Identification of Boundary Pressure Field Exciting a Plate Under Turbulent Flow

Abstract

The characterisation of the aeroacoustic wall pressure field generated by turbulent flow is a difficult task that often requires instrumented panels and huge facilities like wind tunnels. In situ and non-intrusive experiments are rather not possible. In addition, the pressure field is dominated by the aerodynamic component and the experimental dynamics are not sufficient to measure correctly spectra in low wavenumbers by microphones. The present chapter deals with such a separation method by using the Force Analysis Technique (FAT). FAT is based on the use of the equation of motion of the structure (here a plate) and on the approximation of the fourth derivatives by a finite difference scheme. In the case of turbulent flow, the force auto-spectrum can be deduced at one point of the structure by measuring the velocity at 13 points synchronously. To this purpose, an array of 13 pU (acoustic pressure/particle velocity) probes has been made up. This array is moved in the near-field of the plate to identify map of the wall pressure level applied on the surface of the plate. In the present application, it is shown that FAT only identifies the component of the excitation with wavenumber lower than the natural flexural wavenumber of the plate, due to filtering effect of the plate and of the finite difference scheme. In most cases, the convective peak is then canceled and only the acoustic part of the turbulent flow is identified. This property can be of great interest for vehicle manufacturers to quantify the part of the wall pressure that is responsible of the radiated noise or to use FAT as a source separation technique.