cfx12 workshop 03 vortexshedding

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Workshop 3

WS3-1ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

February 23, 2009Inventory #002599

Vortex Shedding

Introduction to CFX



WS3: Vortex Shedding

Workshop Supplement Objectives

• Setup a transient simulation of a transient vortex sheddingbehind a cylinder (Kármán vortex street)

• Get acquired with the post processing of transient results inCFD Post

• Compare the predicted Strouhal number with experimental data






Workshop Supplement Reynolds Number Effects

40 < Re < 150

5-15 < Re < 40

Re < 5

Laminar vortex street

A pair of stable vortices in the

wake

Creeping flow (no separation)



Re > 3.5×106

3×105 < Re < 3.5×106

150 < Re < 3×105

Turbulent vortex street, butthe separation is narrowerthan the laminar case

Boundary layer transition to

turbulent

Laminar boundary layer up tothe separation point, turbulentwake




Workshop Supplement Mesh Import

1. Launch Workbench

2. Drag and drop a CFX componentsystem in the project page

3. Start CFX-Pre by double clicking Setup



. - >>ICEM CFD

5. Set the Mesh Units to m

• For some mesh formats it is important toknow the units used to generate the mesh

6. Import the meshF10_S10_B15_Hex010.cfx5




Workshop Supplement Define Simulation Type

1. Edit the Analysis Type object in the Outline tree

2. Set the Analysis Type Option to Transient

3. Set the Total Time to 20 [s]

4. Set the Timesteps to 0.01 [s] and click OK

The first step is to change the Analysis Type to Transient:



• The simulation will have 2000 timesteps




Workshop Supplement Define New Material

1. Define CEL expressions for Re,Velocity, Density and Viscosity

2. Right Click on Materials> Insert>Material

3. Name = MyFluid

4. Insert CEL expressions for Density



and Viscosity – The idea is to set the properties in

order to reach the target Reynoldsnumber

5. Click OK




Workshop Supplement Edit Default Domain

1. Edit Default Domain from the Outline tree

2.Basic Settings> Material = “MyFluid”

3. Fluid Models> Heat Transfer> Option= None

4. Fluid Models> Turbulence> Option =



5. Click OK




Workshop Supplement Create Boundary Conditions (Wall)

1. Insert a new boundary named “Cylinder”

– Set the Boundary Type to Wall and the Location to “CYLINDER”

– Boundary Details> Option = No Slip Wall

2. Insert a new boundary named “RightWall”

– Set the Boundary Type to Wall and the Location to “RIGHT”

Start by creating the Walls boundary conditions:



–

3. Insert a new boundary named “LeftWall”

– Set the Boundary Type to Wall and the Location to “LEFT”

– Boundary Details> Option = Free Slip Wall




Workshop Supplement Create Boundary Conditions (Outlet & Sym)

1. Insert a new boundary named “Inlet”

– Set the Boundary Type to Inlet and the Location to “IN”

– Boundary Details> Velocity= 20 [m.s-1]

• We are dealing with an incompressible flow

2. Insert a new boundary named “Outlet”

– Set the Boundary Type to Outlet and the Location to “OUT”

– Boundary Details> Relative Pressure = 0 [Pa]



• We are dealing with an incompressible flow

3. Insert a new boundary named “Sym1”

– Set the Boundary Type to Symmetry and the Location to “SYM1”

4. Insert a new boundary named “Sym2”

– Set the Boundary Type to Symmetry and the Location to “SYM2”




Workshop Supplement Create Initial Conditions

1. Create CEL Expressions for the initial flowangle, U and V velocities

– The idea is to create an asymmetry in the

initial velocity field in order to accelerate thegeneration of vortices and reduce thecomputational time

2. Insert a Global Initialisation



. n er artes an e oc ty omponents ,insert the CEL Expressions for U and Vvelocities

4. Set the Relative Pressure to 0 Pa

5. Click OK




Workshop Supplement Solver Control

1. Under Solver Control> Basic Setting, set the following parameters:

– Min. Coeff. Loops = 1

– Max. Coeff. Loops = 5

– Residual Type = RMAX

– Residual Target = 1E-3

• These parameters together with the “Timestep“ are the key numerical inputs for atransient calculations






Workshop Supplement Output Control

1. Under Output Control> Trn Results, do the following steps:

– Insert new transient results

– Option = Selected Variables

– Output Variable List = Pressure, Velocity, Velocity u, Velocity v, Velocity w.

– Timestep Interval = 5

2. Define the following CEL expressions for the Drag and Lift






Workshop Supplement Output Control

1. Under Output Control> Monitor, define the following Monitor Points:

Name X [m] Y [m] Z [m] Variable/CEL

CdCylinder - - - CdCylinderExpressionClCylinder - - - ClCylinderExpression

HighPpt -1 0 0.25 Pressure

LowP t 1 0 0.25 Pressure



Monitor Point 1 -2 2 0.25 Velocity

Monitor Point 2 2 2 0.25 Velocity









Workshop Supplement Run Solver

1. Save the project as Vortex.wbpj

2. In the Project Schematic, Edit the Solution object to start the Solver Manager

3. Start the run from the Solver Manger

• You can monitor the volume of water in the domain during the simulation on theUser Points tab

• The simulation will take about 30 min to complete. Therefore results files havebeen provided with this workshop



4. After a few timesteps, Stop your run5. Select File > Monitor Finished Run in the Solver Manager

6. Browse to the results file provided with the workshop

• Take a look at the Momentum and Mass residuals and at the User Points. The

transient behaviour of the flow is clear.




Workshop Supplement Post-Process Results

1. Using Windows Explorer, locate thesupplied results file Vortex.res , anddrag it into an empty region of the

Project Schematic2. A new CFX Solution and Results cell

will appear. Double-click on theResults object to open it in CFD-



os .





1. Insert> Contour

• Name = myVelocity

• Location = Sym1

• Variable = Velocity• Range = User Specified

• Min = 0 [m s^-1]

• Max = 26 [m s^-1]

•







• Behind the cylinder transient vorticesare formed

• The appearance of these vortices havea certain frequency that depends onthe Reynolds number

• The Strouhal number is a



oscillating flows

• The Strouhal is defined as a functionof the frequency, diameter and velocity

• The frequency will be calculated

through a FFT of the monitoring points U

D f St

⋅

=





1. Go back to Workbench

2. In the Vortex component system, rightclick on Solution and choose Display

Monitors

3. In the solver Manager, go to Workspace>Workspace Properties>Global PlotSettings:

– Plot Data by = Time Step



. n t e ser o nts a , r g t c c on

the Graph>Monitor Properties>RangeSettings >Plot Data By = SimulationTime

5. On the User Points Tab, right click on

the Graph>Export Plot Data6. This file requires further modification in a

text editor so as to keep the “Time” and“Monitor Point 2” columns only.Modification has already been made: the

result file is “Monitor Point 2.csv”





1. In CFD-Post, Insert> Chart

– Name = myFFT

– General Tab

• Type = General XY- Transient

• Fast Fourier Transform = on• Substract mean = on

• Range input Data Min = 10

• Range input Data Max = 20

– Data Series



• Data Source > File = Monitor Point 2.csv file

– X Axis Tab

• Min = 1

• Max = 5

– Y Axis Tab

• Y Function = Magnitude

2. Export chart and save it as a .csv file3. Open the .csv File and locate the frequency that

gives the highest Magnitude

4. Use this frequency together with the diameterand velocity to calculate the Strouhal number





• The CFD calculations can berepeated for several finer Grids inorder to study the discretisation error

• On successive finer grids theStrouhal number will approachasymptotically to a grid independentvalue

Strouhal number

Grid 1 0.1490

Grid 2 0.1657

Grid 3 0.1686

Grid 4 0.1690



• In this case the Grid 4 gives 3 % withrespect the experimental value

Experiment 0.164




Workshop Supplement Summary

• A transient simulation was performed for studying the laminarvortex shedding behind a cylinder

• The computed Strouhal number was compared with theexperimental values for different grids



cfx12 workshop 03 vortexshedding

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