A three-dimensional micromodel of a porous medium is used to investigate the effect of polymer on the pore-level flow and implications for enhanced oil recovery.

By considering a time-dependent flow due to a moving actuator disk, it is found theoretically that one can increase the mean energy conversion beyond the Betz limit for steady flows.

Three-dimensional (3D) fluid-structure-interaction simulations are conducted for real and hypothetical batoid-fish-like swimming, using a unified 3D kinematic model; proposed here. The combined effect of batlike flapping and fishlike undulation results in various types of 3D vortex structures that are correlated with the propulsive performance parameters and can be used for an efficient design of underwater vehicles.

A two-phase model of active nematic matter within a passive polymeric phase is shown to capture important cellular dynamics, such as cell division and motility, in viscoelastic fluids. The results show the suppressing effect of polymer relaxation and viscosity on the dynamics of active matter.

A low-order model is proposed to describe the slow-timescale dynamics of thermoacoustic limit cycles in an idealized annular combustion chamber. A quaternion ansatz for the acoustic pressure field enables a unified description of the impact of system asymmetries (nonuniform distribution of heat release rate or azimuthal mean flow), flame response delay, and stochastic forcing from turbulence upon the standing or spinning nature of the thermoacoustic modes.

The classical Rayleigh-Bénard convection problem where the melting temperature of the material is comprised between the two imposed temperatures is revisited. The equilibrium interface between the two phases can be flat in the purely diffusive regime or corrugated when thermal convection in the liquid phase kicks in. Close to the transition, bistability between the two solutions is observed.

Simulations of rapidly rotating turbulent convection in a cylinder reveal a strong sidewall circulation that contributes significantly to the overall convective heat transfer. However, a separation of the heat transfer into sidewall boundary layer and interior contributions is feasible; simulations and experiments in a cylinder are used to mimic laterally unbounded convective flows in nature.

Slanted vibrations are known to induce directional motion of sessile drops. A quantitative investigation into the influence of the amplitude, frequency, and inclination of vibrations, as well as the liquid viscosity, finds that the rectified motion velocity increases quasilinearly with the forcing amplitude. This is not predicted by existing theories and offers challenging questions for future investigations.

A surface jet during droplet coalescence is identified and studied using two color high-speed cameras with side and bottom views. By introducing a surface tension difference between the droplets, the internal dynamics can be modified, while the surface jet can either be enhanced or suppressed. A mechanism to control mixing is therefore established.

The orbital dynamics of an inertialess deformable prolate capsule in unbounded shear flow are numerically investigated. In certain parameter regimes, the capsule can adopt multiple stable orbits, depending on the initial orientation. A corresponding multiplicity in the rheological properties is predicted for a dilute suspension of such capsules.

Simultaneous effects of rotation and compression or extension of a viscous drop are studied numerically in terms of deformation, stationarity, and stability of the drop. The problem is modeled in terms of creeping flow and is solved using the boundary integral method. The region of stability of the deformed drop under various effects is presented.

Two model-based estimation methods are proposed to estimate the time-resolved cylinder wake based on a single sensor measurement. The two methods are compared at Re = 100 in simulations and at Re = 1036 in experiments. Results show that estimation can be improved when the nonlinear trigonometric relations between harmonics of the vortex shedding frequency are considered. A physical interpretation of the results is also given.

Numerical simulations in two dimensions of the flow past a cylinder restrained by an elastic support find that the vortex-induced vibration can be suppressed using a nonlinear energy sink consisting of a rotating mass inside the cylinder.

Numerical experiments demonstrate that the edge state of pipe flow can be reached by scaling down three-dimensional perturbations that are “too strong” to trigger turbulence. Further analysis reveals the importance of energy amplification in the bulk region for the transition in pipe flow.

Evaporation flux in the thin transition film close to the wall is up to 100 times greater than at the center of a wetting meniscus. Despite higher evaporation flux, the thin film accounts for a relatively small part of the total evaporation from the liquid-vapor interface. The thin-film contribution is directly proportional to the solid wall thermal conductivity and inversely proportional to vapor pressure and Bond number.

Light scattering in turbid liquids induces bulk flows. These flows are simulated and analyzed to show how they can deform flat interfaces up to instability and jetting. Numerical results are compared to experimental ones performed in near-critical phase-separated liquid mixtures. The comparison shows quantitative agreement for interface shapes and allows qualitative retrieval of the behavior of the produced fluid flow rates. These light-induced bulk flows and their contactless actuation of interfaces are well suited for microscale applications.

A new experiment analyses the process driving the fast transition towards turbulence of an impulsively accelerated then decelerated interface between gases of different densities, when the timescale of this transitional process is of the same order as that of the resulting turbulence. This transition to turbulence is revealed as the imprint of the initial condition is lost and the dynamical spectral content covers a wide range of scales compatible with a self-similar trend.

The impact of buoyancy on the solute mass transport in an evaporating liquid mixture confined in a horizontal slit is studied theoretically. Solvent evaporation at one end of the slit induces solute concentration gradients, which, in turn, drive free convection, thus dispersing solutes in a steadily increasing length scale along the slit.

Inertial settling of rigid and flexible fibers in suspension is investigated using numerical simulations. Clumping of fibers in the semidilute regime leads to faster settling; flexible fibers exhibit more clumping and higher settling velocities. At higher fiber concentrations, mobility becomes limited, thus fiber orientation and deformation become insensitive to concentration.

A continuum model is used to study the transport of light particles in a dense bacterial suspension. Universal scaling laws are provided for the diffusion coefficient, mean vortex size, and relaxation time as a function of fluid friction. The findings should apply to transport phenomena in generic active systems such as dense bacterial suspensions, microtubule networks, or even artificial swimmers, to name a few.

We present an experimental study on the preferential concentration of sub-Kolmogorov inertial particles in active-grid-generated homogeneous and isotropic turbulence. Big data techniques able to detect centers and compute Voronoï tessellations 10 times faster than standard algorithms are used. Since preferential concentration depends on multiple parameters we performed experiments varying all except one parameter: volume fraction, Reynolds number based on the Taylor length scale, and particle residence time interacting with turbulence.

Experiments and numerical simulations show how colloidal particles can be trapped permanently in merging flow channels. When two merging colloidal streams contain different solutes, interactions between the particles, solutes, channel wall, and the inlet fluid flow induce a near-wall vortex that leads to stable particle trapping. A unique particle trapping mechanism, potentially found in a wide range of common flow systems, is documented and characterized.

Granular flows naturally occur at high densities and significant shear rates. A kinetic theory, applicable at high densities and arbitrary shear rates, predicting the viscosity of an inelastic hard sphere fluid over many orders of magnitude is proposed. It explains the origin of Newtonian, shear thinning, as well as shear thickening behavior, i.e., Bagnold scaling in granular fluids.

A model based on hydrodynamic feedback mechanisms that is able to correctly reproduce power spectra commonly found in open cavity flows is presented. Using a delay differential equation, it takes into account time lags from the reflection of instability waves and the recirculation region inside the cavity.

High-resolution Direct Numerical Simulations of the Taylor-Green vortex using standard Navier-Stokes (NS) equations (order of the Laplacian $p=1$) and hyperviscosity ($p$ up to 100) show that evolution of the total energy and its dissipation are similar for the two simulations, but that the energy spectrum develops a more pronounced bottleneck. A link between this bottleneck and the thermalized (absolute equilibrium) state is demonstrated. The vortex tubes seen for the NS equations ($p=1$, above) are replaced by less elongated vortex “blobs” for the hyperviscous case ($p=10$, below).

Using measurements of error growth and finite-time Lyapunov exponents, the amount of chaos in a turbulent system is calculated by direct numerical simulation. The statistical distributions of the chaos are dependent on the Reynolds number and the scale, with implications for the measurement of time-averaged properties.

A very hard turbulent regime of the classical Rayleigh-Bénard convection problem is accessed using direct numerical simulations (5.7 billion grid points) with the aim of obtaining a deeper understanding of the small-scale dynamics and flow topology. The outcomes attest to the strong self-growth of strain production augmented by vortex contraction, and the linear amplification of vortex stretching relevant to the strain-dominated structures.

The idea of defining vortex boundaries as material curves that minimize the leakage of vorticity from the fluid mass they enclose when compared to other nearby material curves is put forward. The exact solution to this calculus of variations problem provides a mathematical criterion that unites common features of empirical observations: the material and vorticity-transporting nature of observed vortex cores. Moreover, an algorithm for the automated extraction of diffusive vortex boundaries is proposed and tested on analytical and numerical examples.

Experiments and three-dimensional direct numerical simulations are carried out to investigate the interaction between a turbulent convective layer and a stratified layer in a self-organizing fluid. Internal gravity waves are generated, and a reversing horizontal large-scale flow is seen in the stratified layer, driven by a third intermediate layer.

Unsteady flow modes with intrinsic frequencies unrelated to that of the turbine blades are found to be dominated by inertial waves. In the presence of these waves, rim sealing is less efficient, and conventional Reynolds-averaged Navier-Stokes methods fail to predict the sealing effectiveness. The radial seal, which limits the radial flow motion and, in turn, the circumferential Coriolis force, can suppress inertial waves (unsteady flow modes).