amr-wind
amr-wind copied to clipboard
Exawind
Underestimated power predictions from OpenFAST/ALM turbine models in unsteady flows
I am working with @jfrederik-nrel, @ndevelder, @gyalla, and others at NREL/SNL regarding a turbine performance issue that we see with OpenFAST/ALM runs in unsteady ABL runs.
The problem is observed with the IEA 15MW reference turbine, although we need to investigate if it's seen for other models. In steady, uniform flow, the actuator line parameters (grid spacing, epsilon, number of blade points) can be calibrated so that the turbine predictions are relatively accurate across a range of wind speeds when compared against the design curves. However, when the same turbine is used in an unsteady inflow created through a precursor ABL, the power predictions are noticeably lower.
This case is for a precursor with hub-height wind speed 9m/s, shear exponent ~0.17, and 3.7% TI, and using OpenFAST 3.4.1 with ROSCO 2.8. We've tested several combinations, including using an equivalent Joukowski disk model, and they suggest that the wind power available to the turbine should be close to the uniform flow case, but is not being extracted through the ALM model.
Other debugging tests we've tried indicate that it may not be the fault of the controller (fixing rotor speed to the correct value leads to a similar problem), that shear effects don't have strong effect, and OpenFAST alone calculations with turbsim based inflow are reasonably matching expectations:
Additional tests to be completed are to determine whether this impacts both the OpenFAST ADM models as well as ALM.
We will follow up this github issue with some additional details and information, but let us know if there are any questions.
Lawrence
Just clarifying: the faulty points on the power curve are the orange and cyan? AMR-Wind, precursor and AMR-Wind, precursor, fixed rot-speed?
Here is an example input file that corresponds to the orange dot on the power curve (ARM-Wind, precursor)
Here is an example input file that corresponds to the orange dot on the power curve (ARM-Wind, precursor)
Thanks @gyalla. In case anybody wants to try testing this, we have the precursor and boundary I/O files for this case on Eagle at
/scratch/lcheung/2024/AdvControls/precursor6_4kmX2km_5m.bndrydata and the corresponding precursor input file at
https://github.com/lawrenceccheung/ALCC_Frontier_WindFarm/blob/main/farmruns/MedWS_LowTI/template/MedWS_LowTI_precursor1_spectra.inp
Hi @lawrenceccheung , do you have a plane at the rotor-plane (y-z if flow is in x) of the actuator force field? We just had a long discussion with @mbkuhn and Regis, and found that the value of epsilon and spacing are not in line with recommended values.
Actuator grid spacing recommendation: epsilon/dr > 1
Grid resolution recommendation: epsilon/dx>2
Hi @tonyinme, I took a quick snapshot of the wind speed for precursor (left) and uniform (right) for these simulations in my precursor (neutral, 3.5% TI, 8m/s). See below.
I want to add though that this issue is not exclusive to low values of epsilon. I started with simulations with a higher epsilon of 4-6 (same grid size), which satisfies your recommendations, and also for these values of epsilon I experienced significant underpredictions of power.
Thanks @jfrederik-nrel ! Do you have that same plot but for the actuator force?
@tonyinme unfortunately not, I only sampled velocities on these simulations.
I just thought of another test that we could do. We can make use of ABLForcing.velocity_timetable to ramp the wind speed from a constant $U_1$ to another speed $U_2$ over a controlled amount of time, and see how the turbine responds to it. Ideally it will settle to the constant, steady point of operation that it's supposed to, but we might be able to find out what input is getting messed up in this process.
I've successfully run some cases using 100 actuator points and epsilon of 3.125 on a 1.25 m near-actuator grid. I checked the actuator force field with Tony yesterday for these cases.
This is what it looked like for one of the blades (the other blades are slightly out of the plane shown):
And then I re-run, matching the grid resolution and actuator parameters you guys have, and this is what the actuator force field looked like:
I'd be curious to see how the force field looks like in the case Joeri mentioned the choice of epsilon satisfied the requirements. Tony might have some further comments about the figures above.
@rthedin thanks for running that. For someone new to this (like me), how am I to interpret these plots?
Hi Regis, very interesting, thanks for looking into that. I can definitely see how our settings as we are using here, might not be appropriate. However, as I mentioned before, for different settings I've so far seen the same problem where power is way lower for precursor simulations. Therefore I'd be very interested to learn what powers you are getting for the settings you are using, and how close these are to simulations with similar wind speeds in OpenFAST.
I'm getting more power on the stable case, which has lower turbulence and you could argue is the closest to the uniform inflow. Here are the power values I'm getting:
Cond: stable , Power T1: 8003.547 kW
Cond: neutral , Power T1: 7929.298 kW
Cond: unstable, Power T1: 7857.647 kW
These values were obtained with turbulent precursors with 8 m/s at 150 m. Here is a quick summary of the conditions I used:
Hi @rthedin, thanks for sharing that. This is interesting, because at 8m/s, the IEA 15MW is only supposed to generate around 6.3MW of power (according to the design doc), so there might be a slight overestimation of power.
Hm, that is very interesting. I ran a very similar case using my own 8m/s neutral precursor with refinements down to 1.25m and epsilon = 3.125, and my average power was only 6034.339 kW. That's a lot closer to the expected 6.3MW, but still lower. Also, I didn't run a uniform simulation with these settings, but I would expect that the power would also be much higher than in the precursor case here. I'm running this case now to verify.
I do have to note that there is one important difference between @rthedin and my own simulation: I'm running my case with 50 blade points, not 100. Also, I'm not sure what time step you are using? My simulation ran with a 0.05s time step. Would we really expect a power increase of ~30% when we increase the number of blade points by a factor 2 (and/or possibly decrease the time step)?
More importantly though, changing these settings affects power, but it doesn't necessarily explain why, with the same settings, power is much lower in precursor simulations than in uniform simulations. I think the core of this issue is that we want to be able to tune turbine and grid settings in uniform conditions to match expected power and thrust curves, and than apply those same settings to precursor simulations.
The issue is not that we can't get the power to match design curves, just that we need different settings at different flow conditions, which is undesirable.
@jfrederik-nrel can you please share the case in uniform inflow so we can take a look at the input files and some outputs from that simulation?
@tonyinme yes happy to share those results once they come in. Input file is attached here: simulation.i.txt
@jfrederik-nrel , thanks for sharing. It looks like you have 3 levels of refinement. So the grid resolution should be 1.25m near the turbine is that correct? I believe that the other simulations used only a resolution of 2.5m near the turbine?
Yes correct. In my original simulations, I use 2 refinements for a resolution of 2.5m near the turbine. I added a third refinement to mimic Regis' simulation with 1.25m resolution and an epsilon of 3.125 to verify that the discrepancy between uniform and precursor simulation is unrelated to turbine and grid settings.
ok, can you please share the results / input files of the simulation with 2.5m refinement?
@lawrenceccheung @gyalla The comparison with the Joukowsky disk is informative. As far as I can tell, all actuator methods use the same velocity sampling approach to get the flow velocity at actuator points. There are still a lot of other differences, though, like the position of the points, how the forces are computed using the sampled velocities, and how the forces are applied back to the flow itself. Although there are still a lot of differences between the OpenFAST ALM and ADM, would it be helpful to compare the OpenFAST ALM results to OpenFAST ADM results? Because they both share the OpenFAST interface.
Sorry for the delay in getting the ADM results -- there was a change in the module systems on our end that caused a bit of a slowdown. I've added a point on the power curve I shared earlier for the ADM with precursor as well as a two-minute time series.
The ADM-precursor lands in between the ALM-precursor and ALM-uniform results, with a 12% loss in power (compared to 21% for ALM-precursor). This is with the same epsilon and resolution as the ALM case (epsilon=2.0, dx=2.5m)
@tonyinme @mbkuhn I've also looked at the case with eps=3.125 and grid size=1.25 at turbine rotor to check whether the actuator grid spacing ratio causes the difference in power. It seems like this is not the case, or only barely. For the eps=3.125, dx=1.25 case, I still see power being over 15% lower in a precursor simulation w.r.t. a uniform simulation. See plot below.
For another data point on the ADM runs, we've been running some comparisons using a neutral 6.3 m/s precursor and the NREL 2.8-127 turbine model. Using the OpenFAST+ADM model in AMR-Wind and Nalu-Wind gives consistent power results with the design curve and the uniform flow cases:
We're still looking at thrust and other wake-centric quantities, so I won't say that the ADM model is perfect, but it doesn't drastically underestimate power like the IEA15MW ALM cases do.
Lawrence
Looking at that curve, Lawrence, there's no comparison between the precursor and the uniform. The precursor points are both below the curve and the uniform points are all above it. There could easily be a 12% boost to the precursor points, like in Gopal's run, and it would still look "close". Seems like the same pattern exists for the ADM cases, just to a lesser extent compared with the ALM. Which makes me wonder how we know what is "right" and what is a reasonable amount of error. For the first red square, it's actually a significant percentage above the power curve for the same WS. Should this be a concern?
It may be helpful to run that Nalu-Wind ADM case with a uniform inflow and see how much it deviates. If there is a significant deviation, along the same lines as the AMR-Wind ADM case, it would indicate to me that it is not a problem with the AMR-Wind implementation; it is a problem with how we use OpenFAST or it is model error.
BIG Update, and something that will soon lead us to figuring out this problem.
I ran a uniform flow case (green curve) with the same domain, flow direction, turbine (including the yaw angle) as Joeri's precursor case. This led to getting the same power as the precursor (turbulent ABL inflow) case (orange curve), not the same power as Joeri's uniform case (blue curve). This is a clear indication that the discrepancy between uniform and precursor-based results is not due to the presence of turbulence; it must be due to the difference in the setup between the two cases.
Our current hypothesis is that the yaw angle is being somehow misinterpreted within the AMR-Wind - OpenFAST framework. We have some other runs going to try to confirm that hypothesis. As it stands, if we take the 5.4 MW results from the diminished cases and divide that by cos(30 degrees), we get 6.2 MW, which is in the ballpark of the 6.7 MW results from the original uniform case. It's not enough to confirm anything other than the possibility of a bug related to yaw.
Thanks for the update Michael, this is very very informative!
To clarify: in my cases, the wind direction is 240 for the precursor case, and 270 for the uniform case. As the default in OpenFAST is facing west (270 degrees in AMR), I set the yaw angle (in ElastoDyn > NacYaw) to 30 degrees for the precursor case, and 0 degrees for the uniform case. I've also checked the wakes in these simulations, which indicate that there is no wake steering going on, and the turbines seem to be properly aligned with the wind.
One question I have based on this hypothesis: do the runs from @gyalla also have a wind direction mismatch between the uniform and precursor cases? As far as I am aware, both those cases run at 270 degrees, right? So in that case, we wouldn't expect a power mismatch if @mbkuhn's hypothesis is correct, would we?
Good find Michael, and thanks for the update! @jfrederik-nrel is correct that all of the cases I've done so far are run at 270 degrees.
Dropping in with a comment just FYI. There are two places that yaw is specified for an individual turbine in an AMR-Wind simulation, one is in the .i file (example) and one is in the OpenFAST file. I have the impression that only one of these gets used in the code, and I also have the impression that these are both defined in different coordinate systems. Something to keep in mind.
