Measuring airspeed is going to require a pitot-tube to perform the measurement. Unfortunately, commercial pitot tubes are beyond my price range. Commercial pressure gauges, on the other hand, are significantly cheaper. A pitot-tube attachment can be used to gather the dynamic and static pressures, then a pair of pressure gauges can be used to gather the difference between ambiant and static pressure, as well as ambiant and dynamic pressure. Using ambiant pressure (possibly measured by a third barometer) in conjunction with temperature (observable by a thermometer), and the (now calculable) values of static and dynamic pressure, a reasonable value for air density can be found, resulting in an easy calculation of airspeed. Because the tunnel is decidedly low-speed, the effects of compressibility will be less than 5% and should be inconsequential. [Anderson, 175]
For a max test-section speed of 100 m/s, the max dynamic pressure would be ~25 inches of water, making the 30" pressure-gauge a possibility.
The plans call for two screens (whose loss-coefficients will be tuned). The first screen will be hemispherical with a low loss-coefficient to help prevent boundary-layer separation throughout most of the diffuser. The second screen will be flat with a possibly larger loss-coefficient and will help serve to stabilie the turbulence in the flow before entering the settling chamber. Many of the calculations and decisions in the design of this component were drawn from the information in [Mehta]
The choice of angle is, perhaps, 5-10 degrees too adverse, but seems a decent starting point. If the diffuser fails to perform satisfactorally, 2-theta can be dropped to ~30-degrees where a cluster of successful one-screen diffusers have been built. The hope is that the more adverse angle can be compensated-for by use of an additional screen.[Mehta, 69]
Corner fillets help to increase the flow quality in the test-section by making the boundary layer more uniform. [citation-needed] Given the already crude construction and design of this tunnel, I feel it prudent to accept any advantage I can.
The walls of the test-section will employ the use of either glass or plastic on all walls to allow for 360-degree visibility of the test-section. Barlow et al. recommend that the walls of the test-section fan out at ~.5-degrees per wall to negate the nozzling effects of the thickening boundary layer -- this technique will likely be employed.