Free Surface Interaction Research Articles

Interactions between a toroidal bubble and a free surface

Toroidal bubbles (TBs) represent cases of vortex rings with a gas–liquid interface where a gas vortex ring is encased within a liquid vortex ring, and can serve as effective media for mass conveyance, process mixing, noise reduction and reaction regulation. In this study, we carry out a systematic study on the interaction between a TB and a free surface. According to the high-speed photographic images from the experiments, we identify strong and weak interactions in terms of the normalized maximum free surface deformation $h_{max}^*$ . Then, we perform numerical simulations based on the volume of fluid (VOF) method in the OpenFOAM platform. Based on both the experimental and the numerical results, we conclude that the Froude number, $Fr$ , determines the main characteristics during the interaction process. The TB–free surface interaction is essentially the interaction between the liquid vortex ring enveloping the TB and the free surface, supplemented by the TB's complex behaviour. Next, we establish the scaling law of $h_{max}^*$ based on the energy balance condition. Based on this, we provide the critical $Fr$ and the slenderness of the TB, $\eta$ , for identifying the strong and weak interactions, and a parametric plot of the interactions in terms of $Fr$ and $\eta$ .

Large eddy simulations of bubbly flows and breaking waves with smoothed particle hydrodynamics

For turbulent bubbly flows, multi-phase simulations resolving both the liquid and bubbles are prohibitively expensive in the context of different natural phenomena. One example is breaking waves, where bubbles strongly influence wave impact loads, acoustic emissions and atmospheric-ocean transfer, but detailed simulations in all but the simplest settings are infeasible. An alternative approach is to resolve only large scales, and model small-scale bubbles adopting sub-resolution closures. Here, we introduce a large eddy simulation smoothed particle hydrodynamics (SPH) scheme for simulations of bubbly flows. The continuous liquid phase is resolved with a semi-implicit isothermally compressible SPH framework. This is coupled with a discrete Lagrangian bubble model. Bubbles and liquid interact via exchanges of volume and momentum, through turbulent closures, bubble breakup and entrainment, and free-surface interaction models. By representing bubbles as individual particles, they can be tracked over their lifetimes, allowing closure models for sub-resolution fluctuations, bubble deformation, breakup and free-surface interaction in integral form, accounting for the finite time scales over which these events occur. We investigate two flows: bubble plumes and breaking waves, and find close quantitative agreement with published experimental and numerical data. In particular, for plunging breaking waves, our framework accurately predicts the Hinze scale, bubble size distribution, and growth rate of the entrained bubble population. This is the first coupling of an SPH framework with a discrete bubble model, with potential for cost-effective simulations of wave–structure interactions and more accurate predictions of wave impact loads.

Two-phase free-surface flow interaction with moving bodies using a consistent, momentum preserving method

The numerical prediction of two-phase flows with an interface is challenging, to a considerable extent because of the high density ratio at the interface. Numerical results become affected by momentum losses, diverging spurious interface velocities, free surface distortion, and even numerical instability. To prevent issues like these, consistent momentum and mass transport with an additional continuity equation were introduced.In this article, we describe how a consistent discretization was incorporated into our own method and extended for fluid-structure interaction (FSI) with moving rigid bodies. The new method was tested against benchmark simulations from literature confirming that consistent transport modeling gives a significant improvement compared to non-consistent modeling for the dynamics of two-phase flows.Newly devised proof of principle FSI simulations with momentum transfer from fluid to body in the presence of a high-density ratio between fluids are introduced that could serve as a benchmark for future studies. The simulations demonstrate that consistent modeling gives an order of magnitude improvement in terms of momentum conservation compared to non-consistent modeling.Simulations with the new method are also compared to FSI experiments from literature. Results obtained with the consistent method are closer to the measurements than results of the non-consistent method.The merit of consistent modeling with and without FSI becomes especially apparent for two-phase flows with a high-density ratio between fluids.

Focused wave interaction with a partially-immersed rectangular box using 2-D incompressible SPH on a GPU comparing with experiment and linear theory

Wave interaction with bodies spanning the water surface is a common problem. A basic rectangular box of small draft is considered in two dimensions and focused waves are used to minimise reflections. Overtopping occurs in some cases making the inherently mass conservative smoothed particle hydrodynamics (SPH) method appropriate. This is applied in incompressible (divergence-free) form with a 2-D graphics processing unit (GPU) version of the Chow et al. (2019) code with some improvements. A newly parallelised GPU algorithm for the population of a compressed sparse row matrix storage array for the pressure Poisson equation is presented and provides a 327–446 times function speedup and overall simulation time speedups of 1.45–1.78 times with over 5 million particles. The inclusion of the parallel function leads to a completely parallel ISPH algorithm. Peak pressures on the base and front face are compared with experiment and linear (potential-flow) theory. The experiments used periodic focused waves which showed some variation in form about a peak crest although crest elevations were repeatable, and these are reproduced in the model. Converged incompressible SPH (ISPH) values are in approximate agreement with both. Overtopping of the box shows qualitative agreement with experiment. While linear theory cannot account for overtopping or viscous (eddy-shedding) effects, submerged pressure prediction provides a useful approximation. The ISPH model is single phase and limited video evidence suggests that air entrapment can occur in the initial stages of overtopping near the front face. Quite complex vorticity generation and eddy shedding is predicted with free-surface interaction. The study shows the capability and limitations of single phase ISPH for complex wave-body interaction.

A conservative level set method on unstructured meshes for modeling multiphase thermo-fluid flow in additive manufacturing processes

Additive manufacturing (AM) is an emerging technology that fuses deposited powder materials together layer-by-layer using a localized heat source to create an arbitrarily complex geometry. This technology is an inherently multiphysics problem, involving the simultaneous evolution of fluid flow, heat transfer, free surface, laser-material interaction and material phases including solid, liquid and gas. Multiphysics and multiphase models are typically used to understand the dominant physics driving AM processes through high fidelity simulations resolving the individual powders as they melt onto the substrate. However, these types of high fidelity models have not been applicable to modeling directed energy deposition (DED) process, where powders are delivered through the ambient vapor onto the substrate by a nozzle and subsequently fused. This paper presents a novel multiphase thermo-fluid formulation for modeling DED processes. A diffuse level set formulation coupled with the Navier–Stokes, energy conservation and radiative transport equation allows us to model complex free surface, fluid flow, thermal and laser interaction evolution. In addition, our formulation enables us to resolve the vapor flow field and its effect on deposited material during AM processes. The accuracy of our proposed method is assessed by comparing with literature solutions and experimental benchmarks. Our proposed formulation is then used to model simple DED processes that enable us to visualize the entrainment of powder particles into the melt pool. This process elucidates the dominant physical forces that can drive powders into the melt pool as well as their effect on the melt pool evolution within DED processes.

Wettability control on deformation: Coupled multiphase fluid and granular systems.

Understanding of multiphase flow in porous media is important for a wide range of applications such as soil science, environmental remediation, energy resources, and CO_{2} sequestration. This phenomenon depends on the complex interplay between the fluid and solid forces such as gravitational, capillary, and viscous forces, as well as wettability of the solid phase. Such interactions along with the geometry of the medium give rise to a variety of complex flow regimes. Although much research has been done in the area of wettability, its mechanical effect is not well understood, and it continues to challenge our understanding of the phenomena on macroscopic and microscopic scales. In this paper, therefore, the effect of wettability on the deformation of porous media and fluid-fluid patterns is studied through a series of three-dimensional (3D) simulations. To this end, the discrete element method (DEM) and volume of fluid (VOF) are coupled to accurately model free-surface flow interaction in a cylindrical pack of spheres. The fluid-particle interactions are modeled by exchanging information between DEM and VOF, while the effect of wettability is considered to study how it controls fluid displacement. The results indicate that the drag force and deformation in the pack vary with the change in wettability and capillary number. To demonstrate the effect of both wettability and capillary number, a series of numerical experiments were conducted with two capillary numbers and three wettability conditions. Our results show that the drag force was greatest for near extreme wettability conditions, which resulted in a larger deformation.

The traffic performance of the wheeled carrying liquid materials study

A feature of the wheeled vehicles capable of carrying liquid materials is the presence of the external disturbances in the form of interaction with the roadbed. This applies to peculiar transport vehicles, carrying liquid materials considered as non-Newtonian fluid. The internal disturbances in the form of the liquid material free surface interaction is the presence of the external disturbances in the form of interaction with the roadbed. Internal disturbances in the form of the wheeled concrete transport machine working vessel fluid free surface interaction should also be taken into account. The article describes the first part of the task - the interaction of the system with the roadbed. The simplest form of the system motion equations with rolling is in the accompanying coordinate system when written in the form of equations in quasi-coordinates. Based on the apparatus of nonholonomic mechanics, M.V. Keldysh built the theory of the rolling wheels with an elastic tire. Under certain assumptions, the above-mentioned theory in the article is extended to the case of curvilinear motion. These equations are represented as a system of first-order equations convenient for numerical solution.

A generating and absorbing boundary condition for dispersive waves in detailed simulations of free-surface flow interaction with marine structures

The boundaries of numerical domains for free-surface wave simulations with marine structures generate spurious wave reflection if no special measures are taken to prevent it. The common way to prevent reflection is to use dissipation zones at the cost of increased computational effort. On many occasions, the size of the dissipation area is considerably larger than the area of interest where wave interaction with the structure takes place. Our objective is to derive a local absorbing boundary condition that has equal performance to a dissipation zone with lower computational cost. The boundary condition is designed for irregular free-surface wave simulations in numerical methods that resolve the vertical dimension with multiple cells. It is for a range of phase velocities, meaning that the reflection coefficient per wave component is lower than a chosen value, say 2%, over a range of values for the dimensionless wave number kh. This is accomplished by extending the Sommerfeld boundary condition with an approximation of the linear dispersion relation in terms of kh, in combination with vertical derivatives of the solution variables. For this article, the boundary condition is extended with a non-zero right-hand side in order to prevent wave reflection, while, at the same time, at the same boundary, generating waves that propagate into the domain. Results of irregular wave simulations are shown to correspond to the analytical reflection coefficient for a range of wave numbers, and to have similar performance to a dissipation zone at a lower cost.

Slamming load on trimaran cross section with rigid and flexible arches

In this paper, the mixed mode function–modified MPS method was adopted to simulate the water-structure dynamic interaction during slamming impacts on the cross deck of trimaran. The water was considered as an incompressible inviscid fluid and the “conceptual particle” model was used to enhance the stability of the intense free surface interaction during the “filling-up” process under the cross deck. The numerical model for the coupled rigid and flexible modal superposition model was derived for the dynamics of the trimaran hull with rigid and flexible arches, and the flexible arch was simplified by a beam structure. The fluid and structure solvers were coupled in a standard iterative way. The results for rigid-body arch cases obtained with the use of improved free surface condition show good improvement, in comparison to the experiment data. From the study of flexible arch cases with different flexibilities, it is found that the relatively soft structure can reduce the local pressure and slamming load.

The initial development of a jet caused by fluid, body and free surface interaction with a uniformly accelerated advancing or retreating plate. Part 2. Well-posedness and stability of the principal flow

We consider the problem of a rigid plate, inclined at an angle $\unicode[STIX]{x1D6FC}\in (0,\unicode[STIX]{x03C0}/2)$ to the horizontal, accelerating uniformly from rest into, or away from, a semi-infinite strip of inviscid, incompressible fluid under gravity. Following on from Gallagher et al. (J. Fluid Mech., vol. 841, 2018, pp. 109–145) (henceforth referred to as GNB), it is of interest to analyse the well-posedness and stability of the principal flow with respect to perturbations in the initially horizontal free surface close to the plate contact point. In particular we find that the solution to the principal unperturbed problem, denoted by [IBVP] in GNB, is well-posed and stable with respect to perturbations in initial data in the region of interest, localised close to the contact point of the free surface and the plate, when the plate is accelerated with dimensionless acceleration $\unicode[STIX]{x1D70E}\geqslant -\cot \,\unicode[STIX]{x1D6FC}$, while the solution to [IBVP] is ill-posed with respect to such perturbations in the initial data, when the plate is accelerated with dimensionless acceleration $\unicode[STIX]{x1D70E}<-\cot \,\unicode[STIX]{x1D6FC}$. The physical source of the ill-posedness of the principal problem [IBVP], when $\unicode[STIX]{x1D70E}<-\cot \,\unicode[STIX]{x1D6FC}$, is revealed to be due to the leading-order problem in the innermost region localised close to the initial contact point being in the form of a local Rayleigh–Taylor problem. As a consequence of this mechanistic interpretation we anticipate that, when the plate is accelerated with $\unicode[STIX]{x1D70E}<-\cot \,\unicode[STIX]{x1D6FC}$, the inclusion of weak surface tension effects will restore well-posedness of the problem [IBVP] which will, however, remain temporally unstable.

The initial development of a jet caused by fluid, body and free surface interaction with a uniformly accelerated advancing or retreating plate. Part 1. The principal flow

The free surface and flow field structure generated by the uniform acceleration (with dimensionless acceleration $\unicode[STIX]{x1D70E}$) of a rigid plate, inclined at an angle $\unicode[STIX]{x1D6FC}\in (0,\unicode[STIX]{x03C0}/2)$ to the exterior horizontal, as it advances ($\unicode[STIX]{x1D70E}>0$) or retreats ($\unicode[STIX]{x1D70E}<0$) from an initially stationary and horizontal strip of inviscid incompressible fluid under gravity, are studied in the small-time limit via the method of matched asymptotic expansions. This work generalises the case of a uniformly accelerating plate advancing into a fluid as studied by Needham et al. (Q. J. Mech. Appl. Maths, vol. 61 (4), 2008, pp. 581–614). Particular attention is paid to the innermost asymptotic regions encompassing the initial interaction between the plate and the free surface. We find that the structure of the solution to the governing initial boundary value problem is characterised in terms of the parameters $\unicode[STIX]{x1D6FC}$ and $\unicode[STIX]{x1D707}$ (where $\unicode[STIX]{x1D707}=1+\unicode[STIX]{x1D70E}\tan \unicode[STIX]{x1D6FC}$), with a bifurcation in structure as $\unicode[STIX]{x1D707}$ changes sign. This bifurcation in structure leads us to question the well-posedness and stability of the governing initial boundary value problem with respect to small perturbations in initial data in the innermost asymptotic regions, the discussion of which will be presented in the companion paper Gallagher et al. (J. Fluid Mech. vol. 841, 2018, pp. 146–166). In particular, when $(\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D707})\in (0,\unicode[STIX]{x03C0}/2)\times \mathbb{R}^{+}$, the free surface close to the initial contact point remains monotone, and encompasses a swelling jet when $(\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D707})\in (0,\unicode[STIX]{x03C0}/2)\times [1,\infty )$ or a collapsing jet when $(\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D707})\in (0,\unicode[STIX]{x03C0}/2)\times (0,1)$. However, when $(\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D707})\in (0,\unicode[STIX]{x03C0}/2)\times \mathbb{R}^{-}$, the collapsing jet develops a more complex structure, with the free surface close to the initial contact point now developing a finite number of local oscillations, with near resonance type behaviour occurring close to a countable set of critical plate angles $\unicode[STIX]{x1D6FC}=\unicode[STIX]{x1D6FC}_{n}^{\ast }\in (0,\unicode[STIX]{x03C0}/2)$ ($n=1,2,\ldots$).

The initial development of a jet caused by fluid, body and free surface interaction. Part 5. Parasitic capillary waves on an initially horizontal surface

In Part 3 of this series of papers (Needhamet al.,Q. J. Mech. Appl. Maths, vol. 61, 2008, pp. 581–614), we studied the free surface flow generated in a horizontal layer of inviscid fluid when a flat, rigid plate, inclined at an external angle$\unicode[STIX]{x1D6FC}$to the horizontal, is driven into the fluid with a constant, horizontal acceleration. We found that the most interesting behaviour occurs when$\unicode[STIX]{x1D6FC}>\unicode[STIX]{x03C0}/2$(the plate leaning into the fluid). When$\unicode[STIX]{x03C0}/2<\unicode[STIX]{x1D6FC}\leqslant \unicode[STIX]{x1D6FC}_{c}$, with$\unicode[STIX]{x1D6FC}_{c}\approx 102.6^{\circ }$, we were able to find the small-time asymptotic solution structure, and solve the leading-order problem numerically. When$\unicode[STIX]{x1D6FC}=\unicode[STIX]{x1D6FC}_{c}$, we found numerical evidence that a$120^{\circ }$corner exists on the free surface, at leading order as$t\rightarrow 0$. For$\unicode[STIX]{x1D6FC}>\unicode[STIX]{x1D6FC}_{c}$, we could find no numerical solution of the leading-order problem as$t\rightarrow 0$, and hypothesised that the solution does not exist for any$t>0$for these values of$\unicode[STIX]{x1D6FC}$. At the present time, there is no rigorous proof of this hypothesis. In this paper, we demonstrate that the likely non-existence of a solution for any$t>0$when$\unicode[STIX]{x1D6FC}>\unicode[STIX]{x1D6FC}_{c}$can be reconciled with the physics of the system by including the effect of surface tension in the model. Specifically, we find that for$\unicode[STIX]{x1D6FC}>\unicode[STIX]{x1D6FC}_{c}$, the solution exists for$0\leqslant t<t_{c}$, with$t_{c}\sim Bo^{-1/(3\unicode[STIX]{x1D6FE}-1)}\unicode[STIX]{x1D70F}_{c}$and$\unicode[STIX]{x1D70F}_{c}=O(1)$as$Bo^{-1}\rightarrow 0$, where$Bo$is the Bond number ($Bo^{-1}$is the square of the ratio of the capillary length to the fluid depth) and$\unicode[STIX]{x1D6FE}\equiv 1/(1-\unicode[STIX]{x03C0}/4\unicode[STIX]{x1D6FC})$. The solution does not exist for$t\geqslant t_{c}$due to a topological transition driven by a nonlinear capillary wave, i.e. the free surface pinches off (self-intersects) when$t=t_{c}$. We are also able to compare this asymptotic solution with experimental results which show that, in an experimental case where the contact angle remains approximately constant (the modelling assumption that we make in this paper), the asymptotic solution is in good agreement. In general, the inclusion of surface tension leads to the generation of capillary waves ahead of the wavecrest, which decay as$t$increases for$\unicode[STIX]{x1D6FC}<\unicode[STIX]{x1D6FC}_{c}$but dominate the flow and lead to pinch-off for$\unicode[STIX]{x1D6FC}>\unicode[STIX]{x1D6FC}_{c}$. These capillary waves are an unsteady analogue of the parasitic capillary waves that can be generated ahead of steadily propagating, periodic capillary–gravity waves, e.g. Lin & Rockwell (J. Fluid Mech., vol. 302, 1995, pp. 29–44).

MPM Simulations of the Interaction Between Water Jet and Soil Bed

In offshore engineering, pipelines are often buried in the seabed to avoid the damage caused by ocean waves, currents, fishing activities, etc. For its reliability, the high-speed water jet has been increasingly used for constructing the pipe trenches. The jet-soil interaction is a highly complicated two-phase problem. A three-dimensional Material Point Method (MPM) model, Anura3D (www.anura3d.com), is used to simulate the jet trenching processes. In this preliminary study, the effect of the water jet speed on the trenching process is simulated. The research demonstrates the advantages of the MPM model in handling the free surface and soil-water interaction problems, and the results are useful for the offshore oil and gas industry. In future research, the different controlling parameters of the trenching operation, including the jet size, water pressure, translational speed and soil strength, will be tested to optimize the trenching operation to achieve high efficiency.

Bilge keel–free surface interaction and vortex shedding effect on roll damping

Prediction of the roll damping of a ship with bilge keels using traditional techniques, such as Ikeda’s method, is no longer sufficient when the bilge keels interact with the free surface. The discrepancies occurring between this method and actual roll damping are due to free surface interaction and vortex shedding, which are not considered in this method. Reynolds-averaged Navier–Stokes (RANS) solvers can be used to capture these effects, and the widely used Ikeda’s method can be modified. In this study, two-dimensional (2D) roll damping calculations for a hull section with bilge keels, including the free surface effects, are calculated numerically and experimentally for different draft cases. The normal forces acting on the bilge keels are calculated numerically to show the free surface effect on bilge keel roll damping. The generated vortices and vortex shedding from the bilge keels are analyzed to investigate the effect of the bilge keels–free surface interaction on the roll damping coefficients. The results show that the RANS solver is capable of predicting the roll damping coefficients in good agreement with experimental results and can be further used to modify Ikeda’s method. It is concluded that the bilge keel forces decrease when the bilge keels interact with the free surface, whereas Ikeda’s method gives a constant value for each draft condition. The interaction of the bilge keels and free surface affects the generation of the vorticity and vortex shedding from the bilge keels.

Fluid interface detection technique based on neighborhood particles centroid deviation (NPCD) for particle methods

SummaryParticle‐based CFD methods are powerful approaches to investigate free surface, multiphase flows, and fluid structure interaction problems because of their ability of tracking moving fluid interface even with huge deformations or fragmentation and merging. However, many fluid interface particle detection techniques are simple to implement but with low accuracy or provide relatively good detection results at complicated implementation cost or higher computational time. In case of incompressible flow simulation methods solving the Poisson equation of pressure, such as the moving particle semi‐implicit method, boundary particles detection techniques' accuracy affects precision and stability of pressure computation and interaction between fluid phases. In the present work, a new fluid interface particle detection technique is proposed to improve the accuracy of the boundary particles detection and keep the implementation easy. Denominated as the neighborhood particles centroid deviation technique, it is a two‐criteria technique based on the particle number density and the neighborhood particles weighted geometric center deviation. Compared with other techniques, the proposed neighborhood particles centroid deviation technique shows the best results by eliminating false interface particles inside the fluid domain and keeping the interface particles layer thin and regular. As a result, relatively stable pressure time histories and more consistent pressure and velocity fields are achieved. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Characterization of a micro-hydrokinetic turbine in close proximity to the free surface

Predicting hydrokinetic turbine power generation is difficult due to complex geometry, highly turbulent conditions, and difficulty capturing the transient interface existing between air and water. A three-dimensional finite volume solver was used to capture the effects resulting from free surface interaction with the aid of a Volume of Fluid (VOF) multiphase solver. Depths from free surface level to blade tip with corresponding Froude numbers of 0.71, 0.92, 1.04, and 1.31 were modeled specifically to capture the transition from subcritical to supercritical flow conditions. A sharp decrease in performance was observed at the critical Froude number (Fr=1.0). Results at subcritical conditions showed acceptable agreement with previously published single phase results where the turbine is assumed to be operating in an infinite medium. At subcritical conditions, the propeller-based turbine studied was compared to numerical and experimental results obtained for a traditional marine current turbine (MCT). As the flow became critical, a 32.2% decrease in the power coefficient was predicted and significant wake-free surface interaction was observed.

Hydronium and hydroxide at the air–water interface with a continuum solvent model

The distribution of hydronium and hydroxide ions at the air–water interface has been a problem of much interest in recent years. Here we explore what insights can be gained from a continuum solvent model. We extend our model of ionic solvation free energies and surface interaction free energies to include hydronium and hydroxide. The hydronium cation is attracted to the air–water interface, whereas the hydroxide anion is repelled. If the cavity size parameters required by the model are adjusted to reproduce solvation energies, quantitative agreement with experimental surface tensions is achieved. To the best of our knowledge, this is the most accurate theoretical estimation of this property so far. The results indicate that even if ‘water structure’ is important, its effects can be captured with a relatively simple model. They also contradict the inference from electrophoresis that there is strong hydroxide enhancement at the air–water interface.

Sub-Critical Interaction of Free Surface Motion and Thermocapillary Convection

A numerical study is performed to analyze the effect of the first mode of vibration on subcritical thermocapillary convection in a floating-zone liquid bridge of high Prandtl number fluid. First, the onset of oscillatory thermocapillary flow at a certain critical Marangoni number with an insulated and nondeformable free surface is investigated. The onset of oscillatory flow in this case is characterized by a rotating hydrothermal wave. Then, motion is induced to the free surface of the liquid bridge corresponding to the first mode of vibration of a free–free beam that is similar in configuration to the liquid bridge under study. In this case, an insulated free surface and subcritical Marangoni number are maintained to avoid the hydrodynamic instability associated with the rotating hydrothermal wave. The amplitude of vibration is set at 1.1% and 2.25% of the floating-zone diameter for two series of analyses, whereas the frequency of vibration is varied over a wide range to study the interaction between the imposed vibration amplitude and frequency with thermocapillary convection inside the floating zone. It is found that amplified coupling between the flow and induced free surface motion occurs at a certain frequency () that is indicated by significantly enhanced flow and temperature oscillations. The oscillatory flow under these subcritical conditions is generally consistent with earlier studies done at supercritical conditions in a similar configuration. The possibility of free surface interaction resulting in oscillatory thermocapillary flow is demonstrated in this work.

Performance characterization and placement of a marine hydrokinetic turbine in a tidal channel under boundary proximity and blockage effects

Marine hydrokinetic turbine, when operating in a shallow channel is subjected to the boundary proximity effects from a deformable free surface on top and the channel bottom. A close proximity of turbine to these boundaries modifies the flow-field around the turbine and affects device performance. Significant flow acceleration occurs in and around the turbine rotation plane; the magnitude of which depends on size of the turbine relative to the channel cross-section and is commonly referred to as solid blockage. In addition, the wake behind the turbine creates a restriction to the flow called wake blockage. We focus on unraveling the influence of boundary proximity and blockage on the turbine performance through coupled experimental and computational studies. The experiments were carried out in an open surface water channel with a three bladed, constant chord, untwisted marine hydrokinetic turbine submerged at different depths and performance was evaluated under various operating conditions. The findings were complimented by a steady state computational fluid dynamics study that was carried out to understand the effect of flow Reynolds number and solid blockage on the turbine performance. A reduction in tip-depth of immersion was observed to improve the turbine performance until it reached an optimum depth beyond which a reduction in performance was observed due to free surface interaction with wake and bypass region. A transient CFD analysis with volume of fluid approach was performed to incorporate free-surface and buoyancy effects and augment flow-field characterization behind the turbine in the wake, upper bypass, and lower bypass regions. For low tip clearance ratios, a significant drop (up to 5–10% of channel depth) in free surface was observed behind turbine with complex three dimensional flow structures that lead to a skewed wake affecting its expansion and restoration process.

Problem of elastic anisotropy and stacking faults in stress analysis using multireflection grazing-incidence X-ray diffraction

Multireflection grazing-incidence X-ray diffraction (MGIXD) was used to determine the stress- and strain-free lattice parameter in the surface layer of mechanically treated (polished and ground) tungsten and austenitic steel. It was shown that reliable diffraction stress analysis is possible only when an appropriate grain interaction model is applied to an anisotropic sample. Therefore, verification of the X-ray stress factors (XSFs) was accomplished by measuring relative lattice strains during anin situtensile test. The results obtained using the MGIXD and standard methods (χ and ω geometries) show that the Reuss and free-surface grain interaction models agree with the experimental data. Moreover, a new interpretation of the MGIXD results was proposed and applied for the first time to measure the probability of stacking faults as a function of penetration depth for a polished and ground austenitic sample. The XSF models verified in the tensile test were used in the analysis of residual stress components.