A direct numerical simulation (DNS) database for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow has been considered for this analysis. The nonreacting part of the channel flow is representative of the friction velocity based Reynolds number ${\text{Re}}_{\tau }=120$. A skeletal chemical mechanism with 9 chemical species and 20 reactions is employed for representing hydrogen-air combustion. In this work the flow configuration and the turbulence and flame characteristics are similar to those of Gruber et al. [J. Fluid Mech. 709, 516 (2012)]. The interaction between the flame structure and the turbulent flow has been investigated for boundary layer flashback for a comparison with the earlier work of Gruber et al. [J. Fluid Mech. 709, 516 (2012)]. The statistics of wall shear stress, turbulent kinetic energy, and its dissipation have been analyzed to probe the influence of the flame on the underlying turbulence in the channel flow configuration. Furthermore, the budgets for the individual terms in the turbulent kinetic energy transport equation have also been investigated at a given plane in the channel. It is found that the propagation of the flame into the upstream part of the fully developed turbulent boundary layer introduces a flow reversal in some regions upstream of the flame and these regions lead to negative wall shear stress. Interrogation of the DNS data for the budgets of the turbulent kinetic energy transport has revealed that the aforementioned local flow reversal regions have significant influences on the turbulent kinetic energy production, pressure dilatation, and pressure transport terms. It has been found that the flame propagation into the upstream reactants leads to some weak local compressibility effects as demonstrated by the changes in the pressure related terms in the turbulent kinetic energy transport equation. These results indicate that the pressure dilatation and turbulent transport due to pressure are the two dominant terms in the turbulent kinetic energy equation in the case of wall bounded flashback flames.

The energy cascade and diverse turbulence properties of active-grid-generated turbulence were studied in a wind tunnel via hot-wire anemometry. To this end, two active grid protocols were considered. The first protocol is the standard triple-random mode, where the grid motors are driven with random rotation rates and directions, which are changed randomly in time. This protocol has been extensively used due to its capacity to produce higher values of ${\text{Re}}_{\lambda }$ than its passive counterpart, with good statistical homogeneity and isotropy. The second protocol was a static or open grid mode, where all grid blades were completely open, yielding the minimum blockage attainable with our grid. Center-line streamwise profiles were measured for both protocols and several inlet velocities. It was found that the turbulent flow generated with the triple-random protocol evolved in the streamwise direction consistently with an energy dissipation scaling of the form $\varepsilon =C_{\varepsilon }{u}^{\prime 3}/L$, with $C_{\varepsilon }$ being a constant, $L$ the longitudinal integral length scale, and $u^{\prime }$ the rms of the longitudinal velocity fluctuations. Conversely, for the open-static grid mode, the energy dissipation followed a nonequilibrium turbulence scaling, namely, $C_{\varepsilon }\sim {\text{Re}}_G/{\text{Re}}_L$, where ${\text{Re}}_G$ is a global Reynolds number based on the inlet conditions of the flow and ${\text{Re}}_L$ is based on the local properties of the flow downstream the grid. Furthermore, this open-static grid mode scaling exhibits important differences with other grids, as the downstream location of the peak of turbulence intensity is a function of the inlet velocity; a remarkable observation that would allow one to study the underlying principles of the transition between equilibrium and nonequilibrium scalings, which are yet to be understood. It was also found that a rather simple theoretical model can predict the value of $C_{\varepsilon }$ based on the number density of zero crossings of the longitudinal velocity fluctuations. This theory is valid for both active grid operating protocols (and therefore two different energy cascades).

Electroconvection driven by unipolar charge injection in the presence of cross-flow between two parallel electrodes is investigated in a numerical study. The two-relaxation-time lattice Boltzmann method with a fast Poisson solver is used to resolve the spatiotemporal distribution of flow field, electric field, and charge density. Couette and Poiseuille cross-flows are applied to the solutions with established electroconvective vortices. Increasing cross-flow velocity deforms the vortices and eventually suppresses them when threshold values of velocities are reached. At intermediate flow velocities, partial suppression of the vortices leads to the reduction in electroconvection. This behavior is parameterized by a nondimensional parameter, $Y$—a ratio of the electrical forcing term to the viscous term in the Navier-Stokes equations. For high values of $Y$, the electric force dominates the flow, while for values below the critical threshold, the electric force influence is negligible and the flow is dominated by the shear.

We report the shock tube experiments on Richtmyer-Meshkov instability to evaluate the bubble merger effect in initial stages. The initial interface is specially designed by alternatively arranging the major and minor inverse-chevron shapes with different amplitudes and wavelengths such that the bubble merger effect is highlighted. The results show that the major and minor shapes develop independently at very early stages, and after a short transient the bubble merger occurs. The bubble merger promotes the width growth of the major shape while it inhibits the width growth of the minor one. However, the bubble merger has a greater effect on the development of the minor shape than the major one. As the initial size difference between two shapes increases, the bubble merger occurs earlier, and the minor shape is affected more heavily. The developments of bubble front difference seem to experience two different linear stages before it enters a nonlinear stage, and collapse well in dimensionless form for different cases. As a result, the initial size difference has a limited effect on the bubble merger in the streamwise direction. The ratio of the major bubble diameter in spanwise direction to the bubble width increases with time, and does not reach a stable phase because the flow is far from the turbulent mixing region.

Lifted Hele-Shaw cells typically display viscous fingering of liquids, which in turn leads to branched fractal patterns in the absence of any anisotropies. Recently, experiments involving parallely lifted Hele-Shaw cells with holes in the cell plates, also termed as “multiport lifted Hele-Shaw cells,” have been used to generate more regular meshlike patterns in the liquid film. Although such patterns promise usefulness in several applications, their spatiotemporal evolution needs to be theoretically understood for better synthesis. As a first step, therefore, we examine the stability of fingers evolving from a single hole by focusing on flow of an annular film of liquid placed in a lifted Hele-Shaw cell. We use linear stability analysis to find the growth rate of azimuthally periodic perturbations of the inner and outer interfaces around the evolving base state of the liquid film. To validate the results of our stability analysis, we also perform resolved numerical simulations of the setup via an in-house solver based on lubrication theory, which uses front-tracking method to evolve the interface in time and space. For a wide range of parameters and wave numbers, we find excellent agreement in the growth rates predicted by the linear stability analysis with the numerical simulation. The results of the stability analysis are expressed in terms of the capillary number, initial nondimensional plate separation, and initial ratio of the interface radii. Furthermore, using the results from our linear stability analysis, we generate a phase map to demarcate the flow regimes corresponding to unstable and stable states for the interfaces. Numerical simulations of the interface evolution over finite times are consistent with the results predicted by the proposed analysis. These finite-time simulations successfully capture the presence of shielding of the fingers at both the inner and outer interface. The proposed theoretical analysis and insights obtained through numerical simulations thus provide a framework for accurately predicting and experimentally realizing stable fluid patterns in a multiport Hele-Shaw cell.

Thermal convection in inland waters often occurs where the equation of state is highly nonlinear with temperature. We investigate the impact of this nonlinearity on the evolution of the Rayleigh-Taylor instability by analyzing the initial linear instability, the nonlinear plume growth, and the subsequent mixing resulting from this flow instability. The linear stability theory demonstrates that the thickness of the interface between the two layers of the Rayleigh-Taylor instability changes the wave number of maximum growth from the classical prediction. Our predicted wave number of maximum growth agrees well with two-dimensional direct numerical simulations of the diffused interface Rayleigh-Taylor instability. The nonlinear equation of state introduces asymmetry in the growing plumes about the density interface, preferentially generating kinetic energy in the lower layer. This asymmetry further introduces asymmetry in the location of the mixing. We analyze the energy evolution in the system and argue that the nonlinear equation of state will modify the distribution of heat in temperate lakes.

The effects of isothermal initial stratification on the dynamics of the vorticity for single-mode Rayleigh-Taylor instability (RTI) are examined using two-dimensional fully compressible wavelet-based direct numerical simulations. The simulations model low Atwood number ($A=0.04$) RTI development for four different initial stratification strengths, corresponding to Mach numbers from 0.3 (weakly stratified) to 1.2 (strongly stratified), and for three different Reynolds numbers, from 25 500 to 102 000. Here, the Mach number is based on the Atwood-independent gravity wave speed and characterizes the strength of the initial stratification. All simulations use adaptive wavelet-based mesh refinement to achieve very fine spatial resolutions at relatively low computational cost. For all stratifications, the RTI bubble and spike go through the exponential growth regime, followed by a slowing of the RTI evolution. For the weakest stratification, this slow-down is then followed by a reacceleration, while for stronger stratifications, the suppression of RTI growth continues. Bubble and spike asymmetries are observed for weak stratifications, with bubble and spike growth rates becoming increasingly similar as the stratification strength increases. For the range of cases studied, there is relatively little effect of Reynolds number on bubble and spike heights, although the formation of secondary vortices becomes more pronounced as Reynolds number increases. The underlying dynamics are analyzed in detail through an examination of the vorticity transport equation, revealing that incompressible baroclinicity drives RTI growth for small and moderate stratifications but increasingly leads to the suppression of vorticity production and RTI growth for stronger stratifications. These variations in baroclinicity are used to explain the suppression of RTI growth for strong stratifications, as well as the anomalous asymmetry in bubble and spike growth rates for weak stratifications.

Understanding the force on a finite-sized particle immersed in a turbulent boundary layer close to a rough wall is of fundamental importance in predicting particle motion in a variety of industrial and environmental applications. Here we perform fully resolved direct numerical simulations with an immersed boundary method to investigate the forces on a stationary finite-sized particle in wall turbulence over a rough bed. Results show that for the particle sitting on the rough bed, lift is the main contributor to wall-normal force and can be well predicted with proper application of existing force models. The higher lift force is mainly due to the sweep events which temporarily increase the relative velocity as well as local shear magnitude. In contrast, for a particle located slightly away from the rough bed, the wall-normal force is mainly due to the wall-normal component of drag, so ejections and outward interactions are responsible for the increase in the wall-normal force. The standard drag law with near-wall correction can reasonably predict this wall-normal force on the particle, except for the large fluctuations due to self-induced vortex shedding.

The effect of ambient stratification on flow past a prolate spheroid is investigated using large-eddy simulation. The aspect ratio of the body is $L/D=4$ and the major axis $\left(L\right)$ is aligned with the incoming flow. The Reynolds number based on the minor axis $\left(D\right)$ and the incoming velocity $\left(U\right)$ is $\text{Re}={10}^4$. The stratification is set using a linear density background with constant buoyancy frequency $\left(N\right)$. Three simulations with stratification levels, $\text{Fr}=U/ND=0.5,1,3$ and one unstratified simulation, $\text{Fr}=\infty$, are performed. The influence of the body slenderness ratio is assessed by comparing the present results with previous work on a sphere. An overall result is that the flow past a slender body exhibits stronger buoyancy effects relative to a bluff body. At $\text{Fr}\sim O\left(1\right)$, there is a strong interaction between the body-generated internal gravity waves (IGW) and the flow at the body. This interaction gives rise to a critical Froude number, ${\text{Fr}}_c=L/D\pi$, which is proportional to the aspect ratio. When $\text{Fr}\approx {\text{Fr}}_c$ boundary layer separation is delayed and wake turbulence is strongly suppressed. If $\text{Fr}⪆{\text{Fr}}_c$, wake turbulence is only partially suppressed and, when $\text{Fr}⪅{\text{Fr}}_c$, boundary layer separation is promoted and turbulence reappears. To elucidate the effect of body slenderness on stratification effects, the IGW field, the boundary layer evolution and separation are studied. Analysis of the wake reveals the strong influence of the type of separation, and therefore the body shape, on the dimensions and the energetics of the near and intermediate wake. However, the decay rate of the stratified wake in the nonequilibrium region is the same as that of a sphere.

The closure of a human lung airway is modeled as an instability of a two-phase flow in a pipe coated internally with a Newtonian liquid. For a thick enough coating, the Plateau-Rayleigh instability creates a liquid plug which blocks the airway, halting distal gas exchange. Owing to a bifrontal plug growth, this airway closure flow induces high stress levels on the wall, which is the location of airway epithelial cells. A parametric numerical study is carried out simulating relevant conditions for human lungs, in either ordinary or pathological situations. Our simulations can represent the physical process from pre- to postcoalescence phases. Previous studies have been limited to precoalescence only. The topological change during coalescence induces a high level of stress and stress gradients on the epithelial cells, which are large enough to damage them, causing sublethal or lethal responses. We find that postcoalescence wall stresses can be in the range of 300% to 600% greater than precoalescence values and so introduce an important source of mechanical perturbation to the cells.

The flow stability of a dielectric fluid confined in the cylindrical annulus is investigated by linear stability analysis and direct numerical simulations. The annulus is in solid-body rotation about its axis and it is subject to a radial temperature gradient and a high-frequency electric tension, which produce a radial dielectrophoretic force. We analyze the effects of the rotation of the annulus on the thermoelectric convection and of the alternating electric tension on the onset of the rotation-induced thermal convection. Critical states are predicted by linear stability analysis for different values of the rotation rate of the cylindrical annulus and of the electric tension. Direct numerical simulations allow for the validation of the critical states, for the determination of their bifurcation nature, and for the visualization of instantaneous velocity and temperature fields.

Relating incipient motion of sediments to properties of turbulent flows continues to draw significant research attention given its relevance to a plethora of applications in ecology, sedimentary geology, geomorphology, and civil engineering. Upon combining several data sources, an empirical diagram between a densimetric Froude number $F_{dc}=U_c/\sqrt{gh\mathrm{\Delta }}$ and relative roughness $N=d/h$ was recently reported over some six decades of $N$, where $d$ is the grain diameter, $h$ is the overlying boundary-layer depth, $U_c$ is the bulk velocity at which sediment motion is initiated, $g$ is the gravitational acceleration, $\mathrm{\Delta }=s-1$, and $s$ is the specific gravity of sediments. This diagram featured three approximate scaling laws of the form $F_{dc}\sim N^{-\alpha }$ with $\alpha =1/2$ at small $N, \alpha =1/6$ at intermediate $N$, and $\alpha =0$ at large $N$. The individual $\alpha$ values were piecewisely recovered using a combination of (1) scaling arguments linking bulk to local flow variables above the sediment bed and (2) assumed exponents $\sigma$ for the turbulent kinetic energy spectrum $E_{tke}\left(k\right)\sim k^{-\sigma }$, where $k$ is the wave number or inverse eddy size. To explain the $\alpha =1/2$, the aforementioned derivation further assumed the presence of an inverse cascade in $E_{tke}\left(k\right)$ at large wave number (i.e., $\sigma =3$). It is shown here that a single $F_{dc}-N$ curve can be derived using a cospectral budget (CSB) model formulated just above the sediment bed. For any $k$, the proposed CSB model includes two primary mechanisms: (1) a turbulent stress generation formed by the mean velocity gradient and the spectrum of the vertical velocity $E_{ww}\left(k\right)$ and (2) a destruction term formed by pressure-velocity interactions. Hence, a departure from prior work is that the proposed CSB model is driven by a multiscaled $E_{ww}\left(k\right)$ instead of $E_{tke}\left(k\right)$ characterized by a single exponent. Also, the CSB model does not require the presence of an inverse cascade to recover an $\alpha =1/2$. Last, the CSB approach makes it clear that the scaling parameters linking local to bulk flow variables used in prior determinations of $\alpha$ at various $N$ must be revised to account for bed roughness effects.

We study the effects of connectivity enhancement due to film flow phenomena on the drainage of an artificial porous medium and the relative influence of gravity for such effects. The medium is initially fully saturated with a liquid (wetting phase), which is displaced by air (nonwetting phase). Our setup allows us to directly visualize the dynamics of the flow and, in particular, to pinpoint which pore invasion events are due to film flow phenomena. We have observed the formation of an active zone behind the liquid-air interface, inside which film flow drainage events are more likely to occur. Understanding the basic mechanisms of film flow in artificial porous media is of relevance for the analysis of real three-dimensional porous systems, in which gravity cannot be neglected and film flow is bound to be present.

Large-amplitude oscillations of foils have been observed to yield greater propulsive efficiency than small-amplitude oscillations. Using scaling relations and experiments on foils with peak-to-peak trailing edge amplitudes of up to two chord lengths, we explain why this is so. In the process, we reveal the importance of drag, specifically how it can significantly reduce the efficiency, and how this effect depends on amplitude. The scaling relations and experimental data also reveal a fundamental tradeoff between high thrust and high efficiency, where the drag also plays a crucial role.

The current work presents a computationally tractable simulation methodology combining different model-reduction techniques for resolving non-Boltzmann thermodynamics, chemical kinetics, and radiative transfer in complex three-dimensional flows. The empiricism associated with conventional multitemperature nonequilibrium models is abandoned in favor of the multigroup maximum entropy method combined with kinetics-informed adaptive binning. This group-based approach allows for time-varying optimal reduced-order representation of the internal state population distribution while accounting for all collisional processes included in the state-to-state model. Similarly, radiative transfer calculations are performed efficiently by discretization in the spectral, angular, and spatial spaces using the smeared band, discrete ordinate, and finite-volume methods, respectively. This reduction in computational overhead allows truly non-Boltzmann simulations with two-way coupling between the flow and radiation fields to be realized without simplifying approximations based on the tangent-slab method or local escape-factors. The simulation procedure is used in conjunction with the US3D flow solver to investigate the impact of vibrational nonequilibrium on ${\mathrm{CO}}_2$ wake flows and resultant infrared radiation around the Mars 2020 vehicle. This involves comparing predictions for flow-field properties and radiative transfer obtained using the conventional two-temperature model, the multigroup non-Boltzmann model, and for decoupled/coupled flow-radiation calculations. Conventional two-temperature models overestimate the rate of thermal equilibration in the near-wake region resulting in the population of midlying and upper ${\mathrm{CO}}_2$ vibrational levels being underpredicted by multiple orders of magnitude. Additionally, the two-temperature approach (in comparison to bin-based StS) overpredicts the rate of ${\mathrm{CO}}_2$ dissociation thereby leading to erroneous estimates for flow properties in the postshock region (primary source of afterbody radiative emission). This results in inflated values for surface radiative heat flux with two-temperature modeling, although overall differences in radiative behavior are tempered by the fast characteristic relaxation times for ground vibrational levels.

We consider the long-wave Marangoni instability in a heated liquid layer covered by insoluble surfactant. The system of nonlinear equations derived in our previous work is regularized in the limit of strong surface tension. Recent research shows that, without the surfactant, a large-scale oscillatory instability mode exists in the interval of wave numbers $k=O\left({\text{Bi}}^{1/2}\right)$ (the Biot number $\text{Bi}\ll 1)$. Here we study the influence of the surfactant on the Marangoni oscillations. The bifurcation analysis for traveling waves and counterpropagating waves is performed. The types of bifurcation and selected pattern depend on the elasticity number and on the Biot number. Specifically, at small elasticity number, both types of waves are supercritical.

Transition processes in supersonic Batchelor vortices with small disturbances are investigated numerically at Mach numbers 1.5–5.0. The Batchelor vortices for isentropic and uniform stagnation-temperature vortices are considered in terms of thermodynamic properties. In the evolutions, the former vortices are not influenced by compressibility effects; however, the latter have demonstrated slow growth as the Mach number increases. Compressibility effects primarily mean that fluctuation growth related to turbulence is suppressed as the Mach number increases. In spatial fluctuation energy production, isentropic vortices exhibit only kinetic energy fluctuations and uniform stagnation-temperature vortices cause large entropy fluctuation energy with increasing the Mach number. When the Mach number is large, the kinetic fluctuation energy is greatly decreased and the increase in entropy fluctuations is associated with reduced kinetic fluctuation energy. In addition, it is found that the entropy fluctuation property at uniform stagnation temperatures is consistent with the expression derived from the kinetic fluctuation energy based on Morkovin's hypothesis. The present results support the notion that the coefficient of variation of density is appropriate to evaluate compressibility effects. Entropy fluctuation generalizes fluctuation of thermodynamic quantity such as density fluctuation. Therefore, it follows that the property of entropy is strongly related to intrinsic compressibility.

Turbulent channel flows altered by the combination of flush-mounted spinning rings and vertical-velocity opposition control or hydrophobic surfaces are studied through direct numerical simulations. The two types of distributed control are applied over the surface area that is not occupied by the spinning rings. The turbulent mean skin friction is reduced by 20% through the steady rotation of the rings and the effect is enhanced by the distributed controls, reaching a drag reduction of 27%. The numerically computed combined drag reduction is well predicted by an upper bound obtained by a simple idealized model. The turbulence statistics are highly nonuniform along the spanwise direction, but show a weak dependence along the streamwise direction. The wall-shear stress is highly reduced over the central region of the rings. Narrow streamwise-elongated structures forming between adjacent disks offer a detrimental global contribution to the Reynolds stresses, although locally they reduce drag. A spatially dependent form of the Fukagata-Iwamoto-Kasagi identity helps explain the different influence of the two distributed controls, although the global drag-reduction levels are similar. The opposition control is effective in altering the elongated structures between rings, but it does not contribute to enhance the drag reduction in the central-ring region. Hydrophobicity creates a more nonuniform flow and even enhances the intensity of the streamwise structures between rings, but further reduces the wall-shear stress in the central-ring region compared to the ring-only case. The mean wall-slip velocity is the additional beneficial effect offered by the hydrophobic surface between rings.

The passage of a mode-2 internal solitary wave (ISW) over a broad, isolated ridge was explored using both numerical simulations and laboratory experiments. At sufficient incident wave amplitude and speed, the interaction with the ridge caused a deceleration of the incident wave while also generating three wave types: leading mode-1 ISWs, a trailing mode-1 wave-packet, and a trailing mode-2 ISW. The trailing mode-2 ISW was formed as the second piece of the fissioned incident wave. While mode-mode interactions are not part of typically applied weakly nonlinear theories, the fissioning of the leading mode-2 ISW into two mode-2 waves is predicted qualitatively by weakly-nonlinear theory. However, the present study is, to our knowledge, the first experimental and numerical confirmation of the detailed evolution of this phenomenon. In contrast to the previously studied transit of a mode-2 ISW onto a shelf, the parameters quantifying the strength of each resultant wave are found to be independent, having no singular predictor existing for all waves. Lastly, cross-boundary layer transport was found to be more strongly dependent on Reynolds number than on incident wave or background conditions.

Due to large-scale flow inhomogeneities and the effects of temperature, turbulence small-scale structure in thermal convection is still an active field of investigation, especially considering sophisticated Lagrangian statistics. Here we experimentally study Lagrangian pair dispersion (one of the canonical problems of Lagrangian turbulence) in a Rayleigh-Bénard convection cell. A sufficiently high temperature difference is imposed on a horizontal layer of fluid to observe a turbulent flow. We perform Lagrangian tracking of submillimeter-sized particles on a large measurement volume including part of the large-scale circulation (LSC), revealing some large inhomogeneities. Our study brings to light several insights regarding our understanding of turbulent thermal convection: (i) by decomposing particle Lagrangian dynamics into the LSC contribution and the turbulent fluctuations, we highlight the relative impact of both contributions on pair dispersion; (ii) using the same decomposition, we estimate the Eulerian second-order velocity structure functions from pair statistics and show that after removing the LSC contribution, the remaining statistics recover usual homogeneous and isotropic behaviors which are governed by a local energy dissipation rate to be distinguished from the global dissipation rate classically used to characterize turbulence in thermal convection; and (iii) we revisit the superdiffusive Richardson-Obukhov regime of particle dispersion and propose a refined estimate of the Richardson constant.