Showing papers on "Compressibility published in 2010"

Direct numerical simulation of hypersonic turbulent boundary layers. Part 2. Effect of wall temperature

Abstract: In this paper, we perform direct numerical simulation (DNS) of turbulent boundary layers at Mach 5 with the ratio of wall-to-edge temperature Tw/Tδ from 1.0 to 5.4 (Cases M5T1 to M5T5). The influence of wall cooling on Morkovin's scaling, Walz's equation, the standard and modified strong Reynolds analogies, turbulent kinetic energy budgets, compressibility effects and near-wall coherent structures is assessed. We find that many of the scaling relations used to express adiabatic compressible boundary-layer statistics in terms of incompressible boundary layers also hold for non-adiabatic cases. Compressibility effects are enhanced by wall cooling but remain insignificant, and the turbulence dissipation remains primarily solenoidal. Moreover, the variation of near-wall streaks, iso-surface of the swirl strength and hairpin packets with wall temperature demonstrates that cooling the wall increases the coherency of turbulent structures. We present the mechanism by which wall cooling enhances the coherence of turbulence structures, and we provide an explanation of why this mechanism does not represent an exception to the weakly compressible hypothesis.

Volume of supercooled water under pressure and the liquid-liquid critical point.

Abstract: The volume of water (H(2)O) was obtained at about 200-275 K and 40-400 MPa by using emulsified water. The plot of volume against temperature showed slightly concave-downward curvature at pressures higher than ≈200 MPa. This is compatible with the liquid-liquid critical-point hypothesis, but hardly with the singularity-free scenario. When the critical point is assumed to exist at ≈50 MPa and ≈223 K, the experimental volume and the derived compressibility are qualitatively described by the modified Fuentevilla-Anisimov scaling equation.

A monolithic Lagrangian approach for fluid–structure interaction problems

Abstract: Current work presents a monolithic method for the solution of fluid–structure interaction problems involving flexible structures and free-surface flows. The technique presented is based upon the utilization of a Lagrangian description for both the fluid and the structure. A linear displacement–pressure interpolation pair is used for the fluid whereas the structure utilizes a standard displacement-based formulation. A slight fluid compressibility is assumed that allows to relate the mechanical pressure to the local volume variation. The method described features a global pressure condensation which in turn enables the definition of a purely displacement-based linear system of equations. A matrix-free technique is used for the solution of such linear system, leading to an efficient implementation. The result is a robust method which allows dealing with FSI problems involving arbitrary variations in the shape of the fluid domain. The method is completely free of spurious added-mass effects.

Numerical and experimental investigation of transverse injection flows

Abstract: The flow field resulting from a transverse injection through a slot into supersonic flow is numerically simulated by solving Favre-averaged Navier–Stokes equations with κ − ω SST turbulence model with corrections for compressibility and transition. Numerical results are compared to experimental data in terms of surface pressure profiles, boundary layer separation location, transition location, and flow structures at the upstream and downstream of the jet. Results show good agreement with experimental data for a wide range of pressure ratios and transition locations are captured with acceptable accuracy. κ − ω SST model provides quite accurate results for such a complex flow field. Moreover, few experiments involving a sonic round jet injected on a flat plate into high-speed crossflow at Mach 5 are carried out. These experiments are three-dimensional in nature. The effect of pressure ratio on three-dimensional jet interaction dynamics is sought. Jet penetration is found to be a non-linear function of jet to free stream momentum flux ratio.

The influence of wall properties on the MHD peristaltic flow of a Maxwell fluid with heat and mass transfer

Abstract: This article describes the effects of heat and mass transfer on the magnetohydrodynamic (MHD) peristaltic flow in a planar channel with compliant walls. An incompressible Maxwell fluid occupies a porous space. The mathematical formulation is based upon the modified Darcy’s law. The analytic treatment of the solution is given by choosing a small wave number. The expressions of stream function, temperature distribution, concentration field and heat coefficient are constructed. The variations of several interesting parameters are discussed by sketching plots.

Effects of CO2 compressibility on CO2 storage in deep saline aquifers.

Abstract: The injection of supercritical CO2 in deep saline aquifers leads to the formation of a CO2 plume that tends to float above the formation brine. As pressure builds up, CO2 properties, i.e. density and viscosity, can vary significantly. Current analytical solutions do not account for CO2 compressibility. In this article, we investigate numerically and analyti- cally the effect of this variability on the position of the interface between the CO2-rich phase and the formation brine. We introduce a correction to account for CO2 compressibility (den- sity variations) and viscosity variations in current analytical solutions. We find that the error in the interface position caused by neglecting CO2 compressibility is relatively small when viscous forces dominate. However, it can become significant when gravity forces dominate, which is likely to occur at late times of injection.

A blow-up criterion for classical solutions to the compressible Navier-Stokes equations

Abstract: In this paper, we obtain a blow-up criterion for classical solutions to the 3-D compressible Navier- Stokes equations just in terms of the gradient of the velocity, analogous to the Beal-Kato-Majda criterion for the ideal incompressible flow. In addition, the initial vacuum is allowed in our case. MSC(2000): 76N10

Conservative Models and Numerical Methods for Compressible Two-Phase Flow

Abstract: The paper presents the computational framework for solving hyperbolic models for compressible two-phase flow by finite volume methods. A hierarchy of two-phase flow systems of conservation-form equations is formulated, including a general model with different phase velocities, pressures and temperatures; a simplified single temperature model with equal phase temperatures; and an isentropic model. The solution of the governing equations is obtained by the MUSCL-Hancock method in conjunction with the GFORCE and GMUSTA fluxes. Numerical results are presented for the water faucet test case, the Riemann problem with a sonic point and the water-air shock tube test case. The effect of the pressure relaxation rate on the numerical results is also investigated.

Interpretation of AVO anomalies

Abstract: We investigate the effects of changes in rock and fluid properties on amplitude-variation-with-offset AVO responses. In the slope-intercept domain, reflections from wet sands and shales fall on or near a trend that we call the fluid line.Reflectionsfromthetopofsandscontaininggasorlight hydrocarbonsfallonatrendapproximatelyparalleltothefluidline;reflectionsfromthebaseofgassandsfallonaparallel trendontheopposingsideofthefluidline.Thepolaritystandard of the seismic data dictates whether these reflections from the top of hydrocarbon-bearing sands are below or above the fluid line. Typically, rock properties of sands and shales differ, and therefore reflections from sand/shale interfaces are also displaced from the fluid line. The distance of these trends from the fluid line depends upon the contrast of the ratio of P-wave velocity VP and S-wave velocity VS. This ratio is a function of pore-fluid compressibility and implies that distance from the fluid line increases with increasing compressibility. Reflections from wet sands are closer to the fluid line than hydrocarbon-related reflections. Porosity changes affect acoustic impedance but do not significantly impacttheVP /VScontrast.Asaresult,porositychangesmove theAVO response along trends approximately parallel to the fluidline.TheseobservationsareusefulforinterpretingAVO anomaliesintermsoffluids,lithology,andporosity.

A reliable treatment of a homotopy analysis method for two-dimensional viscous flow in a rectangular domain bounded by two moving porous walls

Abstract: This study investigates the problem of laminar, isothermal, incompressible and viscous flow in a rectangular domain bounded by two moving porous walls, which enable the fluid to enter or exit during successive expansions or contractions. The basic equations governing the flow are reduced to a highly nonlinear ordinary differential equation. This equation is solved analytically by using the homotopy analysis method (HAM). The analytic solutions of non-linear differential equation are constructed in the series form. The convergence of the obtained series solutions is carefully analyzed. The convergence analysis elucidates that the homotopy perturbation method (HPM) does not give the accurate results for the large values of the permeation Reynolds number Re. Graphical results are presented to investigate the influence of the nondimensional wall dilation rate α and permeation Reynolds number on the velocity, normal pressure distribution and wall shear stress. It is noted that the behavior of the HAM solution for the velocity, normal pressure distribution and wall shear stress is in good agreement with the numerical solution.

Influence of radially varying mhd on the peristaltic flow in an annulus with heat and mass transfer

Abstract: In this paper we have investigated the peristaltic flow of an incompressible MHD Newtonian fluid in a vertical annulus. The effects of radially varying MHD is also taken into account. The flow is investigated in a wave frame of reference moving with the velocity of the wave. The governing equations of two-dimensional fluid have been simplified under long wave length and low Reynolds number approximation. Exact and numerical solutions have been carried out. The expression for pressure rise is calculated using numerical integration. The graphical results are presented to interpret various physical parameter of interest.

Pore pressure evolution in deforming granular material; a general formulation and the infinitely stiff approximation

Abstract: [1] The physics of deformation of fluid-filled granular media controls many geophysical systems, ranging from shear on geological faults to landslides and soil liquefaction. Its great complexity is rooted in the mechanical coupling between two deforming phases: the solid granular network and the fluid-filled pore network. Often deformation of the granular network leads to pore fluid pressure (PP) changes. If the PP rises enough, the fluid-filled granular media may transition from a stress-supporting grain network to a flowing grain-fluid slurry, with an accompanying catastrophic loss of shear strength. Despite its great importance, the mechanisms and parameters controlling PP evolution by granular shear are not well understood. A formulation describing the general physics of pore fluid response to granular media deformation is developed and used to study simple scenarios that lead to PP changes. We focus on the infinitely stiff end-member scenario, where granular deformation is prescribed, and the PP responds to this deformation. This end-member scenario illustrates the two possible modes of pore fluid pressurization: (1) via rapid fluid flow when fluid drainage is good and (2) via pore volume compaction when drainage is poor. In the former case the rate of deformation controls PP evolution, while in the latter case, fluid compressibility is found to be an important parameter and the amount of pressurization is controlled by the overall compaction. The newly suggested fluid-induced mechanism (mechanism 1) may help explain observations of liquefaction of initially compact soils and shear zones.

Questionable collapse of the bulk modulus in CrN

Abstract: In this comment we show that the main conclusion in a previous article, claiminga drastic increase in compressibility of CrN at the cubic to orthorhombic phasetransition, is unsupported by first-pr ...

On the role of the ambient fluid on gravitational granular flow dynamics

Abstract: The effects of the ambient fluid on granular flow dynamics are poorly understood and commonly ignored in analyses. In this article, we characterize and quantify these effects by combining theoretical and experimental analyses. Starting with the mixture theory, we derive a set of two-phase continuum equations for studying a compressible granular flow composed of homogenous solid particles and a Newtonian ambient fluid. The role of the ambient fluid is then investigated by studying the collapse and spreading of two-dimensional granular columns in air or water, for different solid particle sizes and column aspect (height to length) ratios, in which the front speed is used to describe the flow. The combined analysis of experimental measurements and numerical solutions shows that the dynamics of the solid phase cannot be explained if the hydrodynamic fluid pressure and the drag interactions are not included in the analysis. For instance, hydrodynamic fluid pressure can hold the reduced weight of the solids, thus inducing a transition from dense-compacted to dense-suspended granular flows, whereas drag forces counteract the solids movement, especially within the near-wall viscous layer. We conclude that in order to obtain a realistic representation of gravitational granular flow dynamics, the ambient fluid cannot be neglected.

Computational fluid dynamics modelling of landslide generated water waves

Abstract: This paper describes the application of detailed computational fluid dynamics (CFD) to simulate the formation and propagation of waves generated by the impact of landslide material with water. The problem is schematised as a multiphase–multicomponent fluid flow: compressible air, water and transported alluvial material. The landslide simulation is performed by means of a hybrid approach: as a rigid solid body slipping down along an inclined slope until it starts penetrating the water body. The CFD model solves the Navier–Stokes equations with the RNG k-ɛ turbulence closure scheme and the volume of fluid multiphase method, which maintains the interface as a sharp front. The governing equations are solved using the commercial CFD code, FLUENT. The computed results are compared with experimental data reported in the literature. The model is then applied to simulate the 1958 Lituya bay Tsunami event with a 2D a simplified geometry and the results are compared to others found in literature.

Velocity gradient invariants and local flow-field topology in compressible turbulence

Abstract: The effect of compressibility on velocity gradient structure and the related flow-field patterns/topology is investigated using direct numerical simulation data. To clearly isolate compressibility effects, the behavior is investigated as a function of local level of dilatation. Importantly, dilatation-conditioned behavior is found to be independent of Mach and Reynolds numbers. Not surprisingly, at low dilatation level, velocity gradient structure and local flow topology are similar to incompressible turbulence. At high dilatation levels, however, the behavior is quite different. A recently developed velocity gradient evolution equation – Homogenized Euler Equation (HEE) – qualitatively captures many of the observed features of compressible turbulence.

Low-Reynolds number swimming in gels

Abstract: Many microorganisms swim through gels, materials with nonzero zero-frequency elastic shear modulus, such as mucus. Biological gels are typically heterogeneous, containing both a structural scaffold (network) and a fluid solvent. We analyze the swimming of an infinite sheet undergoing transverse traveling wave deformations in the "two-fluid" model of a gel, which treats the network and solvent as two coupled elastic and viscous continuum phases. We show that geometric nonlinearities must be incorporated to obtain physically meaningful results. We identify a transition between regimes where the network deforms to follow solvent flows and where the network is stationary. Swimming speeds can be enhanced relative to Newtonian fluids when the network is stationary. Compressibility effects can also enhance swimming velocities. Finally, microscopic details of sheet-network interactions influence the boundary conditions between the sheet and network. The nature of these boundary conditions significantly impacts swimming speeds.

Multiple-Relaxation-Time Lattice Boltzmann Approach to Compressible Flows with Flexible Specific-Heat Ratio and Prandtl Number

Abstract: A new multiple-relaxation-time lattice Boltzmann scheme for compressible flows with arbitrary specific heat ratio and Prandtl number is presented. In the new scheme, which is based on a two-dimensional 16-discrete-velocity model, the moment space and the corresponding transformation matrix are constructed according to the seven-moment relations associated with the local equilibrium distribution function. In the continuum limit, the model recovers the compressible Navier-Stokes equations with flexible specific-heat ratio and Prandtl number. Numerical experiments show that compressible flows with strong shocks can be simulated by the present model up to Mach numbers $Ma \sim 5$.

Main phase transition in lipid bilayers: Phase coexistence and line tension in a soft, solvent-free, coarse-grained model

Abstract: We devise a soft, solvent-free, coarse-grained model for lipid bilayer membranes. The nonbonded interactions take the form of a weighted-density functional, which allows us to describe the thermodynamics of self-assembly and packing effects of the coarse-grained beads in terms of a density expansion of the equation of state and weighting functions that regularize the microscopic bead densities, respectively. Identifying the length and energy scales via the bilayer thickness and the thermal energy scale, kBT, the model qualitatively reproduces key characteristics (e.g., bending rigidity, area per molecule, and compressibility) of lipid membranes. We employ this model to study the main phase transition between the fluid and the gel phase of the bilayer membrane. We accurately locate the phase coexistence using free energy calculations and also obtain estimates for the bare and the thermodynamic line tension.