Answer:
There is no hard set way to determine the layout of a wind farm. Many use software such as WindFarmer where the user enters in topography of the land (height coordinates), specifies wind characteristics (such as frequency and intensity from given directions) and also specifies how many turbines they wish to add.
The software package then models the wind and creates 'power' values for each possible location for a turbine to be placed (directly related to the wind velocity and the type of turbines being used). Then a heuristic program is run which changes the order of arrangement until it finds some local optima (the goal is to maximise the power being produced by the wind farm).
The biggest issue around this analysis is the interference caused by an upwind turbine on a downwind turbine for a given wind direction. This is where the problem becomes computationally expensive.