CFX-Intro 14.5 WS10 Vortex-Shedding

8/13/2019 CFX-Intro 14.5 WS10 Vortex-Shedding

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2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14.5 Release

Introduction to ANSYS

CFX

Workshop 10

Vortex Shedding


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Workshop Description:

Set up a transient simulation of vortex shedding behind a cylinder (Krmn

vortex street) and compare the predicted Strouhal number with experimental

data. The cylinder has a diameter of 2m.

Learning Aims:

This workshop introduces several new skills:

Preparing a simulation for transient analysis

Learning how to post-process a transient simulation, including performing a

FFT

Introduction

Introduction Setup Solution Results Summary


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Reynolds Number Effects

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 toturbulent

Laminar boundary layer up to

the separation point and

turbulent wake

Laminar vortex street

A pair of stable vortices in the

wake

Creeping flow (no separation)



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1. Launch Workbench

2. Drag and drop a CFX component system inthe project page

3. Start CFX-Pre by double clicking Setup

4. Right-click on Mesh> Import Mesh > ICEMCFD

5. Set the Mesh Unitsto m

For some mesh formats it is important to

know the units used to generate the mesh

6. Import the mesh F10_S10_B15_Hex010.cfx5(workshop_input_files\WS_10_Vortex

Shedding)

Mesh Import



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Define Simulation Type

1. Edit theAnalysis Type object in the Outline tree

2. Set theAnalysis Type option to Transient

3. Set the Total Timeto 20 [s]

4. Set the Timestepsto 0.01 [s]and click OK

The simulation will have 2000 timesteps

The first step is to change theAnalysis Type to Transient:



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1. Create the expressions shown to theright. They will be used to define the

inlet velocity and the density and

viscosity of the fluid.

2. Right-click on Materials and select

Insert > Material

3. Name= MyFluid

4. Insert CEL expressions for Density

and Dynamic Viscosity

The expression, FluidViscosity, isdesigned to produce a target Reynolds

number of 100

5. Click OK

Define Expressiond and New Material



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1. Edit Default Domainfrom the Outlinetree

2. On Basic Settingsselect MyFluidfor theMaterial

3. On the Fluid Models tab set the Heat Transfer

option to None

4. Set the Turbulenceoption to None (Laminar)

5. Click OK

Edit Default Domain




Create Boundary Conditions (Walls)

1. Insert a new boundary named Cylinder

Set the Boundary Typeto Walland the Locationto CYLINDER

On the Boundary Detailstab set Option= No Slip Wall

2. Insert a new boundary named RightWall

Set the Boundary Typeto Walland the Locationto RIGHT

On the Boundary Details tab set Option= Free Slip Wall

3. Insert a new boundary named LeftWall

Set the Boundary Typeto Walland the Locationto LEFT

On the Boundary Details tab set Option= Free Slip Wall

Start by creating the Walls boundary conditions:




Create Boundary Conditions (Outlet & Sym)

1. Insert a new boundary named Inlet

Set the Boundary Typeto Inletand the Locationto IN

On the Boundary Detailstab set Normal Speed = FlowVelocity

2. Insert a new boundary named Outlet

Set the Boundary Typeto Outletand the Locationto OUT

On the Boundary Details tab setAverage Static Pressure> Relative Pressure=

0 [Pa]

3. Insert a new boundary named Sym1

Set the Boundary Typeto Symmetry and the Locationto SYM1

4. Insert a new boundary named Sym2

Set the Boundary Typeto Symmetryand the Locationto SYM2




1. Create Expressionsfor the initial flow angle, U and Vvelocities, as shown at the bottom of the slide

The initial velocity field is asymmetric in order to accelerate

the generation of vortices and reduce the computational time

2. Insert Global Initialization

3. Under Cartesian Velocity Componentsinsert theExpressionsfor U and V velocities

4. Set the Relative Pressure to 0 Pa

5. Click OK

Create Initial Conditions



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Solver Control

1. Under Solver Control > Basic Settingsset the following parameters:

Min. Coeff. Loops = 1

Max. Coeff. Loops = 5

Residual Type = MAX

Residual Target = 1E-3

These parameters together with the Timestepare the key numerical

inputs for a transient calculations



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Output Control

1. Under Output Control > Trn Resultsperform 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



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Output Control

1. Under Output Control> Monitordefine the following Monitor Points:

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

CdCylinder - - - CdCylinderExpression

ClCylinder - - - ClCylinderExpression

HighPpt -1 0 0.25 Pressure

LowPpt 1 0 0.25 Pressure

Monitor Point 1 -2 2 0.25 Velocity

Monitor Point 2 2 2 0.25 Velocity


Monitor Point 4 4 2 0.25 VelocityMonitor Point 5 6 2 0.25 Velocity





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Run Solver

1. Save the project as Vortex.wbpj

2. In the Project SchematicEditthe Solutioncell to start the CFX-Solver Manager

3. Start the run from the CFX-Solver Manager

You can monitor the velocity of water in the domain and the coefficients of lift and drag

during the simulation on the User Points tab

The simulation will take about 15 min to complete. Therefore results files have been provided

with this workshop

4. After a few timesteps stop your run

5. Select File > Monitor Finished Runin the CFX-Solver Manager

6. Browse to the results file provided with the workshop

View the mass and momentum residuals and the User Points. Right-click in the User Points

window and select Monitor Properties. On the Range Settingstab check the box by SetManual Scale (Linear). Set the Lower Boundto -10 and the Upper Boundto 30. The transient

behaviour of the flow can then be clearly seen.



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Post-Process Results

1. Using Windows Explorer, locate the

results file supplied, CFX_001.res, and

drag it into an empty region of the

Project Schematic

2. A new CFX Solutionand Resultscell will

appear. Double-click on the Resultscell

to open the file in CFD-Post.



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Insert > Contour

Name= myVelocity

Location= Sym1

Variable= Velocity Range= User Specified

Min= 0 [m s^-1]

Max= 26 [m s^-1]

# of Contours= 27




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1. Use the Timestep Selector to load results from different points in the

simulation

2. With the first Timesteploaded, open the Animation tool

3. Select the Quick Animationtoggle and select Timestepsas the object to

animate

4. Turn off the Repeat Foreverbutton

5. Enable the Save Movie toggle and choose the folder where the video will be

saved

6. Click the Playicon to animate the results and to generate the video file



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Behind the cylinder transient vortices are

formed

The appearance of these vortices have a

certain frequency that depends on the

Reynolds number

The Strouhal number is a dimensionless

number used as a measure of the

predominant frequency. For flow past acylinder the value tends to be about 0.2 over

a wide range of Reynolds number

The Strouhal number is defined as a function

of the frequency, diameter and velocity

The frequency will be calculated through aFFT of the monitoring points


U

DfSt



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1. Go back to Workbench

2. In the Vortex component system, right-click on the

Solutioncell and choose Display Monitors

3. In the CFX-Solver Managergo to Workspace>

Workspace Properties > Global Plot Settings. Set

Plot Data By= Time Step

4. On the User Pointstab right-click in the window and

select Monitor Properties > Range Settings > PlotData By= Simulation Time and pressApply. Then

click on the Plot Linestab and expand the USER

POINTobject in the tree. Switch off all but Monitor

Point 2; only these data are needed. Click OK.

5. On the User Points tab right-click in the window and

select Export Plot Data. You then have the optionto save a csv file.

6. The file requires editing so that it contains only Time

and Monitor Point 2columns. This has already been

done in Monitor Point 2.csv, which is provided.




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1. Close the CFX-Solver Manager and return to CFD-Post

2. Insert > Chart Name= myFFT

General tab

Type = General XY- Transient

Fast Fourier Transform = on

Substract mean = on

Range input Data Min = 10 and Max = 20 Data Series

Data Source > File = Monitor Point 2.csv

X Axis tab

Min = 1 and Max = 5

Y Axis tab

Y Function = Magnitude Apply

3. Export chart and save it as a .csv file

4. Open the .csv File and locate the frequency that gives the highest Magnitude

5. Use this frequency together with the diameter (2 m) and velocity (20 m s-1) to calculate the Strouhal

number



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For this grid the Strouhal number is calculated to be about 0.151 compared with an

experimental value of 0.164.

The CFD calculations can be repeated for several finer grids in order to study the

discretisation error. Finer meshes are provided in the directory GRIDS.

As the grids are refined the Strouhal number will asymptotically approach a grid-

independent value


Introduction Setup Solving Results Summary

Strouhal number

Grid 1 0.151

Grid 2 0.1657

Grid 3 0.1686

Grid 4 0.1690

Experiment 0.164


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Summary

A transient simulation was performed for studying the laminar vortexshedding behind a cylinder.

The computed Strouhal number was compared with the experimental

value.

Introduction Setup Solving Results Summary

Documents

CFX-Intro 14.5 WS10 Vortex-Shedding