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The predictive capability of two way--coupled point-particle Euler-Lagrange model in accurately capturing particle-flow interactions under grid refinement, wherein the particle size can be comparable to the grid size, is systematically evaluated. Two situations are considered, (i) uniform flow over a stationary particle, and (ii) decaying isotropic turbulence laden with Kolmogorov-scale particles. Particle-fluid interactions are modeled using only the standard drag law, typical of large density-ratio systems. A zonal, advection-diffusion-reaction (Zonal-ADR) model is used to obtain the undisturbed fluid velocity needed in the drag closure. Two main types of interpolation kernels, grid-based and particle size--based, are employed. The effect of interpolation kernels on capturing the particle-fluid interactions, kinetic energy, dissipation rate, and particle acceleration statistics are evaluated in detail. It is shown that the interpolation kernels whose width scales with the particle size perform significantly better under grid refinement than kernels whose width scales with the grid size. Convergence with respect to spatial resolution is obtained with the particle size--based kernels with and without correcting for the self-disturbance effect. While the use of particle size--based interpolation kernels provide spatial convergence and perform better than kernels that scale based on grid size, small differences can still be seen in the converged results with and without correcting for the particle self-disturbance. Such differences indicate the need for self-disturbance correction to obtain the best results, especially when the particles are larger than the grid size., Submitted to International Journal of Multiphase Flow

Turbulent flows in porous media occur in a wide variety of applications, from catalysis in packed beds to heat exchange in nuclear reactor vessels. In this review, we summarize the current state of the literature on methods to model such flows. We focus on a range of Reynolds numbers, covering the inertial regime through the asymptotic turbulent regime. The review emphasizes both numerical modeling and the development of averaged (spatially filtered) balances over representative volumes of media. For modeling the pore scale, we examine the recent literature on Reynolds-averaged Navier–Stokes (RANS) models, large-eddy simulation (LES) models, and direct numerical simulations (DNS). We focus on the role of DNS and discuss how spatially averaged models might be closed using data computed from DNS simulations. A Darcy–Forchheimer-type law is derived, and a prior computation of the permeability and Forchheimer coefficient is presented and compared with existing data.

Oxy-fuel based pulse detonation system can be used for direct power extraction when combined with magnetohydrodynamics (MHD). A space-time conservation element solution element (CE/SE) method is used to investigate the operational envelope of oxy-coal detonations with gaseous methane as a surrogate fuel. The CE/SE method results in a consistent multidimensional formulation for structured/unstructured meshes by providing flux conservation in space and time without the need for complex Riemann solvers to capture solution discontinuities. A modified revised Jones-Lindstedt (JL-R) reaction mechanism accounting for radicals such as O, OH, and H was used as a reduced mechanism to simulate detonation waves from CH4−O2 combustion. The numerical scheme is first verified by comparing predictions with the ZND theory and other published data to show excellent agreement. For shock-induced detonation, the effect of driver shock temperature, pressure, stoichiometric ratio (ϕ) and initial driver shock length, on detonation initiation and propagation was investigated. The simulations accurately predicted detonation velocities, at various ϕ values, compared with available experimental data. The results show that higher gas temperatures and velocities are achieved through oxy-detonations compared to air. The chosen reduced chemical kinetic mechanism, that accounts for radical disassociation, is found to be critical in appropriately limiting heat release during oxy-combustion, thereby predicting detonation temperature and velocity accurately.

Direct numerical simulations (DNS) are performed in a triply periodic unit cell of a face-centred cubic (FCC) lattice covering the unsteady inertial, to fully turbulent, flow regimes. The DNS data are used to quantify the flow topology, integral scales, turbulent kinetic energy (TKE) transport and anisotropy distribution in the tortuous geometry. Several unique flow features are observed within this low porosity configuration, where the mean flow undergoes strong acceleration and deceleration regions with presence of three-dimensional helical motions, weak wake-like structures behind spheres, stagnation and jet-impingement-like flows together with merging and spreading jets in the main pore space. The jet-impingement and weak wake-like flow structures give rise to regions with negative total TKE production. Unlike flows in complex shaped ducts, the turbulence intensity levels in the cross-stream directions are found to be larger than those in the streamwise direction. Furthermore, due to the compact nature and confined geometry of the FCC packing, the turbulent integral length scales are estimated to be less than 10 % of the bead diameter even for the lowest Reynolds number studied, indicating the absence of macroscale turbulence structures for this configuration. This finding suggests that even for the highly anisotropic flow within the pore, the upscaled flow statistics are captured well by the representative volumes defined by the unit cell.

Large-eddy simulations (LES) of a particle-laden jet under a range of volume loadings are performed using a modified point-particle Euler-Lagrange (EL) approach to evaluate the effect of the volume/mass displaced by the subgrid particles on the flow. The spatio-temporal variations in the volume fraction of the carrier phase are taken into account giving rise to a zero-Mach number, variable density formulation with source terms in both momentum and continuity equations. The influence of volume loading as well as particle relaxation time on these volumetric displacement effects are investigated by performing numerical tests at different inlet volume loadings (0.047%-37.6%) and Stokes numbers (0.038-11.6). It is shown that for volume loadings above 5%, the volumetric displacement effects tend to become important. For the cases studied, these effects increase the mean and r.m.s. velocities of the carrier phase due to the continuity source term; however, they decrease further downstream due to dispersion of particles and spreading of the jet. It is observed that reducing the particle Stokes number increases the volumetric displacement effects due to their quick response time to the changes in the background flow, lower dispersion and smaller reaction forces.

A general, two-way coupled, point-particle formulation that accounts for the disturbance created by the dispersed particles in obtaining the undisturbed fluid flow field needed for accurate computation of the force closure models is presented. Specifically, equations for the disturbance field created by the presence of particles are first derived based on the inter-phase momentum coupling force in a finite-volume formulation. Solution to the disturbance field is obtained using two approaches: (i) direct computation of the disturbance velocity and pressure using the reaction force due to particles at computational control volumes, and (ii) a linearized, approximate computation of the disturbance velocity field, specifically applicable for low Reynolds number flows. In both approaches, the computed disturbance field is used to obtain the undisturbed fluid velocity necessary to model the aerodynamic forces on the particle. The two approaches are thoroughly evaluated for a single particle in an unbounded and wall-bounded flow on uniform, anisotropic, as well as unstructured grids to show accurate computation of the particle motion and inter-phase coupling. The approach is straightforward and can be applied to any numerical formulation for particle-laden flows including Euler-Lagrange as well as Euler-Euler formulations., 31 pages, 8 figures

A numerical investigation of unsteady hydrodynamic forces on the particle bed in an oscillatory flow environment is performed by means of direct numerical simulations. Statistical descriptions of drag and lift forces for two particle sizes of diameter 372 and 125 in wall units in a very rough turbulent flow regime are reported. Characterization of unsteady forces in terms of spatial distribution, temporal autocorrelation, force spectrum as well as cross-correlations with measurable flow variables is carried out. Based on the concept of impulse, intermittency in the drag and lift forces is also investigated. Temporal correlations show drag and lift to be positively correlated with a time delay that is approximately equal to the Taylor micro-scale related to the drag/lift fluctuations. The force spectra for drag and lift reveal roughly two scaling regions,$-11/3$and$-7/3$; the former typically represents turbulence–mean-shear interactions, whereas the latter indicates dominance of turbulence–turbulence interactions. Particle forces are strongly correlated with streamwise velocity and pressure fluctuations in the near-bed region for both flow cases. In comparison to the large-diameter particle case, the spatial extent of these correlations is 2–3 times larger in homogeneous directions for the small sized particle, a feature that is reminiscent of longer near-bed structures. For both large- and small-particle cases, it is shown that the distributions of drag (lift) fluctuations, in particular, peakedness and long tails, match remarkably well with fourth-order Gram–Charlier distributions of velocity (pressure) fluctuations. Furthermore, it is demonstrated that the intermittency is larger in the case of the lift force compared to that for the drag in both flow cases. Distributions of impulse events are heavily and positively skewed and are well described by a generalized extreme value distribution.

Effects of roughness on the near-bed turbulence characteristics in oscillatory flows are studied by means of particle-resolved direct numerical simulations (DNS). Two particle sizes of diameter 375 and 125 in wall units in a very rough turbulent flow regime are studied. A double-averaging technique is employed to study the nature of the wake field, i.e., the spatial inhomogeneity at the roughness length scale. This introduced dispersive stresses along with additional production and transport terms in the double-averaged Reynolds stress budget, indicating alternate pathways of turbulent energy transfer mechanisms. While overall nature of the elementary processes responsible for energy transport in both flow cases remains identical, there are some discernible differences with regards to the near-bed turbulence structure. It is shown that the roughness modulates the near-bed turbulence, distorts and breaks the streamwise horseshoe structures, and therefore reduces the large-scale anisotropy; a result of redistribution of energy from streamwise component to other two components of the Reynolds stress and is illustrated by studying their individual budgets. In comparison to the small diameter, such effects are found to be significant in the case of large diameter particle-bed. In addition, redistribution of energy in the large diameter particle case shows reduced dependence on the flow oscillations, while for the small diameter particle it is more pronounced towards the end of an acceleration cycle.

Large-eddy simulations (LES) of a single-phase, turbulent flow in a 90° pipe bend are performed at three Reynolds numbers (5300, 27 000, and 45 000) to investigate the correlation between secondary flow motion and wall shear stresses, which is suspected to be a potential mechanism responsible for material erosion. The isothermal flows are validated against available experimental and numerical data first. The snapshot proper orthogonal decomposition (POD) is applied for the medium and high Reynolds number flows to identify the secondary flow motions and the oscillation of the Dean vortices that are found to cause swirl-switching. Distinguished frequencies of the POD time coefficients at Strouhal numbers of 0.25 and 0.28 are identified for Reynolds numbers at 27 000 and 45 000, respectively. Moreover, shear stress on the pipe wall and the associated power spectral density are obtained and shown to have the same oscillating frequency as the swirl-switching.

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.

Particle-resolved direct numerical simulations (DNS) are performed to investigate the behaviour of an oscillatory flow field over a rough bed, corresponding to the experimental set-up of Keiller & Sleath (J. Fluid Mech., vol. 73 (04), 1976, pp. 673–691) for transitional and turbulent flows over a range of Reynolds numbers (95–400) based on the Stokes-layer thickness. It is shown that the roughness modulates the near-bed turbulence, produces streamwise horseshoe structures which then undergo distortion and breaking, and therefore reduces the large-scale anisotropy. A fully developed equilibrium turbulence is observed in the central part of the oscillation cycle, with two-component turbulence in the near-bed region and cigar-shaped turbulence in the outer region. A double averaging of the flow field reveals spatial inhomogeneities at the roughness scale and alternate paths of energy transport in the turbulent kinetic energy (TKE) budget. Contrary to the unidirectional, steady flow over rough beds, bed-induced production terms are important and comparable to the shear production term. It is shown that the near-bed velocity and pressure fluctuations are non-Gaussian, a result of critical importance for the modelling of incipient motion of sediment grains.

The accuracy of Euler-Lagrange point-particle models employed in particle-laden fluid flow simulations depends on accurate estimation of the particle force through closure models. Typical force closure models require computation of the slip velocity at the particle location, which in turn requires accurate estimation of the undisturbed fluid velocity. Such an undisturbed velocity is not readily available when the fluid and particle phases are two-way coupled, due to the disturbance created by the particle's force in the nearby fluid velocity field. A common practice is to use the disturbed velocity to compute the particle force which can result in errors as much as 100% in predicting the particle dynamics. In this work, a correction scheme is developed that facilitates accurate estimation of the undisturbed fluid velocity in particle-laden fluid flows with and without no-slip walls. The model is generic and can handle particles of different size and density, arbitrary interpolation and projection functions, anisotropic grids with large aspect ratios, and wall-bounded flows. The present correction scheme is motivated by the recent work of Esmaily & Horwitz (JCP, 2018) on unbounded particle-laden flows. Modifications necessary for wall-bounded flows are developed such that the undisturbed fluid velocity at any wall distance is accurately recovered, asymptotically approaching the result of unbounded schemes for particles far away from walls. A detailed series of verification tests was conducted on settling velocity of a particle in parallel and perpendicular motions to a no-slip wall. A range of flow parameters and grid configurations; involving anisotropic rectilinear grids with aspect ratios typically encountered in particle-laden turbulent channel flows was considered in detail. When the wall effects are accounted for, the present correction scheme reduces the errors in predicting the near-wall particle motion by one order of magnitude smaller values compared to the unbounded correction schemes.

Surface transient storage (STS) and hyporheic transient storage (HTS) have functional significance in stream ecology and hydrology. Currently, tracer techniques couple STS and HTS effects on stream nutrient cycling; however, STS resides in localized areas of the surface stream and HTS resides in the hyporheic zone. These contrasting environments result in different storage and exchange mechanisms with the surface stream, which can yield contrasting results when comparing transient storage effects among morphologically diverse streams. We propose a fluid mechanics approach to quantitatively separate STS from HTS that involves classifying and studying different types of STS. As a starting point, a classification scheme is needed. This paper introduces a classification scheme that categorizes different STS in riverine systems based on their flow structure. Eight STS types are identified and some are subcategorized based on characteristic mean flow structure: (1) lateral cavities (emergent and submerged); (2) protruding in-channel flow obstructions (backward- and forward-facing step); (3) isolated in-channel flow obstructions (emergent and submerged); (4) cascades and riffles; (5) aquatic vegetation (emergent and submerged); (6) pools (vertically submerged cavity, closed cavity, and recirculating reservoir); (7) meander bends; and (8) confluence of streams. The long-term goal is to use the classification scheme presented to develop predictive mean residence times for different STS using field-measurable hydromorphic parameters and obtain an effective STS mean residence time. The effective STS mean residence time can then be deconvolved from the transient storage residence time distribution (measured from a tracer test) to obtain an estimate of HTS mean residence time.

International audience; Direct numerical simulation of pore-scale turbulence is performed in a unit cell of a face-centered cubic lattice at three different pore Reynolds numbers (300, 500, and 1000). The pore-geometry gives rise to very low porosity resulting in rapid acceleration and deceleration of the flow in different regions. Eulerian statistics of mean velocity and turbulent kinetic energy are first described to provide a good understanding of the overall flow topology. In addition, the spatial variances and probability density functions for the instantaneous and temporal fluctuation velocities are computed to assess the higher-order statistics. The flow field is then analyzed using angular Lagrangian multiscale statistics of fluid particles to study their directional change at different timescales. Two power laws are observed for the evolution of the mean absolute angle as a function of time lag, demarking the early time and an intermediate, inertial range. The effect of the geometric confinement on the asymptotic behavior of the angular statistics is examined in detail. An asymptotic limit different than π/2 (corresponding to equidistribution of the mean angle, observed for three-dimensional isotropic turbulence with periodic boundaries) or 2 3 π (observed for two-dimensional wall-bounded turbulence) is obtained, representing the strong effect of geometric confinement on turbulence. Besides, the probability density functions (PDFs) for the instantaneous curvature angles are computed and the normalized PDFs are fit to a Fisher distribution. Furthermore, a Monte Carlo-based stochastic model is developed to predict the asymptotic curvature angle.

Obtaining highly-resolved velocity data from experimental measurements in porous media is a significant challenge. The goal of this work is to compare the velocity fields measured in a randomly-packed porous medium obtained from particle image velocimetry (PIV) with corresponding fields predicted from direct numerical simulation (DNS). Experimentally, the porous medium was comprised of 15 mm diameter spherical beads made of optical glass placed in a glass flow cell to create the packed bed. A solution of ammonium thiocyanate was refractive-index matched to the glass creating a medium that could be illuminated with a laser sheet without distortion. The bead center locations were quantified using the imaging system so that the geometry of the porous medium was known very accurately. Two-dimensional PIV data were collected and processed to provide high-resolution velocity fields at a single plane within the porous medium. A Cartesian-grid-based fictitious domain approach was adopted for the direct numerical simulation of flow through the same geometry as the experimental measurements and without any adjustable parameters. The uncertainties associated with characterization of the pore geometry, PIV measurements, and DNS predictions were all systematically quantified. Although uncertainties in bead position measurements led to minor discrepancies in the comparison of the velocity fields, the axial and normal velocity deviations exhibited normalized root mean squared deviations ( NRMSD ) of only 11.32% and 4.74%, respectively. The high fidelity of both the experimental and numerical methods have significant implications for understanding and even for engineering the micro–macro relationship in porous materials. The ability to measure and model sub-pore-scale flow features also has relevance to the development of upscaled models for flow in porous media, where physically reasonable closure models must be developed at the sub-pore scale. These results provide valuable data in support of that goal.

Natural lateral cavities in open channels are important because their lower water velocities promote water quality and provide refugia for organisms. Little is known about the influence of natural cavity shapes and roughness on flow structure and exchange dynamics. We investigated the effects of cavity shape (semi-circular, backward conic, and forward conic) and bed roughness on the flow structure and mean residence time (MRT) of a lateral cavity in a flume. All cavity shapes have a flow field dominated by a one-gyre recirculation pattern, contrasting results of rectangular cavities at similar Reynolds number and aspect ratios. Transverse velocity energy spectra indicate the flow is dominated by large-scale quasi-2D coherent structures. Fundamental frequencies of mixing layer vortex shedding are fastest for forward conic cavities and slowest for backward conic cavities. Fluid enters cavities at shallower mixing layer depths, and fluid exits cavities at deeper mixing layer depths. MRTs are smaller for hydraulically smooth cases and forward conic cavities due to higher recirculation velocities. MRTs are larger for rough bed cases and backward conic cavities. Rough flow cases have a strong correlation to a predictive MRT relationship derived by Jackson et al. (Water Resour Res: 10.1002/wrcr.20272 , 2013) (R 2 = 0.77); however, this predictive model does not work well for smooth cavities. Two MRT relationships were derived for smooth lateral cavities and both have strong power-law correlations to normalized MRT. Understanding cavity shape and bed roughness effects will provide a guideline for designing lateral embayments in stream restoration projects.

Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport. To study pickup and transport of sediment, it is necessary to have a detailed knowledge of the small amplitude oscillatory flow over the sediment layer near the sea bed. Fully resolved direct numerical simulations are performed using fictitious domain approach (Apte & Finn, 2012) to investigate the effect of a sinusoidally oscillating flow field over a rough wall made of regular hexagonal pack of spherical particles. The flow arrangement is similar to the experimental data of Keiller & Sleath (1976). Transitional and turbulent flows at Reδ = 95, 150, 200 (based on the Stokes layer thickness, [Formula: see text] are studied. Turbulent flow is characterized in terms of coherent vortex structures, Reynolds stress variation and PDF distributions. The nature of unsteady hydrodynamic lift forces on sediment grains and their correlation to sweep and burst events is also reported. The dynamics of the oscillatory flow over the sediment bed is used to understand the mechanism of sediment pick-up.

DNS coupled with a Point-Particle based model (PP) is used to study and predict particle-turbulence interactions in an open channel flow at Reynolds number of 811 (based on the friction velocity) corresponding to the experimental observations of [Righetti & Romano, JFM 2004]. Large particles of diameter 200 microns (8.1 in wall units) with average volume loading on the order of 0.001 are simulated using four-way coupling with closure models for drag, added mass, lift, pressure, and inter-particle/particle-wall collision forces. The point-particle model is able to accurately capture the effect of particles on the fluid flow in the outer layer where particles are under resolved. However, the dynamical interaction of particle-turbulence is under predicted in the near wall region where particles size are much larger than Kolmogorov scale and grid resolution in wall-normal direction, but smaller in both stream and span wise directions. It is conjectured that due to the large size particles compared to the Kolmogorov length scale near the bed, the effect of disturbances and deflections in the flow due to presence of such large particles is not captured using Lagrangian Point-Particle approach. For this configuration, the point-particle model is not appropriate in the near wall region and a hybrid resolved particle approach may be necessary.

Summary In field studies of solute transport, transient storage within lateral cavities and other stream features generates breakthrough curves (BTCs) with pronounced and persistent skewness. Current solute transport theory requires that the coefficient of skewness ( CSK ) decrease over time because the system eventually reaches Fickian conditions. However, published data show that CSK is constant in time. To aid development of solute transport theory that explains field observations, we quantify the effect of lateral cavities on solute transport under non-Fickian and Fickian conditions. Six hydrodynamics models were developed: one with no lateral cavities, three with lateral cavities in series, and two with lateral cavities in parallel. Results reveal that lateral cavities in series have longer tails and smaller peak concentrations compared to lateral cavities in parallel. Lateral cavities in series cause greater dispersion and require larger distances to reach Fickian conditions ( x Fick ) compared to lateral cavities in parallel. Cavity configuration has a greater influence on longitudinal dispersion and x Fick than the number of cavities present. CSK changes with monitoring location and maximum CSK (= 10–20) near lateral cavities is higher than empirical estimates (≈1.18). We postulate that adding more transient storage zones would increase channel complexity and yield closer results between simulated and empirical CSK , and testing this hypothesis warrants future research. Finally, while current models can obtain good fits to measured BTCs by parameterizing mass exchange rates and volume ratios, these parameters do not adequately describe the fundamental fluid mechanics driving exchange.

The relative performance of two numerical approaches involving body conforming and non-conforming grids for simulating porescale flow in complex configurations of fixed packed beds of spheres at moderate pore Reynolds numbers (12 ⩽ Re ⩽ 600) is examined. In the first approach, an unstructured solver is used with tetrahedral meshes which conform to the boundaries of the porespace. In the second approach, a fictitious domain formulation is used which employs non-body conforming Cartesian grids and enforces the no-slip conditions on the pore boundaries implicitly through a rigidity constraint force. Sphere to sphere contact points, where the fluid gap between solid boundaries becomes infinitesimal, are not resolved by either approach, but this is shown to have a negligible effect on the local flow field at the Reynolds numbers considered. Detailed grid convergence studies of both steady and unsteady flow through simple cubic packings indicate that for a fixed level of uncertainty, significantly lower grid densities may be used with the fictitious domain approach which also does not require complex grid generation techniques. This translates into large savings for simulation of flow through realistic packed beds, which is shown by both analytic estimates and actual CPU timings. The applicability of the fictitious domain approach is demonstrated by simulating unsteady flow through a randomly packed bed of 51 spheres at a pore Reynolds number of 600. The results are used to examine the dominance of helical vortices in the porescale flow field.

[2] Accurate estimates of mass-exchange parameters in transient storage zones are needed to better understand and quantify solute transport and dispersion in riverine systems. Currently, the predictive mean residence time relies on an empirical entrainment coefficient with a range in variance due to the absence of hydraulic and geomorphic quantities driving mass exchange. Two empirically derived relationships are presented for the mean residence time of lateral cavities—a prevalent and widely recognized type of transient storage—in gravel-bed rivers and streams that incorporates hydraulic and geomorphic parameters. The relationships are applicable for gravel-bed rivers and streams with a range of cavity width to length (W/L) aspect ratios (0.2–0.75), shape, and Reynolds numbers (Re, ranging from 1.0 � 10 4 to 1.0 � 10 7 ). The relationships equate normalized mean residence time to nondimensional quantities: Froude number, Re, W/L, depth ratio (ratio of cavity to shear layer depth), roughness factor (ratio of shear to channel velocity), and shape factor (representing degree of cavity equidimensionality). One relationship excludes bed roughness (equation (13)) and the other includes bed roughness (equation (14)). The empirically derived relationships have been verified for conservative tracers (R 2 of 0.83) within a range of flow and geometry conditions. Topics warranting future research are testing the empirical relationship that includes the roughness factor using parameters measured in the vicinity of the cavity to reduce the variance in the correlation, and further development of the relationship for nonconservative transport.

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

When a few bubbles are entrained in a travelling vortex ring, it has been shown that, even at extremely low volume loadings, their presence can significantly affect the structure of the vortex core (Sridhar & Katz, J. Fluid Mech., vol. 397, 1999, pp. 171–202). A typical Euler–Lagrange point-particle model with two-way coupling for this dilute system, wherein the bubbles are assumed subgrid and momentum point sources are used to model their effect on the flow, is shown to be unable to capture accurately the experimental trends of bubble settling location, bubble escape and vortex distortion for a range of bubble parameters and vortex strengths. The bubbles experience significant amounts of drag, lift, added mass, pressure and gravity forces. However, these forces are in balance with each other as the bubbles reach a mean settling location away from the vortex core. The reaction force on the fluid due to the net summation of these forces alone is thus very small and is unable to affect the vortex core. By accounting for fluid volume displacement due to bubble motion, experimental trends on vortex distortion and bubble settling location are captured accurately. The fluid displacement effects are studied by computing various contributions to an effective volume displacement force and are found to be important even at low volume loadings. As the bubble size and hence bubble Reynolds number increase, the bubbles settle further away from the vortex centre and have strong potential for vortex distortion. The net volume displacement force depends on the radial pressure force, the radial settling location of the bubble, as well as the vortex Reynolds number. The resultant of the volume displacement force is found to be roughly at $4{5}^{\circ } $ with the vortex travel direction, resulting in wakes directed towards the vortex centre. Finally, a simple modification to the standard point-particle two-way coupling approach is developed wherein the interphase reaction source terms are consistently altered to account for the fluid displacement effects and reactions due to bubble accelerations.

Particle-resolved direct numerical simulations are performed using fictitious domain approach [1] to investigate the effect of an oscillatory flow field over a rough wall made up of a regular hexagonal pack of fixed spherical particles, in a setup similar to the experimental configuration of [2]. Turbulent flows at Reynolds numbers, Reδ = 200 and 400 (based on the Stokes-layer thickness δ) are studied. The unsteady nature of hydrodynamic forces on particles and their cross-correlations with measurable flow variables are investigated. Temporal correlations showed drag and lift to be positively correlated with a phase difference, which is approximately equal to the Taylor micro-scale related to drag/lift correlations. Spatio-temporal correlations between the flow field and particle-related quantities showed that the lift force is well correlated with the streamwise velocity fluctuations up to distances of the same order as the particle diameter, beyond which the cross correlation decays considerably. On the other hand, the pressure fluctuations are correlated and anti-correlated with the lift force in the front and aft regions of the particle, respectively, as a result of wake effects. Further statistical analyses showed that the near-bed velocity and pressure fluctuations fit poorly with Gaussian distributions. Instead, a fourth order Gram-Charlier distribution model is proposed that may have consequences on the Gaussian descriptions of sediment pick-up functions typically used in quantification of turbulent transport of sediment particles.

This study investigates use of solar thermochemical processing of clean fuels using biomass products (in particular CH4, H2O). To address technological feasibility of a microchannel-based solar receiver/reactor, a combined numerical and experimental study of methane-steam reforming is carried out on a single microchannel with Palladium-deposited channel walls and heat input to facilitate endothermic heterogeneous reactions producing syngas. A simple one-dimensional model solving steady state species mass fraction, energy, and overall conservation of mass equations is developed, calibrated and validated against concurrent experimental data [1, 2]. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The effects of the total heat input, heat flux profile, and inlet flow rate on production of hydrogen are investigated to assess the effectiveness of the microchannel configuration for production of hydrogen. A coupled shape-constrained optimization and Monte-Carlo...

In many applications, a moving fluid carries a suspension of droplets of a second phase which may change in size due to evaporation or condensation. Examples include liquid fuel drops in engines and raindrops or ice-crystals in a thunderstorm. If the number of such particles is very large, and, if further, the flow is inhomogeneous, unsteady or turbulent, it may be practically impossible to explicitly compute all of the fluid and particle degrees of freedom in a numerical simulation of the system. Under such circumstances Lagrangian Particle Tracking (LPT) of a small subset of the particles is used to reduce the computational effort. The purpose of this paper is to compare the LPT with an alternate method that is based on an approximate solution of the conservation equation of particle density in phase space by the method of moments (MOM). Closure is achieved by invoking the assumption that the droplet size distribution is locally lognormal. The resulting coupled transport equations for the local mean and variance of the particle size distribution are then solved in conjunction with the usual equations for the fluid and associated scalar fields. The formalism is applied to the test case of a uniform distribution of droplets placed in a non homogeneous temperature field and stirred with a decaying Taylor vortex. As a benchmark, we perform a direct numerical simulation (DNS) of high resolution that keeps track of all the particles together with the fluid flow.

An Eulerian–Lagrangian approach is developed for the simulation of turbulent bubbly flows in complex systems. The liquid phase is treated as a continuum and the Navier–Stokes equations are solved in an unstructured grid, finite volume framework for turbulent flows. The dynamics of the disperse phase is modeled in a Lagrangian frame and includes models for the motion of each individual bubble, bubble size variations due to the local pressure changes, and interactions among the bubbles and with boundaries. The bubble growth/collapse is modeled by the Rayleigh–Plesset (RP) equation. Three modeling approaches are considered: (a) one-way coupling, where the influence of the bubble on the fluid flow is neglected, (b) two-way coupling, where the momentum-exchange between the fluid and the bubbles is modeled, and (c) volumetric coupling, where the volumetric displacement of the fluid by the bubble motion and the momentum-exchange are modeled. A novel adaptive time-stepping scheme based on stability-analysis of the non-linear bubble dynamics equations is developed. The numerical approach is verified for various single bubble test cases to show second-order accuracy. Interactions of multiple bubbles with vortical flows are simulated to study the effectiveness of the volumetric coupling approach in predicting the flow features observed experimentally. Finally, the numerical approach is used to perform a large-eddy simulation in two configurations: (i) flow over a cavity to predict small-scale cavitation and inception and (ii) a rising dense bubble plume in a stationary water column. The results show good predictive capability of the numerical algorithm in capturing complex flow features. Copyright © 2010 John Wiley & Sons, Ltd.

High-fidelity, predictive fluid flow simulations of the interactions between the rising thermal plumes from forced air warming blower and the ultra-clean ventilation air in an operating room (OR) are conducted to explore whether this complex flow can impact the dispersion of squames to the surgical site. A large-eddy simulation, accurately capturing the spatiotemporal evolution of the flow in 3 dimensions together with the trajectories of squames, is performed for a realistic OR consisting of an operating table (OT), side tables, surgical lamps, medical staff, and a patient. Two cases are studied with blower-off and blower-on together with Lagrangian trajectories of 3 million squames initially placed on the floor surrounding the OT. The large-eddy simulation results show that with the blower-off, squames are quickly transported by the ventilation air away from the table and towards the exit grilles. In contrast, with the hot air blower turned on, the ventilation airflow above and below the OT is disrupted significantly. The rising thermal plumes from the hot air blower drag the squames above the OT and the side tables and then they are advected downwards toward the surgical site by the ventilation air from the ceiling. Temporal history of the number of squames reaching 4 imaginary boxes surrounding the side tables, the OT, and the patient's knee shows that several particles reach these boxes for the blower-on case.

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.

A numerical study based on the Eulerian–Lagrangian formulation is performed for dispersed phase motion in a turbulent flow. The effect of spatial filtering, commonly employed in large-eddy simulations, and the role of the subgrid scale turbulence on the statistics of heavy particles, including preferential concentration, are studied through a priori analysis of DNS of particle-laden forced isotropic turbulence. In simulations where the subgrid scale kinetic energy attains 30–35% of the total we observe the impact of residual fluid motions on particles of a smaller inertia. It is shown that neglecting the influence of subgrid scale fluctuations has a significant effect on the preferential concentration of those particles. A stochastic Langevin model is proposed to reconstruct the residual (or subgrid scale) fluid velocity along particle trajectories. The computation results for a selection of particle inertia parameters are performed to appraise the model through comparisons of particle turbulent kinetic energy and the statistics of preferential concentrations.

Large-eddy simulation of an evaporating isopropyl alcohol spray in a coaxial combustor is performed. The Favre-averaged, variable density, low-Mach number Navier-Stokes equations are solved on unstructured grids with dynamic subgrid scale model to compute the turbulent gas-phase. The original incompressible flow algorithm for LES on unstructured grids by [Mahesh et al., J. Comp. Phys. 197 (2004) 215–240] is extended to include density variations and droplet evaporation. An efficient particle-tracking scheme on unstructured meshes is developed to compute the dispersed phase. Experimentally measured droplet size distribution and size-velocity correlation near the nozzle exit are used as the inlet conditions for the spray. The predictive capability of the LES approach on unstructured grids together with Lagrangian droplet dynamics models to capture the droplet dispersion characteristics, size distributions, and the spray evolution is examined in detail. The mean and turbulent quantities for the gas and particle phases are compared to experimental data to show good agreement. It is shown that for low evaporation rates considered in the present study, a well resolved large-eddy simulation together with simple subgrid models for droplet evaporation and motion provides good agreement of the mean and turbulent quantities for the gas and droplet phases compared to the experimental data. This work represents an important first step to assess the predictive capability of the unstructured grid LES approach applied to spray vaporization. The novelty of the results presented is that they establish a baseline fidelity in the ability to simulate complex flows on unstructured grids at conditions representative of gas-turbine combustors.

A numerical formulation for Eulerian–Lagrangian simulations of particle-laden flows in complex geometries is developed. The formulation accounts for the finite-size of the dispersed phase. Similar to the commonly used point-particle formulation, the dispersed particles are treated as point-sources, and the forces acting on the particles are modeled through drag and lift correlations. In addition to the inter-phase momentum exchange, the presence of particles affects the fluid phase continuity and momentum equations through the displaced fluid volume. Three flow configurations are considered in order to study the effect of finite particle size on the overall flowfield: (a) gravitational settling, (b) fluidization by a gaseous jet, and (c) fluidization by lift in a channel. The finite-size formulation is compared to point-particle representations, which do not account for the effect of finite-size. It is shown that the fluid displaced by the particles plays an important role in predicting the correct behavior of particle motion. The results suggest that the standard point-particle approach should be modified to account for finite particle size, in simulations of particle-laden flows.

Particle-resolved direct numerical simulations are performed using fictitious domain approach [1] to investigate the effect of an oscillatory flow field over a rough wall made up of a regular hexagonal pack of fixed spherical particles, in a setup similar to the experimental configuration of [2]. Turbulent flows at Reynolds numbers, Reδ = 200 and 400 (based on the Stokes-layer thickness δ) are studied. The unsteady nature of hydrodynamic forces on particles and their cross-correlations with measurable flow variables are investigated. Temporal correlations showed drag and lift to be positively correlated with a phase difference, which is approximately equal to the Taylor micro-scale related to drag/lift correlations. Spatio-temporal correlations between the flow field and particle-related quantities showed that the lift force is well correlated with the streamwise velocity fluctuations up to distances of the same order as the particle diameter, beyond which the cross correlation decays considerably. On the other hand, the pressure fluctuations are correlated and anti-correlated with the lift force in the front and aft regions of the particle, respectively, as a result of wake effects. Further statistical analyses showed that the near-bed velocity and pressure fluctuations fit poorly with Gaussian distributions. Instead, a fourth order Gram-Charlier distribution model is proposed that may have consequences on the Gaussian descriptions of sediment pick-up functions typically used in quantification of turbulent transport of sediment particles.Copyright © 2015 by ASME

Characterization of a microchannel solar thermal receiver for a supercritical carbon dioxide (sCO2) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two receivers are fabricated and experimentally characterized — a parallel microchannel design and a microscale pin fin array design. Lab-scale experiments have been used to demonstrate the receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreased with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm2 at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of line pressure.Copyright © 2015 by ASME

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 (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.

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.

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.

Lack of accurate criteria for onset of incipient motion and sediment pickup function remain two of the biggest hurdles in developing better predictive models for sediment transport. To study pickup and transport of sediment, it is necessary to have a detailed knowledge of the small amplitude oscillatory flow over the sediment layer near the sea bed. Fully resolved direct numerical simulations are performed using fictitious domain approach [1] to investigate the effect of a sinusoidally oscillating flow field over a rough wall made of regular hexagonal pack of spherical particles. The flow arrangement is similar to the experimental data of Keiller et al. [2]. Transitional and turbulent flows at Reδ = 95,150,200 and 400 (based on the Stokes layer thickness, Display Formulaδ=2ν/ω) are explored over a range of non-dimensional sphere sizes. Turbulent flow is characterized in terms of coherent vortex structures, Reynolds stress variation, turbulent kinetic energy budget and PDF distributions. The nature of unsteady hydrodynamic lift forces on sediment grains and their correlation to sweep and burst events is also reported. The dynamics of the oscillatory flow over the sediment bed is used to understand the mechanism of sediment pick-up.Copyright © 2014 by ASME

Both experimental and computational methods applied to the study of porous media flows are challenging due to the complex multi-phase geometry and ability to resolve scales over a reasonably large domain. This study compares experimentally obtained results based on refractive index matching of detailed velocity field vectors with those obtained using DNS to evaluate both methods for consistency. Data were obtained in a randomly packed bed using uniformly sized spherical particles. Experimental challenges including refractive index matching errors, magnification uncertainties, and the identification of the proper geometry as well as, the arduousness, of matching the geometry, grid resolution particularly near solid contact points, and proper boundary conditions DNS are presented. Detailed comparison of the numerical simulation with PIV measurements are presented by attention paid to the statistical distribution of velocities, and their deviation from DNS estimations from the measured values. There is reasonable matching the velocity fields except for some regions of constricted flow. The axial velocity results are within 12 percent and the normal velocity within 9%. Streamline details show that both methods agree well. It is found that inlet conditions play a significant role in being able to match results. NOMENCLATURE

Three-dimensional Reynolds averaged Navier-Stokes modeling, validated against experimental data, is used to parameterize the flow features and time scales in idealized rectangular cavities for a wide range of width-to-length ratios, 0.4≤W/L≤1.1, and Reynolds number based on the depth, 5,000≤RD≤20,300, representative of isolated dead zones in small natural streams. The flow features for this parameter range are similar to open cavity flows and consist of a mixing layer spanning the entire length of the dead zone together with a single main recirculation region. The Langmuir time scale (ratio of dead-zone volume to discharge) based on the assumption of a well-mixed dead zone is found to be a function of the mean rotation time scale (inverse of average rotation rate) within the dead zone, the momentum thickness of the upstream boundary layer, and the dead-zone width. The entrainment coefficient, used to relate the exchange velocity to the average free- stream velocity, is shown to be directly related...

This paper discuses the design of several micro-channel solar receiver devices. Due to enhanced heat transfer in micro-channels, these devices can achieve a higher surface efficiency than current receiver technology, leading to an increase in overall plant efficiency. The goal is to design an efficient solar receiver based on use of super-critical carbon-dioxide and molten salt as heat-transfer fluids. The super-critical Brayton cycle has shown potential for a higher efficiency than current power cycles used in CSP. Molten salt has been used in CSP applications in the past. The required inlet and outlet temperatures of the fluid are 773.15 K and 923.15 K for carbon-dioxide and 573.15 K and 873.15 K for molten salt. These temperature values are determined by the power cycles the devices are designed to operate in. The required maximum pressure drop is 0.35 bar for carbon-dioxide and 1 bar for molten salt. These pressure values are intended to be a practical goal for maximum pressure drop. The super-critical carbon-dioxide power cycle requires an operating pressure of is 120 bar. Finally, each device must withstand any mechanical and thermal stresses that may exist. Devices presented range in size from 1 cm2 to 4 cm2 and in heat transfer rates from 200 W to 400 W. The size of the device is based on the output capacity of the solar simulator which will be used for testing. For carbon-dioxide, three designs were developed with varying manufacturability. The low risk design features machined and welded parts and straight parallel channels. The medium risk design features machined and diffusion bonded parts and straight parallel channels. The high risk design features a circular micro-pin-fin array created using EDM and is constructed using diffusion bonding. The absence of high operating pressure for molten salt made structural design much easier than for carbon-dioxide. Conjugate heat-transfer simulations of each design were used to evaluate pressure drop, receiver efficiency, and flow distribution. Two and three dimensional structural analyses were used to ensure that the devices would withstand the mechanical and thermal stresses. Based on the numerical analyses, a receiver efficiency of 89.7% with a pressure drop of 0.2 bar were achieved for carbon-dioxide. The design was found to have a structural safety factor of 1.3 based maximum mechanical stress occurring in the headers. For molten salt, an efficiency of 92.1% was achieved with a pressure drop of 0.5 bar.Copyright © 2014 by ASME

A geyser in a closed conduit system is characterized by an explosive release of a mixture of air and water through a drop shaft. Geysers are often observed in combined sewer systems (CSSs) as an explosive form of combined sewer overflows (CSOs). In the present work, the flow characteristics of sewer geyser phenomenon are studied numerically using a three-dimensional computational fluid dynamics (CFD) model for the first time. Traditionally, investigations of flow processes associated with sewer geysers are conducted through physical experiments and one or two-dimensional numerical models. However, sewer geysers are undeniably three-dimensional, hence 3D models are necessary to gain further understanding of the phenomena. The CFD model was calibrated using an experiment that exhibit characteristics of sewer geysers.

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.

A comprehensive numerical analysis has been conducted to study the combustion of a double-base homogeneous propellant in a rocket motor. Emphasis was placed on the motor internal flow development and its influence on propellant combustion. The formulation is based on the Favre-averaged, filtered equations for the conservation laws and takes into account finite-rate chemical kinetics and variable thermophysical properties. Turbulence closure is obtained using the large-eddy-simulation technique. The contribution of large energy-carrying structures to mass, momentum, and energy transfer is computed explicitly, and the effect of small scales of turbulence is modeled. The governing equations and associated boundary conditions are solved using a time-accurate, semi-implicit Runge-Kutta scheme coupled with a fourth-order central difference algorithm for spatial discretization. The motor internal flowfield is basically determined by the balance between the inertia force and the pressure gradient arising from the mass injection at the propellant surface. The temporal evolution of the vorticity field shows a laminar upstream region, a transition zone in the midsection of the chamber, and a fully developed turbulent regime further downstream. The turbulent mixing proceeds at a rate faster than chemical reactions, and the flame stretch is strong enough to regard propellant combustion as a well-stirred reactor. The combustion wave in the laminar region exhibits a two-stage structure consisting of a primary flame, a dark zone, and a secondary luminous flame. The enhanced energy and mass transport in the turbulent region partially merges the primary and secondary flames, thereby raising the temperatures in the dark zone. In the present study, the smoother axial velocity gradient and vertical flow convection prevent turbulence from deeply penetrating into the primary flame zone. The turbulence energy spectra indicate dominant harmonics in a frequency range capable of triggering combustion instabilities.

Interactions between acoustic waves and the transient combustion response of a double-base homogeneous propellant in a rocket motor have been analyzed numerically. The analysis extends the previous work on gas-phase flame dynamics to include the coupling with condensed-phase processes. Consequently, a more complete description of propellant combustion response to imposed acoustic oscillations can be obtained. Emphasis is placed on the near-surface flame-zone physiochemistry and its coupling with unsteady propellant burning in an oscillatory environment. The formulation treats complete conservation equations and the finite-rate chemical kinetics in both the gas-phase and subsurface regions. The instantaneous propellant burning rate is predicted as part of the solution. Various distinct features of unsteady heat release arising from propellant combustion response in a motor with forced oscillations are studied systematically. As in the pure gas-phase dynamics of the previous case, the dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. However, the propellant combustion response may qualitatively modify the temporal evolution of heat-release distribution in the luminous flame and as a result exerts a significant influence on the global stability behavior. The primary flame structure adjacent to the propellant surface is usually little affected by flow oscillation. This may be attributed to the large thermal inertial of the condensed phase, which tends to restrain the temperature variation in the near-surface zone in the present study of laminar flows. The situation with a turbulent flow may be drastically different, as turbulence may penetrate directly into the, primary flame and substantially change the local flame dynamics and transport phenomena.

This computational study investigates design of microchannel based solar receiver for use in concentrated solar power. A design consisting of a planar array of channels with solar flux incident on one side and using supercritical carbon dioxide as the working fluid is sought. Use of microchannels is investigated as they offer enhanced heat transfer in solar receivers and have the potential to dramatically reduce the size and increase the performance. Designs are investigated for an incident heat flux of 1 MW/m2, up to 3.3 times that of current solar receivers [1], resulting in significant reduction of size and cost. The goal is to design a microchannel receiver with inlet and outlet temperatures of the working fluid of 500°C and 650°C, operating pressure of 100 bar, pressure drop less than 0.35 bar and surface efficiency greater than 90% defined by radiation and convection losses to the environment. Three micro-channel designs are considered: rectangular cross section with high and low aspect ratio (designs A and B) and rectangular cross section with an array of micro pin-fins of various shape spanning the height of the channel (design C). Numerical simulations are performed on individual channels and on a unit cell of the pin-fin design. Structural analysis is performed to ensure that the design can withstand the operating pressure and thermal stresses. The effects of flow maldistribution and header system in an array of channels are also investigated. Preliminary results show that all three designs are capable of meeting the requirements, with the pin-fin design having the lowest pressure drop and highest efficiency.

[1] Surface transient storage (STS) has functional significance in stream ecosystems because it increases solute interaction with sediments. After volume, mean residence time is the most important metric of STS, but it is unclear how this can be measured accurately or related to other timescales and field-measureable parameters. We studied mean residence time of lateral STS in small streams over Reynolds numbers (Re) 5000–200,000 and STS width to length (W/L) aspect ratios between 0.2–0.75. Lateral STS have flow fields characterized by a shear layer spanning the length of the STS entrance, and one primary gyre and one or more secondary gyre(s) in the STS. The study's purpose was to define, measure, and compare residence timescales: volume to discharge ratio (Langmuir timescale); area under normalized concentration curve; and characteristic time of exponential decay, and to compare these timescales to field measureable parameters. The best estimate of STS mean residence time—primary gyre residence time—was determined to be the first characteristic time of exponential decay. An apparent mean residence time can arise, which is considerably larger than other timescales, if probes are placed within secondary gyre(s). The Langmuir timescale is the minimum mean residence time, and is linearly correlated to channel velocity and STS width. The lateral STS mean residence time can be predicted using a physically based hydromorphic timescale derived by Uijttewaal et al. (2001) with an entrainment coefficient of 0.031 ± 0.009 for the Re and W/L studied.