Consider the propagation of a gravity current (GC) released from a lock of length $x_0$ and height $h_0$ into an ambient fluid of height $H h_0$ and density $\rho _{o}$. The lock contains a layer of thickness $H_L h_0$ of density $\rho _L$ overlaid by a layer of thickness $(1-H_L)h_0$ and density $\rho _U$, where $\rho _{o} < \rho _U < \rho _L$ and $H_L \in (0, 1)$. Assume Boussinesq and large Reynolds-number flow. The internal stratification parameter is $\sigma = (\rho _L - \rho _U)/(\rho _L - \rho _{o})$, in the range $(0,1)$; the classical GC is $\sigma =0$. Such GCs were investigated experimentally (Gladstone et al., Sedimentology, vol. 51, 2004, pp. 767–789; Dai, Phys. Rev. Fluids, vol. 2, 2017, 073802; Wu & Dai, J. Hydraul. Res., 2019, pp. 1–14.); we present a new self-contained model for the prediction of the thickness $h$ and depth-averaged velocity $u$ as functions of distance $x$ and time $t$; the position and speed of the nose $x_N(t)$ and $u_N(t)$ follow. We derive a compact scaling upon which, for a given $H$ (height ratio of ambient to lock), the flows differ in only one parameter: $\varPsi = \{ [1 -\sigma (1 - H_L)]/[1 - \sigma (1 - H_L^2)] \} ^{1/2}$. The parameter $\varPsi$ equals $1$ for the classical GC and is larger in the presence of stratification; a larger $\varPsi$ means a faster and a thinner GC. The solution reveals an initial slumping phase with constant $u_N$, a self-similar phase $x_N \sim t^{2/3}$, and the transition at $x_V$ to the viscous regime. Comparisons with published experiments show good data collapse with the present scaling $\varPsi$, and fair-to-good quantitative agreement (the discrepancy and the stability conditions are discussed).

Rayleigh–Bénard convection (RBC) and Taylor–Couette flow (TCF) are two paradigmatic fluid dynamical systems frequently discussed together because of their many similarities despite their different geometries and forcing. Often these analogies require approximations, but in the limit of large radii where TCF becomes rotating plane Couette flow (RPC) exact relations can be established. When the flows are restricted to two spatial independent variables, there is an exact specification that maps the three velocity components in RPC to the two velocity components and one temperature field in RBC. Using this, we deduce several relations between both flows: (i) heat and angular momentum transport differ by $(1-R_{\Omega })$, explaining why angular momentum transport is not symmetric around $R_{\Omega }=1/2$ even though the relation between $Ra$, the Rayleigh number, and $R_{\Omega }$, a non-dimensional measure of the rotation, has this symmetry. This relationship leads to a predicted value of $R_{\Omega }$ that maximizes the angular momentum transport that agrees remarkably well with existing numerical simulations of the full three-dimensional system. (ii) One variable in both flows satisfies a maximum principle, i.e. the fields’ extrema occur at the walls. Accordingly, backflow events in shear flow cannot occur in this quasi two-dimensional setting. (iii) For free-slip boundary conditions on the axial and radial velocity components, previous rigorous analysis for RBC implies that the azimuthal momentum transport in RPC is bounded from above by $Re_S^{5/6}$, where $Re_S$ is the shear Reynolds number, with a scaling exponent smaller than the anticipated $Re_S^1$.

In this note, we establish the applicability and effectiveness of a recently developed approach to the recovery of nonlinear water waves from pressure measurements by proving that it is applicable to the celebrated extreme Stokes wave.

A quantitative understanding of the dominant mechanisms that govern the generation and decay of the counter-rotating vortex pair (CVP) produced by yawed wind turbines is needed to fully realize the potential of yawing for wind farm power maximization and regulation. Observations from large eddy simulations (LES) of yawed wind turbines in the turbulent atmospheric boundary layer and concepts from the aircraft trailing vortex literature inform a model for the shed vorticity and circulation. The model is formed through analytical integration of simplified forms of the vorticity transport equation. Based on an eddy viscosity approach, it uses the boundary-layer friction velocity as the velocity scale and the width of the vorticity distribution itself as the length scale. As with the widely used Jensen model for wake deficit evolution in wind farms, our analytical expressions do not require costly numerical integration of differential equations. The predicted downstream decay of maximum vorticity and total circulation agree well with LES results. We also show that the vorticity length scale grows linearly with downstream distance and find several power laws for the decay of maximum vorticity. These results support the notion that the decay of the CVP is dominated by gradual cancellation of the vorticity at the line of symmetry of the wake through cross-diffusion.

Predicting and controlling entropy-wave-driven combustion instabilities is challenging, because the production, advection and dispersion of entropy waves in practical systems is difficult to model. The present paper aims to shed new light on this problem by considering a highly turbulent configuration with experiments and large eddy simulations. In this configuration, the decay of entropy waves is not only governed by the shear dispersion of an idealized turbulent pipe flow, as assumed in the recent studies on the topic, but also enhanced by the highly three-dimensional dispersion due to large-scale coherent structures. A novel post-processing approach for background-oriented schlieren (BOS) thermometry is proposed and enables the measurement of entropy waves with high spatio-temporal resolution for wide ranges of entropy wave amplitudes and frequencies. Instantaneous BOS snapshots are arranged in a panoramic coordinate frame using velocity data. This work, therefore, contributes to filling the knowledge gap in experimental data on entropy waves. The new dataset is accompanied with large eddy simulations to further elucidate the mechanisms dominating the amplitude decay of the entropy waves. It is shown that shear dispersion models that are only based on mean profiles of the axial velocity significantly underestimate the decay in the present configuration, where the entropy waves are formed by periodically injecting hot air pockets in the main stream. It is shown that the turbulent nature of the coherent hot pockets plays a key role in the dispersion enhancement.

Clusters of inertial particles in turbulence are usually identified from the spatial coherence of the particle concentration field, neglecting their temporal persistence. The latter is in fact essential to the ability of the particles to interact with each other and to modify the flow. Here, we leverage simulations of homogeneous isotropic turbulence laden with small heavy particles and develop a Lagrangian framework to follow them before, during and after their time as part of a coherent cluster. We define a criterion to establish whether a cluster survives over successive time steps, and use it to characterize its lifetime. We find that cluster lives have typical durations of a few Kolmogorov time scales, with positive correlation between cluster size and lifetime. Increasing inertia and gravitational settling both lead to longer lifetimes. Small clusters emerge from the coagulation of non-clustered particles, quickly followed by disintegration into prevalently non-clustered particles. By contrast, large clusters result from the recombination of other large clusters. The birth of a cluster is preceded by an exponential contraction of the particle cloud, and its death coincides with the beginning of a slower exponential expansion, The contraction is simultaneous to a decline in the local small-scale turbulence activity, while the expansion is accompanied by its recovery. Therefore, during their lifetime, the clusters experience lower-than-average enstrophy and strain rate in the fluid. This relatively quiescent state of the flow is thus a necessary condition for the cluster survival, at least in the considered range of turbulence intensity and particle inertia.

The flow past a fixed rigid cylinder becomes unsteady beyond a critical Reynolds number close to $47$, based on the body diameter and inflow velocity. The present paper explores numerically the vortex-induced vibrations (VIV) that may develop for a flexible cylinder at subcritical Reynolds number ($Re$), i.e. for $Re<47$. Flexible-cylinder VIV are found to occur down to $Re\approx 20$, as previously reported for elastically mounted rigid cylinders. A detailed analysis is carried out for $Re=25$, in two steps: the system behaviour is examined from the emergence of VIV to the excitation of the first structural modes; and then focus is placed on higher-mode responses. In all cases, a single vibration frequency is excited in each direction. The cross-flow and in-line responses exhibit contrasting magnitudes (peak amplitudes of $0.35$ versus $0.01$ diameters), as well as distinct symmetry properties and evolutions (e.g. standing/travelling waves). The flow, unsteady once the cylinder vibrates, is found to be temporally and spatially locked with body motion. The synchronization with the cross-flow standing-wave responses is accompanied by the formation of cellular wake patterns, regardless of the modes involved in the vibrations. Body trajectory varies along the span, but dominant orbits can be identified. Despite the low amplitudes of the in-line responses, connections are uncovered between orbit orientation and flow–structure energy transfer, with different trends in each direction.

Kessler (Nature, vol. 313, 1985, pp. 218–220) first showed that plume-like structures spontaneously appear from both stationary and flowing suspensions of gyrotactic microswimmers in a vertical pipe. Recently, it has been shown that there exist multiple steady, axisymmetric and axially uniform solutions to such a system (Bees & Croze, Proc. R. Soc. A, vol. 466, 2010, pp. 2057–2077). In the present study, we generalise this finding by reporting that a countably infinite number of such solutions emerge as the Richardson number increases. Linear stability, weakly nonlinear and fully nonlinear analyses are performed, revealing that each of the solutions arises from the destabilisation of a uniform suspension. The countability of the solutions is due to the finite flow domain, while the transcritical nature of the bifurcation is because of the cylindrical geometry, which breaks the horizontal symmetry of the system. It is further shown that there exists a maximum threshold of achievable downward flow rate for each solution if the flow is to remain steady, as varying the pressure gradient can no longer increase the flow rate from the solution. All of the solutions found are unstable, except for the one arising at the lowest Richardson number, implying that they would play a role in the transient dynamics in the route from a uniform suspension to the fully developed gyrotactic pattern.

We study the effects of nozzle geometry on the dynamics of thin fluid films flowing down a vertical cylindrical fibre. Recent experiments show that varying the nozzle diameter can lead to different flow regimes and droplet characteristics in the film. Using a weighted residual modelling approach, we develop a system of coupled equations that account for inertia, surface tension effects, gravity and a film stabilization mechanism to describe both near-nozzle fluid structures and downstream bead dynamics. We report good agreement between the predicted droplet properties and the experimental data.

Turbulent flows driven by a vertically invariant body force were proven to become exactly two-dimensional (2-D) above a critical rotation rate, using upper bound theory. This transition in dimensionality of a turbulent flow has key consequences for the energy dissipation rate. However, its location in parameter space is not provided by the bounding procedure. To determine this precise threshold between exactly two-dimensional and partially three-dimensional (3-D) flows, we perform a linear stability analysis over a fully turbulent 2-D base state. This requires integrating numerically a quasi-2-D set of equations over thousands of turnover times, to accurately average the growth rate of the 3-D perturbations over the statistics of the turbulent 2-D base flow. We leverage the capabilities of modern graphics processing units to achieve this task, which allows us to investigate the parameter space up to $Re=10^5$. At the Reynolds numbers typical of 3-D direct numerical simulations and laboratory experiments, $Re\in [10^2, 5\times 10^3]$, the turbulent 2-D flow becomes unstable to 3-D motion through a centrifugal-type instability. However, at even higher Reynolds numbers, another instability takes over. A candidate mechanism for the latter instability is the parametric excitation of inertial waves by the modulated 2-D flow, a phenomenon that we illustrate with an oscillatory 2-D Kolmogorov flow.

This experimental study addresses the longstanding question of why inception of attached cavitation on curved surfaces or hydrofoils at incidence is relatively insensitive to the concentration of free-stream nuclei. High-speed imaging and high-resolution particle image velocimetry measurements examine cavitation inception on three curved surfaces with varying pressure minima followed by regions with adverse pressure gradients. When these pressure gradients either thicken the boundary layer or cause local flow separation, thin $(50\text {--}60\ \mathrm {\mu }\textrm {m})$ low-momentum zones form close to the wall. Microbubbles trapped in these regions are generated initially from the collapse of intermittent attachment of travelling bubble cavitation. These bubbles migrate slowly upstream for a few milliseconds either under the influence of the adverse pressure gradients when the flow remains attached or carried by the recirculating flow when the boundary layer is separated. Their speed is only 2 %–4 % of the free-stream velocity, and their trajectories are erratic, indicating near-dynamic equilibrium. Owing to the low local pressure, their diameter increases by two to four times by non-condensable gas diffusion, from $10$ to $30\ \mathrm {\mu }\textrm {m}$ to the thickness of the low-momentum zone. At that time, either they are swept downstream by the free-stream flow or they become nuclei for new attached cavitation events. When the new patches collapse, new microbubbles form and the process repeats itself frequently, and independently of the free-stream nuclei. These phenomena do not occur when the adverse pressure gradients are too mild to create low-momentum zones with sufficient thickness to facilitate the slow upstream migration and growth.

It is well known that perforation of a flat plate reduces its drag when exposed to a flow. However, studies have shown an opposite effect in the case of cylinders. Such a counterintuitive result can have significant consequences on the momentum modelling often used for wind turbine performance predictions, where increased porosity is intrinsically linked to lower drag. Here, a study of the drag of various types of porous cylinders, bars and plates under steady laminar inflow is presented. It is shown that, for most cases, the drag decreases with increased porosity. Only special types of perforations can increase the drag on both cylinders and bars, either by enhancing the effect of the rear half of the models or by organizing the wake structures. These rare occurrences are not relevant to wind turbine modelling, which indicates that current momentum models exhibit the qualitatively correct behaviour.

In most cases, the dominant mechanism of energy dissipation for a bubble in volume oscillations is the thermal energy exchanged with the liquid. The process is subtle and its precise description a matter of some complexity. These features have prevented its ready incorporation in many applications, which forcedly have to rely on the rather inaccurate polytropic pressure–volume relation. This paper develops two approximate models of the thermal interaction, formulated in terms of ordinary differential equations, which can be readily added to standard Rayleigh–Plesset-type formulations at a modest computational cost.

The aim of this paper is to make the formation of liquid plugs as difficult as possible in liquid partially filling a horizontal rectangular tube in a downward gravity field by setting the walls to have differing contact angles. Manning et al.'s method (J. Fluid Mech., vol. 682, 2011, pp. 397–414), extended from Concus–Finn theory, is applied to the existence of capillary plugs in rectangular tubes. The critical Bond numbers ($B_c$) determining the existence of capillary plugs in a rectangular tube are studied for different settings of the non-uniform contact angles, and the influence of the aspect ratio (defined as the width-to-height ratio) of the rectangular cross-section on $B_c$ is examined. Compared to the maximum and minimum of $B_c$ reached for uniform contact angles, the maximum of $B_c$ is higher, which is attained for the bottom contact angle $\gamma _2=135^{\circ }$, the top contact angle $\gamma _4=45^{\circ }$, and the side contact angles $\gamma _1=\gamma _3=90^{\circ }$; while the minimum is considerably lowered to zero, which is reached for $\gamma _1=\gamma _2=45^{\circ }$ and $\gamma _3=\gamma _4=135^{\circ }$. The aspect ratio of the rectangle has no influence on the maximum and minimum $B_c$ for a tube with walls of differing contact angles. There is only one non-occluded liquid topology in a square, while two topologies may occur in a rectangle with aspect ratio 2, and the transition between the two topologies is accompanied by a kink of the curve of $B_c$. Optimization of the non-uniform contact angles can facilitate or effectively block the capillary plugs in rectangular tubes regardless of the aspect ratios.

Fluid deformable surfaces show a solid–fluid duality which establishes a tight interplay between tangential flow and surface deformation. We derive the governing equations as a thin film limit and provide a general numerical approach for their solution. The simulation results demonstrate the rich dynamics resulting from this interplay, where, in the presence of curvature, any shape change is accompanied by a tangential flow and, vice versa, the surface deforms due to tangential flow. However, they also show that the only possible stable stationary state in the considered setting is a sphere with zero velocity.

In the preparation of café latte, spectacular layer formation can occur between the espresso shot in a glass of milk and the milk itself. Xue et al. (Nat. Commun., vol. 8, 2017, pp. 1–6) showed that the injection velocity of espresso determines the depth of coffee–milk mixture. After a while, when a stable stratification forms in the mixture, the layering process can be modelled as a double diffusive convection system with a stably stratified coffee–milk mixture cooled from the side. More specifically, we perform (two-dimensional) direct numerical simulations of laterally cooled double diffusive convection for a wide parameter range, where the convective flow is driven by a lateral temperature gradient while stabilized by a vertical concentration gradient. Depending on the strength of stabilization as compared to the thermal driving, the system exhibits different flow regimes. When the thermal driving force dominates over the stabilizing force, the flow behaves like vertical convection in which a large-scale circulation develops. However, with increasing strength of the stabilizing force, a meta-stable layered regime emerges. Initially, several vertically-stacked convection rolls develop, and these well-mixed layers are separated by sharp interfaces with large concentration gradients. The initial thickness of these emerging layers can be estimated by balancing the work exerted by thermal driving and the required potential energy to bring fluid out of its equilibrium position in the stably stratified fluid. In the layered regime, we further observe successive layer merging, and eventually only a single convection roll remains. We elucidate the following merging mechanism: as weakened circulation leads to accumulation of hot fluid adjacent to the hot sidewall, larger buoyancy forces associated with hotter fluid eventually break the layer interface. Then two layers merge into a larger layer, and circulation establishes again within the merged structure.

A roughness scaling behaviour is tested by performing the direct numerical simulation (DNS) of a turbulent channel flow over three-dimensional sinusoidal rough walls. By systematically varying the roughness height ${{k}^{+}}$ and the roughness steepness S, the results for three groups of cases are considered and compared with those for flat-wall turbulence. The results show that the mean velocity and Reynolds stresses are highly dependent on both ${{k}^{+}}$ and S. To describe these specific relationships, we define a coupling scale ${{k}^{+}} S$. With this coupling scale, all the simulated data for the roughness function (${\rm \Delta} {{U}^{+}}$), the ratio of the pressure drag to the total wall resistance (${{\gamma }_{p}}$), the normalized bulk mean velocity ($U_{b}^{+}$) and the peak of the streamwise turbulent velocity fluctuations ($\overline {u_{p}^{\prime +}}$) collapse onto single curves, which shows that there is a strong direct correlation between them, i.e. ${\rm \Delta} U^{+}, \gamma _{p}, U_{b}^{+}, \overline {u_{p}^{\prime +}} \propto f(k^{+} S)$. Furthermore, a model for the prediction of wall resistance based on the roughness function can be established by defining a drag increasing ratio (DI). Accordingly, the wall resistance coefficient ${{C}_{f}}$ can be estimated directly from ${{k}^{+}}S$ of a given rough surface. These results suggest that this coupling scale provides a useful alternative to the equivalent sand grain roughness ${{k}_{s}}$.

Linearisation of the Navier–Stokes equations about the mean of a turbulent flow forms the foundation of popular models for energy amplification and coherent structures, including resolvent analysis. While the Navier–Stokes equations can be equivalently written using many different sets of dependent variables, we show that the properties of the linear operator obtained via linearisation about the mean depend on the variables in which the equations are written prior to linearisation, and can be modified under nonlinear transformation of variables. For example, we show that using primitive and conservative variables leads to differences in the singular values and modes of the resolvent operator for turbulent jets, and that the differences become more severe as variable-density effects increase. This lack of uniqueness of mean-flow-based linear analysis provides new opportunities for optimising models by specific choice of variables while also highlighting the importance of carefully accounting for the nonlinear terms that act as a forcing on the resolvent operator.

Reconnection plays a significant role in the dynamics of plasmas, polymers and macromolecules, as well as in numerous laminar and turbulent flow phenomena in both classical and quantum fluids. Extensive studies in quantum vortex reconnection show that the minimum separation distance $\delta$ between interacting vortices follows a $\delta(t) \sim t^{1/2}$ scaling. Due to the complex nature of the dynamics (e.g. the formation of bridges and threads as well as successive reconnections and avalanche), such scaling has never been reported for (classical) viscous vortex reconnection. Using direct numerical simulation of the Navier–Stokes equations, we study viscous reconnection of slender vortices, whose core size is much smaller than the radius of the vortex curvature. For separations that are large compared to the vortex core size, we discover that $\delta (t)$ between the two interacting viscous vortices surprisingly also follows the 1/2-power scaling for both pre- and post-reconnection events. The prefactors in this 1/2-power law are found to depend not only on the initial configuration but also on the vortex Reynolds number (or viscosity). Our finding in viscous reconnection, complementing numerous works on quantum vortex reconnection, suggests that there is indeed a universal route for reconnection – an essential result for understanding the various facets of the vortex reconnection phenomena and their potential modelling, as well as possibly explaining turbulence cascade physics.

We explore the near-resonant interaction of inertial waves with geostrophic modes in rotating fluids via numerical and theoretical analysis. When a single inertial wave is imposed, we find that some geostrophic modes are unstable above a threshold value of the Rossby number $kRo$ based on the wavenumber and wave amplitude. We show this instability to be caused by triadic interaction involving two inertial waves and a geostrophic mode such that the sum of their eigenfrequencies is non-zero. We derive theoretical scalings for the growth rate of this near-resonant instability. The growth rate scaled by the global rotation rate is proportional to $(kRo)^2$ at low $kRo$ and transitions to a $kRo$ scaling for larger $kRo$. These scalings are in excellent agreement with direct numerical simulations. This instability could explain recent experimental observations of geostrophic instability driven by waves.