cfd report ali

8/10/2019 CFD Report Ali

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The Hong Kong Polytechnic University

ME4414 Fluids Engineering

Computational Fluid Dynamics (CFD) Tutorial Report

By Ali Jahan Zaib (12063189D)

27thOctober 27, 2014


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Contents1. Objective: .............................................................................................................................................. 2

2. Problem Definition: ............................................................................................................................... 3

3. Methodology: ........................................................................................................................................ 4

3.1 Mesh Generation: ............................................................................................................................... 4

3.2 Solver Setup: ....................................................................................................................................... 7

4. Results and Discussion: ....................................................................................................................... 14

For Re=1 .................................................................................................................................................. 15

For Re=10 ................................................................................................................................................ 17

For Re=100 .............................................................................................................................................. 20

5. Conclusion: .......................................................................................................................................... 24

References and Appendix: .......................................................................................................................... 25


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1.Objective:

The aim of this CFD tutorial is to familiarize with the mainstream CFD software

ANSYS. The software used is ANSYS ICEM and FLUENT. Through solving a sample

problem by CFD software, we will achieve the following objectives:

1.

To be able to create a 2-D mesh

2.

To be able to troubleshoot basic meshing problems

3.

To be able to solve a sample problem via CFD

4. To be able to change parameters to obtain different results

5.

To be able to run a simulation and generate meaningful reports

6.

To apple knowledge learnt to future fluid mechanics problems


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2.Problem Definition:

In this CFD tutorial, a sample problem has been provided which will be solved

using CFD software. Let us define the problem.

Air flows across a cylinder with a uniform velocity of 0.1 m/s in a wind tunnel.The length of the wind tunnel (i.e. Fluid Domain) is 2.5m long and 1m in height. The

diameter of the cylinder is 0.1m. Below is the diagram of the model:

We have to make a few assumptions. We assume that the fluid isincompressible and that the problem is two dimensional. We also assume the

cylinder to be a smooth cylinder.


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3.Methodology:

For the CFD, we will first require to make a mesh in ANSYS ICEM for the

problem and then transport the mesh into ANSYS FLUENT to conduct the

simulation.

It is important to note that we need to carry out more than one simulation

to obtain different results to ensure that the data obtained is reliable and match it

with existing data available.

The boundaries given by the tutorial sheet may or may not be enough to

obtain correct results thus we have the following values we can adjust to obtain

different sets of results.

Test set up for Reynolds number Re= 1, 10 and 100

These different inputs will help us learn more about the CFD software as well

as how the inputs affect the model at hand. By manipulating the inputs, we can

have a better understanding of the fluid flows and become more confident in

analyzing fluid simulations and making good judgment of fluid problems.

In the following sections, we will discuss more about the usage of ANSYS

ICEM for mesh generation and ANSYS FLUENT for the solver setup.

3.1 Mesh Generation:

The idea behind mesh generation is that CFD software utilize numerical

methods to solve for complex fluid mechanics equations through an iterative

process. This is a method that develops approximations of the governing equations

of fluid mechanics and thus applies them onto the mesh which is essentially a grid

that may be 2-dimensional or 3-dimensional. As system of equations are solved,

the solutions correspond to a specific cell on the grid and after the simulation has

ended, the whole mesh contains the various values. The data is post-processed to

extract important information such as drag, lift or pressure.

The mesh has various properties that can be adjusted to obtain varying

degrees of accurate results and the mesh also needs to be altered based on the

complexity of the model system. Parameters such as grid type (Hex or Quad),

resolution of the grid, how many cells required or even if enough memory is


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present all affect the accuracy of the results but also the time required to obtain

the results. A higher resolution mesh may give a more accurate result but it is not

always the case as there may be other limiting factors or one may have simply

obtained an accurate result and further increasing the mesh resolution would be a

waste of resources.

For example problem at hand, we have the following mesh properties:

Mesh Part Nodes

Cylinder 100

Inlet 100

Outlet 100

SideA 250

SideB 250

Diagonal edge near cylinder 10 per edgeTable 1 - Mesh Parameters

The nodes are basically points on the mesh which are used as points for the

finite element analysis used by the solver to solve the under lying fluid mechanics

equations. The more the number of modes, the higher the resolution of the mesh

but this also requires a longer processing time to calculate the results. Thus, the

above values represent the number of node an edge is divided into. Through ICEM,

it is possible to add extra nodes to specific parts of the model to enhance the results

obtained from those parts which are more essential such as areas near the cylinder.

On the next page are images showing part of the setting in ICEM during meshing

process.


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Figure 2-Mesh setup showing cylinder as example

Figure 1 - Mesh parameters of Diagonal Edge


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Figure 3 - Finished Mesh

3.2 Solver Setup:

For any given problem, when we move to the simulation phase, we have to

set up our numerical model. This is very important as if our input is faulty, our

results will be faulty thus we will not be able to solve the problem.

We have to select appropriate physical models and then set the proper

parameters for the turbulence and other factors. We have to define the material

properties such as the density, flow rate, temperature etc. Then prescribe theoperating conditions and boundary conditions at all boundary zones. We may need

to provide an initial solution and then set up the solver controls and finally the

convergence controls.

It is important to set up a properly defined convergence monitor as

sometimes we may not need to do 100 iterations to obtain accurate results. Thus

we need to set up a proper monitor so that we can save time and resources.

For example problem at hand, we have followed the FLUENT tutorial notes

and the provided values from the problem to define all the parameters before

starting the calculation. On the next few pages, the details of the Solver setup are

discussed.


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The list of parameters used are shown in the table below:

Parameters Values

Area (m2) 0.1

Density (kg/m3) 1.225001

Depth (m) 1

Enthalpy (j/kg) 0

Length (m) 1

Pressure (Pascal) 0

Operating Pressure (Pascal) 101325

Temperature (K) 288.16

Velocity (m/s) 0.000146073

Viscosity (kg/m-s) 1.7894e-05

Ratio of Specific Heats 1.4Table 2 - Solver Parameters

Figure 4 - Solver Parameters


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One thing to note for the values shown on the previous page is that they are

the values for the condition of Re=1which is based on the following equation for

Reynolds Number calculation:

() =

where is Density of fluid, V is free stream velocity, D is characteristic length

of particle/object and is viscosity of fluid.

Setting the Re=1 and finding the corresponding velocity that is required,

=

1 = (1.225)(0.1)(1.7894 0.5)

= 0.000146073 /

Therefore, the velocity is out controlled parameter that we adjust to obtain

the results for the simulation at Re=1, Re=10 and Re=100. Rest of the values are

kept constant.

Reynolds Number Velocity (m/s)

1 0.00014607310 0.00146073

100 0.0146073Table 3 - Reynolds number and corresponding Velocities

We set the solver to consider transient analysis as the patterns generated

are changing with time and we want to be able to see the distinct patterns at a

specific time.

Figure 5 - General Solver Settings


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Then, the model chosen for this analysis is selected. We have selected

Viscous Laminar flow situation which is suitable for this analysis.

We are using Air as the fluid in this analysis and we take the default values

for the density and viscosity as provided by FLUENT which have been shown in Fig.

4.

Figure 6 - Model


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We then select the Solution methods in which we select PISO method.The

Transient formulation method employs Second Order Implicit finite element

analysis model. This ensures accuracy for the results and required initial values to

be provided. Thus the step after requires initialization of the values into the solver

as shown in the figures below.

Figure 8 - Solution InitializationFigure 7 - Solution methods


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Next step involves setting the Monitors, also known as congerence monitors

that alow the user to see if the soution has converged or not and it can also allow

the user to monitor various other parameters such as the Drag and Lift coefficients.

We have set our slver to monitor the Drag and Lift coeffitients at specific

parameters such as every 5 time steps.

An optional set is to include the recorder for the animation sequence. This is

a feature that will allow us to generate an animation of the flow after the simulation

has ended. This can be especially useful to visualize the flow and see how the flow

acts around the object. We have set our solver to record the frames of the flow

every 5 time steps that will allow us to generate an animation at the end.

Figure 10 - Solver Monitors

Figure 9 - Animation Recorder


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The final step is to run the calculation which is shown in the next page.

This is the final and an important part of the simulation which determines

how accurate and how long will the simulation run for. For all of our Re values, we

have set the following values:

Parameters Values

Time Step (s) 0.5Number of Time Steps 5000

Max iterations per Time Step 20Table 4 - Time Step Parameters

Finally, the solver it set to calculate and the results are generated. Time taken

varies based on the complexity of the calculation.

Figure 11 - Running the Calculation


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4.Results and Discussion:

After running all the simulations for the 3 Re values, the following results are

obtained. The results have been divided by the Re values and they contain

information for the Pressure Coefficients, Velocity Magnitudes and Path lines. They

are also compared to the existing literature values and diagrams for accuracy of

results.

Results are shown starting next page for better visualization in landscape.


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For Re=1,

Figure 13 - Re=1, Pressure Coefficient Contour

Figure 12 - Re=1, Velocity Contour


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Figure 14 - Re=1, Pathlines

Figure 15 - Reference Diagram for Re


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For the results at Re=1, we can see that the Pressure contour reveals that the pressure is the highest at the

front of the cylinder and that behind it is relatively low pressure. For the velocity contour, we can see a similar case

that the velocity is actually the lowest in the front of the cylinder. Furthermore, seeing the Pathlines, we can see

that the pattern is very similar to the reference diagram for the flow over a cylinder at low Re values such as around

1 or lower. This thus confirms that the results obtained are quite accurate and we have had a successful simulation

for Re=1.

Next we take a look at Re=10 with the velocity increased by a factor of 10.

For Re=10,



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Figure 18 - Re=10, Velocity Contour

Figure 17 Re=10, Pathlines


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We can see above that the results at Re=10 are much more different as we can start to see a laminar wake

region form behind the cylinder. The results are further confirmed when matched with the reference diagram for

the flow at Re=10 over a cylinder. Thus simulation has been successful. Finally, we look at the Re=100 in the next

part.

Figure 19 - Re=10, Pathlines Close-up

Figure 20 - Reference Diagram for Re=10


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For Re=100,


Figure 21 - Velocity Contour


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Figure 24 - Re=100, Pathlines

Figure 23 - Re=100, Pathlines Close-up


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Observing the results, we can see how the Karman Vortices have formed behind the cylinder at Re=100. This

has caused a vibration in the object which is further revealed by the relation of the Drag and Lift coefficients. We

can also see that the Pathlines closely adhere to the reference diagram for Re=100. Lets take a look at the Drag

and Life coefficient chart.

Figure 25 - Reference Diagram for Re=100

Figure 27 - FLUENT Cd and Cl relation w.r.t TimeFigure 26 - Reference Chart of Cd and Cl w.r.t Time


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The above charts comparison shows that the results of the variation of the Cd and Cl with respect to time

are close to the reference values from the notes chart, thus overall the simulation has been accurate for Re=100.

There may be errors in the chart or slight variances in the simulation results. This is normal as the simulation

will calculate new each run and thus is bound to have some variances over time. These errors are negligible in this

research as the results highly resemble the reference values and diagrams thus we can say that the simulation

overall was very accurate.


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5.Conclusion:

To conclude, through this tutorial analysis on the flow over a cylinder at

different Re values, we have managed to learn the basics of the CFD software. WE

have learnt how to generate mesh with different parameters as well as how to

adjust the solver to meet our needs and then calculate the results. We also found

that the results highly depend on the input parameters and may show a significant

change when varying different parameters.

We were able to generate useful and accurate reports and charts which

helped us analyze the problem more efficiently and we have been able to

troubleshoot the simulation with the help of the lab staff and the professors

guidance.

We can now apply this knowledge learnt in future fluid mechanics problemsto help us nurture our engineering sense by being able to visualize the fluid

mechanics problems through the use of CFD software.

For further research, we can adjust the domain size to see how it influences

the results and also adjust the mesh configuration from QUAD to HEX and observe

how the accuracy changes.


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References and Appendix:

CFD notes

ANSYS ICEM tutorial notes

ANSYS FLUENT tutorial notes

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