A novel scaling methodology is presented for unique wind and water tunnel experiments of floating offshore wind farms. Geometric similarity of the floater design is leveraged to improve the hydraulic response and relax Froude scaling. Turbine motion, power, and wake measurements reveal intricate coupled dynamic interactions between wave topology, wake evolution, turbine response, and wind farm power output.

We introduce a novel dimensionless parameter, termed the centrifugal work number, which is derived from the rotational energy equation. This parameter quantifies the ratio of centrifugal force work to gas enthalpy in the flow within rotating channels. Large eddy simulation was employed to validate the influence of this parameter on rotating channel flow. The flow similarity theory considering the fluid compression characteristics inside the rotating channel has been established.

Wetting of surfaces holds a crucial interest for diverse technological and societal areas, from nanotechnology to contagion dynamics. We propose the usage of substrates with time-varying wettability as a novel tool to solve a critical problem, namely the control of dewetting morphologies. A new numerical method, able to handle unprecedented system sizes in this context, is employed. We unveil the existence of a droplet-to-rivulet transition determined by the substrate adaptation rate. Our work paves an avenue for the application of simulations to study the complex dynamics of thin liquid films over switchable and adaptive substrates.

The familiar pouring sound we all hear when preparing tea or coffee has been a rare topic of study so far, despite its importance in several applications. We experimentally find that the sound amplitude increases with the jet corrugation, indicating that thin jets are louder than thick ones for the same given height. When pouring from a high distance, the jet breaks up into impacting drops, and the amplitude increases with the jet length and diameter. Results show that the jet corrugation relates to the volume of entrained air, hence the pouring sound can enter as a practical method to measure water aeration rates.

Dragonfly wings, with their unique corrugated structure, may achieve superior aerodynamic performance in specific flight conditions, although these conditions are not yet fully understood. This led us to investigate the vortex dynamics and lift generation as a corrugated wing transitions from a stationary state to translational motion. The results reveal that suppression of secondary vortices generated on the wing significantly improves the overall performance of corrugated wings. This advances our understanding of flight dynamics and can also contribute to applications in engineering and biomimicry.

This paper reports on a reciprocating flow through a double bifurcation geometry to quantify the potential gas transport in flows similar to those in the human airway during high-frequency ventilation. The roles of nonlinear mean streaming and turbulent diffusion are investigated in appropriate clinical conditions and their dependence on both the upstream and downstream conditions are manifested. We report turbulent bursts occur in the first three generations of the airway when the instantaneous flow velocity exceeds a critical value. Results highlight the importance of considering a model geometry with appropriate complexity for physiological flows.

The beating of cilia and flagella has fascinated biologists and physicists alike for decades. Previous mathematical models designed to capture the corresponding flow fields are typically either highly simplified and do not resolve all lengths scales or are computationally demanding and cannot be easily generalized to larger arrays of cilia. This paper develops an alternative method which uses point torques to represent the cilia and captures the near- and far-field characteristics in an efficient and accurate manner. These singularity solutions can be used to accurately calculate the collective flows and transport properties of larger ciliary arrays.

We present a novel three-dimensional configuration for the numerical study of the flame-vortex interaction. The premixed flame is initially located within a vortex tube with the same trefoil knotted centerline, and it propagates outward and interacts with the knotted vortex tube. This configuration allows the study of flame extinction, flame-flame interaction, and suppression of the vortex reconnection. Using direct numerical simulations, we show that the asymmetric flame propagation along the vortex axis generates helicity, and the baroclinic effect arising from the misalignment between the local density gradient and nonlocal pressure gradient flattens the vortex/flame tubes.

In the presence of shear-enhanced diffusion (Taylor dispersion), flame propagation is effectively anisotropic. The paper clarifies the influence of the direction of a shear flow relative to the direction of propagation on the diffusional-thermal instabilities of premixed flames. Interestingly, the cellular flame instability can now be encountered in mixtures with Lewis numbers larger than one.

Many materials encountered around us in daily life are yield stress materials, which behave solid-like for small applied stresses, but start to flow when the applied stress exceeds a threshold. The shear viscosity of these materials is relatively well understood. However, this is far from a complete and satisfactory description of the mechanical response, and most flows are more complicated. A coherent picture for general flows is still lacking.

Buoyancy-driven flow of an electrically conducting fluid occurs in planets, stars and industrial applications. Here we use simulations of Rayleigh-Bénard convection with an imposed horizontal magnetic field and stress-free mechanical boundary conditions as a simplified model for understanding such systems. As the buoyancy forcing and magnetic field strength are increased we find a sequence of bifurcations, ranging from steady two-dimensional convection cells to three-dimensional anisotropic convective turbulence. Large scale mean flows become dominant as the parameters are made more extreme.

The numerical simulations reported here show that the significant slowdown of many stars can be understood as a subcritical transition to turbulence. It is sustained by a nonlinear mechanism involving the emergence of a strong magnetic field in the radiative region of stars. This turbulence provides a new source of angular momentum transport that persists at very low differential shear, arbitrarily far from any hydrodynamic instability. This scenario is captured by a simple nonlinear model that provides a general description of the transition to turbulence in astrophysical flows, as long as it involves competition between a large-scale magnetic field and a small-scale magnetic instability.

We study the dynamics of a mesoscale current-undercurrent system motivated from the multilayer circulations in the Western North Pacific, attempting to further understand the dynamic origin of the Luzon Undercurrent. We adopt the so-called biglobal analysis, which can deal with realistic basic flows of continuously nonuniform vertical shear and strong horizontal variation. The high vertical shear between the upper and lower currents, and that near the vertical boundaries give rise to two branches of unstable modes of distinct dynamical features. The interplay between baroclinic and barotropic instabilities, as well as the coupling ageostrophic and viscous/diffusive motions are revealed.

This work investigates an idealized model of a finite-volume, cylindrical gravity current, placing it in a context with non-buoyant dipoles and atmospheric ‘modons’. In cross-section, the current presents as a semicircular vortex of dense fluid in an unstratified background. In the frame traveling with the current, the flow is in a steady-state, achieved by a balanced generation of baroclinic vorticity. Numerical experiments demonstrate how the steadiness of a single vortex depends upon this balance, and show that vortex interactions have similarities to those of solitary waves.

Instantaneous flow fields over a NACA 0012 aerofoil at the stall and post-stall angles of attack have been measured using the time-resolved particle image velocimetry technique. The time sequence of the measured data reveals that the massive separation over the upper surface of the aerofoil is intermittent in nature. This intermittent massive separation is due to the interchange of instability, that is, from a convective to an absolute instability and from an absolute to a convective instability. A physical mechanism for this intermittent massive separation has also been proposed.

We fabricate a soluble microfluidic channel within a micro-PIV-based imaging system to explore the pore-scale dissolution dynamics in a gravity field. We find that the evolution of eddies induced by buoyancy-driven convection within the flow-dissolution system governs the dissolution rate and the dissolution regimes. This research enhances our comprehension of the interplay between buoyancy-driven convection and forced convection in etching the solid surface.

In the image, the free surface between a liquid and air is deformed into a sharp cusp by the rapid rotation of the cylinder on the left. We show that such a structure is a generic feature of viscous flow, and study its properties. This is important, since air can enter the fluid through the cusp’s tip, and thus become entrained into the interior of the fluid.

When flowing through disordered media, fluid-fluid interfaces are subjected to sharp capillary jumps that shape displacements at large scales. The intricacies of this phenomenon are hard to decipher due to the inherent stiffness and non-locality of the problem. Here we use an “imperfect” Hele-Shaw cell to elucidate the response to a single topographical defect as a function of the imposed flow rate, keeping to stable Saffman-Taylor conditions. Through imbibition-drainage experiments and numerical integration of a robust two-dimensional model we showcase the profound impact of the proximity to the fingering instability.

Freezing droplets are ubiquitous in many practical applications, like in aircraft and wind turbine icing, spray technologies, food and pharmaceutical engineering, and natural phenomena such as raindrop formation. In the context of sessile droplets, the freezing process unveils fascinating physics, featuring a frost halo on the substrate, the evolution of the liquid-ice interface, and the formation of a cusp-like morphology at the tip of the droplet. Our study explores the novel theoretical aspects of frost halo formation, revealing its intricate link to the inherent evaporation process during the initial freezing phases.

We perform experiments with liquid nitrogen’s gas-liquid interface oscillations in a U-shaped quartz tube and investigate the wetting dynamics in inertia-dominated conditions. The experiments reveal a linear relationship between dynamic contact angle evolution and Capillary number in advancing conditions while the contact angle remains near equilibrium in receding conditions. An equivalent contact angle, derived from a model, shows the overall independence of the capillary pressure from the actual contact angle evolution. Theoretical analysis indicates viscous forces dominate in small tubes, while gravity and inertial forces govern larger tube oscillations.

We introduce a quantum inspired and widely applicable tensor network-based framework for developing reduced order models describing wall-bounded fluid flows. For the examples of the lid-driven and doubly-driven cavities, we find that our method only uses a small fraction of the number of variables parameterizing the solution compared to direct numerical simulation and can improve its runtime by an order of magnitude on comparable hardware. Our work provides a novel path towards efficient high-precision simulations of the Navier-Stokes equation at high Reynolds numbers.

Oscillatory flows in deformable tubes have been of intense interest since Womersley’s work in the 1950s. The solutions for the pressure, flow rate, and wave propagation along the tube are a cornerstone of biofluid mechanics. However, it is assumed that the hydrodynamic pressure can only cause infinitesimal wall deformations; the cross-sectional area cannot change. Yet, oscillatory flows do deform conduits to such an extent that a nonlinear pressure gradient develops. We derive and benchmark a reduced-order model (a single, complex-valued partial differential equation for the pressure) that captures two-way coupling between flow and deformation, without restrictions on the oscillation frequency.

Viscous-flow induced buckling modifies the morphology of sheet-like particles suspended in liquids. Experiments reveal that a pair of sheets can bend at a shear rate ten times lower than the buckling threshold defined for a single sheet. Using simulations and modeling, we demonstrate that this softening is due to hydrodynamic interactions. Our study suggests that the morphology of sheet-like particles in suspensions and the resulting rheology is not a purely material property, but also depends on particle concentration and microstructure.

Granular flows exhibit kinematic patterns reminiscent of turbulence. In discrete element method simulations, we measure the characteristic size of these patterns and relate it to the size of a representative element volume (REV). We find that this size increases and diverges close to jamming. Such large REVs may be the root cause of nonlocal behaviors and finite size effects observed at the continuum level.

Semi-dilute to dense particle-laden inertial pipe flows are studied using magnetic resonance imaging in conjunction with direct numerical simulations. In general, both methods show an excellent agreement for both the time-averaged velocity and concentration profiles. Different flow regimes are identified based on the experimental and numerical data. For higher solid volume fractions a core-peaking concentration distribution is found that explains the observed drag decrease relative to the drag expected for a uniform concentration distribution.

High speed granular flows flowing over inclined surfaces are studied using discrete element method and continuum simulations. Significant slip velocity at the base and strong oscillations in the layer height are observed. The popular inertial-number-based JFP rheological model fails to capture the transient flow dynamics at high inertial numbers. Accounting for the layer dilatancy effect and the presence of normal stress difference is essential to accurately predict the average flow behavior. A modified rheological model recently proposed by the authors, accounting for all these effects describes the evolution of the high-speed granular flow very well for both low as well as high inertial numbers.

Active fluids operating at negligible Reynolds numbers can exhibit spontaneous coherent motion, dynamical vortex patterns, and mesoscale turbulence. We employ tools from nonlinear dynamical systems theory to uncover the global phase space of two-dimensional active nematic channel flow. We compute several Exact Coherent Structures (ECSs), which are exact solutions of the physical dynamics with distinct and regular spatiotemporal structure; examples include unstable equilibria, periodic orbits, and traveling waves. We provide numerical evidence to show that this collection of ECSs and their invariant manifolds act as an organizing template for the complicated spatiotemporal motion of the active fluid.

Prior studies have shown that the eddy diffusivities governing mean passive scalar transport can be nonlocal in space and time. However, nonlocal eddy diffusivities are often cost prohibitive to compute and difficult to implement in reduced-order models. This work proposes a systematic and cost-effective approach for quantifying and modeling nonlocal eddy diffusivities.

A precise relationship between the boundary layer edge velocity components, $U_e$ and $V_e$, and the friction velocity, $u_{\tau }$, is derived from the integral of the continuity and momentum equations for flow over a flat plate. This relationship is validated using experimental and numerical data across a wide range of Reynolds numbers. At higher Reynolds numbers, the integral relation can be approximated as $U_e{V}_e/u_{\tau }^2\approx H_{12}$, where $H_{12}$ represents the shape factor.

Compressible turbulent boundary layers over concave surface curvature are of major interest in the design of high-speed aircraft and propulsion systems. In this work, we investigate the effect of longitudinal concave surface curvature on the statistical and structural properties of the fluctuating wall shear stress and wall heat flux, including probability density function, space-time correlations, and frequency spectra. We also quantitatively demonstrate that the significantly energized outer large-scale structures make an increasingly important contribution to the mean wall shear stress and wall heat flux generation.

Compressible turbulent channel flows exhibit a multitude of vortex structures dispersed throughout the boundary layer. Although the structural similarity between compressible and incompressible flows has been well established, most of these observations are concluded through merely visualizations of instantaneous fields. We dissect the statistical features of vortices in compressible channel flows comprehensively and show a quantitative consistency with incompressible flows by applying semi-local scaling. This work fills a gap in our knowledge about vortex structures in compressible channel flows and first applies Morkovin’s hypothesis to statistics of instantaneous vortex features.

Reynolds number (Re) effects are studied in the outer region of moderate adverse-pressure-gradient (APG) turbulent boundary layers (TBLs). This study finds that the small scale (viscous) energy in an APG TBL at near-equilibrium conditions reduces with increasing friction Re. The origin of this trend is traced back to the production of turbulent kinetic energy in an APG TBL, the small scale contribution to which is also found to decrease with Re in the outer region. The results emphasize that new scaling arguments and spatial-resolution corrections should be tested rigorously across a broad range of Re, particularly for pressure gradient TBLs.

Scalar-field initial conditions can have a strong effect on the evolution(s) of scalar fields and the rate at which mixing occurs. The effects of the scalar field initial conditions are studied by analyzing the evolution of three scalar fields with interfaces oriented normal to the streamwise, wall-normal, and transverse directions. When the interface is aligned normal to the mean velocity vector, higher rates of production and destruction of the scalar dissipation, as well as strong advection and stretching of the interface by the mean flow are observed. It is therefore recommended that scalar interfaces be aligned normal to the mean velocity vector to promote mixing within internal flows.

$A$ $posteriori$ validation and verification of black box machine learned turbulence models is time consuming and is not always fruitful. We discuss a theoretical framework that allows $a$ $priori$ screening of machine-learned models that are based on feed-forward neural networks. It requires no knowledge of the weights and bias and only knowledge of the activation function. The method tells one whether a machine learned model preserves basic calibrations like the law of the wall.

A direct numerical comparison is made of two-dimensional Navier-Stokes flows between two different boundary conditions, namely the whole plane and a periodic domain. We study their differences after adjusting the Reynolds number. The norms generally decay faster on the periodic domain than on the whole space. In the case of a simple vortex merger the difference is appreciable, whereas the difference is small in the case of turbulence. This gives support to studying finite-energy turbulence under periodic boundary conditions.

Internal waves play an important role in the dynamics of the ocean. When propagating in vertically varying stratification, they generate harmonic modes through a nonlinear effect. In this work, measured ocean stratification profiles are analyzed to estimate the propensity of oceanic internal tides to exhibit this effect. As seen in the figure, nonlinear harmonic generation could plausibly occur in the Equatorial Indo-Pacific, but not elsewhere, in the global ocean.