Vibroacoustics of Uniform Structures in Mean Flow

Abstract

AbstractThis chapter is organized as three parts: in the first part, an analytic approach is formulated to account for the effects of mean flow on sound transmission across a simply supported rectangular aeroelastic panel. The application of the convected wave equation and the displacement continuity condition at the fluid-panel interfaces ensures the exact handling of the complex aeroelastic coupling between panel vibration and fluid disturbances. To explore the mean flow effects on sound transmission, three different cases (i.e., mean flow on incident side only, on radiating side only, and on both sides) are separately considered in terms of refraction angular relations and sound transmission loss (STL) plots. Obtained results show that the influence of the incident side mean flow upon sound penetration is significantly different from that of the transmitted side mean flow. The contour plot of refraction angle versus incident angle for the case when the mean flow is on the transmitted side is just a reverse of that when the mean flow is on the incident side. The aerodynamic damping effects on the transmission of sound are well captured by plotting the STL as a function of frequency for varying Mach numbers. However, as the Mach number is increased, the coincidence dip frequency increases when the flow is on the incident side but remains unchanged when in the flow is on the radiating side. In the most general case when the fluids on both sides of the panel are convecting, the refraction angular relations are significantly different from those when the fluid on one side of the panel is moving and that on the other side is at rest.In the second part, the transmission of external jet noise through a double-leaf skin plate of aircraft cabin fuselage in the presence of external mean flow is analytically studied. An aero-acoustic-elastic theoretical model is developed and applied to calculate the sound transmission loss (STL) versus frequency curves. Four different types of acoustic phenomenon (i.e., the mass-air-mass resonance, the standing-wave attenuation, the standing-wave resonance, and the coincidence resonance) for a flat double-leaf plate as well as the ring frequency resonance for a curved double-leaf plate are identified. Independent of the proposed theoretical model, simple closed-form formulae for the natural frequencies associated with the above acoustic phenomena are derived using physical principles. Excellent agreement between the model predictions and the closed-form formulae is achieved. Systematic parametric investigation with the model demonstrates that the presence of the mean flow as well as the sound incidence angles affects substantially the sound transmission behavior of the double-leaf structure. The influences of panel curvature together with cabin internal pressure on jet-noise transmission are also significant and should be taken into account when designing aircraft cabin fuselages.In the third part, a theoretical model is developed to investigate the influence of external mean flow on the transmission of sound through an infinite double-leaf panel filled with porous sound absorptive materials. The sound transmission process in the porous material is modeled using the method of equivalent fluid-structure coupling conditions that are accounted for to ensure displacement continuity at fluid-structure interfaces. Analytic solutions for the sound transmission loss of the whole structure are obtained. For validation, the model predictions are compared with existing experimental results. Numerical investigations with the model are subsequently performed to quantify how a set of systematic parameters affect the sound transmission loss. It is demonstrated that the porous material affects the STL curve in terms of both the absorption effect and the damping effect. Besides, the material loss factor and the thickness of the faceplates also have an influence on the coincidence dip of the STL curve. At frequencies below the coincidence frequency, the external mean flow increases the STL values due to the added damping effect of the mean flow while shifting the coincidence frequency upward because of the refraction effect of the mean flow. In addition, the coincidence frequency decreases with increasing azimuth angle between the sound incident direction and mean flow direction.KeywordsExternal Mean FlowSound Transmission Loss (STL)Coincidence DipResonance CoincidencePanel CurvatureThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.