Reexamination of some direct numerical simulation data for a turbulent differentially heated vertical channel finds that the components of the Reynolds stress scale with the product of the friction velocity and the maximum mean flow.

In direct numerical simulations of spanwise rotating plane Couette flow, hysteresis is found between a 2-pair-roll-cell as the rotation increases to a 3-pair-roll-cell when the rotation decreases.

A low dimensional model for stratified turbulence predicts the existence of invariant manifolds for the evolution of temperature and velocity gradients. Fluid elements evolve preferentially along these manifolds, associated with stable regions and with regions prone to develop local convection.

The representation of self-sustaining processes via resolvent analysis for turbulent flows is improved where the resolvent operator is not low rank by approximating the nonlinear forcing using parasitic modes, with analogy to weakly nonlinear analysis near critical Reynolds numbers.

Using ultrasound Doppler imaging we experimentally and analytically reconstruct the blood flow field in arteries and provide an $in-vivo$-validated, noninvasive, and reliable Wall Shear Stress (WSS) estimation. WSS is a major mechanical modulator of many functions of the cardiovascular system.

A dense planar array of spherical microswimmers is studied analytically and numerically using pairwise lubrication interactions. Suspension dynamics are mediated through gravitational torques exerted on the cells and cell-cell repulsive forces, and they reveal stable, oscillatory, and chaotic states.

A numerical simulation finds that a three-dimensional conical oblique wave takes on a cellular structure on the front, presenting a fish-scale shape. The cell shape becomes relatively irregular as the heat release is increased.

A study finds that for two adjacent buoyant flames the flickering mode transition from in-phase to anti-phase is caused by a transition of the inner-side vortex pattern from symmetric to staggered. This mechanism is similar to the instability in the wake of a bluff body that initiates the Karman vortex street.

Quantitative experimental measurements are used to test the ability of theoretical development to predict the transverse velocity migration of particles in a confined Poiseuille flow, according to the viscoelastic constitutive parameters of dilute polymer solutions.

An accurate and efficient model based on the maximum entropy principle is established for the vibrational excitation and dissociation of oxygen. Good agreement with state-specific calculations and recent experimental data is obtained.

Experiments on the impact of a drop near the edge of an inclined substrate are reported. A liquid sheet forms beyond the edge, then fragments into droplets. This configuration is a minimal model of the crucial raindrop impacts on plant leaves that are responsible for the dispersal of crop diseases.

At low impact velocities, a droplet does not immediately make contact with a surface because of an entrapped air layer, leading to delayed surface wetting. High-speed images are used to investigate the influence of this entrapped air layer and the surface wettability on droplet spreading.

Vortices in a model of the solar tachocline decay into pairs of stable donutlike Alfvén waves. These propagate zonally in opposite directions, colliding periodically. Nonlinear effects distort the waves significantly after each collision, but between collisions their shapes and speed remain fixed.

The deformation and breakup of an axisymmetric electrolyte drop in a dielectric medium stretched by an electric field is studied. An accurate and efficient boundary integral method is developed to solve the time-dependent Stokes flow problem in the case of arbitrary Debye layer thickness.

Two-dimensional flows can be controlled efficiently with spanwise-periodic wall forcing or wall deformation. Optimal “wavy” controls for the linear stability of the laminar flow past a cylinder are obtained with an adjoint method.

The Optimally Time-Dependent (OTD) modes, a set of deformable orthonormal modes that track transient instabilities, are incorporated into a robust, inexpensive control algorithm that can steer any trajectory of a high-dimensional nonlinear system toward a fixed point of the governing equations.

Experiments show that crossflow vortices in the boundary layer on a rotating wide cone start to meander before breakdown. The structural development of the overturning process of these vortices, where high-momentum upwelling of the vortices leads to transition to turbulence, is described in detail.

Experimental measurements of the velocity field around a droplet swimming close to a wall demonstrate the critical impact of confinement.

Channel viscosimetry makes it possible to estimate surface viscosity of a layer of surfactants or metal oxides. A new model is presented with surface curvature effects accounted for, regardless of whether or not the supporting liquid is wetting or if the contaminated surface is Newtonian.

Lyapunov and non-modal stability analysis reveal why stationary and self-similar capillary flows in slender open triangular channels are so stable to disturbances. This robust feature allows superior flow management for applications ranging from space micropropulsion to microfluidic diagnostics.

Adding surfactant to clean fluid stabilizes the inclined flow. A theoretical study shows that beyond a critical level the flow is destabilized as more surfactant is added due to desorption from the surface. If the mass density of the fluid increases with surfactant concentration in the bulk, a later stabilizing stage occurs.

The collective locomotion of two tandem autopropelled flapping foils is greatly affected by the phase difference. Two distinct vortex interactions are observed—merging interaction and broken interaction—which respectively result in the highest efficiency for the follower and the leader.

Using direct numerical simulation it is shown that even when bidisperse particles are settling rapidly, intermittent fluctuations allow turbulence to continue to play a key role in their relative motion, an effect increasing with $Re$. Low-order statistics relevant to collision rates are found to depend only weakly on $Re$.

Mean flow and turbulence in bubble-in-chain induced flows are experimentally observed at various release frequencies and flow rates. We find that horizontal water velocity profiles collapse to a universal Gaussian distribution and one-dimensional velocity spectra to a consistent slope of −3.

We develop a passive method of soft filtration which leverages interfacial forces to prevent the contamination of substrates withdrawn from a liquid polluted by microparticles and micro-organisms.

When approaching a recirculation cell, Brownian aerosols can enter the cell or slip along its border or drift away. Determining which particles do what is very challenging. A general analytical expression for the probability of capture of aerosols in such cells is derived.

Phase diagram showing the variation of the number of long-time solutions with $Re$ and a dimensionless measure of spring stiffness. Each combination of symbol shape and color indicates a different set of such solutions, with the pentagons corresponding to three unsteady solutions and one steady one.

Flow-induced flutter of flexible flapping membranes can greatly increase scalar mixing in channel flows in the inertial microfluidics regime. We use flow-structure interaction simulations to investigate their flow physics and mixing ability and find rapid mixing with relatively low pressure loss.

Three-dimensional viscous fingering for miscible non-Newtonian fluids with and without chemical reactions were studied with a microfocus X-ray computed tomography scanner. We find that the area fraction of injected fluid in the reactive cases was lower than that in the nonreactive cases.

Numerical simulations of turbulent flows over traveling wavy boundaries reveal that very large-scale motions are enhanced by a wavy boundary, but their intensities are decreased as wave phase speed increases. The wave-induced flow provides an extra energy transfer for the very-large-scale motions.

With experiments and direct numerical simulations we study the angular distribution of symmetry axes of nearby small spheroids in a turbulent flow when inertial effects are negligible. We find the angles to be unexpectedly large with a fractal attractor and a distribution with power law tails.

The long short-term memory (LSTM) neural network is used to predict the temporal evolution of a low-order representation of near-wall turbulence. This network leads to excellent predictions of turbulence statistics and of the system dynamics, characterized by Poincaré maps and Lyapunov exponents.

A refinement of the logarithmic law for the mean velocity in canonical wall-bounded turbulent flows is presented. The relevant length scale in the overlap region is found to be based on the weighted mean velocity gradient instead of the classical length scale based on the wall friction velocity.

A study of vortex dynamics in a confined superfluid shows that a system of three co-rotating quantum vortices and one vortex of opposite rotation exhibit chaotic dynamics. We find the onset of chaos is seeded through the close approach and separation of vortices in a scattering event.

An experimental study reveals the formation of a thin circulation layer (TCL) adjacent to the entire interface of a two-fluid swirling flow in a sealed, vertical cylindrical container. The TCL scenario differs from that predicted numerically.

Equilibrium properties of axisymmetric flow in cylindrical (Taylor-Couette) geometries are studied using the methods of statistical mechanics. The system is constrained by an infinite number of conservation laws, leading to an intricate interplay between the toroidal ($\sigma$) and poloidal ($\xi$) flow fields.

Active control of vortex shedding from a blunt trailing edge is achieved experimentally using piezoelectric actuators. The system can suppress and amplify the vortex shedding pattern in the wake, as well as force near-wake symmetry. An application of closed-loop control is also demonstrated.