Comparison of pitot traverses taken at varying distances downstream of obstructions

This study determined the deviations between pitot traverses taken under "i deal" conditions-at least seven duct diameter's lengths (i,e., distance=7D) from obstructions, elbows, junction fittings, and other disturbances to fl ows-with those taken downstream from commonplace disturbances. Two perpendi cular 10-point, log-linear velocity pressure traverses were taken at variou s distances downstream of tested upstream conditions. Upstream conditions i ncluded a plain duct opening, a junction fitting, a single 90 degrees elbow , and two elbows rotated 90 degrees from each other into two orthogonal pla nes. Airflows determined from those values were compared with the values me asured more than 40D downstream of the same obstructions under ideal condit ions. The ideal measurements were taken on three traverse diameters in the same plane separated by 120 degrees in honed drawn-over-mandrel tubing. In all cases the pitot tubes were held in place by devices that effectively el iminated alignment errors and insertion depth errors. Duct velocities range d from 1500 to 4500 ft/min. Results were surprisingly good if one employed two perpendicular traverses. When the averages of two perpendicular travers es was taken, deviations from ideal value were 6% or less even for traverse s taken as close as 2D distance from the upstream disturbances. At 3D dista nce, deviations seldom exceeded 5%. With single diameter traverses, errors seldom exceeded 5% at 6D or more downstream from the disturbance. Interesti ngly, percentage deviations were about the same at high and low velocities. This study demonstrated that two perpendicular pitot traverses can be take n as close as 3D from these disturbances with acceptable (less than or equa l to 5%) deviations from measurements taken under ideal conditions.