Averaged Dynamics of Fluids near the Oscillating Interface in a Hele-Shaw Cell

The steady flow in a Hele-Shaw cell filled with fluids with a high viscosity contrast in the presence of fluid oscillations is experimentally studied.The control of oscillatory dynamics of multiphase systems with interfaces is a challenging technological problem.We consider miscible(water and glycerol)and immiscible(water and high-viscosity silicone oil PMS-1000)fluids under subsonic oscillations perpendicular to the interface.Observations show that the interface shape depends on the amplitude and frequency of oscillations.The interface is undisturbed only in the absence of oscillations.Under small amplitudes,the interface between water and glycerol widens due to mixing.When the critical amplitude is reached,the interface becomes unstable to the fingering instability:Aqueous fingers penetrate the high-viscosity glycerol and induce intensive mixing of miscible fluids and associated decay of the instability.After the disappearance of the fingers,the interface takes a U-shape in the central part of the cell.A similar effect is observed for immiscible fluids:The oscillating interface tends to bend to the side of a high-viscosity fluid.Again,when the critical amplitude is reached,the fingering instability arises at the convex interface.This paper focuses on the causes of bending of the initially undisturbed interface between miscible or immiscible fluids.For this purpose,we measure the steady flow velocity near the interface and in the bulk of a high-viscosity fluid using Particle Image Velocimetry(PIV).

Oscillating field stimulation promotes neurogenesis of neural stem cells through miR-124/Tal1 axis to repair spinal cord injury in rats

Spinal cord injury often leads to severe motor and sensory deficits,and prognosis using the currently available therapies remains poor.Therefore,we aimed to explore a novel therapeutic approach for improving the prognosis of spinal cord injury.In this study,we implanted oscillating field stimulation devices and transplanted neural stem cells into the thoracic region(T9–T10)of rats with a spinal cord contusion.Basso-Beattie-Bresnahan scoring revealed that oscillating field stimulation combined with neural stem cells transplantation promoted motor function recovery following spinal cord injury.In addition,we investigated the regulation of oscillating field stimulation on the miR-124/Tal1 axis in neural stem cells.Transfection of lentivirus was performed to investigate the role of Tal1 in neurogenesis of neural stem cells induced by oscillating field stimulation.Quantitative reverse transcription-polymerase chain reaction,immunofluorescence and western blotting showed that oscillating field stimulation promoted neurogenesis of neural stem cells in vitro and in vivo.Hematoxylin and eosin staining showed that oscillating field stimulation combined with neural stem cells transplantation alleviated cavities formation after spinal cord injury.Taking the results together,we concluded that oscillating field stimulation decreased miR-124 expression and increased Tal1 content,thereby promoting the neurogenesis of neural stem cells.The combination of oscillating field stimulation and neural stem cells transplantation improved neurogenesis,and thereby promoted structural and functional recovery after spinal cord injury.

Density-dependent carrier-envelope phase shift in attosecond pulse generation from relativistically oscillating mirrors

The carrier-envelope phase(CEP)φ_(0)is one of the key parameters in the generation of isolated attosecond pulses.In particular,“cosine”pulses(φ_(0)=0)are best suited for generation of single attosecond pulses in atomic media.Such“cosine”pulses have the peak of the most intense cycle aligned with the peak of the pulse envelope,and therefore have the highest contrast between the peak intensity and the neighboring cycles.In this paper,the dynamics of single attosecond pulse generation from a relativistically oscillating plasma mirror is investigated.We use an elementary analytical model as well as particle-in-cell simulations to study few-cycle attosecond pulses.We find that the phase of the field driving the surface oscillations depends on the plasma density and preplasma scale length.This leads us to a counterintuitive conclusion:for the case of normal incidence and a sharp plasma-vacuum boundary,the CEP required for the generation of a single attosecond pulse phase is closer toφ_(0)=π/2(a“sine”pulse),with the exact value depending on the plasma parameters.

Numerical Modelling of Double Slit Caisson Breakwater Integrated Oscillating Water Column Wave Energy Converter with Modal Superposition Method

Oscillating water column wave energy converter is a power generation device in which ocean waves excite the oscillation of the water surface in an air chamber, which generates fluctuations in air pressure and rotate air turbine generator(s). The oscillation of the fluid in the air chamber is a fluid oscillation phenomenon with a natural period, similar to fluid oscillation in a container such as sloshing. Previous research has shown that for an oscillating water column with a single air chamber submerged in water, the oscillation characteristics can be modeled as a one-degree-of-freedom oscillation system that takes only a single oscillation mode into account. However, a double-slit breakwater integrated oscillating water column wave energy converter using two water columns of the breakwater separated by slit walls, has been verified to have two resonance periods. In this study, the free oscillating motion of the oscillating water column wave energy converter using the double-slit breakwater is modeled by modal superposition method including the first-order and second-order modes of vertical motion of the two water surfaces. The result from the simulation is similar to the result of the free vibration experiment.

Frictional sliding of infilled planar granite fracture under oscillating normal stress

This paper investigates the frictional behavior of the infilled rock fracture under dynamic normal stress.A series of direct shear tests were conducted on saw-cut granite fractures infilled with quartz using a selfdeveloped dynamic shear apparatus,and the effects of normal load oscillation amplitude,normal load oscillation period and sliding velocity were studied.The test results reveal that the shear response can be divided into three stages over a whole loading-unloading process,characterized by different time spans and stress variations.Generally,a smaller oscillation amplitude,a longer oscillation period and a fast shear velocity promote the stability of the friction system,which is also confirmed by the Coulomb failure criterion calculated based on the observed periodic apparent friction coefficient.The dynamic strengthening/weakening phenomenon is dependent on the oscillation amplitude and product of sliding velocity and oscillation period(vT).Also,the rate and state friction law incorporating the parameter a that characterizes the normal stress variation is employed to describe the dynamic friction coefficient but exhibits an incompetent performance when handling intensive variation in normal stress.Finally,the potential seismicity induced by oscillating normal stress based on the observed stress drop is analyzed.This work helps us understand the sliding process and stability evolution of natural faults,and its benefits for relative hazard mitigation.

Investigation on the Oscillating Buoy Wave Power Device

An oscillating buoy wave power device (OD) is a device extracting wave power by an oscillating buoy. Being excitedby waves, the buoy heaves up and down to convert wave energy into electricity by means of a mechanical or hydraulic de-vice. Compared with an Oscillating Water Column (OWC) wave power device, the OD has the same capture width ratio as the OWC does, but much higher secondary conversion efficiency. Moreover, the chamber of the OWC, which is the mostexpensive and difficult part to be built, is not necessary for the OD, so it is easier to construct an OD. In this paper, a nu-merical calculation is conducted for an optimal design of the OD firstly, then a model of the device is built and, a model testis carded out in a wave tank. The results show that the total efficiency of the OD is much higher than that of the OWC andthat the OD is a promising wave power device.

Lagrangian-based numerical investigation of aerodynamic performance of an oscillating foil

The dynamic stall problem for blades is related to the general performance of wind turbines,where a varying flow field is introduced with a rapid change of the effective angle of attack (AOA).The objective of this work is to study the aerodynamic performance of a sinusoidally oscillating NACA0012 airfoil.The coupled k-ω Menter's shear stress transport (SST) turbulence model and γ-Reθ transition model were used for turbulence closure.Lagrangian coherent structures (LCS) were utilized to analyze the dynamic behavior of the flow structures.The computational results were supported by the experiments.The results indicated that this numerical method can well describe the dynamic stall process.For the case with reduced frequency K =0.1,the lift and drag coefficients increase constantly with increasing angle prior to dynamic stall.When the AOA reaches the stall angle,the lift and drag coefficients decline suddenly due to the interplay between the first leading-and trailing-edge vortex.With further increase of the AOA,both the lift and drag coefficients experience a secondary rise and fall process because of formation and shedding of the secondary vortex.The results also reveal that the dynamic behavior of the flow structures can be effectively identified using the finite-time Lyapunov exponent (FTLE) field.The influence of the reduced frequency on the flow structures and energy extraction efficiency in the dynamic stall process is further discussed.When the reduced frequency increases,the dynamic stall is delayed and the total energy extraction efficiency is enhanced.With K =0.05,the amplitude of the dynamic coefficients fluctuates more significantly in the poststall process than in the case of K =0.1.

Fully Nonlinear Time Domain Analysis for Hydrodynamic Performance of An Oscillating Wave Surge Converter

The hydrodynamic behaviour of an oscillating wave surge converter(OWSC) in large motion excited by nonlinear waves is investigated. The mechanism through which the wave energy is absorbed in the nonlinear system is analysed. The mathematical model used is based on the velocity potential theory together with the fully nonlinear boundary conditions on the moving body surface and deforming free surface. The problem is solved by the boundary element method. Numerical results are obtained to show how to adjust the mechanical properties of the OWSC to achieve the best efficiency in a given wave, together with the nonlinear effect of the wave height. Numerical results are also provided to show the behaviour of a given OWSC in waves of different frequencies and different heights.

Numerical Simulation and Experimental Investigation on Performance of the Wells Turbine in Irregular Oscillating Flow

The Wells turbine is an axial-flow air-turbine designed to extract energy from ocean waves. An important considera-tion is the self-starting capability of the Wells turbine, a phenomenon encountered where the turbine accelerate by itselfup to a certain speed for the best turbine performance. In order to clarify the self-starting characteristic and running per-formance of the Wells turbine in an irregular oscillating flow, a numerical simulation process is established in this paperon the rational assumption of quasi-steady flow conditions. Both self-starting characteristics and running performanceare obtained through the numerical simulation and subsequently compared with the experimental data achieved on acomputer-controlled oscillating flow test rig which could realize some irregular oscillating flow according to the specifiedspectrum. Results show that the self-starting time decreases with the increase of the significant wave height and the meanfrequency of the irregular oscillating flow. Therefore, it is possible to predict accurately the performance of the Wells tur-bine by computer simulation.

SELF STARTING PROCESS SIMULATION OF WELLS TURBINE IN REGULAR AND IRREGULAR OSCILLATING AIR FLOWS

Self-starting performance is very important for the design of the Wells turbine. In order to analyse the self-starting performance of the Wells turbine in regular and irregular waves, a mathematical model was established to simulate the self-starting process of the Wells turbine in regular and irregular oscillating air flows. Based on the turbine’s force coefficients and the restraint moment function of electric generator, the self-starting performance was investigated in different oscillating air flow conditions and the turbine’s inertia. Digit results show that the self- starting time is shortened with the increase of the amplitude of oscillating air flow or the decrease of the period of oscillating air flow and the turbine’s iner tia. The key to the improvement of the starting performance of the Wells turbine is to improve the turbine’s performance in the stalling region.

Transport of dissolved oxygen at the sediment-water interface in the spanwise oscillating flow

The distribution and concentration of dissolved oxygen(DO)play important roles in aerobic heterotroph activities and some slow chemical reactions,and can affect the water quality,biological communities,and ecosystem functions of rivers and lakes.In this work,the transport of high Schmidt number DO at the sediment-water interface of spanwise oscillating flow is investigated.The volume-averaged Navier-Stokes(VANS)equations and Monod equation are used to describe the flow in the sediment layer and the sediment oxygen demand of microorganisms.The phase-averaged velocities and concentrations of different amplitudes and periods are studied.The dependence of DO transfer on the amplitude and period is analyzed by means of phase-average statistical quantities.It is shown that the concentration in the sediment layer is positively correlated with the turbulence intensity,and the DO concentration and penetration depth in the sediment layer increases when the period and amplitude of the oscillating flow increase.Moreover,in the presence of oscillating flow,a specific scaling relationship exists between the Sherwood number/oxygen consumption of aerobic heterotrophs and the Reynolds number.

Resonance in flow past oscillating cylinder under subcritical conditions

Flow around an oscillating cylinder in a subcritical region are numerically studied with a lattice Boltzmann method(LBM). The effects of the Reynolds number,oscillation amplitude and frequency on the vortex wake modes and hydrodynamics forces on the cylinder surface are systematically investigated. Special attention is paid to the phenomenon of resonance induced by the cylinder oscillation. The results demonstrate that vortex shedding can be excited extensively under subcritical conditions, and the response region of vibration frequency broadens with increasing Reynolds number and oscillation amplitude. Two distinct types of vortex shedding regimes are observed. The first type of vortex shedding regime(VSR I) is excited at low frequencies close to the intrinsic frequency of flow, and the second type of vortex shedding regime(VSR II)occurs at high frequencies with the Reynolds number close to the critical value. In the VSR I, a pair of alternately rotating vortices are shed in the wake per oscillation cycle,and lock-in/synchronization occurs, while in the VSR II, two alternately rotating vortices are shed for several oscillation cycles, and the vortex shedding frequency is close to that of a stationary cylinder under the critical condition. The excitation mechanisms of the two types of vortex shedding modes are analyzed separately.

A NUMERICAL STUDY OF THE VORTEX MOTION IN OSCILLATING FLOW AROUND A CIRCULAR CYLINDER AT LOW AND MIDDLE Kc NUMBERS

A new hybrid model, which is based on domain decomposition andproposed by the authors is used for calculating the flow around a circular cylinderat low and middle Keulegan-Carpenter numbers (Kc=2～18)respectively.Thevortex motion patterns in asymmetric regime,single pair(or transverse)regime anddouble pair(or diagonal)regime are successfully simulated.The calculated drag andinertial force coefficients are in better agreement with experimental data than otherrecent computational results.

Epidural oscillating field stimulation increases axonal regenerative capacity and myelination after spinal cord trauma

Oscillating field stimulation(OFS)with regular alterations in the polarity of electric current is a unique,experimental approach to stimulate,support,and potentially guide the outgrowth of both sensory and motor nerve fibers after spinal cord injury(SCI).In previous experiments,we demonstrated the beneficial effects of OFS in a 4-week survival period after SCI.In this study,we observed the major behavioral,morphological,and protein changes in rats after 15 minutes of T9 spinal compression with a 40 g force,followed by long-lasting OFS(50μA),over a 8-week survival period.Three groups of rats were analyzed:rats after T9 spinal compression(SCI group);SCI rats subjected to implantation of active oscillating field stimulator(OFS+SCI group);and SCI rats subjected to nonfunctional OFS(nOFS+SCI group).Histopathological analysis of spinal tissue indicated a strong impact of epidural OFS on the reduction of tissue and myelin loss after SCI in the segments adjacent to the lesion site.Quantitative fluorescent analysis of the most affected areas of spinal cord tissue revealed a higher number of spared axons and oligodendrocytes of rats in the OFS+SCI group,compared with rats in the SCI and nOFS+SCI groups.The protein levels of neurofilaments(NF-l),growth-associated protein-43(marker for newly sprouted axons),and myelin basic protein in rats were signifiantly increased in the OFS+SCI group than in the nOFS+SCI and SCI groups.This suggests a supporting role of the OFS in axonal and myelin regeneration after SCI.Moreover,rats in the OFS+SCI group showed great improvements in sensory and motor functions than did rats in the nOFS+SCI and SCI groups.All these findings suggest that long-lasting OFS applied immediately after SCI can provide a good microenviroment for recovery of damaged spinal tissue by triggering regenreative processes in the acute phase of injury.

Investigation of steady plasma actuation effect on aerodynamic coefficients of oscillating airfoil at low Reynolds number

In this work, numerical study of two dimensional laminar incompressible flow around an oscillating NACA0012 airfoil is proceeded using the open source code Open FOAM. Oscillatory motion types including pitching and flapping are considered. Reynolds number for these motions is assumed to be 12000 and effects of these motions and also different unsteady parameters such as amplitude and reduced frequency on aerodynamic coefficients are studied. For flow control on airfoil, dielectric barrier discharge plasma actuator is used in two different positions on airfoil and its effect is compared for the two types of considered oscillating motions. It is observed that in pitching motion, imposing plasma leads to an improvement in aerodynamic coefficients, but it does not have any positive effect on flapping motion.Also, for the amplitudes and frequencies investigated in this paper, the trailing edge plasma had a more desirable effect than other positions.

Oscillating field stimulation promotes axon regeneration and locomotor recovery after spinal cord injury

Oscillating field stimulation(OFS)is a potential method for treating spinal cord injury.Although it has been used in spinal cord injury(SCI)therapy in basic and clinical studies,its underlying mechanism and the correlation between its duration and nerve injury repair remain poorly understood.In this study,we established rat models of spinal cord contusion at T10 and then administered 12 weeks of OFS.The results revealed that effectively promotes the recovery of motor function required continuous OFS for more than 6 weeks.The underlying mechanism may be related to the effects of OFS on promoting axon regeneration,inhibiting astrocyte proliferation,and improving the linear arrangement of astrocytes.This study was approved by the Animal Experiments and Experimental Animal Welfare Committee of Capital Medical University(supplemental approval No.AEEI-2021-204)on July 26,2021.

Torsional Vibration of Roller Oscillating Tooth Gear Drive

Torsional vibration of roller oscillating tooth gear drive (ROTGD) is studied in this paper. On the basis of conservation law for kinetic energy and potential energy, the mathematical expressions are developed which describe transformation of moment of inertia of inertial components into input shaft. Also, the formula is derived which expresses transformation of contact stiffness of elastic components into input shaft torsional stiffness. Besides, torsional vibration model of ROTGD is presented by using the transfer matrix method, and natural frequencies and vibration mode shapes are determined. Eventually, an example is given.

Effect of Oscillation Parameters to Flow Field in the Pool during the Oscillating Twin-Roll Strip Casting Process

During the oscillating twin?roll strip casting process, the quality of final products is directly influenced by the flow field distribution of molten metal in the pool. The variation in the flow field is caused by oscillating roller benefits, for homogeneous distribution of strip impurity, and decreasing the grain size. Thus, the quality of the strip could be improved. A numerical model was developed using the multiphase flow technology, coupled with heat transfer, fluid flow, solidification, and oscillation. Furthermore, a transient algorithm was adopted for simulating the oscillating twin?roll strip casting process of AlSi9Cu3 and 3104 aluminum alloy. This paper focuses on the flow distribution in the pool, in comparison with the traditional vertical twin?roll strip casting process, while the amplitude or frequency is chang?ing with the definite value of casting velocity, roller diameter, nozzle angle, and the strip thickness. Consequently, the conclusions were experimentally validated by oscillating twin?roll 3104 aluminum alloy strip casting. Vibrating casting technology can change the flow field in the pool by vibration, which can improve the quality of the strip core.

Surface Instability of a Vertically Oscillating Granular Layer

In the study of the surface instability of a vertically oscillating granular layer,we obtained experimentally the phase diagram for the surface states of the layer in the driving frequency-acceleration plane,and measured the dispersion relation for the surface waves in a granular layer in comparison to that in viscous fluids.Our experiments show that the onset dimensionless acceleration increases with the driving frequency,and the wavelength of the surface waves increases with the depth of granular layer.These experimental results are in agreement with our theoretical model qualitatively.

AN EXPERIMENTAL STUDY OF FLUCTUATING LIFT ON A SQUARE-SECTION CYLINDER OSCILLATING TRANSVERSELY IN A UNIFORM STREAM

Experiments on a square section cylinder fixed and forced to oscillate transversely in a uni-form stream were conducted in a water tank.The Reynolds numberof the experiments is in the range of 3·10~3tO 10~4,the amplitude to side length ratio A/D is up to 0.7 and the range of reduced velocity is4.5【Vr【12.This study aims at investigating the lock-in phenomenon,the fluctuating lift and the phaseshift between fluctuating lift and displacement of the oscillating cylinder.The problems on the aeroelastic insta.bility relating to present experimental results have been discussed.The flow visualization clearly shows thatthere are drastic changes of vortex-shedding from cylinder at the resonance point and the upper end of thelock-in range.The results of the flow visualization give better understanding of the physical mechanism of thephase shift.