Turbulence deep within the interior of stars bears many qualitative similarities with turbulence in the ocean and atmosphere on Earth. In both contexts, various forms of convective instabilities and shear instabilities govern the transport of heat, momentum (angular momentum) and chemical species. However, an important distinction between stellar fluids and geophysical fluids is the value of the Prandtl number, which is of order unity in oceanic and atmospheric flows, and asymptotically small in stars. As a result, many well-known scaling laws for turbulent transport in geophysical flows must be revisited at low Prandtl number, as reviewed in this paper.

The authors study how pathogens can be exposed to damaging hydrodynamic stressors during the aerosolization process.

Propulsive performance and acoustic emission of undulatory swimmers are investigated using an integrated unsteady potential flow and acoustic boundary element solver. Anguilliform and carangiform swimming gaits are modeled by a deforming NACA 0012 airfoil section. The effect of a ground plane on undulatory swimming and associated noise generation is examined as a function of reduced frequency and above plane altitude with the method of images. Ground effect becomes pronounced when swimming within half of a chord length from the ground plane, where the thrust, power, propulsive efficiency, and peak acoustic pressure all increase for both gaits.

Using the discrete vortex method we perform numerical simulations of the Anguilliform mode of swimming dynamics of a one-dimensional flexible filament. Various parameters are varied to quantify the coefficient of thrust and swimming efficiency. We show that wake vortices are in a Benard–von Karman (BvK) configuration in the drag producing regime and rearrange to reverse-BVK (rBvK) when thrust producing. The resultant wake vortex distribution contour map and associated velocity field clarify differences between BvK in drag regime, the transition region, and rBvK in thrust regime. Optimal parameters for high thrust and swimming efficiency of two-dimensional flexible filaments are identified.

Catalytic effects on a copper sphere impacted by a hypersonic flow at very high enthalpy and very low pressure conditions have been investigated by using both a classical multi-temperature model and a state-to-state approach considering three different recombination statistics of molecules: one reproducing the incoming distribution, one considering a uniform distribution, and the third populating only the highest vibrational level. Comparisons with experimental findings strongly suggest that, at the investigated flow and wall conditions, the use of a partial catalysis model is mandatory.

Dense suspensions can behave like a fluid or a solid, in which a running person can stay afloat on top of it, while a walking person sinks. A free-falling impactor into a dense suspension is simulated using a particle-based simulation that incorporates the free-surface of the suspension. It is found that the shear stress of the suspension is not affected by the impact, and persistent homology also elucidates a distinct topological structure of the force chains. These findings distinguish the impact-induced hardening from the shear-induced phenomena, such as shear thickening and shear jamming.

Significant shear localization, observed as substantial concentration gradients, is discovered in large amplitude oscillatory shear (LAOS) simulations of a complex fluid in a planar Couette setup. The localization is demonstrated to occur due to the inertial effects imposed by the oscillatory shear in LAOS, which serve as perturbations in the nonlinear equations describing the structural response of the fluid to shear. Additionally, a criterion for the onset of this shear localization is presented.

Coiling of a fluid filament is observed when the fluid viscosity is sufficiently large. The coiling frequency depends on several parameters (shown in the image) but most notably on viscosity. In this study we found that viscoelastic fluid filaments coil at a much smaller frequency than that of Newtonian fluids, under equivalent conditions. The reduction results from the increased value of the extensional viscosity typically observed for these liquids.

The accumulation of stresses as polymeric chains cross successive pores creates streaks of high polymeric stress. Highly stretched polymeric chains in the regions of high polymeric stress resist the flow crossing these streaks, leading to eddy formation in the different regions of the pores. Multiple distinct unstable flow structures occur inside the pore above a critical Weissenberg number.

Using recently developed boundary-layer equations for turbulent Rayleigh-Bénard convection that take into account fluctuations and buoyancy, we derive analytical results for heat flux scaling dependencies in low- and high-Prandtl-number limits. Our theoretical results are supported by direct numerical simulation data, and further reveal a surprising close resemblance of the heat flux scaling dependencies in turbulent Rayleigh-Bénard and steady forced convection. This finding resolves the paradox of the apparent applicability of the scaling results in steady forced convection even though the boundary layer theory in that case is not applicable to turbulent Rayleigh-Bénard convection.

Using long-time direct numerical simulations, we analyze reversals of the large scale zonal flow in two-dimensional Rayleigh-Bénard convection with a rectangular geometry. These reversals appear and disappear as bifurcations on a turbulent background flow.

Surfactants are often added to sprays to improve spray deposition. We show how the dynamic surface tension of surfactants solutions can be evaluated from drop impact experiments, allowing for a better understanding of drop deposition in the presence of surfactants.

Dispersions of bubbles with a compound interface in liquids are ubiquitous in nature and various industrial processes. Here, we experimentally investigate the formation of oil-coated bubbles at submerged coaxial orifices in quiescent liquids. A force balance model is developed to accurately predict the bubble size, which is determined by the revised Bond number, size ratio of the coaxial orifices, and the interfacial tension ratio of oil-water and oil-air interfaces. The results may be useful in estimating the size distribution of oil-coated bubbles as an input to characterize the transport and dynamics of bubbly flows where compound interfaces are present.

The interaction between the electrohydrodynamic instability and an imposed flow is studied. The imposed flow distorts the concentration field causing a sheltering effect which hinders the ion transport from low to high concentration regions, and therefore suppresses the electroconvective instability below a certain wavenumber. The removal of the morphological modes resulting from the hydrodynamic instability can change the wavenumber of the unstable electrodeposition.

Spherical Couette flows in liquid metals are a suitable candidate for generating magnetic dynamo states in the laboratory. However, results in our 3 meter diameter experiment have shown that an enhancement of the flow’s helicity is likely required. Previous works suggested roughening the inner sphere boundary by adding baffles in order to achieve these goals. In the present work, we perform hydrodynamic studies of the effect of different baffle designs in a 40 cm water experiment. Results point to one of these models as a promising upgrade that we will use in our 3 m experiment to effectively increase our chances of obtaining dynamo action.

Universality in the behavior of multilayer radial Hele-Shaw flows is discovered by semi-analytical methods. In particular, it is found numerically that the maximum injection rate for a stable flow decreases proportional to $t^{-1/3}$ regardless of the number of interfaces and increases at a rate proportional to the number of interfaces to the two-thirds power at large time $t\gg 1$. However, at earlier times the number of interfaces can increase the maximum stable injection rate by a much greater amount.

For mid-latitude salt-fingering staircases in the oceans, the staircase step-sizes are always observed to be smaller at vertical positions of relatively higher background gradients than at vertical positions characterized by relatively lower gradients. We extend the gamma instability theory of Radko (2003) to a system with inhomogeneous background gradients for temperature and salinity to explain this observed trend. On the basis of our three-dimensional turbulence analyses and mean-field model simulation, we successfully explain the origins of such step-size differences and test our proposed mechanism against the historical staircase data recorded in the Tyrrhenian Basin.

We investigate droplets evaporating on flat pinning-free substrates with smooth wettability patterns. Symmetric patterns lead to a hierarchy of bifurcations in the three-dimensional parameter space represented by the droplet cross sectional area $A$, midpoint $\ell$, and footprint $R$. In asymmetrical patterns, the presence of disconnected stable branches forces the droplet to move towards one direction, hence showing that a droplet’s motion can be controlled upon evaporation.

The azimuthal velocity profile in the Taylor-Couette flow is linearly stable if the density of angular momentum increases with the radial distance. However a temperature difference between the two cylinders can change the stability condition of the flow. The problem is studied through a linear stability analysis, with a particular attention given to the Keplerian regime and the regime where the inner cylinder is at rest.

We investigate the influence of heating on the shape and dynamics of a droplet on a liquid substrate. The problem is studied numerically in the framework of longwave amplitude equations. The effect of gravity is taken into account. We show that the thermocapillary stresses produce a significant deformation of the droplet’s interfaces. Heating the droplet from above can create an oscillatory instability generating periodic and quasiperiodic oscillations, change of droplet shape, and droplet decomposition. Heating from below can lead to the substrate layer’s rupture due to its monotonic instability.

Liquids can be sculpted by spreading along predefined rough hydrophilic lanes surrounded by superhydrophobic backgrounds. We find that two distinct types of liquid spreading arise depending on the lane width. While only a fringe film wicks into the nanoroughness of very narrow lanes, a thick bulk film spreads along wide lanes. We theoretically analyze the spreading rates of films, which change power laws in the course of spreading, and corroborate the theory with experiments.

When an object suspended in fluid moves past a soft substrate it experiences a lift force due to the deformability of the substrate. Here, we find this lift force analytically for a general elastic or viscoelastic substrate. For oscillatory relative motion, the lift force is time-dependent and its temporal Fourier modes carry signatures of the viscous and elastic components of the substrate response. The theory may be useful to characterize the mechanical properties of soft materials without solid-to-solid contact via lift force measurements.

Depositing and obtaining liquid films of uniform thickness is a problem integral to numerous applications that require multilayer films where each layer has distinct properties. Through a lubrication-theory-based model, this work studies the mechanisms that may initiate dewetting in miscible two-layer two-component films. Numerical solutions reveal that disparities in initial solute concentration between the film layers couple with circulatory flows to produce significant film-height nonuniformities. Several scaling relationships are developed to shed light on the underlying physical mechanisms.

Experiments show that thin film flow down a fiber exhibits beading patterns whose symmetry about the fiber depends upon the fiber diameter and liquid surface tension. Both symmetric and asymmetric morphologies exhibit absolute (isolated, Plateau-Rayleigh) and convective instabilities, with the asymmetric beading dynamics resembling that of the free viscous jet indicating a minimal interaction between the liquid and fiber.

A most important challenge on the way towards reliable biotechnological systems is to find biocompatible, long lasting, and precisely controllable methods of propulsion $in$ $vivo$. In the present theoretical work we explore possibilities of actuating a soft droplet by small internal motors, which either operate autonomously, or are externally driven, or both. Motors and external drives are modeled as point forces.

In this work we investigate the collective dynamics of a colloidal monolayer sedimenting down an inclined plane with experiment, theory, and simulations. We find that the evolution of the density profile of the monolayer is well described by a Burgers-like model, and that this description is quite robust.

We investigated the liquid slip in a planar confined flow channel by proposing an exponentially decaying interaction force between fluid particles and two flat walls in the mesoscopic lattice Boltzmann model framework. In this way, we can explicitly link density profile, velocity profile, and apparent slip length with the mesoscale interaction parameters (force strength and decay length), and formulate the permeability-enhancement ratio as a function of two dimensionless numbers that indicate the role of interaction strength and interaction distance (relative to gap size of flow channel) in the slip-flow system.

A liquid-solid fluidized bed is investigated by means of optical techniques involving index matching. Statistics of velocity fluctuations of both phases are determined. A physical model distinguishing between interstitial and wake regions is introduced. It reproduces the velocity fluctuation distributions up to the third-order-moment, explaining the reversal of the skewness when increasing the solid concentration.

Sediment settling in inclined fractures (channels with high aspect ratio) is studied numerically. Unlike the conventional Boycott effect, heterogeneous particle-clustering plumes are observed from the simulation results. Granular-induced Kelvin-Helmholtz and Rayleigh-Taylor instabilities are found to be the dominating mechanisms in this process. This work deepens the understanding of the Boycott effect in fractures, and provides various quantitative relationships of acceleration ratio versus inclination angle.

By using molecular dynamics simulations we demonstrate that at high Péclet numbers a nanographene suspended in a shear flow aligns at a constant orientation angle, in contrast with the rotary motion predicted by Jeffery’s theory. This instantaneous alignment with the flow of the nanographene is due to hydrodynamic slip at the fluid-particle interface, and produces a marked reduction in suspension viscosity compared to the no-slip case.

The periodic wake flow past a bluff body immersed in a moving fluid can be synchronized to the harmonic vibrations of the body. A low-dimensional, phase-based analysis has been adopted to uncover the theoretical conditions necessary for synchronization between the wake and body oscillations. The present phase-based methodology allows for modifying the transient features of unsteady flows, thereby enabling the online control and analysis of aerodynamic forces on flyers/swimmers, vortex-induced vibrations, and fluid-structure interactions.

We train deep convolutional autoencoders to learn highly efficient embeddings of two-dimensional turbulence. We define a new technique, latent Fourier analysis, that decomposes these representations into a set of interpretable recurrent patterns, and show how these recurrent patterns are closely related to the simple invariant solutions populating the turbulent attractor. By examining a series of bursting episodes with this framework we are able to identify large numbers of new simple invariant solutions that characterize these events and which have avoided previous detection methods.

We studied the role of solutal free convection on the transport of a buoyant solute at the microfluidic scales, 5–500 µm, both for a two-dimensional slit and a three-dimensional microchannel. Our theoretical predictions using both asymptotic analytical solutions and accurate numerical resolutions describe the different solute transport regimes.

We investigate the dynamics of turbulent dispersion by means of direct numerical simulations of a passive tracer released in a homogeneous isotropic turbulent flow. We focus on the link between the probability density function (PDF) of the passive scalar concentration and its mixing properties. In particular, we show how the gamma distribution can be used as a suitable model for the PDF, as has been previously verified in wind tunnel experiments in wall-bounded turbulent flows. Finally, we develop a simple mixing model to estimate the time scale that regulates the decay rate of the intensity of concentration fluctuations.

Using direct numerical simulation, the shock wave–turbulent boundary layer interaction (STBLI) generated by a compression ramp in Mach 7 flow is investigated. The behavior of turbulence in hypersonic STBLI has not been investigated as extensively in the literature as in supersonic interactions. This work describes the evolution of mean and fluctuating quantities, including velocity and temperature fluctuations, providing insight into the behavior of turbulence in a hypersonic STBLI.

Scaling of near-wall turbulence intensities has been a subject of many investigations over several decades. Emerging evidence suggests that the near-wall streamwise turbulence intensity may not precisely depend on the logarithm of the friction Reynolds number, challenging the scaling obtained with Towsend’s attached eddy model. Here, we present new mathematical evidence that it is instead proportional to the inverse of the outer velocity scale using a novel quasilinear approximation made to the Navier-Stokes equations.

High-resolution direct numerical simulations are conducted to analyze the flow dynamics of a wall-mounted rectangular prism with a high depth-ratio at different incidence angles. The results demonstrated that changing the incidence angle leads to the creation of transient helical structures in the wake of the prism. The characteristic length and dominant frequencies associated with the unsteady flow are determined, and the origin of different peak frequencies are identified. Finally, a skeleton model is established for the unsteady wake of the prism.

In a viscous fluid, a reciprocal motion does not lead to net propulsion by virtue of the Scallop theorem. In a granular medium, we observe that the same reciprocal motion generates a large drift of the moving object. This study investigates the specific mechanisms underlying this propulsion.

The effect of weak stratification on turbulent plane Couette flows is studied by means of direct numerical simulations at the Prandtl number of air. We demonstrate that the stratification tends to reduce the intensity of Couette streamwise streaks in the channel center, eventually breaking the characteristic Couette large-scale rolls. By analyzing the bursting mechanism (i.e. sweeps and ejections) responsible for the vertical momentum transfer, we observe weaker events in a disorganized spatial configuration. This scenario no longer supports the generation of Couette large-scale streaks and rolls in the channel center.

Monin-Obukhov similarity theory (MOST) is the theoretical foundation for parameterizing surface-atmosphere exchanges in nearly all weather, climate, and hydrological models. According to MOST, the classic logarithmic profiles of mean temperature break down as the buoyancy effects become important. Compared to MOST, we show that the buoyancy force does not modify the logarithmic nature of the mean potential temperature profile in sheared and unstably stratified atmospheric boundary layers, but instead modulates its slope, which is no longer universal and differs from 1/κ, where κ is the von Kármán constant.

A simple potential flow model sheds light on the behavior of detached recirculation bubbles as their initial conditions vary.

This study investigates the characteristics of streamline curvature and torsion in wall-bounded turbulence using DNS data from channel and boundary layer flows. The analysis includes characterizing the curvature of probability distributions as a function of the wall-normal distance. Far from the wall, the probability density function of the curvature decays with a -4 power dependence for the extreme events that are comparable with the Kolmogorov length scale. Consistent with previous studies, these large values of curvature are hypothesized to reflect the flow in the vicinity of stagnation points in the fluctuating field.

Reduced-order models are derived for a simplified description of transition and turbulence in Waleffe and Couette flow. Twelve nonlinear ordinary differential equations model the dynamics of coherent structures in fair agreement with direct numerical simulation. Compared to previous models, these low order systems display longer turbulence lifetimes and lower transition thresholds. Two spanwise wavelengths, $L_z$ and $L_z/2$, for roll and streak modes is a key feature. The models show that interaction between structures with either wavelength is crucial to maintain longer turbulence lifetimes, as a neglect of such interaction leads to order-of-magnitude lifetime reductions.

The attached-eddy framework is extended to turbulent coherent structures over a traveling wavy boundary. The results provide evidence for the presence of the hierarchically distributed self-similar wall-attached structures of streamwise velocity fluctuations in the presence of a wavy boundary. This is helpful for the modeling and prediction of flow properties in wind-wave interactions.

In turbulent free-shear flows, layers of intense shear bound regions of nearly uniform momentum. The thickness of these layers scales as the Kolmogorov length scale and the velocity across these layers presents a jump of approximately 10% of the characteristic large-scale velocity of the flow. In layers of intense shear, rotation dominates, whereas in layers of intense scalar gradient, strain is prevalent.

We investigate the effects of approach flow conditions (uniform flow (UF): Case A and thick turbulent boundary layer (TBL): Case B) on the unsteady three-dimensional wake characteristics and bimodality of a square back Ahmed body. Using improved delayed detached eddy simulations we show that TBL induces a much higher ground clearance momentum deficit than UF, which significantly alters wake asymmetry orientation in the wall-normal plane and completely suppresses bimodal wake behavior in the spanwise plane. Both time-averaged and time-resolved turbulence statistics are used to explore the differences between the wake structure of the Ahmed body subject to the two approach flow conditions.

Wall-resolved large eddy simulations and wall-modeled large eddy simulations are still separate strategies in the field of wall-turbulence applications, and no significant attempts have been documented to blend them in order to exploit the full potential of the two methods. This work enables a smooth transition between these two techniques, designing a robust algorithm that dynamically adapts the wall treatment. In particular, we propose a unified method that employs augmented turbulent viscosity and diffusivity at the wall location and allows for preservation of both the no-slip and isothermal/adiabatic conditions.

Airfoils operating at low Reynolds number have a proclivity to induce a laminar separation bubble (LSB) on their upper surface. We investigate the effects of angle of attack on the characteristics of the LSB and the flow field around a NACA0012 airfoil. Large-eddy simulations show that there are three distinct angle-of-attack regimes: a pre-stall attached flow, a near stall separating-attaching flow, and a full stall separated flow without reattachment.

Small conical-shaped jets may emanate from a liquid bath a short while after a small drop has hit a liquid pool. Here we perform Particle Image Velocimetry (PIV) measurements of the liquid flow inside upward jets after drop impact and show that fluid elements inside the jets may decelerate up to 5-20 times the gravitational acceleration. The measurements show that both the shape of the jet and the velocity profile are self-similar. A theoretical model including surface tension, fluid inertia, and gravity correctly predicts the self-similar velocity profile and shape of the jet, allowing us to provide the first quantitative explanation of the shape and dynamics of the emanating jets.

The spectra plotted in this work demonstrate suppression of random wind-generated waves by a deterministic long wavemaker-generated wave in a moderately sized wind-wave facility. Experiments performed for a wide range of experimental conditions show that the effect of a regular wave on the local wind-wave field excited by steady wind remains essential as long as the local dominant frequency of random wind waves remains below that of the regular waves.

Tsunami run-up in bays is essentially a wave diffraction problem in three dimensions. In this work we are showing that it is possible to treat the problem in one dimension and still obtain satisfactory results for the run-up.