**3.5. Fans and blowers**

The fans and blowers employed for wind tunnels are of two primary types. Axial fans (Fig‐ ure 4a) are composed of fixed or adjustable pitched blades arranged radially around the axis of rotation, which is often aligned with the axis of flow through the wind tunnel. Although axial fans are highly efficient at inducing flow, the flow tends to spiral and this problem must be addressed [53] if the flow conditions of Zingg's first criterion are to be met. Centri‐ fugal blowers (Figure 4b) have fixed pitch blades or impellers that are arranged parallel to the axis of rotation at the circumference of a blower cage. The axis rotation is commonly nor‐ mal to the axis of air flow down the wind tunnel. Centrifugal blowers tend to be more flexi‐ ble with respect to design, are more stable and efficient over a variety of flows, and produce less spiraling in the flow than axial fans [53].

Some portable field wind tunnels are too compact for adequate flow conditioning. This shortcoming is very problematic as flow considerations are the most important factor in the successful operation of the wind tunnel [31]. Wind tunnels may not reach true trans‐ port capacity or overshoot true transport capacity if flow conditioning upwind of the working section is inadequate [54] and wind tunnel height may limit the amount of up‐ ward mixing during strong turbulent diffusion [23]. The height of the working section affects the depth of the boundary layer that may be achieved. Upper limits of the Froude number F have been proposed for wind tunnel design of from 10 [55] to 20 [24]. The Froude number is defined by:

$$F = \text{U}^2 / \text{gH} \tag{2}$$

Where *U* is the wind tunnel design wind speed, *g* is the acceleration due to gravity, and *H* is the wind tunnel height. A well developed boundary layer at least 50 cm thick is recom‐ mended to ensure initiation of vertical particle uplift [45]. For this reason, mini-tunnels and micro-tunnels may be too small to allow results that can be scaled up to field scales [56].

**Figure 4.** An axial fan (a) and a centrifugal blower (b) typical of those used in construction of portable field wind tunnels.

### **3.6. Flow conditioning**

[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.

Power sources have ranged from external sources such as the power take-off shaft of a trac‐ tor as input to a transmission that output to drive chains [35, 36], to self-contained direct drive internal combustion engines [24, 25, 28, 31, 38-40], self contained internal combustion engines driving hydraulic pumps to provide for a hydraulic drive motor at the blower [46], and electric motors supplied by portable generators [45, 47]. All these power sources are field tested and reliable. The wind speed may be adjusted by varying the engine or motor

The fans and blowers employed for wind tunnels are of two primary types. Axial fans (Fig‐ ure 4a) are composed of fixed or adjustable pitched blades arranged radially around the axis of rotation, which is often aligned with the axis of flow through the wind tunnel. Although axial fans are highly efficient at inducing flow, the flow tends to spiral and this problem must be addressed [53] if the flow conditions of Zingg's first criterion are to be met. Centri‐ fugal blowers (Figure 4b) have fixed pitch blades or impellers that are arranged parallel to the axis of rotation at the circumference of a blower cage. The axis rotation is commonly nor‐ mal to the axis of air flow down the wind tunnel. Centrifugal blowers tend to be more flexi‐ ble with respect to design, are more stable and efficient over a variety of flows, and produce

Some portable field wind tunnels are too compact for adequate flow conditioning. This shortcoming is very problematic as flow considerations are the most important factor in the successful operation of the wind tunnel [31]. Wind tunnels may not reach true trans‐ port capacity or overshoot true transport capacity if flow conditioning upwind of the working section is inadequate [54] and wind tunnel height may limit the amount of up‐ ward mixing during strong turbulent diffusion [23]. The height of the working section affects the depth of the boundary layer that may be achieved. Upper limits of the Froude number F have been proposed for wind tunnel design of from 10 [55] to 20 [24]. The

Where *U* is the wind tunnel design wind speed, *g* is the acceleration due to gravity, and *H* is the wind tunnel height. A well developed boundary layer at least 50 cm thick is recom‐ mended to ensure initiation of vertical particle uplift [45]. For this reason, mini-tunnels and micro-tunnels may be too small to allow results that can be scaled up to field scales [56].

<sup>2</sup> *F U gH* = / (2)

speed or by changing the pitch of the fan or blower blades.

66 Wind Tunnel Designs and Their Diverse Engineering Applications

less spiraling in the flow than axial fans [53].

Froude number is defined by:

**3.4. Power sources**

**3.5. Fans and blowers**

Flow conditioning sections of various designs have been used to straighten the flow and remove or reduce the scale of eddies in the flow, to initiate a logarithmic wind speed profile and turbulence, and to initiate saltating abrader material into the air flow down the wind tunnels. A typical honeycomb flow straightener with 10 mm screen layers used to create an even logarithmic wind speed profile is presented in Figure 5. If the flow is properly conditioned and the height of the wind tunnel is not limiting the depth of the boundary layer may be estimated from the wind speed profile in the wind tunnel work‐ ing section [46]. Investigators have stated that although boundary layer thickness is a poorly defined concept, it may be estimated as the height at which the wind speed pro‐ file attains 99 percent of its maximum value [57]. Finally, the proper regulation of care‐ fully chosen abrader material allows for saltation clouds representing different rates of erosion and surface abrasion although rates consistent with those noted in the field for natural sand movement [58] are commonly used. Portable field wind tunnel may be used to estimate the threshold wind velocity necessary to initiate particle movement us‐ ing impact sensors [18] or optically based sensors [59]. The technique of using the per‐ centage of seconds in which moving particles are noted [60] is easily employed in a portable wind tunnel if the wind speed can be slowly and evenly increased.

**Figure 5.** A flow conditioner showing the large cell honeycomb used to break the scale of eddies and straighten flow and also the 10 mm screen layers used to even the flow and create a logarithmic wind speed profile in the wind tunnel.
