Wall pressure and shear stress measurements beneath an impinging jet

This paper presents comprehensive measurements of wall pressure and surface shear stress beneath a plane, two-dimensional, turbulent jet impinging normally onto a flat surface. The results cover a wider range of Reynolds number and ratio of impingement height (H) to nozzle gap (D) than do previous studies. The pressure distributions are nearly Gaussian, independent of Reynolds number, and closely balance the momentum flux from the jet nozzle as H/D varies. Particular attention was paid to probe size in measuring the wall shear stress because this has a significant effect on the results. A range of Preston tubes and Stanton probes were tested from which it was found that a 0.05-mm-high Stanton probe—the smallest that we could make—appeared to give accurate results. As expected, the shape of the wall shear stress distributions depended both on H/D and on Reynolds number. Furthermore, the relation between wall pressure and shear stress from Hiemenz's theoretical solution for stagnation flow is not in agreement with the results. It is postulated that the discrepancy is due to the relatively high free-stream turbulence level in the jet. Future papers will document the mean flow field and turbulence and the time dependence of the surface pressure.