Sound sources of low‐speed jet impingement on a large flat plate

Abstract

Jet impingement on a large flat normal plate produces a noisy turbulent divergent wall jet. As in the more general case, the image flow in the plate can be postulated and the “three sound‐pressures theorem” applied (neglecting dissipation effects): For long wavelengths (“compact” flow), the aerodynamic sound sources must be reducible to lateral quadrupoles alone, with maxima normal to and in that plane. Additional (usually weak ??) shear dipoles associated with the viscous traction forces lie in the plane of the plate. Specifically, the large scale axisymmetric unsteady flow associated with the flow resonances that sometimes occur resembles a periodic series of ring vortices that approach the plate and then spread out, symmetrically. Regardless of the details of this symmetrical unsteady flow, the associated source field must be periodic annular distributions of the lateral quadrupoles and traction dipoles; in the long‐wavelength (compact) low‐speed case these degenerate into symmetrical point quadrupoles, one lateral and one longitudinal, yielding “U8 laws” for simple similarity. Dissipation results in “U8‐type” monopoles due to heat addition fluctuations.