High Frequency Pressure Research Articles

High-frequency pressure fluctuations in vertical jet impingements: fast pressure-sensitive paint analysis

High-frequency pressure fluctuations in vertical jet impingements: fast pressure-sensitive paint analysis

Plume and wall temperature impact on the subsonic aft-body flow of a generic space launcher geometry

Experimental and numerical simulation of launcher base flows are crucial for future launcher design. In experiments, exhaust plume simulation is often limited to cold or slightly heated gases. In numerical simulations, multi-species reactive flow is often neglected due to limited resources. The impact of these simplifications on the relevant flow features, compared to real flight scenarios, needs to be characterized in order to enhance the design process. Experimental and numerical investigations were carried out within the framework of the SFB/TRR 40 Collaborative Research Centre to study the impact of plume and wall temperature on the base flow of a generic small-scale launcher configuration. Wind tunnel tests were performed in the Hot Plume Testing Facility (HPTF) at DLR, Cologne, using subsonic ambient flow and pressurized air or hydrogen–oxygen combustion as exhaust gases. The tests were numerically rebuilt using the DLR TAU code employing a scale-resolved IDDES approach, including thermal coupling and detailed chemistry. The paper combines the experimental and numerical findings from the SFB/TRR 40 base flow studies and highlights the most prominent influences on the mean flow field, the pressure field, the dynamic wake flow motion, and the resulting aerodynamic forces on the nozzle. High-frequency pressure measurements, high-speed schlieren recordings, and time-resolved CFD results are evaluated using spectral and modal analysis of the one- and two-dimensional flow field data.

To plasma electrons motion theory in high-frequency fields

The article is dedicated to the kinetic theory of plasma, where the problem of interaction of high-frequency electric fields with weakly non-uniform plasma is investigated using kinetic equations with binary collision integrals for plasma particles. A new methodology for determining the expression of the averaged highfrequency pressure force is proposed based on solving the kinetic equation and using the method of successive approximations (separation of slow motions and fast oscillations) under the limiting conditions. An expression for the averaged quasi-potential force is derived based on the kinetic equation for the electron distribution function in weakly inhomogeneous magnetoactive plasma, taking into account electrons collisions with fixed ions and the presence of a longitudinally inhomogeneous high-frequency electric field using wellknown methods of theoretical and mathematical physics, such as successive approximations, averaging over the effective field oscillation period, and integration over trajectories. The amplitude of the field is a slowly varying function in both time and space coordinates. The obtained expression allows us to estimate the influence of collisions of plasma particles on the Miller force, and under limiting conditions it coincides with the known expressions for the high-frequency pressure force derived from the equation of plasma electrons motion in high-frequency fields. In all calculations, the contribution of the magnetic component of the electromagnetic field is neglected, which is quite justified for the longitudinal electric field.

Optimization of Composition, Cold Rolling Scheme, and Structure of Brass Strip Billets for Continuous High-Frequency Pressure Welding

Optimization of Composition, Cold Rolling Scheme, and Structure of Brass Strip Billets for Continuous High-Frequency Pressure Welding

Formulation of justifiable wind loading distributions up tall buildings derived from HFFB measurements

For the general case of non-linear and combined sway and twist mode shapes, analysis of HFFB measurements gives rise to serious challenges and various techniques have been developed using significant assumptions. This is because it is not possible to directly obtain the distribution of varying fluctuating wind loads throughout the height of tall buildings. This paper proposes a straightforward HFFB-based wind load/twist distribution approach in the time domain to predict the wind-induced dynamic response of tall buildings. Using this framework, it is possible to overcome the deficiencies of mode shape correction factor approach, and the complexity and impracticality of alternative approaches for tall buildings. Two 1:300 scale rigid models of benchmark tall buildings, IAWE Buildings A and B, have been selected to perform a detailed wind tunnel investigation as subjects to evaluate this proposed HFFB approach study. Building A has complex 3D mode shapes combining sway and twist, whereas the simpler Building B has uncoupled linear mode shapes in sway and twist. The high frequency pressure integration (HFPI) wind tunnel technique was utilised to measure surface pressure information in separate tests to obtain the reference wind loading for validation of the HFFB predictions. Furthermore, a commonly used HFFB mode shape correction factor approach was used to estimate the dynamic wind responses which were compared to those from the proposed approach. From the comparison between the aerodynamic and dynamic wind results from proposed HFFB approach and reference HFPI results, it is found that the proposed HFFB approach can predict the vertical distributions of the wind forces and torque (wind loads) with acceptable accuracy for carrying out the conceptual stage of tall building design.

Cavitating flows in microchannel with rough wall using a modified microscale cavitation model

Cavitation can induce drastic disruption and significantly enhance heat transfer, especially in microchannels. However, due to the scale effect, the cavitation characteristics in micro-scale can be significantly different from those in conventional scale. A modified cavitation model according to the Rayleigh-Plesset bubble kinetic equation is formulated by including the effects of surface tension, and temperature difference between the local environment and the far-field environment. The impacts of latent heat of vaporization and condensation are introduced into the energy equation. The micro-scale cavitation model proposed in the paper is verified through comparison with experimental data. The influence of surface roughness on cavitation flow in microchannel with restrictor is then investigated. The results demonstrate that bottom surface roughness in microchannels significantly increases flow resistance, suppresses cavitation flow and causes high frequency pressure fluctuations.

A new method for obtaining wind loads by robustly combining part-model test results

The authors propose a new method to synthesise the time series of wind loads up the height of a building by robustly combining part-model test results and show that they are essentially identical to those sampled simultaneously from a complete model. A pressure-tapped circular cylinder was built to study and validate this segmented model test method using high frequency pressure integration. The detailed mathematical derivation of the synthetic wind load (SWL) method is presented. The dynamic response of a hypothetical chimney using uncorrected wind loads from two independent wind tunnel tests of the circular cylinder model is found to be underestimated considerably compared to using wind loads from simultaneously sampled pressure taps. However, the dynamic response of the chimney using the SWL method to robustly combine the wind loads from two independent wind tunnel tests is almost identical to the dynamic response of the chimney using pressure tap data sampled simultaneously over the whole model. The establishment of this method is of great importance for the wind tunnel investigation of very high aspect ratio buildings where a model of the entire building cannot be tested within typical wind tunnel test sections because the Reynolds number would be too low, or the model section would be too narrow to contain the pressure tubes, and part-models need to be tested.

The influence of vegetated marshes on wave transformation in sheltered estuaries

Assessing the influence of marshes on mitigating flooding along estuarine shorelines under the pressures of sea level rise requires understanding wave transformation across the marsh. A numerical model was applied to investigate how vegetated marshes influence wave transformation. XBeach non-hydrostatic (XB-NH) was calibrated and validated with high frequency pressure data from the marsh at China Camp State Park in San Pablo Bay, California (USA). The model was used to examine how marsh and hydrodynamic characteristics change the potential for marshes to mitigate wave driven flooding. Model results demonstrate that hydrodynamics, vegetation, and marsh width influence wave transformation most, while marsh morphology parameters such as elevation and slope had least effect. Results suggest that in the range of settings explored here (incident wave heights ranging from 0.5 to 3 m and water levels ranging from current mean higher high water to 3 m above current mean higher high water), in comparison to wave propagation over an unvegetated mudflat, marsh vegetation reduces runup by a median of 40 cm and wave height by a median of 35 cm. Results illustrate how marshes can be strategically utilized to provide flood reduction benefits.

Propagation characteristics analysis of high-frequency vibration in pumped storage power station based on a 1D fluid-solid coupling model

High-frequency pressure fluctuation is a common hydraulic phenomenon in pumped storage power station (PSPS), which is caused by the rotor-stator interaction in the pumped turbine. The propagation of the high-frequency vibration (HFV) could transmit the vibration energy to the upstream headrace tunnel, inducing severe environmental vibration and noise pollution. This study aims to establish an applicable numerical model to investigate the propagation characteristics of HFV, and evaluate the vibration magnitude of both solid and fluid in the headrace tunnel of the PSPS. In this study, a one-dimensional (1D) numerical model is established for the fluid solid interaction (FSI) behavior of the pipe, where the equations of solid and fluid are solved intuitively in the time domain by the Finite Difference Method (FDM) and Method of Characteristics (MOC), respectively. Detailed discretization schemes and solution procedures are deduced and presented in this study. Four validation cases are carried out to examine the established model. The results demonstrate that the established mode is applicable for predicting the vibration in the fluid-filled pipe (FFP). The non-reflect boundary conditions are proposed to eliminate the influence of the reflected wave from the boundary. The propagation characteristics of the high-frequency vibration (HFV) in FFP are comprehensively analyzed and summarized based on a real PSPS. And magnitude distributions of the pressure fluctuation, axial and radial vibration along the pipeline axis are presented and discussed.

Experimental Study on Carbon Fiber-Reinforced Polymer Groove Machining by High-Power Water-Jet-Guided Laser.

Due to the excellent properties of carbon fiber-reinforced polymers (CFRPs), such as high strength and strong corrosion resistance, the traditional water-jet-guided laser (WJGL) technology has problems with fiber pull-out and has a small cutting depth when processing CFRPs. Therefore, in this study, we used high-power water-jet-guided laser (HPWJGL) technology to perform groove processing experiments on CFRPs. The effects of four key process parameters, high laser power, pulse frequency, feed rate, and water-jet pressure, on the cutting depth were investigated by a single-factor experiment. The formation mechanism of groove cross-section morphology and the processing advantages of high-power water-jet-guided lasers were analyzed. On this basis, the mathematical prediction model of cutting depth was established by using the response surface method (RSM), and the optimal combination of process parameters was obtained. The mathematical prediction model was verified by experiments, and the error was only 1.84%, indicating that the model had a high reference value. This study provides a reference for the precision machining of HPWJGL technology.

Leakage Detection in Water Distribution Networks Based on Multi-Feature Extraction from High-Frequency Pressure Data

Leakage detection is an important task to ensure the operational safety of water distribution networks. Leakage characteristic extraction based on high-frequency data has been widely used for leakage detection in experimental networks. However, the accuracy of single-feature-based methods is limited by the interference of background pressure fluctuations in networks. In addition, the setting of leakage diagnosis thresholds has been insufficiently studied, but influences leakage detection performance greatly. Hence, a new method of leakage detection is proposed based on multi-feature extraction. The multi-features of leakage are composed of instantaneous characteristics (ICs) and trend characteristics (TCs), which constitute comprehensive leakage information. The levels of the instantaneous and trend pressure drops in background pressure fluctuations in network environments are quantified for the setting of leakage diagnosis thresholds. In addition, ICs and TCs are used for leakage degree prediction. The proposed method was applied to an experimental network. Compared with the single-feature-based method and the cumulative sum (CUSUM) method, the proposed method achieved increases of 6.01% and 13.66% in F-Scores, respectively, and showed better adaptability to background pressure fluctuations in complex network environments.

Cavitation-induced shock wave behaviour in different liquids

This paper follows our earlier work where a strong high frequency pressure peak has been observed as a consequence of the formation of shock waves due to the collapse of cavitation bubbles in water, excited by an ultrasonic source at 24 kHz. We study here the effects of liquid physical properties on the shock wave characteristics by replacing water as the medium successively with ethanol, glycerol and finally a 1:1 ethanol–water solution. The pressure frequency spectra obtained in our experiments (from more than 1.5 million cavitation collapsing events) show that the expected prominent shockwave pressure peak was barely detected for ethanol and glycerol, particularly at low input powers, but was consistently observed for the 1:1 ethanol–water solution as well as in water, with a slight shift in peak frequency for the solution. We also report two distinct features of shock waves in raising the frequency peak at MHz (inherent) and contributing to the raising of sub-harmonics (periodic). Empirically constructed acoustic pressure maps revealed significantly higher overall pressure amplitudes for the ethanol–water solution than for other liquids. Furthermore, a qualitative analysis revealed that mist-like patterns are developed in ethanol–water solution leading to higher pressures.

Characterization and predictive modeling of a trajectory-oriented dual-mode scramjet combustor

Linear and nonlinear characteristics of a dual-mode scramjet combustor are investigated in ground-simulated acceleration and deceleration trajectory experiments. The experiments were conducted in the direct-connected transient flight trajectory simulator 1 at the Institute of Mechanics, Chinese Academy of Sciences. High-frequency pressure measurements, the schlieren, and CH* chemiluminescence high-speed imaging were applied for the diagnostics. Based on the quantitative analysis, the physical processes of acceleration and deceleration in general represent similar nonlinear characteristics. The linear characteristics are limited to the low-frequency oscillation period in the presence of physical governing mechanisms. A nonlinear predictive model of the dual-mode scramjet combustor based on historical measurements is proposed due to its generality for the acceleration and deceleration trajectory.

Inlet Gap Influence on Low-Frequency Flow Unsteadiness in a Centrifugal Fan

In this study, unsteady low-frequency characteristics in a voluteless low-speed centrifugal fan operating at a high mass flow rate are studied with improved delayed detached eddy simulation (IDDES). This study is motivated by a recent finding that the non-uniformly distributed pressure inside this type of fan could be alleviated by improving the gap geometry. The present simulation results show that the velocity magnitudes of the gap have distinct low and high regions. Intensive turbulent structures are developed in the low-velocity regions and are swept downstream along the intersection between the blade and shroud, on the pressure side of the blade. Eventually, the turbulence gives rise to a high-pressure region near the blade’s trailing edge. This unsteady flow behavior revolves around the fan rotation axis. Additionally, its period is 5% of the fan rotation speed, based on the analysis of the time history of the gap velocity magnitudes and the evolution of the high-pressure region. The same frequency of high pressure was also found in previous experimental measurements. To the authors’ knowledge, this is the first time that the trigger of the gap turbulence, i.e., the unsteady local low velocity, has been determined.

High temperature fuel impacts on combustion characteristics of a swirl-stabilized combustor

Future aircraft will require advanced thermal management systems to manage the increased heat loads resulting from the avionics, turbomachinery and other subsystems of higher performance, and high speed flight systems. Fuel is the primary coolant to alleviate these thermal management challenges. Heated fuel entering the combustor is likely to affect combustion performance due to dramatic changes in properties that impact fuel atomization, distribution, vaporization and fuel/air mixing. In this study, the combustion performance, emissions, and flame characteristics of a single-nozzle, swirl-stabilized combustor operating with high temperature fuels were investigated. Four fuels representing a wide range of jet fuel physical and chemical properties were studied at injection temperatures of 32, 121 and 260 °C. The test fuels included a conventional Jet A (A-2), a low cetane number alcohol-to-jet (ATJ) alternative fuel (C-1), n-dodecane (n-C12) and a high viscosity jet fuel blend (C-3A). Tests were conducted at a combustor pressure of 2 atm and inlet air temperature of 121 °C at global equivalence ratios (ϕ) of 0.10–0.24 for most fuel temperatures. Particulate matter (PM) emissions (concentrations and size distributions), and primary gaseous emissions were measured. High frequency pressure measurements and high-speed video were collected to assess fuel temperature impacts on combustion stability and flame characteristics, respectively. Results show that the heated fuel impacts on combustion were highly dependent on fuel type, temperature and ϕ. Qualitative results show that as fuel temperature increased, the flame in the primary zone appeared more compact, and less thermally radiant (lower soot). PM emissions were significantly impacted with reductions exceeding 95 % in particle number density for all fuels, and between 20 and 55 % reductions in mean particle diameters for aromatic-containing fuels. Gaseous emissions show up to ∼ 90 % reduction in unburned hydrocarbons (UHC) and up to 60 % reduction in carbon monoxide (CO), and corresponding moderate increases in NOx and CO2 with the heated fuel. The combustor acoustic response was slightly suppressed in frequencies of interest as fuel temperature increased. In general, the impacts of heated fuel on combustion were mostly beneficial and are primarily attributed to improved fuel atomization and reduced fuel evaporation timescales of the heated fuel. Specific impacts of fuel physical and chemical properties, and potential implications of heated fuel combustion are discussed.

Oxidant starvation under various operating conditions on local and transient performance of proton exchange membrane fuel cells

Oxidant starvation in proton exchange membrane fuel cells could cause severe fluctuation on dynamic performance and the significant reduction in durability. In this work, dynamic transfer and distribution characteristics of reactants and water under oxidant starvation are studied by measuring local current densities, temperatures, and the cell voltage in situ. Besides, high frequency resistance and pressure drop along the flow channel at the cathode are obtained to determine the membrane hydration state, and mass transfer and distribution. The effects of cathode humidity, cell temperature and operating modes on the local and overall transient behavior of the cell under oxidant starvation are analyzed. The experimental results show that with the air stoichiometry of 0.8, the current fluctuation distribution is much non-uniform and hydrogen pumping occurs in the downstream. When oxidant starvation occurs under potentiostatic mode, current densities in the upstream gradually increase to the steady-state; while under galvanostatic mode, local current densities in the upstream show overshoots and current variations coefficient along the flow channel are about two times of potentiostatic mode. As cell temperature increases, severe oxygen starvation occurs in the downstream and local temperature differences tend to be larger. When the unhumidified air is supplied into the cell, the combined effect of local membrane hydration state and local oxygen concentration plays a dominant role, leading to greater magnitudes of current fluctuation and higher change rate of current variation coefficient in the downstream.

Experimental study of sharp noise caused by rotating detonation waves

Previous studies of rotating detonation engines (RDEs) focused on combustion, heat transfer, and propulsion, but not noise, which is considered here. High-frequency pressure sensors, such as dynamic sensors, are often used as contact measurements to determine the detonation pressure and rotating detonation cycle time, but they obtain data for only a few seconds. Here, a simple and inexpensive acoustic measurement method is used to obtain the high-frequency sound pressure of the detonation and cycle time over more than a few hours and is not affected by high-temperature products. The results show that data of non-contact acoustic measurements, which can reflect the rotating detonation characteristics, agree well with those of picocoulomb pressure sensors and high-speed imaging. The production mechanism of sharp noise caused by rotating detonation waves is analyzed theoretically and compared with experimental results. Kinematic equations for vibrations are derived, and disturbance trajectories caused by oblique shock waves are found to be Archimedes spirals. This constitutes the first study on noise from RDEs. The study also promotes acoustic measurements, which have rarely been used to study RDEs.

Spatiotemporal analysis of sheet and cloud cavitation and its damage potential

The cavitation regime has a substantial influence on the damage potential, thus it has to be considered in any specific investigation. For this purpose, we set up a test rig at the Technische Universität Darmstadt using a Circular Leading Edge hydrofoil (CLE) to analyse the damage potential of sheet and cloud cavitation. Exceeding a critical Reynolds number Re c, the cavitation regime transitions from harmless sheet cavitation to aggressive cloud cavitation. High-speed recordings of the cavitation regime are correlated with high frequency pressure data from a wall-mounted piezoelectric pressure transducer. Spatial and temporal content of the cavitating flow are captured applying proper orthogonal decomposition (POD) to the high-speed recordings. In order to determine the damage potential of the cavitation regime we apply a copper foil on the hydrofoil surface, on which plastic, crater-shaped deformations due to bubble collapses occur. Images of the surface are recorded before and after each run via two-dimensional Pit-Count microscopy. We correlate spatial modes from the cavitating flow field with the eroded surface rate from pitting tests leading to the result that cloud cavitation associated with increasing cloud size is more aggressive. A power law is identified where pitting rate increases with fourteenth power of the Reynolds number.

Refined 1D–3D Coupling for High-Frequency Forced Vibration Analysis in Hydraulic Systems

High-Frequency Pressure Fluctuation (HFPF) is an extensively observed hydraulic phenomenon in pumped-storage power stations and water conveyance projects. The investigation of the propagation characteristics of the pressure perturbation is of great significance for the safe operation of hydraulic facilities. In this study, a one-dimensional (1D)–three-dimensional (3D) coupling model is established based on the combination of the Method of Characteristics (MOC) and Computational Fluid Dynamics (CFD) and implanted in the open source software OpenFOAM. The established model in this study implants the dynamic mesh module into the original OpenFOAM solver sonicLiquidFoam and presents the complete solution procedure of the CFD model with the dynamic mesh considered. The vibration of the pipe walls modeled by the mesh motion is employed to numerically generate the HFPF in the hydraulic system, which could not be implemented in the traditional MOC model. The independence of the pressure perturbation in the pipeline system is validated by the time-domain pressure variation. The graphical method is applied to describe the multiple reflection and superposition characteristics of the traveling wave in a simplified hydraulic system. Based on this, the mechanism of the superimposed characteristic of the traveling and standing pressure waves in the hydraulic system are analyzed, and the theoretical superimposed time-domain processes and the variations of the pressure oscillation magnitude are analyzed and presented. The 1D–3D coupling method and the theoretical analysis method could be referenced by other complex hydraulic systems.

Understanding Primary Blast Injury: High Frequency Pressure Acutely Disrupts Neuronal Network Dynamics in Cerebral Organoids.

Blast exposure represents a common occupational risk capable of generating mild to severe traumatic brain injuries (TBI). During blast exposure, a pressure shockwave passes through the skull and exposes brain tissue to complex pressure waveforms. The primary neurophysiological response to blast-induced pressure waveforms remains poorly understood. Here, we use a computer-controlled table-top pressure chamber to expose human stem cell-derived cerebral organoids to varied frequency of pressure waves and characterize the neurophysiological response. Pressure waves that reach a maximum amplitude of 250 kPa were used to model a less severe TBI and 350 kPa for a more severe blast TBI event. With each amplitude, a frequency range of 500 Hz, 3000 Hz, and 5000 Hz was tested. Following the 250 kPa overpressure a multi-electrode array recorded organoid neural activity. We observed an acute suppression neuronal activity in single unit events, population events, and network oscillations that recovered within 24 h. Additionally, we observed a network desynchronization after exposure higher frequency waveforms. Conversely, organoids exposed to higher amplitude pressure (350k Pa) displayed drastic neurophysiological differences that failed to recover within 24 h. Further, lower amplitude "blast" (250 kPa) did not induce cellular damage whereas the higher amplitude "blast" (350 kPa) generated greater apoptosis throughout each organoid. Our data indicate that specific features of pressure waves found intracranially during blast TBI have varied effects on neurophysiological activity that can occur even without cellular damage.