Unstedy Flow Past a Cylinder

8/12/2019 Unstedy Flow Past a Cylinder

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Unsteady Flow Past a Cylinder - Problem Specification

Problem Specification

Consider the unsteady state case of a fluid flowing past a cylinder, as illustrated

above. For this tutorial we will use a Reynolds Number of 120. In order to simplify the

computation, the diameter of the cylinder is set to 1 m, the x component of the

velocity is set to 1 m/s and the density of the fluid is set to 1 kg/m^3. Thus, the

dynamic viscosity must be set to 8.333x10^-3 kg/m*s in order to obtain the desired

Reynolds number.

Compared to the steady case, the unsteady case includes an additional time-

derivative term in the Navier-Stokes equations:

The methods implemented by FLUENT to solve a time dependent system are very

similar to those used in a steady-state case. In this case, the domain and boundary

conditions will be the same as the Steady Flow Past a Cylinder. However, because

this is a transient system, initial conditions at t=0 are required. To solve the system,we need to input the desired time range and time step into FLUENT. The program

will then compute a solution for the first time step, iterating until convergence or a

limit of iterations is reached, then will proceed to the next time step, "marching"

through time until the end time is reached.

2. Pre-Analysis & Start-Up


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Please complete the "Steady Flow past a Cylinder tutorial before completing this

tutorial. Clickhereto go to the problem statement of the "Steady Flow Past a

Cylinder" tutorial.

Alternatively, clickhereto download the completed project files for the "Steady Flow

Past a Cylinder" tutorial.

The pre-analysis is the same for both steady and unsteady flow past a cylinder.

Click [here]to go to the pre-analysis of the "Steady Flow Past a Cylinder" tutorial.

To start-up, open your completed "Steady Flow Past a Cylinder" project file. (If using

the completed version in the zip file above, extract the files and open

"Cylinder.wbpj".)

Right-click on Fluid Flow (FLUENT)and then click Duplicate. Enter "Unsteady

Flow" in the highlighted field to rename it. Your Project Schematic should now

appear as below.

4. Setup (Physics)

Launch FLUENT.

(Do u b l e C l i c k ) Se t u p in "Unsteady Flow", the duplicate project. Select Dou b l e

P r e c i s i o n , and if using a computer with multiple cores, select parallel, and set the

number of cores to be used.
https://confluence.cornell.edu/display/SIMULATION/FLUENT+12.1+-+Steady+Flow+Past+a+Cylinder+-+Problem+Specificationhttps://confluence.cornell.edu/display/SIMULATION/FLUENT+12.1+-+Steady+Flow+Past+a+Cylinder+-+Problem+Specificationhttps://confluence.cornell.edu/display/SIMULATION/FLUENT+12.1+-+Steady+Flow+Past+a+Cylinder+-+Problem+Specificationhttps://confluence.cornell.edu/download/attachments/144972258/Cylinder.zip?version=1&modificationDate=1335902066000https://confluence.cornell.edu/download/attachments/144972258/Cylinder.zip?version=1&modificationDate=1335902066000https://confluence.cornell.edu/download/attachments/144972258/Cylinder.zip?version=1&modificationDate=1335902066000https://confluence.cornell.edu/download/attachments/144972258/Cylinder.zip?version=1&modificationDate=1335902066000https://confluence.cornell.edu/display/SIMULATION/FLUENT+12.1+-+Steady+Flow+Past+a+Cylinder+-+Problem+Specification


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Then click OK

Transient

In this step here we will, tell FLUENT to solve for the unsteady flow. As you can see,

by default FLUENT will solve for the steady flow.

P ro b l em Se t u p > Gene r a l . Set T ime to T r a n s i e n t .

Specify Material Properties

To achieve a Reynolds number of 120, as required in the problem statement, we will

change the material viscosity, to 8.333*10^-3 kg/m*s.


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P ro b l em Se t u p > Ma t e r i a l s > F l u i d > C re a t e /E d i t . ... Set the v i s c o s i t y to

8.333*10^-3 kg/m*s. Click Change /C r e a t e .

Then click C l o s e .

Save Project

5. Solution

Convergence Criterion: Turn off Drag, Turn on Lift

So l u t i o n > Mon i t o r s > D ra g > Ed i t . ... Then uncheck P r i n t to Co n s o l e and

uncheck P l o t . Click o k.

So l u t i o n > Mon i t o r s > L i f t > Ed i t . ... Then check P r i n t t o

C o n s o l e , P l o t and Wr i t e . Click o k. The last option writes the lift coefficient data to

a file that is buried in one of the subfolders that FLUENT creates in the working

folder. You'll have to dig around to find it.

Solution Initialization

First, let's set the initial condition in all of the cells to a velocity of 1 m/s in the X-

direction. So l u t i o n > So l u t i o n I n i t i a l i za t i o n . Set Compu t e

F r omto f a r f i e l d 1 .. Click I n i t i a l i z e.

Next, we'll change the velocity in some of the cells to more quickly reach a sinusoidal

variation of the lift coefficient. Ad ap t > Reg i o n . ...


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Then set X M into 0.5 m, set X Maxto 32 m, set Y M into 0 m, and set Y Maxto

32m.

Click Ma r k then click C l o s e . This will select the cells bounded by these four points,

so we can change the initial condition in them.

Next, click Pa t c h .


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Complete the patching menu as shown below. This will change the initial Y

component of velocity in the selected region from 0 to 0.2 m/s.

Click Pa t c h ,then click c l o s e .


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Setting Up Data Export to Create Animation

We would like to create an animation of the vorticity magnitude after the solution has

been calculated. To do so, we will need to export data from FLUENT to CFD-Post,

the post processor used to view results. To do so, go to Solution > CalculationActivities > Automatic Export > Create > Solution Data Export....

Next, change File Type to CFD-Post compatible, as this is the program we will use

for post processing. Then, select Vorticity Magnitude from the list of variables on the

right, so we can make an animation of contours of vorticity. Finally, click Browse, and

choose a convenient file location to place the data files. Make note of this location for

later use.


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Advance Solution in Time

So l u t i o n > Ru n Ca l c u l at i o n . Set T ime S t e p S i z e to 0.2 seconds and set

the Numb e r O f T ime S t ep s to 400.

Now, click Ca l c u l a t e . (You may have to hit Ca l c u l a t e twice.) Now, have a cup of

coffee. When complete, close FLUENT to return to the main project window.

Save Project

Results

Open CFD-Post

We'll create a separate CFD-Post module, as this is the easiest way to load the

results for this project.

On the left of the main project window, expand Component Systemsand double-

click Results.


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Your project schematic window should now appear as below.

Double click on the Resultsmodule that was just created to open CFD-Post.

Results

Now, we need to load the results of our FLUENT simulation.

After opening CFD-Post, click the Load Resultsbutton in the upper left corner of the

screen.

Next, browse to the location where you chose to save the FLUENT data files. Selectthe .cas file that is in this folder, which should be named "FFF-1-0001.cas", or


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similar. In the bottom right of this window, select Load complete history

as:and Single Case. Finally, click Open.

Click OKin the popup window if one appears.

h4. Load Timesteps

Click Tools > Time Step Selectorto open the Time Step Selector.

Select the first time step, and click Apply. Leave the Time Step Selector window

open, but continue to the next step.


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h4. Create Vorticity Contour

Now, let's insert a contour of vorticity, in order to animate it.

While leaving the Time Step Selector window open, click Insert > Contour. Name it

"Vorticity Contour".

Under Details of Vorticity Contour, select symmetry 1from Locations.

Next, ensure that Variableis set to Vorticity.

Change Rangeto User Specified. Set the Minto 0.01 s^-1and Maxto 2 s^-1.

Enter 25for Number of Contours. You should now see the following:


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Click Applyto create the contour.

Next, let's set up the view we would like for the animation. You can see that we are

currently viewing the 2D surface from a 3D, isometric perspective. To fix this, click

the Z-axis in the axes triad in the lower corner.

Now let's zoom in to the are of interest. Select the zoom box tool from the upper

toolbar.

Using the zoom box tool, click and drag a box that roughly encompasses the area

shown below to zoom in on it.


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Now we're ready to animate the vorticity contour over this zoomed-in area.

h4. Create Animation

Return to the Time Step Selector Window, which should still be open. Click

the Animate Timestepsbutton.

Select Keyframe Animation, and click the insert new keyframe button, .

Change the number of frames to equal the number of data files we saved to animate,

in this case 400. Your Animation window should look like this:


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Keeping the Animation window open, click back to the Time Step Selector window.

Select time step #400, and click Apply. The Vorticity Contour on the right half of your

screen should now have changed. Click back to the Animation window, and insert

another new keyframe. This time, leave the number of frames set to 10.

We're now ready to set up the saving options for the animation. Click the arrow in the

bottom right of the window to expand the options.


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Check the box labeled Save Movie, and use the folder icon to set the desired file

location and type.

Next, maximize your CFD-Post window, and click the play button in the Animation

window to create the animation!

Your video should turn out similar to the one below.

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Unstedy Flow Past a Cylinder