# Javier Urzay's research while affiliated with Stanford University and other places

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## Publications (66)

An integrated computational framework is introduced to study complex engineering systems through physics-based ensemble simulations on heterogeneous supercomputers. The framework is primarily designed for the quantitative assessment of laser-induced ignition in rocket engines. We develop and combine an implicit programming system, a compressible re...

The interaction between a weakly turbulent free stream and a hypersonic shock wave is investigated theoretically by using linear interaction analysis (LIA). The formulation is developed in the limit in which the thickness of the thermochemical nonequilibrium region downstream of the shock, where relaxation toward vibrational and chemical equilibriu...

Breaking waves generate a distribution of bubble sizes that evolves over time. Knowledge of how this distribution evolves is of practical importance for maritime and climate studies. The analytical framework developed in Part 1 (Chan, Johnson & Moin, J. Fluid Mech., vol. 912, 2021, A42) examined how this evolution is governed by the bubble-mass flu...

A Mach-10 hypersonic boundary layer of air overriding a cold, isothermal, non-catalytic flat wall, and with a stagnation enthalpy of 21.6 MJ kg −1 , is analysed using direct numerical simulations. The calculations include multicomponent transport, equilibrium vibrational excitation and chemical kinetics for air dissociation. The initially laminar b...

Rocket engines and high-power new generations of gas-turbine jet engines and diesel engines oftentimes involve the injection of one or more reactants at subcritical temperatures into combustor environments at high pressures, and more particularly at pressures higher than those corresponding to the critical points of the individual components of the...

The interaction between an incident shock wave and a Mach-6 undisturbed hypersonic laminar boundary layer over a cold wall is addressed using direct numerical simulations (DNS) and wall-modeled large-eddy simulations (WMLES) at different angles of incidence. At sufficiently high shock-incidence angles, the boundary layer transitions to turbulence v...

The interaction between an incident shock wave and a Mach-6 undisturbed hypersonic laminar boundary layer over a cold wall is addressed using direct numerical simulations (DNS) and wall-modeled large-eddy simulations (WMLES) at different angles of incidence. At sufficiently high shock-incidence angles, the boundary layer transitions to turbulence v...

Breaking waves generate a distribution of bubble sizes that evolves over time. Knowledge of how this distribution evolves is of practical importance for maritime and climate studies. The analytical framework developed in Part 1 examined how this evolution is governed by the bubble-mass flux from large to small bubble sizes, which depends on the rat...

In this study, the open-source Hypersonics Task-based Research (HTR) solver for hypersonic aerothermodynamics is described. The physical formulation of the code includes thermochemical effects induced by high temperatures (vibrational excitation and chemical dissociation). The HTR solver uses high-order TENO-based spatial discretization on structur...

The interaction of an incident shock wave with a Mach-6 undisturbed laminar boundary layer is addressed using DNS and equilibrium wall-modeled LES (WMLES). The wall temperature is cold compared to the free-stream stagnation temperature, such that the mean temperature profile develops a peak near the wall due to viscous heating. The consequences of...

In this work, the turbulence statistics in highly supersonic boundary layers downstream of an incident shock wave are investigated by DNS. The objective is to examine the transformations and scaling laws of spatially evolving highly supersonic boundary layers with a canonical setup. This can allow for the development of efficient reduced-order mode...

It is known that the dynamics of particles dispersed in turbulent flows can be significantly altered by electric charges and external electric fields. The next step therefore involves the investigation of whether these electric interactions can be exploited in engineering applications to control processes involving the transport of particles in tur...

The interaction of an incident shock wave with a Mach-6 undisturbed laminar boundary layer is addressed using DNS and equilibrium wall-modeled LES (WMLES). The wall temperature is cold compared to the free-stream stagnation temperature, such that the mean temperature profile develops a peak near the wall due to viscous heating. The consequences of...

This paper is associated with a video winner of a 2018 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2018.GFM.V0027

This work addresses the question of the validity of self-similar formulations in describing the structures of methane/air laminar counterflow diffusion flames subjected to incident sub-breakdown DC electric fields. The electric field is induced by two flat porous electrodes located on the oxidizer and fuel sides of the burner and arranged parallel...

A new subgrid-scale (SGS) model for turbulent velocity fluctuations is proposed for large-eddy simulations (LES) of dispersed multi-phase flows. The modeled velocity contains scales smaller than the LES grid resolution, thereby enabling the prediction of small-scale phenomena such as the preferential concentration of particles in high-strain region...

Wall-modeled LES for shock-induced aerodynamic heating in hypersonic boundary layers

In this study, equilibrium WMLES has been compared against DNS for addressing
the performance of the former in predicting shock-induced transition and heating in
hypersonic boundary layers. In this flow, the equilibrium wall model underperforms in a
number of different metrics. At low shock-incidence angles, the post-shock boundary layer
in DNS is...

It is known that the dynamics of particles laden in turbulent flows can be significantly altered by electric charges and external electric fields. The next step therefore involves the investigation of whether these electric interactions can be exploited in engineering applications to control processes involving the transport of particles in turbule...

The generation of microbubbles upon the collision and interaction of liquid bodies in a gaseous environment is a ubiquitous process in two-phase flows, including large-scale phenomena like ship wakes, breaking waves and rain showers. These collision and interaction events involve the relative approach of pairs of liquid-gas interfaces. As these int...

Fuselages of high-speed aerospace vehicles are often subject to large thermal loads related to shock-induced phenomena. One important case is that of a hypersonic transitional boundary layer interacting with an oblique shock overriding an isothermal wall. In this study, use of DNS and equilibrium wall-modeled LES (WMLES) is made in order to investi...

Direct numerical simulations of incompressible homogeneous-isotropic turbulence laden with a dilute suspension of inertial point particles are performed in conjunction with a wavelet multiresolution analysis of the results. The use of spatially localized wavelet basis functions enables the simultaneous consideration of physical and scale spaces in...

A three-dimensional wavelet multi-resolution analysis of direct numerical simulations of a turbulent premixed flame is performed in order to investigate the spatially localized spectral transfer of kinetic energy across scales in the vicinity of the flame front. A formulation is developed that addresses the compressible spectral dynamics of the kin...

The original version of this Article contained an error in the last sentence of the second paragraph of the 'Atmospheric rarefaction effects' section of the Results, which incorrectly read 'The other one emulates the rarefied, CO2-rich Martian atmosphere (μ♂ = 1.3 × 10-5 N s m-2) at 6.9 mbar and 210 K, which gives ρ♂ = 1.6 × 10-12 kg m-3.' The corr...

Self-induced electricity, including lightning, is often observed in dusty atmospheres. However, the physical mechanisms leading to this phenomenon remain elusive as they are remarkably challenging to determine due to the high complexity of the multi-phase turbulent flows involved. Using a fast multi-pole method in direct numerical simulations of ho...

Numerical simulations of counterflow laminar diffusion flames impinged by sub-breakdown DC electric fields are performed in this work using multi-component transport and a detailed chemical mechanism for methane-air combustion that includes elementary steps for the conversion of six electrically charged species. The electric field is induced by two...

Great efforts have been dedicated during the last decades to the research and development of hypersonic aircrafts that can fly at several times the speed of sound. These aerospace vehicles have revolutionary applications in national security as advanced hypersonic weapons, in space exploration as reusable stages for access to low Earth orbit, and i...

Aerospace vehicles flying at supersonic and hypersonic speeds are subject to increased wall heating rates caused by viscous friction with the gas environment. This extra heat is commonly referred to as convective aerodynamic heating. In wall-modeled large-eddy simulations, the near-wall region of the flow is not resolved by the computational grid....

Submitted for the DFD17 Meeting of The American Physical Society The effects of incident electric fields on counterflow diffusion flames. 1 MARIO DI RENZO, PIETRO DE PALMA, MARCO DONATO DE TUL-LIO, GIUSEPPE PASCAZIO, Univ Politecnico di Bari, JAVIER URZAY, Stanford University — The impingement of electric fields on flames is known to have potential...

Rocket engines and new generations of high-power jet engines and diesel engines of-tentimes involve the injection of one or more reactants at subcritical temperatures into combustor environments at high pressures, and more particularly, at pressures higher than those corresponding to the critical points of the individual components of the mixture,...

In this study, a wavelet-based method for extraction of clusters of inertial particles in turbulent flows is presented that is based on decomposing Eulerian particle-number-density fields into the sum of coherent (organized) and incoherent (disorganized) components. The coherent component is associated with the clusters and is extracted by filterin...

A number of micro-scale biological flows are characterized by spatio-temporal chaos. These include dense suspensions of swimming bacteria, microtubule bundles driven by motor proteins, and dividing and migrating confluent layers of cells. A characteristic common to all of these systems is that they are laden with active matter, which transforms fre...

In this study, a dynamic model for large-eddy simulations is proposed in order to describe the motion of small inertial particles in turbulent flows. The model is simple, involves no significant computational overhead, contains no adjustable parameters, and is flexible enough to be deployed in any type of flow solvers and grids, including unstructu...

The study of transcritical dynamics finds important applications in recent designs of gas turbine engines for jet propulsion. In particular, current trends in ultra-low emission technologies for aviation industry are gearing combustors toward lean burn and high pressure ratios. Lean burn aims at decreasing nitrogen oxides by avoiding their peak pro...

Multi-scale interactions and kinetic-energy transfer between turbulence and flames are fundamental to understanding and modeling premixed turbulent reacting flows. In order to investigate these phenomena, direct numerical simulations of a turbulent premixed flame are analyzed in this study. The results reveal a flux of kinetic energy that involves...

Spectral kinetic energy transfer by advective processes in turbulent premixed reacting flows is examined using data from a direct numerical simulation of a statistically planar turbulent premixed flame. Two-dimensional turbulence kinetic-energy spectra conditioned on the planar-averaged reactant mass fraction are computed through the flame brush an...

This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of propo...

This paper extends to transonic mixing layers an analysis of Lighthill ("Reflection at a Laminar Boundary Layer of a Weak Steady Disturbance to a Supersonic Stream, Neglecting Viscosity and Heat Conduction," Quarterly Journal of Mechanics and Applied Mathematics, Vol. 54, No. 1, 1950, pp. 303-325.) on the interaction between weak shocks and laminar...

At subcritical pressures, atomization devices in chemical-propulsion systems must ensure proper rupture of the liquid volume through aerodynamic shearing and homogeneous dispersion of the liquid droplets in the combustion chamber. The gas environment in the combustor is typically hot as a result of the heat released by chemical reactions, with part...

Ignition in a supersonic mixing layer interacting with an oblique shock wave is investigated analytically and numerically under conditions such that the post-shock flow remains supersonic. The study requires consideration of the structure of the post-shock ignition kernel that is found to exist around the point of maximum temperature, which may be...

Liquid propellants are often used in propulsion systems. In subcritical conditions, atomization involves the rupture of the liquid volume through the competition between aerodynamic shearing and surface tension. In contrast, the classic atomization description becomes inadequate at supercritical conditions when the characteristic temperature and pr...

This study addresses the optical performance of a plasma adaptive lens for aero-optical applications by using both axisymmetric and three-dimensional numerical simulations. Plasma adaptive lenses are based on the effects of free electrons on the phase velocity of incident light, which, in theory, can be used as a phase-conjugation mechanism. A clos...

In wind pollination, the release of pollen from anthers into airflows determines the quantity and timing of pollen available for pollination. Despite the ecological and evolutionary importance of pollen release, wind-stamen interactions are poorly understood, as are the specific forces that deliver pollen grains into airflows. We present empirical...

This study addresses the dynamics of backscatter of kinetic energy in the context of large-eddy simulations (LES) of high-speed turbulent reacting flows. A priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen-air turbulent mixing layers with complex chemistry and multicomponent diffusion...

This study addresses the influences of residual radical impurities on the computation and experimental determination of ignition times in H2/O2 mixtures. Particular emphasis is made on the often-times encountered problem of the presence of H-atoms in the initial composition of H2/O2 mixtures in shock tubes. Two methods are proposed for quantifying...

The present paper deals with the description of the interacting multiscale processes governing spray vaporization and combustion downstream from the near-injector atomization region in liquid-fueled burners. One of the main objectives is to emphasize the progress made in the mathematical description and understanding of reactive spray flows by inco...

Conditions are identified under which analyses of laminar mixing layers can shed light on aspects of turbulent spray combustion. With this in mind, laminar spray-combustion models are formulated for both non-premixed and partially premixed systems. The laminar mixing layer separating a hot-air stream from a monodisperse spray carried by either an i...

In this study, a flamelet-based framework is proposed for quantifying the uncertainties induced by chemical-kinetic rates on the aerothermochemical field in numerical simula-tions of high-speed turbulent combustion. For this purpose, this report is divided into the following sections. In Section 2, a short background on flamelet physics is provided...

Approximately 10 percent of plant species rely on wind for pollination
(anemophily). These include many taxa of economic importance: e.g.
cultigens such as wheat and maize; species like grasses and ragweed that
trigger allergies; and the conifers, our most important species for the
forest industry in the mid- latitudes. It has often been assumed th...

The vaporization and combustion of a monodisperse fuel spray in a
laminar counterflow mixing layer is investigated under conditions such
that the droplet Stokes number is smaller than 1/4, so that the droplets
do not cross the stagnation plane, but instead tend to accumulate there
because of their inertial slip motion. Vaporization is confined to t...

The study of the physical processes that lead to extinction of flames in
gaseous hydrogen-air non-premixed combustion is of paramount importance
for the reliable design of power plants and advanced propulsion systems
in automobiles and hypersonic aircrafts. However, there remain several
uncertainties in the experimental quantification of reaction r...

This analysis addresses the propagation of spiral edge flames found in von Kármán swirling flows induced in rotating porous-disk burners. In this configuration, a porous disk is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame a...

This analysis makes use of asymptotic analyses and numerical methods to address, in the limit of small Reynolds and ionic Péclet numbers and small clearances, the canonical problem of the forces exerted on a small solid spherical particle undergoing slow translation and rotation in an incompressible fluid moving parallel to an elastic substrate, su...

Applied mathematics techniques are used in this investigation to gain
insight into three different physical processes of current interest in
combustion and fluid dynamics. The first problem addresses the
propagation of spiral edge flames found in von Karman swirling flows
induced in rotating porous-disk burners. In this configuration, a porous
disk...

Flames advected by swirling flows are subject to large strain rates that may cause local extinction. In this investigation, a porous-disk burner is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame adjacent to the disk, resulting...

Plant reproduction depends on pollen dispersal. For anemophilous (wind-pollinated) species, such as grasses and many trees, shedding pollen from the anther must be accomplished by physical mechanisms. The unknown nature of this process has led to its description as the 'paradox of pollen liberation'. A simple scaling analysis, supported by experime...

The influence of soft boundaries on the forces experienced by a small sphere undergoing slow translation and rotation near a wall is investigated using asymptotic and numerical methods. The clearance between the sphere and the wall is assumed to be small, so that the lubrication approximation holds in the gap. The forces induced by boundary deforma...

Uncertainties in elementary reaction rates of chemical mechanisms have traditionally been reported by experimentalists in terms of constant-temperature prefactors to ac-count for observed sparsity in measurements (Konnov 2008; Hong, Davidson & Hanson 2011b). These uncertainty prefactors have been used in uncertainty quantification and optimization...

## Citations

... Thermochemistry is firmly rooted in the study of combustion problems, high-speed flows, reactive and non-reactive shocks, rocket engine performance, and high explosives [41,42]. For instance, strong hypersonic shocks involve changes in the molecular structure of the gas, including vibrational excitation leading to dissociation [43,44], and later electronic excitation leading to ionization [45], which eventually transform the gas into a plasma. Turbulent combustion and gaseous detonations have also been the topic of intense research due to their high thermodynamic efficiency in propulsion applications [46,47]. ...

... Some studies using Direct Numerical Simulations (DNS) were able to model every turbulent scale and each bubble individually for small domains and low Reynolds numbers [29][30][31][32], but this type of approach is unfeasible for most practical applications [21]. A technique that has been gaining popularity in the last few years consists in using the interface-capturing/tracking algorithm to track only the largest pockets of air. ...

... Turbulent combustion and gaseous detonations have also been the topic of intense research due to their high thermodynamic efficiency in propulsion applications [46,47]. But they often exhibit strong deviations from equilibrium due to the wide range of length and time scales involved, making it necessary to rely on complex fluid dynamical analyses and numerical simulations with a high computational cost [48,49]. Despite the deep understanding provided by the latter approach, there are still cases in which a proper physical explanation can not be found based only on numerical results. ...

... being = 1/3 the recovery factor [Zhang et al., 2018], where is the Prandtl number. However, in hypersonic boundary layers, the recovery temperatures are so high that the wall temperature is usually lower [Urzay and Di Renzo, 2021], generating large heat fluxes to the wall. This affects the flow dynamics in concurrency with the Mach number, enriching the physical effects that have to be accounted for when developing theoretical relations and reduced order models. ...

... In so-called transcritical conditions, flows operate in virtually multi-phase conditions, i.e., there is coexistence of both gas-like and liquid-like states. However, unlike subcritical two-phase flow, single-component transcritical flows are always in the continuous regime, as a consequence of the small Knudsen numbers (Kn ≪ 1) resulting from small mean free paths at high pressures [2,3]. Trans-and supercritical fluids are relevant in many engineering applications, including internal-combustion and rocket engines, among others. ...

... The shock-induced separated flow can support various local and global flow instabilities (Robinet 2007;Egorov, Neiland & Shvedchenko 2011;Guiho, Alizard & Robinet 2016;Sansica, Sandham & Hu 2016;Bugeat et al. 2022) that may lead to unsteadiness and a laminar-turbulent transition. Experiments and direct numerical simulations (DNSs) with many canonical configurations (Chapman, Kuehn & Larson 1958;Ginoux 1960;Simeonides & Haase 1995;Benay et al. 2006;Willems, Gülhan & Steelant 2015;Roghelia et al. 2017;Currao et al. 2020;Fu et al. 2021) have shown that strong interactions can promote the laminar-turbulent transition near flow reattachment, which is frequently accompanied by streamwise streaks in the reattached boundary layer. These instabilities can also be understood by examining the spectrum of the linearised Navier-Stokes operator. ...

... The resulting system of ordinary differential equations is integrated in time using a third order strong stability preserving Runge-Kutta scheme [55]. The calculations presented in this work are carried out with the described numerical procedure in the Hypersonic Task-based Research (HTR) solver [56][57][58]. The HTR solver leverages the runtime Legion [59][60][61] and the programming language Regent [62] to execute the described calculations on high-performance supercomputers with heterogeneous architectures. ...

... Despite significant advances in the last 30 years from experiments and simulations, numerical and theoretical models of multiphase flows are still not suitable for predictive capabilities in many realistic applications. Current state-of-the-art point-particle Lagrangian models coupled with an Eulerian flow description can be used to simulate thousands to millions of particles at the expense of computational power [15], [16], [17], but these numbers are still far from practical real-life situations involving billions or trillions of particles [18], [19]. In addition, the fidelity of those simulations depends on the range of particle-fluid nondimensional parameters that dominate the two-way coupled interactions being modeled. ...

... Similar to the work described by Chan et al. (2018Chan et al. ( , 2019Chan et al. ( , 2021c, an ensemble of numerical simulations of breaking waves was generated to obtain the desired statistics. Details of the simulation parameters may be found in the aforementioned references. ...

... Improved particle statistics are observed with models that apply a deconvolution op-erator on the LES velocity field [24][25][26], but these models merely modify the velocity field on the LES grid and do not augment the range of modeled scales. Bassenne et al. [27] combine the dynamical deconvolution of Park et al. [26] with the subgrid extrapolation of Domaradzki and Loh [9] and obtain realistic particle clustering with LES of homogeneous isotropic turbulence (HIT) and for a wide range of Stokes numbers. The high computational cost, originating from a divergence-free projection on a very fine grid make, this model unsuitable for LES. ...