Pressure Fluctuation Measurements Research Articles

Experimental Study of the Cavitating Flow on an Independently Heated Venturi Nozzle

Abstract Despite the observation of change in the cavitation regime on a heated surface, the specific section of the wall surface that plays a more dominant role in this transition phenomenon remains unknown. This study experimentally investigated the effect of surface temperature of different regions on the cavitating flow in terms of the cavitation regime. The experiments were conducted using a convergent-divergent Venturi nozzle comprising two parts that could be heated independently. The Venturi nozzle could be fully or selectively heated at either the front, where the leading edge of the cavity sheet was located, or the rear, where the cavity sheet developed. The cavitation behavior under different heating conditions was investigated using high-speed visualization and fluctuating pressure measurements. Compared with the non-heated case, which exhibited a sheet-cloud cavitation regime, the cavitation regime on the completely heated Venturi nozzle exhibited transient cavitation. The same transition phenomenon was observed when only the front of the Venturi nozzle was heated. A liquid film was observed beneath the cavity sheet of transient cavitation when only the front portion was heated. In contrast, heating the rear part alone did not induce a change in the cavitation regime. Thus, it appeared that the transition of the cavitation regime on a heated surface was mainly influenced by the temperature increase at the leading edge of the cavity sheet.

Observation of exceptionally strong near-bottom flows over the Atlantis II Seamounts in the northwest Atlantic

Knowledge of near-bottom ocean current velocities and especially their extreme values is necessary to understand geomorphology of the seafloor and composition of benthic biological communities and quantify mechanical energy dissipation by bottom drag. Direct measurements of near-bottom currents in deep ocean remain scarce because of logistical challenges. Here, we report the results of flow velocity and pressure fluctuation measurements at three sites with depths of 2573–4443 m in the area where the Gulf Stream interacts with the New England Seamounts. Repeated episodes of unexpectedly strong near-bottom currents were observed, with the current speed at 4443 m of more than 0.40 m/s. At 2573 m, current speeds exceeded 0.20 m/s approximately 5% of the time throughout the entire eight-week measurement period. The maximum flow speeds of over 1.10 m/s recorded at this site significantly surpass the fastest previously reported directly measured current speeds at comparable or larger depths. A strong correlation is found between the noise intensity in the infrasonic band and the measured current speed. The noise intensity and the characteristic frequency increase with the increasing current speed. Machine-learning tools are employed to infer current speeds from flow-noise measurements at the site not equipped with a current meter.

Empirical Modeling of Subcritical Hopf Bifurcation of Thermoacoustic Stirling Engine

This study models the subcritical Hopf bifurcation in thermoacoustic Stirling engines using the Stuart–Landau model, highlighting the role of nonlinear dynamics. By inducing self-sustained oscillations and measuring pressure fluctuations across different temperature gradients imposed on the regenerator, we reveal the engine’s transition to a nonlinear domain, characterized by heightened oscillation amplitudes and unique periodic patterns. Interpreted Landau constants and growth rates illuminate the stabilizing effects of nonlinear dynamics, demonstrating the Stuart–Landau model’s applicability in thermoacoustic engine analysis. Our research confirms that this empirically refined model reliably describes oscillation amplitudes and transient phenomena, contributing valuable perspectives for advancing thermoacoustic engine design and operational understanding.

Hybrid rich- and lean-premixed ammonia-hydrogen combustion for mitigation of NOx emissions and thermoacoustic instabilities

The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen-doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen-air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetry-induced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and self-excited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observe that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects – such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96 % reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.

Investigation of sound pressure leakage effect for primary calibration down to 10−2 Hz using a laser pistonphone system

Recently, the demand for sensor calibration in the infrasonic range has increased to obtain accurate information when monitoring infrasound generated by large-scale natural disasters. The laser pistonphone method is a primary calibration method to evaluate infrasound sensors in the low-frequency range. In this method, sound pressure is generated in a fixed sealed volume of air, by a volume change via an attached piston movement. By measuring the piston displacement using laser interferometer, the generated sound pressure can be calculated by multiplying the velocity of volume change by acoustic transfer impedance of a pistonphone. The main concern with this method is that sound pressure leakage from a gap adjacent to piston significantly changes the acoustic transfer impedance at lower frequencies. To apply the laser pistonphone method in the low-frequency range down to 10−2 Hz, it is essential to accurately evaluate and compensate for the leakage effect, i.e. the gap’s acoustic transfer impedance. In this study, we propose a technique for experimentally evaluating the gap impedance. The main idea is to determine the total acoustic transfer impedance by dividing the sound pressure by the volume velocity and then deducing the gap impedance by subtracting the chamber impedance from the obtained total acoustic transfer impedance. A digital pressure sensor was used to precisely measure sound pressure below 100 Hz because pressure sensors are suitable for accurate measurement of pressure fluctuations at low frequencies. We validated the proposed approach by calibrating an analog pressure sensor that outputs an analog voltage proportional to the absolute pressure. As a result, the sensitivity calibrated by the laser pistonphone method at 0.02 Hz agreed with the static sensitivity provided by the manufacturer within 0.02 dB.

Experimental investigation about the aerodynamic noise caused by hypersonic turbulence

In this study, experimental investigations pertaining to aerodynamic noise are conducted in a hypersonic quiet wind tunnel, which possesses the unique capability of operating in both quiet and conventional modes, achieved by controlling the bleed slot to regulate the boundary layer flow. The measurement of aerodynamic noise and analysis of coherent structures of hypersonic turbulence are accomplished employing pressure probes and the nano-tracer-based planar laser scattering technique. The characterization of aerodynamic noise within the freestream unveils that noise generated by turbulent structures on the nozzle wall is the primary factor affecting the turbulence intensity of the freestream flow. Regarding the acoustic radiation noise generated by the turbulence on the nozzle wall, a peak frequency ranging from 54 to 57 kHz is observed in its spectrum within a unit Reynolds number range of 0.3×107–1.1×107 m−1. Under different operation modes, the freestream pitot-pressure data feature the similar spectral slope of f−2.2 and f−12 before and after the aforementioned peak. Through wavelet analysis, scales of the coherent structure in the turbulent boundary layer are examined, revealing that the maximum energy streamwise scale of the coherent structure is approximately 2.47 mm under the unit Reynolds number of 0.7×107 m−1. The product of this scale with the characteristic frequency of 56 kHz obtained from fluctuating pressure measurements is close to the local sound speed of about 140 m/s.

Research on a Pressure Control Method for a Liquid Supply System Based on Online Updating of a Radial Basis Function Neural Network

In order to solve the problem of frequent pressure fluctuations caused by fluid quantity variation in hydraulic support liquid supply systems and the pressure response lag caused by long-distance pipelines, an online updated radial basis function neural network (RBF neural network) control method was proposed for the long-distance liquid supply system. Based on the analysis of the measured pressure fluctuations of the mining face and the process of the stable pressure liquid supply system, the influencing factors of the stable pressure liquid supply flow demand were obtained. The flow set of the stable pressure liquid supply system was established and fitted in the SimulationX–Simulink co-simulation model and the online correction was carried out by using the characteristics of the repeated action of the hydraulic support. Finally, the online updating RBF neural network regulator was established to realize the pressure regulator control of the pumping station, and the experimental platform was set up for verification. The results show that this method can effectively reduce the pressure fluctuations caused by the change in the flow demand of the mining face, and can adjust the flow rate of the mining face, reduce the pressure impact, and improve the efficiency of the machine.

Cavitation identification in a hydraulic bulb turbine based on vibration and pressure fluctuation measurements

Cavitation can lead to reduced efficiency, increased vibration and noise in hydraulic turbines. Accurate and rapid identification of cavitation states in a turbine is important. In this work, a test system has been constructed for a hydraulic bulb turbine, to synchronously obtain cavitation images on runner blades, vibrations on both the runner hub and the runner chamber, and pressure fluctuations on the draft tube of the turbine at different cavitation conditions. The multifractal characteristics of the collected signals obtained by using the multifractal detrended fluctuation analysis method have been analysed and compared in detail. It is found that the multifractal characteristics of the vibrations and pressure fluctuations depend strongly on the cavitation state. The multifractal strength of the pressure fluctuations in the draft tube is stronger than that of the vibrations on the runner, induced by cavitation. Furthermore, some multifractal parameters of vibrations and pressure fluctuations have been extracted and used for cavitation state identification in the turbine, which is important for safe operation of a turbine.

Thermal Interference When Recording Turbulent Pressure Fluctuations on the Surface of a Floating Device

Thermal interference is studied when recording turbulent pressure fluctuations on the surface of a floating device at specified experimental parameters of temperature stratification of an aquatic medium. The effect of distortion of the spectral levels of pressure fluctuations recorded by a sound receiver in the field of temperature inhomogeneities is studied using the example of measurements of turbulent pressure fluctuations in the boundary layer during the vertical ascent of a device from a specified depth. At moderate flow velocities exceeding 1–2 m/s, the temperature susceptibility of a piezoceramic receiver is shown to be decisively determined by its characteristic “thermal” frequency. The parameters of the threshold critical frequency, below which the temperature signal (thermal interference) prevails over the useful signal generated by pressure fluctuations, are determined. With respect to the receivers used in experiments on a floating device [7], the values of the threshold critical frequency are 130 and 215 Hz.

Evaluation of uncertainties in pressure fluctuations measurement on low-head Francis turbine model during part-load operation

The Francis turbines are often operated at the part load (PL) conditions to meet the electricity demand while maintaining system stability. During PL operations, the turbine experiences flow instabilities and vortex formation in the draft tube, leading to potential damage to the turbine. This experimental study focused on measuring pressure fluctuations in a model low-head Francis turbine at PL operation, i.e. 70% of rated load condition. Eight pressure sensors were installed at various locations to measure the pressure fluctuations. In-situ calibrations were performed for various measuring instruments used in testing, such as flow meters, pressure transducers, torque transducers, and speed sensors. The uncertainty evaluation for hydraulic turbine efficiency and pressure fluctuations in the draft tube cone was carried out at the PL operations. Quantifying the uncertainty for both Type A and Type B was conducted. The maximum estimated uncertainty for measuring hydraulic turbine model efficiency is ±0.272% and for pressure fluctuations during PL operation is ±1.358%.

Slope Stability Evaluation Due to Reservoir Draw-Down Using LEM and Stress-Based FEM along with Mohr–Coulomb Criteria

Rapid Draw-Down (RDD) in an earthfill dam has serious implications for dam safety regarding slope stability issues. The evaluation of reservoir draw-down impact on slope stability was carried out with the Limit Equilibrium Method (LEM) and stress-based Finite Element Method (FEM), using GeoStudio. The time-dependent Factor of Safety (FOS) and nonlinear behavior were evaluated considering 8 h of RDD. The resulting FOS values of 1.28 and 1.27 using LEM and stress-based FEM were classified as unsafe. The minimum allowable draw-down factor of safety value is 1.3, as per the guideline. The suggested two designs, with upstream horizontal filters and increased upstream dam permeability, provided an adequate FOS. However, the nonlinear analysis with coupled FEM has shown that the upstream slope is unstable in all three cases (i.e., as-built design, increased upstream dam shell permeability, and suggested application of horizontal filter layers) considering 8 h of RDD. Several gradual draw-down rates were also tested and it has been found that the FOS increases with decreased draw-down rates. FOS charts, pressure fluctuation, and flow measurements in the upstream dam shell have revealed that slope stability is highly influenced by pore water pressure and draw-down rate. The safe allowable draw-down rate of 20 h was identified, considering the as-built design of the dam.

Near-field pressure and wake velocity coherence of a circular cylinder

Aerodynamic noise and unsteady loads resulting from the vortex shedding of a circular cylinder pose significant challenges in engineering applications. Understanding these challenges is closely related to pressure fluctuations on the cylinder surface. This experimental study conducted simultaneous measurements of surface pressure and velocity fluctuations within the subcritical Reynolds number range (14.7×103≤Re≤30×103) to investigate the influence of vortex shedding on near-field pressure. The experiments utilized a highly instrumented cylinder with mini-pressure transducers. The results revealed that surface pressure fluctuations exhibit maximum energy content near the cylinder's shoulders at the fundamental vortex shedding frequency (f0), aligning with pronounced lift fluctuations. The analysis of pressure–velocity coherence indicated that the most energetic flow structures resulting from vortex shedding significantly contribute to generating surface pressure fluctuations at the f0-peak frequency, extending over a considerable distance from the near- to far-wake regions. Additionally, the pressure fluctuations responsible for drag fluctuations are predominantly imposed at the base of the cylinder, primarily at the second harmonic (2f0), arising from flow structures shed at the end of the vortex formation region. Wavelet analysis provided insights into the temporal characteristics of surface pressure fluctuations, revealing amplitude modulation over time with multiple repetitive patches around the f0-peak frequency and close to the cylinder's shoulders, where the highest energy level predominates due to vortex shedding.

Thermal Interference When Recording Turbulent Pressure Fluctuations on the Surface of a Floating Device

Thermal interference is studied when recording turbulent pressure fluctuations on the surface of a floating device at specified experimental parameters of temperature stratification of an aquatic medium. The effect of distortion of the spectral levels of pressure fluctuations recorded by a sound receiver in the field of temperature inhomogeneities is studied using the example of measurements of turbulent pressure fluctuations in the boundary layer during the vertical ascent of a device from a specified depth. At moderate flowvelocities exceeding 1–2 m/s, the temperature susceptibility of a piezoceramic receiver is shown to be decisively determined by its characteristic “thermal” frequency. The parameters of the threshold critical frequency, below which the temperature signal (thermal interference) prevails over the useful signal generated by pressure fluctuations, are determined. With respect to the receivers used in experiments on a floating device [7], the values of the threshold critical frequency are 130 and 215 Hz.

Simultaneous measurement of unsteady surface pressure and leading edge noise of a NACA0012 airfoil

Airfoil-turbulence interaction noise, which is created whenever turbulent flow encounters an airfoil, is a major source of unwanted noise emitted byaircraft and turbomachinery. Despite being a major research focus overthepast decades, the accurate prediction and understanding of airfoil–turbulence interaction noise remain an open question. This experimental investigation involved simultaneous measurements of unsteady pressure fluctuations at 61 locations across the surface of a NACA0012 airfoil and far field sound pressure at a phased 64-microphone array. Various square-mesh turbulence grids were placed upstream of the airfoil to vary the isotropic free stream turbulence. Additionally, the influence of the airfoil’s angle of attack as well as different mean flow speeds on the unsteady surface pressure and radiated far field noise was investigated. Ultimately, the links between the simultaneously acquired unsteady surface pressure on the airfoil and the microphone array sound pressure were studied. This provided insight into the mechanisms of leading edge noise source generation and their influence on the radiated far field noise.

Experimental investigation of pressure-reducing instabilities in perforated plates for compressible and cavitating flows

The pressure losses through sharp-edged, multi-orifice circular perforated plates with two different ratios of plate thickness to orifices’ diameter are experimentally investigated using both water and dry air as working fluids. The choice of a thickness for which fluid-dynamic instabilities related to flow reattachment are known to occur allows to investigate the effects of cavitation and compressibility on such instabilities. It is found that both cavitation and compressibility may be the cause of an early reattachment of the flow within the plate’s perforation, modifying the pressure loss characteristics of the device. Wall pipe pressure fluctuations measurements for subsonic air flows allow linking changes in whistling frequencies to changes in the pressure loss regime of the plate.

Pressure fluctuations in a gas-solid fluidized bed at temperatures up to 1650 °C

Fluidized beds operating at ultra-high temperatures have great application potential for synthesizing essential materials and chemicals, but they are relatively unexplored. This study, for the first time, investigates fluidization characteristics at temperatures up to 1650 °C by measurement and analysis of pressure fluctuations in a laboratory fluidized bed of 30 mm diameter with corundum particles of an average size of 900 μm. Standard statistical and spectral methods are used to analyze pressure fluctuation signals and characterize fluidization behavior based on the effect of temperature on pressure fluctuation parameters, including the probability density function, standard deviation, autocorrelation, power spectra density, amplitude, dominant frequency, and average cycle frequency. The results indicate that, for the coarse corundum particles used in this study, fluidization behavior inverts at a crucial temperature of approximately 1400 °C. Below this temperature, gas–solid flows in the bed are predominantly controlled by gas bubble movements. In this temperature range, periodicity and maximum amplitude decrease and dominant frequency increases with temperature. Above 1400 °C, interparticle forces, such as van der Waals and viscous forces, are significantly strengthened due to changes in the structural and physiochemical properties of solid materials. The greatly enhanced interparticle forces at ultra-high temperatures promote the dominance of microstructured flows over the overall pressure fluctuations, making interparticle forces the controlling factor influencing gas–solid flows in fluidized beds at ultra-high temperatures. These findings provide a better understanding of ultra-high temperature fluidized beds and can promote their research and development for industrial applications.

Correlation between retinal vein pulse amplitude, estimated intracranial pressure, and postural change

Spaceflight associated neuro-ocular syndrome (SANS) is common amongst astronauts on long duration space missions and is associated with signs consistent with elevated cerebrospinal fluid (CSF) pressure. Additionally, CSF pressure has been found to be elevated in a significant proportion of astronauts in whom lumbar puncture was performed after successful mission completion. We have developed a retinal photoplethysmographic technique to measure retinal vein pulsation amplitudes. This technique has enabled the development of a non-invasive CSF pressure measurement apparatus. We tested the system on healthy volunteers in the sitting and supine posture to mimic the range of tilt table extremes and estimated the induced CSF pressure change using measurements from the CSF hydrostatic indifferent point. We found a significant relationship between pulsation amplitude change and estimated CSF pressure change (p < 0.0001) across a range from 2.7 to 7.1 mmHg. The increase in pulse amplitude was highest in the sitting posture with greater estimated CSF pressure increase (p < 0.0001), in keeping with physiologically predicted CSF pressure response. This technique may be useful for non-invasive measurement of CSF pressure fluctuations during long-term space voyages.

Uncertainty and validation of unsteady pressure-sensitive paint measurements of acoustic fields under aero engine-like conditions

Fast response pressure-sensitive paint (PSP) allows optical measurements of pressure fluctuations on a surface with high spatial and temporal resolution. This technique is evaluated for aeroacoustic measurements inside an aeroacoustic wind tunnel (AWT). The AWT is a test rig especially designed for investigating the excitation and propagation of sound under conditions typical for turbomachinery. The aim of this work is to compare the results of sound pressure measurements of tonal sound fields in a circular duct conducted with PSP and microphone arrays in order to assess the applicability of PSP in turbomachinery acoustics applications. A data analysis process is presented, which projects the camera image of the PSP data onto a given surface. To analyze the spatial pressure fluctuations, the PSP data are transformed in the frequency domain using pixel-wise fast Fourier transform. Measurements with a mean Mach number up to 0.109 and 5 kHz excitation frequency are conducted. An acoustic mode generator is used to excite the sound field with specific circumferential mode order. The pressure fluctuations obtained with the PSP measurement visualize the measured acoustic field well and allow early interpretation. The pressures of PSP and microphones are in good agreement; for example, the maximum detected deviation in pressure at 2700 Hz is 30 Pa. A preview on using radial mode analysis to decompose the acoustic field, measured by PSP, into acoustic modes is provided. The results are confirmed by a decomposition using conventional arrays of flush-mounted microphones.Graphical

Multiparametric experimental analysis of the pin disc rotational cavitation generator

The alarming increase in water pollution is driving research into novel, environmentally friendly treatment solutions such as hydrodynamic cavitation. This study is part of the research on the pin disc rotational generator of hydrodynamic cavitation, which utilises the low pressure wake behind the rotor pins to induce cavitation and the short gap between the rotor and stator pins to enhance cavitation cloud fluctuation. Due to the lack of understanding of the effects of cavitation treatment, a laboratory device was built to investigate the mechanisms of cavitation generation and the effects of various geometric features such as the diameter, number, and shape of the rotor pins and the gap size between the rotor and stator pins. Using simultaneously measured pressure fluctuations and high-speed visualisation, a method was developed to quantify the extent of cavitation, and it was found that throttling the high-pressure side had an order- of- magnitude smaller effect on cavitation than the number of rotor pins. It was found that a smaller number of rotor pins with large downstream area produced the most aggressive cavitation conditions. The weak spectral response and lower mean vapour cloud area, as well as the lower fluctuation in the case without stator, demonstrated the key role of the stator in the onset and aggressiveness of cavitation.

Active control of coupling and its effect on near-field pressure fluctuations in supersonic rectangular twin jets

Supersonic rectangular twin jets (SRTJ) are of interest for current and future generations of tactical aircraft. However, the adverse effects of screech-loop coupling have previously been documented to significantly increase the near-field (NF) pressure fluctuations. These high-amplitude pressure fluctuations have the potential to fatigue and damage nearby aft components of the aircraft. Previous studies have documented that the NF pressure fluctuation level depends on the coupling of the jets: in-phase coupling along the twin jets’ minor axes produces stronger NF pressure fluctuations than that of out-of-phase coupling. The objective of this work is to further investigate the effects of coupling modes on NF pressure fluctuations in SRTJ and to mitigate the adverse effects of coupling using active flow control. Localized arc filament plasma actuators are employed to alter the SRTJs’ coupling mode by leveraging natural flow instabilities with minimal power input. A NF microphone array is used for simultaneous coupling and NF pressure fluctuation measurements. Schlieren imaging and spectral proper orthogonal decomposition are used to assess the effects of control on the flow field. The effect of excitation at various frequencies and actuation patterns on coupling and NF pressure fluctuations in different flow regimes are explored and discussed.