Steady state Frozen Rotor Simulation with different RPM
 

Hi All

I am doing a rotor stator simulation using CFX and I want to plot the power curve for 5 different RPM. But my doubt is with steady state RANS and Frozen rotor simulation can I get the power curve as we change the RPM ? Since our rotor is frozen there wornt be any difference in the moment value or correct me if I am wrong.

Correct me if I am wrong, but are you referring to your horizontal windmill?
If so, do you keep wind speed constant upon increasing RPM?

Hi Gert

Yes its about the wind mill only. I keep the wind speed constant for now. And trying with different RPMs.

If you have a too high RPM, you add momentum to the air. So the torque (power) you gain from the wind will be negative.
At too low RPM, your wind mill will be too much of an obstacle. So you get a lof of pressure on the blades, at low RPM. So, it will be a positive torque (power), but not optimal as well.

The optimum is what you need to derive from the test you describe. Perform a series of test with multiple RPM's and then determine the optimal TSR (Tip Speed Ratio).

Hi Gert

Yes exactly this is why I am trying to plot the pressure curve using different RPM values keeping the Velocity constant. But my concern is my simulation is a steady state simulation with Frozen rotor option in CFX, that means the Rotor doesnt rotate at all in this case. So will it still make sense to do this power curve test ? or should I run it in transient case ?

Frozen rotor is an approximation. Very well suited to span a nice curve with 5 points (or as many as you need) around your optimum. Thereafter perform 3 transient calculations around the optimum.

Perfect in that case I will try this only and come to an optimum value.

Hi Gert

I have some problem here . I tried with different RPM from 100,200,300,400,500,600 till 1000 and till then the frozen rotor RANS computation is giving me power in increasing order, so should I try to run for higher RPM like 2000 or 1500 so that the power value will come down ? so that I get this parabola curve ? or am I doing something wrong here ?

Try to image what is happening here. In real life I mean. You are running your rotor with 1000 rpm (What is your tip speed? Did you calculate this?) and still expect it to generate energy? That is not realistic unless you are in a hurricane. But you are in Germany. No hurricane.
You are adding energy. So go to the other side if the spectrum: N<100rpm.

(It has nothing to do with the frozen rotor. You have so many blades that I doubt if you'll see a large difference with transient)

In normal condition I was using 400 as my RPM I used -400 in the rotor domain setup (image is attached for the setup as well as the power calculation)


Tip speed I calculated using this following link below

https://www.hmicronpowder.com/calculator
and for 100 RPM its showing 4m/s and for 200 its 8m/s

The simulation was running for 10000 iterations and the power value was given on a monitor and its almost constant, but with the same setting for 300 and 200 RPM the monitor plot looks but fluctuating, dont know why. What else could I check ?

I have also attached the power monitor plot for different RPM values, but for some reason the 500 one is having some wierd graph and the value as well. But the setting is completly the same as others.

Also my concern is should I give the RPM as -400 with global Z coordinate or +400, how does this sign make a difference ? The thickness of 2mm is in the possitive z direction. But both sides are given as symmetry boundary condition.

Also I tried for RPM 100 both -100 and 100 in the Rotor domain rotation spped with z as direction and I could see that the power value is different for both. for -100 am getting 0.5686 and for +100 am getting 0.881W