Turbulent channel flow past a moving array of spheres

\u3cp\u3eWe have performed a particle-resolved direct numerical simulation of a turbulent channel flow past a moving dilute array of spherical particles. The flow shares important features with dilute vertical gas solid flow at high Stokes number, such as significant attenuation of the turbulence kinetic energy (TKE) at low particle volume fraction. The flow has been simulated by means of an overset grid method, using spherical grids around each particle overset on a background non-uniform Cartesian grid. The main focus of the present paper is on the TKE budget, which is analysed both in the fixed channel frame of reference and in the moving particle frame of reference. The overall (domain-integrated) TKE and turbulence production due to mean shear are reduced compared to unladen flow. In the fixed frame, the interfacial term, which represents production due to relative (slip) velocity, accounts for approximately 40 % of the total turbulence production in the channel. As a consequence, the total turbulence production and the overall turbulence dissipation rate remain approximately the same as in the unladen flow. However, a comparison with laminar flow past the same particle configuration reveals that significant parts of various fixed-frame statistics are due to non-turbulent structures, spatial variations that are steady in the moving particle frame. In order to obtain a clearer picture of the modification of the true turbulence and in order to reveal the rich three-dimensional (3-D) statistical structure of turbulence interacting with particles, time averaging in the moving frame of reference of the particle is used to extract the fluctuations entirely due to true turbulence. In the moving frame, the turbulence production is positive near the sides and in the wake, but negative in a region near the front of the particle. The turbulence dissipation rate and even more the dissipation rate of the 3-D mean flow attain very large values on a large part of the particle surface, up to approximately 400 and 4000 times the local turbulence dissipation rate of the unladen flow, respectively. Very close to the particle, viscous diffusion is the dominant transport term, but somewhat further away, in particular near the front and sides of the particle, pressure diffusion and also convection provide large and positive transport contributions to the moving-frame budget. A radial analysis shows that the regions around the particles draw energy from the regions further away via the surprising dominance of the pressure diffusion flux over a large range of radii. Spectra show that (very) far away from the particles all scales of the (true) turbulence are reduced. Near the particles enhancement of small scale turbulence is observed, for the streamwise component of the velocity fluctuation more than for the other components. The most important reason for turbulence reduction and anisotropy increase appears to be particle-induced non-uniformity of the mean driving force of the flow.\u3c/p\u3

Direct numerical simulation of the motion of particles in rotating pipe flow

In this paper the motion of particles in rotating pipe flow is studied for various flow cases by means of direct numerical simulation. Compared to flow in a non-rotating pipe, the Navier-Stokes equation contains as only extra term the Coriolis force when the equation is considered in a rotating frame of reference. Particles in the flow also experience a centrifugal force, which drives them to one side of the wall of the pipe. The flow is characterized by two Reynolds numbers for the mean axial velocity and the rotation rate, respectively. Among the cases studied are one in which the flow without rotation would be laminar and rotation leads to turbulence and another one for which Poiseuille flow is unstable but instead of transition to a turbulent state, a time-dependent laminar flow results. In all cases studied a counter-rotating vortex is present. The simulation results are used to calculate the collection efficiency of the rotational phase separator (RPS) under turbulent flow conditions. The RPS is a device to separate liquid or solid particles from a lighter or heavier fluid by means of centrifugation in a bundle of channels which rotate around a common axis. The results show that, compared to Poiseuille flow, the collection efficiency for larger particles decreases due to the combined action of the vortex and turbulent velocity fluctuations, while it is unchanged for smaller particle

The turbulent rotational phase separator

The Rotational Phase Separator (RPS) is a device to separate liquid or solid particles from a lighter or heavier fluid by centrifugation in a bundle of channels which rotate around a common axis. Originally, the RPS was designed in such a way that the flow through the channels is laminar in order to avoid eddies in which the particles become entrained and do not reach the walls. However, in some applications the required volume flow of fluid is so large, that the Reynolds number exceeds the value for which laminar Poiseuille flow is linearly stable. Depending on the Reynolds numbers the flow can then be turbulent, or a laminar time-dependent flow results. In both cases a counter-rotating vortex is present, which might deterioratethe separation efficiency of the RPS. This is studied by means of direct numerical simulation of flow in a rotating pipe and particle tracking in this flow. The results show that the collection efficiency for larger particles decreases due to the combined action of the vortex and turbulent velocity fluctuations, while it is unchanged for smaller particles

Thermodynamics and hydrodynamics of \u3csup\u3e3\u3c/sup\u3eHe-\u3csup\u3e4\u3c/sup\u3eHe mixtures

\u3cp\u3eThe specific heat of liquid \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe mixtures is usually written in terms of the sum of the specific heat of a \u3csup\u3e3\u3c/sup\u3eHe-quasiparticle gas and the specific heat of the pure \u3csup\u3e4\u3c/sup\u3eHe component. The thermodynamics based on this starting point is derived. Relations of important quantities and their low- and high-temperature limits are given. These are used to derive expressions for the velocity of second sound. This latter quantity is a very important source of information for the Fermi gas properties. Finally, the Fermi gas parameters are summarized in the chapter. The experimental aspects of the \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe hydrodynamics are treated. The appearance of mutual friction that has long been neglected in this field is discussed, together with the properties of the critical velocities. The phenomenological equations of motion are given. The occurrence of mutual friction is a strong indication that \u3csup\u3e4\u3c/sup\u3eHe vortices play an important role in \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe hydrodynamics. From the equation of motion of quantized \u3csup\u3e4\u3c/sup\u3eHe vortices, the observed cubic velocity dependence of the \u3csup\u3e4\u3c/sup\u3eHe chemical potential difference is explained on purely dimensional grounds. A differential equation is given from which the temperature profile in a cylindrical tube in which \u3csup\u3e3\u3c/sup\u3eHe flows through superfluid \u3csup\u3e4\u3c/sup\u3eHe can be calculated.\u3c/p\u3

Highly scalable DNS solver for turbulent bubble-laden channel flow

\u3cp\u3eWe present an efficient and highly scalable solver for direct numerical simulation (DNS) of dispersed gas-liquid flow, containing a large number of deformable bubbles. We apply this to O(10\u3csup\u3e4\u3c/sup\u3e) bubbles in a turbulent flow. This was accomplished by a well-considered combination of state-of-the-art numerical methods, as well as fast and scalable numerical algorithms that have their origin in single-phase and two-phase flows. The features key-elements of the algorithm of curvature computation, bubble collision and variable-coefficient Poisson equation solver are presented. Resolution requirements for an accurate advection of a rising bubble have been established by comparison with the (Hysing, 2009) benchmark. The Generalized Height Function (GHF) method, was adopted for the curvature computations. We observed agreement with theoretical convergence rates, as well as a significant reduction of spurious velocities when employing GHF, compared to a finite difference approach. The main interaction mechanisms between bubbles and walls of the domain were analyzed, showing second order convergence of the underlying numerical methods. A detailed analysis of the parallel performance of the Navier–Stokes (NS) solver and the solver for the gas volume fraction was carried out. The analysis revealed close to linear scaling up to ≈ 18000 cores on a computational grid with 1 billion cells for the NS solver, and an ideal scaling of the gas volume fraction solver up to O(10\u3csup\u3e3\u3c/sup\u3e) bubbles. Beyond that, acceptable overhead for up to O(10\u3csup\u3e4\u3c/sup\u3e) bubbles was found. A simulation of a downflow configuration of a turbulent channel loaded with a total of 10000 bubbles illustrates the computational capabilities. First and second order statistics of the velocity field were computed as well as the profiles of the average gas volume fraction field in the statistically steady state. In the case considered, a 47% increase of the wall shear stress was observed, brought about by turbulence modification arising from the embedded bubbles. The interaction between turbulence and bubbles at high volume fraction resulted in a strong attenuation of the rms of the velocity fluctuations in a wide region of the core of the channel.\u3c/p\u3

Direct numerical simulation of biomass combustion in a turbulent particle-laden channel flow

Renewable energy is the key to meet the ever-increasing global energy needs in a climate-constrained world. Biomass, being the only carbon based renewable energy fuel, is gaining importance in order to satisfy environmental concerns about fossil fuel usage. Biomass co-firing with coal is one of the main methods in achieving the objectives of increasing sustainable energy production. The present paper is aimed at the development of a computational model for biomass pyrolysis and combustion in a compressible gas flow

Low-mach algorithm for heated droplet-laden turbulent channel flow including phase transition

In this contribution a turbulent channel flow with dispersed droplets is investigated. The dispersed phase undergoes phase transition, which leads to heat and mass transfer between the phases, and correspondingly modulates turbulent flow properties. As a point of reference we investigate the flow of water droplets in air, containing also water vapor. A full simulation was done using a time-explicit numerical model applying second order accurate finite volume discretization. However, simulation at realistic Mach numbers becomes very time consuming with the explicit method. A new low Mach number algorithm is proposed to simulate turbulent multiphase flow efficiently at realistic conditions. The explicit code is used as a point of reference for the results obtained with the new algorithm

DNS of turbulent channel flow subject to oscillatory heat flux

In this paper we study the heat transfer in a turbulent channel flow, which is periodically heated through its walls. We consider the flow of air and water vapor using direct numerical simulation. We consider the fluid as a compressible Newtonian gas. We focus on the heat transfer properties of the system, e.g., the temperature difference between the walls and the Nusselt number. We consider the dependence of these quantities on the frequency of the applied heat flux. We observe that the mean temperature difference is quite insensitive to the frequency and that the amplitude of its oscillations is such that its value multiplied by the square root of frequency is approximately constant. Next we add droplets to the channel, which can undergo phase transitions. The heat transfer properties of the channel in the case with droplets are found to increase by more than a factor of two, compared to the situation without droplets

Lagrangian network analysis of turbulent mixing

\u3cp\u3eA temporal complex network-based approach is proposed as a novel formulation to investigate turbulent mixing from a Lagrangian viewpoint. By exploiting a spatial proximity criterion, the dynamics of a set of fluid particles is geometrized into a time-varying weighted network. Specifically, a numerically solved turbulent channel flow is employed as an exemplifying case. We show that the time-varying network is able to clearly describe the particle swarm dynamics, in a parametrically robust and computationally inexpensive way. The network formalism enables us to straightforwardly identify transient and long-term flow regimes, the interplay between turbulent mixing and mean flow advection and the occurrence of proximity events among particles. Thanks to their versatility and ability to highlight significant flow features, complex networks represent a suitable tool for Lagrangian investigations of turbulent mixing. The present application of complex networks offers a powerful resource for Lagrangian analysis of turbulent flows, thus providing a further step in building bridges between turbulence research and network science.\u3c/p\u3