Fluent-Intro 14.5 WS07b Vortex Shedding

2012 ANSYS, Inc. November 13, 2012 1 Release 14.5

14.5 Release

Introduction to ANSYS Fluent

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 didnt 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.



The computational domain was created in ANSYS DesignModeler and 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 > 3.5106

3105 < Re < 3.5106

40 < Re < 150

150 < Re < 3105

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


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

The mesh will be read in and displayed, and the zone names will be shown in the TUI window.


Final domain

extent

Mesh The mesh needs scaling.

- Select Scale (Solution Setup > General > Scale), and enter the values shown for the scaling factors, then press Scale. Be careful only to press Scale 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 the 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 and 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 1m/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



Solution Monitor Set up residual monitors to monitor convergence

- 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 Initialization Initialize the flow field based on the inlet 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


Set the number of requested iterations to 400 then Calculate


Steady state solution is asymmetric

Run Calculation Choose Graphics and Animations > Vectors

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



Save the case and data files

- File > Write Case and Data

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

Under Solution Setup > General, change the time option to Transient


Save the case & data files and make transient


Run Calculation For the transient scheme, it is recommended to change Solution 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 pressure under-relaxation factor as shown in the image.



Solution Monitor Edit the Surface monitor

- Change Get Data Every to Time Step. Also set X Axis to Time Step

- OK



sVSt

D

fperiod

V

fDSt 06.6

.

1

Run Calculation


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 the time step size (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

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 Save the transient case and 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

- NITA is an algorithm used to speed up the transient solution process - NITA runs about twice as fast as the ITA scheme - NITA scheme reduces the splitting error to O(t2) 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


2


x

V

y

U

y

V

x

UQ

.

Result Analysis Save the transient case and data files with the name transient-detail.

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 Timesteps

- 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 Select Calculation Activities > Solution Animation > Create/Edit

Increase number of sequences to 1

Sequence 1, every 2 Timesteps

Define, which will open the Animation Sequence window

Set window to 3, press Set to enable this window, and type to Contours

... Continued on next slide



Saving Images on-the-fly 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



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)

- 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] 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



Post-Processing [Fluent] From the Plots Menu, select FFT then Set Up

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 [FFT] 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


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.


Post-Processing [FFT] You may need to change the graphic window so that Spectral Analysis is visible


Close Fluent Run CFD-Post Close Fluent

Open a CFD-POST session

- We will create an animation




Post-Processing [CFD-Post] Animations done in CFD-Post can be based on all the data files already saved.

Thus, you can create animations of anything after the 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



Post-Processing [CFD-Post] Insert a vector

- Keep default name Vector 1

- Location symmetry 1

- Apply

- Click on the Z axis to

align the view angle


Post-Processing [CFD-Post]


Activate the 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.


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 Meshing application, or

from adaptive meshing in Fluent

Reynolds number effects

For lower Reynolds numbers, steady state, laminar analysis is possible.

For increasing Reynolds numbers, unsteady transitional turbulent models (k-kl-omega, Transition SST) have to be considered.

For Reynolds numbers above 3.5106 , the standard or SST k-omega turbulence models would be used.


You can investigate other flow patterns 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. - Post-processing 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 the predicted frequency is in good agreement with results from literature. Knowing your aims from the start will help you make sensible decisions of how large to make the domain, the level of mesh resolution needed, and which numerical schemes should be selected.



Braza, M., Chassaing, P., and 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. and 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 1996. 28:447-539

References

Documents

Fluent-Intro 14.5 WS07b Vortex Shedding