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USDA-ARS Wind Tunnel

A wind tunnel in a field.

Collecting real time soil erosion data presents several problems. One major concern is that data cannot easily be collected from a large number of field sites given the instrument cost and field space required for their operation, i.e., they do not lend themselves to rapid deployment over a wide geographic region. However, analytical requirements mandate erosion data be collected from a large variety of surface conditions and soil types. An easy solution to this problem lies in using a portable wind tunnel in the field that can easily and quickly be moved from site to site and from field to field for detailed evaluations of numerous field surfaces.

Our wind tunnel was designed and built at the USDA/ARS Palouse Conservation Field Station near Pullman, WA. It is 13.4 m long and has a working section 7.3 m long, 1.2 m high and 1.0 m wide. Power is supplied by a 1953 40 hp Ford industrial gas engine which drives a 1.4 m Joy axivane fan. There is a transition from the fan inlet height to the ground level after which the airflow undergoes an intensive flow conditioning. Fan-induced turbulence and swirl are eliminated using 2 perforated plates, a honeycomb and a small mesh screen spaced over a distance of about 2 m. At this point, the flow is laminar and devoid of turbulence.

The natural wind that is being simulated using the wind tunnel contains turbulence. Therefore, it becomes necessary to introduce turbulence back into the airflow. This is accomplished by using a rod-grid placed at the entrance to the working section. The rods are logarithmic-spaced vertically and linearly in the horizontal direction. This grid produces shearing in the flow from the tunnel floor to a distance of about 4 cm below the tunnel roof. Detailed flow information obtained using a hot-film sensor indicates that the shear profile in the lower 1 m of the tunnel is closely matched to shear contained in the lower 1 m of a natural wind. Use of the shear-generating device in the wind tunnel has greatly enhanced its applicability for gathering wind erosion data.

Data are collected over a several minute run using an increasing wind velocity each minute.

Soil erosion and PM10 data are collected 1 m inside the tunnel exit using BSNE samplers at four heights, an isokinetic slot sampler (0.75 m high) connected to a variable speed vacuum and a cyclone separator and two, low volume PM10 samplers at 0.5 and 0.75 m height.

After each test run, the tunnel is moved to the next test plot by disconnecting the outermost working sections and using a 4 m crane, swinging the transition and contraction sections through a hinge-point. The tunnel is then pulled forward to the next plot.