General Sir John Kotelawala Defence University

COMPUTATIONAL FLUID DYNAMICS

Modelling a Motor Vehicle

by

LLY SRIMAL (ENG/AE/12/0044)

Supervised By

Mr RMPS Bandara

GENERAL SIR JOHN KOTELAWALA DEFENCE UNIVERSITY

RATMALANA, SRI LANKA.

i

Contents

1 Introduction .................................................................................................................................... 1

2 Problem statement ......................................................................................................................... 1

3 Parameters ...................................................................................................................................... 1

4 Methodology ................................................................................................................................... 1

5 Results ............................................................................................................................................. 4

6 Interpretation.................................................................................................................................. 7

7 Conclusion ....................................................................................................................................... 7

ii

List of Figures

Figure 1 Model of the Motor Vehicle ..................................................................................................... 1

Figure 2 Domain ..................................................................................................................................... 2

Figure 3 Grid View ................................................................................................................................. 4

Figure 4 Iterations of the Model ............................................................................................................. 5

Figure 5 Contour of Static Pressure ........................................................................................................ 5

Figure 6 Contours of Velocity Vectors ................................................................................................... 6

Figure 7 Velocity And Turbulence ......................................................................................................... 6

List of Tables

Table 1 ..................................................................................................................................................... 2

Table 2 ..................................................................................................................................................... 2

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1

1 Introduction

The purpose of this project is to compute an air flow over a body of the motor vehicle and to

observe variations of flow patterns over this motor vehicle for a given velocity magnitude.

Here I have used the k-epsilon viscous model. In the following chapters will demonstrate the

procedure of the simulation with assigning conditions to this modelled motor vehicle.

2 Problem statement

The problem to be considered is shown schematically in Figure 1.1. A flow velocity of

50km/h (13.88m/s) flows in to the face and the surfaces of the motor vehicle and leaves the

motor vehicle at the rear.

Figure 1 Model of the Motor Vehicle

3 Parameters

Flow Velocity=50km/h (13.88m/s)

4 Methodology

First create a drawing as shown in the figure in AutoCAD. And then export this drawing to

the Gambit as an iges file. After importing it to the Gambit user platform it is essential to

check for discontinuities of the model. If any discontinuities are present in the model, needed

to be corrected. Initially this model which has drawn form AutoCAD was present with an

interference with the domain of this model. This interference was corrected. Then a face has

created using the CREATE FACE FORM WIREFRAME command and labelled as FLUID.

After that I have created the mesh form FACE MESH COMMAND. Select the available face

from the face list. Then apply the elements as QUAD and select the TYPE as PAVE. Interval

size has been selected as lower as possible to have maximum cells in the mesh. In this project

I have created 958850 quadrilateral cells for interval size of 0.005. In order to specify the

boundary conditions following table has referred.

2

Table 1

Name Type Edges

Velocity Inlet Velocity_Inlet f_edge.3

Pressure Outlet Pressure_Outlet f_edge.4

Symmetry Symmetry f_edge.5

Wall Domain Wall f_edge.6

Wall Car Wall f_edge.1 Wall f_edge7

Then the case data of this file has been saved and then it has been opened in FLUENT. In

simulation phase first I have check for the grid in GRID CHECK command. If the grid has

been created correctly it will indicate as DONE after running the list of checks. For further

accuracy it is better to check the case also in the CASE CHECK command. Then I have

scaled the grid to meters using the SCALE command.

According to the following table I have defined the parameters to be assigned for this

simulation

Table 2

` Models Solver Solver-Pressure Based

Space-2D

Velocity Formulation-Absolute

Figure 2 Domain

3

Gradient Option-Green-Gauss Cell

Based

Energy Select the Energy Equation Check Box

Viscous Model-k epsilon (2 eqn)

K epsilon Model-Standard

Near Wall Treatment-Standard Wall

Functions

Materials

Fluent Fluid

Materials Air

Density Constant

Operating

Conditions No changes

Boundary

Conditions

Velocity_Inlet

Velocity Magnitude =13.88m/s

Velocity Specification Method-

Magnitude, Normal to Boundary

Reference Frame-Absolute

Specification Method-K and Epsilon

Pressure_Outlet Specification Method- K and Epsilon

Symmetry No change

Wall Motion-Stationary Wall

Shear Condition-No Slip

Roughness Height=Zero meters

Roughness Constant=0. 5 Wall Domain

Controls

Wall Car Same as above

Solve Solution Discretization-Pressure->Second Order

Density,Momentum,Modified turbulent

Viscosity, Energy-> Second Order

Upwind

Pressure Velocity Coupling-Simple

Monitors

Residual-Check the Print and Plot

Boxes

Convergence Criterion-Absolute

Absolute Criteria for

continuity=0.0001

Only check convergence for the

continuity

Initialize Compute From-Velocity Inlet

Iterate Number of Iterations=1000

4

After completing assigning the values I have initialize this simulation. Same as the above fist

method I have selected these commands. Solve Initialize Compute from Velocity Inlet.

After initializing I have started the simulation for 1000 iterations.

5 Results

Figure 3 Grid View

This figure shows us the how the grid is been created. We can observe that the grid cells are

well aligned at the surfaces of the car.

5

It was observed that the solution is converged at 549 iterations. In the above figure shows the

residuals of each parameter.

Figure 5 Contour of Static Pressure

Figure 5 shows the contours of static pressure of the surface of the motor car. We can see

that the static pressure is higher at the face of the motor vehicle and at the edges of the roof

static pressure is low.

Figure 4 Iterations of the Model

6

Figure 6 shows the contours of velocity vectors when the free stream velocity meets the

motor vehicle. We can observe that the edges will create high velocity magnitude. In the

right fpicture we can see that the turbulence will create at the rear of the motor vehivle and

the nose of the the motor vehicle. Likewise we can observe contours of density, temperature.

Above figure shows the vectors of velocity and turbulence. In the left hand picture the circled point

shows us the vectors which are separated from the flow and the right hand figure, the circled point

shows where the turbulence will occur. High turbulence will create due to the vacuum in the flow.

This higher turbulence will reduce the body stability and cause body vibrations. Likewise for the other

parameters of the flow we can observe vectors of velocity coloured for each parameter such as

velocity, temperature, density and turbulence.

Figure 7 Velocity And Turbulence

Figure 6 Contours of Velocity Vectors