The coordinated motion of active filaments, most notably cilia and flagella, are used by numerous micoorganisms and swimming cells to facilitate swimming. This Letter explores how the coordinated states of active filaments depend on the topology of the surface to which they are attached, as well as whether the surface is held fixed or free to move. Most notably, the Letter demonstrates that for spherical surfaces, the coordinated state changes dramatically when the sphere is released due to defect-induced alterations in filament motion.

An experimental study is performed to investigate the influence of crystal growth on the contact line behavior of a droplet on an undercooled surface. We show the effect that the growing nucleus has on the arrest of the contact line and the underlying dependence on nucleation rate. Finally, based on classical nucleation theory and scaling relations for droplet spreading, we calculate the temporal evolution of the solidifying area.

To investigate large-scale magnetic field generation in highly conducting magnetohydrodynamic (MHD) regimes typical of rotating astrophysical systems, we conduct high-resolution MHD simulations in the dynamical regime of large magnetic Reynolds numbers, of turbulence with an inhomogeneous kinetic helicity distribution. We obtain a nonlinear helical dynamo solution, characterized by a weak large-scale magnetic wave slowly propagating over saturated small-scale MHD turbulence, bypassing the catastrophic resistive quenching problem of homogeneous helical dynamos by developing turbulent fluxes of magnetic helicity through the system equator. Hints of fast-reconnection plasmoids are found.

Numerical simulations have been used to estimate the validity range of the linear potential theories of Fernandez-Feria and the classical Garrick model on the propulsion of a pitching airfoil and a heaving airfoil in forward flight, in terms of the Reynolds, Re, and Strouhal number, St.

Microscale intercellular pores of tree conduit contain flexible elements that locally restrict flow in response to mechanical stress. This facilitates autonomous flow control in which the cells collectively steer water across the tissue in strikingly complex patterns. With this insight, we are able to design physically programmable inline flow devices that operate without a central control unit. We expect the new approach will strongly impact flow-engineering, due to its conceptual simplicity and low cost.

Infants born with single-ventricle physiologies pose significant challenges for mechanical circulatory support devices. We investigated the existence of a hemodynamically optimized geometry for an artificial right atrium (ARA) that can act as a reservoir for circulatory support and may potentially lessen the risk of implementing biventricular supports in Fontan patients. Our results indicate that a hemodynamically optimum shape for an ARA is convex at the outlet and concave at the opposite side that resembles a healthy anatomical right atrium.

Surface-driven flows are ubiquitous, from subcellular cytoplasmic streaming to organ-scale ciliary arrays. Here, we model how confined geometries can be used to engineer complex hydrodynamic patterns driven by activity prescribed solely on the boundary. We simulate light-controlled surface-driven active matter, probing the emergent properties of a suspension of active colloids that can bind and unbind from surfaces of a closed microchamber, together creating an active carpet. Switching the particle velocities with light, we program the active suspension and demonstrate a rich design space of flow patterns. Our results point towards design and control of surface-driven active fluids.

Optically trapped colloidal probes have been widely used for active microrheology of viscoelastic fluids over the last decade. A significant issue which arises is measurement of complex viscoelastic parameters over a wide frequency range and a short time. We use a combination of square and sinusoidal waves to spatially modulate the trapped probe over a frequency range spanning five decades, and use the phase response to calculate complex viscoelastic parameters with high signal to noise in a little over three minutes. We test our method over a wide variety of linear viscoelastic samples at different concentrations, and find good agreement of the measured parameters with known values.

Thermodynamically constrained averaging theory was used to derive a macroscale model for the flow of generalized Newtonian fluids through porous media. The parameters needed for the model were determined from Newtonian flow systems. Predictions of flow for several shear-thinning Cross model fluids flowing in a variety of isotropic and anisotropic porous media agreed with microscale simulations within the numerical error of the simulator. Equating to the shift-factor, we found that it may be predicted a priori from geometric system and rheological properties. The proposed model obviates the need for experimental flow data for generalized Newtonian fluids in the porous medium of interest.

Many planetary core fluid processes can be modeled with low Prandtl number convection in a rapidly rotating sphere. Close to the onset a regime of multimodal convection is typically found. We analyze azimuthally drifting periodic flows from which multimodal convection arises in terms of bifurcation theory.

We measure the entrainment coefficient of a turbulent plume containing dense particles. When the particle buoyancy flux opposes that of the plume fluid, the entrainment coefficient is increased by up to 40%. In contrast, when the particle buoyancy flux and the plume fluid buoyancy flux act in the same direction, the entrainment coefficient is unaffected. The effect on the entrainment coefficient and difference between the two configurations is linked to particle clustering within the plume.

In this paper, asymptotic analysis and simulations are synergistically used to investigate the dynamics of thin two-dimensional droplets that spread on chemically heterogeneous surfaces and are subjected to mass transfer effects. A set of integrodifferential equations is derived for the motion of the two contact points of the droplet, which is shown to accurately capture the underlying dynamics in a variety of representative cases examined.

This paper presents the development of an asymptotic model which governs the evolution of the Fourier harmonics of the contact line of a thin droplet moving on a chemically heterogeneous surface, while being subjected to prescribed changes in its volume. Numerical experiments demonstrate the capacity of our model to accurately capture the dynamics, and, importantly, at a small fraction of the computing resources that would have been required for simulating the governing long-wave equation.

In this study, we numerically and theoretically investigate the electrowetting-induced detachment of liquid droplets. An analytical model adopting the energy balance approach is derived to predict the jumping velocity of an electrowetting-actuated droplet, which comprehensively accounts for the influence of liquid properties, droplet size and applied voltage. Our main contribution is to reformulate the models of four energy components including the surface energy, kinetic energy, gravitational potential energy at detachment, as well as the viscous dissipation of entire recoiling. The necessity to correct these energy components has also been physically explained.

The application of a transverse magnetic field to a turbulent free jet can not be reduced to a damping or smearing effect. A detailed analysis shows surprising effects such as the development of two strong reverse jet flow zones adjacent to the main jet (flowing upward in the picture).

Fluidic interfaces between conducting and dielectric media disintegrate under sufficiently strong electric fields, leading to electrohydrodynamic tip streaming, often in form of a Taylor cone. In this article, the governing mechanism of an alternative interface deformation mode is studied, where the interface is pushed away from the electrode, and additional cone structures emerge from the rim of the dimple. Using experimental and numerical methods, we show that the droplets of the conducting liquid play a crucial role in the dimple formation, as they induce a background flow, which in turn leads to the interfacial deformation.

In this work we perform reactive control of axisymmetric disturbances in turbulent jets. The disturbances are produced by an external forcing and possess stochastic phases and amplitudes, akin to turbulent fluctuations found in unforced jets. The control law is based on an existing inverse feedforward scheme developed for transition control. Here we apply it to control convective growth mechanisms in fully turbulent jets and we build on previous works that considered control of harmonic disturbances. We demonstrate the successful implementation of real-time reactive control of the disturbances and achieve order-of-magnitude attenuations of velocity fluctuations.

Motivated by the transport of cargo inside cells, we study the hydrodynamic interactions between a point-force and a rigid, slender filament at a separation distance between the filament radius and its length. Using asymptotic calculations and boundary element computations, we show that the leading-order interactions are captured by a new resistive-force theory with modified drag coefficients.

A Janus particle is an example of a micro swimmer which converts chemical energy present in the environment to its mechanical energy using surface reaction. In this work, we theoretically study the weak effect of solute advection on the swimming velocity of an active particle. Using the method of matched asymptotic expansion we provide a general framework for calculating first order corrections to the concentration field and the swimming velocity of an active particle in terms of Peclet number relative strength of advection to diffusion. The O(Pe) corrections reduce the magnitude of swimming velocity independent of other parameters

Using theory and Direct Numerical simulations, we show that gravitational settling can contribute strongly to particle concentrations in wall-bounded turbulent flows even when the settling number Sv is very small. This occurs because the other competing mechanisms, e.g. the turbophoretic drift velocity, become very small in certain regions of the flow, such as very close to the wall, which we demonstrate using asymptotic analysis.

The drag anisotropy experienced by cylindrical rods moving through granular sediments is measured and found to be far greater than in viscous fluids at comparable aspect ratios. The flows while moving perpendicular and parallel to the cylinder axis are visualized and related to the observed dependence on aspect ratios, speed, and material properties.

Network analysis has recently been adopted as a framework to investigate complex turbulent dynamics. We apply these concepts to a set of particle trajectories in turbulent channel flow at different frictional Reynolds numbers (Re) from 180 to 950, building networks accounting for the transition between discrete states of particle distribution. Two main mechanisms determine a reduction in dispersion near the walls, namely the weaker velocity fluctuations at lower Re and the cyclic patterns of motion at higher Re. When exceeding Re of 395, many network properties behave independently of Re when scaled with outer flow variables.

What is the maximum speed that can be achieved for a given wind intensity and direction? The answer is detailed in our article using a new expression for the aerodynamic power dissipation. This evaluation of the aerodynamic power in crosswinds is established via precise wind tunnel experiments performed with a time trial specialist completed with a theoretical approach which allows determination of the maximum speed in any crosswind.

We present a data-driven framework that uses very limited data to learn generalizable bypass transition models. Trained on just two flat plate cases, the augmented model is shown to generalize to other flat plate cases and turbomachinery configurations.

Wall heat-flux effects on turbulence statistics and structures in supersonic turbulent boundary layers with the heated and cooled wall are investigated. To study effects of the semi-local Reynolds number (Re${}_{\tau }^{*}$), we conduct large eddy simulation with constant Re${}_{\tau }^{*}$ by imposing a dynamic viscosity proportional to the square root of the density and comparing to flows with a conventional viscosity given by Sutherland’s law. For constant Re${}_{\tau }^{*}$, wall heated and cooled cases show collapsed turbulence statistics and structures when scaling with the semi-local length scale. The results suggest that Re${}_{\tau }^{*}$ is a similarity parameter of the flows with thermophysical property variations.

A turbulent axisymmetric bluff body wake is investigated using large eddy simulation to uncover key flow dynamics and structures. The effect of low-frequency and high-frequency pulsed jet forcing at the point of separation is studied to reveal the underlying mechanisms involved in the base pressure changes, which provide a reference for bluff body drag reduction and active flow control.

The more the number of large-scale rolls, the larger is the heat flux. Here, the large-scale flow structures transform into multiple rolls due to the presence of tall roughness elements. These rolls efficiently wash-out the cavities leading to frequent thermal plumes. In the present work, this process is exploited to enhance the heat flux.

Using direct numerical simulations we study the spatiotemporal correlations of passive scalars advected both by Navier-Stokes flow and by synthetic velocity fields. Beyond their global behavior, which is either a Gaussian or an exponential decay in time, depending on the carrier flow, we determine with accuracy the prefactors in the exponents, and find a remarkable agreement with recent results from functional renormalization group.

We cast the sub-grid velocity field in large eddy simulations as a sum of spatially localized stochastic modes, or Gabor modes, which are dynamically coupled to the resolved large-scale field. We evaluate the ability of the Gabor representation to reconstruct second order space-time statistics of the pressure field in homogeneous isotropic turbulence and find the reconstruction to be remarkably accurate in both a priori and a posteriori settings. The small number of modes required results in significant compression in degrees of freedom relative to solving the governing equations on a mesh capable of resolving all scales.

Integrable turbulence theoretically describes system states (in optics, hydrodynamics) where nonlinear phenomena dominate. Fourier analysis is challenged by the richness of such states also featuring solitons. Bidirectional soliton gases are generated in a straight 34 m long shallow water wave flume. The soliton content is quantified with a direct scattering transform provided by the finite-gap theory of the KdV equation. The conditions of soliton gas occurrence are discussed with regards to the wave-maker energy input and the viscous dissipation. The results of this study open up perspectives, for instance, to the understanding of sea states in shallow water.

In nonlinear dynamical systems one finds many examples with the occurrence of a signal that alternates randomly between long periods of regular behavior and relatively short irregular bursts. In other words, the motion is intermittent, nearly periodic with occasional irregular bursts. Such intermittency scenarios are in particular observed within the transition to turbulence and classified into different types. In the present paper an intermittency scenario of type III is observed.