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jason August 20, 2013 13:19

Rotor Torque Prediction
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Dear Forum,

I am studying a hydropower turbine using pimpleDyMFoam and AMI. A quick sketch of the setup is attached, however, this is just a simple model to replicate my larger problem.

I know that the fluid acting on the rotor will cause it to rotate anticlockwise around the x-axis when viewed along the positive x axis from the origin as shown, until the rotor reaches a certain free rotation speed, at which point the net torque on the rotor will be zero (Mxx = 0). A pressure plot is included indicating the high pressures on one side of the blades. As the rotor rotates faster and faster in the fluid the relative velocity between the moving fluid and the rotor blade surface will tend towards zero and thus the dynamic pressure created at the blade will tend to zero. To account for these moving blades I have additionally used the rotatingWall boundary condition.

When I run the solver in ‘locked’ mode, ie I specify zero rotation of the rotor in the dynamicMeshDict and zero rotatingWall boundary, I obtain a torque on the rotor for Mxx which is negative (right hand rule, thumb in +ve x-direction, positive torque is clockwise). So far so good, a negative Mxx will cause the rotor to rotate anticlockwise if the rotor was allowed to rotate. The torque magnitude is also in the right ball park so I am happy.

Next I set the rotor to rotate a little in the anticlockwise direction, my understanding is that the torque will decrease to zero as I increase the rotor speed anticlockwise due to the decreasing relative velocity and dynamic pressure above. However, OpenFOAM reports that the torque increases in magnitude (and is still negative).

Conversely, if I set the rotor to rotate a little in the clockwise direction OpenFOAM reports that the torque decreases in magnitude and will hit zero net torque at a certain rotor speed eg somewhere between +200 and +300 RPM as shown in the results graph...I haven’t pinned it down exactly yet but I do know from experiments that a similar rotor of this diameter will freely rotate around 230 RPM…..this also makes me happy. However, the rotor is now rotating in the wrong direction with zero net torque.

My questions are …

1) a) Has anyone experienced this and if so what was the problem?
2) b) Am I reading this correctly? Or is it possible that the forces/torques contribution from the rotating rotor and the pressure/viscous forces been summed incorrectly somehow in the code?

Any advice would be highly appreciated…my mesh is quite large but I could setup a simpler blockMesh case if that would help, please let me know.

Many thanks in advance


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