Dependence of pressure losses on angle of attack for flow through perforated plates

We have measured the dependence of pressure losses on angle of attack in pipe flow through a set of seven perforated metal plates. Pressure losses were measured in air flow at temperature and pressure about 24°C and 736 mm Hg. The nominal pipe diameter is 3.5″ resulting an internal pipe diameter of 90.17 mm. Specifically, the plates differ in type of metal, thickness, hole size, and hole spacing. Using a pipe-flow apparatus in which the angle of attack for the plate was set at either 0°, or 45°, or 22.5°, we measured the pressure losses over speeds ranging up to approximately 30 m/s. Generally, the pressure loss increases linearly with the square of the air speed; and, in this context, the ratio of pressure loss to air speed squared represents a classical loss coefficient, K, for perforated plates. For a specific air speed, the magnitude of the loss coefficients correlates positively with the ratio of the distance between hole centers to the hole diameter, s/D. The geometric effects associated with changing the angle of attack are complex, and a single dimensionless ratio such as s/D provides insufficient information to describe this effect. However, the measurements do indicate that the loss coefficient K depends on the ratio, s/D, raised to a power of approximately 4.8. Flow visualization may provide some more insight into the specific physical phenomena responsible for these pressure losses.