workshop 7: cavitation for flow over a hydrofoil

© 2015 ANSYS, Inc. WS7-1 Release 16.0

16.0 Release

Multiphase Flow Modelingin ANSYS CFX

Workshop 7: Cavitation forFlow Over a Hydrofoil


Introduction

• This example demonstrates the use of the Rayleigh-Plessetcavitation model in ANSYS CFX.

• In order to shorten run-time, a two dimensional planar mesh is used

• This example features:

– Eulerian-Eulerian multiphase flow (continuous-continuous morphology)

– Cavitation Model


Background

• This example involves flowof liquid water around a hydrofoil. The reduction in local absolute pressure due to local velocity increases as the water flows around the hydrofoil causes the water to cavitate

• In order to aid convergence, a non-cavitating solution with negligible vapor volume fraction will be obtained first and used as a restart for the cavitating case


Computational Mesh• The hexahedral mesh is one

cell thick in the z-direction (the thickness extent is 0.6 in)

• The mesh near the hydrofoil has been highly refined


Importing the Mesh• Copy the file TASCFlow grid file HydrofoilGrid.grd

from the input files folder into a working directory

• Start CFX-Pre from the CFX Launcher and create a

new case (File/New Case)

• Right-click on Mesh in the Outline and select

Import Mesh with the type set to Other. When the Import

Mesh form appears, set the Files of Type filter to CFX-

TASCflow and specify the file as HydrofoilGrid.grd. Set the

Mesh Units to m and click Open to import the mesh.


Imported Mesh


Importing Materials

• The two materials used to set the properties of the fluids for this simulation are Water at 25 C and Water Vapour at 25 C. Neither material is in the default CFX-Pre library and must be loaded from a file.

• Right-click on Materials in the Tree View and select Import Library Data. On the Select Library Data to Import form, highlight Water Data and click the + sign. Highlight Water at 25 C and Water Vapour at 25 C and click OK. These materials will now appear in the Outline.


Defining the Domain Fluids (liquid)•Double-click the Default Domain

created for your imported mesh to edit it

•On the Basic Settings tab of the Details form for the domain:

– Set the Domain Type to Fluid Domain

– Highlight Fluid 1 in the Fluid and Particle Definition window and click on the Delete icon to remove it

– Click on the New icon in the Fluid and Particle definition window and insert a new fluid named liquid

– Click on the Expand List icon next to the Material entry box and select the material Water at 25 C under Water Data and click OK. Set theMorphology Option for this fluid to Continuous Fluid


Defining the Domain Fluids (vapor)•On the Basic Settings tab of the

Domain form:

– Click on the New icon in the Fluid and Particle definition window and insert a new fluid named vapor

– Click on the Expand List icon next to the Material entry box and select the material Water Vapour at 25 C under Water Data and click OK. Set theMorphology Option for this fluid to Continuous Fluid

– Still on the Basic Settings tab, under Domain Models set the Reference Pressure to 0 Pa and leavethe simulation non-buoyant


Fluid Domain: Fluid Models

• On the Fluid Models tab.

– Enable the Homogeneous Model toggle

– Set Free Surface Model to None.

– Set Heat Transfer Model to Isothermal with a Fluid Temperature of 25 C

– Set Turbulence Model to k-Epsilon


• Click on the Fluid Pair Models tab and verify that the Mass Transfer Option is set to None for this initial non-cavitatingsolution

• Click OK to complete the specification of the domain.

Fluid Pair Models


Boundary Conditions• For this simulation, the

boundary conditions include:

– an Inlet on the left (low x-face)

– Symmetry Planes on the high and low z-faces

– an Outlet on the right(high x-face)

– Free-slip Walls at thebottom and top of the domain.

– No-slip Walls for the hydrofoil surface

Outlet

Symmetry

Free-Slip Walls

Inlet

No-Slip Walls


Boundary Conditions: Inlet• Create a boundary condition and give it

a name (Inlet)

• On the Basic Settings tab, set the Boundary Type to Inlet and the Location to IN

• On the Boundary Details tab, set the Mass and Momentum Option to Normal Speed and enter a value of 16.91 m/s. Set the Turbulence Option to Intensity and Length Scale and enter values of 0.03 and 0.0076 m.

• On the Fluid Values form:

– Highlight liquid and set the Volume Fraction to 1.0

– Highlight vapor and set the Volume Fraction to 0.0

• Click OK to complete the boundary definition


Boundary Conditions: Outlet

• Create a Boundary Condition named Outlet

• On the Basic Settings tab, set the Boundary Type to Outlet and the Location to OUT.

• On the Boundary Details tab panel, set Mass and Momentum Option to Average Static Pressure and set a Relative Pressure of 51957 Pa. Leave the other settings at their default values

• Click OK


Symmetry Boundary Conditions• Create a new boundary condition and

give it a name (sym1). On the Basic Settings tab, set the Boundary Type to Symmetry and the Location to SYM1 and click OK

• Create a new boundary condition and give it a name (sym2). On the Basic Settings tab, set the Boundary Type to Symmetry and the Location to SYM2 and click OK


Boundary Conditions: SlipWalls

•Create a new boundary condition named SlipWalls.

– On the Basic Settings tab, set the Type to Wall and the Location to BOT and TOP

– On the Boundary Details tab, set the Mass and Momentum Option to Free SlipWall

•Click OK to create the boundary

•The only external boundaries left inthe domain are the surfaces of thehydrofoil. These are already defined as no-slip walls in the default boundary created for the domain so the creation of boundary conditions for this simulation is complete.


Boundary Condition Summary


Global Initialization

•Click on the Global Initialization icon. On the Global Settings tab, set the Static Pressure Option to Automatic

•Set the Velocity Type to Cartesian and the Option for the Velocity Components to Automatic with Value. Enter 16.91 m/s for U and set V and W to 0.0 m/s. Set the Turbulence Option to Medium Intensity (5%)

•On the Fluid Settings tab:

– Highlight liquid and set the Volume Fraction Option to Automatic with Value and enter 1.0.

– Highlight vapor and set the Volume Fraction Option to Automatic with Value and enter a value of 1.0

• Click OK to complete the initialization


Solver Parameters• Click on the Solver Control icon from the

menu bar

• On the Basic Settings form:

– Set the Advection Scheme Option to High Resolution

– Set the Timescale Control to Physical Timescale and set avalue of 0.01 s

– Set the Max. Iterations to 35 (thegoal here is just to provide a reasonable starting point for the cavitating solution)

– Enter 1e-4 for the Residual Target

– Leave the other settings at their default values and click OK to apply the solver settings


Write Solver Input File and Define Run

• Click the Define Run icon from the menu bar

– When the Write Solver Input File form appears, enable the Quit CFX-Pre toggle, enter the desired file name, and click Save to write the definition file

– Click Save and Quit when prompted to save the simulation file


Running the Solver• When the Define Run form of the

Solver Manager appears, click Start Run


Convergence for the Initial Simulation

Residuals for Volume Fraction are low since the boundary conditions and the choice of fluid models combined to give a simple solution that matched the initial condition (liquid.vf = 1.0)

• The non-cavitating solution should take less than 2 minutes to run 35 iterations.


Initial Postprocessing: Absolute Pressure• Start CFD-Post and load the results file Select the +z view. Make sym2 visible

and color it according to Absolute Pressure (click on the … icon next to the Color Variable to specify this variable). Zoom in on the area around the hydrofoil. Clearly, negative absolute pressures are predicted on the upper curved surface which are not physically realistic.


Post-Processing: liquid Velocity• Color sym2 according to liquid.Velocity. The increase in velocity as

the liquid flows around the hydrofoil gives rise to the computed negative absolute pressures.


Calculating Lift and Drag Forces• The lift on the hydrofoil is

just the net force in the y-direction on the hydrofoil surfaces (Default Domain Default) while the drag is the corresponding force in the x-direction Use Tools/Function Calculator to calculate the net lift and drag forces.

• These values could also have been obtained by summing the pressure and viscous forces in the Wall Force and Moment Summary in the output filefor the run.

Lift Force Drag Force


Forces from the Solver Manager•Forces can also be monitored from the Solver Manager:

– Workspace/New Monitor/Plot Lines/Force...


Need for a Cavitation Model

•The initial solution is essentially a non-cavitating single-phase solution since the

boundary conditions and flow models used resulted in a constant value for

the volume fraction field (Water at 25 C. vf = 1.0).

•The non-cavitating model is physically not realistic since it predicts negative

absolute pressures on the upper surface of the hydrofoil. In reality, water

vapor would form in these regions and the absolute pressure should

approach the vapor pressure of water at the local temperature.

•We can rerun this case and turn on the cavitation model for interphase mass

transfer. We should expect the lift force to decrease since the negative

absolute pressures on the upper surface of the hydrofoil should be

mitigated.

•Start by reopening the previous simulation file in CFX-Pre


•Double-click on the flow Domain in theTree View to bring up the Domain details form

•Click on the Fluid Pair Models tab and set the Mass Transfer Option to Cavitation

•The Cavitation Option will default toRayleigh Plesset with a mean diameterof 2 microns

•Enable the Saturation Pressure toggleand enter a Saturation Pressure of 3574 Pa, which is the vapor pressure of water at 25 [C].

•Leave the other settings for theCavitation Model at their default values

•Click OK to modify the Domain

Fluid Pair Models: Adding Cavitation


Solver Parameters•Click on the Solver Control icon from

the menu bar

•On the Basic Settings form:

– Change the Max. Iterations to 150

– Leave the other settings at their previous values and click OK to apply the form


Write Solver Input File and Define Run•Click the Define Run icon from the menu bar

– When the Write Solver Input File form appears, enable the Quit CFX-Pre toggle,enter the desired file name hydrofoil_cavit.def, and click Save to write the definition file

– Click Save and Quit when prompted to save the simulation file


Running the Solver for the Cavitation Run

•When the Solver Manager appears, specify the Definition File as the cavitation definition file that you just wrote

•Enable the Initial Values Specification toggle. Click the New icon and specify the non-cavitating results file as the File Name for Initial Values 1

•Choose to continue History from Initial Values 1

•Click Start Run to initiate the simulation

•The simulation should require less than 3 minutes to converge


Convergence for the Cavitating Solution

Residuals for Volume Fraction increased due to cavitation

Net cavitationrate near zero


Postprocessing the Cavitating Solution

• Load the results file. Select the +z view. Make sym2 visible and color it according to Absolute Pressure. Zoom in on the area around the hydrofoil. Negative absolute pressures are no longer observed (for solver runs with the cavitation model, absolute pressures will be clipped to the vapor pressure)

Specified vapor pressure


Water Vapor Formation• Color sym2 according to vapor.Volume Fraction. Water vapor is formed in

the low pressure region at the top of the hydrofoil but condenses as the vapor moves to higher pressure regions where the vapor pressure is exceeded, resulting in an overall net cavitation rate near zero.


Lift and Drag for the Cavitating Solution•Use Tools/Function Calculator to calculate the net lift (y-direction) and drag

(x-direction) forces on the hydrofoil and compare them with the values obtained from the noncavitating solution

•As expected, the lift force has decreased from the value for the noncavitatingsolution (121.64 N to 100.3 N). The drag force has increased more than a factor of 2 from the value from the noncavitating solution (3.05 N to 7.29 N)

Lift Force Drag Force


Pressure Clipping for the Cavitation Model

•As mentioned previously, absolute pressures are clipped to the vaporpressure for cavitation model runs. For post-processing purposes, the relative pressure is clipped to the value which corresponds to zero absolute pressure. The true relative pressure computed by the solver is stored as Solver Pressure.


Cavitation Rate

•Color sym2 by liquid | vapor.Bulk Interphase Mass Transfer Rate. The formation of water vapor near the top surface of the hydrofoil and rapid collapse as it moves into the regions of higher pressure away from the hydrofoil can clearly be seen


Further Details

•Further details for this example including comparison to experimentalpressure coefficient curves can be seen by going through the ANSYS CFX Tutorial: Cavitation Around a Hydrofoil supplied as part of the on-line documentation for ANSYS CFX

workshop 7: cavitation for flow over a hydrofoil

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