Wave Flow Research Articles

Computational study of a helium-propellant microwave electrothermal thruster

The microwave electrothermal thruster (MET) utilizes wave-exited microdischarges to heat gas flows, enhancing the specific impulse of the thruster. Our computational study investigates a 17.5 GHz helium-propellant MET, employing a two-dimensional, axisymmetric fluid model of plasma coupled with electromagnetic wave and gas flows. The discharges operate in the glow regime, remaining weakly ionized, and in thermal non-equilibrium. The plasma densities reach approximately 1020m−3, and the gas temperature is around 2000 K. Even a slight off-resonant frequency operation results in a significantly lower plasma density and gas temperature. Gas heating, primarily driven by electromagnetic Joule heating, plays a critical role in influencing the overall discharge behavior. The measured peak thrust and specific impulse are 8.24 mN and 292 s, respectively, at a mass flow rate of 3.2 mg/s with 30 W of power. Compared to a cold gas thruster, there is a significant increase in the specific impulse by a factor of approximately 1.7. The enhanced performance trades off with propulsive efficiency, which decreases by a factor of 1.5 from the peak 65% at 10 W. This is due to higher energy losses to cavity walls from heat conduction with increased power. These findings underscore the critical balance between the input power and mass flow rate to improve the MET performance, highlighting the importance of power management to maximize thrust and efficiency. Furthermore, the predicted thrust and specific impulse agree well with experimental values for nominally similar MET thruster studies in the literature.

Extreme wave impacts on a suspended box above free surface

A systematic approach incorporating precision-controlled experiments and high-fidelity numerical simulations has been employed to delve deeper into the physics of extreme wave impact pressures. Using a suite of high-speed imaging and high-resolution pressure sensing techniques that are accurately synchronized with the wave generation, a consistent and well-correlated set of data providing details of the focusing incident wave front, flow velocities, and impact pressure time histories has been successfully obtained. The numerical simulations using a validated incompressible smoothed particle hydrodynamics (ISPH) provided further details that were not tractable in the experiments. Based on the results obtained, there were five broad impact scenarios identified, ranging from focusing crest impacts to plunging jet impacts with varying entrapped air. Among these impact types, the highest impact pressures were those associated with a focusing wave crest. It has been found that high impact pressures were correlated with a convergence of the horizontally impinging overturning crest and the vertical surge of the wall boundary water mass, but with the overturning crest impingement just ahead of the arrival of the wall boundary water mass, which in essence amounted to the impingement of a focusing wave front. Depending on the relative arrival times of the impinging crest and the wall boundary upsurge, the impact scenarios could vary from an up-slosh to a jet impingement with a clear entrapped air pocket, followed by a flip-through of the incident crest. For the scenario with the highest impact pressures, broadly classified as type II impact in this paper, the peak pressures could reach as high as 85ρC2, an order of magnitude higher than impact scenarios without the focusing effect, such as the scenario with a plunging jet impact with entrapped air (classified as types III and IV impact in this study). As the volume of entrapped air for this scenario was relatively small, the one-phase ISPH model used in this study was able to capture the peak pressure characteristics, including the peak pressure amplitudes. However, in scenarios with significantly highly entrapped air during impact, a two-phase model would be necessary.

Penalty Sponge Layers (PSL) for hyperbolic systems. General formulation, well-posedness and stability

In this work we design and analyze an alternative to the Absorbing Boundary Conditions (ABC) or Perfectly Matched Layers (PML) technique to solve symmetric first order hyperbolic systems in unbounded domains. We propose a general formulation of absorbing layers that we call Penalty Sponge Layer (PSL). Well-posedness as well as stability properties are quite simply inherited from the original problem. Such a formulation is written in terms of a set of auxiliary variables similar to those encountered in the Complex Frequency Shifted PML (CFS-PML) technique, but this time they have the role of a penalty term, hence the name PSL. This second role of penalty, ensured by stretching an artificial frequency to infinity in the vicinity of the interface, makes the layer asymptotically non-reflecting. Numerical tests are performed to compare them with Standard Sponge Layers (SSL) and PMLs, and to validate their success, even in severe situations where the latter fail (e.g., the Euler system at oblique subsonic/critical or supersonic flow and anisotropic waves).

Numerical study on improved mass and heat transfer performance in a solid oxide electrolysis cell with sine wave flow field

In this paper, the periodic variation of sine wave is used to promote the transport and diffusion process of reactive gases; and the relationship between the performance of a solid oxide electrolysis cell (SOEC) and the mass and heat transfer capabilities of its flow field is thoroughly examined. The impact of variations in the amplitude and frequency of the sine wave on the SOEC's electrolysis efficiency, its mass and heat transfer properties, and fluid flow dynamics are explored. The results indicate that longitudinal flow velocity of the gas in the sine wave flow channel is increased by 26.96-80.76% at a current density of 10000 A/m2, and this kind of flow channel exhibits superior gas transport performance. Moreover, the temperature-uniformity index and the maximum thermal stress of the positive-electrolyte-negative (PEN) structure are decreases by 18.93-22.67% and 20%, respectively. It indicates that this configuration is capable of reducing thermal stress and enhancing the uniformity of temperature distribution at high current-densities. Increasing the amplitude and frequency improves the comprehensive performance of the flow field. More specifically, frequency has a greater impact on flow field performance than amplitude.

Coupling SPH-DEM method for simulating the dynamic response of breakwater structures under severe free surface flow

This paper introduces a novel smoothed particle hydrodynamics (SPH) - discrete element method (DEM) to enhance our understanding of the impact of free-surface flow on breakwater structures. SPH is employed to simulate incompressible fluids, while DEM is utilized to track the trajectories of solid particles. Advanced strategies including polyhedral particle modeling and contact model are adopted to enhance the model's capability in solving the dynamic behavior of irregular particle. Several cases are analyzed to validate the accuracy of coupling model, showing a high level of agreement. Additionally, the impact of wave flow on coastal breakwaters composed of irregular rocks is simulated. This work reveals that the coupling model effectively provides extensive information about the flow field and dynamic response of multi-particle systems, such as velocity, fluid force and force chain distribution. This model exhibits significant potential for analyzing the dynamic response of severe free surface flow on the multi-block structures.

Coupling Effects of a Top-Hinged Buoyancy Can on the Vortex-Induced Vibration of a Riser Model in Currents and Waves

In order to investigate the effects of the top-end dynamic boundary of risers caused by floater motions on their vortex-induced vibration (VIV) characteristics, a combined model comprising a buoyancy can with a relatively simple structural form and a riser is taken as the research object in the present study. The aspect ratios of the buoyancy can and the riser model are 5.37 and 250, respectively. A set of experimental devices is designed to support the VIV test of the riser with a dynamic boundary stimulating the vortex-induced motion (VIM) of the buoyancy can under different uniform flow and regular wave conditions. Several data processing methods are applied in the model test, i.e., mode superposition, Euler angle conversion, band pass filter, fast Fourier transform, and wavelet transform. Based on the testing results, the effect of low-frequency VIM on the high-frequency VIV of the riser is discussed in relation to a single current, a single wave, and a combined wave and current. It is found that the coupling effect of VIM on the riser VIV presents certain orthogonal features at low current velocities. The effect of the cross-flow VIM component on VIV is far more prominent than that of its counterpart, the in-line VIM, with increasing flow velocity. The VIM in the combined wave–current condition significantly enhances the modulation of vibration amplitude and frequency, resulting in larger fluctuation peaks of vibration response and further increasing the risk of VIV fatigue.

Overtaking Effects on Spherical Shock Wave Propagation in Metallic Mediums

The propagation of spherical shock waves through metallic mediums have been considered in this paper. The effect of overtaking disturbances in the propagation of spherical shock waves through solids, especially metals such as Aluminium and Copper, has been analysed by employing Chisnell-Chester-Whitham method. Under the influence of spherical shock waves flow variables have been obtained analytically as well as estimated numerically. Expressions for the pressure and the particle velocity behind the shock wave have also been obtained. It is found that shock strength in both the metals decreases as shock propagates through them. Shock velocity and modified shock velocity depict a decreasing pattern with increasing propagation distance. These observations suggest many industrial and medical applications of shock waves.

Global patterns in river water storage dependent on residence time

AbstractAccurate assessment of global river flows and stores is critical for informing water management practices, but current estimates of global river flows exhibit substantial spread and estimates of river stores remain sparse. Estimates of river flows and stores are hampered by uncertainties in land runoff, an unobserved quantity that provides water input to rivers. Here we leverage global river flow observations and an ensemble of land surface models to generate a globally gauge-corrected monthly river flow and storage dataset. We estimate a global river storage mean (± monthly variability) of 2,246 ± 505 km3 and a global continental flow of 37,411 ± 7,816 km3 yr−1. Our global river water storage time series demonstrates that flow wave residence time is a fundamental driver that can double or halve river water stores and their variability. We also reconcile the wide range in previous estimates of monthly variability in global river flows. We identify previously underappreciated freshwater sources to the ocean from the Maritime Continent (Indonesia, Malaysia and Papua New Guinea) amounting to 1.6 times the Congo River and illustrate our capability of detecting severe anthropogenic water withdrawals.

Study on Dynamic Response Characteristics of Offshore Floating Wind Turbine Pitch System

Due to its special working conditions, offshore wind turbine will bear large direct and indirect loads under the combined action of air flow and wave flow. In this paper, a variable pitch system composed of variablepitch motorand variable pitch bearing is improved, and the characteristics of system's bending moment, torque, vibration and other physical quantities under the action of multiple physical loads are verified, and the mechanical response characteristics of floating wind turbine under the control of unified variable pitch and independent variable pitch are studied under the running conditions at sea. The results show that mechanical structure of uniform pitch is compared with that of independent pitch, the independent variable pitch structure can effectively reduce the mean oscillation value of wind turbine tower in the parallel direction of air flow by optimizing control strategy, and reduce the thrust at the hub of wind turbine and the bending moment at the root of tower, but increase the vibration frequency and fatigue load of offshore wind turbine tower along parallel direction of air flow. Reduce the fatigue life of equipment. The research results can be used as a reference to reduce the variable pitch control and vibration suppression of offshore wind turbines and improve the reliability of wind turbines.

Cardiac dysfunctions in children with drug-resistant epilepsy.

There were reports of cardiac dysfunction that led to sudden unexpected death in epilepsy (SUDEP) in patients with epilepsy. Early detection of cardiac dysfunction can lead to early management to prevent sudden cardiac death in these patients. The objective of our study is to assess cardiac functions in children with drug-resistant epilepsy (DRE) compared with the normal population by using a standard echocardiogram (SE), tissue Doppler imaging (TDI) and myocardial strain evaluations (MSE). Twenty-seven children who have been diagnosed with DRE based on the International League against Epilepsy (ILAE) were included in the study, along with 27 children whose ages match those of the normal control group. Seventeen children, median age 12 years old, were using more than four anti-seizure medications. Structural brain lesions were the most common cause of epilepsy, 55.6% (15). Generalized tonic-clonic seizures were the most common seizure type, 55.6% (15). Children with DRE had a lower early mitral valve E wave inflow velocity compared with the control group (p < 0.05). They also had lowered early diastolic velocities (e') and myocardial performance index (MPI) when compared with the control group (p < 0.05). There was a statistically significant difference in left ventricular myocardial strain in children with DRE, with an average of -21.1 (IQR -23.5 and -19.4) and control, -25.5 (IQR -27.3 and -24.2). Children with DRE have an impairment of left ventricular diastolic function and myocardial strain, which could indicate decreased myocardial deformation and contraction compared with controls. These cardiological assessments can be used to evaluate children with DRE for early diagnosis and management of their cardiac dysfunction.

Analysis of the Oscillatory Flows of Multiple Shock Waves in a Constant Area Duct

Analysis of the Oscillatory Flows of Multiple Shock Waves in a Constant Area Duct

Low-order modelling of three-dimensional surface waves in liquid film flow on a rotating disk

Using low-dimensional numerical simulations, we investigate the characteristics of complex and three-dimensional surface waves in a liquid film flowing over a rotating disk, focusing on large flow rates from a nozzle. Existing integral boundary layer (IBL) models, which are based on spatially averaged variables along the direction normal to the disk surface, have primarily focused on the formation of axisymmetric waves under relatively small flow rates. In this study, an extended IBL model that accounts for both laminar and turbulent regimes is developed by considering the non-uniformity of the local flow rate in the spreading film flow and incorporating closure models dependent on the local Reynolds number. Our numerical results successfully capture the generation of concentric waves by an impinging circular liquid jet and their transition into three-dimensional solitary waves. These findings are in good agreement with visualization images and time-series data of free-surface fluctuations from a displacement sensor. The backscattering of small-scale three-dimensional turbulence into large-scale horizontal turbulence inside the film plays a critical role in determining the transition of wave modes and the nonlinear dynamics of the waves in the turbulent regime. Furthermore, the behaviour of three-dimensional waves in the downstream region, including frequent wave coalescence in the transition region and the breakup of small-scale solitons, is distinct from that of gravity-driven falling film flows. The amplitude of the three-dimensional waves is inversely related to the generalized Reynolds number defined for rotating films.

Numerical study on performance of submerged permeable breakwater under impacts of multi-directional focused wave groups

Multi-directional waves focusing is considered a crucial method for generating extreme waves, posing a significant threat to coastal infrastructure and personal property. In recent years, there has been widespread construction of permeable submerged breakwaters as a preventive measure against extreme wave damage. Therefore, this study aims to assess the attenuation of multi-directional focused waves by both permeable and impermeable submerged breakwaters based on nonhydrostatic numerical wave model (NHWAVE). Effects of several key parameters are carefully examined. According to research findings, permeable submerged breakwaters offer significant benefits in reducing the wave amplitude, flow velocity, water elevation, and energy flux of multi-directional focused wave groups, while also effectively inhibiting the transfer of wave energy to higher frequencies. Additionally, we observe alternating positive and negative vortices occur at the boundary of the permeable submerged breakwater. The findings of this study are expected to advance the investigation into the hydrodynamic characteristics of submerged permeable breakwaters subjected to focused waves.

Meta-barriers for ventilated sound reduction via transformation acoustics

Resolving the trade-off problem between wave obstructing and flow transport is an intriguing but challenging task that may find potential applications in many noise control scenarios. Herein, we present a meta-barrier for achieving broadband sound reduction yet keeping efficient ventilation based on transformation acoustics. The proposed meta-barrier consists of an internal core and a coating made of double-negative acoustic metamaterial with high damping dissipation. Unlike conventional barriers, the meta-barrier has a magnified scattering cross-section far beyond its actual size, allowing it to provide the coherent interference effect in addition to the enhanced blocking effect. The coherent interference effect can directly reduce the sound radiation efficiency of sound waves radiating from an aperture. Moreover, the inherent material loss in the coating has a positive effect on the sound reduction performance of the meta-barrier over a wide frequency range. The robust sound reduction performance of the meta-barrier is demonstrated numerically for the cases of large placed distances and wide-range incident angles. Experiments were conducted to confirm the effectiveness of a meta-barrier sample constructed of a plate-type double-negative acoustic metamaterial with multiple constituent layers for sound reduction and ventilation. The fabricated meta-barrier sample, with the wind-speed ratio of 75 %, can achieve superior sound attenuation in the frequency range of 500–1000 Hz. This work may inspire the design and development of passive acoustic devices to suppress low-frequency airborne noise while supporting large-area ventilation.

Regular reflection of shock waves in steady flows: viscous and non-equilibrium effects

Numerical analysis of a steady monatomic gas flow about the point of the regular reflection of a strong oblique shock wave from the symmetry plane is conducted with the Navier–Stokes–Fourier (NSF) equations, the regularized Grad 13-moment (R13) equations and the direct simulation Monte Carlo (DSMC) method. In contrast to the inviscid solution to this problem completely defined by the Rankine–Hugoniot (RH) relations, all three models predict a complicated flow structure with strong thermal non-equilibrium and a long wake with flow parameters not predicted by the RH relations. The temperature $T_y$ related to thermal motion of molecules in the direction normal to the symmetry plane has a maximum inside the reflection zone while in a planar shock wave the maximum is observed for the $T_x$ temperature. The R13 equations predict these features much better than the NSF equations and are in good agreement with the benchmark DSMC results. An analysis of the flow with the conservation equations was conducted in order to evaluate the effects of various processes on a fluid element moving along the symmetry plane. In contrast to the shock wave where effects of viscosity and heat conduction are one-dimensional with zeroth net contribution to the fluid-element energy across the shock, the flow across the zone of the shock reflection is dominated by two-dimensional effects with positive net contribution of viscosity and negative contribution of heat conduction to the fluid-element energy. These effects are believed to be the main source of the wake with parameters deviating from the RH values.

Monte Carlo simulation of O2− ion transport and rate properties in noble gases with O2 addition

The kinetics of O2− ions were numerically studied in He and Ar with a small admixture of O2 in a strong electric field through a Monte Carlo simulation. Cross sections for elastic scattering between O2− ions and He and Ar atoms were determined by adjusting cross section values to obtain good agreement between calculated and measured ion mobilities in pure helium and argon. Ion transport coefficients, mean energy, and detachment rate were calculated for various mole fractions of O2 and values of the reduced electric field. It was shown that the O2− ion mobility, mean energy, and detachment rate in He:O2 mixtures are much higher than those in Ar:O2 mixtures and air. A great difference between the properties of O2− ions in He–O2 and Ar–O2 mixtures was explained on the basis of the difference in the polarizability and mass for He and Ar atoms. Calculated detachment rates can be used for simulating guided ionization waves in He and Ar flows ejected into ambient air.

Downhole detection of gas kick using low-frequency elastic wave: Multiphysics modeling and its implications

Gas kick is a common and high-risk drilling trouble in geo-energy engineering, especially for deep-water drilling and ultra-deep well drilling. The most favorable way of early detection of gas kick is to move the detection from the wellhead or near surface to the downhole. In this paper, a new conception of downhole detection of gas kick using low-frequency elastic wave was put forward. The framework of coupling the multiple physic processes of gas-liquid two-phase annulus flow, bubble migration and low-frequency elastic wave propagation during gas kick was developed. The characteristic responses of low frequency elastic wave to gas kick were quantitatively analyzed based on multiphysics modeling under the ideal case and different levels of background gas (BGG), respectively. The modeling results indicated that: (i) The velocity decreases cliff-like and the attenuation coefficient increases significantly at the gas-kick bubble front. The change magnitudes of the velocity and the attenuation coefficient increase with the gas intrusion rate, and decrease with the pump displacement. The change magnitude of attenuation coefficient decreases with mud density, while the increase of velocity is not sensitive to the mud density. (ii) For the ideal case of the vertical well, the effective detection within the depth section of (2014–2568) meter could identify gas kick about (14.3–29.2) minutes earlier than the conventional pit-gain method. The gas kick could be identified earlier when the deviation angle is smaller and/or the monitoring point goes deeper. (iii) The changing magnitudes and rates of the velocity and the attenuation coefficient decrease with the BGG. The downhole detection of gas kick using low-frequency elastic wave should be reliable when the BGG is lower than 5%. The effects of rock cuttings, phase transition of the gas, and drill-string vibrations should be further investigated in future study.

Effect of perforation density distribution on production of perforated horizontal wellbore

To address the issue of horizontal well production affected by the distribution of perforation density in the wellbore, a numerical model for simulating two-phase flow in a horizontal well is established under two perforation density distribution conditions (i.e. increasing the perforation density at inlet and outlet sections respectively). The simulation results are compared with experimental results to verify the reliability of the numerical simulation method. The behaviors of the total pressure drop, superficial velocity of air-water two-phase flow, void fraction, liquid film thickness, air production and liquid production that occur with various flow patterns are investigated under two perforation density distribution conditions based on the numerical model. The total pressure drop, superficial velocity of the mixture and void fraction increase with the air flow rate when the water flow rate is constant. The liquid film thickness decreases when the air flow rate increases. The liquid and air productions increase when the perforation density increases at the inlet section compared with increasing the perforation density at the outlet section of the perforated horizontal wellbore. It is noted that the air production increases with the air flow rate. Liquid production increases with the bubble flow and begins to decrease at the transition point of the slug–stratified flow, then increases through the stratified wave flow. The normalized liquid flux is higher when the perforation density increases at the inlet section, and increases with the radial air flow rate.

Enhancing the efficiency of gas sensing on perovskite BaTiO3 nanoparticles using dynamic shock wave flow environment

The effect of nonlinear dynamic shock waves on perovskite BaTiO3 nanoparticles (NPs) is investigated using a table top pressure-driven shock tube (Reddy Tube) for ammonia gas sensing applications with the Mach number of 2.2. In general, shock waves are a nonlinear high-pressure phenomenon characterized by high dynamic pressure and temperature. Raman and powder X-ray diffraction (XRD) methods were utilized to evaluate the molecular and structural characteristics of the BaTiO3 (BTO) NPs under the influence of dynamic shock waves. The surface absorption was assessed using UV- diffuse reflectance spectroscopy (UV-DRS). XRD data shows that BaTiO3 NPs crystallized in tetragonal crystal structure with P4mm space group and stable throughout the shocks, without undergoing any phase change. The average particle size of the control BaTiO3 was 31 nm, while those of shocked BaTiO3 were 62 nm (100 shocks) and 55 nm (200 shocks), demonstrating the significant change in morphology and showing that particles have irregular spherical morphology and are agglomerated by shock wave loaded conditions. Moreover, BaTiO3 nanoparticles were shocked after being tested with various concentrations of different gases at a temperature of 200°C. Under shock loaded conditions, the heightened sensing response for ammonia gases was between 5 and 20 ppm; also, sensing properties such as sensitivity, response, and recovery time indicated that BaTiO3 nanoparticles are a good potential ammonia gas sensing material for real-time sensor applications with enhanced characteristics.

Wind Profiles in Peninsular Malaysia: A Comprehensive Upper Air Analysis

Understanding the atmospheric properties and patterns is crucial in empowering Malaysia's national aerospace blueprint, national space, and legislation. While various policies have been enacted and implemented, there is a lack of information on the wind profile, specifically the upper air across Peninsular Malaysia. Realising the need to establish a standard guideline for national reference, future research, space-aerospace application, and legislation, this study was performed to develop the first wind profile analysis of upper air in Peninsular Malaysia. Relevant data from the Malaysian Meteorology Department was collected for analysis and evaluation. Specifically, a meteorology balloon attached with a sounding radiosonde was used to record data at 0000UTC (0800 h LT) and 1200UTC (2000 h LT) on the 15th day of each month for 7 years (from 2015 to 2021) at the KLIA and Kuantan Meteorology Stations for the wind profiling analysis. The daily overall data collection was recorded accurately once the balloon's rising rate stabilises from vertical air current (katabatic or anabatic winds). Subsequently, the collected data were evaluated in terms of the minimum, maximum, and average wind speeds for each year and time. Finally, the average wind speed of each year and time were combined to generate the Peninsular Malaysia wind profile. Based on the results, the projected wind profile for both stations identified three peaks of discrete sine wave flow type with low-speed wind profile in Peninsular Malaysia. The three peaks amplified the highest air velocity, whereby the 1st and 2nd peaks were located at the troposphere layer from 9,000 m to 12,000 m altitude (average wind speed of 10.8 ms-1) and 12,000 m to 18,000 m altitude (average wind speed of 13.7 ms-1). The 3rd peak was located at the stratosphere layer from 18,000 m to 32,000 m altitude (average wind speed of 15.2 ms-1). Since East Malaysia is located on the same equatorial line, the wind profile is hypothetically the same and exhibits only slight differences. In short, the established wind profile of upper air in Peninsular Malaysia in this study would facilitate other future studies and assist long-term planning of Malaysia's airspace legislation.