# Hub reference with Turbsim

Hello,

I am developing an offshore wind turbine platform and I am using Turbsim to generate the wind. I have a doubt about how the rotor (or the hub) is located at the grid. I have read from the manual the following information:

HubHt: Turbine Hub Height [m]:
The HubHt parameter is hub height of the turbine for which the inflow is being generated. TurbSim uses the metric system so enter the value in meters. This parameter is used as a reference height for determining the grid location.
GridHeight: Height of the Grid [m]:
This parameter is the distance (in meters) between the top and bottom of the grid. The top of the grid is assumed to be aligned with the top of the rotor disk (see Figure 5), and because all points of the grid must be above ground level, 1/2*GridHeightHubHt.

I have a hub height of 90 m, a rotor diameter of 126 m. I have set the GridHeight to be 179 m and the GridWidth to be 180 m (I know it is big, but as it is an offshore wind turbine I need a big grid to cover the movements of the platform).
My problem is that I am not sure about where the hub is located, if it is located with the rotor disk aligned with the top of the grid or if it is located 90 m up from the bottom of the grid letting at the top space for the possible movement of the platform (this last option would be right for my case)

Thank you, best regards.

Dear Aitor,

See Figure 5 from the TurbSim User’s Guide: nwtc.nrel.gov/system/files/TurbSim.pdf. In your case, GridHeight < GridWidth, so the right-most image applies. The hub-height will be located at the center of the grid (at 90 m), so, the top of the grid will be 90+179/2 = 179.5 m, well above the top of the actual 126-m diameter rotor. The statement about “the top of the grid is assumed to be aligned with the top of the rotor disk” refers to TurbSim’s assumed rotor diameter, as the actual diameter is not entered in TurbSim.

Best regards,

Dear Jason,