Pressure Amplitude Distribution Research Articles

Experimental Investigation of Part Load Vortex Rope Mitigation With Rod Protrusion in an Axial Turbine

Abstract The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target.

Deep-water ambient sound on near-bottom receivers in the New England Seamount Chain

Ambient sound was recorded for about two months by three synchronized, single-hydrophone receivers that were moored at depths of 2573, 2994, and 4443 m on seamount flanks. Hydrophones were located within 5 m from the seafloor. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.5 to 4000 Hz. Statistical distribution of pressure amplitude exhibits much heavier tails than the expected Rayleigh distribution throughout the frequency band of observations. Spatial variability of the observed ambient sound is controlled by the seafloor properties, bathymetric shadowing, and nonuniform distribution of the noise sources on the sea surface due to the Gulf Stream and its meanders. Interferometry of the ambient sound recorded by the receivers with the horizontal separations of 7.0 km shows strong variations of the acoustic travel time between the receivers along a surface-reflected path due to the evolution of the Gulf Stream position. The magnitude of the variations of the passively measured acoustic travel time is consistent with the available contact measurements of the sound speed profiles. Environmental inferences derived from the ambient, shipping, and flow noise data will be discussed. [Work supported by the ONR TFO DRI.]

Dynamic modelling and engineering simulation of fluid mechanics in water injectors

To study the fluid dynamic response mechanism under the working condition of water injection well borehole, based on the microelement analysis of fluid mechanics and the classical theory of hydrodynamics, a fluid microelement pressure-flow rate relationship model is built to derive and solve the dynamic distribution of fluid pressure and flow rate in the space of well borehole. Combined with the production data of a typical deviated well in China, numerical simulations and analyses are carried out to analyze the dynamic distribution of wellbore pressure at different injection pressures and injection volumes, the delayed and attenuated characteristics of fluid transmission in tube, and the dynamic distribution of wellbore pressure amplitude under the fluctuation of wellhead pressure. The pressure loss along the wellbore has nothing to do with the absolute pressure, and the design of the coding and decoding scheme for wave code communication doesn't need to consider the absolute pressure during injecting. When the injection pressure is constant, the higher the injection flow rate at the wellhead, the larger the pressure loss along the wellbore. The fluid wave signal delay amplitude mainly depends on the length of the wellbore. The smaller the tubing diameter, the larger the fluid wave signal attenuation amplitude. The higher the target wave code amplitude (differential pressure identification root mean square) generated at the same well depth, the greater the wellhead pressure wave amplitude required to overcome the wellbore pressure loss.

Exergy-based methodology to develop design guidelines for performance optimization of a coupled co-axial thermoacoustic refrigerator

Performance of thermoacoustic (TA) systems are very complex, and their performance optimization is a difficult task, and most TA systems have relatively very low efficiencies. The main objective of this study was to develop an exergy analysis to introduce a guide for a designer, how to optimize the performance of a TA system. We have applied this methodology to a coupled thermoacoustic engine and refrigerator with a standing wave in the engine and a traveling wave in the refrigerator. In an optimization procedure, a lumped systems numerical analysis was performed to predict the main parameters of the thermoacoustic system. The numerical method was validated by comparison with numerical and experimental results in the literature. The cost function was the total system coefficient of performance, and design parameters include the refrigerator regenerator length, location, and hydraulic radius. The performance index was decomposed into three main subsystem indices, i.e., the efficiency of the engine, the efficiency of the resonator, and the coefficient of performance of the refrigerator. The optimum point for each component was found, and the total system optimal point which is a result of thermoacoustic interaction of all segments were analyzed. It was illustrated how a change in design parameters could affect the pressure and velocity amplitude and phase distribution in different subsystems, which help the designer deal with the challenge of the optimal parameter selection to enhance the system performance. Finally, an exergy analysis was introduced to assess loss mechanism's contribution to each refrigerator's component. This result in a roadmap for optimization of the system. A new parameter called non-dimensional exergy loss portion (NELP) was introduced, which may significantly show the improvement potential in each system component.

Numerical study of high-intensity focused ultrasound (HIFU) in fat reduction.

This study aimed to investigate the effect of fat-layer thickness and focal depth on the pressure and temperature distribution of tissue. Computer simulations were performed for the skin-fat layer models during high-intensity focused ultrasound (HIFU) treatment. The acoustic pressure field was calculated using the nonlinear Westervelt equation and coupled with the Pennes bioheat transfer equation to obtain the temperature distribution. To investigate the effect of the thickness of the fat layer on pressure and thermal distributions, the thickness of the fat layer behind the focal point (z=13.5mm) changed from 8 to 24mm by 2mm step. The pressure and temperature distribution spectra were extracted. The simulated results were validated using the experimental results with a 98% correlation coefficient (p<0.05). There was a significant difference between the pressure amplitude and temperature distribution for the 8-14mm thickness of the fat layer (p<0.05). By changing the focal point from 11.5 to 13.5mm, the maximum acoustic pressure at the focal point increased 66%, and the maximum temperature was 56%, respectively. Considering the specific treatment plan for each patient, according to the skin and fat layer thicknesses, can help prevent side effects and optimize the treatment process of HIFU.

A study on finite amplitude standing waves in stepped acoustic resonator

Investigation of finite amplitude standing waves generated by boundary driving in the stepped acoustic resonator has been carried out both theoretically and experimentally to derive an appropriate tool to design resonators that have characteristics suitable for specific applications such as a microphone calibration system with a higher-pressure level. Initially, a modified resonance condition is proposed, which is deduced from the transfer matrix method and the characteristics of the standing wave resonator. The predictions of the resonance frequencies in a cylindrical resonator are in excellent agreement with those obtained by analytic formula in previous studies. Then, the modified resonance condition is applied to calculate the resonance frequencies of a stepped resonator. The boundary driving is provided by a piston at the open end. The resonant response in the cylindrical and stepped resonator, including the pressure amplitudes, the pressure waveform and the distribution of pressure amplitude along the axis, is calculated numerically by the finite volume method, which is a preliminary prediction of finite amplitude standing waves in the resonators. The calculated results of the stepped resonator are verified in the experiment, in which a stepped acoustic resonator is connected to a loudspeaker functioning as the piston.

Thermoacoustic oscillations in a can-annular model combustor with asymmetries in the can-to-can coupling

Can-annular combustors are equipped with a set of nominally identical cans, circumferentially arranged around the shaft. Adjacent cans are coupled acoustically via a small gap at the downstream end, where the circular cross-section transitions into the annular turbine inlet. Recent experimental and theoretical work has shown that the coupling strongly affects thermoacoustic system stability and, hence, may give rise to damaging pressure oscillations that originate from a constructive interference of pressure waves and heat release rate fluctuations. A laboratory-scale can-annular combustor has been operated with premixed CH4–H2–air mixtures to study these instabilities. The combustor consists of eight identical cans, connected acoustically to their respective neighbours via size-adjustable side branches; the latter allow for a variation of the coupling strength. New experimental results are presented in which five asymmetric sets of coupling strengths are investigated and compared to a previously studied symmetric configuration. The sets are chosen such that the symmetry is gradually reduced until a fully asymmetric configuration is realized. The perturbations affect the observed mode type, frequency of oscillation and the pressure amplitude distribution over the eight cans. The perturbations via the can-to-can couplings do not trigger mode localization. The symmetric configuration considered previously, shows activity in an azimuthal mode of third and fourth order, with unsteady switching between the two. The mode shape is characterised through novel phase-averaged OH* chemiluminescence images capturing all eight cans.

Alternator noise reduction based on claw-pole optimization

The high-frequency aerodynamic noise of claw-pole alternator presents a harsh sound in human subjective perception, which has an important impact on the car passengers’ comfort. Therefore, experimental and optimization studies have been conducted to investigate the high-frequency aerodynamic noise during the high-speed operation of claw-pole alternator. In this study, a claw-pole alternator was first tested under no-load conditions. The experimental results revealed that the 6th, 9th, 12th, 18th, 30th and 36th orders are the main noise orders. Meanwhile, the dipole was the main noise source at a speed range of 8000–14,000 rpm. Subsequently, a hybrid simulation method was applied to solve the acoustics field problem. The simulation and experimental results of the acoustics field characteristics satisfactorily agreed for all main orders of the aerodynamic noise at both speeds of 8000 and 12,000 rpm. The minimum error for the 9th order was 0.35dBA at 8000 rpm, whereas the minimum error of the 8th order reached 0.51 dBA at a speed of 12,000 rpm. Thus, the proposed simulation model could be considered a reliable approach. Additionally, the pressure amplitude distribution on the rotor surface for different orders was analyzed. The higher-pressure amplitudes at higher orders, such as the 30th and 36th orders, are mainly distributed at the claw-pole near the stator and root of the claw-pole, indicating that higher acoustics levels corresponding to the higher orders are mainly associated with the claw-pole. Finally, the claw-pole notching was parametrically modeled, and the eigenvalues of the radial, the notch inclination angle and the axial stretch length were optimized by using Latin hypercube sampling method. After notching, the air mass flow through the front and rear blades exhibited small changes, which slightly affected the cooling performance of the alternator. The claw-pole notching yielded the highest optimization, when the eigenvalues of radial, notch inclination angle and axial stretch length were 46.82 mm, 63.3° and 14.65 mm respectively.

EXTRACTION OF PRESSURE AND TEMPERATURE DISTRIBUTION OF HIGH INTENSITY FOCUSED ULTRASOUND CONSIDERING NONLINEAR PROPAGATION

The aim of this study is the extraction of acoustic pressure distribution in the target tissue layers based on the nonlinear behavior of waves. The nonlinear behavior effect of high intensity focused ultrasound (HIFU) on the temperature distribution of the tissue was extracted and compared with the linear behavior. The acoustic pressure field was calculated using the Westervelt equation and was coupled with Pennes thermal transfer equation. The simulations were performed for three layers of skin, fat and muscle using Comsol software. The disagreement between two linear and nonlinear models was analyzed with Kolmogorov–Smirnov test. The pressure and temperature distributions were calculated in nonlinear model by changing the acoustical parameters of the transducer including intensity, effective radiation area, focal length and sonication time. Model results were validated with experimental results with 98% correlation coefficient ([Formula: see text]). There is no significant difference between the pressure amplitude and temperature distribution in linear and nonlinear models at low intensity ([Formula: see text]), but with increasing intensity to 10[Formula: see text]W/cm2, in nonlinear model, maximum pressure and maximum temperature increased 40% and 20% compared with linear model. For input intensities of 1.5, 2, 8 and 10[Formula: see text]W/cm2, the maximum pressure (at focal point) increased 10%, 12%, 22%, 40% and maximum temperature increased 1%, 2%, 12%, 20% in nonlinear model compared to linear model. At 0.8[Formula: see text]cm2 and 1.5[Formula: see text]cm2 effective radiation area, the maximum acoustic pressure and temperature in nonlinear model increased from 12[Formula: see text]MPa to 30[Formula: see text]MPa and 43∘C to 79∘C, respectively. By decreasing the focal lengths from 10[Formula: see text]mm to 7.5[Formula: see text]mm, the maximum temperature increased from 45∘C to 87∘C. It is concluded a change in the input parameters of the transducer; it can be very effective in treating. The results emphasize the effects of nonlinear propagation and acoustical radiation parameters to improve the HIFU treatment.

Investigating heterogeneous distribution of fluid pressure in hydraulic fractures during pulsating hydraulic fracturing

Pulsating hydraulic fracturing (PHF) has been proven to be a potential fracturing method. Compared with conventional hydraulic fracturing, the PHF can generate a more complex fracture network with lower breakdown pressure and less induced seismicity. A distribution of fluid pressure in hydraulic fractures determines the initiation and propagation of new fractures, which has not been fully investigated in the PHF. In this paper, a distribution of fluid pressure in pulsating hydraulic fractures is innovatively investigated through a transient flow model (TFM). This model is solved by the method of characteristics (MOC). This solution method is then verified against experimental data. Through this model, the mechanism of the pressure distribution is analyzed and the optimization method for PHF is proposed. The effects of input frequency, friction, fracture roughness and fracture length on the fluid pressure distribution during the PHF are investigated, respectively. Results show that the fluid pressure at each point along hydraulic fractures fluctuates in the same frequency, and the pressure fluctuation amplitudes are heterogeneously distributed in the hydraulic fractures during the PHF. This phenomenon results from the fact that a standing wave is formed in the hydraulic fractures during the PHF. Moreover, according to the number of wave nodes in the standing wave, the heterogeneous distribution of fluid pressure amplitudes can be divided into three categories for input frequency from 1 Hz to 20 Hz. The frequency ranges of these three pressure distribution categories are centered by three orders of resonance frequency, and the number of wave nodes for these three pressure distribution categories equals the order of resonance frequency. Although the standing wave in the hydraulic fractures is partially changed by a friction effect, the fluid pressure amplitude distribution is still heterogenous. In addition, the friction has a dissipation effect on a resonance amplitude. The increasing fracture roughness reduces the heterogeneity of a fluid pressure distribution in hydraulic fractures during the PHF. The first maximum fluid pressure amplitude near a fracture entrance may not exist for high fracture roughness. Finally, the effect of the heterogeneous distribution of fluid pressure on the initiation and propagation of new fractures during the PHF is addressed, and then a novel and effective design method of input frequency and amplitude for the PHF is proposed. Multiple initiations of new fractures can be obtained by optimizing an input frequency and amplitude.

Acoustic field generated by an innovative airborne power ultrasonic system with reflectors for coherent radiation

In order to obtain the maximum efficiency in industrial processes assisted by airborne power ultrasound, the ultrasonic systems must be capable of generating an acoustic field with a maximum energy concentration in the desired areas. In this paper, the acoustic performance of two different ultrasonic systems is presented. The first system corresponds to an airborne power ultrasonic transducer with a flat rectangular plate radiator, and the second system is composed by the same transducer mounted in a set of reflectors that allow the generation of coherent radiation. The acoustic pressure field for each system has been determined numerically and the spatial pattern has been experimentally measured. In the experiment, the system with reflectors obtained higher pressure amplitude in a wider area, due to the coherent radiation achieved. The directivity pattern obtained in the experiment confirms this coherent radiation field. This is the first time that the acoustical behavior of two different ultrasonic systems with flat rectangular radiator, vibrating in a complex flexural mode, has been numerically and experimentally described and compared in terms of pressure amplitude distribution and directivity pattern.

Investigation of Rotor-Stator Interaction and Flow Unsteadiness in a Low Specific Speed Centrifugal Pump

Instantaneous flow dynamics induced by rotor-stator interaction are detrimental to the stable operation of centrifugal pumps. In this study, unsteady rotor-stator interaction and flow structures within a low specific-speed centrifugal pump are analysed using the Large Eddy Simulation (LES) method. For that purpose, pressure pulsation and the evolution process of a vortical structure are combined to investigate rotor-stator interaction in order to clarify the inherent correlation between pressure amplitude and vorticity distribution. The results show that distinct peaks at blade passing frequency (fBPF) are closely associated with the positions of the monitoring point due to rotor-stator interaction. An unsteady vortical structure at the near tongue region is related to the relative position of the impeller with respect to the tongue, and the upstream effect of the volute tongue significantly affects the vorticity distribution on the blade pressure side. Rotor-stator interaction is dominated by vortex shedding in the wake of the blade trailing edge and their impingement on the volute tongue with subsequent cutting and distortion. Moreover, the high-pressure amplitude is generated with the corresponding high vorticity magnitude observed as well. Therefore, it is confirmed that pressure amplitude is significantly associated with the corresponding vorticity magnitude.

Prediction and Reduction of Aerodynamic Noise of the Multiblade Centrifugal Fan

An aerodynamic and aeroacoustic investigation of the multiblade centrifugal fan is proposed in this paper, and a hybrid technique of combining flow field calculation and acoustic analysis is applied to solve the aeroacoustic problem of multiblade centrifugal fan. The unsteady flow field of the multiblade centrifugal fan is predicted by solving the incompressible Reynolds-averaged Navier-Stokes (RANS) equations with conventional computing techniques for fluid dynamics. The principal noise source induced is extracted from the calculation of the flow field by using acoustic principles, and the modeled sources on inner and outer surfaces of the volute are calculated with multiregional boundary element method (BEM). Through qualitative analysis, the sound pressure amplitude distribution of the multiblade centrifugal fan in near field is given and the sound pressure level (SPL) spectrum diagram of monitoring points in far field is obtained. Based on the analysis results, the volute tongue structure is adjusted and then a low-noise design for the centrifugal fan is proposed. The comparison of noise tests shows the noise reduction of improved fan model is more obvious, which is in good agreement with the prediction using the hybrid techniques.

Impact of load impedance on the performance of a thermoacoustic system employing acoustic pressure amplifier

An acoustic pressure amplifier (APA) is capable of improving the match between a thermoacoustic engine and a load by elevating pressure ratio and acoustic power output. A standing-wave thermoacoustic engine driving a resistance-and-compliance (RC) load through an APA was simulated with linear thermoacoustics to study the impact of load impedance on the performance of the thermoacoustic system. Based on the simulation results, analysis focuses on the distribution of pressure amplitude and velocity amplitude in APA with an RC load of diverse acoustic resistances and compliance impedances. Variation of operating parameters, including pressure ratio, acoustic power, hot end temperature of stack, etc., versus impedance of the RC load is presented and analyzed according to the abovementioned distribution. A verifying experiment has been performed, which indicates that the simulation can roughly predict the system operation in the fundamental-frequency mode.

Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer.

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Compact piezoelectric transducers for lithotripsy

Self-focusing piezoelectric transducers are widely used in extracorporeal shockwave lithotripsy. This application demands high focal pressure pulse amplitudes, small focal beamwidths and large penetration depths, which results in large transducers with typical dimensions of 350-mm focal distance and 500-mm diameter. Against the background of the development of smaller and therefore more economical devices it is still desirable to reduce the transducer size but keep the acoustic properties. This can be achieved only by an increased initial pressure amplitude at the transducer surface. Since the single piezoelectric elements are driven close to their electrical limit, a different mechanical design is necessary. Piezoelectric elements placed in a layered manner on both sides of a spherical cap could provide a significant increase of the pressure pulse amplitude generated at the transducer surface. Three-dimensional FEM simulations of the transducer structure including all important design parameters, like the electrical drive for both sides, show the ability of generating higher amplitudes by keeping similar pulse shapes. Measurements made on a prototype transducer with a diameter of 270 mm and a focal distance of 200 mm show comparable results to the standard single-layered design regarding peak focal pressure amplitude and spatial amplitude distributions.

Investigation of speech intelligibility in building with curved surfaces by means of objective measures

There are two aspects in an acoustics design process that have to be taken care of in order for the room and the sound system to be intelligible. The first is the electronic part of the design. The second is the environment in which the system is to be installed. With the assumptions that the selection of sound system equipment and installation are correct and thorough, the sound reflection pattern and its delay in building with curved surfaces has to be investigated and understood. This paper investigates via measurements as well as through the simulation software the distribution of sound pressure level, sound delay (RT60), time and amplitude of early reflection (Rearly), and late reflection (Rlate) in a symmetrical room and in a room with curved surfaces. Rearly is categorized as the first-arriving wave-front that happens before 35 ms and Rlate is that within 70 and 120 ms. Preliminary investigation has shown that the equal distribution of these objective measures is destroyed with the existence of curved surfaces in the room. This unequal distribution hints that speech intelligibility of a room with curved surfaces is also varied and altered.

Ultrasonic manipulation of particles in microgravity

The manipulation of yeast cells and latex particles of diameters 1.3, 12 and in aqueous suspension in 1 and 3 MHz ultrasonic standing waves has been examined and compared in microgravity (0 g), 1 g and 1.8 g. The experiments were carried out during the 23rd ESA parabolic flight campaign. The suspended particles concentrated to form bands at half wavelength separation in the axial direction of a vertical tubular sample holder with a Bessel pressure amplitude distribution profile. At 1 g small ( latex) particles formed bands but these broke up within a few seconds. In contrast throughout 0 g bands of these particles formed and remained stable. The transition from 0 to 1.8 g during flight induced streaming which broke up the bands of latex. Bands formed with yeast cells were more stable at 1 g but, during transitions from 0 to 1.8 g, some bands broke up. Bands of the larger (12 and ) particles were stable at 0, 1 and 1.8 g and during all transitions between the fields. Thermal gradient convective flow rather than acoustic streaming was identified as the main source of flow in the sample holder at 1 g. The absence of thermal streaming at 0 g allowed manipulation of smaller particles in that situation. A frequency ramping technique appropriate for removal of particles from suspension in 1 g had a better performance in 0 g.

Ultrasonic propagation, scattering, and defocusing in suspensions

Fine resolution narrow-band pressure amplitude distribution measurements have been performed using a hydrophone in the field of a weakly focused bowl transducer radiating into castor oil and 1% and 2% by volume suspensions of 320-μm polystyrene beads in castor oil over the frequency range 1.0 to 2.5 MHz at 20 °C. The axial distributions permitted determination of the excess attenuation due to scattering in the suspensions as a function of frequency. The frequency range included a resonance. The excess attenuation measured was compared with the theoretical predictions of Waterman and Truell [J. Math. Phys. 2, 512–537 (1961)] and good agreement was obtained. The values of the excess attenuation obtained were used to predict axial pressure amplitude distributions in the suspension. Although the excess attenuation was determined from far-field measurements, the agreement between the predicted axial distributions and those measured experimentally extended well into the near field. Lateral distributions were measured at seven frequencies at the position of the geometrical center of curvature of the source and at the position of the true focus in the 1% suspension. Comparison with similar scans in castor oil alone revealed no detectable defocusing due to the presence of the scatterers.

A systematic experimental investigation of a highly intense pressure field generated by a plane circular transducer

This work is part of a study of highly intense sound beams with application to medical therapy in mind. A systematic experimental investigation of a sound beam propagated into water from a plane circular transducer was for the first time, it is believed, performed for a large range of cw excitation levels. The fundamental frequency was 1 MHz and the on-source pressure ranged from 0.8 to 24 bar (shock formation distance to Rayleigh distance ratio ranging from 1.8 to 0.06). The pressure amplitude distributions of the five first harmonic components, along and across the acoustic axis, are presented and discussed. New nonlinear effects are uncovered: a nonlinear self-defocusing of the beam and an alteration of the near-field structure of the harmonic components.