Flow Structure, Pressure Fluctuations and Resulting Vibrations Caused by the Interaction of a Turbulent Jet With Cross Flow Over a Flexible Flat Plate

Abstract

The objective of this study is to characterize the velocity, vorticity, wall pressure fluctuations and resulting structural vibrations caused by injection of a round, turbulent jet into a turbulent boundary layer. The experiments are performed in a quiet water channel with back ground noise well below the local pressure fluctuations. One of the channel walls is replaced by a vibration isolated, 1m long, aluminum plate from which the 1cm-diameter jet is injected. The cross flow velocity is fixed at 2 m/s and the velocity ratio, r (ratio of mean jet velocity to the cross flow), varies from 0.5 to 2.5 and Re based on cross flow and jet diameter is 20,000. High-resolution PIV is used to measure the flow field and high sensitivity, low-noise pressure sensors are used for the wall pressure measurements. The flush-mounted transducers are installed at several locations ranging from 2–15 diameters behind the jet. Auto-spectra of the pressure signals show that the effect of the jet is in the 15–100Hz range, and increase the wall pressure levels by 25dB for r=2.5. The fluctuations increase with velocity ratio and decrease with distance from the jet, although there is only a 6dB increase in overall levels at r=2.5 as compared to r=1. Hilbert-Huang “amplitude” spectrum shows the frequency content of the signal as it evolves in time, and is found to be a useful tool to characterize such unsteady phenomena. Velocity and pressure measurements have been performed simultaneously and thousands of frames have been recorded. Analysis of these frames demonstrates the relationship between the wall pressure fluctuations and the vortical structures. Several striking differences in the flow structure between high and low velocity ratios are described in the paper. Acceleration measurements describe the effect of the jet and cross flow on the vibrations of the side-wall. Cross flow boundary layer dominates structural vibrations below 1000Hz, and jet velocity effects are visible at 1000Hz–2000Hz. At higher jet velocities effects are seen even below 1000Hz and large narrow band frequency peaks occur. (CD ROM version includes color figures).