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Underwater Propeller CFD analysis with Ansys CFX

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Old   March 13, 2014, 05:42
Post Underwater Propeller CFD analysis with Ansys CFX
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Trabelsi Alaeddine
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Hello,

I'm new to CFD analysis. My first application consist of an underwater 3 bladed propeller analysis in order to determine its thrust and get its characteristics (Kt=f(J), 10Kq=f(J) and η=f(J) where: J=advance ratio; Kt=thrust coefficient; Kq=torque coefficient).

I will show you how I proceeded and give you some screenshots illustrating my work and I hope you can guide me in order to improve my result (which looks not really reasonable).Because attatchments are limited in size you can download them from this link: http://www.2shared.com/file/nxFfFIlN/Screenshots.html (file size=3.25 Mo)

Step 1: I created the propeller geometry then I defined the rotational domain and the stationary domain with Solidworks.

Step 2: I exported the geometry to Pointwise in order to mesh the domains.

Step 3: I exported the mesh to Ansys CFX-Pre

STEP 4: I defined the analysis mode type as transient (see attatchment)

Step 5: I set up the boundary conditions as follows:

-The stationary domain entrance is defined as Inlet (velocity component U=V=W=0 m.s^-1) and turbulent intensity 5%.

-The top, bottom and side walls are defined as Opening Pres. and Dirn. with pressure = 0.

-The exit also is defined as Opening Pres. and Dirn. with pressure = 0.

-The propeller surfaces were defined as a rotating non slip wall.

-I defined the rotational domain motion with 2500 RPM.

-I defined fluid-fluid interfaces between the stationary default domain and the small rotating propeller domain.

-The frame change option is set on both fluid-fluid interfaces to frozen rotor.

STEP 6: I set up the global initialisation as shown in the attatchment

STEP 7: I run the solver then I launched CFX-Post to visualise results (see attatchments)

Now my questions are as follows:

*Any comments on the meshing quality ?
*Are the Pre-settings correct ?
*Why the streamline don't seem to be reasonable ? What should I change ?
*How can I get the plots I mentioned above from CFX-Post ? (please have a look at the last page of the attatched PDF where you can find an example of what I intend to get )
*Does the values shown in the image called "CFX-Post Thrust" really present the thrust and torque produced by the propeller ?

Any help would be great! Thanks

Waiting for your replies

Trabelsi Alaeddine
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Old   March 13, 2014, 08:07
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Glenn Horrocks
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Lots of things to fix in your analysis. You have a bit of work to do before you get an accurate result.

* Why is the inlet U=V=W=0? Surely you want to model the prop in a flow, so U should be the speed you wish to model.
* Your simulation has not converged very well. See FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
* Why have you modelled this as transient? If you are using a frozen rotor approach use a steady state simulation.
* Why have you modelled 3 blades? Why not model 1 blade with periodic boundaries?
* If you must use a transient simulation, then use adaptive time steps, homing in on 3-5 coeff loops per iteration. Make sure the max and min values are wide enough to not restrict the time step size, let it find the time step size it needs.
* You need to do sensitivity studies on convergence and mesh size (especially mesh size). See FAQ for details: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F and http://journaltool.asme.org/Template...umAccuracy.pdf
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Old   March 13, 2014, 12:29
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Trabelsi Alaeddine
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Thanks a lot ghorrocks for your pieces of advice. I'm working to improve my results following your recommendations. However, I found a tutorial in which the transient type, the frozen rotor option and the U=V=W=0 condition are used but the result seems to be logic. You can download the tutorial from this link:

http://www.mediafire.com/download/kk...rial_elica.zip (file size:22.95 Mo)

I'm wondering why the simulation in the tutorial gives a good result.

Anyway, I will be back with the new results as soon as possible.

A last question: how can I model 1 blade with periodic boundaries ? have you any documents or links that could help me to try it ?

Thanks a lot another time.
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Old   March 13, 2014, 17:34
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Your link is to an executable. I do not run executables from download sites.

Have a look at the tutorials on modelling single blade passages. There are a few examples of this in rotating machinery.
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Old   March 14, 2014, 10:49
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You can download the tutorial as *.SWF file from this link:

http://www.mediafire.com/download/7o...r6j0o/Tuto.swf
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Old   March 15, 2014, 05:31
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The SWF did not work for me but don't worry about fixing it, I do not wish to see it. The whole point of a frozen rotor approach is that you can model it is steady state. If your tutorial uses a transient model then it has missed the point.

So I would start by doing a steady state model with a frozen rotor frame change model. This might not correctly account for the rotation of the disc, so you will probably need to use the stage frame change model for a better result. But you should not need to use transient rotor/stator which is good because that will take a LONG time to run.

If you have no idea what I am talking about with frozen rotor and stage interfaces then you better look in the documentation. This is all explained in the modelling guide, section 5.3.3 (General Connection and frame change models).
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