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Step 7: Refine Mesh

It our mesh refine enough? Will the current mesh give us an accurate representation of the physical flow? Mesh density determine the truncation error of our simulation. With too little mesh density, the result we obtain will be inaccurate. With too many mesh elements,  we will be wasting unnecessary computational power (long computational time!). To get the optimum mesh density, we do mesh sensitivity analysis.

For this purpose, we will create three model with different mesh density. We will create a coarser mesh and a finer mesh and compare all the three meshes together.

For coarse mesh, reduce the mesh density by half of the original mesh. For fine mesh, double the mesh density compared to the original density.

Follow Step 1 through Step 5. At step 2, use the following edge mesh properties instead.


Label

Medium Mesh (Original)

Coarse Mesh

Fine Mesh

A

Interval Count: 36, Double First Length: 0.5

Interval Count: 24, Double First Length: 0.7

Interval Count: 54, Double First Length: 0.3

B

Interval Count: 36, Double First Length: 0.2

Interval Count: 24, Double First Length: 0.3

Interval Count: 54, Double First Length: 0.12

C

Interval Count: 30, First Length: 0.1

Interval Count: 20, First Length: 0.2

Interval Count: 45, First Length: 0.09

D

Interval Count: 18

Interval Count: 12

Interval Count: 27

E

Interval Count: 90, First Length: 0.1

Interval Count: 60, First Length: 0.15

Interval Count: 135, First Length: 0.07

F

Interval Count: 36

Interval Count: 24 

Interval Count: 54

G

Interval Count: 72

Interval Count: 48

Interval Count: 108

H

Interval Count: 30

Interval Count: 20

Interval Count: 45

I

Interval Count: 36, Double First Length: 0.05

Interval Count: 24, Double First Length: 0.07

Interval Count: 54, Double First Length: 0.03

To double the mesh density, the interval count is increased by factor of 1.5. The first length is decreased by a factor of about 1.5. To half the mesh density, the interval count is decreased by factor of 1.5. The first length is increased by a factor of about 1.5. Think about why it is so. 

After that, you should have three meshes with following mesh elements.


Coarse

Medium

Fine

Mesh Elements

6240

14040

31590

Your mesh should differ slightly from the above table. The mesh properties given to you in this tutorial are of less significant figures. Whereas the meshes used by the author were generated using script with exact accuracy.

Drag Coefficient Comparison

For each mesh, take note of the drag coefficient and compare with each other.


Coarse

Medium

Fine

Cd

2.4981

2.4941

2.4927

% dif

0.160378

0

0.056132



Pressure Coefficient Comparison

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