1

Computational Fluid Dynamics Analysis of airflow around a car model

using Solidworks Flow Simulation

Version 13.02.2015, 01:35

To display the CFD catalogue, click View in menu, and tick the Navigation

Pane in the “show” tab. Now chapters, steps, and tasks are shown.

Introduction

The SolidWorks Flow Simulation is the fluid flow analysis tool that is fully

embedded in SolidWorks. The flow analysis includes the following steps:

1) Create an assembly in SolidWorks;

2) Create a project in SolidWorks Flow Simulation. The project contains all

the simulation settings and computation results;

3) Run the analysis: Solidworks solves the equations describing the fluid

flow;

4) Viewing the Flow Simulation results which include:

Flow visualisation:

» Vectors, Contours, Isolines

» Cut Plots, Streamlines, Isosurfaces

Numerical outputs:

» XY Plots (can be exported to MS Excel)

» Goals (can be exported to MS Excel)

» Surface Parameters

» Reference Fluid Temperatures

1. Build an Assembly of a Car in a Wind tunnel

First, build a car and a wind tunnel of realistic dimensions as described in Step

1, 2, and 3.

Step 1. Build a car of a basic shape

The object should be recognisable as a car, but not necessarily possessing all the

small features, such as rear view mirrors, and not necessarily being well

streamlined. Choose realistic car dimensions. The bottom of tyres should be flat

to mate them with the floor of the wind tunnel.

Create a new ‘Part’. Set Y as the vertical axis and Z aligned with the direction

of the car motion. Choose the right plane and click the sketch button to draw the

car sketch.

2

Use the ‘three arc point’ tool to draw wheels, then draw a horizontal line in the

half circle as the bottoms of the tyres.

Remove the rest arc of tyres bottom by using ‘trim to closet’ in ‘Trim entities’

tool and clicking the surplus arc at the bottom.

3

Similarly, remove segments of the car bottom line above the wheels so that a

close contour is completed. Use ‘Extruded boss/base’ tool in features ribbon are

to make the car 3-dimensional: extrude the sketch in x direction and set the car

width.

Now the tyres must be cut. There are many ways to create wheels such as draw

a car body and add four wheels separately. Here, we draw a rectangular contour

like in the picture and use the ‘Extruded cut’ tool remove the surplus matter

“between wheels”.

4

Finally, a body with four wheels is created as shown below.

5

Step 2. Build a wind tunnel

Use the same way as above to create a new solid part document where build a

wind tunnel. Note that the wind tunnel should be large enough to reduce the

influence of walls on the flow around the car model as much as possible. For

example the wind tunnel dimensions can be 26m×9m×7m, and its wall

thickness can be, say, 500 mm.

There are many ways to build a wind tunnel. One simple way is to make a block

and hollow it by using the extruded cut command. First create a block

26m×9m×7m. And, insert a reference plane in the Features menu (‘Reference

Geometry’ tab) 0.5 m away from one end of the wind tunnel. Then, sketch a

smaller rectangle (8m×6m) on the reference plane and use the ‘Extruded Cut’

command (25 m) to remove the inside of the block. Please use the extrude cut

only for 25 m, so that and the windtunnel wall thickness is 0.5 m. Make sure

both ends of the wind tunnel are closed. Otherwise, you need to add lids on both

ends. Some steps are shown as below.

1) Using the ‘Corner rectangle command’ to draw the wind tunnel on

the front plane. Make sure the length of wind tunnel aligns with Z-axis.

6

2) Dimensions of the rectangle are 7000 mm×9000m.

3) Extrude the rectangle by 26000mm (Extruded Boss/Base command).

The block is ready. Now, need to make it into an empty box.

7

5) Insert a reference plane to sketch a rectangle as the end wall of the inner

surface of the wind tunnel.

The reference plane should be placed inside the body of the block 500

mm away from the end wall. If the plane is outside of the block, then tick

the flip and the reference plane will move inside the block.

6) Sketch the rectangle on the newly created reference plane.

8

7) Use the ‘extruded Cut’ command to hollow the block (25000mm). The

wind tunnel is now ready as shown below.

9

Step 1.3. Place the car inside the wind tunnel

1) Create a new Assembly file for the car in the wind tunnel.

Insert the ‘Car part’ and ‘Wind Tunnel part’ in, by ‘Insert Components’

command in ‘Insert Components’ tap of assembly ribbon.

2) In order to be able to see the car inside the wind tunnel, right click on the

‘Wind Tunnel’ part and choose ‘Component displayHidden Lines

Visible’ to make it transparent.

10

3) Mate the side surface of the car model and the wall surface of the wind

tunnel: use the ‘Mate’ tool from the ‘Assembly’ toolbar. Select the

wind tunnel surface and car side surface. Mate by parallel and define

distance (say 4.2 m). Make sure the car is placed in the middle of the

wind tunnel.

4) Mate the bottom surface of one of the wheels of the car model and the

inner surface of the bottom of the wind tunnel by using Coincident

command. Other wheels will level with the floor automatically. To select

the inner, not outer surface of the wind tunnel. If not right click and

choose ‘select other’ option, then you can finally find the right surface.

11

Mate the front surface of the car model and the inner surface of the end of the

wind tunnel by using Parallel command and defining distance (say 8.0 m).

Finally, the car is placed in the wind tunnel.

Note:

Position the car exactly in the middle between the walls (by using ‘Mating’

command again) and slightly closer to the inlet of the wind tunnel to enable

better resolution of the wake behind the car.

2. Aerodynamics Analysis of the Car using Solidworks Flow Simulation

Check before using SolidWorks Flow Simulation. If the flow simulation

toolbox is not installed by default, click Tools, Add-ins, and then click

SolidWorks Flow Simulation 2013 to load SolidWorks Flow Simulation.

12

Now, perform the aerodynamic analysis of the car.

2.1 Create a Flow simulation Project

Step 1. Set up the Flow Simulation

1) Click Flow Simulation, and go to Wizard .

2) Configuration name for the project: select create new to create a

new configuration, name it in an illustrative way, for example,

‘Car_non_streamlined_20mps’, and click Next.

3) Unit System: Choose SI (m-kg-s) in the Unit system column, then

click Next.

13

4) Analysis Type and Physical Features: Leave all the setting as it is,

do not tick any physical features. Then click Next.

5) Default Fluid: under Gases, select Air and then click Add. Flow type

will be Laminar and Turbulent with no regard to compressibility

and humidity. Then click Next.

14

6) Wall Conditons: Default thermal condition is the Adiabatic wall (no

heat exchange) and zero roughness. Click Next.

7) Environmental Conditions: First leave Pressure 101325 Pa

(standard pressure), Temperature 293.2 K (room temperature) and

some Turbulence intensity (say, 2%). As mentioned in a briefing

lecture, the “Turbulence intensity” is a somewhat arbitrary parameter

15

which in reality varies across the flow domain. Being unable

predicting it, one usually choses the value providing results most

similar to experimental.

8) Set Velocity in Z direction 20 m/s, and Turbulence length 0.05 m.

Again, a somewhat arbitrary parameter characterising turbulent eddy

size. Click Next.

Note that in real world, the car would be moving through the stationary air. In a

wind tunnel, the car is stationary and air is moving. These set-ups are identical

except for the air behaviour in the gap under the car.

9) Results and Geometry Resolution: Manually specify the minimum

gap size (say, 0.05m), and the minimum wall thickness (say,

0.05m). Click Finish.

The “result resolution” should be set to yield acceptably accurate results within

a reasonable amount time. Ideally, compare results obtained at different result

resolutions to find the acceptable minimum.

16

Step 2. Set up Boundary Conditions and Simulation Goals

Flow simulation analysis tree

A tab for the Flow Simulation analysis tree is added

to the Feature Manager area. Click on the Flow

Simulation analysis tree tab . Expand the Input

Data listing.

Set up boundary conditions

In the Flow simulation drop-out menu select the Boundary conditions. It

includes two setting ups: inlet and outlet.

17

1) Set up the Inlet boundary conditions

Select the inner face of wind-tunnel’s front end (right clickselect other

option) and set the inlet velocity flow, 20 m/s (72 km/h, 45 mph). Then a range

of arrows appear, if not, left click and select ‘Show’. Now, the arrows will turn

up.

2) Set up the Outlet boundary conditions

On the outlet, set the environment pressure. Select the inner face (inlet and

outlet), right clickselect other and then choose from the planes in the list.

Type: inlet-

Inlet Velocity

18

Setting up the simulation Goals

Insert global goals

Now set up simulation goals. These are used as a criteria

of when to stop simulation. The software assumes that

when the values called goals stop changing, the flow

field is calculated with the sufficient precision and the

calculation can be stopped. A number of parameters can

be used, for example the “Turbulent Energy” goal.

Right click Goals in the Flow Simulation analysis tree

and select the Insert Global Goals from the shortcut

menu. Select the Turbulent Energy, or Forces, or some

other parameter which seems appropriate. If the

simulation runs for long enough, the answer does not

depend on which exactly parameter was used for

monitoring of the progress.

Insert Surface goals

Now, the task is to find forces acting on the car: drag (the force in streamwise

direction), lift (vertical force), and the side force.

Surface goal is a physical parameter calculated on a user-specified face (or

faces) of the model.

19

Select the whole car without flat bottoms of the wheels mated to the wind tunnel

as they are not in contact with the flow and insert X, Y, and Z forces as surface

parameters. Then, separately, select a front panel of the car (for example, as

shown in the figure below) and set the force acting on it in streamwise direction

as a Surface Goal.

Step 3. Running the Analysis

All is set up now, it is time to run the simulation.

Run the analysis

Right-click the project name and click Run.

Alternatively, click Flow Simulation, Solve,

Run, or click Run Solver on the Flow

Simulation toolbar.

20

Solver information

The Solution Monitor window appears after few seconds or so. On the right of

the window, the log steps taken in the solution process is displayed. On the left

is the information window with mesh information and any warnings.

Usually, the simulation takes less than half an hour. You may spend this time

performing back-of-the-envelope estimates required in the task (6) in the “lab

report” section.

Goal Plot

When you click Goal Plot , the Add/Remove Goals dialog appears. Select

the goals whose plots you want to view and click OK.

21

For each goal selected in the Add/Remove Goals dialog box, the Goal Plot

shows the goal convergence diagram. The goal plot is an illustration how the

iterative solver works and does not represent any physical values.

Goal Table

Goal Table shows the list of all specified goals and contains the same

information as the upper portion of the Goal Plot window.

Save the file

After investing time in running the analysis it is prudent to save your work.

2.2 Viewing the Results

Once the calculation is finished, you can view the saved calculation results

through numerous Flow Simulation options in a customized manner directly

within the graphics area. The results options are:

» Cut Plots (section view of parameter distribution)

» Surface Plots (parameter distribution on a selected surface)

» Isosurfaces

22

» Flow Trajectories (streamlines and particle trajectories)

» Goal Plot (behaviour of the specified goals during the calculation)

» XY Plots (parameter change along a curve, sketch)

» Surface Parameters (getting parameters at specified surfaces)

» Point Parameters (getting parameters at specified points)

» Report (project report output into Microsoft Word)

» Animation of results

We will view the flow trajectories and surface plots the next.

Step1. Load the results.

Right-click Results in the Flow Simulation analysis tree and select Load. If

Unload appears in the list, the results have already been loaded.

The Load Results window opens. Select 3.fld (or whichever file appropriate)

and click Open to load the results file.

Step 2. Mesh

23

Insert the Mesh plot: Right click Mesh3D view (below the results). Note

that it is only possible to perform when the simulation is finished as the mesh

construction is a part of the simulation. For the Fluid cells, set its value All.

Then click Apply and OK.

Zoom in the mesh plot and note that the mesh is finer around the model corners

and curve surfaces.

Note: If you cannot see the mesh, right-click the wind tunnel and go to the

Component DisplayWireframe.

24

Save the results by using screenshots which will be included to your report.

Step 3. Flow Trajectories

Plotting flow trajectories is a way to qualitatively understand the computed flow

structure. They are analogous to the streamers of smoke in a wind tunnel.

25

To illustrate the flow structure around the car,

insert the flow trajectories adjacent to different

panels. Present a set of plots sufficient to

recognize main features of the flow. Check

whether the flow separates from the back of

the car. It does separate from the back of a car

in real life.

Insert a flow trajectory

Right-click Flow Trajectories in the Flow

Simulation analysis tree, and select Insert.

Select one or several faces of the car body, for

example, the front panel and the windscreen as

shown below. The streamlines located close to

the selected

surfaces will be

plotted.

Set the Number of Points to, for example, 50. For

Draw trajectories as, select Line with Arrow and

set value, for example, 0.01m. Leave the other

settings at their default values. Select the car front

panels for the panel of Starting Point. Then click

tick.

26

Adjust the colour scale to visualise pressure.

The colour if streamlines represents pressure. Click on the top value of the

legend and enter the value relevant for your simulation (in this example it is

101575 Pa). Repeat this process for the bottom value (here, 100930 Pa).

Select the legend, then right clickEdit, set the top and bottom value in the

blanket.

Note: The main reason not to use the default values but to enter specific ones is

because when the design is changed, the minimum and maximum pressure

27

values will be different. That means red would represent one pressure on one

plot and a different pressure on another plot. Using the same minimum and

maximum settings for each analysis allows for meaningful comparisons

between different iterations of the design.

Experiments with streamlines

There are two ways to alter the streamline plots:

» Edit the definition of the existing plot

» Insert a new plot

When multiple sets of flow trajectories are present at a plot, they can be

displayed selectively.

To hide the flow trajectory, right-click Flow Trajectories 1 and select Hide.

To insert a new flow trajectory, right-click Flow Trajectories, and select

Insert.

Select left side panel of the car. Set the Number of Points. For Draw

trajectories as, select Line, set value 1. Click OK.

28

Examples of streamline plots are shown below.

29

The plot with lines of the flow separation marked. In this case, the flow leaves

the car surface at almost every sharp corner. The picture will be more

complicated for rounded shapes.

30

4) Cut Plots

To insert a cut plot is just like the way of inserting a new flow trajectory. Right

side plane cut plot of the car and its results.

Select appropriate colour scheme, so that the pressure features will be visible.

Think why the pressure is higher in some areas and lower in the others.

Compare pressure with its free stream value.

If too few or none contours are seen on the plot, try to narrow the limits of the

pressure scale. For example, if the whole plane looking like above figure almost

all is light green, change the scale limits to 101480 and 101150.

Then, insert a cut plot for the back of car and the legend limits is changed into

101329 Pa and 101279 Pa.

31

32

3. Lab report

1) Present a screenshot of the computational mesh.

2) Illustrate the flow structure around the car by presenting few to several

streamline plots. Among others, the flow structure in the wake behind the car

must be visualised.

3) Present a set of pressure cut plots. Explain why in some area the pressure is

higher and in some lower than the free stream pressure.

4) Identify lines of the flow separation i.e. the lines where the flow leaves the

car surface. Mark the lines by hand (using Paint or any other image editing

software) at the car surface;

5) Present aerodynamic forces acting on the model

The preceding examples of flow trajectories and cut plots were excellent tools

for visualizing how the air flows and pressure around the car. However, they are

more qualitative than quantitative. Let’s move on to a more quantitative

interpretation of results.

In the analysis tree, expand the Results listing and right clickGoal Plots.

Select Insert from the shortcut menu. Click All. Click OK.

33

When click Export to Excel, the MS Excel is launched and a spreadsheet opens.

The first three columns show the name of the goal, the units and the computed

value.

6) Select a panel, for example, the front one. Cite the total force acting on the

selected panel (a surface goal) and compare it with the order-of-magnitude

estimate 𝐹 ≈ 𝐴 ∙1

2𝜌𝑉2 , where 𝐴 is the area of the panel, 𝜌 is the air density

(1.2kg/m3), and V is the free stream flow speed. Dose the calculation make

sense?

Note: To calculate the area of select surface, you can use the dimension you

sketch at very beginning. Or you can click the Evaluate tab and select the

Measure, then the area of selected face will show.

7) Cite the drag force acting on the model and find the power required to

overcome the drag (both in KW and in Horse Powers);

8) Cite the lift force acting on the model. Compare it with the car weight. Is the

anti-wing (also known as spoiler) useful in this case?

9) Improve the car aerodynamics

To minimize the drag, a car should have a shape which keeps the flow attached.

In other words, the flow separation should be avoided. Using lines, arcs and

splines to smooth the edges to prevent flow detachment, make the faces oblique

to the flow etcetera.

When optimising the shape, a restriction is imposed:

DO NOT change the car frontal area, i.e. the area of the car silhouette as view

from the front. For example, rounding corners like in Fig. 3.1, has changed the

area. The modification shown in Fig 3.2 keeps frontal area constant. Perform

34

calculation with the streamlined model and discuss the results. Watch carefully

the flow structure in the wake behind the car: only for a very streamlined shape

the flow will remain attached to the car. You may need to introduce an artificial

kink at the rear of the model to ‘tell’ the software that the flow must detach

from the car body.

Fig. 3.1

Fig. 3.2

10) Plot and compare the flow structure around the basic and the streamlined

model and comment on the differences. Indicate lines of the flow separation

for the streamlined model.

11) Cite and compare lift and drag force acting on the basic and streamlined

cars. Calculate the drag coefficients 𝐶𝐷 of the basic and streamlined cars from

the formula 𝐹𝐷 = 𝐹𝑟𝑜𝑛𝑡𝑎𝑙𝐴𝑟𝑒𝑎 ∙ 𝐶𝐷 ∙1

2𝜌𝐴2 Here the area of the car frontal

silhouette is used. Note that for a reasonably shaped car 0.2>CD>1. If the

coefficient obtained is not in this specific range, either find the error or explain

the computation outcome.

12) Calculate and compare the engine power required to overcome the drag for

basic and streamlined cars. Calculate the fuel consumption in miles per gallon

35

if the energy loss occurs only due to the aerodynamic drag. Assume the engine

efficiency 25% and the petrol calorific value of 45 MJ/kg.

Avoid the over-precision in answers. At the level of aerodynamic modelling

the SolidWorks provides, 3 significant figures are more than enough when

citing calculated values.

The Lab report should contain streamline plots, cut plots, and address the

tasks from 1 to 12 and should not exceed 6 pages.