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st268 August 29, 2012 05:46

Modelling Tidal Turbine, general momentum source, body forces
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I am trying to model a tidal turbine (HATT) in CFX via a subdomain containing a general momentum source. I am interested in a very specific case, I know the free stream velocity, the TSR and the lift and drag coefficients for iterative 2d sections of my blade. From these I have the equations attached

Where Theta rel is the angle of the relative velocity taking into account the rotation. Fazim is around the plane of rotation and the z axis is my rotation axis.

I am stuck trying to write the above in a form that will be accepted by the general momentum source options in CFX. If anybody could point me in the right direction your help would be greatly appreciated.

ghorrocks August 29, 2012 07:53

Are you intending replacing the whole turbine with a single momentum source to model the drag it imposes on the flow?

st268 August 29, 2012 08:28

Thank you for replying first off.

Yes, I have my body forces in terms of unit volume. I wished to apply this to a subdomain of my flow. I am now trying manually inputting my expressions from which I got the force equations above and letting ANSYS start from there. When I try and simple put in the above equations as they stand I of course get errors regarding the units.

So I have figured it out. I had done all my initial work in Matlab and tried to simply transfer my final x,y,z forces to CFX. ANSYS did not like that, so I built them up from scratch using user expressions and functions. Only thing I am pondering is whether I have to set my subdomain to rotating or whether I have described this aspect sufficiently in my body forces.

ghorrocks August 29, 2012 18:29

if you are replacing the turbine with a momentum source then no, do not put this in a rotating frame of reference. If the turbine puts a swirl in the flow coming off it then you should include this in the momentum source, but in a stationary frame of reference.

Felggv September 3, 2012 14:37

Wouldn't a interface do the job?

ghorrocks September 3, 2012 18:36

Do what job? This simulation can be done as a single domain so there should not be any need for an interface. However interfaces can sometimes be useful as they are 2D regions which you can apply source terms to.

Felggv September 3, 2012 20:11

You guys were discussing about doing a domain with a source of momentum... that's why I suggested an interface with this source term!


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