sam lindop l018852a assignment 2

7/30/2019 Sam Lindop L018852A Assignment 2

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1. CFD is an abbreviation of computational fluid dynamics, this is the prediction and analysis of

the behaviour of fluid motion passing objects by numerical methods rather than model

experiments. The package that is used by Staffordshire University is CHAM pheonics. This

package uses the Navier-stokes equations to solve how the movement of the fluid, pressure

temperature and density are all related. There are three stages that this package works in.

the first stage is setting up the domain. This is where you define the velocities and density of

the fluid. You also set up the grid, this grid divides the domain up and in each square of the

grid the package solves the Navier-stokes equation. You can set the number of interations;

interations are the number of times the equations are solved, the more interations you set

the more reliable the result will be. The next stage is the solver stage, this is where CHAM

pheonics solves the equations and the past stage is the post processor stage and this is

where it gives a visual representation of what is happening to the flow of the fluid.

CFD can be used in many different ways. For example the ACUSIM software is used in many

different applications, from automotive design and aerofoils to, corrugated pipe modellingand the design of blood handling devices. To do all of these calculations it uses the Reynolds

averaged Navier-Stokes (RANS) equations. The difference between CHAM pheonics and

ACUSIM is that ACUSIM doesn’t use the grid system that CHAM pheonics offers; instead

ACUSIM uses Galerkin/Least-Squares. This is s a higher-order accurate, yet stable

formulation that uses equal order nodal interpolation for all variables, including pressure

according to ACUSIM.

Another type of CFD package is the open FOAM. This package is similar in the way CHAM

pheonics in that it uses pre and post processors. To start work on the solver its uses simple

laplace e.g. thermal diffusion in a solid then solves potential flow which are the bases for the

Navier Stokes equations. However the difference between the two is that open FOAM can

calculate more multiphase flow calculations for example gas bubbles in a liquid and mixing

of incompressible fluids. Both CHAM pheonics and open FOAM can model combustion, this

can used to model the flow of fuel in both petrol and diesel four stroke combustion engines.

It can also be used to run a solver for fires and turbulan diffusion flames. They can also be

used for combustions with chemical reactions and combustion with chemical reactions using

density based thermodynamics package. However the ACUSIM package does not use these

solvers.

Another CFD package is TYCHO, this is a multi-dimensional, compressible hydrodynamics

code. This is a Lagrangian remap version of the Piecewise Parabolic Method developed by

Paul Woodward and Phil Colella (1984). What this offers you is gas interaction with

obstacles, heat diffusion and thermal exchange. It also has gravity as a constant background

field. This is different to the others because they have the option to turn gravity off and

change how strong the effects of gravity are. TYCHO and ACUSIM share the same feature in

that they have the option to change the boundary conditions. None of the other CFD

packages give this option.

Another CFD company is Zeus numerix who produce FlowZ. They describe the package as anadvanced CFD solver. This is because it offers six different numerical schemes. HLLC, Roe,



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Figure 1-Geometry

Vanleer, StegarWarming, AUSM, AUSMPW. This gives the option to compare results

obtained. This makes results more reliable and accurate and this is the only CFD package

looked at with this feature.

Some CFD packages integrate CAD modelling into the program, this means that you can

design, model and analyse the model. This makes cuts down production time because you

don’t have to keep importing the model over from the modelling software to the analysis

software which can be time consuming. You can make all the changes in the software so you

don’t have to keep consistently saving and importing the model when changes have been

made. An example of a CFD package that does this is NUMECA.

NUMECA is similar to CHAM pheonics in that it uses the grid generation to do the

calculations, however there are some CFD packages that generate the optimal grid

automatically, an example of this is Design Builder. They use the geometry from the model

and the boundary conditions to promote optimal solution convergence. Another differencebetween Design Builder and the others is that it uses a different algorithm, it uses the

SIMPLER algorithm, and this means Semi-Implicit Method for Pressure-Linked Equations.

This allows the Navier stokes equations to be coupled with an interactive procedure.

To conclude there are many different types of CFD packages all over the world and all

offering a range of different features. CFD is very useful is the designing process, it can

improve reliability, economy and improve the working life of different parts. This is why it

has become such a large part of engineering. It also cuts cost in the designing because you

don’t have to pay for a wind tunnel, and the cost of building a model to test in a wind tunnel

also a large amount of packages are free to download.

2. To do a simulation in CHAM pheonics first you have to set up the domain that is doing to be

used. The domain for this simulation needs to be large enough for the car to fit it and have

space at the rear of the car to see if there are any changes to air as it passes the car. To set

up the domain you need to go through the menus and to

geometry. Figure 1 shows the menu that will come up.

Here you can change the domain size and number of cells

in the grid. The domain size is 20m x 5m x5m and the

number of cells is 20 x 50 x 20.

The cells are how the grid is divided up and how many

cells there are equals how many calculations the

programme will do of the Navier-stokes equations in the

grid. Then the number of interations is how many

calculations it will do in one cell. This can be found in

the numerics section of the main menu. The number of

interations for these simulation runs is 500. The next step in the main menu is to set the

NAMGRD setting, this is found in the GROUND menu and the box next to NAMGRD, f1



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needs to be keyed in. what this does is give the drag, side and lift

coefficients after the equations have be solved in solver. Figure

3 shows this.

After this has been done the next stage is to start bringing in the

objects that will be involved in the testing. The first one is the

inlet, this is where the air comes into the domain. To set the size

of the inlet there is a size tab in the object options box. The next

step is to set the velocity of the air flow, this is in the attributes

section and is measured in m/s. for the tests the speeds will be

13.41m/s, 22.53m/s and 31.29. This is because these are the

speed limits in the city, urban and the motorway.

The next stages are to place the outlet and the floor in. they are

done in a similar way to the inlet, however in the place tag you

can choose where to place them so the outlet will be on the

opposite side of the inlet and the floor will be placed on the bottom of the domain. After

these are in place the next stage is to bring in the car that is going to be tested. The car that

will be tested is the Audi A4. To do this, in the object options menu there is a section where

you can bring in CAD files and by using the rotation options and the size options you can get

it in the right place and the correct size for the simulation, this is what happened when the

Audi was imported.

After the Audi is in place the solver stage can begin.

His is where the software does the calculations for the

air flow. Figure 4 shoes where the solver can be found

in the software.

Once the solver is running the image in figure 5 will

show up. This shows the solver monitoring the progress

of the solver when it is solving the Navier Stokes

equations. Another box that appears up after the solver

has completed the equations, this is the box that gives

the details of life side and drag coefficients. Figure 6

shows this.

After the solver has been completed, the post

processing stage can begin. In this section there is a

visual representation of the flow of fluid. From the

results of pressure and velocity you can see how the air

is flowing and how fast it is traveling. And you can set

up stream lines to give a visual representation of what is

happening.

Figure 2-numerics

Figure 3

Figure 4

Figure 5

Figure 6



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The first test that I did was at 30mph (13.41m/s) the results are as follows.

The next test that was simulated is with the air velocity at 50mph (22.53m/s).

From the results we can see the areas

of high pressure on the car. This is at

the front of the car, this is becausethis part of the car to hit the air and is

forcing it to rise above the car which

gives the low pressure on the roof of

the car because of the angle of the

windscreen. This low pressure

creates and area of faster moving air

because of pressure gradient created.

There is also an area of low velocity

at the rear of the car, this is because

the air is being forced away from the

car as it travels through the air, this

leaves an area that cannot be

affected by the velocity of the air

flow.

Figure 7-pressure at 13.41m/s

Figure 8-velocity at 13.41m/s

Even though the velocity of the air flow

has been increased there is not much

change to the flow of air with regards to

pressure. At the front of the car there is

an increase in pressure because the air is

flowing faster therefore more air is

hitting the front of the car creating

higher pressure. There is also not a lot of

change in the velocity, the speeds are

higher however there is no change to the

flow of the air.

Figure 9- pressure at 22.53m/s

Figure 10- velocity at 22.53



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The final speed that is simulated is 70mph (31.29m/s). The results are as follows:

There isn’t much visual change in velocity and pressure because the car is not moving at and angletherefore the flow of air will be taking the same course, therefore another way to visualise the

changes of pressure is in graphs.

Figure 13- average pressure

From figures 11 and 12 we can see in

more detail how the pressure and

velocity changes as the air flows over

the car. There is a greater amount of

air flowing at a slower speed at the

front of the car as more air is being

forced over the car. This increase in

velocity increases the pressure at the

front because there is more air is

being forced to flow away from the

car.

Figure 11- pressure at 31.29m/s

Figure 12- velocity at 31.29m/s

From figure 13 we can see that there isn’t

a large amount of change to the average

pressure, there is just 0.6082 difference

between 13.41m/s and 31.29m/s. the

pressure is negative because it is relative

to the pressure in the atmosphere. From

the results we can see that the high the

velocity the lower the pressure around

the car will reach. This is because the

faster the air is flowing the more drag

there will be, and it is this low pressure

that is the drag effect.



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3. Pressure coefficient means the ratio of pressure forces to the internal forces. The equation

to work out pressure coefficients is :

The pressures that needed to be recorded needed to be spread equally along the car aboveit and below it, to gain reliable data there are 5 points where the pressure will be recorded

from. The results for 30mph are:

30mph

Above car Below car

45.70772 25.90243

-5.999194 -46.22072

-29.66726 -3.021159

-25.59049 -26.82663

-6.536892 -11.42372

When this is put into the equation and put into a graph it looks like this:

Figure 14- pressure coefficients for 30mph

From figure 14 we can see that the pressure coefficient with the air flowing over the car is a steady

arc where as there are peaks and troughs is the results, this could be because the wheels are

creating areas of low pressure which would affect the pressure coefficient.

Figure 15-pressure coefficients for 50mph



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When we compare figure 14 and 15 the lines and the coefficients above and below the car are very

similar, and it is the same results for the simulation at 70mph as shown in figure 16.

The results for the pressure coefficients vary slightly, however when plotted on a graph they all take

the same form. This is because pressure coefficients are relative to the pressure in the domain and

as the car has not moved from its original placement. Therefore from the graphs we can see that the

higher amounts of pressure are found at the front of the car, and as the air flows over the car the

pressure is decreasing , however as the air starts to rear the rear end of the car the pressure

increases slightly.

The pressure that is being recorded below the car has two peaks of a high pressure coefficient. Thismeans that the pressure at those two points were lower. The reason for this is the way the wheels

cut through the air; they leave an area of low pressure behind them. The reason why the first peak is

0.033 larger than the second is because these are the front wheels and they are hitting the air first,

therefore by the time the air has reached the rear wheels the pressure is not as high as it was when

it first hit the front wheels this means that they have a lower pressure difference because the air

pressure is already lower. Figure 17 shows the car at a close up view making clearer to see the

pressure behind the wheels. It is very clear that

the pressure behind the front wheels is lower

than that behind the rear wheels.

To conclude, the higher the velocity of air passing over the car, the higher the pressure will be at the

front of the car and the lower the pressure will be at the rear of the car. We can also see that

changing the velocity will not affect the pressure coefficient because the cars position is not moving

and the pressure coefficient is relative to the pressure in the domain. The pressure coefficients

below a car are not as laminar as you would expect, it is widely affected by the wheels creating areasof low pressure.

Figure 16- pressure coefficients for 70mph

Figure 17



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References

ACUSIM Software » Applications. 2013. ACUSIM Software » Applications. [ONLINE] Available

at: http://www.acusim.com/html/applications.html. [Accessed 9 April 2013].

Standard Solvers. 2013. Standard Solvers. [ONLINE] Availableat:http://www.openfoam.com/features/standard-solvers.php#incompressibleFlowSolvers.

[Accessed 10 April 2013].

ACUSIM Software » AcuSolve. 2013. ACUSIM Software » AcuSolve. [ONLINE] Available

at: http://www.acusim.com/html/acusolve.html?phpMyAdmin=FRtr5SghpEUhhg%2CH2nBQB3F8g0f

. [Accessed 10 April 2013].

Welcome to TYCHO - TYrolian Computational HydrOdynamics. 2013.Welcome to TYCHO - TYrolian

Computational HydrOdynamics. [ONLINE] Available at: http://www.tycho-cfd.at/. [Accessed 11 April

2013].

Zeus Numerix - FlowZ - Pressure and Density based Solver. 2013. Zeus Numerix - FlowZ - Pressure

and Density based Solver . [ONLINE] Available

at:http://www.zeusnumerix.com/products/cfd/flowz/#pressure_based. [Accessed 11 April 2013].

Numeca International: Products. 2013. Numeca International: Products. [ONLINE] Available

at: http://www.numeca.be/index.php?id=16. [Accessed 11 April 2013].

Modelling Capabilities . 2013. Modelling Capabilities . [ONLINE] Available

at:http://www.cham.co.uk/products/modellingcapabilities.php. [Accessed 10 April 2013].

http://www.acusim.com/html/applications.html



http://www.openfoam.com/features/standard-solvers.php#incompressibleFlowSolvers



http://www.acusim.com/html/acusolve.html?phpMyAdmin=FRtr5SghpEUhhg%2CH2nBQB3F8g0f


http://www.tycho-cfd.at/



http://www.zeusnumerix.com/products/cfd/flowz/#pressure_based



http://www.numeca.be/index.php?id=16



http://www.cham.co.uk/products/modellingcapabilities.php










sam lindop l018852a assignment 2

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