Current Shear Research Articles

Observational Evidence of Surface Wave‐Generated Strong Ocean Turbulence

AbstractBy using an acoustic Doppler velocimeter mounted on the seabed of the continental shelf of the northern South China Sea, high frequency velocity fluctuations were measured for 4.5 days. The turbulent kinetic energy dissipation rate was estimated. During the observation, the strong ocean response to Typhoon Rammasun was recorded to compare the turbulent characteristics before and during the typhoon. The results show that the turbulence near the seabed is mainly generated by the tidal current shear and exhibits a quarter diurnal variation during the period before the typhoon. During the typhoon period, the dissipation rate ε dramatically increased from 1 × 10−6 to 1 × 10−2 m2 s−3 within a short time, and the significant wave height and the surface wave orbital velocity showed the same tendency. This finding suggests that the turbulence is dominantly generated by the surface waves near the seabed.

Reduction of Burr in Rolling Process Through Shear Mechanism Analysis

This study analyzes the burrs and defects generated on a shear surface during the billet shear process and the effects of the shearing conditions such as the blade shape, knife gap, align between top and bottom knife and angle. We believe that our study makes a significant contribution to the literature because it provides a means of overcoming the limitations of current methods used to reduce defects. Further, we believe that this paper will be of interest to the readership of your journal because of its focus on improving the current shearing process to obtain better quality products. The objective of this study is to develop an reduction of burr in rolling workpiece. This paper presents the method to evaluate the kind of blade types. In addition, the results of a field test in a real, large factory are introduced in this paper.

Stability analysis of deep-water waves on a linear shear current using unsteady conformal mapping

This paper describes linear stability analysis of the two-dimensional steady motion of periodic deep-water waves with symmetric non-overhanging profiles propagating on a linear shear current, namely a vertically sheared current with constant vorticity. In order to investigate numerically with high accuracy the stability of large-amplitude waves, we adopt a formulation using conformal mapping, in which the time-varying water surface is always mapped onto the real axis of a complex plane. This formulation allows us to apply numerical methods developed for large-amplitude irrotational waves without a shear current directly to the present problem, and reduces the linear stability problem to a generalized eigenvalue problem. Numerical solutions describe both super- and sub-harmonic instabilities of the periodic waves for a wide range of wave amplitudes and clarify how the behaviours of dominant eigenvalues change with the strength of the shear current. In particular, it is shown that, even in the presence of a linear shear current, the steady periodic waves lose stability due to superharmonic disturbances at the wave amplitude where the wave energy attains an extremum, similarly to the case of irrotational waves without a shear current. It is also found that re-stabilization with an increase in wave amplitude characterizes subharmonic instability for weak shear currents, but the re-stabilization disappears for strong shear currents.

Particle trajectories under interactions between solitary waves and a linear shear current

ABSTRACT This paper is concerned with particle trajectories beneath solitary waves when a linear shear current exists. The fluid is assumed to be incompressible and inviscid, lying on a flat bed. Classical asymptotic expansion is used to obtain a Korteweg-de Vries (KdV) equation, then a forth-order Runge-Kutta method is applied to get the approximate particle trajectories. On the other hand, our particular attention is paid to the direct numerical simulation (DNS) to the original Euler equations. A conformal map is used to solve the nonlinear boundary value problem. High-accuracy numerical solutions are then obtained through the fast Fourier transform (FFT) and compared with the asymptotic solutions, which shows a good agreement when wave amplitude is small. Further, it also yields that there are different types of particle trajectories. Most surprisingly, periodic motion of particles could exist under solitary waves, which is due to the wave-current interaction.

A comprehensive finite element examination of Carreau Yasuda fluid model in a lid driven cavity and channel with obstacle by way of kinetic energy and drag and lift coefficient measurements

In current pagination the flow problems involving shear rate dependent nonlinear viscosity have been treated successfully in the sense of space discretization and solvers. The stable finite element pair Q2/P1disc is employed to approximate the velocity and pressure spaces independently. Discretized form of non-linear expressions is linearized by implementing Newton’s procedure and the resulting systems are solved using a geometric multigrid approach. The flow generated by way of driven cavity and by an obstacle are very important benchmarks of computational fluid dynamics. In current pagination shear rate reliant viscosity model renowned as Carreau Yasuda fluid is capitalized. The obtained results are demonstrated and analyzed with the help of velocity and viscosity plots. In addition, we have produced new reference data for kinetic energy (K.E) for driven cavity problem and drag and lift coefficients for circular obstacle problem. The obtained results for driven cavity problem reveal the fact that K.E is an increasing function of the relaxation parameter (λ) and power law exponent (n) whereas a decreasing function of the model parameter (a). For the case of flow around obstacle the drag coefficient (CD) and the lift coefficient (CL) show strong dependence on (λ) and (n), however a weak dependence on the parameter (a).

Environmental controls on marine productivity near Cape St. Francis, South Africa

Abstract. This study considers ocean–atmosphere influences on marine productivity over the shelf near Cape St. Francis, South Africa. Multiday estimates of chlorophyll fluorescence in the period 2006–2017 with an area outlined by 34.5–33.75∘ S and 24–26.5∘ E provide the basis for evaluation using data from high-resolution reanalyses. Correlations with the mean annual cycle of chlorophyll fluorescence were significant for salinity, linking marine productivity and the coastal hydrology. A strengthened Agulhas Current induces cyclonic shear that lifts water at the shelf edge. Composite high-chlorophyll-fluorescence events were dominated by a large-scale midlatitude atmospheric ridge of high pressure. The resultant easterly winds caused offshore transport and the upwelling of cool nutrient-rich water in multiday events at the beginning and end of austral summer. Environmental controls on interannual fluctuations of the commercial fishery were also explored. Southwestward currents and diminished heat fluxes favored the squid catch, while anchovy and sardine catches were linked with upper northerly wind, consistent with large-scale weather patterns that underpin coastal upwelling and river discharge. Productivity lags a few days behind cyclonic wind and current shear and the upstream coastal hydrology, which shares a common atmospheric driver.

An Improved Method for Determining Near‐Surface Currents From Wave Dispersion Measurements

AbstractA new inversion method for determining near‐surface shear currents from a measured wave spectrum is introduced. The method is straightforward to implement and starts from the existing state‐of‐the‐art technique of assigning effective depths to measured wavenumber‐dependent Doppler shift velocities. A polynomial fit is performed, with the coefficients scaled based on a simple derived relation to produce a current profile that is an improved estimate of the true profile. The method involves no user‐input parameters, with the optimal parameters involved in the polynomial fit being chosen based on a simple criterion involving the measured Doppler shift data only. The method is tested on experimental data obtained from a laboratory where current profiles of variable depth dependence could be created and measured by particle image velocimetry, which served as “truth” measurements. Applying the new inversion method to experimentally measured Doppler shifts resulted in a >3 times improvement in accuracy relative to the state of the art for current profiles with significant near‐surface curvature. The experiments are dynamically similar to typical oceanographic flows such as wind‐drift profiles, and our laboratory thus makes a suitable and eminently useful scale model of the real‐life setting. Our results show that the new method can achieve improved accuracy in reconstructing near‐surface shear profiles from wave measurements by a simple extension of methods, which are currently in use, incurring little extra complexity and effort. A novel adaptation of the normalized scalar product method has been implemented, able to extract Doppler shift velocities as a function of wavenumber from the measured wave spectrum.

Post-cracking shear behaviour of concrete beams strengthened with externally prestressed tendons

External prestressing technology has been successfully adopted for enhancing the load-carrying capacity of beams in bridge engineering. Previous studies on the shear behaviour of externally prestressed concrete (EPC) beams mainly focused on the ultimate shear strength. The post-cracking shear behaviour of EPC beams remains unclear. This study aims to investigate the post-cracking shear behaviour of EPC beams. To this end, nine EPC beams with different design parameters were tested to failure. The investigated parameters included the prestressing condition, shear span to depth ratio, shear reinforcement ratio, and bend angle of external tendons. In contrast to reinforced concrete beams, the EPC beams demonstrated shear tension sliding with concrete spalling failure. The applied prestressing increased the concrete shear contribution by 68%. The percentage of the concrete shear contribution out of the total shear strength of the test beams ranged from 45% to 79%. The post-cracking shear strength reserve ability index, defined as the ratio of the shear strength and shear cracking strength, ranged from 1.72 to 2.96 in the EPC beams. A new ductility index, defined as the ratio of the ultimate deflection and the deflection corresponding to first shear cracking, was used to evaluate the post-cracking deformability of EPC beams. The ductility index of the EPC beams ranged from 3.0 to 7.6, indicating considerable post-cracking deformability. The beam’s stiffness at first shear cracking was 39–81% of the beam’s initial stiffness; the beam’s stiffness at the ultimate state was 18–41% of the beam’s initial stiffness. Finally, the current shear provisions were evaluated by the test results.

Simulation of wave-current interaction with a sinusoidal bottom using OpenFOAM

A two-dimensional numerical model in OpenFOAM, an open-source computational fluid dynamics software package, is used to investigate Bragg resonance under a wave-current field. An internal mass source wavemaker and a numerical sponge layer are implemented for wave-current generation and wave absorption, respectively. To ensure the validity of the internal wavemaker, a series of examinations is carried out. A simulation of wave-current interaction with a sinusoidal bottom is then carried out. The numerical results are compared with available experimental data and theoretical results. Moreover, the reflectivity and flow field in the presence of uniform and shear currents are investigated.

The Southern Caribbean upwelling system off Colombia: Water masses and mixing processes

The upwelling system off the southern Caribbean coast is probably the main source of the nutrients that support biological productivity in the oligotrophic Caribbean Sea. Subtropical underwater (SUW) that forms the subsurface salinity maximum in the Caribbean Sea is the main source of upwelled waters in this system. Profiles of salinity and temperature with depth derived from four oceanographic cruises and Argo floats showed that upwelled waters have a salinity that is ~0.11 g kg−1 lower than the SUW in the central Caribbean Sea and a seasonal variation of approximately 0.09 g kg−1 that reflects the rainy/dry seasons. In addition, the SUW is ~50 m shallower on the continental shelf slope (~100 m) compared to the depth of the SUW in the central Caribbean Sea. The origin of these modified SUW was analyzed using the Mercator numerical model, which reproduces the main vertical characteristics of the subsurface salinity maximum. The modeled data showed that SUW upwelling off of the La Guajira Peninsula and Venezuela arrive into the system via an intense Caribbean Coastal Undercurrent (CaCU, mean speed ~0.28 m s−1). This current is formed in front of the Nicaragua platform from the divergence of subsurface water flow at the salinity maximum depth. The lower salinity observed in the upwelled waters may be the result of intense vertical mixing processes that could occur when the SUW are transported by the CaCU below the Panama-Colombia Gyre (PCG) region before reaching the upwelling zones. The mixing processes—involving double diffusion and mechanical turbulence driven by vertical shear of horizontal currents—were analyzed using the Turner Angle and the Thorpe scale, respectively. Below a depth of 200 m, double diffusion by salt fingers (diffusivities > 5 x 10−5 m2 s−1) was the main process of salt diffusion, generating a downward salt flux of >2 x 10−2 g kg−1 m d−1 between the SUW and the North Atlantic Central Waters (NACW). Above a depth of 100 m, mechanical turbulent diffusion generates a salt flux towards the surface ranging 0.5–4 x 10−2 g kg−1 m d−1, where double diffusion by salt finger is not possible. The diluted SUW is subsequently transported by the CaCU, connecting—at the subsurface level—the PCG region with the upwelling zones off of Colombia and Venezuela. As well as modifying the salt content of the coastal SUW, these mixing processes may also alter the nutrient content of upwelling waters, the ecosystem effects of which are still unknown.

The nonmodal kinetic theory for the electrostatic instabilities of a plasma with a sheared Hall current

The kinetic theory for the instabilities driven by the Hall current with a sheared current velocity, which has the method of the shearing modes or the so-called nonmodal approach as its foundation, is developed. The developed theory predicts that in the Hall plasma with the inhomogeneous electric field, the separate spatial Fourier mode of the perturbations is determined in the frame convected with one of the plasma components. Because of the different shearing of the ion and electron flows in the Hall plasma, this mode is perceived by the second component as the Doppler-shifted continuously sheared mode with time-dependent wave numbers. Due to this effect, the interaction of the plasma components forms the nonmodal time-dependent process, which should be investigated as the initial value problem. The developed approach is applied to the solutions of the linear initial value problems for the hydrodynamic modified two-stream instability and the kinetic ion-sound instability of the plasma with a sheared Hall current with a uniform velocity shear. These solutions reveal that the uniform part of the current velocity is responsible for the modal evolution of the instability, whereas the current velocity shear is the source of the development of the nonmodal instability with exponent growing with time as ∼(t−t0)3.

Impacts of subtidal motions and the earth rotation on modal characteristics of the semidiurnal internal tide

Subtidal motions, such as large-scale circulations and mesoscale eddies, frequently occupy on the propagation path of internal waves in the ocean. Through solving a modified Taylor–Goldstein equation with subtidal currents, density stratification, and the earth rotation, this study investigates the impacts of subtidal motions and the earth rotation on the modal characteristics of the semidiurnal internal tide (SIT) based on long-term mooring measurements in the northern South China Sea (SCS). It is shown that the modal characteristics of the SIT are significantly influenced by the time-varying subtidal current and density stratification associated with energetic subtidal motions in the northern SCS. The earth rotation plays a minor role in modulating the modal characteristics of the SIT in the northern SCS, but it becomes effective at middle and high latitudes when strong subtidal current shear associated with subtidal motions is present. Moreover, the strong subtidal current associated with subtidal motions may significantly affect the critical latitude of the SIT. The observational results reported here will help to improve our understanding of the modal characteristics of internal tides under the influence of subtidal motions in global oceans.

Pressure–strain terms in Langmuir turbulence

This study investigates the pressure–strain tensor ($\unicode[STIX]{x1D72B}$) in Langmuir turbulence. The pressure–strain tensor is determined from large-eddy simulations (LES), and is partitioned into components associated with the mean current shear (rapid), the Stokes shear and the turbulent–turbulent (slow) interactions. The rapid component can be parameterized using existing closure models, although the coefficients in the closure models are particular to Langmuir turbulence. A closure model for the Stokes component is proposed, and it is shown to agree with results from the LES. The slow component of $\unicode[STIX]{x1D72B}$ does not agree with existing ‘return-to-isotropy’ closure models for five of the six components of the Reynolds stress tensor, and a new closure model is proposed that accounts for these deviations which vary systematically with Langmuir number, $La_{t}$, and depth. The implications of these results for second- and first-order closures of Langmuir turbulence are discussed.

Extrapolating Eulerian ocean currents for improving surface drift forecasts

ABSTRACT Predictions of drift trajectories based on four drift models were compared with observations from drifting buoys deployed in 2014 and 2015 in the Estuary and Gulf of St. Lawrence to show the impact of the current vertical shear on the surface drift predictions. Input of ocean currents and winds are obtained from ISMER's 5 km resolution ocean circulation model and from the Canadian Regional Deterministic Prediction System, respectively. The control drift model A considers depth-averaged near-surface currents (0–5 m) provided by the top grid cell of the ocean circulation model. Model B performs a linear extrapolation assuming a constant vertical shear equal to that between the first two cells of the ocean model. Models C and D perform a dynamic extrapolation assuming an Ekman layer with a constant or linearly increasing vertical viscosity, respectively. Model performance is evaluated based on several error metrics. Drift models based on extrapolated surface currents reduce separation distances relative to the control model by 25% (model B), 31% (model C) and 35% (model D) on average, for lead times from 3 h to 72 h. We thus recommend the use of extrapolation methods of near-surface ocean currents for improving surface drift forecasting skills in support of emergency response.

Coupling vibration response of a curved flexible riser under the combination of internal slug flow and external shear current

Apart from the vortex-induced vibration (VIV) caused by flow over flexible risers, internal slug flow-induced vibration (FIV) is extensively encountered in practice as risers are conveying oil–gas two-phase flow. This paper presents an experimental study of the coupling response of a flexible catenary riser under the combination of internal slug flow and external shear current. The vibration displacement and the internal flow regime are synchronously captured using the non-intrusive technique of high speed imaging method. The experimental results illustrate that the contribution of internal FIV and external VIV depends on the phase difference and the water depth. For the in-plane response, the contribution of internal slug flow is greater than the external flow at the upper part, while the out-of-plane response is dominated by the external current. Unlike the standing wave behavior in the internal FIV case, a traveling wave occurs with the participation of external flow, associated with the multimodal response. Additionally, the response frequencies vary over time, possessing oscillating characteristics. Thus unstable response and mode switching are observed in the deflected shapes of the flexible riser. Due to the vigorous out-of-plane vibration, the longer liquid slugs are split into shorter segments. As the gas–liquid ratio increases, the liquid slug length is decreased gradually, resulting in the reduction of system mass. Both the in-plane and out-of-plane responses are enhanced accordingly.

Long-lived predator-prey dynamics in the pedestal of near-zero torque high performance DIII-D plasmas

Quiescent high performance plasmas (often termed QH-mode) are attractive due to the replacement of potentially damaging energy and particle releases known as edge localized modes by relatively benign edge harmonic oscillations (EHOs). These EHOs are believed to be driven unstable by edge current and/or edge toroidal rotational shear and contribute to edge particle transport. Decreasing the applied neutral beam torque in standard QH-mode discharge leads to an improved quiescent phase of higher and wider pedestal, also known as the wide-pedestal QH-mode [Burrell et al., Phys. Plasmas 23, 056103 (2016)]. This work expands upon the observed limit cycle oscillation (LCO) dynamics [Barada et al., Phys. Rev. Lett. 120, 135002 (2018)] in this wide pedestal QH-mode. The onset of these LCOs after wide-pedestal transition is found to be correlated with the disappearance of coherent EHOs which happens either when the edge maximum bootstrap current decreases after the transition or when the toroidal rotation is decreased consistent with simulation predictions. Sustainment of this quasistationary oscillating regime is found to be possible due to a predator-prey type competition between E × B velocity shear and turbulence density fluctuations facilitated by an inward propagation of nonzonal flow like toroidally and poloidally symmetric E × B velocity perturbations from these LCOs. These LCO dynamics are further controlled by adding electron cyclotron heating (ECH) to a neutral beam heated wide-pedestal QH-mode discharge which led to a surprising increase in energy confinement correlated with a concomitant decrease in edge turbulence in contrast to normally observed confinement degradation in H-mode with ECH.

Modulation instability and rogue waves for shear flows with a free surface

The effect of shear currents on rogue waves on a free surface is modeled by the nonlinear Schr\odinger equation. Curvature of velocity profiles (or vorticity gradient) and the relative motion of the wave packet and shear current play critical roles, through enhanced growth rates of disturbances.

Nonlinear hydroelastic waves on a linear shear current at finite depth

This work is concerned with waves propagating on water of finite depth with a constant-vorticity current under a deformable flexible sheet. The pressure exerted by the sheet is modelled by using the Cosserat thin shell theory. By means of multi-scale analysis, small amplitude nonlinear modulation equations in several regimes are considered, including the nonlinear Schrödinger equation (NLS) which is used to predict the existence of small-amplitude wavepacket solitary waves in the full Euler equations and to study the modulational instability of quasi-monochromatic wavetrains. Guided by these weakly nonlinear results, fully nonlinear steady and time-dependent computations are performed by employing a conformal mapping technique. Bifurcation mechanisms and typical profiles of solitary waves for different underlying shear currents are presented in detail. It is shown that even when small-amplitude solitary waves are not predicted by the weakly nonlinear theory, we can numerically find large-amplitude solitary waves in the fully nonlinear equations. Time-dependent simulations are carried out to confirm the modulational stability results and illustrate possible outcomes of the nonlinear evolution in unstable cases.

Current density dependent shear performance and fracture behavior of micro-scale BGA structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under coupled electromechanical loads

The shear performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under coupled electromechanical loads with the increasing current density (from 6 × 103 to 1.1 × 104 A/cm2) were investigated systematically. Severe Joule heating and current crowding effects on actual temperature, shear strength, damage and fracture behavior of the joints under coupled electromechanical loads were studied by theoretical formulation and experimental characterization, as well as finite element simulation. Results demonstrate that severe Joule heating and current crowding effects lead to significantly increased temperature in the joints, which is much higher than the ambient temperature. The shear strength of joints under coupled electromechanical loads shows strong dependence on current density, in terms of a relatively rapid monotonic decrease with the increasing current density. Moreover, a fully coupled finite element model was developed to characterize the degree of damage of joints under coupled electro-mechanical loads. The results show that electric current leads to aggravated damage in the joints, and damage occurs and accumulates much more easily in joints subjected to high density current stressing. The maximum viscoplastic dissipation energy density increases monotonically with current density. With increasing current density, there is a transition in fracture position (path) from the solder matrix to the solder/IMC interface of the joints, which corresponds to a shift in fracture mode from ductile fracture to brittle fracture.

Upper ocean shear in the northern South China Sea

The shear in the upper ocean of the northern South China Sea (SCS) is examined based on current observations from six moorings in 2010–2011. Spectral analysis results indicate that the sub-inertial currents, near-inertial waves (NIWs), diurnal and semidiurnal internal tides (ITs) dominate the current and shear in the northern SCS. Through comparing current variance and shear caused by these motions, this study shows the great contribution of NIWs to shear: Although NIWs only account for 2–7% of the total current variance, the shear caused by NIWs is approximate one fifth to one-quarter of the total shear. Moreover, the NIWs are dominated by the component with upgoing phase and downgoing energy, whereas the upgoing and downgoing components are comparable in both diurnal and semidiurnal ITs. Because the incoherent component has a larger contribution to shear than the coherent component, the shear of both diurnal and semidiurnal ITs exhibits significant signals with frequencies larger than the spring-neap cycles of approximate 14 days. The larger contribution to shear and smaller proportion in current variance suggest that the incoherent component of ITs has a larger vertical wavenumber than the coherent component. In addition, a case study shows that the mesoscale eddy pair occurring between 22 October and 2 December 2010 does not significantly enhance the ocean shear at two moorings especially below 150 m depth, although it contributes a lot to the current variance.