Turbsim spatial averaging

Wind & Water Inflow Turbulence
1

Hi,

I noticed, by chance, that if I had an even number of grid points in the turbulence box, then the turbulence intensity calculated at the hub (Vhub stddev/ Vhub mean) would be much smaller than what I would specify in turbsim.in. (both in *outb and in *.wnd)
I tested the same case with odd number of points in the domain to find the turbulence intensity to be correct at the hub. I assume because in this case there is a grid point exactly at the hub.

I played around with the spatial coherence parameters but that only seems to change the turbulence spectrum, not really the turbulence intensity.

  • I would like to know if there is any spatial averaging done in the inflow turbulence. I did not find any info in the user guide. Any kind of spatial averaging will result in some degree of destruction of the signal and a reduction in the TI because no two neighboring points are 100% coherent, right? How doe you address this issue?
  • If the blade elements are smaller than the distance between two grid points, what inflow turbulence does an element located between two points experience?
  • If I use very few grid points in the domain, would there be a significant impact on the loads? Especially the fatigue loads?

All three questions may have just one answer in the end!

Thanks and best regards.

2

Dear Deepali,

When you calculate the turbulence at the hub, are you referring to an output from the InflowWind module of OpenFAST? The InflowWind module will perform 3D interpolation (2 spatial + time) when calculating the wind inflow at points (user-specified points or aerodynamic analysis nodes) that are not coincident with the TurbSim grid. This interpolation can smooth the turbulence, as you are noticing.

From our experience performing convergence studies, we have found that it is important to use a spatial discretization of the turbulent wind field in TurbSim equal to the maximum blade chord (i.e., dY and dZ = c_max, e.g., 5 m for the NREL 5-MW baseline turbine). Using finer resolution does not improve convergence. Using coarser resolution than that can overly smooth the turbulence (because of interpolation smoothing) and reduce fatigue loading. This is a good rule of thumb for specifying spatial discretization in TurbSim.

I hope that helps.

Best regards,