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At least 685 records · Page 38Linked to original sources
arXiv2014

The Numerical Simulation of Ship Waves Using Cartesian-Grid and Volume-of-Fluid Methods

Cartesian-grid methods in combination with immersed-body and volume-of-fluid methods are ideally suited for simulating breaking waves around ships. A surface panelization of the ship hull is used as input to impose body-boundary conditions on a three-dimensional cartesian grid. The volume-of-fluid portion of the numerical algorithm is used to capture the free-surface interface, including the breaking of waves. The numerical scheme is implemented on a parallel computer. Various numerical issues are discussed, including implementing exit boundary conditions, conserving mass using a novel regridding algorithm, improving resolution through the use of stretched grids, minimizing initial transients, and enforcing hull boundary conditions on cartesian grids. Numerical predictions are compared to experimental measurements of ship models moving with forward speed, including model 5415 and model 5365 (Athena). The ability to model forced-motions is illustrated using a heaving sphere moving with forward speed.

physics.flu-dyn↗
arXiv2014

The Numerical Simulation of Ship Waves Using Cartesian Grid Methods with Adaptive Mesh Refinement

Cartesian-grid methods with Adaptive Mesh Refinement (AMR) are ideally suited for simulating the breaking of waves, the formation of spray, and the entrainment of air around ships. As a result of the cartesian-grid formulation, minimal input is required to describe the ships geometry. A surface panelization of the ship hull is used as input to automatically generate a three-dimensional model. No three-dimensional gridding is required. The AMR portion of the numerical algorithm automatically clusters grid points near the ship in regions where wave breaking, spray formation, and air entrainment occur. Away from the ship, where the flow is less turbulent, the mesh is coarser. The numerical computations are implemented using parallel algorithms. Together, the ease of input and usage, the ability to resolve complex free-surface phenomena, and the speed of the numerical algorithms provide a robust capability for simulating the free-surface disturbances near a ship. Here, numerical predictions, with and without AMR, are compared to experimental measurements of ships moving with constant forward speed, including a vertical strut, the DDG 5415, and a wedge-like geometry.

physics.flu-dyn↗
arXiv2014

Numerical Simulation of Wakes in a Weakly Stratified Fluid

This paper describes some preliminary numerical studies using large eddy simulation of full-scale submarine wakes. Submarine wakes are a combination of the wake generated by a smooth slender body and a number of superimposed vortex pairs generated by various control surfaces and other body appendages. For this preliminary study, we attempt to gain some insight into the behavior of full-scale submarine wakes by computing separately the evolution the self-propelled wake of a slender body and the motion of a single vortex pair in both a non-stratified and a stratified environment. An important aspect of the simulations is the use of an iterative procedure to relax the initial turbulence field so that turbulent production and dissipation are in balance.

physics.flu-dyn↗
arXiv2014

The Numerical Simulation of Ship Waves using Cartesian Grid Methods

Two different cartesian-grid methods are used to simulate the flow around the DDG 5415. The first technique uses a "coupled level-set and volume-of-fluid" (CLS) technique to model the free-surface interface. The no-flux boundary condition on the hull is imposed using a finite-volume technique. The second technique uses a level-set technique (LS) to model the free-surface interface. A body-force technique is used to impose the hull boundary condition. The predictions of both numerical techniques are compared to whisker-probe measurements of the DDG 5415. The level-set technique is also used to investigate the breakup of a two-dimensional spray sheet.

physics.flu-dyn↗
arXiv2014

Two Dimensional Subsonic Euler Flow. Past a Wall or a Symmetric Body

The existence and uniqueness of two dimensional steady compressible Euler flows past a wall or a symmetric body are established. More precisely, given positive convex horizontal veloicty in the upstream, there exists a critical value $ρ_{cr}$ such that if the incoming density in the upstream is larger than $ρ_{cr}$, then there exists a subsonic flow past a wall. Furthermore, $ρ_{cr}$ is critical in the sense that there is no such subsonic flow if the density of the incoming flow is less than $ρ_{cr}$. The subsonic flows possess large vorticity and positive horizontal velocity above the wall except at the corner points on the boundary. Moreover, the existence and uniqueness of a two dimensional subsonic Euler flow past a symmetric body are also obtained when the incoming velocity field is a general small perturbation of a constant velocity field and the density of the incoming flow is larger than a critical value. The asymptotic behavior of the flows is obtained with the aid of some integral estimates for the velocity field and its far field states.

math.AP↗
arXiv2014

A Numerical Formulation for Simulating Free-Surface Hydrodynamics

Cartesian-grid methods in combination with immersed-body and volume-of-fluid methods are ideally suited for simulating breaking waves around ships. A surface panelization of the ship hull is used as input to impose body-boundary conditions on a three-dimensional cartesian grid. The volume-of-fluid portion of the numerical algorithm is used to capture the free-surface interface, including the breaking of waves. The numerical scheme is implemented on a parallel computer. Recent improvements to the numerical scheme are discussed, including implementation of a new multigrid procedure and conversion to MPI communication. Numerical predictions are compared to laboratory measurements of a towed transom-stern model.

physics.flu-dyn↗
arXiv2014

The Impact of Oceanic Heat Transport on the Atmospheric Circulation: a Thermodynamic Perspective

The present study investigates how global thermodynamic properties of the climate system are affected by the changes in the intensity of the imposed oceanic heat transport in an atmospheric general circulation model in aqua-planet configuration. Increasing the poleward oceanic heat transport results in an overall increase in the surface temperature and a decrease in the equator-to-pole surface temperature difference as a result of the ice-albedo feedback. Following the classical ansatz by Stone, the atmospheric heat transport changes in such a way that the total poleward heat transport remains almost unchanged. We also find that the efficiency of the climate machine, the intensity of the Lorenz energy cycle and the material entropy production of the system decline with increased oceanic heat transport which suggests that the climate system becomes less efficient and turns into a state of reduced entropy production, as the enhanced oceanic transport performs a stronger large-scale mixing between geophysical fluids with different temperature, thus reducing the availability in the climate system and bringing it closer to a state of thermal equilibrium.

physics.ao-ph↗
arXiv2014

A theoretical and numerical determination of optimal ship forms based on Michell's wave resistance

We determine the parametric hull of a given volume which minimizes the total water resistance for a given speed of the ship. The total resistance is the sum of Michell's wave resistance and of the viscous resistance, approximated by assuming a constant viscous drag coefficient. We prove that the optimized hull exists, is unique, symmetric, smooth and that it depends continuously on the speed. Numerical simulations show the efficiency of the approach, and complete the theoretical results.

math.OC↗
arXiv2014

Hodograph Method and Numerical Integration of Two Hyperbolic Quasilinear Equations. Part I. The Shallow Water Equations

In paper [S.I. Senashov, A. Yakhno. 2012. SIGMA. Vol.8. 071] the variant of the hodograph method based on the conservation laws for two hyperbolic quasilinear equations of the first order is described. Using these results we propose a method which allows to reduce the Cauchy problem for the two quasilinear PDE's to the Cauchy problem for ODE's. The proposed method is actually some similar method of characteristics for a system of two hyperbolic quasilinear equations. The method can be used effectively in all cases, when the linear hyperbolic equation in partial derivatives of the second order with variable coefficients, resulting from the application of the hodograph method, has an explicit expression for the Riemann-Green function. One of the method's features is the possibility to construct a multi-valued solutions. In this paper we present examples of method application for solving the classical shallow water equations.

physics.flu-dyn↗
arXiv2014

Conservation of writhe helicity under anti-parallel reconnection

Reconnection is a fundamental event in many areas of science, from the interaction of vortices in classical and quantum fluids, and magnetic flux tubes in magnetohydrodynamics and plasma physics, to the recombination in polymer physics and DNA biology. By using fundamental results in topological fluid mechanics, the helicity of a flux tube can be calculated in terms of writhe and twist contributions. Here we show that the writhe is conserved under anti-parallel reconnection. Hence, for a pair of interacting flux tubes of equal flux, if the twist of the reconnected tube is the sum of the original twists of the interacting tubes, then helicity is conserved during reconnection. Thus, any deviation from helicity conservation is entirely due to the intrinsic twist inserted or deleted locally at the reconnection site. This result has important implications for helicity and energy considerations in various physical contexts.

math-ph↗
arXiv2014

Quantifying thermally induced flowability of rennet cheese curds

Conversion of liquid milk to cheese curds is the first stage in cheese manufacture. Changing the rigidity of cheese curds through heating and pH control is an established method for preparing fresh curds, whereas a similar method to prepare fully coagulated curds is largely unknown. This study elucidated the effect of temperature variation on the viscoelastic moduli of fully coagulated curds under different pH conditions. The results showed that rennet curds treated at pH 4.8 exhibited drastic changes in the viscoelasticity at 43 degrees C, above which the degree of fluidity exceeded the degree of rigidity. The viscoelastic moduli exhibited exponential decay as a function of temperature, which was independent of pH.

cond-mat.soft↗
arXiv2014

Theoretical models in low-Reynolds-number locomotion

The locomotion of microorganisms in fluids is ubiquitous and plays an important role in numerous biological processes. In this chapter we present an overview of theoretical modeling for low-Reynolds-number locomotion.

physics.flu-dyn↗
arXiv2014

The mean velocity profile of near-wall turbulent flow

The issue of analytical derivation of the mean velocity profile in a near-wall turbulent flow is revisited in the context of a two-dimensional channel flow. An approach based on the use of dispersion relations for the flow velocity is developed. It is shown that for an incompressible flow conserving vorticity, there exists a decomposition of the velocity field into rotational and potential components, such that the restriction of the former to an arbitrary cross-section of the channel is a functional of the vorticity and velocity distributions over that cross-section, while the latter is divergence-free and bounded downstream thereof. By eliminating the unknown potential component with the help of a dispersion relation, a nonlinear integro-differential equation for the flow velocity is obtained. It is then analyzed within an asymptotic expansion in the small ratio v*/U of the friction velocity to the mean flow velocity. Upon statistical averaging in the lowest nontrivial order, this equation relates the mean velocity to the cross-correlation function of the velocity fluctuations. Analysis of the equation reveals existence of two continuous families of solutions, one having the near-wall asymptotic of the form U \sim ln^p (y/y0), where y is the distance to the wall, p>0 is arbitrary, and the other, U \sim y^n, with n>0 also arbitrary except in the limit n \to 0 where it turns out to be a universal function of the Reynolds number, n \sim 1/ln(Re). It is proved, furthermore, that given a mean velocity distribution having either asymptotic, one can always construct a cross-correlation function so as to satisfy the obtained equation. These results are discussed in the light of the existing controversy regarding experimental verification of the law of the wall.

physics.flu-dyn↗
arXiv2014

Sequential tasks performed by catalytic pumps for colloidal crystallization

Gold-platinum catalytic pumps immersed in a chemical fuel are used to manipulate silica colloids. The manipulation relies on the electric field and the fluid flow generated by the pump. Catalytic pumps perform various tasks, such as the repulsion of colloids, the attraction of colloids, and the guided crystallization of colloids. We demonstrate that catalytic pumps can execute these tasks sequentially over time. Switching from one task to the next is related to the local change of the proton concentration, which modifies the colloid zeta potential and consequently the electric force acting on the colloids.

cond-mat.soft↗
arXiv2014

Hamiltonian Hydrodynamics and Irrotational Binary Inspiral

Gravitational waves from neutron-star and black-hole binaries carry valuable information on their physical properties and probe physics inaccessible to the laboratory. Although development of black-hole gravitational-wave templates in the past decade has been revolutionary, the corresponding work for double neutron-star systems has lagged. Neutron stars can be well-modelled as simple barotropic fluids during the part of binary inspiral most relevant to gravitational wave astronomy, but the crucial geometric and mathematical consequences of this simplification have remained computationally unexploited. In particular, Carter and Lichnerowicz have described barotropic fluid motion via classical variational principles as conformally geodesic. Moreover, Kelvin's circulation theorem implies that initially irrotational flows remain irrotational. Applied to numerical relativity, these concepts lead to novel Hamiltonian or Hamilton-Jacobi schemes for evolving relativistic fluid flows. Hamiltonian methods can conserve not only flux, but also circulation and symplecticity, and moreover do not require addition of an artificial atmosphere typically required by standard conservative methods. These properties can allow production of high-precision gravitational waveforms at low computational cost. This canonical hydrodynamics approach is applicable to a wide class of problems involving theoretical or computational fluid dynamics.

gr-qc↗
arXiv2014

A Solver for Massively Parallel Direct Numerical Simulation of Three-Dimensional Multiphase Flows

We present a new solver for massively parallel simulations of fully three-dimensional multiphase flows. The solver runs on a variety of computer architectures from laptops to supercomputers and on 65536 threads or more (limited only by the availability to us of more threads). The code is wholly written by the authors in Fortran 2003 and uses a domain decomposition strategy for parallelization with MPI. The fluid interface solver is based on a parallel implementation of the LCRM hybrid Front Tracking/Level Set method designed to handle highly deforming interfaces with complex topology changes. We discuss the implementation of this interface method and its particular suitability to distributed processing where all operations are carried out locally on distributed subdomains. We have developed parallel GMRES and Multigrid iterative solvers suited to the linear systems arising from the implicit solution of the fluid velocities and pressure in the presence of strong density and viscosity discontinuities across fluid phases. Particular attention is drawn to the details and performance of the parallel Multigrid solver. The code includes modules for flow interaction with immersed solid objects, contact line dynamics, species and thermal transport with phase change. Here, however, we focus on the simulation of the canonical problem of drop splash onto a liquid film and report on the parallel performance of the code on varying numbers of threads. The 3D simulations were run on mesh resolutions up to $1024^3$ with results at the higher resolutions showing the fine details and features of droplet ejection, crown formation and rim instability observed under similar experimental conditions. Keywords:

physics.flu-dyn↗
arXiv2014

Scaling properties of viscous fingering

We present a study of viscous fingering using the Volume Of Fluid method and a central injection geometry, assuming a Laplacian field and a simple surface tension law. As in experiments we see branched structures resulting from the Saffman-Taylor instability. We find that the area $A$ of a viscous-fingering cluster varies as a simple power law $A \sim L^α$ of its interface length $L$. Our results are compared to previously published simulations in which the viscosity of the invading fluid is vanishing. We find differences in exponent $α$ and in the appearance of detached droplets and bubbles.

physics.flu-dyn↗