**3.3. Simulating saltation**

**3.** It must be durable.

mantling.

ment when available.

ria are listed below:

eled.

high surface stress.

has been described by:

quired to initiate erosion than to sustain it.

of an eroding field is to be simulated.

**4.** It must be safe to use.

eroding materials over field surfaces.

64 Wind Tunnel Designs and Their Diverse Engineering Applications

**6.** It must have ready portability.

**3.2. Aerodynamic design criteria**

**5.** It should have sufficient size to afford free movement and representative sampling of

**7.** It should be light in weight and amenable to quick and positive assemblage and dis‐

Another criterion that he used but did not list was the use of commercially available equip‐

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‐

**1.** The flow must reproduce the logarithmic wind speed profile in the natural atmosphere,

**2.** The surface shear stress must scale correctly with the wind speed above the surface so

**3.** The vertical turbulence intensity and scale in the region close to the ground must be re‐ alistic, ensuring that vertical turbulent dispersion of suspended grains is properly mod‐

**4.** The flow must be spatially uniform to avoid local scouring by anomalous regions of

**5.** Gusts should be simulated in the tunnel due to the fact that higher shear stress is re‐

**6.** A portable wind tunnel simulation of erosion should allow for the introduction of sal‐ tating grains at the beginning of the working section if more than the very upwind area

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

\*/ / ( ) ( ) *U u k ln z z z o* = (1)

thus ensuring realistic aerodynamic forces on saltating grains.

that realistic aerodynamic forces act on grains of all sizes at the surface.

Although criterion 6 is not truly aerodynamic, it is very necessary in order to simulate well developed steady state saltation of sand grains over an eroding surface. However, it also raises more questions as to the design and operation of the portable wind tunnel such as how much material to introduce, what the size distribution should be, and how to distribute it realistically in the flow before it strikes the ground surface tested in the working section. An orifice controlled gravity fed saltation initiator that drops the sand abrader into inclined tubes for acceleration before striking a sandpaper surface and bouncing into the flow stream is shown in Figure 3.

**Figure 3.** A complex flow conditioning section showing the abrader hopper and inclined tubes used to initiate salta‐ tion into the flow stream.

Saltation has been shown to reach a maximum at about 7 m length in wind tunnels [51] and decreases at longer distances, reaching equilibrium at between 10 and 15 m into the working section [45]. Longer working sections have limited utility however due to their lower trans‐ portability [47] and require a substantially longer uniform level surface on which to be set [52]. Working section lengths of portable field wind tunnels have varied from 3 m [19, 47] to almost 12 m [38, 39]. Recently, a small circular device named the Portable In-Situ Wind Ero‐ sion Research Laboratory (PI-SWERL) [52] has been used to develop shear stress over a sur‐ face and entrain particles using radially induced rather than linearly induced shear stress.
