Boundary Layer Streaming Research Articles

Unsteady 3D MHD Boundary Layer Stream for Non-Newtonian Power-Law Fluid Near Stagnation Point of Moving Surfaces

An unsteady three-dimensional MHD boundary layer is a fluid flow region near a surface where magnetic fields are present and interact with the fluid flow, causing it to become unsteady. This type of flow is commonly found in various astrophysical and technological applications, such as in plasmas and fusion reactors. The 3D nature of the flow introduces additional complexities to the flow dynamics, making the study and modeling of unsteady MHD boundary layers a challenging and active area of research. The unsteady boundary layer flow of fluid over a moving stagnation surface is theoretically examined in the current work with the impression of a magnetic field. The exact outcomes of the governing equations for the flow domain are obtained by utilizing the shooting phenomena. The specified analytical outcomes are also obtained for some cases. Detailed discussions of the parameters involved are confirmed both physically and graphically. Numerical results for both profiles are presented graphically. The study and modeling of unsteady 3D MHD boundary layers is imperative for a thorough understanding of various physical phenomena, improving the performance of technological systems, and advancing our knowledge of fluid dynamics.

Effect of Wave Skewness and Sediment Particle Size on Sediment Transport Due to Combined Wave–Current Seabed Boundary Layer Streaming

Effect of Wave Skewness and Sediment Particle Size on Sediment Transport Due to Combined Wave–Current Seabed Boundary Layer Streaming

Linear and quadratic convection significance on the dynamics of MHD Maxwell fluid subject to stretched surface

The heat transmission process is a prominent issue in current technology. It occurs when there is a temperature variation between physical processes. It has several uses in advanced industry and engineering, including power generation and nuclear reactor cooling. This study addresses Maxwell fluid’s steady, two-dimensional boundary layer stream across a linearly stretched sheet. The primary objective of this research is to investigate the impact of the non-Newtonian fluid parameter (Deborah number) on flow behavior. The secondary objective is to investigate the effect of linear and quadratic convection to check which model gives higher heat transfer. The flow is caused by the surface stretching. The mathematical model containing the underlying partial differential equations (PDEs) is built using the boundary layer estimations. The governing boundary layer equations are modified to a set of nonlinear ordinary differential equations (ODEs) using similarity variables. The bvp4c approach is employed to tackle the transformed system mathematically. The impacts of numerous physical parameters like stretching coefficient, mixed convective parameter, heat source/sink coefficient, magnetic coefficient, variable thermal conductance, Prandtl number, and Deborah number over the dimensionless velocity and temperature curves are analyzed via graphs and calculated via tables. After confirming the similarity of the present findings with several earlier studies, a great symmetry is shown. The findings show that the linear convection model gains more heat transport rate than the quadratic convection model, ultimately giving a larger thermal boundary layer thickness. Some numeric impacts illustrate that boosting the magnetic coefficient elevates the fluid’s boundary layer motion, causing an opposite phenomenon of Lorentz force because the free stream velocity exceeds the stretched surface velocity.

Heat Transfer Analysis of Micro Polar Fluid with AL2O3 and CuO Hybrid Nanofluid Over a Plate with and Without Out Viscous Dissipation

Abstract: This work analysis investigated with the boundary layer stream and heat transfer aspects of a micropolar nanofluid over a porous shrinking sheet with thermal radiation. The boundary layer equations governed by the partial differential equations are transformed in to a set of ordinary differential equations with the help of suitable local similarity transformations. The coupled nonlinear ordinary differential equations are solved by the commercial MATLAB code bvp4c.The solutions of dimensionless velocity ,velocity gradient and temperature profiles are analyzed by the effect of various controlling flow parameters nonlinear parameter ,material property and Eckert number. and temperature and Prandtl number. Physical quantities such as skin frication coefficient, local heat, computed. Keywords: Viscous effects .flat plate, Nanofluid, hybrid nanofluid, micro polar fluid , local Nusselt number, skin friction, micro rotation

An Eulerian two-phase flow model investigation on scour onset and backfill of a 2D pipeline

The capability of an Eulerian two-phase model, SedFoam, in simulating the onset of scour underneath a pipeline and the backfill process is investigated. When a pipeline is slightly buried in the sediment bed, the scour onset can be caused by the piping process, which is due to seepage flow moving underneath the pipeline driven by the upstream–downstream pressure difference. To directly resolve piping as part of the scour simulation has been a challenge in the single-phase models. Alternatively, the two-phase models may be capable of simulating piping and backfill as it can resolve the interactions between the flow, structure, sediment transport, and seepage flow using a single set of governing equations and closures. To prove this point, SedFoam is validated by two laboratory experiments for the onset of scour underneath a pipeline. For piping driven by a prescribed upstream–downstream pressure difference, the model captures the temporal evolution of the pore-pressure gradient and the resulting fine-scale bathymetric change around the pipe consistent with the measured data. The model further provides insight into the seepage flow and the creeping movement of sediments during the onset of scour. When simulating piping driven by a unidirectional current, SedFoam successfully predicts piping driven by the upstream–downstream pressure gradient due to the incoming flow deceleration by the presence of pipeline and flow separation. As a proof-of-concept application, SedFoam is applied to simulate pipeline scour driven by an oscillatory flow. Although the boundary layer streaming effect is neglected, the model result shows realistic scour onset and the development of a scour hole. To demonstrate the model’s capability to simulate the backfill process, the pipeline is then lowered artificially into the scour hole as an idealized treatment of the complex pipeline sinking process during scour. The resulting burial depth due to backfill is similar to that predicted by the empirical formula.

Magnetic influence on the acoustic streaming boundary layer at a solid wall

The effect of acoustic streaming generated due to a standing wave over a flat plate in the presence of a magnetic field has been discussed. Due to this unsteadiness, the flow is divided into two regions: the boundary layer region and the core flow region. Invoking successive approximation method, we found the velocity expression for the core region confirming that the velocity is independent of viscosity but depends on a magnetic field and is used to demonstrate the Couette flow.

On the mixed convective carbon nanotube flow over a convectively heated curved surface

AbstractIn this article, mixed convective boundary layer stream of nanofluid flow with carbon nanotube as nanoparticles and transmission of heat over a coiled stretched surface are studied. The influence of magnetic orientation and velocity slip is also encountered in this problem. Two classes of carbon nanotubes, SWCNT and MWCNT, are considered as nanoparticles and water as a pure liquid. The foremost leading partial differential equations (PDEs) are formulated through curvilinear coordinate system subjected to proper boundary conditions. To simplify this nonlinear PDE‐associated model, we have employed a compatible similarity conversion and acquired the nonlinear dimensionless ordinary differential equations (ODEs). To determine the requisite numerical solution of the transformed problem, a shooting procedure embedded with RK‐4 technique has been applied. Various pictorial attempts have been initiated against different parametric inputs to reveal the hydrothermal scenario. Some physical quantities like skin friction and Nusselt numbers are calculated to investigate flow distribution inside the preferred system. A comparison with earlier research depicts parallel outcomes. Results assured that velocity is a cumulative function with positive increment of curvature parameter, but an opposite scenario is shown for temperature for both type of nanofluids. The amount of heat transition has been declined against the improvement of the magnetic parameter.

Preliminary Study on the Development of a Platform for the Optimization of Beach Stabilization Measures Against Beach Erosion III - Centering on the Effects of Random Waves Occurring During the Unit Observation Period, and Infra-Gravity Waves of Bound Mode, and Boundary Layer Streaming on the Sediment Transport

Preliminary Study on the Development of a Platform for the Optimization of Beach Stabilization Measures Against Beach Erosion III - Centering on the Effects of Random Waves Occurring During the Unit Observation Period, and Infra-Gravity Waves of Bound Mode, and Boundary Layer Streaming on the Sediment Transport

Particle transport induced by internal wave beam streaming in lateral boundary layers

Quantifying the physical mechanisms responsible for the transport of sediments, nutrients and pollutants in the abyssal sea is a long-standing problem, with internal waves regularly invoked as the relevant mechanism for particle advection near the sea bottom. This study focuses on internal-wave-induced particle transport in the vicinity of (almost) vertical walls. We report a series of laboratory experiments revealing that particles sinking slowly through a monochromatic internal wave beam experience significant horizontal advection. Extending the theoretical analysis by Beckebanze et al. (J. Fluid Mech., vol. 841, 2018, pp. 614–635), we attribute the observed particle advection to a peculiar and previously unrecognized streaming mechanism in the stratified boundary layer originating at the lateral walls. This vertical boundary layer streaming mechanism is most efficient for significantly inclined wave beams, when vertical and horizontal velocity components are of comparable magnitude. We find good agreement between our theoretical prediction and experimental results.

NUMERICAL INVESTIGATIONS OF THE MECHANISMS ASSOCIATED WITH THE ONSHORE RIPPLE MIGRATION

Quantification of cross-shore sediment transport is one of most intriguing challenges in shoreline and coastal geomorphology. During the past decades, several key mechanisms associated with onshore/offshore sediment transport have been identified, such as wave skewness/asymmetry, progressive wave streaming and undertow current. However, applying these mechanisms to the migration of wave formed bedforms (ripples) is not straightforward. For example, recent field observations off Fire Island, NY showed that ripples migrated onshore even during periods of offshore directed wave skewness, which is contradictory to the prediction of empirical sediment transport formulations. The physical processes driving ripple vortex formation, ejection and boundary layer streaming associated with rippled bed can further complicate the bedload/suspended load sediment transport over ripples. To fully understand these mechanisms, a comprehensive model that can resolve the ripple dynamics and interactions between free surface wave and rippled bed is examined.

Numerical analysis of the flapping mechanism for a two-phase coaxial jet

A numerical study of a coaxial liquid jet sheared by an annular high-speed stream is performed at moderate density and velocity ratio between phases. The destabilization mechanism of the jet is studied : due to the shear induced by the high-speed stream, annular interfacial waves are generated near the jet injection, which subsequently connect and yield a sinuous organization for the whole jet, leading to a global flapping phenomenon. The influence of the high-speed stream boundary layer thickness on this flapping is investigated, as well as the influence of the velocity and the momentum flux ratio between phases. It appears that it is not sufficient to consider only the momentum flux ratio between phases to characterize the flapping dynamics or the steps of the atomization process. On the other hand, the Reynolds and Weber numbers based on the high-speed stream properties display a strong influence on these phenomena.

Sediment flux based model of instantaneous sediment transport due to pure velocity-skewed oscillatory sheet flow with boundary layer stream

Velocity-skewed oscillatory sheet flow transport is studied by an approximate model for instantaneous sediment transport resulting from exponential profiles of sediment concentration and velocity over a mobile bed. The phase lag effect, and asymmetry in wave friction factor, oscillatory flow orbital amplitude, roughness height, bed shear stress and wave boundary layer thickness are modelled so that the net boundary layer stream of pure velocity-skewed oscillatory flow can be obtained. The proposed model is found consistent with the previous classical formulas, and qualitatively refines a series of exponents of velocity power function for the approximation of instantaneous sediment transport rate under different flow conditions. The sediment fluxes can be well predicted with accuracy as that of a two-phase numerical model. The net boundary layer stream and sediment transport rates are also well predicted, especially the negative net stream and rates with large phase residual. The phase residual is shown a useful but insufficient factor in the net sediment transport in pure velocity-skewed flows. In addition, the net boundary layer stream resulting from velocity-skewed wave boundary layer development difference between onshore and offshore flow durations is an essential factor as well.

Paraffin Diffusion Flame Attachment at a Small Backward-Facing Step

ABSTRACTA boundary layer approximation is used to describe the flow in the region around a small backward-facing step at the base of an air stream boundary layer that flows parallel to a solid fuel. This region may hold a diffusion flame between the air and the fuel vapor that is generated by gasification of the solid fuel under the action of the heat flux reaching its surface from the flame. Local extinction leading to flame detachment is shown to occur at a small distance downstream of the step when a Damkohler number equal to the ratio of the diffusion time across the height of the step to a characteristic chemical time falls below a certain critical value. The selfsimilar flow that develops far downstream of the step when the Damkohler number is above its critical value is described.

Numerical study of a flapping liquid sheet sheared by a high-speed stream

A numerical study of a liquid sheet sheared on both sides by a high-speed stream is performed in this work, at moderate density and velocity ratio between phases. Near the injection, an interfacial wave develops on both interfaces of the liquid sheet. A vortical detachment of the high-speed stream is observed behind this wave and modifies the pressure field around the sheet. The global flapping mechanism is a consequence of the pressure difference between the two sides of the liquid sheet. The flapping dynamics is characterized and compared to existing correlations available in the literature. A sensitivity study of the flapping dynamics to the high-speed stream boundary layer thickness is performed and a relevant Strouhal number is proposed.

Suspended sediments due to random waves including effects of second order wave asymmetry and boundary layer streaming

The paper provides a practical stochastic method by which the suspended sediment concentration due to long-crested (2D) and short-crested (3D) nonlinear random waves can be calculated. The approach is based on assuming the waves to be a stationary narrow-band random process, and by using the parameterized formulas valid for regular waves presented in Soulsby, R.L., 1997. Dynamics of Marine Sands. Thomas Telford, London. The Forristall, G.Z., 2000. Journal of Physical Oceanography. 30, 1931–1943. wave crest height distribution representing both 2D and 3D nonlinear random waves is also adopted. The model covers sediment suspension over rippled beds and for sheet flow. Comparisons are made with random wave data from Thorne, P.D., Williams, J.J., Davies, A.G., 2002. Journal of Geophysical Research. 107 (C8), 3178 for flow over rippled beds. An example for sheet flow using data typical to field conditions is also included to illustrate the approach.

Experimental and theoretical study of wave–current turbulent boundary layers

AbstractAn experimental study of turbulent wave–current boundary layer flows is performed using a state-of-the-art oscillating water tunnel (OWT) for flow generation and a particle image velocimetry system for velocity measurements. The current velocity profiles in the presence of sinusoidal waves indicate a two-log-profile structure suggested by the widely-used Grant–Madsen model. However, for weak currents in the presence of nonlinear waves, the two-log-profile structure is contaminated or even totally obliterated by the boundary layer streaming which is produced by the asymmetry of turbulence in successive half-periods of nonlinear waves. To interpret experimental results, a semi-analytical model which adopts a rigorous way to account for a time-varying turbulent eddy viscosity is developed. The model can accurately predict turbulence asymmetry streaming, which leads to successful predictions of the mean velocity embedded in nonlinear-wave tests and the current velocity profiles in the presence of either sinusoidal or nonlinear waves. Since the Longuet-Higgins-type streaming due to wave propagation is absent in OWT flows and not included in the semi-analytical model, future work is necessary to extend this study for applications in the coastal environment.

Experimental study of turbulent oscillatory boundary layers in an oscillating water tunnel

A high-quality experimental study including a large number of tests which correspond to full-scale coastal boundary layer flows is conducted using an oscillating water tunnel for flow generations and a Particle Image Velocimetry system for velocity measurements. Tests are performed for sinusoidal, Stokes and forward-leaning waves over three fixed bottom roughness configurations, i.e. smooth, “sandpaper” and ceramic-marble bottoms. The experimental results suggest that the logarithmic profile can accurately represent the boundary layer flows in the very near-bottom region, so the log-profile fitting analysis can give highly accurate determinations of the theoretical bottom location and the bottom roughness. The first-harmonic velocities of both sinusoidal and nonlinear waves, as well as the second-harmonic velocities of nonlinear waves, exhibit similar patterns of vertical variation. Two dimensionless characteristic boundary layer thicknesses, the elevation of 1% velocity deficit and the elevation of maximum amplitude, are found to have power-law dependencies on the relative roughness for rough bottom tests. A weak boundary layer streaming embedded in nonlinear waves and a small but meaningful third-harmonic velocity embedded in sinusoidal waves are observed. They can be only explained by the effect of a time-varying turbulent eddy viscosity. The measured period-averaged vertical velocities suggest the presence of Prandtl's secondary flows of the second kind in the test channel. Among the three methods to infer bottom shear stress from velocity measurements, the Reynolds stress method underestimates shear stress due to missed turbulent eddies, and the momentum integral method also significantly underestimates bottom shear stress for rough bottom tests due to secondary flows, so only the log-profile fitting method is considered to yield the correct estimate. The obtained bottom shear stresses are analyzed to give the maximum and the first three harmonics, and the results are used to validate some existing theoretical models.

Transient force generation during impulsive rotation of wall-mounted panels

AbstractSquare and triangular shape actuator panels mounted on the wall of a wind tunnel beneath an air flow have been impulsively rotated with an angular velocity between 3 and $26~\mathrm{rad} ~{\mathrm{s} }^{- 1} $. A custom-designed balance was used to measure the time-dependent lift and drag forces during the deployment of the actuator, the position of which was monitored by a digital encoder. The measured forces have been compensated for inertia effects which are significant. The results indicated that all lift and drag force coefficients during the transient deployment are different than the corresponding coefficients under stationary conditions at the same deployment angle. It was found that these dynamic effects are augmented with increasing velocity ratio $\mathit{Str}$. The square actuator was found to have better aerodynamic performance than the triangular ones. Additional experiments within different boundary layers reveal that the generated unsteady forces on the moving panels are affected by the characteristics of the incoming boundary layers. The results showed that the thinner the boundary layer is the higher the forces are. Time-resolved flow visualization studies indicated that during the deployment of the panel the upstream turbulent boundary layer structures and the free stream fluid are decelerated and squeezed in the longitudinal direction as they approach the moving plate. A very thin and highly sheared wall layer develops over the moving panel, it generates a substantial amount of vorticity and it subsequently separates from the three edges of the panel to form a large-scale ring-like vortical structure which is responsible for the transient augmentation of the aerodynamic forces. This structure consists of wrapped around separated shear layers which contain pockets of compressed eddies and free stream fluid originated in the upstream incoming boundary layer and free stream. A horseshoe vortex starts to form over the moving plate and during the final stages of deployment it has been moved upstream while the incoming boundary layer turbulent structures are pushed and diverted upwards.

Practical sand transport formula for non-breaking waves and currents

Many existing practical sand transport formulae for the coastal marine environment are restricted to a limited range of hydrodynamic and sand conditions. This paper presents a new practical formula for net sand transport induced by non-breaking waves and currents. The formula is especially developed for cross-shore sand transport under wave-dominated conditions and is based on the semi-unsteady, half wave-cycle concept, with bed shear stress as the main forcing parameter. Unsteady phase-lag effects between velocities and concentrations, which are especially important for rippled bed and fine sand sheet-flow conditions, are accounted for through parameterisations. Recently-recognised effects on the net transport rate related to flow acceleration skewness and progressive surface waves are also included. To account for the latter, the formula includes the effects of boundary layer streaming and advection effects which occur under real waves, but not in oscillatory tunnel flows. The formula is developed using a database of 226 net transport rate measurements from large-scale oscillatory flow tunnels and a large wave flume, covering a wide range of full-scale flow conditions and uniform and graded sands with median diameter ranging from 0.13mm to 0.54mm. Good overall agreement is obtained between observed and predicted net transport rates with 78% of the predictions falling within a factor 2 of the measurements. For several distinctly different conditions, the behaviour of the net transport with increasing flow strength agrees well with observations, indicating that the most important transport processes in both the rippled bed and sheet flow regime are well captured by the formula. However, for some flow conditions good quantitative agreement could only be obtained by introducing separate calibration parameters. The new formula has been validated against independent net transport rate data for oscillatory flow conditions and steady flow conditions.

RANS-based simulation of turbulent wave boundary layer and sheet-flow sediment transport processes

A numerical model coupling the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations with two-equation k−ω turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes. The hydrodynamic model is additionally coupled with bed and suspended load descriptions, the latter based on an unsteady turbulent-diffusion equation, for simulation of sheet-flow sediment transport processes. In addition to standard features common within such RANS-based approaches, the present model includes: (1) hindered settling velocities at high suspended sediment concentrations, (2) turbulence suppression due to density gradients in the water–sand mixture, (3) boundary layer streaming due to convective terms, and (4) converging–diverging effects due to a sloping bed. The present model therefore provides a framework for simultaneous inclusion of a number of local factors important within cross-shore wave boundary layer and sediment transport dynamics. The hydrodynamic model is validated for both hydraulically smooth and rough conditions, based on wave friction factor diagrams and boundary layer streaming profiles, with the results in excellent agreement with experimental and/or previous numerical work. The sediment transport model is likewise validated against oscillatory tunnel experiments involving both velocity-skewed and acceleration-skewed flows, as well as against measurements beneath real progressive waves. Model capabilities are exploited to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity-skewed conditions. For the medium sand grain conditions considered, the model results suggest that streaming effects can enhance onshore sediment transport rates by as much as a factor of two. Moreover, for fine sand conditions streaming (and related convective) effects are demonstrated to potentially reverse the direction of net transport (i.e. from offshore to onshore) relative that predicted in oscillatory tunnel conditions. The developed model is implemented within the popular Matlab environment, and hence may be attractive for both research and educational purposes.