Fluent-Intro 15.0 WS08b Vortex Shedding

8/20/2019 Fluent-Intro 15.0 WS08b Vortex Shedding

1/39

© 2014 ANSYS, Inc. February 28, 2014 1 Release 15.0

15.0 Release

Introduction to ANSYS

Fluent

Workshop 8b

Vortex Shedding


2/39


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-dependent

(transient) simulations. Topics covered include: – Selecting a suitable time step - 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-

dependent) simulation, as well as additional skills in using custom field

functions and fast Fourier transforms.

I Introduction

Introduction Model Setup Solving Post-Processing Summary


3/39

© 2011 ANSYS, Inc. February 28, 20143 Release 14.0

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



4/39


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



5/39


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 toturbulent

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



6/39


Start a Fluent Project (standalone)


• Launch Fluent from the Start Menu

• Start Menu > ANSYS 15.0 > Fluid Dynamics> Fluent

- Select '2D' and 'Display Mesh After

Reading'

- Select the working directory you are using

on your machine (may be different to that

shown here)


7/39© 2014 ANSYS, Inc. February 28, 2014 7 Release 15.0

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

extents

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 havechanged 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

- Keep default settings



11/39


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

‐ (unselect the 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



12/39


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



13/39


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’


14/39


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’



15/39


• 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


16/39


Run Calculation• For the transient scheme, the default pressure-velocity coupling (SIMPLE) may

require more iterations to converge than other available choices

- 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



17/39


Solution Monitor• Edit the Surface monitor

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

- OK



18/39


sV St

D

f period

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 time step.

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 willbe 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 time steps, Hence we will run the solverusing a time step size of 0.1s.


19/39


• 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 outputWindows

Run Calculation

The ‘ Extrapolate Variables’ option will speed up

convergence. Without this option, each time step

would start with the solution at the previous time step.

This option provides a better starting point for the new

time step based on how the solution is changing with

time. Notice that as the solver runs, convergence is

attained in 5-10 iterations at each time step.



20/39


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 time step. 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 eachcycle, 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



21/39


1

- NITA is an algorithm used to speed up the

transient solution process- NITA runs about twice as fast as the ITA scheme

- Two flavors of NITA schemes available

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

About 20% cheaper than NITA/PISO on a pertime-step basis

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

- With Fractional Step for Pressure-Velocity Coupling


2


22/39


x

V

y

U

y

V

x

U Q

.

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’



23/39


Extracting Transient DataUnless 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) time steps 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 ananimation) 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.



24/39


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



25/39


Saving Images On-the-fly• 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 type to Contours

... Continued on next slide



26/39


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



27/39


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



28/39


Run Calculation for Creating Animation• Run the calculation:

- Use a smaller time step 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.



29/39

© 2014 ANSYS, Inc. February 28, 2014 29 Release 15.0Introduction Model Setup Solving Post-Processing Summary

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 desired, this can be written out as an mpeg movie


30/39


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 time steps. 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


31/39


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


32/39


33/39


Close Fluent – Run CFD-Post• Close Fluent

• Open a CFD-POST session

- We will create an animation



34/39



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


35/39



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


36/39


Post-Processing [CFD-Post]


• Activate the Timestep Selector panel

Recall that in Fluent, we

generated a contour plot

every 2 time steps. We

saved the data files used

here every 5 time steps.

• Pick a time value from

the list then Apply to

see the result at that

time step• Click on the film icon,

then the play button,

for a quick animation

of all saved time steps


37/39


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 3.5×106 , the standard or SST k-omega turbulence

models would be used


You can investigate otherflow patterns by changing

the Reynolds number.


38/39


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

casesOne 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 assessingthe 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 howlarge to make the domain, the level of mesh resolution needed, and whichnumerical schemes should be selected.



39/39

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 15.0 WS08b Vortex Shedding