluid here refers to a lot of substances as well which behave like fluids depending on the circumstances they are in, for example, winds blowing around buildings, cooling air flows in a laptop, drugs in asthma inhalers etc. It mainly focuses on affects made by pressure, temperature, density and the velocity of the fluids. It is powerful technique software with a wide range of applications. It is very commonly used software in automotive industry (the method has an ability to calculate the air flows around a vehicle or car), especially in motorsports because of the massive competition. It first came in light in 1990's.The software is now owned by Bar Honda Formula 1 team. The software has always been updated at times as the as the new computer technologies come in. since the software runs of a computer, the speed of the computer processor also plays an important role in improving the performance, in other words, more powerful the supercomputer, more accurate result of CFD process. As the time passes it has become cheaper and advanced.Staffordshire University logo
Streamlines under a stock car (http://strangeholiday.com/oops/stuff/annurev.fluid.38.050304.092016.pdf)
Working process of computational software is divided into phases:
First of all, it divides the geometric structure of the object into a set of elements or cell which named as a mesh. The accuracy of the result of CFD totally depends upon the number of cells as higher their number is, more accurate result will be.
Secondly, mesh representing volume occupied by the fluid is created. Now, when fluid flow comes into contact with the object, it gets separated into as many cells as possible.
Then, computer starts finding out the problem it faced during the second phase. Besides this, it also set up a clear image of interaction of object and fluid flow. Finally, simulation is started and result obtained is analyzed.
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INSTRUMENTED ROAD OR TRACK TESTING:
This testing method has earned lots of popularity in short time as it a very cost effective testing method as all we need to perform this is good weather and cost of renting a track and instrumenting. Aerodynamic drag plays an important role to improve the performance (fuel consumption, speed) of car. These tracks are constructed especially for the vehicles which are new or are under research. The tracks are made under such circumstances that when testing a car it gives the most relevant or most accurate results.
These tests are used to identify the different properties or aspects in different conditions such as traffic flow, weather (raining, dusty, snowing or at different wind flows) and time of day. The car is made run on the track at different and constant speeds to check the flow ratios with the help of the sensors fixed on both sides of the track. The sensors track the dynamic path of the car which give information about the flows and performance of the cars. The aerodynamic behaviour of the vehicle is then analyzed and the car is again made run on the track with improved or modified aerodynamic properties.
Part 2Staffordshire University logo
Computational Fluid Aerodynamics (CFD)
The science that tries to verify a numerical solution to the equations that govern the fluid flow is called CFD (Computational Fluid Dynamics). In the recent years, CFD methods have been utilized as an important tool of design analysis within research organisation and industry with the help of supercomputers. In fact, science is now trying for the theoretical-experimental treatment so that results which are obtained in wind tunnel testing can be used to standardize and prove the numerical models.
As everything have its dual aspects, so as CFD. It has its own lots of advantages and some disadvantages too which are discussed as follows:
A large number of CFD simulations can be performed rapidly and of course it is way cheaper than making a real model and running it in a wind tunnel or an instrumented track. With CFD there is always a greater probability of getting design right first time before manufacturing the actual vehicle because overall concept and shape can be tested. CFD enables the designer to analyze and visualize the flow around the vehicle. It can be done a number of times to facilitate the design optimization. It is much cheaper than to perform a wind tunnel test.
Auto optimization is the advanced form of CFD and is considered to be more accurate. In it computational methods that accesses mathematically the obtained results and makes changes to the configuration and the shape of the vehicle. It automatically selects the most suitable design.
Automatic mesh deformation is one of the advancement in auto optimization. Performing computational optimization method and automatic mesh together, it is possible to explore geometry changes and the software alters the mesh along with the geometry.
CFD technology helps to improve the aerodynamic performance of vehicles and also helps in reducing co2 emmision.Thus, this testing method not only serve a duty for automotive purposes but also helps in keeping environment pollution free.
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No doubt CFD is a best tool among all other ways of determining vehicle aerodynamics but scaling issues can create some limitations to it such as reduicton of transition from laminar to turbulent flows (e.g. boundary layers) or the calculation of separated flow and unsteady wakes.)
CFD can make millions of calculations to achieve an accurate result but the final result can only be appropriate for some reasons like: non accurate or less accurate data, scientific knowledge base or reduction in number of calculations by supercomputer.
So, there are more chances of making numerical errors since everything relays on computers. Besides this, approximation of result can cause Truncation errors and round off errors are also possible due to the word size available on a particular computer
The bottom line is that solutions find by CFD rely upon user-defined elements like: grid generation and turbulence modelling, which will be soon an obstacle facing code developers. It is not yet possible to create full flow field model economically as in mostly vehicles the large scale flow regimes depends on inevitability of transition and turbulence.
Pressure image:E:\25MPH\25mph Pressure.gif
The pressure is very low (-21.71446) on upper and lower surface of wing .It is more (33.87485) on front tip of wing and also it is less( 23.76770) on front upper and lower surface, it also same on back upper and lower surface of wing.
Velocity image:E:\25MPH\25mph Velocity.gif
The velocity is very low (1.854647)on front tip of wing and it is high (6.536719) on upper and lower front surface of it. It is also same on back upper and lower surface of wing. Velocity is really low (9.658100) on lower surface and upper surface of wing but on mid portion of upper surface it is (7.317064)
The pressure is very low (-16.66089) on middle upper and lower surface of wing .It is more (38.92842)on front tip of wing and also it is less( 23.76770) on front upper and lower surface, it also same on back upper and lower surface of wing.
The velocity is very low (2.634993)on front tip of wing and it is high (8.097410) on upper and lower front surface of it. It is also same on back upper and lower surface of wing. Velocity is really high (9.658100) on lower surface and upper surface of wing but on mid portion of upper surface it is (7.317064)
Pressure image:E:\65MPH\65mph pressure.gif
The pressure is very low (-1.500165) on upper and lower surface of wing .It is more (33.87485) on front tip of wing and also it is less( 18.71413) on front upper and lower surface, it also same on back upper and lower surface of wing.
The velocity is high (6.536719)on upper and lower front surface of wing. It is also same on back upper and lower surface of wing. Velocity is really high (9.658100) on lower surface and upper surface of wing but on mid portion of upper surface it is (7.317064)
Pressure image:E:\85MPH\85mph pressure.gif
The pressure is very low (-16.66089) on upper surface and also low on lower surface(-11.60731) of wing .It is more (33.87485) on front tip of wing and also it is less( 23.76770) on front upper and lower surface, it also same on back upper and lower surface of wing.
The velocity is high (8.097410) on upper and lower front surface of it. It is also same on back upper and lower surface of wing. Velocity is really high (9.658100) on lower surface and upper surface of wing but on mid portion of upper surface it is (7.317064)
Part 4: Conclusion: Staffordshire University logo
As the technology improves day by day the things (automobiles) get cheaper, smaller (size) and advanced. Since there are a lot of ways of aerodynamic testing, but the CFD software is the best way of evaluating the aerodynamic performance of the car.
There are a lot of thing which make CFD the best. Like we can also evaluate the aerodynamic performance of the car using methods like wind tunnels but using wind tunnels it becomes far more expensive (making arrangements for the sample model), whereas in CFD we just need a CAD model and everything is done more quickly than any other method. Also if the model needs any changes or modifications it is very easy as compared to making new models for wind tunnels.CFD is more affordable and there is no maintenance required whereas wind tunnels and instrumented tracks needs a lot of maintenance at the time of operation or when free.
There are very less or no chances of any errors in CFD (since everything is done with the help of computer) only a human error could affect the results, whereas in the case of track testing and wind tunnels the probability of making errors is more as big machinery or equipments are used which are analyzed by humans (no doubt computers are more reliable).
There are lots of models (heavy) which are dangerous to test in wind tunnels in full scale, whereas in CFD the simulation can be performed without worrying about anything.
Using the mesh deformation makes the calculations more clear, one can focus on each and every single part the vehicle or model tested as compared to an overall result obtained from track testing or wind tunnels. It takes very less time to carry out the results.
So CFD is the best method for testing the aerodynamic performance and it will become better and cheaper as the computers get more powerful and cheaper.
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