[AS_Aero]

Hi All

I am doing a steady state RANS computation for a rotor stator and its quasi2d with 2mm thickness in z direction. My simulation is running for 2 minutes and the residual is going to 1e-6 and its converging. although my monitor point values are not constant yet. It ran for 200 iterations and then due to the residual its stopping. So should I reduce the residual limit from 1e-4 to 1e-7 or something and run for 1000 iterations and see ? if my power value gets constant or not ?. I have attached the images .

[ghorrocks]

Yes, your power is still changing. But make sure you look at the magnitude of the change - from iterations 140 to 240 the change is about 1%. Is a 1% change significant in your case? If so then you need to converge tighter. If not you can handle a looser convergence criteria.

This is a type of sensitivity analysis and allows you to establish the sensitivity of your simulation to convergence criteria. It allows you to set a convergence criteria which will give you a known level of accuracy in your results.

It is important to also do sensitivity analysis on mesh size, time step size (if transient) and any other tunable parameters (such as boundary condition proximity).

[AS_Aero]

Yes I ran it longer and there is not much difference in the power values.
but again now I am puzzled
I ran for a speed of 10m/s and different rpm to get a power curve but unfortunately the power goes on increasing and then it starts to go down and suddenly it increases at next rpm. As shown below in tne image
50 0.3814
75 0.5074
100 0.5808
150 0.6326
175 0.6044
200 0.5710
250 0.3776
300 0.0063
400 0.92

so why is it increasing after this 300 rpm ? Is it normal or ?
Also if I see my power plot for 500 iterations it has a particular pattern attached on the image. is it normal as well ?

[ghorrocks]

How are you calculating the power? I recall on another thread that you are taking the absolute value of the torque or the power or something - this is exactly the problem which doing that will cause

[AS_Aero]

Hi Glenn

ShaftPower = abs(rpm)*2*pi*moment/60[s/min]/360[degree]
moment = abs( torque_z()@Rotor_Blade_Walls)
rpm = 50 [rev min^-1]

This is the formula am using to calculate the power. Do you think I am making some mistake on this ?

[ghorrocks]

Your function moment = abs( torque_z()@Rotor_Blade_Walls) is the culprit. Get rid of the abs function. While you are at it, why have the abs function in the rpm either?

[AS_Aero]

Hi Glenn

Since my rpm is given as -200 rev /min on the rotor domain as its rotating in the anti clock wise. So I gave the abs of it. And for moment I was told to give abs or I saw somewhere here in the forum that abs of torque has to be giveen. But even if I dont give abs on both rpm and moment then the both negatives will cancel each other right ? And does it going to affect my power value by any chance ?
And I must tell you in this case we are doing steady state frozen rotor.

[Gert-Jan]

If you take the abs value of the torque, then the outcome will be always positive. Then how are you ever going to find out if the your rotor will deliver or consume energy?

[AS_Aero]

But here we are considering the maximum free RPM right without any generator unit. So am giving a RPM of 400 (its negative since the rotation of the rotor is expected in Anticlockwise) and then a speed or wind speed, which drives the rotor to rotate. So this rpm will be the maximum rpm the rotor can rotate right ? So you expect to have the maximum power out of it or the power we get is the maximum what we will get. So by using this rpm vs power, I get the power curve and then decided the optimum rpm for the specific speed. Also for some reason when my speed is increased the power value is also increased for particular RPM. Is it physical ?
And I will try to do without any absolute value and see how the power is changing. And should I remove the absolute for rpm as well or ? Kindly let me know how the function should look like ?

[ghorrocks]

Simply
Power = rpm*torque_z()@Rotor_Blade_Walls

Note that you don't need all the units conversion stuff you had there, CFX does that for you providing you correctly specify the units.

If this gives you a negative number and you don't like it then just stick a "-" out the front. But as Gert-Jan says, you need to know when this thing consumes and generates power and you tell that by the change in sign of the power.

[AS_Aero]

That mean only when I get a positive Power for particular Speed , I can tell that my rotor is generating power and not extracting power ? And then how should I plot the Power curve to get the optimum rpm values ? For example now I am running for a speed of 10m/s and my rpm is given as -300 rev/min (This negative is given on the rotor domain definition as the rotor has to rotate in negative z direction) . But in power calculation I am giving it as positive value. and then still am getting negative power value. It could also be because I am using a quasi2d right ? If I increase the Z direction thickness of the rotor , will I get a positive power value ? And in whatever case should I use this power value for the power curve or only after the rpm where I get positive power ?

[ghorrocks]

Positive and negative power, torque and rotation directions is just from your definitions. If this concerns you you should check the rotation definitions of all these functions so you know what positive or negative means in your geometry.

If this comes from a 2d simulation then changing the thickness of the simulation domain will just scale the results in proportion with the thickness. It will not change the sign.

[AS_Aero]

Dear Glenn

Thanks a lot for your suggestions. Yes thats true. I have attached the image of my rotation domain and the power value calculation.
But now my main consideration is about the optimum RPM for particular speed ? How to calculate that. I know its based on the power curve and in power curve should I take this negative power value also or only rpms which has positive power ? and from that I should draw a power curve and get the optimum rpm ?
And this Optimum RPM changes based on the wind speed as well right ?

To give more details
for speed of 10m/s I tried with RPM of 50, 100,150,200, 300,400,500 and for all the RPMs except 400 and 500 the Power is negative. That means the Power is generated only from an RPM of 400 and above. So in my power curve where Power vs RPM should I only plot from 400 or from 50 to see the optimum one ?

[AS_Aero]

So also after running the same simulation for speed from 10m/s to 15m/s even for 400 RPM and 500RPM the power value is negative. What actually does that mean physically ?

[ghorrocks]

It is possible (likely actually) that you have the sign the wrong way around and for 10m/s it is generating power below 400rpm and absorbing power above 400rpm, and that as the air speed increases the zero power speed increases. So for 15m/s at 400rpm and 500rpm it is generating power and you have not gone fast enough to see the zero power point yet.

Looks very closely at your torque definitions and rotation directions to check the sign to ensure you know which is generating power and which is absorbing power.

[AS_Aero]

Hi Glenn

Sorry I dint understand your explanation properly. Maybe I try to give more details.
So for 10m/s speed, I ran from RPM of 50 to 500 and only 400 and 500 gave me positive power value. The Magnitude remain the same from previous equation, only sign got changed. Then I increased the speed from 10 to 15m/s and then ran the same with 50 to 400 RPM and ALL are giving negative values. And except RPM of 500.

Now coming back to your information. I dint understand about this Zero Power value, should I find that point (RPM for particular speed where the Power is zero ) ?

Your term Generating power means it should be positive right ? And how come going fast means going to zero power. I dint understand this. Regarding the setup am not completly sure if I have done some mistake. Would be great if you can have a look on the def file since you have good experience on this. I have attached the def file link.


https://wetransfer.com/downloads/f96...0090416/f60788

Kindly have a look and let me know, if there is anything wrong.

[Gert-Jan]

The sign is just a sign and depends on how you setup your simulation in your coordinate system. So it is impossible for us to check.

But you should just take a step back, go back to real life. Then you can imagine that at low rpm any wind can drive the rotor, so it can drive your generator. At high rpm, the rotor will drive the wind, so you need a motor with power.

This is the logic that you will find in real life. There is not reason to think that CFX will fool you. So trust CFX and trust yourself. But think for yourself and try to understand what is going on and make connection to real life.

So, it does not matter what sign the outcome of your calculation is: At low rpm the rotor will generate energy, at high rpm it will cost energy. And as long as you don't take the absolute value of the torque, the point that the outcome will change sign, is the point of Zero Power.

[AS_Aero]

Hi Gert

I totally got your point. So when you say it doesnt matter what sign the outcome of my calculation is . In the begning it was told that if the power value is negative, that means the power is being used to rotate the rotor, or ? In any case as I said in the previous comment, my rotor is producing possitive value of the power after 400 RPM till then it is negative for a speed. But if I increse the speed then the power value is positive for higher RPM . Now my question is how will we know that 500 rpm is too high for a rotor to consume energy to rotate at 500 rev /minute and its not wind going to rotate the turbine ? I am confused about it .
Second thing is with respect to the power sign, what does this change from negative to positive at higher rpm means then ? It does has some significance right ?
It would be great if either of you could have a look into my files and tell me if I am doing some mistake or what exactly is wrong.

[AS_Aero]

Again an small clarification in my Power Calculation

ShaftPower = rpm*2*pi*moment/60[s/min]/360[degree]
moment = torque_z()@Rotor_Blade_Walls
rpm = 50 [rev min^-1]

This is what I am giving , but if I omit the units like

Power = rpm*moment
moment= torque_z()@Rotor_Blade_Walls
rpm= 50

The results are different secondly here the unit for moment is in Joules and power is also in jules since rpm is constant value. I dont see that cfx directly gives the dimension for this.

If I give it like the first one its showing in Watt else its showingi n jules if I just multiply 50*moment

If I give rpm =50 [rev min^-1] and then multiply rpm*moment its showing in [kg m^2 s^-3 degree] as units.

So here also am puzled what to give or if the first one itself is correct ?

[AS_Aero]

addition to that if you dint have time to have a look on the file, I have attached the images the first one shows that the thickness of 2mm is in the positive Z axis direction and the second image shows how the rotor setup is with angular velocity specified. its in negative value because the rotor rotates in anticlock wise as its blades are positioned like that, you can see from the image.
So if something here is wrong could be the reason for negative power value (I suppose )

I have uploaded the streamlines for three different rpms as well. Kindly have a look and let me know if we could infer from this the reason for the power values.