Acoustic Energy Production Research Articles

Vortex dynamics and acoustic sources in the wake of finned cylinders during resonance excitation

The flow–sound interaction mechanism and its effect on the vortex dynamics in the wake of circular finned cylinders are experimentally investigated using phase-locked particle image velocimetry at Reynolds numbers between 7 × 104 and 9.5 × 104. In addition, a hybrid experimental–numerical technique using the theory of vortex sound is employed to quantify the acoustic sources and sinks in the vicinity of finned cylinders with different fin-to-root diameter ratios, Df/Dr = 1.5, 2.0, and 2.5. The results show that changing the diameter ratio of the fins induces fundamental changes in the wake structure and the vortex shedding process downstream of the cylinder. Finned cylinders induce stronger vortex cores with a shorter formation length compared to their equivalent bare cylinders. Moreover, the flow topology over the spanwise direction shows that acoustic resonance results in uniform cylindrical vortex cores with less three-dimensional distortion, which demonstrates that the flow field becomes highly two-dimensional during resonance excitation. Quantification of the energy transfer between the flow and the sound fields reveals an enhancement in the acoustic energy production closer to the cylinder with a significant dependence on its fin-to-root diameter ratio.

The Morávka meteorite fall: 2. Interpretation of infrasonic and seismic data

Abstract— The sound production from the Morávka fireball has been examined in detail making use of infrasound and seismic data. A detailed analysis of the production and propagation of sonic waves during the atmospheric entry of the Morávka meteoroid demonstrates that the acoustic energy was produced both by the hypersonic flight of the meteoroid (producing a cylindrical blast wave) and by individual fragmentation events of the meteoroid, which acted as small explosions (producing quasispherical shock waves). The deviation of the ray normals for the fragmentation events was found to be as much as 30° beyond that expected from a purely cylindrical line source blast. The main fragmentation of the bolide was confined to heights above 30 km with a possible maximum in acoustic energy production near 38 km. Seismic stations recorded both the direct arrival of the airwaves (the strongest signal) as well as air‐coupled P‐waves and Rayleigh waves (earlier signals). In addition, deep underground stations detected the seismic signature of the fireball. The seismic data alone permit reconstruction of the fireball trajectory to a precision on the order of a few degrees. The velocity of the meteoroid is much less well‐determined by these seismic data. The more distant infrasonic station detected 3 distinct signals from the fireball, identified as a thermospheric return, a stratospheric return, and an unusual mode propagating through the stratosphere horizontally and then leaking to the receiver.

Absorption of sound at a slot in a splitter plate in a mean-flow duct

Perforated screens are often deployed to attenuate aerodynamic sound in heat-exchanger cavities and other ducts conveying mean flow. The dissipation is caused by vorticity production in the perforations. This mechanism is investigated theoretically for the case of a thin rigid plate lying along the centreline of a duct and having a single transverse slot, a configuration that is to be studied experimentally at the Institute of Sound and Vibration Research of Southampton University. Time-harmonic acoustic waves are incident on the slot in the presence of equal parallel mean flows on either side of the plate. A linearized theory of unsteady shearing flow over a slot, which incorporates the influence of vorticity ejection into the flow, is used to examine mean-flow/acoustic energy exchanges. According to this theory acoustic energy is absorbed provided that the Strouhal number based on slot width and mean-flow velocity is sufficiently small. At higher frequencies there exists an infinite set of discrete frequency intervals within which there is a net production of acoustic energy at the slot at the expense of the kinetic energy of the mean flow.

On the production and absorption of sound by lossless liners in the presence of mean flow

The interaction of sound with the leading and trailing edges of a nominally lossless acoustic liner is examined in the presence of mean flow. It is argued that acoustic energy is conserved only if the whole flow is irrotational. In practice the sound induces vorticity production at the liner, and in particular that generated at the edges of the liner leads to a net transfer of energy between the acoustic field and the mean flow. Analytical results are given for a “pressure release” liner in a rigid plane wall, it being concluded that there is a net production of acoustic energy at a trailing edge for all subsonic mean flow Mach numbers M. At a leading edge there is a net production or absorption of sound, depending on both the direction of propagation of the incident wave and the value of M.

Nonlinear resonant interactions in internal cavity flows

F the suggestion of Flandro and Jacobs that periodic vortex shedding from internal obstructions could produce significant oscillating pressure levels in turbulent duct flow, Culick and Magiawala, Dunlop and Brown, and Brown et at. presented a variety of results verifying the phenomenon of acoustic energy production by vortex shedding and interaction. In further investigation of some of the flow mechanisms involved in acoustic energy production by vortex shedding, Isaacson and Marshall presented measurements of oscillating velocity frequency spectra which indicated that in certain flow regimes, velocity'oscillations occur which satisfy the frequency resonant interaction. This Note presents results of bispectral cross-power measurements of trios of frequencies occurring in an internal cavity in a turbulent flow which provide additional evidence that these trios of frequencies are of quadratic interaction origin. These measurements were made in a subsonic wind tunnel which is driven by a centrifugal fan, driving air into a settling chamber which is 0.61 m on each side and 1.22 m long. The settling chamber is segmented and contains four damping screens, which act to reduce the turbulence of the flow into the test section. A converging section connects the settling chamber to the test section which is 0.178 m on each side and 2.44m long. The internal flow cavity is made up of four plexiglas restrictors, with the forward set of restrictors located 1.89 m downstream of the entrance to the test section. A schematic diagram of the flow cavity is shown in Fig. 1. For the measurements reported here, the gap between the forward pair of restrictors was 53.3 mm and the distance between the two pairs of restrictors was 101.6 mm. A TSI Model 1050 constant-temperature hot-film anemometer, together with a TSI Model 1055 linearizer, and a TSI Model 1057 signal conditioner were used for these measurements. The single sensor, located a distance of 16 mm behind the forward restrictor pair along .the flow axis of zero vertical velocity, was used to measure the velocity frequency spectrum presented in Fig. 2. The output of this sensor was also used to generate the bispectral measurements indicated in Fig. 3. The frequency spectra were obtained with a HewlettPackard Model 3590A wave analyzer and an HP Model 3594A sweeping local oscillator. The analog output of the analyzer was digitally processed with, a Hewlett-Packard Model 2240A measurement and control processor, with an HP-85 computer as the controller. The data were plotted on a Hewlett-Packard Model 7225A plotter. Note that the cavity configuration used in this experiment produces two converging, almost parallel, shear layers at the forward baffles. Kelly has shown that shear layers of this type produce instabilities of both subharmonic and superharmonic frequencies relative to the most unstable frequency determined by the Orr-Sommerfeld equations. Phillips has shown that waves of this type can be turned away from the critical region of the shear layer and

Experimental Method to Measure Low-Frequency Sound Radiation—Nearfield Acoustical Holography

Abstract : A technique called Nearfield Acoustical Holography (NAH) has been developed. The technique, a significant improvement over conventional holography, is not subject to the wavelength resolution limitation associated with conventional techniques. An exact formulation of the Green's function propagator allows reconstructions of the entire pressure, particle velocity, and vector intensity fields. The vector intensity may be plotted to show the flow of acoustic energy around and away from a source. On some sources, these mappings reveal areas of non-radiating, circulating energy flow. These areas may have large pressure or velocity amplitudes and negative intensity regions that previously led to misconceptions about the production of acoustic energy.

Acoustic energy in non-uniform flows

The concept of acoustic energy is extended to non-uniform fluid flows. In general, the resulting energy balance equation exhibits acoustic energy production or dissipation within the flow, but in an important special case the production term is zero, implying conservation of acoustic energy. The acoustic energy equations used in classical and geometric acoustics are recovered from the general formulation by making the appropriate assumptions.