Entrainment is the process by which ambient fluid is incorporated into the flow and remains poorly understood. In cumulus clouds, entrainment governs the altitude attained by the cloud and the resulting droplet size distribution, which together determine the radiative contribution to the global energy balance. The release of latent heat by the condensation of water vapor drives the flow in cumulus clouds, which often resemble a series of isolated parcels of buoyancy, or thermals. Here, using direct numerical simulation (DNS), we study the effects of condensation heating on the entrainment in a moist thermal in contrast with a dry thermal that has no buoyancy sources.

We present direct quantification of the scale-dependent eddy-diffusivity for homogeneous isotropic turbulence. This analysis has implications in closure modeling of transport equations involving turbulence. The computational methodology is based on the previously developed macroscopic forcing method which uses macroscopic forcing in transport equations and collects the field response. Eddy diffusivity is then obtained from statistics of the forced system after mapping the response field to the average space. In the limit of large scales the eddy diffusivity is consistent with the Boussinesq approximation; however, it deviates proportional to the scale in the limit of small scales.

Cilia and flagella are used throughout the natural world to facilitate microscale fluid motion. These active structures often appear in groups and their motion is coordinated. This paper explores the synchronized states of a pair of hydrodynamically coupled filaments and characterizes in detail the recently discovered bistability of two states. This study identifies the unstable edge state that exists between the two basins of attraction and shows how the bifurcations exhibited by the filament system can be recovered using an extension of Adler’s equation for coupled oscillators.

Multicellular magnetotactic bacteria (MMB) – the only known obligately multicellular species of bacteria – use the geomagnetic field to navigate the pore space of water-saturated sediment. To investigate how MMB navigate around obstructions, we orient a magnetic field to direct thousands of MMB toward the hard surface of a microfluidic chamber. MMB in a sub-millitesla field distribute exponentially about the surface as an active gas with a penetration length $\lambda$ proportional to the distance a colony swims before aligning with the applied field. At a critical magnetic field $\lambda$ shrinks to the size of a single MMB colony and the active gas condenses into an active two-dimensional fluid.

Understanding how time-dependent flows affect the propagation of a flame is of prime importance for engineering applications. We experimentally study the case of a quasi-two-dimensional flame propagating in a flow that varies periodically in time. The flow acts as a parametric forcing and induces parametric restabilization and parametric destabilization of the flame front. The comparison of our experimental measurements of these destabilization and restabilization thresholds with a low frequency theory indicates how the flame response varies with the time scale associated with the dynamics of the flow.

In this experimental work we analyze various turbulent premixed flames under the effects (or lack thereof) of the Darrieus-Landau hydrodynamic instability. In particular, the effects of an increasing level of turbulence are considered by means of different observables. On the one hand we obtain results which can improve the understanding of fundamental mechanisms ruling the interaction between flame fronts, either unstable and stable, and turbulence. On the other hand, conclusions we draw may lead to new definitions or improvement of closure models for numerical simulation of turbulent premixed flames under the effects of hydrodynamic instability.

Mucus is a complex biological fluid covering the human respiratory tract and protecting the bronchial epithelium from fine particles and pathogens. In pathological conditions, it typically tends to dehydrate, becoming viscous, sticky and difficult to evacuate by natural mechanisms. Various devices to aid in the clearance of airways based on the use of pressure waves are being developed, and in that context there is a strong need to better understand the interactions between mucus and oscillating pressure waves. This paper investigates the interactions between a yield stress shear-thinning/thickening fluid and different types of pressure waves in a two-dimensional channel.

Fingering double diffusive convection plays an important role in the vertical mixing of the ocean, and is inevitably affected by background shear. Here we show that with a very weak shear salt fingers become horizontally well-organized, and the salinity flux can be enhanced. While for strong shear salt fingers are replaced by sheet-like structures and fluxes are suppressed compared to the cases without shear.

The performance of Phase Change Material (PCM) devices in microgravity can be significantly improved by thermocapillary convection. However, the melting process in this case is affected by a series of instabilities and mode transitions due to the evolving size and shape of the liquid domain. We perform a numerical investigation of pattern selection for thermocapillary flow in ideal rectangular containers of liquid n-octadecane in microgravity and show how this can be applied to explain the more complex dynamics of melting PCMs. In particular, the locations of travelling and standing wave instabilities predicted in this way show very good agreement with numerical simulations of PCM melting.

The effect of interfacial disturbances on instabilities of buoyant/thermocapillary convective flows in rectangular cavities is studied in a series of numerical experiments. The relation between interface deformations and the Boussinesq approximation is discussed. It is shown that including interface disturbances in the model can alter the critical temperature difference by approximately 10%, producing either a destabilizing, or a stabilizing effect.

The interaction between turbulence and deformable interfaces occurs in many environmental phenomena and industrial applications, from bubble generation in breaking waves, to combustion and atomization processes. We use direct numerical simulations to unveil the complex interaction among turbulence, interfaces, and bubble properties, namely density, viscosity, and surface tension. We observe that while density differences have a negligible effect on breakage and coalescence of bubbles, viscosity differences play a key role in the interactions among bubbles and between bubbles and turbulence.

Magnetic manipulation of a deionized (DI) water droplet on a slippery liquid-infused porous surface (SLIPS) is realized by a single magnet. The ultra-low friction nature of SLIPS enables a small magnetic reprelling force to actuate the droplet forward successfully at a velocity of 1 mm/s. In addition, this method exhibits great potential for transporting low-surface-tension fluids such as sodium dodecyl sulfate (SDS) droplets, bubbles, and magnetic nanofluids, which offer potential benefits for the design and applications of magnetically actuated droplets in droplet microfluidics.

The effect of electric field on the flow of viscoelastic fluids has been investigated and the bifurcation and flow pattern transition processes have been mainly studied through numerical simulation considering a coaxial cylinder model. Different bifurcations have been found with transition among them and accordingly critical criteria of bifurcation also change. Plus, among these bifurcation paths, the swinging, rotating, splitting, and vanishing behaviors of charge void regions which are quite different from those of Newtonian fluids, have been observed.

The hypersonic boundary-layer transition is affected crucially by surface imperfections such as heating or cooling sources. In order to quantify this effect we develop an asymptotic theory, which not only reveals the interaction mechanism between oncoming instability modes and surface heating/cooling strips, but also provides quantitatively the change of the transition onset due to the scattering effect. The asymptotic predictions agree favorably with harmonic linearized Navier-Stokes calculations and direct numerical simulations.

Shallow layer equations are variously employed to describe flows of turbulent water, grains, mixed suspensions, mud and other media by selecting an appropriate model for basal drag. In each case, the equations admit steady ‘monoclinal’ traveling wave solutions that connect uniform flows of different depths. We conduct a ‘model-agnostic’ linear stability analysis of these solutions and their associated modes. By adopting a generalized perspective, the fundamental reasons for differences in properties between particular systems may be teased out.

A controlling protocol for the traditional viscous fingering instability is presented. By coupling time-dependent injection rates with time-varying electric currents, it is shown via boundary integral simulations that the strategy allows control of the self-similar regime (delay, promote, or suppress) and the relative finger instability size without altering the system’s physical parameters.

High skin friction and thermal loads are inevitable consequences of turbulent flows in supersonic flights. Here, the use of boundary-layer streaks is studied by direct numerical simulations in order to prevent/delay turbulence from occuring at M_∞=2.0. The stabilizing/destabilizing effects (y/δ_in=0.517 in the image) of the streaks under the influence of various thermal boundary conditions are examined and the effective amplitude range in delaying transition is revealed.

We study the coupled hydrodynamics between a force- and torque-free slender microswimmer and an interface with a deformation that is influenced by both surface tension and bending elasticity. For an interface separating two fluids with arbitrary viscosities, the role of the swimmer orientation, the fluid and interface properties on the swimmer migration is examined. The nature of the short-time swimmer migration depends crucially on its orientation and the fluid viscosity ratio. When a swimmer is in the more viscous fluid and parallel or perpendicular to the interface, the short-time swimmer migration is opposite to that of the long-time migration.

The coating of rotating objects with varying surface curvature is an important manufacturing step for a wide variety of products. We develop a lubrication-theory-based model for flow on two-dimensional cross sections of rotating noncircular cylinders with variations in the radius of curvature comparable to the characteristic cylinder radius. The model provides more accurate solutions than prior models which assumed small cylinder curvature variations. Because our model is more computationally efficient than full two-dimensional simulations, it is used to perform a parametric study for rotating elliptical cylinders, leading to the identification of four distinct regimes of coating behavior.

Lubricated surfaces are formed by coating surfaces with a thin layer of oil lubricant. We consider a model problem to understand how lubricated surfaces respond to the deposition of a droplet, which induces forces on the lubricant via its Laplace pressure and the triple line (the Neumann balance). This forcing leads to the formation of a meniscus around the droplet, or ‘skirt’. We show that the properties of the skirt depends on the amount of lubricant available in the lubricant reservoir and also that the the evolution towards the equilibrium is extremely slow.

The capability of droplet absorption into a funnel-like capillary tube is enormously affected by the combined influence of the droplet size, the opening angle, and the intrinsic wettability. Through a quantitative investigation of the penetration criteria, we demonstrate that a finite-volume droplet can be totally absorbed by a hydrophobic funnel-like structure depending on the opening angle. This result is sharply contrary to the previous conclusion for infinite-volume droplets in straight capillary tubes and our predictions extend the limit of the maximum contact angle for the complete penetration of droplets into capillary tubes in the literature.

Transient natural convection boundary layer flow is studied at Pr>1 and the curvature effect is specifically and fundamentally explored. Important scale laws, i.e. boundary layer thickness δt and characteristic velocity $u_z$ of the transient and steady states and the cut-off time $t_s$ of the initial growth, are proposed and validated. We find that for cylinder radius much larger than the boundary layer thickness, the present scaling relations reduce to those of the classic flat boundary layer. The most curved boundary layer we examine is 26 times thicker than the cylinder radius. We show that the present scaling set is accurate for all flow conditions bounded by the two limiting scenarios.

We investigate hydrodynamic interactions between droplets and the interface of two co-flowing immiscible streams in a microchannel. We find that small droplets of confinement ratio (drop size to suspending stream width), $\beta <0.5$ migrate towards the co-flow interface while larger droplets of $0.5<\beta <1$ unexpectedly drift away from the interface. Large droplets with $\beta >1$ cause deformation waves in an initially stable and flat interface that propagate downstream akin to traveling peristaltic waves. The deformation wave amplitude associated with a droplet grows spatially downstream but remains the same for a train of droplets crossing a fixed location, indicating convective instability.

In a simple silo flow, the presence of an obstacle has been found to suppress clogging, but little systematic work has been done to test how general this is. By systematically varying the size and position of an obstacle, we find that generally an obstacle does suppress clogging, but in some cases may enhance clogging. By examining micro/mesoscale features of the flow we find that, even when comparing scenarios where clogging is suppressed, the reasons for the suppression may be different – the intruder introduces both geometric and dynamic effects which compete differently.

Simulations link nanoscale particle properties to bulk rheology via microscopic stress-activated constraints. The framework shows quantitative agreement with experimental rheology data.

We explore a model for compaction of a two-phase medium in which the solid is allowed to deform elasto-viscoplastically. Previous models for compression of colloidal and cellulose fiber suspensions often prescribe the solid stress as a material function of the local solid concentration. Our current model distinguishes between purely elastic or plastic deformation and allows a solid viscous response. This is crucial for model comparison to experiment where a dramatic improvement over inelastic constitutive models is demonstrated.

Intrusion into fluid-grain mixtures can lead to very large stresses and surprising solid-like behavior. One important mechanism, captured by Darcy-Reynolds theory, involves a feedback effect between the expansion of the granular material (Reynolds dilation) and the flow of fluid through the pores between grains (Darcy flow). We build on prior work to demonstrate the effectiveness of Darcy-Reynolds theory in capturing the forces over a wide range of physical parameters. We also demonstrate a breakdown of the theoretical predictions for very large viscosity fluids, and we suggest a hypothesis for this deviation.

In this work, we realize the mathematically predicted universal mechanism of the local emergence of Peregrine solitons in water tank experiments, with a particular aim to control the point of the soliton occurrence in space-time by employing the inverse scattering transform for the synthesis of the initial data. Using this approach, we are able to engineer a localized wave packet with a prescribed solitonic and radiative content, evolving in a rogue wave at a predicted position from the wave maker.

Turbulent flows generate large skin friction over surfaces, dissipating significant energy and causing a significant waste of fuel. Various skin-friction control techniques have been proposed in the past, among which transverse wall oscillation (TWO) has been proven one of the most effective approaches. We examine the key modifications induced by the TWO technique in the turbulent flow field with the aim of elucidating the physical mechanism responsible for mitigating skin friction. We find that traveling-wave TWO significantly damps turbulence production and coherent structures, resulting in a decrease in turbulence intensity and hence skin friction.

We investigate the joint dynamics of the near-wake velocity and the base pressure of a square back Ahmed body. We identify two modes that are responsible for most of the pressure drag variations. We show that the turbulent large-scale velocity field in the wake can be recovered from base pressure measurements.

Spanwise modification of the Reynolds shear stress (RSS) of a turbulent boundary layer modified by miniature vortex generators (MVGs) is investigated. The methodology is based on quadrant analysis of RSS to first identify the spatial and temporal information of the RSS. We then apply the Fourier mode decomposition and triple velocity decomposition to obtain various statistics of the decomposed RSS. In addition, the interactions between small-scale and large-scale RSS are further investigated and we showed that the large scales tend to strengthen due to the inverse energy transfer of small scales.

Wavelet-based adaptive large-eddy simulation uses wavelet filtering to separate resolved from residual (more from less energetic) turbulent eddies, overcoming limitations of traditional lowpass filter-based methods that are not able to investigate the multi-resolution features and intermittency of turbulence. This is further developed by introducing a novel localized dynamic kinetic-energy-based model for wall-bounded turbulent compressible flows. Application to supersonic channel flow demonstrates the efficiency and accuracy of the method that is characterized by a high dynamic spatial-temporal adaptation, paving the way for modeling and simulation of complex engineering turbulent flows.

Turbulence in gases and liquids manifests in the form of spontaneously developing chaotic dynamics in a laminar flow without any measurable external disturbances. For gas flows, we propose that turbulence is created by the average effect of an intermolecular potential, leading to a system of equations with novel coupling between density and momentum. Numerical simulations show that turbulent dynamics in this system emerges spontaneously in the absence of external disturbances, in agreement with observations. In addition, the time-averaged kinetic energy spectra of our numerical solutions decay at the Kolmogorov rate, which is also an intriguing feature of observed turbulent flows.

For oscillating grid turbulence (OGT) in homogeneous fluids, the turbulence intensity decreases with the distance to the vibrating grid used to generate the forcing, and it is usually described in terms of depth profiles of turbulent quantities (kinematic energy, integral length scale, and eddy viscosity) . Here we propose revised parametric laws for these quantities in homogeneous fluid, and present new results for a two-layer fluid for which the turbulent quantities are strongly altered by the fluid interface. In the case of a two-layer fluid, we also investigate the mixing and discuss the role of turbulent flow properties at the interface as well as weak mean flow features.

A spectral decomposition method is proposed to segregate the contributions of wall-attached/wall-detached and self-similar/non-self-similar eddies by taking advantage of the proper orthogonal decomposition. It is applied to analyze the compressible and incompressible turbulent channel flows to verify the Reynolds number and compressibility effects on velocity fluctuations and investigate whether the temperature fluctuations share similar features. The statistical properties of eddies of each category have been discussed in detail.

We investigate the planar flow past a circular cylinder for Reynolds numbers between 1000 and 10,000. The flow is studied as a dynamical system and the force-time signal is considered when the system goes from the periodic to the chaotic regime. Two-dimensional numerical simulations were performed with a Vortex Particle Method called Diffused Vortex Hydrodynamics (DVH). This computational approach allows high spatial resolution with an accurate description of different vortical scales shed in the flow field. During the transition of the system from a regular to a chaotic regime, the lift time signal shows intermittent irregular patterns.

Understanding path instability mechanisms of freely falling annular disks is a complex fluid-structure interaction problem. It is well known that disk size, inertia, and surrounding flow properties play significant roles in governing trajectories and wake structures. These factors control the solid to fluid relative intensity, which in turn gives rise to path and wake instability. We present a numerical investigation of factors influencing the free-fall of annular disks. Combining kinetic and dynamic characteristics, three dimensionless parameters are taken into account: the inner and outer diameter ratio, the dimensionless moment of inertia, and the body/fluid density ratio.

This paper proposes a hierarchical clustering method which can analyze the behavior of vortices in detail by dividing volumetric vortical regions obtained by a direct numerical simulation into individual groups of vortical points and automatically tracking them. In addition to visualizing and tracking the instantaneous spatial distribution and temporal motion of each point set of vortices linked by a data hierarchy (inclusivity), which is called “p-clusters” in a unified manner, it is now possible to mechanically search for common local dynamics among many different vortical structures found in transitional and turbulent flows.

Tsunamis generated by landslides represent a threat which has long been assessed through the maximum amplitude of the leading wave. To predict this amplitude, the latter must be related to the initial conditions of landslides. This is achieved by modeling the core physics of wave generation by a viscous collapse, founded upon a systematic study of the influence of the slide inertia and dissipation on the wave formation in three-phase numerical simulations.

A fully coupled numerical model is developed to study wave interactions with perforated elastic disks. The flow past the perforated surface is represented by a quadratic pressure discharge condition with practical validity. The nonlinear nature of the pressure drop condition results in a dependency of hydrodynamic responses on wave steepness.