**3.2. Aerodynamic design criteria**

Mike Raupach and John Leys [31] suggested six aerodynamic criteria that should be consid‐ ered in addition to the seven practical criteria proposed by Zingg. These aerodynamic crite‐ ria are listed below:


They noted that criteria 1 to 4 are satisfied if the air flow near the ground surface is a well developed equilibrium boundary layer sufficiently deep to contain particle motion in the inner region where the mean wind speed profile is logarithmic and uniform over the eroding area. The logarithmic wind speed profile for neutral atmospheric stability has been described by:

$$
\Delta L\_z = \begin{pmatrix} \mu \ast \,/k \end{pmatrix} \ln \left( z \,/ \, z\_o \right) \tag{1}
$$

where *Uz* is the wind speed at height *z* above the surface, *u\** is the friction velocity, *zo* is the aerodynamic roughness length of the underlying surface, and *k* is the von Karman constant, usually assigned a value of approximately 0.4.

Criterion 5 requires turbulence with length scales greater than possible within the practical dimensions of portable wind tunnels and cannot be naturally generated by shear forces within either the working sections or flow conditioning sections of a portable wind tunnel. They tried to simulate gustiness using mechanical interruption of air flow in the flow condi‐ tioning section of their tunnel but discovered that the turning vane they employed for this purpose reduced the mean wind speed without increasing the vertical turbulence.
