Search

We investigate the dynamic interfacial deformation induced by micrometric particles exerting a periodic force on a planar interface or on a bubble, and the resulting lateral capillary interactions. Assuming that the deformation of the interface is small, neglecting the effect of viscosity and assuming point particles, we derive analytical formulas for the dynamic deformation of the interface. For the case of a planar interface the dynamic point force simply generates capillary waves, while for the case of a bubble it excites shape oscillations, with a dominant deformation mode that depends on the bubble radius for a given forcing frequency. We evaluate the lateral capillary force acting between two particles, by superimposing the deformations induced by two point forces. We find that the lateral capillary forces experienced by dynamically forced particles are non-monotonic and can be repulsive. The results are applicable to micrometric particles driven by different dynamic forcing mechanisms such as magnetic, electric or acoustic fields.

This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. Two-dimensional sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The Discrete Element (DEM) technique is coupled with the Lattice Boltzmann Method (LBM), for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using Multiple- Relaxation-Time LBM (LBM-MRT) for numerical stability. In order to simulate interconnected pore space in 2D, a reduction in the radius of the grains (hydrodynamic radius) is assumed during LBM computations. The collapse of granular column in fluid is compared with the dry cases to understand the effect of fluid on the runout behaviour. A parametric analysis is performed to assess the influence of the granular characteristics (initial packing) on the evolution of flow and run-out distances for slope angles of 0{\deg}, 2.5{\deg}, 5{\deg} and 7.5{\deg}. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass. The mechanism of energy dissipation, shape of the flow front, water entrainment and evolution of packing density is used to explain the difference in the flow characteristics of loose and dense granular column collapse in fluid.

The present study aims to implement the software ANSYS AQWA to discuss the hydrodynamic analysis of the DeepCwind semi-submersible floating platform in waves based on the potential flow theory by considering the second-order wave exciting force. In this study, the linearized potential-flow hydrodynamic radiation and diffraction problems in the frequency domain were firstly solved by adopting the three-dimensional panel method. Subsequently, the hydrodynamic coefficients and wave loading data were transformed to time domain forms by the Cummins time domain equation as a system loading input. Furthermore, the quadratic transfer function (QTF) matrices with different frequencies and directions deduced based on the near field integration over the mean wetted hull surface were adopted for the calculation of slow-drift forces. In order to represent the damping in a real system for modeling potential flow without Morison&rsquo, s elements, an additional quadratic damping matrix was added to capture the viscous drag. Eventually, both of the dynamic mooring model based on the lump-mass (LM) approach and the quasi-static mooring model based on the multi-segmented, quasi-static (MSQS) approach are introduced to discuss the mooring effect on the platform hydrodynamics. The effect of wave heading angles on the platform motion is considered as an influential parameter as well.

The dynamic response of floating horizontal axis wind turbines (FHWATs) are affected by the viscous and inertia effects. In free decay motion, viscous drag reduces the amplitude of pitch and roll fluctuation, the quasi-static mooring system overestimates the resonant amplitude values of pitch and roll. In this paper, the quasi-static mooring system is modified by introducing linear damping and quadratic damping. The dynamic response characteristics of the FHAWT modified model of the DTU 10 MW tension leg platform (TLP) were studied. Dynamic response of the blade was mainly caused by wind load, while the wave increased the blade short-term damage equivalent load. The tower base bending moment was affected by inclination of the tower and the misaligned angle &beta, wave between wind and wave. Except the yaw motion, other degrees of freedom motions of the TLP were substantially affected by &beta, wave. Ultimate tension of the mooring system was related to the displacement caused by pitch and roll motions, and standard deviation of the tension was significantly affected by the wave frequency response. Under the action of wave load, the viscous drag would stimulate the mooring system and increase the resonance of the platform motion.

Two of the most common modes of oscillation of single degree of freedom wave energy converters are heave and surge, which are, respectively, exploited by heaving point absorbers (HPAs), and oscillating wave surge converters (OWSCs). Given major hydrodynamic differences between HPAs and OWSC, different nonlinear forces may be more or less relevant. Likewise, the scaling properties of such nonlinear forces may be different, according to the type of device, introducing uncertainties. This paper studies different nonlinear effects, and the relevance of different hydrodynamic force components, in HPAs and OWSCs. Nonlinear Froude–Krylov forces, as well as viscous drag effects, are represented and both prototype and full-scale device sizing are considered. Results show that HPAs are predominantly affected by nonlinear Froude–Krylov forces, while the most important hydrodynamic forces in OWSCs are diffraction and radiation effects. In addition, viscous drag appears to have little relevance in HPAs, but a significant influence in OWSCs. Finally, nonlinearities are shown to significantly affect the phase of the different force components.

This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier&ndash, Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled, the response reduction is: (i) 3% due to the nonlinear mooring response and the Froude&ndash, Krylov force, (ii) 1&ndash, 4% due to viscous forces, and (iii) 18&ndash, 19% due to induced drag and non-linear added mass and radiation forces. The effect of the induced drag is shown to be largely scale-independent. It is caused by local pressure variations due to vortex generation below the body, which reduce the total pressure force on the hull. Euler simulations are shown to be scale-independent and the scale effects of the WEC are limited by the purely viscous contribution (1&ndash, 4%) for the two waves studied. We recommend that experimental model scale test campaigns of WECs should be accompanied by RANS simulations, and the analysis complemented by scale-independent Euler simulations to quantify the scale-dependent part of the nonlinear effects.

The input of energy by wind to water waves is compared with the observed growth of the waves using a suite of microphysical measurement techniques in the laboratory. These include measured tangential stresses in the water and air immediately adjacent to the interface with corresponding form drag measurements above wind-forced freely propagating waves. The drag data sets are consistent but the comparison has highlighted important issues in relation to the measurement of fluctuating pressures above freely propagating waves. Derived normalized wind input values show good collapse as a function of mean wave steepness and are significantly in excess of the assembly of net wave growth measurements by Peirson & Garcia (J. Fluid Mech., vol. 608, 2008, pp. 243–274) at low steepness. Sheltering coefficients in the form of Jeffreys (Proc. R. Soc. Lond. Ser. A, vol. 107, 1925, pp. 189–206) are derived that are consistent with values previously obtained by Donelan & Pierson (J. Geophys. Res., vol. 92, 1987, pp. 4971–5029), Donelan (Wind-over-Wave Couplings: Perspectives and Prospects, Clarendon, 1999, pp. 183–194) and Donelan et al. (J. Phys. Oceanogr., vol. 36, 2006, pp. 1672–1689). The sheltering coefficients exhibit substantial scatter. By carefully measuring the associated growth of the surface wave fields, systematic energy budgets for the interaction between wind and waves are obtained. For non-breaking waves, there is a significant and systematic misclose in the radiative transfer equation if wave–turbulence interactions are not included. Significantly higher levels of turbulent wave attenuation are found in comparison with the theoretical estimates by Teixeira & Belcher (J. Fluid Mech., vol. 458, 2002, pp. 229–267) and Ardhuin & Jenkins (J. Phys. Oceanogr., vol. 36, 2006, pp. 551–557). Suitable normalizations of attenuation for wind-forced wave fields exhibit consistent behaviour in the presence and absence of wave breaking. Closure of the surface energy flux budget is obtained by comparing the normalized energy loss rates due to breaking with the values previously determined by Banner & Peirson (J. Fluid Mech., vol. 585, 2007, pp. 93–115) and Drazen et al.(J. Fluid Mech., vol. 611, 2008, pp. 307–332) when expressed as a function of mean wave steepness. Their normalized energy loss rates obtained for non-wind forced breaking wave groups are remarkably consistent with the levels found during this present study when breaking waves are subject to wind forcing.

A novel method similar to marching technique was used to model wind tunnel tests. ► Sail trim criteria based on their interactive effect are identified. ► Areas of separated flow were characterised. ► Local flow field was correlated with sail surface pressures. ► An aerodynamic model based on potential flow with viscous correction is proposed. The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using Computational Fluid Dynamics. A Reynolds-averaged Navier-Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where thepressure distributions on the sails were measured. An original approach was employed byusing two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered. the third author received a financial support from Brest Métropole Océane and the ERASMUS scholarship

Spacers are designed to generate significant secondary flow structures and create directional changes in the flow through membrane modules. Shape of the spacers used in membrane modules strongly influences the resulting flow and therefore performance of the module. In this work fluid dynamics of rectangular channels similar to membrane modules and containing different spacers was simulated using a three-dimensional computational fluid dynamics (CFD) model. A ‘unit cell’ approach was evaluated and used for this purpose. In addition to predicting the pressure drop, the simulated results provided significant insight into fluid dynamics of spacer filled channels. The validated CFD model was used to evaluate performance of different spacer shapes and understand the role of spacer shape and resulting fluid dynamics. The models were extended for the first time to simulate flow in spacer filled curvilinear channels, which could be useful in understanding the fluid behavior in spiral modules. The results were compared with those obtained with the flat channel. The approach and results presented in this work will have significant implications for identifying improved spacers with higher propensities to reduce fouling in membrane modules.