Surface Oscillations Research Articles

The influence of parameters on the hydrodynamic characteristics of oscillating water column devices: A numerical study

The influence of parameters on the hydrodynamic characteristics of OWC (oscillating water column) devices exhibits a coupling phenomenon, which is frequently neglected. To elucidate the comprehensive influence and its mechanism of structure parameters on the hydrodynamic characteristics of offshore OWC devices, a structure with a bottom plate was selected in this study. Through numerical simulation, the hydrodynamic characteristics of the chamber with variations in PTO (power take off) coefficient α, chamber length Bs, internal depth Jn, and under water length of front wall Je with different wave conditions are investigated. The following conclusions are primarily drawn: The research method of using the relative wavelength L/Bs as a non-dimensional parameter and solely varying the wavelength L has its drawbacks. In cases that L/Bs is held constant, the efficiency of chambers differing only in Bs can vary by more than 70%. The influence of each parameter on the effectiveness is interrelated. Chambers with a lower α exhibit a more pronounced impact of Bs. The efficiency of device with a smaller relative wavelength L/Bs is more significantly affected by the Jn. The mechanism by which the parameters affect efficiency is primarily since chambers with shorter Bs or greater Jn exhibit smaller deformations during internal liquid surface oscillation, resembling a more uniform oscillating water column. This allows more energy to be utilized for pushing air in and out of the chamber above, thereby enhancing the energy conversion capability. The results of this research can provide valuable insights for the design of OWC devices.

Interaction of a Taylor bubble with the liquid–liquid interface in a notched tube

Injection of air into stratified liquids for improved mixing and for the removal of oil droplets from water is some well-established applications for augmentation of heat and mass transfer and purification of water. The present study investigates the dynamics of a Taylor bubble passing through the liquid–liquid interface in a vertical tube provided with rounded notch of different radii using the Eulerian approach-based volume of fluid method. The complete bypass of the Taylor bubble across the liquid–liquid interface and the notch is observed for dimensionless notch radius, r* = 0.0388, whereas the Taylor bubble has bypassed in the form of smaller volumes periodically pinched-off near the notch for higher notch radius. The drag coefficient has significantly increased during the bypass of the bubble across the liquid–liquid interface, its peak has an order of magnitude ≅102 for r* = 0.0388, and it is ≅ 103 for a larger notch radius. This increase in the drag coefficient is also manifested in terms of significant increase in the wall shear stress and capillary pressure drop across the notch. The increase in liquid viscosity ratio has resulted in a slight increase in pinched-off bubble volumes and their surface oscillations.

VOF-based modeling study of surface oscillation behavior and gas–liquid stirring dynamics in the bottom-blown reactors

Gas bottom-blown reactors are widely employed in the chemical, metallurgical, and biochemical industries due to their superior mixing capabilities. This study numerically investigates the gas–liquid flow hydrodynamics and bubble dynamics in a bottom-blown system with a circular boundary using the volume of fluid approach. Following the experimental validation of the mathematical model, this study evaluates the effects of gas flow rates, liquid heights, and fluid medium properties on the dynamics of the bubble swarm, as well as on the surface behavior and the mixing characteristics of the flow field. The results indicate that: (1) Rising bubbles disturb the fluid flow, generating numerous vortices that facilitate momentum transfer for effective gas stirring; (2) The height of the liquid surface oscillation during the bottom-blown process exhibits significant temporal variation, with the peak showing clear lateral swings; (3) Due to the nonlinear interactions involving bubble coalescence and breakup during their ascent, along with constructive and destructive interference between liquid surface waves, the liquid surface can exhibit violent fluctuations at certain moments; (4) Bubble size and aspect ratio significantly influence liquid surface oscillations; (5) The bubble rising path displays instability due to vortex shedding, with some vortices forming at locations where the bubble shape changes abruptly. A prediction equation for bubble velocity is proposed: uc=σ5.2737Lcρc4.2737/16156μl9.5475.

Application of the Rossiter Model for Predicting the Frequency of Vortex Shedding and Surface Oscillations in Rectangular Shallow Reservoirs

Application of the Rossiter Model for Predicting the Frequency of Vortex Shedding and Surface Oscillations in Rectangular Shallow Reservoirs

Coupling analysis between wave resonance in the moonpool and heave motion of the twin hulls with mooring effect

Fluid resonance in the moonpool formed by two identical rectangular hulls in water waves is investigated by employing the Computational Fluid Dynamics (CFD) package OpenFOAMⓇ. The influence of vertical stiffness on the behavior of moonpool resonance coupling with the heave motion response is presented. Numerical simulations show that the free surface oscillation in the moonpool exhibits a two-peak variation with the incident wave frequency, defined as the first and second peak frequencies. A local Keulegan–Carpenter (KC) number is introduced for describing the influence of fluid viscosity and flow rotation on the fluid resonance and heave motion resonance. At the first peak frequency, the free surface oscillation and heave motion response show an in-phase relationship, where increase in the vertical stiffness can increase the relative motion between them. This finally leads to an increase in the KC number, indicating the increased effect of energy dissipation with increase in the vertical stiffness. At the second peak frequency with an out-of-phase relationship between the free surface oscillation and heave motion response, the variation of the KC number is not sensitive to the vertical stiffness. Correspondingly, the influence of energy dissipation is not strongly dependent on the vertical stiffness.

Defining and Mitigating Flow Instabilities in Open Channels Subjected to Hydropower Operation: Formulations and Experiments

A thorough literature review was conducted on the effects of free surface oscillation in open channels, highlighting the risks of the occurrence of positive and negative surge waves that can lead to overtopping. Experimental analyses were developed to focus on the instability of the flow due to constrictions, gate blockages, and the start-up and shutdown of hydropower plants. A forebay at the downstream end of a tunnel or canal provides the right conditions for the penstock inlet and regulates the temporary demand of the turbines. In tests with a flow of 60 to 100 m3/h, the effects of a gradually and rapidly varying flow in the free surface profile were analyzed. The specific energy and total momentum are used in the mathematical characterization of the boundaries along the free surface water profile. A sudden turbine stoppage or a sudden gate or valve closure can lead to hydraulic drilling and overtopping of the infrastructure wall. At the same time, a PID controller, if programmed appropriately, can reduce flooding by 20–40%. Flooding is limited to 0.8 m from an initial amplitude of 2 m, with a dissipation wave time of between 25 and 5 s, depending on the flow conditions and the parameters of the PID characteristics.

A numerical study on the oscillatory dynamics of tip vortex cavitation

In this paper, we numerically study the mechanism of the oscillatory flow dynamics associated with the tip vortex cavitation (TVC) over an elliptical hydrofoil section. Using our recently developed three-dimensional variational multiphase flow solver, we investigate the TVC phenomenon at Reynolds number $Re = 8.95 \times 10^5$ via dynamic subgrid-scale modelling and the homogeneous mixture theory. To begin, we examine the grid resolution requirements and introduce a length scale considering both the tip vortex strength and the core radius. This length scale is then employed to non-dimensionalize the spatial resolution in the tip vortex region, the results of which serve as a basis for estimation of the required mesh resolution in large eddy simulations of TVC. We next perform simulations to analyse the dynamical modes of tip vortex cavity oscillation at different cavitation numbers, and compare them with the semi-analytical solution. The breathing mode of cavity surface oscillation is extracted from the spatial-temporal evolution of the cavity's effective radius. The temporally averaged effective radius demonstrates that the columnar cavity experiences a growth region followed by decay as it progresses away from the tip. Further examination of the characteristics of local breathing mode oscillations in the growth and decay regions indicates the alteration of the cavity's oscillatory behaviour as it travels from the growth region to the decay region, with the oscillations within the growth region being characterized by lower frequencies. For representative cavitation numbers $\sigma \in [1.2,2.6]$ , we find that pressure fluctuations exhibit a shift of the spectrum towards lower frequencies as the cavitation number decreases, similar to its influence on breathing mode oscillations. The results indicate the existence of correlations between the breathing mode oscillations and the pressure fluctuations. While the low-frequency pressure fluctuations are found to be correlated with the growth region, the breathing mode oscillations within the decay region are related to higher-frequency pressure fluctuations. The proposed mechanism can play an important role in developing mitigation strategies for TVC, which can reduce the underwater radiated noise by marine propellers.

Acceleration is the key to drag reduction in turbulent flow

A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes, and Reynolds numbers. The drag was reduced by as much as 35%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, and/or the Reynolds number. Here, we find that the key parameter is the acceleration, which greatly simplifies the complexity of the phenomenon. This result is shown to apply to channel flows with spanwise surface oscillation as well. This insight opens potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems.

Dynamics of Fourier's and Fick's laws on the convectively heated oscillatory sheet under Arrhenius kinetics: The finite-difference technique

The significance of chemical reaction with activation energy and convective boundary conditions on the fluid flow via an oscillatory stretchy surface in the presence of permeable media and radiation is analyzed in this study. This inspection presents Fourier and Fick's laws-based equations for heat, mass transport, and liquid flow through an oscillating stretchy sheet. Understanding these dynamics aids in the optimisation of catalytic reaction settings, where gradients greatly influence reaction rates in concentration and temperature. The governing differential equations of the current study are modelled and changed into their non-dimensional form by employing suitable similarity variables. The finite difference method (FDM) is also used to numerically solve the obtained dimensionless equations. The influence of many factors on the several profiles is portrayed with graphical representations. The outcome of the unsteadiness and porosity parameters on the velocity profile with time coordinate is depicted. The increase in the radiation parameter and Biot number upsurges the thermal profile. The temperature reduces as the unsteadiness parameter and temperature relaxation time parameter grow. The upsurge in the activation energy parameter intensifies the mass transport. The rise in concentration relaxation time parameter diminishes the concentration profile.

Efficient numerical modeling of time-fractal tangent hyperbolic fluid flow with heat and mass transfer

This paper introduces a precise method for solving time-fractal parabolic equations specifically designed to represent complex physical phenomena using fractal calculus. The suggested technique attains third-order accuracy by utilizing the fractal Taylor series and implements a concise scheme for spatial discretization. Stability and convergence analyses are offered for scalar and system fractal parabolic equations. The mathematical model analyzes fluid flow over both flat and oscillatory surfaces, considering the effects of heat and mass transfer. This involves transforming the governing equations into dimensionless partial differential equations, which are then solved using the suggested method. From the calculated results, it can be verified that the proposed scheme produces less error than the existing two schemes. This study enhances computational techniques for fractal calculus and introduces novel opportunities for comprehending non-Newtonian fluids in diverse physical contexts.

Flow Response of a Laminar Shock–Boundary Layer Interaction to Prescribed Surface Motions

The response of an impinging laminar shock–boundary layer interaction (SBLI) to prescribed surface motions is investigated numerically at M∞=2, Rea=120,000, and a shock-associated pressure ratio of 1.5. Parametric sweeps over classical standing and traveling mode shapes are considered at low-, moderate-, and high-frequency conditions for fluid–structure interaction. The specific values are chosen based on compliant panel deformations and frequencies reported in the literature. At low surface oscillation frequencies, the SBLI responds to the deformations in a quasi-steady fashion, with standing wave forcing displaying both breathing and sloshing of the separated region depending on structural mode shape. As the oscillation frequency is increased, the flow transitions to an unsteady response with pronounced separation bubble undulations in time. Higher-order modes and frequencies lead to the largest reductions in the time-mean separation bubble size, more so with traveling surface waves. Modal decompositions show that pressure fluctuations, which arise due to dynamic interaction with the surface, persist downstream and increase in amplitude with the surface oscillation frequency.

Can β-blockers prevent intracranial aneurysm rupture? - insights from Computational Fluid Dynamics analysis.

Hypertension is a risk factor for intracranial aneurysm rupture. We analyzed whether the intake of drugs from specific classes of anti-hypertensive medications affects hemodynamic parameters of intracranial aneurysm dome. We recorded medical history including medications and the in-hospital blood pressure values. We then obtained 3D reconstruction of each patients' aneurysm dome and the feeding artery. Using OpenFOAM software we performed Computational Fluid Dynamics analysis of blood flow through the modeled structures. Blood was modeled as Newtonian fluid, using the incompressible transient solver. As the inlet boundary condition we used the patient-specific Internal Carotid Artery blood velocity waves obtained with Doppler ultrasound. We calculated haemodynamic parameters of the aneurysm dome. All presented analyses are cross-sectional.We included 72 patients with a total of 91 unruptured intracranial aneurysms. The history of β-blocker intake significantly influenced hemodynamic parameters of aneurysm dome. The patients on β-blockers had significantly smaller aneurysm domes (5.09 ± 2.11 mm vs. 7.41 ± 5.89 mm; p = 0.03) and did not have aneurysms larger than 10 mm (0% vs 17.0%; p = 0.01). In the Computational Fluid Dynamics analysis, walls of aneurysms in patients who took β-blockers were characterized by lower Wall Shear Stress Gradient (1.67 ± 1.85 Pa vs. 4.3 ± 6.06 Pa; p = 0.03), Oscillatory Shear Index (0.03 ± 0.02 vs. 0.07 ± 0.10; p = 0.04) and Surface Vortex Fraction (16.2% ± 5.2% vs. 20.0% ± 6.8%; p<0.01). After controlling for covariates, we demonstrated difference of Surface Vortex Fraction (F[1, 48] = 4.36; p = 0.04) and Oscillatory Shear Index (F[1, 48] = 6.51; p = 0.01) between patients taking and not taking β-blockers, respectively. Intake of β-blockers might contribute to more favorable hemodynamics inside aneurysmal sac. Other antihypertensive medication classes were not associated with differences in intracranial aneurysm parameters.

Influence of activation energy and variable thermal conductivity on magneto-convective nonlinear thermo-radiative Casson nanofluid containing motile microorganisms over a sinusoidal surface

The activation energy of convective-radiative nanoliquids with sinusoidal walls has garnered significant academic interest in recent years. This focus arises due to the pivotal applications of nanoparticles in thermal engineering, industrial processes, and medical sciences. This article investigates the magnetohydrodynamic heat transfer characteristics of Casson nanofluids containing microorganisms influenced by an oscillatory surface. The study incorporates temperature-dependent viscosity and utilizes Buongiorno’s mathematical model to account for nonlinear thermal radiation, Brownian motion, and thermophoretic effects from using nanomaterials in Casson fluids. These effects enhance thermal conductivity, improving the heat transportation phenomenon while considering gyrotactic microorganism density distributions. The boundary layer flow problem is formulated using partial differential equations and associated boundary conditions, subsequently numerically simulated using the bvp4c function. The investigation explores the thermal behavior of nanoparticles in the presence of bioconvection phenomena, Lorentz force, chemical reactions, and activation energy. As a result, this model addresses variable thermal conductivity and bioconvection aspects in Casson nanofluids over a sinusoidal surface. Numerically computed results for the Nusselt number, motile density, and Sherwood number provide insights into several essential features. The findings reveal that an increase in the Casson fluid parameter correlates with a decreased velocity profile. The assumptions of variable thermal conductivity and temperature-dependent viscosity prove more effective in raising the temperature of nanoparticles. The nanoparticle concentration profile rises noticeably for smaller Schmidt number Sc values when the activation energy parameter is higher than larger Sc values. Additionally, the presence of microorganisms in Casson nanofluids leads to increased temperatures due to an elevation in the bioconvection Rayleigh number. Moreover, heightened oscillating frequency parameters decrease nanoparticle concentration and microorganism density. Including activation energy and oscillating frequency parameters further enhances the understanding of nanoparticle concentration and microorganism density dynamics, offering valuable contributions to theoretical research and practical applications across multiple interdisciplinary fields.

Experimental Validation of Power Output and Efficiency for an Oscillating Water Column (OWC)

With the global energy demand escalating and concerns over the environmental impact of fossil fuels, there's a pressing need for cleaner, sustainable alternatives. This study highlights the potential contribution of wave energy to the power needs of coastal structures in the context of renewable energy. The research evaluates the power output and efficiency for an Oscillating Water Column (OWC) system that can be integrated into coastal structures to meet part of their power needs. Findings from both theoretical calculations and a 1:10 scale model experiment are presented. The mechanical power output and efficiency of the system for a full-scale prototype were calculated for deep water conditions with a wave height of 3m. The water surface oscillation inside the chamber is assumed to reflect the oscillation occurring outside the chamber. The maximum average mechanical power output for the full-scale prototype, corresponding to a wavelength of 22.5 m, was determined to be 64.8 kW, achieving a mechanical efficiency of 64.4%. The overall efficiency of the system is calculated as 55% by assuming the generator efficiency to be 85%, resulting in an average power output of approximately 55 kW. A 1:10 scale model of the OWC system with a Wells turbine was constructed and tested in a tank for deep water conditions. Froude similarity and Keulegan-Carpenter similarity were used, ensuring a seamless transition from the model to the prototype. The OWC model was subjected to controlled heaving motion with a period of T=1.2 s. The power generated by the OWC model illuminated four integrated 3.4V LEDs on the Wells turbine, which were used to measure the power output produced. The power output of the model was measured to be a minimum of 0.12 W for a rotational speed of 107 rpm, which corresponds to a power output of 12 kW for the scaled-up prototype. This system has the potential for further enhancement by incorporating multiple OWCs into coastal structures exposed to wave action. Such development could facilitate meeting the power requirements of coastal structures, thereby contributing to the promotion of both renewable energy generation and a sustainable environment. Future research will focus on optimizing OWC chamber sizes for specific sites and refining the model to better capture water surface oscillation dynamics. l

Interface characteristics of selective laser melted stainless steel 316L and Inconel 718

Additive manufactured dissimilar metal constructs of alternating Inconel 718 and Stainless Steel 316L layers were fabricated by using selective laser melting. The solidified interfaces between two materials were examined to understand the interface characteristics. No distinct horizontal layers have been observed and undulating dissimilar interfaces at meso-scale. The undulations of the interfacing are understood a result of the melt pool surface oscillations driven by forces such as surface tension. The shape of the interface and the chemical distribution across the interface implied that the mass transport by the flow can significantly influence the interface morphology.

Water structural effects on DNA–DNA interactions and homologous recognition

Comprehending the principles underlying the interaction between DNA molecules in electrolyte solution is of crucial importance for understanding the phenomena of DNA condensation, recognition of homologous genes, and properties of DNA liquid crystals. Although DNA was considered in all its helical complexity over the last two decades, only a primitive, local, electrostatic model of the aqueous solution was considered. The forces acting between DNA molecules however must be influenced by the structure of the liquid separating them. In this paper we make a step towards unravelling earlier neglected effects of water structure on DNA–DNA interactions. We develop a model of DNA interaction accounting for the nonlocal dielectric response of water and of the electrolyte plasma dissolved in it. We focus particularly on the study of the dipolar overscreening effect, which leads to decaying oscillations of the electrostatic potential about DNA, and how it will affect electrostatic energy surfaces as a function of interaxial separation and mutual azimuthal orientation of the parallelly aligned interacting DNAs. We found that going beyond the classical electrostatic description of water (the so-called primitive model) reveals a complex oscillatory interaction surface in space, with many possible metastable configurations. These however vanish rapidly with smearing of the DNA charge distribution, due to dephasing of the oscillatory components of the interaction. This dephasing results in the suppression of oscillation amplitudes and leads to the dominance of non-oscillatory components in spatial dipolar correlations and the Debye screening. These dominant correlation patterns are shown to lead to a substantial enhancement of the difference between the interaction of homologous and nonhomologous DNA sequences, deepening and sharpening the homology recognition well.

Improving the quality of mustard seeds during pre-sowing preparation

Improving the quality of mustard seeds during pre-sowing preparation

Control of cylinder wake using oscillatory morphing surface

In this study, the wake of a cylinder was actively controlled by the cylinder's oscillatory morphing surface. Experiments were conducted in a closed-loop water channel. A cylinder of diameter 36 mm was placed in 0.09 m/s water flow, resulting in the Reynolds number 3240 and the vortex shedding frequency around 0.5 Hz. The cylinder's morphing surface oscillated at four different frequencies, i.e., 0.5, 1, 2, and 4 Hz. It was found that, compared to the rigid circular cylinder, the cylinder with oscillatory morphing surface can generally produce a smaller vortex formation length, especially at intermediate oscillation frequencies. The shear layers developed from the cylinder transit and roll up earlier due to enhanced flow instabilities. With the highest-frequency oscillations, the shear layer develops into a train of many small vortices that follow the trace of undisturbed shear layer. This study reveals some physical insights into this novel flow control method, which could be useful in future engineering applications.

Free surface oscillation driven by rotating stirrer

To gain insights into the mechanisms of free surface oscillation in a rotating mixing container, we observe the free surface deformation and measure the torque acting on the bar. The container was half-filled with liquids. Periodic surface oscillation occurs. At the rotational speed where the amplitude of the oscillation reaches its maximum, the time-averaged torque also takes the local maximum values. To account for the sloshing mechanism, an equation of motion is derived using the Lagrangian mechanics; we found that the sloshing occurs when the collision frequency of bar on the surface is consistent with the natural frequency of the system and the damping coefficient is sufficiently smaller than unity. The time-averaged torque increases when the sloshing becomes violent. We conclude that the hydrodynamics of oscillation is successfully modeled using point-mass mechanics, and thus we can reasonably capture the rotation speed at which violent oscillation occurs.Graphical Free surface deformation driven by the rotating arm in the cylindrical container which is half-filled with liquids

Multiple moonpools within a vessel free to heave and pitch: An experimental study

Moonpools in marine vessels and offshore platforms are vertical openings through the structure used to support underwater operations. Experimental investigations of multiple moonpool resonant responses coupled with vessel heave and pitch motions are presented in this study. For three moonpools at forward, central, and aft locations of a rectangular vessel, the simultaneous location-dependent free surface response amplitudes and oscillation phases are compared for a range of incident wave frequencies in the piston mode range. For the vessel free to heave and pitch, the central moonpool has the highest response amplitude; whereas, a previous study with fixed vessel (Garad et al. 2023) showed that the largest free surface responses are observed in the forward moonpool. Due to coupled hydrodynamic interactions between the moonpool oscillations and the vessel motions, the moonpool resonance frequency is linked with the vessel heave response. Multiple response amplitudes peaks are observed in the forward and aft moonpools due to body motions. The non-dimensional response amplitude increases nonlinearly with decreasing wave steepness close to the resonance frequency. The multiple moonpool oscillations show distinct phase differences from one another and with the outer wave, the individual values depending on the incident wave frequency. Considering the vessel motions, comparison with experiments without moonpools shows that the pitch response is increased due to multiple moonpool oscillations. The hydrodynamic interactions between multiple moonpools and a floating vessel have prospective applications in the case of marine platforms, where the resonant motions can be used for wave energy conversion through oscillating water column devices.