No abstract

We present a wave transmission line model developed to understand the transmission of energy through fluid-filled piping systems. The piping systems are represented as a sequence of components, e.g. valves, bends, and other components, connected with straight pipe sections. The transmission line model makes use of experimentally- or analytically-determined scattering coefficients to represent component behavior. The coefficients capture important coupling between fluid and structure, and among different structural wave types. The measurement of these coefficients is the subject of a separate paper [1]. The straight pipe segments are modeled analytically using fluid-filled, thick shell theory. Their motion is described in terms of amplitudes of freely traveling, left-and-right propagating waves. Results are presented which compare transfer functions measured on a piping system to predictions from the transmission line model, where each component is modeled with experimentally determined scattering coefficients. Initial results highlight important issues regarding the use of reciprocity, passivity, and causality to improve the quality of coefficients which are difficult to measure (for example, where certain frequency bands had high signal to noise). Algorithms for determining whether measured coefficients meet constraints on passivity, reciprocity, and causality are introduced. Predictions comparing analytical and measured coefficients are shown for a single-component (elbow) piping system.

The acceleration to force transfer functions across a shaker-excited thin band under tension with a deep notch cut through its center is examined up to the point of fatigue-induced failure. An analytical model is developed to track the changes in modal properties evident in the experimental results as a function of band parameters. The progression of the structure toward failure is documented by plots of rotational stiffness and crack length as a function of fatigue time.

Computational verification of the Crighton and Oswell predictions for the response of a line excited, doubly infinite, submerged plate in mean flow are presented. The phenomena highlighted for validation include: the occurrence of absolute instability above a critical flow speed, the presence of convectively unstable waves downstream from the driver for all flow velocities below this critical speed, and the existence of anomalous propagating waves with group velocities pointed towards the driver over a narrow range of excitation frequencies. Extensions of the Crighton and Oswell model to include the effects of compressibility in the fluid, finite thickness in the structure and the presence of compliant coatings are discussed. Extensions of this model to three dimensions are also presented through analytical studies and numerical simulations of instability waves on a point excited 2-D plate submerged in a 3-D fluid in mean flow. [Work supported by ONR and St. John College, Cambridge.]

The contribution of enclosure-borne sound to a loudspeaker’s total radiated sound was determined indirectly by a combination of analytical and experimental techniques. The transfer functions that relate a loudspeaker enclosure’s average vibratory response both to point force excitation and to external acoustic excitation were determined experimentally for several different enclosure prototypes. A reciprocity relation that equates the radiated pressure to force transfer function, which was not measured directly, to the acoustically induced vibrational velocity to point source volume velocity transfer function, determined experimentally, was used in evaluating the loudspeaker enclosure radiation resistance. Finally, enclosure-borne radiated sound power, as derived from measured enclosure vibrational velocity, was related to radiated sound power due to rigid body loudspeaker cone vibration. Some of the enclosures exhibited certain structural modes which, through these studies, were identified as efficient sound radiators, and as such, candidates for further study in order to limit their radiated output.