Experimental Investigation of Flow-Induced Panel Vibrations at Cruise Mach Number

Surface structures exposed to a fluctuating pressure field are excited best, if the pressure field contains characteristics that match the eigenmodes and wave speeds of the structural vibration. A pressure field can be caused either by acoustical sources, or a turbulent boundary layer, or both. The aim of this paper is the experimental determination of the excitation characteristic of the boundary layer and the corresponding vibration response of an underlying structure under like conditions. The motivation behind this is to obtain experimentally a joint model for the excitation and the related response of the vibration. Individual measurements of the vibration response of a generic aircraft panel exposed to the pressure field of a turbulent boundary layer have been conducted in the past. Individual characterization of a field of fluctuating pressure on a surface has been investigated as well. However, measurements of the excitation characteristics of the turbulent boundary layer of a generic aircraft panel at the same test setup are scarce. The excitation characteristics of the turbulent boundary layer have been measured individually in several speed ranges. A wavenumber decomposition is oftentimes used to display the features of the surface pressure. In the low-speed range, Arguillat^1 detected the convective propagation on a at plate in the wavenumber domain. Smith^2 used a generic setup of a at plate and a generic car side mirror in order to find turbulenceinduced acoustic waves propagating over the surface. These acoustic waves were believed to be responsible for a part of the excitation of the surface structure. In the high speed measurements by Ehrenfried & Koop^3 in the wind tunnel and Haxter & Spehr^4 in a flight test, the convective velocity of the turbulent structures in the boundary layer were considered to be the main cause for excitation of the surface structure. At high speeds, the modes of the structure match the convective speed of the turbulent vortices in the boundary layer, which is called aerodynamic coincidence. The vibration of airplane fuselage subject to excitation by a turbulent boundary layer has been measured by Wilby and Gloyna.^5 In their measurement, accelerometers were placed in lengthwise and streamwise direction on panels, stringers, and frames of a Boeing model 737 aircraft. The distribution was carried out with large distances in between the sensors in order to find correlations in between adjacent panels and structure