# VEHICLE AERODYNAMICS PDF

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 brocapazbebuh.cf 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.

Aerodynamics is extremely important to get past that limiting barrier that you go through all the time on the highway. Although spoilers may look cool and increase handling and downforce, but the limiting factor is that spoilers make aerodynamics come to play quicker, but decreases aerodynamic part of it by the bulky shape of it flying through the air. Comparison with Aircraft Aerodynamics[ edit ] Automotive aerodynamics differs from aircraft aerodynamics in several ways.

First, the characteristic shape of a road vehicle is much less streamlined compared to an aircraft. Second, the vehicle operates very close to the ground, rather than in free air.

Third, the operating speeds are lower and aerodynamic drag varies as the square of speed. Fourth, a ground vehicle has fewer degrees of freedom than an aircraft, and its motion is less affected by aerodynamic forces. Fifth, passenger and commercial ground vehicles have very specific design constraints such as their intended purpose, high safety standards requiring, for example, more 'dead' structural space to act as crumple zones , and certain regulations.

Methods of Studying Aerodynamics[ edit ] Play media One of the side effects of automotive aerodynamics is seed dispersal. Automotive aerodynamics is studied using both computer modelling and wind tunnel testing. 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.

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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. 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 [78] 8 Surface NA only for Richard Wood [78] 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 [24] 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.

## 1st Edition

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 [25]. 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.

## Aerodynamics of Road Vehicles in the Computational Domain: A Summary

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.

## Automotive Aerodynamics

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.

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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.

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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. Turbulence experienced by road vehicles under normal driving conditions.

Wolf Heinrich Hucho — Aerodynamics of road vehicles. Butterworth,Boston July , 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: Increase of driving stability 1 reducing aerodynamic lift Effect of the gap size on the lift Airfoils, side box provided with wing increase the negative lift: The area of side walls covered by mud can be reduced considerably by reducing the deceleration of underbody flow.

In optimisation besides wind tunnel investigations numerical simulation of flow field has become more and more important. Related Papers. By Mohd Shukry Abdullah. Review on Vehicle's Aerodynamic Drag Reduction.Skinfriction coefficient Fig In case of cylinder non- symmetrical flow field. The flow topology around the wheels in a wheel arch is revealed. Gradient effects on drag due to boundary layer suction in Automotive wind tunnels SAE Half Rankine Oval around the car body.

Next at the rear end due to separation the COP falls and remains more or less constant in the rear vertical. In case of Fast back designs the flow is complex due to interaction of the longitudinal vortices and separation in the rear. Hatchback can be Squareback or Fastback. However one curve corresponding to Notcback with Threebox version rear spoiler.

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