Low power coefficient in turbine simulation

efe011 · January 8, 2016, 10:19

Hi everybody,

I'm simulating a tidal turbine as validation case with a moving rotating frame in steady state and I need some advice.

My model has a k-w turbulence model with gamma-re-theta transition model, and I've already simulated a quasi-2D foil succesfully getting error below 7% for lift and less than 1% for drag in the Re range where the turbine works (500k aprox), which is good enough for my purpose.

My mesh is poly with a Y+<1, and I've performed a sensibility analysis to the mesh size, going from 4M to 11M cells, staying finally with a 6M due to independence of the solution.

So, the issue is that, despite I've checked the sensibility to many things, I'm still getting about a 23% smaller power coefficient than the experimental results. Moreover, this skewed results seems to be pretty robust, since after altering several parameters I'm still getting the same. The parameters I've changed:

*Changing the flow speed and turbine rotational speed, keeping their ratio constant (TSR). The idea was to modify the Reynolds number.
*Using half the turbulence intensity for initial conditions and the inflow.(0.1 to 0.05)
*Changing max Y+ from 1 to 0.5
*Using low Y+ instead of All Y+ wall treatment
* By having a Stream Edge Function for Gamma Re Theta as: $WallDistance > a?1:0. I changed the "a" variable from 5mm to 1m.

Any hints on how to get more accurate results??

Thank you very much.

Kind regards.

efe011 · January 11, 2016, 13:22

During the weekend I performed an analysis over the blockage ratio and I didn't find anything...

Any idea or thought would be really appreciated.

fluid23 · January 11, 2016, 15:39

Is this an horizontal or vertical axis-turbine? My immediate thought is that your calculation of Cp doesn't jive with what done in the validation case. If you are seeing a nearly constant error of 23%, it could be that your reference area is off by 23%. One way to check would be compare dimensional power (so not Cp, but P) and see what happens.

efe011 · February 3, 2016, 09:42

It is an horizontal axis turbine.

I'm afraid to say that it's not as simple as an area mistake, and I don't have the power data. Instead, I only have the power coefficient as experimental data.

Any other hints? Thank you.

January 8, 2016, 10:19	Low power coefficient in turbine simulation	#1
efe011 New Member Join Date: Oct 2012 Posts: 9 Rep Power: 13	Hi everybody, I'm simulating a tidal turbine as validation case with a moving rotating frame in steady state and I need some advice. My model has a k-w turbulence model with gamma-re-theta transition model, and I've already simulated a quasi-2D foil succesfully getting error below 7% for lift and less than 1% for drag in the Re range where the turbine works (500k aprox), which is good enough for my purpose. My mesh is poly with a Y+<1, and I've performed a sensibility analysis to the mesh size, going from 4M to 11M cells, staying finally with a 6M due to independence of the solution. So, the issue is that, despite I've checked the sensibility to many things, I'm still getting about a 23% smaller power coefficient than the experimental results. Moreover, this skewed results seems to be pretty robust, since after altering several parameters I'm still getting the same. The parameters I've changed: Changing the flow speed and turbine rotational speed, keeping their ratio constant (TSR). The idea was to modify the Reynolds number. Using half the turbulence intensity for initial conditions and the inflow.(0.1 to 0.05) Changing max Y+ from 1 to 0.5 Using low Y+ instead of All Y+ wall treatment * By having a Stream Edge Function for Gamma Re Theta as: $WallDistance > a?1:0. I changed the "a" variable from 5mm to 1m. Any hints on how to get more accurate results?? Thank you very much. Kind regards.

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January 11, 2016, 13:22		#2
efe011 New Member Join Date: Oct 2012 Posts: 9 Rep Power: 13	During the weekend I performed an analysis over the blockage ratio and I didn't find anything... Any idea or thought would be really appreciated.

January 11, 2016, 15:39		#3
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	Is this an horizontal or vertical axis-turbine? My immediate thought is that your calculation of Cp doesn't jive with what done in the validation case. If you are seeing a nearly constant error of 23%, it could be that your reference area is off by 23%. One way to check would be compare dimensional power (so not Cp, but P) and see what happens.

February 3, 2016, 09:42		#4
efe011 New Member Join Date: Oct 2012 Posts: 9 Rep Power: 13	It is an horizontal axis turbine. I'm afraid to say that it's not as simple as an area mistake, and I don't have the power data. Instead, I only have the power coefficient as experimental data. Any other hints? Thank you.