Kevin J. Bastyr

Development of a velocity gradient underwater acoustic intensity sensor

A neutrally buoyant, underwater acoustic intensity probe is constructed and tested. This sensor measures the acoustic particle velocity at two closely spaced locations, hence it is denoted a “u-u” intensity probe. A new theoretical derivation infers the acoustic pressure from this one-dimensional velocity gradient, permitting the computation of one component of acoustic intensity. A calibration device, which produces a planar standing-wave field, is constructed and tested. In this calibrator, the performance of the u-u intensity probe compares favorably to that of an acoustic intensity probe which measures both pressure and velocity directly.

High-frequency thermoacoustic-Stirling heat engine demonstration device

A small thermoacoustic-Stirling engine demonstration device that can produce sound in excess of 100 dB at 560 Hz has been constructed. The engine consists of a quarter wavelength acoustic resonator with a smaller diameter coaxial regenerator positioned toward the resonator’s closed end, thereby forming an acoustic feedback path around the regenerator. Acoustic oscillations begin spontaneously when the hot heat exchanger adjoining one end of the regenerator is heated to a sufficient temperature. A water stream in a second heat exchanger maintains the opposite end of the regenerator near ambient temperature. This device was inspired by the Backhaus-Swift engine and is a preliminary step in the investigation of regenerator operation at frequencies much higher than may be practical with mechanical or free-piston Stirling engines.

Sound‐speed determination in a fluid‐filled elastic waveguide

Theory and test methods are presented for determining the longitudinal sound speed inside a fluid‐filled elastic waveguide. The waveguide is a vertically oriented open‐ended column of water equipped with a moving coil driver. The elastic properties of the waveguide reduce the sound speed to the point where a passive anechoic termination can be used effectively. This facilitates a compact device that exhibits plane progressive waves for sensor calibration at low frequencies. Once the sound speed is determined, then the size of the device and usable bandwidth are known. The sound speed is determined through measurement of the longitudinal resonance frequencies associated with the free vibration case of a duct having ideal rigid and pressure‐release boundary conditions. In practice, however, the moving coil driver presents an impedance boundary condition (e.g., not ideally rigid) that inadvertently couples the mechanical system of the driver to the acoustic system of the waveguide. To circumvent this issue, ...

Calibration and testing of a neutrally buoyant u‐u intensity probe

The theory of operation and the methodology for calibration of an underwater acoustic intensity probe are explained. The intensity probe which is classified as a u‐u probe, consists of two moving coil velocity sensors mounted separately in neutrally buoyant, collinearly oriented bodies. This two‐velocity sensor probe is used to determine intensity in a manner analogous to that of pressure gradient hydrophones. The acoustic intensity is shown to be the imaginary part of the cross spectrum measured between the output voltages of the velocity sensors, scaled by a complex, frequency‐dependent calibration constant. Due to the vector nature of intensity, these complex calibration constants contain magnitude and phase information. To establish the calibration constant, an experiment was conducted that employed the u‐u intensity probe and a reference pressure transducer in a standing wave tube. The intensity in the tube is known, and measurements made of this field with the u‐u probe are in agreement with the pre...

The performance of a high‐frequency thermoacoustic‐Stirling engine

A thermoacoustic‐Stirling engine that operates at 400 Hz with a working fluid of 1‐MPa helium is constructed. For proper acoustic phasing in this engine’s regenerator, an acoustic power feedback path exists in the form of an annulus surrounding the regenerator. This feedback path is obtained by suspending an insulated, stainless steel sleeve containing a wire mesh regenerator, which is flanked by two heat exchangers, a short distance from one end of the larger diameter resonator. The ambient heat exchanger is a shell and tube exchanger, while the hot heater consists of nichrome ribbon wound on an aluminum silicate frame. Gedeon streaming is prevented by a diaphragm covering the end of the stainless steel sleeve adjacent to the ambient heat exchanger. A variable acoustic load provides a convenient means of testing this engine at various hot heater temperatures, while operating at different acoustic pressure amplitudes effects the acoustic power generated by the engine. [Work supported by ONR.]

Bias error due to flow‐induced noise on a velocity gradient underwater acoustic intensity sensor

A neutrally buoyant underwater acoustic u‐u intensity probe employs discrete sensors for the direct measurement of acoustic particle velocity at two locations. Active intensity is calculated from the cross spectrum between the two velocity sensors [K. J. Bastyr and G. C. Lauchle, J. Acoust. Soc. Am. 103, 2755(A) (1998)]. Under quiescent conditions the sensor has no bias error; however, in the presence of mean flow, this is no longer true. A derivation of the bias error that results from performing acoustic intensity measurements in the presence of mean flow is presented. Results show that the bias error due to the turbulent boundary layer generated by fluid flow over the cylindrical body that encapsulates the sensors is proportional to the cross spectrum of the turbulent velocities measured by the sensors. This theoretical derivation is quantified with preliminary experimental data obtained by towing the sensor at low speeds through an open channel of water. [Work supported by Office of Naval Research, Mu...