Traveling Surface Waves Research Articles

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.

Spatiotemporal imaging and manipulation of surface plasmons.

Surface plasmon polaritons (SPPs) are travelling surface waves that have shown promise for applications in nanophotonics as they provide a direct route toward photon-mediated electrical signal generation - a central paradigm for speeding up and scaling down photonic elements. SPP waves have also generated fundamental interest due to their high-field strength and sub-wavelength confinement, properties that have enabled the surface-enhanced Raman effect. Over the last decade, photoemission electron microscopy (PEEM) has emerged as a pioneering technique for imaging surface electric fields through ultrashort laser pulse mediated electron emission and has therefore become an indispensable tool for characterizing plasmonic phenomena at interfaces in a variety of materials. PEEM offers nanometer spatial resolution and femtosecond temporal resolution, allowing SPPs to be prepared, monitored, and manipulated on the nanometer-femtosecond scale. Through a brief review of recent reports, we aim to introduce PEEM-based SPP imaging and manipulation modalities and highlight their utility in the context of emerging nanoscale and quantum materials science advancements.

Влияние типа магнитостатической волны на спиновую накачку в микроструктурах жиг/pt

Using the inverse spin Hall effect (ISHE), spin pumping by traveling surface waves (MSSW) and backward volume waves (BVMSW) in YIG/Pt microstructures has been experimentally studied. It has been discovered that the frequency dependence of the emf UISHE(f), generated due to the ISHE, in the case of BVMSW is characterized by a maximum near the frequency of the long-wave (f0) spectrum boundary. In the case of a MSSW, the dependence UISHE (f) exhibits maxima both at the frequency f0 and at the frequency fs of the short-wave boundary of the MSSW spectrum. The indicated differences in the dependences UISHE (f) for MSSW and OOMSW are explained by the difference in the functions of the density of states of spin waves in the spectrum of MSSW and BVMSW.

Friction on layered media: How deep do phonons reach?

We theoretically study the frictional damping of a small probe object on a coated planar surface, analyzing the resulting phonon modes via a theory of viscoelasticity. Three different types of excitations are found to contribute to friction in distinct ways: traveling (3D) spherical waves, traveling (2D) surface waves, and evanescent waves. While traveling waves transport energy away from the probe, determined by long range elastic properties (wavelength), evanescent waves transform energy into heat in a near-field range, characterized by the size of the probe. Thus, fundamentally different behaviors are predicted, depending on coating thickness and material properties.

Pulsating fields of a thin film in a static magnetic field under vertical vibrations

In the present paper a dynamics of a thin ferrofluid film under the vertical vibration in a static magnetic field is examined. The vibrational amplitude is assumed to be greater than film thickness so that vibrational force is greater than magnetic and gravitational forces. The pulsating part and the averaged part of the hydrodynamics fields are obtained. The solution of pulsating part for the traveling surface wave is found. The equation for the averaged surface profile is found.

Mechanowetting drives droplet and fluid transport on traveling surface waves generated by light-responsive liquid crystal polymers

In nature, capillary forces are often driving microfluidic propulsion and droplet manipulation, and technologies have been developed to utilize these forces in applications such as lab-on-a-chip biosensors and microfluidic systems. At the same time, responsive materials have been developed that can be activated by a variety of external triggers, including light, electric fields, and temperature, to locally deform and create dynamic surface structures, such as traveling waves. Here, we combine these developments into a system that enables capillary-driven droplet transport and fluid propulsion generated by light-induced surface waves in azobenzene-embedded liquid crystal polymers. We demonstrate that the traveling waves are able to efficiently propel fluids by means of mechanowetting. We couple the wave profiles to the fluid simulations using a multiphase computational fluid dynamics approach. We study three different fluid propulsion systems, i.e., peristaltic flow, liquid slug transport, and free-standing droplet transport. The first system operates on a fluid-filled single channel and achieves relative flow speeds of u/uwave<0.01. In contrast, the slugs and droplets are transported at two orders of magnitude higher speed equal to the wave speed (u/uwave=1) by exploiting the mechanowetting effect. We quantify the capillary forces generated by the traveling surface waves. Our method opens new avenues in light-driven (digital) microfluidic systems with enhanced control of fluid flow.

The Existence of Three-Dimensional Multi-Hump Gravity-Capillary Surface Waves on Water of Finite Depth

The paper considers three-dimensional (3D) traveling surface waves on water of finite depth under the forces of gravity and surface tension using the exact governing equations, also called Euler eq...

A bottom mounted wavemaker in water wave flumes

Reflections of the surface waves in hydraulic wave flumes could affect the performance of the common piston, hinged and plunger type wavemakers. This paper proposes a new wavemaker to produce surface waves where the possible reflections are barely able to affect its performance. The proposed wavemaker includes an oscillating paddle at the bottom of the flume where the produced upward flow induces travelling surface waves. An analytical study is carried out on the wave generation of the system. It is shown that the paddle length could significantly affect the height of the generated waves, which could not happen in conventional wavemakers. Effects of the water depth and the excitation frequency on the induced surface waves are also investigated. It is demonstrated that for a specific paddle length, water depth and forcing period the best performance of the system could be achieved.

Anisotropic Spin-Acoustic Resonance in Silicon Carbide at Room Temperature.

We report on acoustically driven spin resonances in atomic-scale centers in silicon carbide at room temperature. Specifically, we use a surface acoustic wave cavity to selectively address spin transitions with magnetic quantum number differences of ±1 and ±2 in the absence of external microwave electromagnetic fields. These spin-acoustic resonances reveal a nontrivial dependence on the static magnetic field orientation, which is attributed to the intrinsic symmetry of the acoustic fields combined with the peculiar properties of a half-integer spin system. We develop a microscopic model of the spin-acoustic interaction, which describes our experimental data without fitting parameters. Furthermore, we predict that traveling surface waves lead to a chiral spin-acoustic resonance that changes upon magnetic field inversion. These results establish silicon carbide as a highly promising hybrid platform for on-chip spin-optomechanical quantum control enabling engineered interactions at room temperature.

Microfluidic slug transport on traveling-wave surface topographies by mechanowetting

Nature often uses capillary forces to manipulate fluids, which has inspired scientists to develop new applications, such as in microfluidics and laboratory-on-a-chip systems. Here we present a method to transport fluids in microfluidic channels, by exploiting the capillary interaction of fluid interfaces with traveling surface waves, called mechanowetting. We found that the three-phase lines of fluid slugs dynamically attach to the crests of the waves, resulting in fluid velocities that are equal to the wave speed. By comparing this microfluidic slug flow to conventional peristaltic fluid propulsion, we demonstrate that fluid velocities can be reached that are one order of magnitude larger. We quantified the efficiency numerically and theoretically in terms of the generated pressure gradient using a closed channel and measured the evolution of the pressure distribution as the wave progresses. The method was shown to work for a very wide range of contact angles. We anticipate that our results will lead to new microfluidic applications based on switchable topography technology.

Mean velocities in an irrotational equatorial wind wave

In this paper, we investigate the mean velocities in an irrotational periodic geophysical traveling surface wave, which propagates in equatorial oceanic regions. Employing the harmonic and complex analysis in combination with exploiting some of the physical structures of the problem, we prove that the horizontal mean velocity is less than the depth-averaged velocity in the frame of reference in which the wave is stationary.

Closed‐loop control of turbulent boundary layers using spanwise traveling surface waves

AbstractThe drag reduction technique of spanwise traveling surface waves in turbulent boundary layers is investigated in a closed‐loop control setup. The results show a dependence of the local skin friction on the spanwise coordinate and a successful application of the controller to determine the optimal amplitude.

Hyporheic exchange in a gravel bed flume with and without traveling surface waves

Solute exchange between the pore water of a streambed and overlying flowing surface water can have important effects on the chemical mass balance and biological productivity in the aquatic environment. The base mechanism for driving this hyporheic exchange in a moving current is turbulence at the water- sediment interface and the water surface slope. Many previous studies have shown that the nature of the exchange can be significantly modified when spatial pressure variations, induced by standing surface waves or bed forms, are present in the system. In contrast to these studies, we consider how transient pressure variations induced by moving waves modify hypothetic exchange. This is achieved experimentally by using a vertical array of electrical conductivity probes to track the movement of a conservative solute tracer in the gravel bed of a recirculating laboratory flume under a variety of flow conditions. Our experimental observations indicate that the addition of traveling waves greatly increases the initial rate and depth of solute penetration into the bed. When the analytical solution of a one-dimensional (vertical) advection–dispersion model is fit to the experimental measurements it appears that the presence of traveling surface waves results in a more dispersive hyporheic transport. Through experimental observation, model fitting, and direct simulation we demonstrate that this apparent dispersion dominated transport is due to the vertical “pumping” of interstitial fluid in the presence of traveling surface waves. From these simulations we develop a relation that for a given set of flow and wave characteristics allows us to estimate the apparent dispersion due to traveling waves.

Mechano-nanofluidics: water transport through CNTs by mechanical actuation

Mechano-nanofluidics, defined as the study of mechanical actuation effects on the properties of nanofluidics, have received broad interest recently in the field of nanofluidics. The coupling between phonons in carbon nanostructures and fluids under confinement is verified to enhance the diffusion of fluids. Especially, carbon nanotubes (CNTs) are applied as perfect nanochannels with fast water mass transport, making them to be one of the next generation of membranes. Here, we investigated water permeation through CNT membranes with the mechanical vibrations using non-equilibrium molecular dynamics simulations. The simulation results reveal that the water flux is highly promoted by a travelling surface waves at 1 THz. The water flux is enhanced by as large as 20 times for the single-file structured water at 20 MPa. The vibration effect is verified to be equivalent to a pressure drop with \({{{\Delta}}}P\) up to 295 MPa. We show that the role of vibration diminishes for water in a larger CNTs. The water structure and hydrogen bond network are analyzed to understand the phenomena. The results of the present work are applied to provide guidance for the development of high-performance membranes.

Parametric investigation of friction drag reduction in turbulent flow over a flexible wall undergoing spanwise transversal traveling waves

Active friction drag reduction by spanwise transversal traveling surface waves is investigated experimentally in a fully developed zero-pressure gradient (ZPG) turbulent boundary layer (TBL). The spanwise transversal traveling wave of an aluminum surface is generated by an electromagnetic actuator system. A parametric study focusing on the influence of the wave amplitude (A+) and wave period (T+) is performed to analyze the impact of the wave parameters on drag reduction. Within the range of the parameters investigated, the maximum local drag reduction of 4.5% is found at A+ = 11.8 and T+ = 110. Furthermore, the TBL flows above the wave crest and trough are investigated by phase-locked PIV and µ-PTV measurements. The results evidence that the drag reduction effect is not only enhanced by increasing the amplitude, but also by reducing the period in the range of the current parameters. The turbulence statistics show that the velocity fluctuations and the Reynolds shear stresses in the streamwise and in the wall-normal direction are damped by the traveling surface wave motion in the near-wall region. The outer velocity distribution deviates from the inner scaling based on the actuated friction velocity, i.e., it possesses a slight tendency of a varying slope in the log region. The phase-locked measurements of the velocity profiles above the crest and the trough show that only above the crest the inner scaling property is valid. Above the moving surface a non-zero spanwise secondary flow is induced. The quadrant decomposition of the turbulent productions shows that the sweep and ejection events are weakened.

Drag reduction of spanwise transversal traveling surface waves with stepwise amplitude increase

AbstractThe effect of sudden amplitude changes of a spanwise traveling surface wave on turbulent boundary layers is investigated numerically using high resolution large‐eddy simulation (LES). The results show drag reduction of up to 9 percent and an adaptation of the turbulent flow to the changed input parameter within 100 convective times. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of surface waves through friction: FEM investigation

AbstractSliding friction between two bodies can generate elastic vibration. This study uses a finite‐element model comprising an elastic body sliding against a flat rigid surface with constant coefficient of friction. For the elastic body a structured topography is taken into account. The model shows traveling surface waves, which depend on the asperities of the sliding surface. It can be shown that the surface structure and its inertia are the cause for elastic waves in the contact region. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical Investigation of the Surface Wave Formation in a Microwave Plasma Torch

The microwave plasma torch (MPT) is an attractive plasma source in both industry and scientific research because of its advantages in electrodeless operation ability, high energy efficiency, and flexibility under different operational conditions. However, relatively few numerical studies have been carried out for the MPT fed by a rectangular waveguide in two or even three dimensions. Therefore, this paper presents a 2-D, axisymmetric, and stationary fluid models to characterize the MPT, with an emphasis on the mechanism of the surface wave formation and the sustentation of the plasma column. Nine partial differential equations are included in this research, coupling microwave radiation, heat transfer, plasma bulk flow, and transport of different species with each other. The obtained results reveal how the traveling surface wave occurs and elucidate the consequent influence of the surface wave formation. Furthermore, this paper demonstrates the extension of the plasma column length and the propagation of the surface wave with an increase in the total plasma-absorbed microwave power. The dependence of the plasma column length on the plasma-absorbed microwave power is found to be motivated by the surface wave propagation. The limitations of the model and the difficulties in its numerical implementation are also discussed in the paper.

Influence of travelling surface waves on nanofluidic viscosity

The viscosity of nanofluidics in the presence of travelling surface wave has been investigated systematically by molecular dynamics (MD) simulations in this paper. The travelling surface wave propagating on the walls of nanochannel can affect the viscosity of nanofluidics sufficiently. The nanoscale fluid mechanism in nanochannels has been influenced by both amplitude and frequency of travelling surface wave, and the hydrodynamic characteristics have been affected significantly including the degree of boundary slip and mass flow rate (MFR). The MD results show that the viscosity of fluid at the nanoscale is an increasing function of the amplitude of travelling surface wave, but is reduced dramatically at the ultra-high frequency of travelling surface wave. The boundary slip illustrates the same trend as the variation of nanofluidic viscosity. For the mass flow rate, it depends on both the apparent viscosity of fluid and slip length at the boundary. Usually the viscosity and slip length are both independent on the pressure gradient, but are enlarged as the increment of pressure gradient in the presence of travelling surface wave. In contrast, when the travelling surface wave propagates on the walls of nanochannel, the mass flow rate is reduced as the pressure gradient increases. It has also been found that the viscosity, slip length and mass flow rate are influenced easily by the travelling surface wave on the hydrophobic surface (weak fluid-wall interaction). In addition, the stress distribution of fluid across the nanochannel has been shown, which indicates that the averaged total stress has been decreased under the condition of travelling surface wave. It can be concluded that our results provide the apparent viscosity of nanofluidics and hydrodynamic characteristics in the presence of travelling surface wave.

Sound wave propagation on the human skull surface with bone conduction stimulation

BackgroundBone conduction (BC) is an alternative to air conduction to stimulate the inner ear. In general, the stimulation for BC occurs on a specific location directly on the skull bone or through the skin covering the skull bone. The stimulation propagates to the ipsilateral and contralateral cochlea, mainly via the skull bone and possibly via other skull contents. This study aims to investigate the wave propagation on the surface of the skull bone during BC stimulation at the forehead and at ipsilateral mastoid. MethodsMeasurements were performed in five human cadaveric whole heads. The electro-magnetic transducer from a BCHA (bone conducting hearing aid), a Baha® Cordelle II transducer in particular, was attached to a percutaneously implanted screw or positioned with a 5-Newton steel headband at the mastoid and forehead. The Baha transducer was driven directly with single tone signals in the frequency range of 0.25–8 kHz, while skull bone vibrations were measured at multiple points on the skull using a scanning laser Doppler vibrometer (SLDV) system and a 3D LDV system. The 3D velocity components, defined by the 3D LDV measurement coordinate system, have been transformed into tangent (in-plane) and normal (out-of-plane) components in a local intrinsic coordinate system at each measurement point, which is based on the cadaver head's shape, estimated by the spatial locations of all measurement points. ResultsRigid-body-like motion was dominant at low frequencies below 1 kHz, and clear transverse traveling waves were observed at high frequencies above 2 kHz for both measurement systems. The surface waves propagation speeds were approximately 450 m/s at 8 kHz, corresponding trans-cranial time interval of 0.4 ms. The 3D velocity measurements confirmed the complex space and frequency dependent response of the cadaver heads indicated by the 1D data from the SLDV system. Comparison between the tangent and normal motion components, extracted by transforming the 3D velocity components into a local coordinate system, indicates that the normal component, with spatially varying phase, is dominant above 2 kHz, consistent with local bending vibration modes and traveling surface waves. ConclusionBoth SLDV and 3D LDV data indicate that sound transmission in the skull bone causes rigid-body-like motion at low frequencies whereas transverse deformations and travelling waves were observed above 2 kHz, with propagation speeds of approximately of 450 m/s at 8 kHz.