Sourabh V. Apte

Large-eddy simulation of swirling particle-laden flows in a coaxial-jet combustor

Abstract Large-eddy simulation (LES) of particle-laden, swirling flow in a coaxial-jet combustor is performed. A mixture of air and lightly loaded, spherical, glass-particles with a prescribed size-distribution enters the primary jet, while a swirling stream of air flows through the annulus. The incompressible, spatially filtered Navier–Stokes equations are solved on unstructured grids to compute the turbulent gas-phase. A Lagrangian formulation and an efficient particle-tracking scheme on unstructured meshes is developed to compute the dispersed phase. The particles are treated as point sources and influence the gas phase only through momentum-exchange terms. The particle-dispersion characteristics are examined in detail; in particular, the dependence of particle trajectories and residence times upon particle sizes is emphasized. The mean and turbulent quantities for the gas and particle phases are compared to experimental data and good agreement is obtained. The LES results are significantly more accurate than the Reynolds-averaged Navier–Stokes equation (RANS) predictions of the same problem. Insight into the two-phase swirling flows is obtained through the residence-times and particle velocity-diameter correlations.

LES of atomizing spray with stochastic modeling of secondary breakup

Abstract A stochastic subgrid model for large-eddy simulation of atomizing spray is developed. Following Kolmogorov’s concept of viewing solid particle-breakup as a discrete random process, atomization of liquid blobs at high relative liquid-to-gas velocity is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. Kolmogorov’s discrete model of breakup is rewritten in the form of differential Fokker–Planck equation for the PDF of droplet radii. Along with the Lagrangian tracking of spray dynamics, the size and number density of the newly produced droplets is governed by the evolution of this PDF in the space of droplet-radius. The parameters of the model are obtained dynamically by relating them to the local Weber number with two-way coupling between the gas and liquid phases. Computations of spray are performed for the representative conditions encountered in idealized diesel and gas-turbine engine configurations. A broad spectrum of droplet sizes is obtained at each location with co-existence of large and small droplets. A novel numerical algorithm capable of simultaneously simulating individual droplets as well as a group of droplets with similar properties commonly known as parcels is proposed and compared with standard parcels-approach usually employed in the computations of multiphase flows. The present approach is shown to be computationally efficient and captures the complex fragmentary process of liquid atomization.

Large-Eddy Simulation of Reacting Turbulent Flows in Complex Geometries

Large-eddy simulation (LES) has traditionally been restricted to fairly simple geometries. This paper discusses LES of reacting flows in geometries as complex as commercial gas turbine engine combustors. The incompressible algorithm developed by Mahesh et al. (J. Comput. Phys., 2004, 197, 215-240) is extended to the zero Mach number equations with heat release. Chemical reactions are modeled using the flamelet/progress variable approach of Pierce and Moin (J. Fluid Mech., 2004, 504, 73-97). The simulations are validated against experiment for methane-air combustion in a coaxial geometry, and jet-A surrogate/air combustion in a gas-turbine combustor geometry.

A numerical method for fully resolved simulation (FRS) of rigid particle-flow interactions in complex flows

A fictitious-domain based formulation for fully resolved simulations of arbitrary shaped, freely moving rigid particles in unsteady flows is presented. The entire fluid-particle domain is assumed to be an incompressible, but variable density, fluid. The numerical method is based on a finite-volume approach on a co-located, Cartesian grid together with a fractional step method for variable density, low-Mach number flows. The flow inside the fluid region is constrained to be divergence-free for an incompressible fluid, whereas the flow inside the particle domain is constrained to undergo rigid body motion. In this approach, the rigid body motion constraint is imposed by avoiding the explicit calculation of distributed Lagrange multipliers and is based upon the formulation developed by Patankar [N. Patankar, A formulation for fast computations of rigid particulate flows, Center for Turbulence Research Annual Research Briefs 2001 (2001) 185-196]. The rigidity constraint is imposed and the rigid body motion (translation and rotational velocity fields) is obtained directly in the context of a two-stage fractional step scheme. The numerical approach is applied to both imposed particle motion and fluid-particle interaction problems involving freely moving particles. Grid and time-step convergence studies are performed to evaluate the accuracy of the approach. Finally, simulation of rigid particles in a decaying isotropic turbulent flow is performed to study the feasibility of simulations of particle-laden turbulent flows.

Unsteady Flow Evolution in Porous Chamber with Surface Mass Injection, Part 1: Free Oscillation

Unsteadye owevolutioninaporouschamberwithsurfacemassinjectionsimulatinganozzlelessrocketmotorhas been investigated numerically. The analysis is based on a large-eddy-simulation technique in which the spatially e ltered and Favre averaged conservation equations for large, energy-carrying turbulent structures are solved explicitly. The effect of the unresolved scales is modeled semi-empirically by considering adequate dissipation rates for the energy present in the resolved scale motions. The e owe eld is basically governed by the balance between the inertia force and pressure gradient, as opposed to viscouseffects and pressuregradient corresponding to channel e ows without transpiration. It accelerates from zero at the head end and becomes supersonic in the divergent section of the nozzle. Three successive regimes of development, laminar, transitional, and fully turbulent e ow, are observed. Transition to turbulence occurs away from the porous wall in the midsection of the motor, and the peak in the turbulence intensity moves closer to the wall farther downstream as the local Reynolds number increases. Increase in pseudoturbulence level at the injection surface causes early transition to turbulence. As the e ow develops farther downstream, the velocity proe le transits into the shape of a fully developed turbulent pipe e ow with surface transpiration. The compressibility effect also plays an important role, causing transition of the mean velocity proe les from their incompressible e ow counterparts as the local Mach number increases. The e ow evolution is characterized primarily by three nondimensional numbers: injection Reynolds number, centerline Reynolds number, and momentum e ux coefe cient.

Unsteady Flow Evolution in Porous Chamber with Surface Mass Injection, Part 2: Acoustic Excitation

Our earlier work on injection-driven eows in a porous chamber is extended to explore the effect of forced periodic excitations on the unsteady eoweeld. Time-resolved simulations are performed to investigate the effects of traveling acoustic waves on large-scale turbulent structures for various amplitudes and frequencies of imposed excitations. The resultant oscillatory eowe eld is decomposed into mean, periodic (or organized), and turbulent (or random)motions using a time-frequency localization technique. Emphasis is placed on the interactions among the three components of the e oweeld. The primary mechanism for the transfer of energy from the mean to the turbulentmotionisprovidedbythenonlinearcorrelationsamongthevelocityeuctuations,asobservedinstationary turbulent eows.Theunsteady, deterministiccomponent gives risetoan additional mechanismforenergyexchange between the organized and turbulent motions and, consequently, produces increased turbulence levels at certain acoustic frequencies. The periodic excitations lead to earlier laminar-to-turbulence transition than that observed in stationary eows. The turbulence-enhanced momentum transport, on the other hand, leads to increased eddy viscosity and tends to dissipate the vortical wave originating from the injection surface. The coupling between the turbulent and acoustic motions results in signie cant changes in the unsteady eow evolution in a porous chamber.

A large-eddy simulation study of transition and flow instability in a porous-walled chamber with mass injection

The unsteady flow evolution in a porous chamber with surface mass injection simulating propellant burning in a nozzleless solid rocket motor has been investigated by means of a large-eddy simulation (LES) technique. Of particular importance is the turbulence-transition mechanism in injection-driven compressible flows with high injection rates in a chamber closed at one end and connected to a divergent nozzle at the exit. The spatially filtered and Favre-averaged conservation equations of mass, momentum and energy are solved for resolved scales. The effect of unresolved subgrid scales is treated by using a dynamic Smagorinsky model extended to compressible flows. Three successive regimes of flow development are observed: laminar, transitional, and fully developed turbulent flow. Surface transpiration facilitates the formation of roller-like vortical structures close to the injection surface. The flow is essentially two-dimensional up to the mid-section of the chamber, with the dominant frequencies of vortex shedding governed by two-dimensional hydrodynamic instability waves. These two-dimensional structures are convected downstream and break into complex three-dimensional eddies. Transition to turbulence occurs further away from the wall than in standard channel flows without mass injection. The peak in turbulence intensity moves closer to the wall in the downstream direction until the surface injection prohibits further penetration of turbulence. The temporal and spatial evolution of the vorticity field obtained herein is significantly different from that of channel flow without transpiration.

Unsteady Flow Evolution and Combustion Dynamics of Homogeneous Solid Propellant in a Rocket Motor

A time-resolved numerical analysis of combustion dynamics of double-base homogenous solid propellant in a rocket motor is performed by means of a Large-Eddy Simulation (LES) technique. The physiochemical processes occurring in the flame zone and their influence on the unsteady flow evolution in the chamber are investigated in depth. A five-step reduced reaction mechanism is used to obtain the two-stage flame structure consisting of a primary flame, a dark zone, and a secondary flame in the gas phase. It is observed that, for homogeneous solid propellant combustion, the chemical time scale is much greater than the smallest turbulence time scale, rendering a highly stretched and thickened flame. The chemical reactions proceed at a slower rate than turbulent mixing, and propellant combustion may be locally treated as a well-stirred reactor. The flowfield in the chamber consists of three regions of evolution: the upstream laminar regime, the central transitional section, and the fully developed turbulent regime further downstream. A theoretical formulation exploring the chamber flow and flame dynamics is established to study the intriguing phenomenon of combustion instability. The work done by Reynolds stresses, vorticity-flame interactions, and coupling between the velocity field and entropy fluctuations may cause resonance effects and excite pressure oscillations leading to self-sustained unsteady motions within the chamber.

A Variable-Density Fictitious Domain Method for Particulate Flows with Broad Range of Particle-Fluid Density Ratios

A numerical scheme for fully resolved simulation of uid-particle systems with freely moving rigid particles is developed. The approach is based on a ctitious domain method wherein the entire uid-particle domain is

The importance and challenge of hyporheic mixing

The hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact. The hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater, including reactions which depend upon mixing of source waters. Some models assume that hyporheic zones are well-mixed and conceptualize the hyporheic zone as a surface water-groundwater mixing zone. But what are the controls on and effects of hyporheic mixing? What specific mechanisms cause the relatively large (>∼1m) mixing zones suggested by subsurface solute measurements? In this commentary, we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater, and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multi-scale physical and chemical spatial heterogeneity. We encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations.