express introductory training in ansys fluent

© 2012 ANSYS, Inc. September 19, 2013 1 Release 14.5

PRACE Autumn School 2013 - Industry Oriented HPC Simulations, September 21-27,

University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia

Express Introductory Training in ANSYS Fluent

Workshop 08

Vortex Shedding

Dimitrios Sofialidis

Technical Manager, SimTec Ltd.

Mechanical Engineer, PhD


Introduction to ANSYS Fluent

14.5 Release

Workshop 7b Vortex Shedding


Workshop Description:

The purpose of this workshop is to introduce good techniques for transient flow modeling.

Learning Aims:

This workshop teaches skills for running Fluent for time–dependant (transient) simulations. Topics covered include:

– Selecting a suitable timestep. – Using Custom–Field–Functions (CFF).

– Auto–saving results during the simulation. – Generating Fast–Fourier Transforms (FFT).

– Generating images during the simulation. – Transient post–processing in CFD–Post.

Learning Objectives:

To show how to set up, run and post–process a transient (time–dependant) simulation, as well as additional skills in using custom field functions and Fast–Fourier–Transforms.

I Introduction

Introduction Model Setup Solving Post–Processing Summary


Simulation to be Performed

• The case considered here is flow around a cylinder with a Reynolds number of 100.

• Vortex shedding will be observed. However the workshop starts with a steady state analysis assuming that the user didn’t anticipate this behavior.

• This workshop demonstrates iterative and non–iterative time advancement, Fast Fourier Transforms (FFT) and animations.

• The tutorial is carried out using Fluent and CFD–Post in standalone mode.



Computational domain created in ANSYS DesignModeler has the following dimensions.

Name Location Dimension

Cylinder D1 2 m (dia.)

Inlet Length D2 20 m = 10 D

Outlet Length D3 30 m = 15 D

Width D4 40 m = 20 D

Computational Domain



Re>3.5×106

3×105 < Re < 3.5×106

40 < Re < 150

150 < Re < 3×105

5–15 < Re < 40

Re < 5

Turbulent vortex street, but

the separation is narrower

than the laminar case.

Boundary layer transition to

turbulent.

Laminar boundary layer up to

the separation point, turbulent

wake.

Laminar vortex street.

A pair of stable vortices in the

wake.

Creeping flow (no separation).

Reynolds Number Effects



Start a Fluent Project (Standalone)


• Launch Fluent from the Start Menu:

"Start Menu>ANSYS 14.5>Fluid Dynamics>Fluent".

Select "2D" Display Mesh After Reading.

Select the working directory you are using on your machine (may be different to that shown here).


Mesh [1]


• Read the Fluent mesh file: "vortex–shedding–coarse.msh" ("File>Read>Mesh").

• The mesh will be read in and displayed, and the zones will be shown in the TUI window.


Final domain

extent.

Mesh [2] • The mesh needs scaling.

Select "Scale" ("Problem Setup>General>Scale"), and enter the values shown, then press "Scale". Be careful only to press "Scale" only once.

• Close the scale panel and "Check" the Mesh.

"General>Check".

"General>Report Quality".

• Display the grid again once scaling has been performed.

"General>Display".



• Select "General" in the navigation pane and keep the "Steady–State" "Pressure–Based" solver.

• Keep "Laminar" setting for the "Viscous Model".

• The properties to be used for the material "air" need to be set.

For "Density", enter "1 (kg/m3)".

For "Viscosity", enter "0.01 (kg/m–s)".

Select "Change/Create".

Solver & Models


Later on we will compare the Fluent results with those from a literature search. We have changed the default material properties for air to aid that comparison.


Boundary Conditions/Solution Methods • Boundary Conditions.

"Inlet":

• Select boundary "in".

• Set velocity to be "1 [m/s]" "normal" to boundary.

"Outlet":

• Select boundary "out".

• Keep default of "0 [Pa]".

"Other boundaries":

• "cylinder" is set to a "wall", no action needed.

• "sym1" and "sym2" are set to "symmetry", no action needed.

• Solution Methods.

Select "QUICK" scheme for "Momentum" equation.



• Set up residual monitors so the convergence can be monitored.

"Monitors>Residuals>Edit".

Make sure "Plot" is on, then "OK".

• Create points to monitor quantity.

"Surface (top menu)>Point".

• Specify coordinates (2 , 1).

• "Activate" point tool to check location on the grid.

• (check out point tool before closing panel).

• "Create" then "Close".

• Surface monitor on point.

"Monitors>Surface Monitors>Create".

• Select "Vertex–Average" on report type and "Velocity" "Y–velocity" in field variable.

• Select "point–6" (the point created above at co–ordinates [2,1]).

• Options: "Print to Console" & "Plot", then "OK".

Solution Monitor



Solution Initialization • Initialize the flow field based on the far–field boundary.

Select "Standard Initialization"

"Compute from" "in" (inlet zone).

"Initialize".

• Save the case file.

"File>Write Case".

You can write case and data files with extension .gz, the files will be compressed automatically.



We have tried to solve this vortex–shedding problem in a steady–state manner. Note that solution is not converging and monitor shows a regular periodic behavior.

Run Calculation [1]


• Set the number of requested iterations to "400" then "Calculate".


Steady state solution is asymmetric.

Run Calculation [2] • Choose Graphics and "Animations>Vectors".

Since this is a 2D simulation, there is no need to pick a surface, just "Display".



Save Case&Data Files and Make Transient • Save the Case&Data files.

"File>Write Case&Data".

• You can write case and data files with extension .gz – the files will be compressed automatically.

To obtain a more realistic solution to this problem we will solve it again, but in a transient (time dependant) manner.

• Under "Problem Setup>General", change the time option to "Transient".



Run Calculation • For the transient scheme, it is recommended to change Solver Methods. The

default pressure velocity coupling (SIMPLE) may require more iterations to converge.

Change to the "PISO" scheme and "2nd Order Implicit" "Transient Formulation" as shown in the image below .

Also change the "Under Relaxation Factors" as shown in the image.



Solution Monitor • Edit the Surface monitor.

Change "Get Data Every" to "Time Step". Also set the "X Axis" to be "Time Step".

"OK"



sVSt

D

fperiod

V

fDSt 06.6

.

1

Run Calculation [1]


• Save the transient case file before starting the computation.

We need to identify a suitable time step size for this problem.

1) A quick way is to do a hand–calculation to see how long it takes for the flow to pass through a typical grid cell. Run this, and check that convergence occurs in less that 20 iterations per timestep.

2) Another approach is to determine the characteristic response of the system. By performing a literature search, we believe that for this problem, the Strouhal number will be approximately 0.165 at this Reynolds number. From this, we can predict the period of the oscillation:

For each oscillation cycle, we will aim to solve 60 timesteps. Hence we will run the solver using a timestep size of 0.1s.


• Specify "Time Step" ("0.1 s") and "Number of Time Steps" ("120').

• Click on the "Extrapolate Variables" option.

• "Calculate" the solution.

Use this option to change the display to show both output Windows.

Run Calculation [2]

The "Extrapolate Variables" option will speed up convergence. Without this option, each timestep would start with the solution at the previous timestep. This option provides a better starting point for the new timestep based on how the solution is changing with time. Notice that as the solver runs, convergence is attained in 5–10 iterations at each timestep.



Run Calculation [3] • Save the transient case&data files.

Note if you add the string %t to the filename ("vortex–shedding–transient–%t.gz") then Fluent will append the current time value to the filename. Note also that this file just contains the results at the current timestep. If you require interim results as the solution progresses, use the "Autosave" feature prior to running the model. We will do this shortly.

Although we now have simulated a couple of oscillations, in order to obtain a true representation of the vortex shedding we need to simulate many more cycles. With each cycle, the "starting position" converges with time until eventually all cycles are identical.

It will take many cycles to achieve this, so we have provided case and data files that has already been converged (simulation time of 84secs). You will then run this on for a further couple of cycles to extract the detail of the fluctuating flow patterns.

• So, read in the supplied Case and Data file:

"vortex–shedding–converged.cas.gz" and ".dat.gz".



1

2

– NITA is an algorithm used to speed up the transient solution process. NITA runs about twice as fast as the ITA scheme.

– NITA scheme reduce the splitting error to O(Δt2)

by using sub–iterations per time step.

– Two flavors of NITA schemes available.

- PISO (NITA/PISO). - Fractional–step method (NITA/FSM).

About 20% cheaper than NITA/PISO on a per time–step basis.

Truncation

error: O(Dt2)

Splitting error (due

to eqn segregation):

O(Dtn)

Overall time–discretization

error for 2nd–order

scheme: O(Dt2)

= +

NITA • Enable the "Non Iterative Time Advancement Method" (NITA).

With "Fractional Step" for "Pressure–Velocity Coupling".



x

V

y

U

y

V

x

UQ

.

Result Analysis • Save the transient case&data files.

One of the ways of quantifying the wake vortices is through the use of the "Q–Criterion". The formula for this is below. It is not a standard quantity computed by Fluent, however since we know the formula, we can ask Fluent to compute it at each grid cell.

• "Define>Custom Field Functions"

Select solver quantities using the pull down list at the right hand side to construct this function as shown, then press "Define".



Extracting Transient Data Unless specifically requested, Fluent will not save interim results during a transient

simulation. There are two ways you may want to consider doing this:

1) Saving the results data every (n) timesteps to disk. This will give a collection of files that can be post–processed at a later date, either using Fluent or CFD–Post. However having to load in a large number of files can be time consuming.

2) The alternative is to extract the required result (like an image from which to build an animation) from Fluent during the solution process. Since all the data is in memory at that instant, this is very quick to perform.

We will do both in this example.



Save Interim Results • To save interim results:

Select "Calculation Activities", and save "Every 5 Time Steps".

Press "Edit", and specify the name of the file to be saved.

Note that the file name will be appended with the current time value (e.g. "transient–detail–00845.dat.gz").

"OK".



Saving Images On–The–Fly [1] • Select "Calculation Activities>Solution Animation>Create/Edit".

• Increase "Number of Sequences" to "1".

• "Sequence 1", "Every" "2 Time Steps".

• "Define", which will open the "Animation Sequence" window.

• Set "Window to 3", press "Set" to enable this window, and "Display Type" to "Contours".

Continued on next slide...



• Set up the contour panel as shown in the image below, then "Display".

Set the graphics window to display screen "3".

Draw a zoom–box with the middle mouse button to zoom in on the cylinder.

Note that the file name will be appended with the current time value.

• Close the contour panel, then "OK" to both panels opened on previous slide.


Saving Images On–The–Fly [2]


Solution Monitors • Edit the Surface monitor again.

Check the box next to "Write" and specify a name for the file. This type of file can be used for Fourier Transform analysis.

"OK".



Run Calculation for Creating Animation • Run the calculation:

Use a smaller timestep for NITA (0.05s).

Solve for 240 Time Steps.

Calculate (this corresponds to roughly 2 periods).

• Save the Case and Data File.

Remember that if you add the string %t to the filename ("vortex–shedding–transient–%t.gz") then Fluent will append the current time value to the filename.




Post Processing (Fluent) [1] • To run the animation (Graphics and Animation in the navigation pane on the left,

then choose Solution Animation Playback and Set Up…)

Use the Play button to view a movie of the series of images.

If required, this can be written out as an mpeg movie.



• From the Plot Menu, select "FFT" then "Setup".

• From the Fourier Transform Window, "Load Input File" and pick the supplied file "fft–data–2000–timesteps.out"(this file was generated after running the simulation for 2000 timesteps. Tip: You may need to alter the file selection filter to "All Files" to see this).

• Pick "Magnitude" for "Y–Axis Function".

• Pick "Strouhal Number" for "X–Axis Function".

Continued on next slide…

Post Processing (Fluent) [2]



Post Processing (Fluent) FFT [1] • Pick "Axes", and for the "X–Axis" turn off "Auto–Range".

• Set bounds from "0.05" to "1". "Apply", then "Close".

• Select "Plot FFT".

Continued on next slide…



• You may need to change the graphic window so that "Spectral Analysis" is visible.

Post Processing (Fluent) FFT [2]

The peak Strouhal number is 0.171, which is close to the 0.165 that was suggested by the literature search. To extract the exact peak value from this graph, enable "Write FFT to file" and look at the text file on disk. The second peak is a harmonic as the input signal is not perfectly sinusoidal.


Close Fluent – Run CFD–Post • Close Fluent.

• Open a CFD–POST session.

We will create an animation.




Post Processing (CFD–Post) [1] Animation in CFD post can be done based on all the data files already saved

Thus, you can create any animation once calculation is finished.

• "File>Load Results".

Select last time step data file (Make sure you select the files generated from the autosave feature, with a filename "transient–detail–1–nnnnn.dat.gz", rather than the results that you have saved manually whilst working though the instructions.

Select "Load complete history as": "A single case".



• Insert a vector.

• Open a CFD–POST session.

Keep default name "Vector 1".

Location "Symmetry 1".

"Apply".

Click on the "Z" axis to align the view angle.

Post Processing (CFD–Post) [2]



• Activate Timestep Selector panel.

Recall that in Fluent, we

generated a contour plot

every 2 timesteps. We

saved the data files used

here every 5 timesteps.

• Pick a time value from the list then Apply to see the result at that timestep.

• Click on the film icon, then the play button, for a quick animation of all saved timesteps.

Post Processing (CFD–Post) [3]


Optional Further Work • There are many ways the simulation in this tutorial could be extended.

• Mesh independence.

Check that results do not depend on mesh.

Re–run simulations with finer mesh(es).

• Generated in ANSYS Meshing application, or,

• from adaptive meshing in Fluent.

• Reynolds number effects.

– For lower Reynolds number, steady state analysis with laminar model is possible.

– For higher Reynolds numbers, unsteady transitional turbulent models (k–kl–omega, SST) have to be considered.

– While for Reynolds number higher than 3.5×106 , the standard or SST k–omega turbulence models would be used.


You can investigate other flow pattern by changing the Reynolds number.


Wrap–Up

This workshop has shown the basic steps that are applied in all CFD simulations: Setting boundary conditions and solver settings. Running steady and transient models. Using iterative and non–iterative advancement schemes. Postprocessing the results, both in Fluent and CFD–Post for transient cases.

One of the important things to remember in your own work is, before even starting the ANSYS software, is to think WHY you are performing the simulation: What information are you looking for. What do you know about the boundary conditions.

In this case we were interested in calculating flow around a cylinder, and assessing the vortex shedding frequency. We checked with FFT analysis that predicted frequency is in good agreement with results from literature. Knowing your aims from the start will help you make sensible decisions of how much of the part to simulate, the level of mesh refinement needed, and which numerical schemes should be selected.



Braza, M., Chassaing, P., & Minh, H. H., "Numerical Study and Physical Analysis of the Pressure and Velocity Fields in the Near Wake of a Circular Cylinder", J. Fluid Mech., 165:79–130, 1986.

Coutanceau, M. & Defaye, J. R., "Circular Cylinder Wake Configurations – A Flow Visualization Survey", Appl. Mech. Rev., 44(6), June 1991.

Williamson, C. H. K, "Vortex Dynamics in The Cylinder Wake", Annu. Rev. Fluid Mechanics, 28:447–539, 1996.

References

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