Have tried for days to find help within the fluent documentation concerning impellers..... How the hell do I set up a 3D problem so as to get an impeller to rotate under the influence of a moving fluid passing over the impeller?

I am working with new water turbine ideas, have been self training on gambit & fluent for several months and have hit a brick wall on this aspect of simulation setups. Would be very greatful for answers.... Cheers John Alloway new zealand

You can use the MRF model (in the Boundary Conditions settings, go to the fluid, and define this as a moving zone, define your rotation axis, and rotation speed... define the impeller as a moving zone with a relative rotation rate of 0). The downfall is that you have to define the rotation rate. I'm sure there's a way to use a UDF to vary the rotation speed until you get no moment on the impeller, but I wouldn't know how to actually do it.

Jason

Thanks for your reply Jason... Yes I can see what you are suggesting however the RPM and the amount of force exerted upon the impeller are the core qu's I'm wanting to get answered from the problem. Surely fluent has it's correct (ha.. probably hard to find) manner of calculating for movement of solids in a fluid flow....??!!

Unfortunately, what you take for sure in your post is really not supported by FLUENT. It's a kind of fluid-structure interaction problem although you probably wouldn't consider deformations of the solid, but only its movement.

I think that the only way of doing what you need rigorously is as Jason said. Otherwise you might be able to find an answer by doing few simulations with different impeller speeds and find the RPMs you need by plotting a graph of force (moment) against RPM. This could save you the trouble of learning how to code the necessary UDF.

George

thanks for writing George, you are a good bloke!...

Well bugger me! DOH and any other number of exclaimations! This is a worry indeed and to think I've invested hundreds of study hours to get here. (Gambit was a major pain!) Well does anyone know which CFD software is suitable for the task?

Hi John,

perhaps a small correction... Fluent is currently capable of modeling this type of flow. If you have access to www.fluentusers.com, on the fluent 6.1 page choose to go to > More Tutorials: and click Application Tutorials. This gets you to a page with lot of examples and somewhere down you find a lot of moving-deforming mesh stuff (which is what you will need for your problem). Have a look at "Projectile Moving Inside a Barrel". Or contact your support engineer and ask for some examples.

Basically Fluent includes a solver for solid body motion, which is coupled to your flow field: this will allow you to model the impeller movement due to the flow around it.

No matter which code you use, it will be difficult to do with CFD. There may be other, faster ways to calculate the lift and drag on the blade.

good luck

Sounds interesting, but I'm really afraid one needs to prescribe the movement of the impeller, at least that's what I'm seen people doing so far. Anyway, if you'll find otherwise, then let us know, John.

George