Coupling of a compressible vortex particle-mesh method with a near-body compressible discontinuous Galerkin solver

A hybrid approach, coupling a compressible vortex particle-mesh method (CVPM, also with ecient Poisson solver) and a high order compressible discontinuous Galerkin Eulerian solver, is being developed in order to eciently simulate flows past bodies; also in the transonic regime. The Eulerian solver is dedicated to capturing the anisotropic flow structures in the near-wall region whereas the CVPM solver is exploited away from the body and in the wake. An overlapping domain decomposition approach is used. The Eulerian solver, which captures the near-body region, also corrects the CVPM solution in that region at every time step. The CVPM solver, which captures the region away from the body and the wake, also provides the outer boundary conditions to the Eulerian solver. Because of the coupling, a bound- ary element method is also required for consistency. The approach is assessed on typical 2D benchmark cases

Can we always neglect the bulk viscous pressure in variable- density flows

In linearly isotropic (Newtonian) fluids, the bulk viscous pressure is proportional to the velocity divergence, with the bulk viscosity of the fluid being the proportionality coefficient. Stokes' hypothesis states that the bulk viscosity of a Newtonian fluid can be set equal to zero. Although not valid for many fluids, it is common practice to invoke this hypothesis in the study of low-Mach-number, variable-density flows. In this talk, based on simple scaling arguments we provide a necessary condition for neglecting the bulk viscous pressure from the governing equations of low-Mach number flows. More specifically, we show that the Reynolds number defined with respect to the bulk viscosity must be very large. We further show that even when this condition is not satisfied, the bulk viscous pressure does not need to be taken explicitly into account because it can be combined with the dynamic pressure

Structure of detonation waves in mixtures containing chemical inhibitors

n this talk we present numerical results of detonation transmission in mixtures containing chemical inhibitors. In our study, the chemical kinetics model consists of a three-step chain-branching mechanism for the fuel combustion and an one-step mechanism for the endothermic reaction between inhibitor and radical. Results from both one and two-dimensional simulations are discussed. It is shown that the absorption of heat and the consumption of radical in the endothermic step result in longer induction zones. This leads to a temporary detachment of the reaction zone from the precursor shock. Eventually, the material behind the shock starts to burn rapidly, thus producing pressure waves which reach the precursor shock and re-initialize the detonation. This is followed by large over-pressures and highly irregular oscillations of the shock. The talk concludes with a discussion on the necessary inhibitor concentration in the initial mixture to achieve permanent detonation suppression

Influence of electrostatic charges and solid-mass loading on particle dynamics in duct

The modulation of particle-laden flows through electrostatic forces is utilized in various industrial applications, for example powder coating and electrostatic precipitation. However, excessive charge accumulation also results in the formation of deposits on component surfaces and spark discharges which may lead to dust explosions. In order to elucidate the dynamics of charged particles, we performed large-eddy simulations of a fully-developed turbulent gas flow (Re = 10 000) through a generic squared-shaped duct. The flow was seeded by charged monodisperse particles. In our talk we will discuss the influence of the solid-mass loading and the electrostatic charges carried by the particles on the particle concentration over the duct’s cross section. Electrostatic charges of 0 pC, 0.125 pC and 0.25 pC were assigned to the particles whereas solid-gas mass loading ratios between 0.01 and 0.04 were considered. Based on our previous findings we expect an increase of each of these parameters to counteract the turbophoretic drift and smoothen the particle concentration profiles

Numerical study of detonations in mixtures of gases and solid particles

In this talk we present results from a numerical study on the structure and stability of detonations in mixtures of gases and solid particles. Cases with both reactive and inert particles are considered. First, the two-phase flow model is described and the various assumptions that it is based upon are discussed. Next, we present an analysis of the steady-wave structures that are admitted by the flow model. Subsequently, numerical results for both 1-D and 2-D detonations are presented and discussed. These results predict that the mass, momentum, and heat transfer between the two phases result in in detonation structures that are substantially different from those observed in purely gaseous flows. The presentation concludes with a discussion of the effect of certain important parameters, such as particle reactivity, particle volume fraction, and particle diameter

A two-phase model for compressible flows of granular mixtures based on non-equilibrium thermodynamics

In this talk we present a new two-phase model for compressible, viscous flows of mixtures consisting of a carrier fluid and a granular material. According to standard mixture-theory approaches, the mixture is treated as a multicomponent fluid, with a set of thermodynamic variables assigned to each of its constituents. The volume fraction occupied by the granular phase and its spatial gradient are included as thermodynamic variables. By applying directly the classical theory of non-equilibrium thermodynamics we derive constitutive relations for the viscous stresses, heat flux vectors, the momentum and energy exchanges between the two phases, and a parabolic partial differential equation for the volume fraction. In the proposed model, thermal non-equilibrium between the two phases emerges as a source term in this equation, in contrast with earlier models

Numerical study of axisymmetric collapses of submarine granular > columns

In this talk, we report on the results of a numerical study of the axisymmetric collapse of subaqueous granular columns. Our study is based on a 2-pressure, 2-velocity continuum flow model for fluid-saturated granular materials. This model is integrated via a multi-phase projection method that incorporates a regularization method for the treatment of material interfaces. In our simulations, a dense column of a granular material immersed in water is placed on a horizontal plane and is allowed to collapse and spread due to its weight. Emphasis is placed on the run-out distance and the termination height and their correlation with the aspect ratio, the volume fraction and the diameter of the grains. Comparisons against experimental measurements and previous numerical predictions are also performed. Finally, in order to examine and quantify the role of the interstitial fluid, we compare our numerical predictions against experimental results from column collapses of dry granular materials

Numerical simulation of unsteady chute flows of two-phase granular mixture

The unsteady gravity-driven flow of a fluid-saturated granular material on an inclined plane is investigated numerically. Our studies are based on a continuum two-phase flow model for the mixtures of interest. The governing equations are integrated via a predictor-corrector algorithm that employs a generalized projection method for the computation of the phasial pressures. Further, it incorporates an interface detection and capturing method to account for the steep gradients of particle concentration across material interfaces. In our numerical setup, a dense granular layer of constant thickness is placed on the surface of an inclined plane, whereas the rest of the domain is filled with an interstitial fluid. Initially the mixture is assumed to be at rest and is accelerated by gravity. A representative sample of these simulations is presented and discussed. Since the flows of interest are susceptible to Kapitza instability, emphasis is placed on the spatio-temporal evolution of the granular layer's free surface and the interplay between inertia and gravity. Also, we discuss the flow characteristics inside the granular layer and we compare the predicted profiles for the phasial variables with those obtained from previous studies

Direct simulation of the flow over a porous layer of large porosity

In this talk we report on direct numerical simulations of constant-density flow over and through a layer of a porous medium with large porosity. Initially the fluid is at rest and the flow is driven by a constant pressure gradient. Periodic boundary conditions are used along the streamwise direction, whereas no-slip conditions are specified on the bottom boundary which also coincides with the lower end of the porous strip. Further, outflow conditions are imposed on the top boundary of the computational domain, which is located sufficiently far from the porous medium. As the flow evolves, a boundary layer is formed on the lower end of the porous strip and an additional transition zone is formed right above its upper end. Due to the steep velocity gradients across this zone, a Kelvin-Helmholtz instability is onset which leads to the formation of a mixing layer. We present and analyze the characteristics of vortex pairing and growth rate of this mixing layer. Finally, we discuss the results of a parametric study with respect to the porosity of the medium