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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
8/13/2019 CFX-Intro 14.5 WS10 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)
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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:
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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:
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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
Introduction Setup Solution Results Summary
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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 3 3 2 0.25 Velocity
Monitor Point 4 4 2 0.25 VelocityMonitor Point 5 6 2 0.25 Velocity
Monitor Point 6 8 2 0.25 Velocity
Monitor Point 7 28 2 0.25 Velocity
Introduction Setup Solution Results Summary
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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.
Introduction Setup Solution Results Summary
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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.
Introduction Setup Solution Results Summary
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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
Post-Process Results
Introduction Setup Solution Results Summary
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Post-Process Results
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
Post-Process Results
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.
Post-Process Results
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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
Post-Process Results
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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
Post-Process Results
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