## About

143

Publications

20,815

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3,022

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Introduction

I am interested in understanding turbulent, multiphase flows using predictive numerical methods such as direct and large-eddy simulations. Various applications, involving such flows, are of interest; namely, particle-laden flows and sediment transport, sprays and droplet atomization, inertial and turbulent flows through porous beds, among others.

Additional affiliations

June 2016 - December 2016

September 2000 - October 2005

**Stanford University**

Position

- Research Associate

Education

September 2000 - October 2005

**Stanford University**

Field of study

- Mechanical Engineering

August 1996 - July 2000

August 1994 - February 1996

## Publications

Publications (143)

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 b...

Modeling of spray systems using the point-particle approach requires estimation of the undisturbed fluid flow quantities such as velocity, pressure, species mass fraction, and temperature at the droplet location, to accurately capture the droplet dynamics. However, in a typical two-way coupled computation, the droplets affect the fluid flow through...

This manuscript aims to explore a potential mechanism responsible for the material erosion observed in supercritical co2 power cycles. Supercritical Carbon Dioxide sco2 has been proposed as working fluid for power generation cycles owing to its non-toxic, non-flammable properties, high density and low cost. However, the severe erosion has become a...

Direct numerical simulation is used to investigate effects of turbulent flow in the confined geometry of a face-centred cubic porous unit cell on the transport, clustering and deposition of fine particles at different Stokes numbers ( $St = 0.01, 0.1, 0.5, 1, 2$ ) and at a pore Reynolds number of 500. Particles are advanced using one-way coupling a...

Pore-resolved direct numerical simulations of turbulent open channel flow are performed comparing the structure and dynamics of turbulence over impermeable rough and smooth walls to a porous sediment bed at permeability Reynolds number ($Re_K$) of 2.6, representative of aquatic beds. Four configurations are investigated; namely, (i) permeable bed w...

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...

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...

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...

Direct numerical simulation is used to investigate effects of turbulent flow in the confined geometry of a face-centered cubic porous unit cell on the transport, clustering, and deposition of fine particles at different Stokes numbers ($St = 0.01, 0.1, 0.5, 1, 2$) and at a pore Reynolds number of 500. Particles are advanced using one-way coupling a...

Correcting for self-disturbance created by a particle in two-way coupled particle-laden flows using point-particle model. The approach developed in recent work (DOI: 10.1016/j.jcp.2021.110381) (JCP 2021) is extended to multiple particles, heat transfer, and is straightforward to implement in any flow solver.
https://mfix.netl.doe.gov/wp-content/up...

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 b...

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 mul...

Accurate prediction of spray atomization process using an Euler-Lagrange (EL) approach is challenging because of high volume fraction of the liquid phase in dense regimes. This would in reality displace a remarkable portion of the gaseous phase which is commonly ignored in the standard EL approaches. In addition, deformation of droplet due to the i...

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 veloci...

Accurate prediction of a dense spray using an Euler-Lagrange approach is challenging because of high volume fraction of the dispersed phase due to subgrid cluster of droplets. To accurately model dense sprays, one needs to capture this effect by taking into account the spatio-temporal changes in the volume fraction of the carrier phase due to the m...

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 veloci...

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 re...

Modeling of a dense spray regime using an Euler-Lagrange approach is challenging because of local high volume loading. A cluster of droplets, that are assumed subgrid, can lead to locally low void fractions for the fluid phase. Under these conditions, spatio-temporal changes in the fluid volume fractions should be considered in an Euler-Lagrange, t...

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. S...

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 g...

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...

Accurate prediction of a dense spray using an Euler-Lagrange approach is challenging because of high volume fraction of the dispersed phase due to subgrid cluster of droplets. To accurately model dense sprays, one needs to capture this effect by taking into account the spatio-temporal changes in the volume fraction of the carrier phase due to the m...

High-fidelity, predictive fluid flow simulations of the interactions between the rising thermal plumes from forced air warming blower and the ultraclean ventilation air in an operating (OR) are conducted to explore whether this complex flow can impact the dispersion of squames to the surgical site. A large-eddy simulation (LES), accurately capturin...

A numerical investigation of unsteady hydrodynamic forces on the particle-bed in an oscillatory flow environment is performed by means of direct numerical simulations (DNS). 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. Characterizati...

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,...

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 corresponding to the large size gravel and the small size sand particle, respectively, in a very rough turbulent flow regime are r...

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...

Modeling of a dense spray regime using an Euler-Lagrange approach is challenging because of local high volume loading. A cluster of droplets, that are assumed subgrid, can lead to locally low void fractions for the fluid phase. Under these conditions, spatio-temporal changes in the fluid volume fractions should be considered in an Euler-Lagrange, t...

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-mi...

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 4...

Sand transport and morphological change occur in the wave bottom boundary layer due to sand particle interactions with an oscillatory flow and granular interactions between particles. Although these interactions depend strongly on the characteristics of the particle population, i.e. size and shape, little is known about how natural sand particles b...

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(125) in wall units corresponding to the gravel (sand) particle in a very rough turbulent flow regime are investigated. A double-averaging technique is e...

A sewer geyser is an explosive overflow of a mixture of gas and water through a vertical pipe of a sewer system. According to reported field observations, the explosive overflows occurred in a form of ejecting water columns that persisted for several seconds. In previous studies, the presence of entrapped air pockets in sewer systems were argued to...

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 St...

Supercritical Carbon Dioxide (sCO2) has been used for power generation cycles owing to
its non-toxic, non-flammable properties and low cost. The high pressure (� 200-350 bar) and
high density fluid also enables extremely compact and high efficiency turbomachinery designs.
Owing to high pressures and temperatures (� 700�C); however, there is evidenc...

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). Experimental...

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 4...

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 d...

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 forw...

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,...

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 usi...

Particle resolved direct numerical simulations are performed using fictitious domain approach (Apte et al., JCP 2009) to investigate oscillatory turbulent flow over a layer of fixed particles representative of a sediment layer in coastal environments. Five particle Reynolds numbers in the range, ReD=660−4240 are studied and results are compared aga...

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...

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...

We perform simulations of sand ripple evolution in an oscillatory boundary layer flow typical of the ripple regime. The simulation framework is a parallel implementation of a three dimensional, variable density, incompressible flow solver, which solves the ensemble averaged Navier-Stokes equations on a fixed, structured grid. The sediment phase is...

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...

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...

Large Eddy Simulations (LESs) for a lean-direct injection (LDI) combustor are performed and compared with experimental data. The LDI emissions characteristics, and radiation-spray coupling effect on the predictions are analyzed. The flamelet progress variable approach is employed for chemistry tabulation coupled with a stochastic secondary breakup...

Accurate prediction of sediment transport in the presence of bedforms such as sand ripples requires an advanced understanding of how dynamic sediment beds interact with turbulent oscillatory flows. In this paper we propose a new approach for simulating these interactions, based on a fixed grid multiphase Euler-Lagrange simulation, that fully couple...

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 t...

Presented this work in Oregon Best Fest held in Portland, OR USA.

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...

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 re...

A numerical scheme for fully resolved simulation of particle–fluid systems with freely moving rigid particles is developed. The approach is based on a fictitious domain method wherein the entire particle–fluid domain is assumed to be an incompressible fluid but with variable density. The flow inside the particle domain is constrained to be a rigid...

[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 quantitie...

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 re...

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 syste...

The computation of Lagrangian coherent structures typically involves post-processing of experimentally or numerically obtained fluid velocity fields to obtain the largest finite-time Lyapunov exponent (FTLE) field. However, this procedure can be tedious for large-scale complex flows of general interest. In this work, an alternative approach involvi...

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 residen...

The characteristics of pore scale vortical structures observed in moderate Reynolds number flow through mono-disperse packed beds of spheres are examined. Our results come from direct numerical simulations of flow through (i) a periodic, simple cubic arrangement of 54 spheres, (ii) a wall bounded, close packed arrangement of 216 spheres, and (iii)...

Natural stream systems contain a variety of flow geometries which contain flow separation, turbulent shear layers, and recirculation zones. This work focuses on stream dead zones. Characterized by slower flow and recirculation, dead zones are naturally occurring cutouts in stream banks. These dead zones play an important role in stream nutrient ret...

The relative performance of (i) a body-fitted unstructured grid Navier-Stokes solver [Moin and Apte, AIAA J. 2006], and (ii) a fictitious domain based finite-volume approach [Apte et al. JCP 2009] is examined for simulating flow through packed beds of spheres at moderate flow rates, 50 ≲ Re ≲ 1300. The latter employs non-body conforming Cartesian g...

Bubble interactions with vortical structures are important to better understand the mechanisms of bubble induced boundary layer drag reduction and chemical mixing. Traditionally, many studies of disperse bubble or particle-laden flows have utilized an Euler-Lagrange two-way coupling approach, wherein the dispersed phase is assumed subgrid and its d...

Turbulent radiation interaction (TRI) effects are associated with the differences in the time scales of the fluid dynamic equations and the radiative transfer equations. Solving on the fluid dynamic time step size produces large changes in the radiation field over the time step. We have modified the statistically homogeneous, non-premixed flame pro...

Direct numerical simulations were performed to study the effect of an elastically mounted trailing edge actuator on the unsteady ow over a plunging, thin airfoil at Reynolds num- ber of 14700 based on the chord length. The goal is to investigate potential benefits of ow-induced passive actuation of the trailing edge to the lift and drag characteris...

This study investigates use of solar thermochemical processing of clean fuels using biomass products (in particular CH 4 , H 2 O). 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-deposite...

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 mod...

Large-eddy simulation (LES) is a promising technique for accurate prediction of reacting multiphase flows in practical gas-turbine
engines. These combustors involve complex physical phenomena of primary atomization of liquid sheet/jet and secondary breakup,
droplet evaporation, turbulent mixing of fuel vapor with oxidizer, and combustion dynamics....

Inertial flows in porous media occur in both natural (e.g., at stream
bed interfaces with the hyporheic zone) and engineered (e.g., near well
bores or in packed-bed reactors) systems. There are a number of
approaches for representing the inertial effects of flow in a porous
medium, and most commonly these laws relate the pressure gradient to the
sq...

Transient storage is the sum of surface transient storage (STS) and
hyporheic transient storage (HTS) and separating the two storage
components is challenging. A number of studies have attempted to
determine the relationship between transient storage and stream channel
properties; however, difficulties ensue when attempting to calculate
STS. The pr...

The effects of Turbulence Radiation Interactions (TRI) in particulate
laden flows can significantly influence thermal radiation fields and
corresponding material heating. Most combustion problems contain strong
heterogeneities which can be treated stochastically. In pulverized coal
combustion these heterogeneities include particulate such as coal,...

Direct numerical simulations are performed to investigate the effect of
an elastically mounted leading edge actuator on the unsteady flow at
high angles of attack over a flat, thin airfoil at Reynolds number of
14700 based on the chord length. The leading edge actuator is mounted
with a torsion spring at one-third the chord length allowing dynamic...

Natural stream systems contain a variety of dead zones characterized by
flow separation, a mixing layer, and a recirculation zone. These dead
zones play an important role in stream solute transport studies.
Previous published work has focused on idealized storage zone geometries
studied in laboratory flumes. Using RANS simulations, this study first...

The pore scale character of moderate Reynolds number, inertial flow
through mono-disperse packed beds of spheres is examined using numerical
experiments. Direct numerical simulations are performed for flow through
(i) a periodic, 3x3x6 simple cubic arrangement at Rep=529,
and (ii) a realistic randomly packed tube containing 326 spheres with
dtube/d...

A mutiscale numerical approach is developed for the investigation of bubbly flows in turbulent environments. This consists of two different numerical approaches capable of capturing the bubble dynamics at different scales depending upon the relative size of the bubbles compared to the grid resolution: (i) fully resolved simulations (FRS) wherein th...

An experimental study of steam methane reforming in a microchannel is presented. Palladium nanoparticles, deposited on a porous aluminized FeCrAlY felt, served as catalyst sites for the reforming reactions. Parametric studies of steam-methane ratio, residence time, average reactor temperature, and temperature distribution were performed. Results de...

Simulations of bubble entrainment and interactions with two dimensional vortical flows are preformed using a discrete element model. In this Eulerian-Lagrangian approach, solution to the carrier phase is obtained using direct numerical simulation whereas motion of subgrid bubbles is modeled using Lagrangian tracking. The volumetric displacement of...

Numerical modeling of methane-steam reforming is per-formed in a microchannel with heat input through Palladium-deposited channel walls corresponding to the experimental setup of Eilers [1]. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-s...

At modest flow rates (10 ≤ Re ≤ 300) through porous media and
packed beds, fluid inertia can result in complex steady and unsteady
recirculation regions, dependent on the local pore geometry. Body fitted
CFD is a broadly used design and analysis tool for flows in porous media
and packed bed type reactors. Unfortunately, the inherent complexities
of...

Direct numerical simulations are performed to investigate the effect of a movable leading edge on the unsteady flow at high angles of attack