Vertical Axis Tidal Turbine - How to Understand Pressure Moment Results

Using SimScale Fluid Flow / CFD
1

I am conducting an incompressible study of a vertical axis tidal turbine. My objective is to determine the output torque from a flow of 3m/s. I understand that simscale cannot conduct a ‘Motion Driven Flow’ study which other CFD packages can, ansys, autodesk CFD ultimate etc, so therefore the angular velocity of the study cannot be a direct output from simscale.

See study - https://www.simscale.com/workbench/?pid=3316415732020668216&mi=spec%3Ad88cbb83-5e6c-44f3-9820-2473880ee83e%2Cservice%3ASIMULATION%2Cstrategy%3A41

image
image1232×722 17 KB

Reading various existing posts, I understand that the method in simscale is to incrementally change the angular velocity of a rotating zone until the moment forces resut in zero. However, despite various attempts in understanding the force plots of my study, I am unsure exactly I am looking for on the plots. I have assumed that I am either;

1 - Looking for the pressure moment in Y to be as converge about as close to 0 as possible. It’s worth noting that the force plots have been set to all faces of the turbine fins, including the ones that face away from the flow during rotation, as they recieve a pressure force negative to Y axis during rotation. Therefore my pressure moment in Y plots have a fluctuating frequency. See below;

image
image1608×849 45.7 KB

2 - OR looking for the pressure moment in Y to converge about the pressure force in Z (flow direction). In this example the force plot is just on the faces of the blades that recieve the flow, hence no/limited fluctuation.

image
image1585×789 31.7 KB

See existing forum post for confirmation of others using this process - Torque and speed from rotating turbine? - #7 by 1318980)

and tutorials that take the same process, but don’t indicate on how to interpret results - Transient Flow Through Water Turbine (Subsonic) | Tutorial | SimScale

Any help would be much appreciated.

Thanks

2

Hi, so it is a little more complex than that. If we were to assume no load on the turbine axis, you would be looking for a moment in the Y direction to converge around 0. But most turbine blades drive something, so you need to know the resistance torque, and you should instead converge on that value.

Also, it’s quite hard to get an exact RPM that matches your force, you would be better off testing say 10 points, and interpolating the location the RPM curve crosses the torque line.

Sometimes fluctuations are inevitable, but other times they are caused by bad convergence, and 9/10 of bad convergence cases are caused by bad mesh. In this case, you have a very coarse setting and get a mesh of around 1.2 million. If I were tackling this, I would look at where in my mesh I get high cell density, and defeature or simplify them.

Best,
Darren

1 Like