Verification & Validation

Check mass flow

It's always good to check the mass flow rate after CFD simulation. The solver tries to keep it satisfied, but sometimes a representative imbalance is obtain, showing that something has to be done in order to get more accurate results.

To do this, we will use FLUENT. Open FLUENT from Solution.

Highlight "Reports" in the left box. Then select "Fluxes" and click "Set Up...".

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You should get an imbalance of 3.42e-10kg/s which is essentially zero. Cool!

Tip speed ratio (TSR)

In practice, this is extracted directly from the Boundary Conditions, since we will essentially check that the velocity at the wall is zero. Therefore the purpose of this check is more to verify if we had correctly inputted the mathematical model into Fluent.

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But we're not done! To calculate the TSR we still have to perform one short step. Recall that TSR=veloc blade/veloc wind, so all we have to do is divide the calculated velocity by 10m/s, the wind speed.

So, our TSR is 0.01676. We will use this value to estimate the value of Cp.

compare blade velocity (TSR?)  This is a BC, so it will essentially just check whether we had inputted the correct mathematical model into the tool.

Verification

Compare Cp with the expected

mesh refinements (different mesh. Summarize, don't need to show steps) Cp might be bad, refine the mesh and check.

Validation

check against FDRL data? Might have to re-scale, as it is a little larger, flow max at 7m/s and much greater rpm.... If you scale it up to xxx this is the results you'd get. Compare Cp, try using the actual wind speed and angular velocity.

Actually the angular velocity is coupled with the incoming velocity. To capture that with fluent, we would have to use 6DOF which is too complex. So a trick would be..... see jeremy email.

Angular velocity in Steady state

One could have noticed that the moment coefficient (Cm) is not zero, which means that there are some extra torque applied on the turbine. That is in fact true, and we could even use this Cm to calculate the power coefficient (Cp) for this setup. 

It's possible to find at which velocity a VAWT like that would spin in case we applied the very same conditions as of this tutorial. At that angular velocity, there would be no extra torque applied on the turbine: if it were to spin faster, a negative torque would slow it down, if it were to spin slower, a positive torque would spin it up.

To find that velocity using the Moving Frame of Reference approach, you can vary the angular velocity (under "Cell zone conditions", open the "inner" condition and change its angular velocity) until Cm is approximately zero. 

If you do that, you will find an angular velocity of about 80RPM.

Mesh Refinement

One can also perform a mesh refinement study. Duplicate your project, import the new refined mesh (found here), "Clear Generated Data" for Results cell, refresh the project and launch Fluent. Remember to use Parallel (2, 3 or 4 cores, no more than that) if available. Also, compute the initialization from farfield1. Run the simulation for about 2000 iterations. This is the new velocity contours.

Image Added

The (absolute value of) mass imbalance between farfield1 and farfield2 also change, dropping to about 1.48e-9. This is a reduction of exactly 5 times! Yay.

Note: this new mesh uses completely different meshing techniques. The ideal would be using the very same method as before. This new mesh is more "professional made" than the previous one.

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