CFD Project- Shreyas Ragavan- MSc Advanced Mechanical

Engineering (2010-11)

Aerodynamic Drag Sensitivity Analysis of a Land Speed Record (LSR) Racing Vehicle

Target Speed : 490.346 kmph

(1% more than the current record)

Rolls Royce Viper 522 series Turbojet+

afterburner engine (~5000 lbs thrust)

Half- Vehicle Model (Gambit)Length= 8m Wheel Radius = 0.1 m Jet engine is roughly half the total length

PROJECT BACKGROUND :

• The vehicle is part of the ‘Stay Gold’ Jet Car Project and the objective is to set a new British Land Speed Record

• The average speed over two straight passes of 402 m with acceleration and deceleration zones of 1300 m is taken into account.

• Current Record : 485.49 kmph over a 500m stretch• Target Speed : 490.47 kmph

PROJECT GOALS :

A. To understand external vehicle aerodynamics and flow representation along with the established methods of reducing aerodynamic drag.

B. Explore the methodology and limitations behind applying CFD. Setup a Simulation, investigate parameters like grid independence as well as different turbulence models.

C. Investigate and apply strategic geometric modifications and optimisation techniques of the chosen parameters

APPROACH & SETUPPossible modifications ?

Suitable Domain Size ?

Efficient Mesh distribution over the volume?

VOLUME MESH DISTRIBUTION

PARAMETERS AND BOUNDARY CONDITIONS

1. Pressure Based Solver

2. 2nd Order solutions with assumed smooth wall

3. Turbulence defined : Length scale and Intensity

4. Operating speeds : 136.207 m/s and 100m/s

5. Rotating wheels and moving ground

6. Energy Equation with Mass continuity used for the Jet with its inlet at ambient conditions

Grid improvement- Reduction in Skewness

Difference between coarsest and finest mesh

GRID INDEPENDENCE

1 e+06 1.25 e+06 1.48 e+06 1.56 e+06 1.98 e+060.32

0.33

0.34

0.35

0.36

0.37

0.38

0.39

0.4

0.41

0.42

KE realizable

SST KW

SA

KE- RNG

Turbulence Models Tested

Coeffi

cie

nt

of

Dra

g (

Cd

)

Domain Cell Count

AREAS OF FLOW DISTURBANCE

Regions of Turbulence

APPLYING MESH MORPHING (USING OPTIMAL SOLUTIONS- SCULPTOR)

• Creating an ‘ Arbitary Shape Volume’ (ASD) over the desired area of deformation

• Controlling the mesh Deformation

• Preparing Acceptable mesh files for Fluent

Final ASD

Initial ASD

Illustration of deformation limitation by converting the middle plane into a ‘Super plane’

ASD shown with Mesh.Length-wise planes in RedTransverse planes in Green

ILLUSTRATION OF THE CELLS BEING DEFORMED

Stretched Cellson pushing the nose downwards

Original Model

PRESSING THE NOSE DOWN-WARDS

Smaller stagnant flow Zone

Original Model

PINCHING THE BODY INWARDS

Original Model

ADDING GEOMETRY : CYLINDERS

Adding Two cylinders , Front and Rear.

• Heights : Front- 0.02 m Rear - 0.05 m • Radius : 50% of the wheel rim width it is

placed in front of• Bottom of both cylinders are aligned with

lower surface of the vehicle

Cylinder heights Reduced by 50% from the top.

Smaller low velocity area

Baseline

With cylinder

Variation of Cp

SUMMARY OF RESULTSType of change Change in Geometry and

dimensions

Percentage increase

or reduction in Cd

Addition of Geometry

through remodelling and

re-meshing

Cylinders with the same height

as the vehicle.

+27.4 %

  Cylinders with 50% height

reduction

+19.5%

Mesh Morphing/ Geometry

Deformations

Pressing the nose down

0.02m

0.04 m

0

-3.2%

  Pressing the Back inwards

0.02m

0.03m

+4%

+7.5

  Extending the nose

(0.005- 0.025m)

0

 

Pushing out the side

(0.01- 0.03m) 0.53%

CONCLUSIONS OF THE STUDY :

Even relatively minor geometric changes have an effect on the overall drag coefficient and the forces being experienced by the vehicle.◦ Provided they are implemented at the right position in the vehicle.

High mesh densities are required to perform appreciable deformations in the mesh.

The exposed wheels have been established to contribute to the drag on a very large scale. It is also established that the wheel covers must be carefully designed so as to not contribute further to the drag.

Areas of improvement required: ◦ More accurate CAD representation

◦ Obtaining reliable Test and Design specifications of the jet engine

◦ Improved Boundary conditions including running the compressible solver at lower Reynolds Numbers