True wind and apparent wind

As a side effect of the boat's velocity, the apparent wind seen by an observer on the boat differs from the true wind seen by a fixed observer.
However up to flow5 v7.02, the boat's velocity was ignored, which meant that the wind's properties used in the analysis were implicitely the Apparent Wind Speed (AWS) and the Apparent Wind Angle (AWA).

Due to friction effects, the True Wind Speed (TWS) is lower at sea level than it is at higher altitude. The first consequence is that the AWS is also a function of altitude. The other consequence is that the AWA which depends on the fixed boat speed and the varying wind speed is also a function of altitude.

To account for this, the boat speed has been added as an analysis parameter in flow5 v7.03. The input parameters have been chosen to be the boat speed, the AWS and the AWA.

Notes: This choice is still experimental and driven by considerations of consistency with previous versions. The input parameters may switch to true wind properties in a future version.

This effect can be described in flow5 with the use of a spline. The wind gradient is applied as a multiplier to the wind speed.

Note: As of v7.03 the wind gradient is ignored in the far field plane, so that the induced lift and drag are calculated using the apparent wind properties at high altitude.