Rotation of the velocity measurements

Measured velocities must be rotated in the flow's frame of reference since inertial subrange model differs between the velocity components, and the effects of anisotropy are more pronounced in the transverse and vertical direction.

The velocities are measured in either the instrument's coordinate system, beam coordinates or more rarely in the earth's coordinate system (east, north and up).

• Describe how the rotation is done (a sketch of an arrow, and a new frame of reference where the new x axis matches the mean flow direction).
• What is used as the mean flow

Examples of measured velocities from two ADVs highlighting that their x-axis may be un-aligned with the flow's. The blue dots and lines that represent the principal axes are for a 5-min subset of the entire timeseries shown in red.