Aeroacoustics of the Vortex Ring

Development of a test rig for the investigation of flow-induced mechanisms of sound generation at radial fans

Two-dimensional interaction between two Taylor vortices is simulated systematically through solving the two-dimensional, unsteady compressible Navier-Stokes equations using a fifth order weighted essentially nonoscillatory finite difference scheme. The main purpose of this study is to reveal the mechanism of sound generation in two-dimensional interaction of two Taylor vortices. Based on an extensive parameter study on the evolution of the vorticity field, we classify the interaction of two Taylor vortices into four types. The first type is the interaction of two counter-rotating vortices with similar strengths. The second type is the interaction of two co-rotating vortices without merging. The third type is the merging of two co-rotating vortices. The fourth type is the interaction of two vortices with a large difference in their strengths or scales. The mechanism of sound generation is analyzed.

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BackgroundMale parasitic wasps attract females with a courtship song produced by rapid wing fanning. Songs have been described for several parasitic wasp species; however, beyond association with wing fanning, the mechanism of sound generation has not been examined. We characterized the male courtship song of Cotesia congregata (Hymenoptera: Braconidae) and investigated the biomechanics of sound production.Methods and Principal FindingsCourtship songs were recorded using high-speed videography (2,000 fps) and audio recordings. The song consists of a long duration amplitude-modulated “buzz” followed by a series of pulsatile higher amplitude “boings,” each decaying into a terminal buzz followed by a short inter-boing pause while wings are stationary. Boings have higher amplitude and lower frequency than buzz components. The lower frequency of the boing sound is due to greater wing displacement. The power spectrum is a harmonic series dominated by wing repetition rate ∼220 Hz, but the sound waveform indicates a higher frequency resonance ∼5 kHz. Sound is not generated by the wings contacting each other, the substrate, or the abdomen. The abdomen is elevated during the first several wing cycles of the boing, but its position is unrelated to sound amplitude. Unlike most sounds generated by volume velocity, the boing is generated at the termination of the wing down stroke when displacement is maximal and wing velocity is zero. Calculation indicates a low Reynolds number of ∼1000.Conclusions and SignificanceAcoustic pressure is proportional to velocity for typical sound sources. Our finding that the boing sound was generated at maximal wing displacement coincident with cessation of wing motion indicates that it is caused by acceleration of the wing tips, consistent with a dipole source. The low Reynolds number requires a high wing flap rate for flight and predisposes wings of small insects for sound production.

Large-scale, dust-trapping vortices may account for observations of asymmetric protoplanetary discs. Disc vortices are also potential sites for accelerated planetesimal formation by concentrating dust grains. However, in 3D discs vortices are subject to destructive `elliptic instabilities', which reduces their viability as dust traps. The survival of vortices in 3D accretion discs is thus an important issue to address. In this work, we perform shearing box simulations to show that disc self-gravity enhances the survival of 3D vortices, even when self-gravity is weak in the classic sense (e.g. with a Toomre $Q\simeq5$). We find a 3D, self-gravitating vortex can grow on secular timescales in spite of the elliptic instability. The vortex aspect-ratio decreases as it strengthens, which feeds the elliptic instability. The result is a 3D vortex with a turbulent core that persists for $\sim 10^{3}$ orbits. We find when gravitational and hydrodynamic stresses become comparable, the vortex may undergo episodic bursts, which we interpret as interaction between elliptic and gravitational instabilities. We estimate the distribution of dust particles in self-gravitating, turbulent vortices. Our results suggest large-scale vortices in protoplanetary discs are more easily observed at large radii.

By solving time-dependent Navier-Stokes equations, the high-order accuracy flow field was obtained by direct numerical simulation of temporal mixing layer, and the sound generation mechanism was studied. The distortion characteristics of two-dimensional Gaussian pulse propagating through temporal mixing layer was preliminarily studied. We found that the various degrees interaction between acoustic wave and the vortex structure in the mixing layer results in the distortion of acoustic wave.

Techniques of scanning acoustic microscopy generally rely on local variations of such solid state parameters influencing generation or propagation of acoustic waves. Depending on the manner of impressing acoustic waves into the sample various methods are distinguished. In conventional scanning acoustic microscopy ultrasound is generated by a lens-transducer arrangement outside the sample and focussed onto or below its surface. Changes in the propagation of this ultrasound wave, like absorption and reflexion or temporal propagation delays, enable analysis of the mechanical or elastic response. At very high frequencies and with additional time-resolving detection techniques applications of this technique to surface analysis become possible. Other scanning acoustic microscopes imply the generation of sound or ultrasound directly within the sample itself due to the impact of temporarily modulated particle or photon beams. These are presently laser, electron, or ion beams. With these methods the acoustic signal as detected by a transducer attached to the sample is on principle affected by propagation properties, too, but it is dominated by local changes of the generation process for the acoustic wave, mainly because the frequency ranges used presently are associated with very long acoustic wavelengths. Depending on the physical nature of the primary probe used many sound generation mechanisms are given resulting in a large amount of different applications. By adjusting the probe parameters in a suitable manner the sound generation process can be confined to the direct vicinity of the specimen surface, which makes this technique feasible for surface characterization. The principles of the various techniques are described, and their usability for surface analysis is discussed.

This article presents an extension of Liepmann’s characterization of an aeroacoustic source in terms of the motion of a bounding surface containing the source region. Rather than using an arbitrary surface, we express the problem in terms of bounding material surfaces, identified by Lagrangian coherent structures (LCSs), which demarcate flow into regions with distinct dynamics. The sound generation of the flow is written in terms of the motion of these material surfaces using the Kirchhoff integral equation, so that the flow noise problem now appears like that of a deforming body. This approach provides a natural connection between the flow topology, as revealed through LCS analysis, and sound generation mechanisms. As examples, we examine two-dimensional cases of co-rotating vortices and leap-frogging vortex pairs and compare estimated sound sources to vortex sound theory.

We have developed a novel osteosynthesis plate with bone union detection using a wire's natural frequency (BUDWF) to provide the quantitative result of bone union detection. The concept for detecting bone union is measuring the rate of frequency change. The frequency is measured from sound generated from the wire attached to a modified plate. The plate is modified from a Syncera ADLER B0409.10 and attached with 0.3 mm diameter 316L stainless steel wire. The sound generation mechanism was created by PEEK and installed on the plate to generate the sound. The preliminary experiments were conducted on a Sawbones tibia composite mimic. We used the cut Sawbones to create fracture samples with a 0, 0.5, 1-, 2-, and 5-mm gap representing the fractured bone with different gap sizes and prepared uncut Sawbones as a union sample. These samples were tested five times, and the sound was recorded from a condenser microphone and analyzed. We found that the BUDWF can differentiate samples with a fracture gap above 2 mm from the union sample, as the differences in the rates of frequency change between samples with a fracture gap above 2 mm and union samples were statistically significant. However, there was a limitation that the BUDWF plate was still unable to differentiate the 0 mm fracture gap and the union sample in this study.

The audible noise (AN) of DC transmission lines is a crucial factor affecting environmental assessment and electromagnetic design. In recent years, the public has paid increasing attention to the audible noise after the emergence of the HVDC technique. This paper emphatically reviewed the work investigating audible noise characteristics in both time and frequency domains, especially the correlation between sound and discharge. The proposed mechanisms of DC audible noise and the mitigating techniques are summarized, including the noise generation process, the physical models, the measurement method, and the mitigating technologies. It is found that the existing literature mainly focused on the prediction and characteristics of AN, the environmental factors which influence AN, and the methods to minimize AN emission. However, existing achievements still need to be improved to fully understand the mechanism of AN generation and solve the adaptability problem of AN prediction methods. The following aspects are valuable in future research: The correlation between audible noise and other corona effects will help solve the problem of difficulty in measuring audible noise in field condition; The corona discharge mechanism and weather resistance anti-corona coating when raindrops are attached to the surface of the transmission line, which will help guide the development and application of anti-corona coatings for the transmission line. Future research should also understand the mechanism of sound wave generation when considering the space charge effect.

The paper delves into the detailed sound generation and propagation mechanisms associated with tripping-induced flow perturbations and trailing-edge scattering using wall-resolved large-eddy simulations. Two distinct boundary-layer tripping techniques, namely a geometrically resolved stair strip and an artificially modeled trip using suction and blowing, are investigated. To facilitate comparison, the natural boundary-layer transition is also simulated as a baseline scenario. The analysis takes into account a Reynolds number of 4×105, a Mach number of 0.058, and a nonzero angle of attack of 6.25° over a NACA 0012 airfoil configuration. The mechanisms for sound generation and propagation related to trailing-edge noise remain consistent across the three transition scenarios. However, boundary-layer tripping notably leads to intricate, scenario-specific noise generation: there is an interaction between the laminar separation bubble and tripping for the stair strip case, whereas laminar boundary-layer instability is evident for the suction and blowing scenario. Aerodynamic flowfields involving acoustic noise sources, their propagating natures near the wall, and far-field acoustics are cross-examined in detail. The comprehensive analysis of observed phenomena provides valuable insights for understanding nonlinear flow and acoustic interactions relevant to airfoil noise and designing new types of trips under adverse pressure gradient flows.

The influence of temperature, size and season on the sounds produced by the sonic muscles of the weakfish Cynoscion regalis are categorized and used to formulate a hypothesis about the mechanism of sound generation by the sonic muscle and swimbladder. Sounds produced by male weakfish occur at the time and location of spawning and have been observed in courtship in captivity. Each call includes a series of 6-10 sound pulses, and each pulse expresses a damped, 2-3 cycle acoustic waveform generated by single simultaneous twitches of the bilateral sonic muscles. The sonic muscles triple in mass during the spawning season, and this hypertrophy is initiated by rising testosterone levels that trigger increases in myofibrillar and sarcoplasmic cross-sectional area of sonic muscle fibers. In response to increasing temperature, sound pressure level (SPL), dominant frequency and repetition rate increase, and pulse duration decreases. Likewise, SPL and pulse duration increase and dominant frequency decreases with fish size. Changes in acoustic parameters with fish size suggest the possibility that drumming sounds act as an 'honest' signal of male fitness during courtship. These parameters also correlate with seasonally increasing sonic muscle mass. We hypothesize that sonic muscle twitch duration rather than the resonant frequency of the swimbladder determines dominant frequency. The brief (3.5 ms), rapidly decaying acoustic pulses reflect a low-Q, broadly tuned resonator, suggesting that dominant frequency is determined by the forced response of the swimbladder to sonic muscle contractions. The changing dominant frequency with temperature in fish of the same size further suggests that frequency is not determined by the natural frequency of the bladder because temperature is unlikely to affect resonance. Finally, dominant frequency correlates with pulse duration (reflecting muscle twitch duration), and the inverse of the period of the second cycle of acoustic energy approximates the recorded frequency. This paper demonstrates for the first time that the dominant frequency of a fish sound produced by a single muscle twitch is apparently determined by the velocity of the muscle twitch rather than the natural frequency of the swimbladder.

The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered (125 Hz). The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.

The sound emission of low-pressure axial fans is substantially influenced by the fan blade geometry and the inflow conditions, induced by the fan installation system. However, the combined impact of these parameters has not yet been comprehensively investigated or understood. Hence the motivation for this thesis was to undertake a compact, systematic experimental study on the sound emission of axial fans with different blade geometry parameters under distorted inflow conditions. The impact of the fan blade design was investigated on the basis of nine fans with different blade loading distributions and different fan blade skew, but otherwise identical geometric parameters. Additionally, the effectiveness of leading edge serrations in reducing axial fan noise was examined with a parametric study of a generic flat-plate fan and eleven sets of fan blades with leading edge modifications that included single-sine (sinusoidal), double-sine and random amplitude leading edge serrations. The inflow conditions were altered to incorporate either an increased inflow turbulence intensity or an inhomogeneous inlet velocity profile. These conditions were realised by grids - three turbulence grids and two velocity gradient grids - that were mounted upstream of the fans. A large increase in the sound radiation with elevated tonal and broadband components was observed with the modified inflow conditions. Thereby, the sound emission of the forward-skewed fans showed a greater susceptibility for distorted inflow conditions than that of the backward-skewed fans. It was found that owing to the fan blade shape, the forward-skewed fans are prone to higher unsteady blade forces and increased pressure fluctuations on the fan blade leading edges. These factors determine the tonal and broadband sound radiation. For the backward-skewed and unskewed fans, dominant subharmonic components, originating from flow phenomena in the tip region, occurred under both free and distorted inflow conditions. All types of leading edge serrations achieved a sound reduction compared with the reference fan with straight leading edges - for both free and distorted inflow conditions. The greatest reduction was observed for the sinusoidal leading edges, followed by the double-sine and random amplitude leading edges. The reduction was mainly dependent on the serration wavelength rather than the serration amplitude: the smaller the leading edge wavelength, the greater was the overall sound reduction. Overall, the investigations showed that the sound emission of low-pressure axial fans is highly dependent on the combined impact of the fan blade geometry and the inflow conditions. The findings contribute to a better understanding of the sound generation mechanisms in axial fans and show means for designing low-noise fans in complex cooling or ventilation systems.

Attention was focused on the interaction between vortex and cavitation as a basic study on modeling unsteady turbulent flows with cavitation. First, separated shear layer with cavitation were directly simulated. Cavitation caused by spanwise and streamwise vortices, which are typical features in high shear layer, was represented by a simple model. Experimentally observed tendencies in the location of vortex formation, the frequency of vortex shedding and the intensity of Reynolds stresses were reasonably reproduced. Influence of cavitation on a typical example of streamwise vortex was observed. It was found that the vorticity in the core of Burgers type vortex was significantly reduced. To establish a description of it, we used an artificially maintained Burgers vortex. As a result, an assumption of constant circulation along a closed circle expanding due to sudden development of cavity could represent the modification of the vortex. INTRODUCTION Flows in hydro-machineries are affected by various types of cavitation. Since 1990’s, several methods have been proposed for the numerical simulation of flow fields including unsteady cavitation. Attached (sheet) cavitation and cloud (bubble) cavitation have been reasonably reproduced. However, the influence of turbulence has not been taken into account completely in previous simulations although most of cavitating flows are turbulent. This causes inaccuracy in predicting the cavitation inception because the local minimum of pressure is thought to be corresponding to the core of turbulence vortices. On the other hand, the effect of cavitation on turbulence has usually been omitted. Therefore, to establish the computational method for turbulent cavitating flows, the interaction between cavitation and turbulence vortices should be correctly modeled. The aim of our study is to address the modeling strategies considering two questions. (1) How are fine-scale vortices in turbulence related to cavitation inception? (2) How does the cavitation modify turbulent vortices and turbulence statistics? Since both are interactive phenomena, they must be analyzed by two-way methodology. One of typical and appropriate examples for our objective is the separated flow in the wake of a thin fence in a two-dimensional channel. Iyer and Cessio (2002) visualized various types of vortex cavitation and they reported the turbulence statistics in the wake region. In the first half of this report, we show the results of the direct numerical simulation with cavitation model, which was developed by Okita and Kajishima (2002). Our result is compared with experimental observation by Iyer and Cessio (2002) qualitatively, because the flow configuration was simplified in our simulation. Then, we investigate the relationship between cavitation and vortical structure; namely, primary (spanwise) vortices and the secondary (streamwise) vortices. In the second half, the interaction between a Burgers vortex and cavitation is directly simulated in an idealized situation. One reason is that streamwise vortices in a shear layer, simulated in the former part, has a profile of Burgers vortex. Moreover, the finest scale eddies in fully developed turbulence has been found to be Burgers type. A Burgers vortex in a uniform stream is given at the inlet cross section of the computational domain for this purpose. Reducing the cavitation number, the structure of vortex decaying by cavitation inception is analyzed. Then we give a phenomenological model that represents the vortical structure during this process. OUTLINE OF COMPUTATION The procedure including cavitation model and numerical method should fit in with the spatio-temporal scale of unsteady motion of vortices in the turbulent shear layer. In