Investigation on trailing-edge noise sources by means of high-speed PIV

Kurzfassung

The noise generation of turbulent flows over surfaces and around edges of airplanes and automobiles is a general design problem and its importance increases in times of growing traffic in this globalizing world. Turbulent boundary layers near the trailing-edge of a surface are known to generate intense, broadband scattering noise as well as surface pressure fluctuations. The wake of vortices shed depends on the trailing edge thickness and results in the emission of narrow band noise. The noise generating mechanisms depend essentially on the radial components of the velocity fluctuations in respect to the trailing edge. Resulting forces may also cause model vibrations, which are additional low frequency noise sources. In this feasibility study the high-speed PIV (HS-PIV) technique is applied to an investigation of the spatial and temporal development of coherent structures in a turbulent flat-plate boundary layer flow in the vicinity of the trailing-edge (TE). It is expected that with time resolved PIV data obtained in the region immediately up- and downstream of the TE an identification of noise sources and their correlation with the flow structure movement will be possible. On the basis of a large number of time resolved instantaneous velocity vector fields the technique enables also the possibility of determining several statistical quantities of fluid mechanical significance: average velocity profiles, velocity fluctuations (u´, v´), rms-fields, probability density functions and space-time-correlations of the velocity fluctuations and the z-component of vorticity. The far field noise intensity, which is radiated locally at the position of the TE, measured simultaneously with the instantaneous velocity fields by a directional microphone at 80 kHz sample rate. A phenomenological link between noise and flow structure movement by a comparison of the pressure -time -history of the far field with the time-velocity vector fields directly at the noise source is one objective of this investigation. A connection of the velocity fluctuation structures to the far field noise by means of space-time-correlations is a first step towards an identification of the noise generating flow structures.