Investigation of the modulated turbulence above bias flow liner perforations with laser-optical measurements

Abstract

The investigation aims at the sound-turbulent interaction in the turbulent shear layers of slit-liners with bias ow, grazing ow, and with mixed con�gurations. Measurements of the 2D time resolved total velocity �eld above the slits are conducted by means of the laser-optical FM-DGV sensor. The measurement system simultaneously delivers the three spatial velocity components which have been correlated in order to evaluate the full turbulent stress tensor. Both the temporal average as well as the coherent sound-induced oscillation of the stress components have been determined. The coherent shear stresses in combination with the gradients of the coherent velocities describe the power transfer between the turbulence and the sound-induced coherent �eld. This power transfer has been analyzed within the near-�eld of the slit for a number of 42 operating conditions including various combinations of bias and grazing mean ow, various sound frequencies, and various dimensions of the slit perforate. The investigation was aimed, among others, at the signi�cance of the power transfer with regard to the acoustical e�ciency of the liner. The latter has been measured separately by microphones in the hard walled sections of the ow duct. In the majority of the various conditions appreciable transfer of energy from the coherent �eld into the turbulence has been observed, which seems to support the signi�cance of the energy transfer. However, a convincing correlation between the total sound dissipation of the liner and the energy transfer has not been found.