ace tut 02 turbulent backstep

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Tutorial 2 Turbulent Flow Past a Backward Facing Step

Objectives

This tutorial covers the following subjects in detail:

• Importing a grid from CFD-GEOM.

• Setting the model title.

• Setting constant values for fluid density and viscosity.

• Setting boundary values appropriate for inlets and outlets in turbulent flow.

• Setting initial conditions.

• Setting solution iterations.

• Setting the spatial differencing scheme.

• Setting the relaxation parameters.

• Requesting printed and graphical output.

• Submitting the model for solution.

Assumptions (requirements) for working this tutorial:

1. You are working in the directory where the tutorial problem resides.

Problem Description

Solve for the flow over a backward facing step at a Reynolds number of 100,000. The goal of the simulation is to find the reattachment length (i.e., the point where the separation bubble disappears on the channel floor.) The Reynolds number can be calculated as:

03 C
ACE-GUI Tutorials 2-1

Tutorial 2: Turbulent Flow Past a Backward Facing Step

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,

where U is the average inlet velocity, and D is the hydraulic diameter (equal to 2h).

The geometry is given in the figure below:

Use fluid properties of water (density = 1000 kg/m3, kinematic viscosity = 1.0x10-6 m2/s).

The average inlet velocity required to obtain a Reynolds number of 100,000 can be calculated as:

m/s

ReUDν

---------=

1.01 cm

20 cm4 cm

s = 0.49 cm

h = 0.52 cm

UνReD

----------1.0

6–×10 100000( )2 0.0052( )--------------------------------------------- 9.62= = =

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Modules and Features Used

Modules Major Features Other Features

• Flow Gas Phase Reaction 3D

Heat Transfer (Heat) Surface Reaction • 2D Planar

• Turbulence (Turb) 2D Axisym

Chemistry Arbitrary Interface BC

User Scalar (Scalar) Thin Wall BC Transient

Radiation (Rad) Cyclic BC

Spray

Free Surface (VOF) Fan Model

Two-Fluid (Fluid2) Momentum Resistance

Cavitation (Cav) Rotating System

Grid Deformation (Deform)

Stress Parallel Processing

Plasma User Subroutines

Electric (Electr)

Magnetic (Magnet)

Kinetic

Semi Device

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Procedure

1. Load the DTF File.

From the File Menu, select Open.

A File Open dialog panel appears.

Select the file named “tbstep.DTF” and press the “Accept” button to read the file.

The DTF file is read into CFD-ACE-GUI and a wireframe outline of the model appears in the viewing window.

The control panel is presented in the Problem Type setting mode.



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2. Specify the Problem Type settings.

Under the Available Modules section, activate the Flow and Turbulence Modules.

Activating the Flow Module implies solution of the U, V, and Pressure Correction equations for this 2D simulation.

Activating the Turbulence Module implies solu-tion of the turbulence kinetic energy (K) and tur-bulence dissipation rate (D) equations.



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3. Specify the Model Options.

4. Specify the Volume Condition Properties.

Press the Model Options [MO] tab to activate the Model Options setting page.

The Model Options page has a Shared tab which contains parameters that are available globally; and a tab for each of the Modules that were acti-vated earlier. In this case, only the Flow and Tur-bulence tabs will be visible.

Under the Turbulence tab ensure that the K-Epsilon Turbulence Model is used.

K-Epsilon turbulence is the recommended model because it is the most robust and is applicable to the widest range of problems.

Please see the “Turbulence Module” chapter of the CFD-ACE-GUI Module Manual for details on the other available turbulence models.

From the Control panel, select the Volume Con-ditions [VC] tab to activate the Volume Condi-tions setting page.

The Volume Condition Page is presented and the Model Explorer changes to the VC mode to list all of the volume conditions in the currently active simulation.

Ensure that the Setting Mode is set to Properties. The Volume Condition Page has several setting modes. The Properties mode allows us to assign the properties of each volume condition.



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Select all of the volume conditions in the simu-lation.

The volume conditions picked are highlighted in reverse video in the Model Explorer list. They are also highlighted by thick lines in the viewing window.

To permanently group the selected items together, press the Group button located in the lower left corner.

The items are now part of a permanent property group. A group name is given in the property column of the model explorer.

• Ensure that the Volume Condition Type is set to Fluid by clicking on the Properties pull down menu.

• Under the Phys tab, set the density to a con-

stant value of 1000 kg/m3.

A fluid type can model gas or liquid flows.

Select All



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5. Specify the Boundary Condition Values.

• Under the Fluid tab, set the viscosity to a

constant kinematic value of 1.0E-006 m2/s.

Press the “Apply” button to accept the values. Volume Condition settings are now complete.

Press the Boundary Conditions [BC] tab to acti-vate the Boundary Conditions setting page.

The Boundary Condition Page is presented and the Model Explorer changes to the BC mode to list all of the boundary conditions in the cur-rently loaded simulation.

Ensure that the Setting Mode is set to General. The Boundary Condition Page has several set-ting modes. The General mode allows us to assign the values of each boundary condition.



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5.a. Select the inlet boundary.

Select the Inlet boundary condition from either the viewing window or the Model Explorer list.

The boundary condition picked is highlighted in reverse video in the Model Explorer list. It is also highlighted by thick lines in the viewing window.

The Control Panel fills to show the boundary condition type and the boundary condition value settings are shown in a tabbed list. There will be one tab for every module activated. In this particular case only the Flow and Turbu-lence tabs will be shown.



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5.b.Set the inlet boundary values for the Flow Module.

• Ensure that the Flow tab is active.

• Ensure that the boundary condition subtype is set to Fixed Velocity (Cartesian).

• Set the X-Direction Velocity to 9.62 m/s.

• All other inlet boundary condition values for the flow module can be left at their default settings.

All of the boundary values that apply to the Flow Module are displayed.

The fixed velocity subtype allows us to fix the velocity at the inlet boundary condition location. This effectively fixes the mass flow rate.



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5.c. Set the inlet boundary values for the Turbulence Module.

Note Turbulence quantities are calculated as follows:

In order to determine the inlet turbulence values we need to assume a value for the turbulence intensity. For internal flows the turbulence intensity can be somewhat large (1-5%). For this simu-lation we will assume 2% turbulence intensity. The free stream turbulence kinetic energy (K) can then be calculated as follows:

where u‘ is the turbulent fluctuation velocity and is equal to the turbulence intensity multiplied by the free stream velocity, assuming u’, v’, and w’ are all equal to 2% of the inlet velocity, we can calculate the inlet turbulent kinetic energy as,

m2/s2

The dissipation rate can then be determined from the following equation,

where

and L is a reasonable length scale. For this case we will choose L as the inlet height (0.0052 m)

Solving for the dissipation rate,

J/(kg-s)

We will round this value to 1 J/(kg-s).

K12--- u'

2v'

2w'

2+ +( )=

K32--- 0.02 9.62( )( )2

0.056= =

DCµ

0.75K

1.5

κL----------------------=

Cµ 0.09=

κ 0.4=

DCµ

0.75K

1.5

κL----------------------

0.09( )0.750.056( )1.5

0.4( ) 0.0052( )------------------------------------------------ 1.05= = =



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5.d.Select the two outlet boundaries.

• Press the Turbulence tab to make the turbu-lence values active.

• Select Fixed Value from the menu

• Set the turbulence kinetic energy (K) to

0.056 m2/s2.

• Set the turbulence dissipation rate to 1.0 J/(kg-s).

• Press the “Apply” button to accept the changes and deselect the Inlet boundary condition.

All of the boundary values that apply to the Tur-bulence Module are displayed.

Select the two outlet boundary conditions by picking either from the viewing screen or Model Explorer list.

The boundary conditions picked are highlighted in reverse video in the Model Explorer list. It is also highlighted by thick lines in the viewing window.

To permanently group the selected items together, press the Group button located in the lower left corner.

The items are now part of a permanent property group. A group name is given in the general col-umn of the model explorer.



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5.e. Set the outlet boundary Flow Module values.

5.f. Set the outlet boundary Turbulence Module values.

• Ensure that the boundary condition subtype is set to Fixed Pressure.

• Ensure that the Pressure value is set to 0 N/

m2.

The fixed pressure subtype allows us to fix the static pressure at the outlet boundary condition location. This effectively sets the “anchor” pres-sure for this simulation.

Ensure that the Temperature value is set to 300 K.

The temperature value is only needed at an out-let in case there is inflow through this boundary.

Press the Turbulence tab to make the turbulence values active.

All of the boundary values that apply to the Tur-bulence Module are displayed.

Set K and D equal to the same values that were given at the inlet (K=0.056, D=1).

The turbulence values are only needed at an out-let in case there is inflow through this boundary.

Press the “Apply” button to accept the values and deselect the outlet boundary conditions.

Boundary Condition settings are now complete.

All other boundary conditions for this model are either Walls or Interfaces and their default val-ues are acceptable.



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6. Specify the Initial Condition Settings.

Press the Initial Conditions [IC] tab to activate the Initial Condition setting page.

• Ensure that the Initial Source is set to Con-stant.

Constant implies that the every cell in the com-putational domain will use the specified values as an initial condition.

Press the Flow tab to activate the initial condi-tion values for the Flow Module.

• Ensure that the X and Y-Direction Veloci-ties are set to 9 and 0 m/s

• Set pressure set to 0 N/m2

• Set temperature are 300 K

Reasonable initial conditions will help prevent divergence in the first few iterations.

Press the Turbulence tab to activate the initial condition values for the Turbulence Module.

Set the initial condition values for turbulence equal to the same as the inlet boundary condi-tion (K=0.056, D=1).

Reasonable initial conditions will help prevent divergence in the first few iterations.



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7. Specify the Solver Control Settings.

7.a. Specify the number of solution iterations.

7.b.Specify the type of spatial differencing to be used.

Press the Solver Control [SC] tab to activate the Solver Control setting page.

The Solver Control Page is presented.

There is a tab for each major type of solver con-trol setting. There are also tabs for the solver output options available from this page.

Under the Iterations tab, set the number of solver iterations to 200.

This instructs the solver to run through the main iteration loop 200 times. The full set of equa-tions will be solved each iteration and we will look for a 3-5 order of magnitude drop in the residuals for each equation solved.

Under the Spatial tab, set the spatial differencing method to Central for the Velocity variable. Set the blending to 0.01 for both sets of variables.

Central differencing will produce a more accu-rate result than the default Upwind differencing method. The blending is the amount of upwind differencing to be blended with the higher order method, 0.01 indicates that a 99% central differ-encing and 1% upwind differencing method will be used.

Note: It is recommended to always leave the spatial differencing set to Upwind for the Turbu-lence variables. The higher order methods can often cause convergence problems for the turbulence equations and do not add significant accuracy to the solution. Please see the “Numerical Methods” chapter of the CFD-ACE-GUI User’s Manual for more details on the differencing methods available.



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7.c. Specify the amount of relaxation to be used.

8. Specify the Output Options.

Select the Relax tab. The relaxation parameters appear.

Under the Relax tab, set the inertial relaxation values for the velocity and turbulence equations to 0.1. Also, verify that the linear relaxation val-ues for the auxiliary variables (pressure, temper-ature, density, and viscosity) are set to 1.0.

The default inertial relaxation parameters are 0.2 for velocity and pressure correction equa-tions. This particular case is simple enough to solve that we may decrease this value to 0.1 in order to speed convergence. Note that decreas-ing this value can make the solution unstable so caution should be exercised.

See the “Numerical Methods” chapter of the CFD-ACE-GUI User’s Manual for more details on the use of relaxation.

Under the Output tab, select output results with the “End of Simulation” option.

The results will be written to the DTF file only at the very end of the simulation. Alternatively you may request that the results be output at a specified iteration frequency.



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8.a. Request printed output.

8.b.Request graphical output.

Under the Print tab, select any desired text based output to be printed to the model.out file.

For this case the Mass Flow summary is often beneficial to determine a level of convergence.

See the CFD-ACE-GUI User’s Manual for more details on the types of printed output that are available.

Under the Graphics tab, select any desired vari-ables to be output to the DTF file for post-pro-cessing in CFD-VIEW.

For this case the velocity vector (U, V, and W), static pressure, total pressure, and stream func-tion are of interest. Also, the turbulence kinetic energy, dissipation rate, and effective viscosity are useful. Y+ is useful to determine that the appropriate wall spacing has been used.

These variables will be written to the DTF file at the frequency specified under the Output tab. The variables will be interpolated to the compu-tational nodes from the cell-centered values computed by the solver.



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9. Run the Simulation.

Press the RUN tab to activate the Run Control page.

The Run Control Page is presented.

Press the “Submit to Solver” button to start the solution process.

A dialog panel is presented.

Because we have modified the simulation data during the solution setup process, the data must first be saved to a DTF file before the solver can start. Press the “Submit Job Under Current Name” button to save the information back to the tbstep.DTF file and to launch CFD-ACE-GUI using that same file.

The data is saved to tbstep.DTF and the solver is started.



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You can press the “View Residuals” and “View Output” buttons to see real-time displays of the residual history and output file contents.

We are looking for a three to five order of mag-nitude drop in the solution residuals.

Figure 2-1. Residual Plotter



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10. Post Processing.

11. This Tutorial is now finished.

Quick Start Instructions

If you are already comfortable with CFD-ACE-GUI operations, then you may follow these “quick start” instructions to very quickly work through the steps of this tutorial. If you have any questions about how to perform these steps, then consult the relevant step in the “Procedure” section of this tutorial.

1. Load the DTF file.• Se

The mass flow summary indicates that the mass flow into and out of the system are equal to 50.02 kg/s/m.

The amount of mass imbalance is a good indica-tion of the level of convergence.

CFD-VIEW can be used to post-process the results:

• Maximum U-Velocity ≈ 10.173 m/s

• Minimum U-Velocity ≈ -2.2659 m/s.

• Inlet Total Pressure ≈ 30,867 N/m2.

• Max. Turb Kinetic Energy ≈ 5.1 m2/s2

• Max. Turb Dissipation Rate ≈ 5543 J/(kg-s)

A recirculation bubble can be seen just down-stream of the step.

The reattachment position can be determined by using the point-probe to locate where the recir-culation bubble ends on the lower channel wall.

The reattachment position for this simulation should be somewhere near 0.339 m.

Place the point-probe at a y-value just off of the lower wall and then change it’s x-location until you find the point where the U-velocity changes sign.

The Y+ range along the upper channel wall is between 35 and 75.

The K-Epsilon turbulence model works best when the Y+ values on the walls are between 30 and 150.

Please see the “Turbulence Module” chapter of the CFD-ACE-GUI Module Manual for more details on Y+.


lect File -> Open and read tbstep.DTF.


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2. Specify the Problem Type Settings.• PT -> Modules -> Activate Flow and Turbulence

3. Specify the Model Options.• MO -> Change title to "Reynold's Number = 100000".

Use Default Settings

4. Specify the Volume Condition Properties.• VC -> Properties -> Shared

• Select all VC's and group

• Density: Constant 1000 kg/m3

• Viscosity: Constant (Kinematic) 1E-06 m2/s

5. Set the Boundary Conditions.• BC -> General

• Select INLET ->

• Flow -> Fixed Vel. (Cartesian) -> X-Direction Velocity: Constant 9.626 m/s

• Turb -> Kinetic Energy: Constant 0.056 m2/s2

• Turb -> Dissipation Rate: Constant 1.05 J/kg-s

• Group Outlets

• Select OUTLET ->

• Flow -> Use Default Settings



6. Specify the Initial Conditions.• IC -> Group VC's

• Flow -> X-Direction Velocity: Constant 9 m/s



7. Specify the Solver Control Settings.• SC


-> Iterations = 200


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• SC -> Spatial: Velocity 0.01 Blending, Turbulence (Upwind)

• SC -> Relaxation: 0.1 for Velocities and Turbulence

8. Specify the Output Options.• SC -> Printed -> Activate Mass Flow Summary

• SC -> Graphic -> Use Default Settings plus all Turbulence Quantities

9. Run the Simulation.• RUN -> Submit to Solver -> Submit Job Under Current Name

10. Post Processing the Simulation.• CFD-VIEW can be used to post-process the solution

• Maximum U-Velocity = 10.17 m/s

• Minimum U-Velocity = -2.2659 m/s

• Delta Pressure = 30,867 Pa

• Maximum Kinetic Energy = 5.1 m2/s2

• Maximum Dissipation Rate = 5543 J/kg-s

• Reattachment Length (measured from step) X = 0.339 m

• 35 < Y+ < 75 at channel upper wall


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