Basics of vehicle aerodynamics. Prof. Tamás Lajos. Budapest University of Technology and Economics. Department of Fluid Mechanics. University of Rome „La. 2. □Course layout. □Importance of vehicle aerodynamics. □Historical review. □Aerodynamics as part of the design process. In fluid mechanical terms, road vehicles are bluff bodies in very close of applied fluid mechanics, the situation in vehicle aerodynamics is.
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Aerodynamics of bacttemcocani.gq Vivek Yakkundi. Uploaded by. Vivek Yakkundi. Table of Contents Sr No Description Page Numbers 1 List of Tables 2 List of. Basics of vehicle aerodynamics Prof. Tamás Lajos Budapest University of Technology and Economics Department of Fluid Mechanics University of Rome „ La. PDF | On Nov 12, , Jason Moffat and others published Aerodynamic Vehicle Design and Analysis.
In the location like stagnation point. As a consequence the overprediction of forebody drag is attributed to the excessive turbulent viscosities predicted near the stagnation point. It is based on the solution of equations for turbulent K. The damping functions lead to turbulence equations with stiff source terms. This aspect and necessary high grid resolution nearby walls in order to resolve the viscous sublayer requires the utilization of atleast point implicit or better f ull implicit time stepping schemes.
The SST model seeks to combine the positive features of both models. One distinct feature of SST turbulence model is the modified turbulent visc osity function.
The purpose is to improve the accuracy of prediction of flows with strong adverse pressure gradients and of pressure induced boundary layer separation. The modification accounts for the transport of the turbulent shear stress. These pertain to scale invariant phenomenon lacking externally imposed characteristic length and time scales.
The concept is to remove a narrow band of high wave number nodes. RNG theory results in decreasing the turbulent Kinetic energy and increasing rate of turbulent dissipation leading to a smaller value of eddy viscosity value. The primary goal of this work is to carry out static pr essure pressure measurement on the 1: In the present work a CAD model is generated in Pro-e, wildfire 2.
Then by using variable section sweep command the model is extruded. Similarly 1: The low speed wind tunnel at I. T, Bombay, Aerospace dept. Refer Fig 17 and In all 53 tappings were taken except two for pitot tube readings for measuring freestream velocity. The car model is screwed in the centre of the test section so as not to have distortion during testing.
However the values fairly match.
In the experiment three velocities of 9. Ahmed et al  Pressure coefficient distribution curves for all the three velocities are plotted. However one curve corresponding to Low speed automotive wind Fig. Tot Drag 0 Poly. Pr Coeff 27 36 45 60 Refer fig These are the plots of pressure distribution coefficient versus X-distance along the vehicle length on a symmetry plane. From these plots the critical areas of drag can be located for the purpose in all 53 pressure tappings are taken plus two more for pitot tube for measuring free stream velocity.
Again at stagnation point there is slight rise in the COP due to change of topology of the body profile. The rise in COP at stagnation point can also be supported by theory of presence of artificial viscosities. This rise continues till the bonnet windshield junction. At this junction there is again interference and the COP dips slightly and rises subsequently upto roof top at the end of wind shield.
Thereafter there is decline in COP as from this point the velocity increases and the situation refers to flow past flat plate where viscous drag dominates. Next at the rear end due to separation the COP falls and remains more or less constant in the rear vertical.
Similarly for the base profile the COP remains steady throughout the length and there is very small decline in the value till end. For establishing linear and second order relation between COP and pressure values ten points including minimum and maximum are considered Fig 23 shows the relations between Static Pressure and Pressure Coefficient, which seems to be linear. These two parameters give us an insight into the beheaviour of a car in windy atmosphere. Table VII shows correlations obtained from the graph fig Polynomial fitting has been done as the R 2 values are better.
The above table VIII shows pressure coefficient values found by various researchers. It is observed that Hatchbacks or the two box version of cars are becoming popular than contemporary Sedan or the three box version inspite of lower drag value of Sedans. Thus creating a conflict of choice amongst customers.
The reasons cited are convenience of luggage boot space inside, smaller and compact size due to reduction in wheel base, better power to weight ratio, better manovurability and lesser parking space. Refer fig 25 and The results of Hatchback and Sedan are compared and it is ascertained that the drag value of Sedan is lower t han Hatchback.
The drag values are also compared with those of generic shapes from literature. Morel 0. More DF l exptl Total Drag 27 36 45 60 Pr Drag 0. Total Drag Pr Drag 0. Figs 33 to From skin friction coefficient images it is observed that the drag value for Hatchback lies between 1 to 1. However these figures pertain to skin friction coefficient i.
The total drag values found from force report are 1. The discrepancies are explained as under. Generally skin friction coefficient is computed for fully attached flows in Aerospace applications. Automotive shapes being bluff bodies therefore do not give realistic values of drag as a whole.
Therefore the total drag values obtained from force report can be assumed as the realistic. The drag values of Hatchback are not much in agreement with those of literature for generic shapes. The other reasons being translation of accurate CAD geometry for the model, wind tunnel corrections and use of robust Numerical scheme.
An accurate CAD model through digital reconstitution is po ssible. The process of white light scanning for mapping to the vehicle points in Cartesian coordinates system followed by conversion of these points to surface possible. These surfaces are then stitched to form panels for numerical simulation.
These are compared with the available literature values for 21 0 rear slant angle. The Sedan values of literature are mentioned only to demonstrate the ir lower drag compared to Hatchbacks. It is also observed that for Hatchbacks the CFD values are little lower.
Aerodynamics of Road Vehicles
These images enable us to track excessive lift, which is not desirable. These images indicate how pronounced the trailing vortices are. It is also observed that the static pressure and velocity values complement each other. It is seen that in Fig 39 of Hatchback trailing vortices are prominent, whereas in Fig 40 of Sedan there are no vortices seen as there is no separation of flow and as a result there is downwash which also provides necessary down force.
Refer Figs 29 and 30 which indicate that the total drag increases as the speed increases. However the total drag is a polynomial as seen from the trendlines. The Sedan or the three box version has a boat tail ramp which avoids separation and therefore the pressure drag reduces. The Sedan version is said to be an optimized version. The wind tunnel experiment is conducted to perform pressure distribution around the 1: Subsequently the same model is tested with CFD simulation package.
Therefore to tackle this problem zonal approach is required.
Also to track the flow physics appropriately the validated software codes are required. Generally the validated software codes are customized for specific application and may not respond accurately in variety of applications. Codes developed for internal flows may not work for external flows. Another conventional tool for analysis is the wind tunnel.
The most crucial amongst these being drag and downforce. In addition the effect of wheel rotation and generated noise are also to be addressed.
In case of bluff bodies the pressure drag is prominent. Road tests represent the most realistic simulation of the environment in which the vehicles operate. However the difficulties associated with ever changing environments often make the results obtained open to debate.
Aerodynamics of Road Vehicles in the Computational Domain: A Summary
However there are also limiations of computational methods , which are as given below. Secondly the modeling of virtual wind tunnel, inlet conditions of velocity , turbulence and also boundary layer development in the wind tunnel has to be matched. Descretisation Error While solving continuous derivatives of the governing equations using finite difference schemes such as higher order upwind schemes Taylor series expansion is generally used.
In such cases Descretisation error may be seen as the effect of truncating the higher order terms. Turbulence modelling Error Modelling of complex flow physics of the flow has been a real challenge.
These models which are used for complete closures are however not very accurate in describing the flow. Separation, Pressure distribution etc. The intricacies of boundary layer development needs to be properly addressed in order that the near wall stresses are properly modeled. Residual Errors These errors can be attributed to the accuracy of the computations. Generally the convergence criteria employed is of the order 10 -4 to 10 -5 Therefore this process enables to find shapes having favourable aerodynamic characteristics with minor geometric modifications.
The main objective is to find out rapidly the best configurations within certain small range of shapes, which are considered to be acceptable. Refer the optimization loop depicted in next chapter on optimization. Richard Wood  8 Surface NA only for Richard Wood  external flow application 5. Also it is noticed that by adding a spoiler either front or rear the Drag value increases. There is also variation in the Drag with differing Turbulence models.
Also it is observed that this turbulence model does not predict the separation accurately as it is ideally not suited to complex geometry. Lower base pressures. In general the Square back Hatchback design suffers from a large recirculation region behind the body due to flow separation, which results in higher drag coefficients owing to lower base pressures at the rear, as seen in the graphic image.
In case of Fast back Hatchback designs the flow is complex due to interaction of the longitudinal vortices and separation in the rear. Results are already discussed in Methodology. The wake analysis is also done from velocity vector diagrams which indicate downwash, and therefore the car is stable and pressure loss at the base is moderate. Francis Chometon , Patrick Gillieron  5.
The important conclusion which can be drawn is that the drag of Sedan Three box version is lower than Hatchback Two box versions for the same car front, which is ascertained from values computed. The Results are discussed in Methodology. Even simple basic vehicle configuration free of all add ons and having smooth surfaces generate a variety of quasi two-dimensional and fully three dimensional regions of separated flow. Extensive research is going on in the field of automobile aerodynamics, especially of cars, and there is a clear indication of the ever increasing popu larity of numerical techniques and CAE tools such as CFD to predict automobile characteristics in comparison to the typical wind tunnel test methods.
With the rapid progress of computer hardware and software components, these simulation methods will become more predictable and accurate and may replace wind tunnel testing in future. On the same lines two sets of orthogonal Bezier curves can be used to design an object surface by specifying an input mesh of control points. The parametric vector function for the Bezier surface is formed as the Cartesian product of Bezier blending functions.
Therefore to change the frontal area or the shape of the car the Bezier points need to be displaced in a prescribed way to alter the geometry. Carmassi . The following block diagram Fig 1. With the help of library of acceptable variant surface shapes the optimized can be found out.
However wind tunnel procedures are cumbersome owing to complexities of measurements and hardware testing involved. Therefore many of the companies are using computer simulations using validated CFD codes.
The last four dacades have witnessed an exponential growth in the speed of arithmetic operations which can be performed on the computers. However there are hardware limitations as the number of operations grows as Re 3. The complex domains can be mapped by unstructured mesh generation which is known as numerical grid. It is observed that all these cars are Notchback versions, however hatchback versions are more popular even though these have little higher values of drag.
The obvious reason is their maneuverability in traffic. Following are the advantages of CFD over experiment based approaches for fluid systems designs.
CFD Codes can produce very large volumes of results at virtually no additional costs. These CFD codes are structured around numerical algorithms that can tackle fluid flow problem. Efforts are under way to develop CFD codes having self a daptive meshing capability. The availability of high end computing and advanced manufacturing techniques in future will help development of vehicles to be faster.
The use of CFD to predict internal and external flows has risen dramatically in the past few years. Efforts are under way to develop CFD codes having self adaptive meshing capability.
The lattice gas automata LGA method can be viewed as a descr ete version of kinetic theory for a dense gas. The motion of particles is simulated on a face centred hyper cubic lattice FCHC which ensures macroscopic isotropy. However the computational approach completely failed to predict the phenomena of vortex breakdown and formation of large separation bubble for slant angles beyond 30 0. They obtained excellent experimental results which were reproduced using CFD codes.
Several mesh resolutions were used with limited success. Therefore the results showed strong dependence on grid structures and questioned the generality of the scheme.
The scope for use of these technologies and initiatives required to reduce energy consumption in overcoming aerodynamic drag of road vehicles has already been discussed in scope in the begining. This theoretical aspect can be leveraged in Deign of Hatchbacks, which are gaining popularity nowadays. A geometry based view point is to distinguish the contributions upstream and downstream of the maximum cross section of a vehicle i.
In the ideal case of inviscid flow i. Viscous effects in real fluids disturb this balance and even without separation the displacement effect of the boundary layer causes a lower pressure level on the afterbody , leading to a resulting force pointing downstream i.
If separation can be avoided on the forebody then the theoretical lower limit of forebody drag is nearly reached , because boundary layer displacement effects are very small in this region. On road vehicles which are bluff bodies massive separation occurs on the afterbody and leads to a drastic deviation of the pressure distribution compared to inviscid flow, also upstream of separation.
A Semi infinite body or Half Rankine Oval which is theoretically assumed to have formed around the vehicle body is as shown in the Fig. Fig 1. Half Rankine Oval around the car body. It is called half body because it has only the leading point and it trails to infinity at the downstream end. As shown in the above figure it is a combination of plane source and uniform flow.
Therefore equating the radial and tangential components to zero.
Also included in literature review. Dominy The aerodynamic forces acting on a passenger vehicle in response to a transient cross wind gust at a relative incidence of 30 0. SAE 2. SAE 4.
Digital Physics Analysis of the Morel body in ground proximity. SAE SAE 8. On the calculation of external aerodynamics: Industrial benchmarks — SAE B Hethenington , D. Some effects of free stream turbulence and the presence of close ground on the flow around bluff bodies. Symposium on aerodynamic drag mechanisms. Plenum press Baker Ground vehicles in high cross winds. Part I: Steady aerodynamic forces. Van Dam , Recent experience with different methods of drag prediction.
Elsavier,Pergamon , The influence of engine cooling airflow on car performance and stability. Car G. Influence of moving belt dimensions on vehicle aerodynamic forces. David C Wilcox — Turbulence modeling. Dwinnel , James H — Principles of aerodynamics Houghton , P. Carpenter- Aerodynamics for Engineering students.
Butterworth Heinman Active suppression of buffeting at the Audi aeroaccoustic wind tunnel operational experiences and enhancements of the control schemes SAE Ferziger J.
Computational methods for fluid dynamics , Springer , Berlin. Aerodynamic design of high performance cars: Discussion and examples on the use of optimization procedures — SAE Lee Good ,J. Howell , M.
Limitation: The Most Aerodynamic Structure
Passmore , A. Correction of nozzle gradient effects in open jet wind tunnel SAE Gradient effects on drag due to boundary layer suction in Automotive wind tunnels SAE Fluid dynamic drag.
Hoerner fluid dynamics. CFD online JSAE Rev , Blazek Computational fluid dynamics: Elsevier Science Ltd Jack Williams — Ford Motor co. Aerodynamic drag of a engine cooling airflow with external interface SAE Jochen Weidman — Audi AG The influence of ground simulation and wheel rotataion on aerodynamic drag optimization — potential for redeucing fuel consumption.
Marcel Deckker Springer Verlag Mc Grawhill John D Andersen — Fundamentals of aerodynamics. Fletcher Computational technics for fluid dynamics , Springer Kevin R. Cooper , Simon Watkins , The unsteady wind environments of road vehicles ,Part 1: A review of the on road turbulent wind environment. Aerodynamics of propulsion. McGrawHill Company Mohammadi — Analysis of k-epsilon model Stagnation pressure SAE Unsteady wake analysis of the aerodynamic drag of notchback model with critical afterbody geometry.
Taylor The finite element method , Fluid dynamics 5th edition Butterworth Heinman , Patankar S. University of Maryland. Springer Ryan — University of Durham , UK An improved wind tunnel configuration for the investigation of aerodynamic cross wind gust response SAE King-Introduction to practical fluid flow.
Butterworth Heinman , Gilhaus , Aerodynamics of vehicle cooling systems. Proceedings 6th colloquium on industrial aerodynamics and vehicle aerodynamics. Fachhochschulle,Aachen, pp Rhie C.
Richard W Johnson — Handbook of Fluid dynamics. T General motors Flowfield simulations of three simplified shapes and comparison with experimental measurements — SAE Scribor Rylski A. Shigeru Haruna, Takahide Nouzawa,Ichiro Kamioka,Hiroshi Sata — An experimental analysis and estimation of aerodynamic noise using a production vehicle.
Spalding D. International Journal of Numerical Methods. England , Volume 4 , Page Stephen B Pope — Turbulent flows. Cambridge University Press Chung Computational fluid dynamics Cambridge University press Morel , Aerodynamic drag of bluff body shapes Charecteristics of hatchback cars. Skip to main content.
Log In Sign Up. Basics of vehicle aerodynamics. Pathak Krishna.
Basics of vehicle aerodynamics Prof. Classification of flow field: In case of cylinder symmetrical flow field. In case of cylinder non- symmetrical flow field.
The share of pressure forces in drag force component of aerodynamic force parallel to undisturbed flow is small. Drag is caused mainly by shear stresses. Since shear forces are small cD is relatively small. Bluff bodies are characterized by boundary layer separation and separation bubbles.
Bus model:The rise in COP at stagnation point can also be supported by theory of presence of artificial viscosities. The response of instantaneous surface pressures to yaw angle fluctuations is reduced for higher frequency fluctuations as small-scale turbulence gusts are less able to modify the flow over the entire vehicle. As of today a better understanding of the aerodynamic phenomena which generate drag is made possible by extensive measurements done in wind tunnel with improved techniques based on the numerical processing of experimental data.
Help Center Find new research papers in: T General motors Flowfield simulations of three simplified shapes and comparison with experimental measurements — SAE These characteristics are achievable with generic shapes like airplane win gs and pontoons. Slipstream effects of passing trains cause aerodynamic loads on objects and passengers waiting at platforms. Road tests represent the most realistic simulation of the environment in which the vehicles operate.