Based on the Onsager reciprocal relation in the linear response regime, we first clarify the equivalence of thermodynamic and electric circuit analyses for electrokinetic energy conversion. Then we present a streaming-potential-based nanofluidic energy conversion system which comprises two immiscible fluids that form a flat interface in a slit-like channel. The validity of the Onsager reciprocal relation to such a two-fluid system is verified. The performance of such an energy converter is illustrated by considering two concrete oil–water systems with different properties. In both cases, we predict that the binary system with a thin oil layer increases both the maximum output power and the energy conversion efficiency, and this enhancement depends strongly on the mobile charges present at the oil–water interface, the salt concentration and the interface location. Concretely, for negatively charged interfaces, we find that the optimal efficiency increases with the interfacial charge for relatively thin oil layers; while for relatively thick oil layers, the interfacial charge has the opposite effect (i.e. reduction effect) on the energy conversion efficiency in the ranges of the parameters. We further investigate these systems from the viewpoint of energy transfer by deriving the related energy equation. We find that viscous dissipation consumes most of the power (more than 90 %), in both single-phase and two-fluid flows. However, the ratio of the viscous dissipation to the power input decreases with increase of the interfacial charge density for the case of a relatively thin oil layer in two-fluid flows. Meanwhile, although the presence of interfacial charges can lead to an increase in electrical dissipation, the amount of the increased power consumption is less than that of the reduced viscous dissipation in the case of a thin oil layer. Therefore, for two-fluid energy converters, the total power consumption can be reduced and the efficiency is improved.

In this work, we study the effect of the compression-corner angle on the streamwise turbulent kinetic energy (sTKE) and structure in Mach 2.8 flow. Krypton tagging velocimetry (KTV) is used to investigate the incoming turbulent boundary layer and flow over $8^{\circ }$, $16^{\circ }$, $24^{\circ }$ and $32^{\circ }$ compression corners. The experiments were performed in a 99 % $\text{N}_{2}$ and 1 % Kr gas mixture in the Arnold Engineering Development Complex (AEDC) Mach 3 Calibration Tunnel (M3CT) at $Re_{\unicode[STIX]{x1D6E9}}=1750$. A figure of merit is defined as the wall-normal integrated sTKE ($\overline{\text{sTKE}}$), which is designed to identify turbulence amplification by accounting for the root-mean-squared (r.m.s.) velocity fluctuations and shear-layer width for the different geometries. We observe that the $\overline{\text{sTKE}}$ increases as an exponential with the compression-corner angle near the root when normalized by the boundary-layer value. Additionally, snapshot proper orthogonal decomposition (POD) is applied to the KTV results to investigate the structure of the flow. From the POD results, we extract the dominant flow structures and compare each case by presenting mean-velocity maps that correspond to the largest positive and negative POD mode coefficients. Finally, the POD spectrum reveals an inertial range common to the boundary-layer and each compression-corner flow that is present after the first ${\approx}10$ dominant POD modes.

Flow-induced oscillations of a wire-mounted, freely yawing axisymmetric round bluff body and the induced loads are regulated in wind tunnel experiments (Reynolds number $60\,000<Re_{D}<200\,000$) by altering the reciprocal coupling between the body and its near wake. This coupling is controlled by exploiting the receptivity of the azimuthal separating shear layer at the body’s aft end to controlled pulsed perturbations effected by two diametrically opposed and independently controlled aft-facing rectangular synthetic jets. The model is supported by a thin vertical wire upstream of its centre of pressure, and prescribed modification of the time-dependent flow-induced loads enables active control of its yaw attitude. The dynamics of the interactions and coupling between the actuation and the cross-flow are investigated using simultaneous, time-resolved measurements of the body’s position and phase-locked particle image velocimetry measurements in the yawing plane. It is shown that the interactions between trains of small-scale actuation vortices and the local segment of the aft-separating azimuthal shear layer lead to partial attachment, and the ensuing asymmetric modifications of the near-wake vorticity field occur within 15 actuation cycles (approximately three convective time scales), which is in agreement with measurements of the flow loads in an earlier study. Open- and closed-loop actuation can be coupled to the natural, unstable motion of the body and thereby affect desired attitude control within 100 convective time scales, as is demonstrated by suppression or enhancement of the lateral motion.

Droplets can be levitated by their own vapour when placed onto a superheated plate (the Leidenfrost effect). It is less known that the Leidenfrost effect can likewise be observed over a liquid pool (superheated with respect to the drop), which is the study case here. Emphasis is placed on an asymptotic analysis in the limit of small evaporation numbers, which indeed proves to be a realistic one for millimetric-sized drops (i.e. where the radius of the drop is of the order of the capillary length). The global shapes are found to resemble ‘superhydrophobic drops’ that follow from the equilibrium between capillarity and gravity. However, the morphology of the thin vapour layer between the drop and the pool is very different from that of classical Leidenfrost drops over a flat rigid substrate, and exhibits different scaling laws. We determine analytical expressions for the vapour thickness as a function of temperature and material properties, which are confirmed by numerical solutions. Surprisingly, we show that deformability of the pool suppresses the chimney instability of Leidenfrost drops.

We study the evolution of velocity fluctuations due to an isolated spatio-temporal impulse using the linearized Navier–Stokes equations. The impulse is introduced as an external body force in incompressible channel flow at $Re_{\unicode[STIX]{x1D70F}}=10\,000$. Velocity fluctuations are defined about the turbulent mean velocity profile. A turbulent eddy viscosity is added to the equations to fix the mean velocity as an exact solution, which also serves to model the dissipative effects of the background turbulence on large-scale fluctuations. An impulsive body force produces flow fields that evolve into coherent structures containing long streamwise velocity streaks that are flanked by quasi-streamwise vortices; some of these impulses produce hairpin vortices. As these vortex–streak structures evolve, they grow in size to be nominally self-similar geometrically with an aspect ratio (streamwise to wall-normal) of approximately 10, while their kinetic energy density decays monotonically. The topology of the vortex–streak structures is not sensitive to the location of the impulse, but is dependent on the direction of the impulsive body force. All of these vortex–streak structures are attached to the wall, and their Reynolds stresses collapse when scaled by distance from the wall, consistent with Townsend’s attached-eddy hypothesis.