Ejercicio 7b - Desprendimiento Torbellinos

WS5-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.

June 12, 2009Inventory #002601

Introductory FLUENT Training

Workshop 5

Vortex shedding around a cylinder



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WS5: Vortex Shedding

Goals

• The purpose of this tutorial is to introduce the us er to good techniques for modelling transient flow

• The case consider here is flow around a cylinder wi th a Reynolds number of 100

• Vortex shedding should be observed. But tutorial st arts with a steady state analysis assuming that the user didn’t antici pate this behavior

• This tutorial demonstrates iterative and non-iterat ive time advancement, FFT and animations

• The tutorial is carried out using FLUENT and CFD Po st in standalone mode



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Introduction

• Computational domain created in Design Modeller had the following dimensions

• For a dimensionless analysis you may prefer a diame ter of 1 m. Note that Fluent uses the physical dimension. But if you specify a cylinder diameter of 1m, inlet velocity = 1 m/s, density =1 kg/m 3, viscosity will be 1/Re = 0.01 [Pa.s]

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



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Reynolds Number Effects

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)



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• Launch Fluent and choose your working directory.• Choose ‘2D’• Choose parallel with 2 processors to take advantage of multi core processor

Note that a parallel token is required in your license

Start a Fluent project (standalone mode)

Choose your workingdirectory here



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Read a mesh that was generated in ICEM CFD

• Read the FLUENT mesh file : vortex-shedding-coarse.msh– File�Read� Mesh



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Mesh

• The mesh will read in and display, and the zones will be writte n out.– Partition with metis method is automatically performed



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Mesh

• The mesh needs scaling, in order to have dimensionl ess format

• Close the scale panel and Check the Mesh– General > Check– General > Report Quality

• Display the grid again once scaling has been perfor med– General > Display

Final domainextent



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Solver & model

• Keep the steady-state pressure-based solver

• Keep laminar model



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Materials

• The properties to be used for the material ‘air’ ne ed to be set– For Density, select 1 kg/m3

– For Viscosity, select 0.01 kg/m.s, Select Change/Create

• Assign the material ‘air’ to the grid cells– Select ‘Cell Zone Conditions’– Highlight ‘fluid’ then ‘Edit’– Observe ‘air’ is already selected



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Boundary Conditions

• Inlet– Set the inlet boundary conditions ‘in’ with V = 1m/s

• Outlet– The zone named ‘out’ is a pressure outlet with a constant gauge pressure of 0 Pa

• Note that default operating pressure is correct but has no impact on calculation since fluid density is constant

• Wall and symmetry boundary conditions keep the defa ult values



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Solution Methods

• Select Solution Methods in the LHS tree

– Use QUICK discretization scheme in order to avoid nu merical diffusion



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Solution Monitors

• Set up residual monitors so the convergence can be monitored– Monitors > Residuals > Edit– Make sure ‘plot’ is on

• Create points to monitor quantitySurface �� Point– Specify coordinates (2 , 1)– Activate point tool to check location on the grid – (check out point tool before closing panel)

• Surface monitor on point– Select “vertex-average” on report type and “velocity” “Y-velocity” in field variable– Options: Print to console & Plot



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Solution Initialization

• Initialize the flow field based on the far-field boundary– Compute from > “in” (inlet zone)



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Save the Case File

• Save the case file

– File > Write Case• You can write case and data files with extension .gz

– the files will be compressed automatically



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Run Calculation

• Set the number of requested iterations to 400

Note that solution is not converging and monitor shows a regular periodic behavior



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Post-Processing

• Choose Graphics and animations > Vectors

Steady state solution is asymetric



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Save the Case&Data Files

• Save the Case&Data files

– File > Write Case&Data..• You can write case and data files with extension .gz




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Transient Run Calculation

• This suggests that hydrodynamics instability may occur

• This requires to solve the problem in a transient modeProblem setup> General



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Run Calculation

• It is also recommended to change solver Methods. Default pre ssurevelocity coupling (SIMPLE) may require more iterations to c onverge– Use PISO et second order time stepping (under Solution methods)– This implies also to change under relaxation factors



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Solution Monitors

• Edit the Surface monitor– Get Data every time step and display Flow time on X axis– Click write button and specify a name for the file. This type of file can be used later for

Fourier Transform analysis



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Save the Transient Case File

• Save the transient case file

– File > Write Case• You can write case and data files with extension .gz




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Run Calculation

• Method 1 : Choose a time step on Total Length/inlet_velocity /200– Then you can check if the convergence occurs in less than 20 subiterations– You can also display courant number …

• (Graphics and Animation > Contours of velocity – Cell courant number)

… And adjust time step so that courant number is close to one– Time step is proportional to courant number

• Method 2: Choosing appropriate time step for such a problemrequires some literature study and can be more efficient tha nabove method– From literature about vortex shedding we know that Strouhal number is between

0.165 for this Reynolds number .This gives an order of magnitude of time step

• One can decide to choose around 60 points in the period.This leads to a time step of 0.1 s

sVSt

D

fperiod

V

fDSt 06.6

.

1 ===⇒=



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Run Calculation

• Specify time step (0.1 s) and number of time step ( 120)

• Click on the Extrapolate Variables option– The solution will be speed Up

Note that between 5 and 10 subiterationsare required to converge each time step



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Save the Transient Case&Data Files

• Save the transient case&data files





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Run Calculation

• Many time-steps have to be run before to get a peri odic signal– it can be time consuming

• So, we propose you to read the final case and data files:File�� Read Case & Data

« vortex-shedding-unsteady.cas.gz »

– This case has been run for 50.4 s until a periodic signal is obtained. And poor initialisation has been convected several time in the domain

• And to use this new starting point to continue the convergence process



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NITA

1

2

• Enable the Non Iterative Time Advancement Method (NITA)– With Fractional Step for Pressure-Velocity Coupling

- 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(∆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( ∆∆∆∆t2)

Splitting error (due to eqn segregation):

O(∆∆∆∆tn)

Overall time-discretizationerror for 2 nd-order scheme: O(∆∆∆∆t2)

= +



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Run Calculation

• Select Calculation Activities in LHS menu

1

2



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Post Processing [FLUENT]

• Create Q-criterion . This is usually used to caract erize vortex

• Define �� Custom Field Function– In 2D incompressible flow it can be written:

– Use calculator and select field function (Derivatives) to create this new variable:

x

V

y

U

y

V

x

UQ

∂∂

∂∂−

∂∂

∂∂= .



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Post Processing [FLUENT]• Animation sequence will be selected before running calculation• Select Calculation Activities in LHS menu

1

2

3 4



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Solution Monitors

• Edit Surface monitor again– Add write option

• This data acquisition on the point will be used for a FFT analysis

– UnSelect the Plot option



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Solution Monitors

• Edit Residuals monitor again– UnSelect the Plot option



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Run calculation for creating animation

• NITA requires a smaller time step– Choose 0.05 s

• And 240 time steps– This corresponds roughly to 2 periods



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Save the Transient Case&Data Files

• Save the transient case&data files





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• To run the animation (Graphics and Animation in the LHS menu)

You can play the animation and write it to a mpeg format



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• FFT analysis (Fast Fourier Transform)Display �� Plot

Load input file named« point-4-y-velocity-final.out »�� This file countains more than 2000 time steps



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• FFT analysis : choose magnitude on Y axis and change X range.

Write the FFT to a file and edit this file The main peak is located for a Strouhalnumber of 0.1698- 3% higher than experiment

The second peak is the harmonicSignal is not strictly sinusoidal



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Close FLUENT – Run CFD-Post

• Close FLUENT

• Open a CFD-POST session– We will create an animation



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Post Processing [CFD-Post]

• 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:



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• Insert a vector



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• Activate time step selector

Select successive time steps, graphics window is updated



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• Activate time step selector

Play the animation and Create file

Turn off the Repeat Forever button



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Summary

• In this tutorial we have used FLUENT with a transient case

• We have learned how to use iterative and non iterativeadvancement scheme

• We checked with FFT analysis that predicted frequency is ingood agreement with results from literature

• We learned how to create animation within Fluent and withCFD-Post



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References

• Braza, M., Chassaing, P., and Minh, H.H., Numerical Study and Physical Analysis of the Pressure and Velocity Fiel ds 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

Ejercicio 7b - Desprendimiento Torbellinos

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