The violent hydrodynamic impact in water entry is a well-known phenomenon. Various methods have been applied to mitigate this “destructive force.” We experimentally investigate a method for reducing the impact force of water entry, which uses a gas jet to impinge on the liquid surface prior to the object entry. The blowing of the gas jet flow forms a cavity in the water and the penetration of the object into the water is thus less impactful. This method also significantly alters the flow field structure in the impact region, resulting in a reduction of the added fluid mass, the counterthrust of the gas jet that can further attenuate the impact intensity. We reported four cavity regimes and six impact modes formed in the gas-liquid interaction for a range of the modified Froude numbers. In particular, the regime of jet cavity only has a jet-assisted water entry with almost no impact event, and the maximum reduction in impact acceleration can reach nearly 90% with the reduction in entry velocity being about 10% relative to the impact without gas jet. The impact force reduction mechanism is considered to be a combination of the reduction of the added fluid mass and the presence of gas jet momentum.

We present a numerical analysis of the lateral movement of a deformable spherical capsule in a pulsatile channel flow, with a Newtonian fluid in an almost inertialess condition and at a small confinement ratio $a_0/R=0.4$, where $R$ and $a$ are the channel and capsule radius. We find that the speed of the axial migration of the capsule can be accelerated by the flow pulsation at a specific frequency. The migration speed increases with the oscillatory amplitude, while the most effective frequency remains basically unchanged and independent of the amplitude. Our numerical results form a fundamental basis for further studies on cellular flow mechanics, since pulsatile flows are physiologically relevant in human circulation, potentially affecting the dynamics of deformable particles and red blood cells, and can also be potentially exploited in cell focusing techniques.

This study aims to understand the impact of instabilities and turbulence arising from canopy mixing layers on wind-driven wildfire spread. Using an experimental flume (water) setup with model vegetation canopy and thermally buoyant plumes, we study the influence of canopy-induced shear and turbulence on the behavior of buoyant plume trajectories. Using the length of the canopy upstream of the plume source to vary the strength of the canopy turbulence, we observed behaviors of the plume trajectory under varying turbulence yet constant cross-flow conditions. Results indicate that increasing canopy turbulence corresponds to increased strength of vertical oscillatory motion and variability in the plume trajectory/position. Furthermore, we find that the canopy coherent structures characterized at the plume source set the intensity and frequency at which the plume oscillates. These perturbations then move longitudinally along the length of the plume at the speed of the free stream velocity. However, the buoyancy developed by the plume can resist this impact of the canopy structures. Due to these competing effects, the oscillatory behavior of plumes in canopy systems is observed more significantly in systems where the canopy turbulence is dominant. These effects also have an influence on the mixing and entrainment of the plumes. We offer scaling analyses to find flow regimes in which canopy induced turbulence would be relevant in plume dynamics.

Based on the phenomenological extension of Darcy's law, two-fluid flow is dependent on a relative permeability function of saturation only that is process or path dependent with an underlying dependency on pore structure and wettability. For applications (fuel cells to underground ${\mathrm{CO}}_2$ storage), it is imperative to determine the effective phase permeability relationships where the traditional approach is based on the inverse modeling of time-consuming experiments. The underlying reason is that the fundamental upscaling step from pore to Darcy scale, which links the pore structure of the porous medium to the continuum hydraulic conductivities, is not solved. Herein, we develop an artificial neural network (ANN) that relies on fundamental geometrical relationships to determine the mechanical energy dissipation during creeping immiscible two-fluid flow. The developed ANN is based on a prescribed set of state variables based on physical insights that predict the effective permeability of 4500 unseen pore-scale geometrical states with $R^2=0.98$.

We analyze the scaling properties of the energy spectra in fully developed incompressible turbulence in forced, rotating fluids in three dimensions (3D), which are believed to be characterized by universal scaling exponents in the inertial range. To elucidate the scaling regimes, we set up a scaling analysis of the 3D Navier–Stokes equation for a rotating fluid that is driven by large-scale external forces. We use scaling arguments to extract the scaling exponents, which characterize the different scaling regimes of the energy spectra. We speculate on the intriguing possibility of two-dimensionalization of 3D rotating turbulence within our scaling theory. Our results can be tested in large-scale simulations and relevant laboratory-based experiments.

Red blood cells (RBCs) change shape and deformability with age, making them more likely to be trapped in the splenic slits (IESs) and removed from circulation. Existing evidence suggests that cell size and viscoelastic properties are altered during aging, making it challenging to identify the determinants of mechanical retention of aged RBCs in the spleen. Herein, we present a comprehensive computational study to investigate the flow dynamics, morphological changes, and mechanical retention of age-associated RBCs in the splenic IESs. Our results show that, before reaching the critical size for the age-associated RBC to be trapped in the IESs, the reduced size of the RBC can shorten the transit time it takes to pass through the IESs, while the increased membrane viscosity slows it down. A narrower IES requires the RBC to undergo greater deformation to pass through. Furthermore, during the shrinkage associated with normal aging, a more significant critical pressure gradient is required for RBCs to deform more to pass through the IESs, resulting in more intense damage. These quantitative findings provide unique insights into the traversal dynamics and retention mechanism of age-associated RBCs.

A theoretical model is presented for the locomotion of an aquatic vehicle propelled by a flexible foil elastically mounted to translational and torsional springs and dampers at an arbitrary pivot axis and actuated by a harmonic torque. The work extends a previous model by the authors for an elastically mounted rigid foil [Lopez-Tello et al., Appl. Math. Model. 116, 236 (2023)], allowing for a passive flexural motion of the foil in addition to the passive pitching and heaving motions of the rigid foil, all of them generated by the actuating torque and the fluid-foil interaction. The Euler-Bernoulli beam equation is used together with linearized results from the potential flow theory, valid for small pitch, heave, and flexural deflection amplitudes. The problem is governed by four ordinary differential equations (ODEs) for the temporal evolutions of the swimming velocity, and the pitch, heave, and flexural motions of the flexible foil. In addition to numerical results of these ODEs, we also present an analytical perturbation solution, which provides a valuable quick insight about the propulsion performance, but which is additionally restricted to very small swimming velocities. The vehicle's propulsion performance is discussed in terms of the foil stiffness ratio and the remaining nondimensional parameters, particularly the translational and torsional spring constants, the pivot axis location, and the Lighthill number. It is found that, except for very low Lighthill numbers, the maximum swimming velocity is reached for a rigid foil actuated at the leading edge with the resonant combination of the translational and torsional springs constants for the given frequency. However, higher propulsive efficiencies and lower costs of transport, but with slightly smaller swimming velocities, are obtained for flexible foils with the same resonant combination of the elastic supports at the leading edge. As a validation of the model, the Strouhal number for optimal propulsion efficiency is found in a narrow band around 0.32, in agreement with many experimental and numerical works on optimal propulsion by flapping foils. Additionally, the relation between Strouhal and Lighthill numbers for optimal propulsion is favorably compared with experimental data for fishes where the primary mechanism for producing thrust is an oscillatory prominent caudal fin.

We study experimentally the enhancement of splashing due to solidification. Investigating the impact of water drops on dry smooth surfaces, we show that the transition velocity to splash can be drastically reduced by cooling the surface below the liquid melting temperature. We find that at very low temperatures (below $-60{}^{\circ }\mathrm{C}$), the splashing behavior becomes independent of surface undercooling and presents the same characteristics as on ambient temperature superhydrophobic surfaces. This resemblance arises from an increase of the dynamic advancing contact angle of the lamella with surface undercooling, going from the isothermal hydrophilic to the superhydrophobic behavior, a phenomenon that we relate to ice growth near the contact line. Finally, we show that the transition from hydrophilic to superydrophobic behavior can also be characterized quantitatively on the dynamics of the ejecta.

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertioelastic instabilities. The nonlinear evolution of these instabilities generates a state of turbulence with significantly different spatiotemporal features compared to Newtonian turbulence, termed elastoinertial turbulence (EIT). We explore EIT by studying the dynamics of a submerged planar jet of a dilute aqueous polymer solution injected into a quiescent tank of water using a combination of schlieren imaging and laser Doppler velocimetry (LDV). We show how fluid elasticity has a nonmonotonic effect on the jet stability depending on its magnitude, creating two distinct regimes in which elastic effects can either destabilize or stabilize the jet. In agreement with linear stability analyses of viscoelastic jets, an inertioelastic shear-layer instability emerges near the edge of the jet for small levels of elasticity, independent of bulk undulations in the fluid column. The growth of this disturbance mode destabilizes the flow, resulting in a turbulence transition at lower Reynolds numbers and closer to the nozzle compared to the conditions required for the transition to turbulence in a Newtonian jet. Increasing the fluid elasticity merges the shear-layer instability into a bulk instability of the jet column. In this regime, elastic tensile stresses generated in the shear layer act as an “elastic membrane” that partially stabilizes the flow, retarding the transition to turbulence to higher levels of inertia and greater distances from the nozzle. In the fully turbulent state far from the nozzle, planar viscoelastic jets exhibit unique spatiotemporal features associated with EIT. The time-averaged angle of jet spreading, an Eulerian measure of the degree of entrainment, and the centerline velocity of the jets both evolve self-similarly with distance from the nozzle. The autocovariance of the schlieren images in the fully turbulent region of the jets shows coherent structures that are elongated in the streamwise direction, consistent with the suppression of streamwise vortices by elastic stresses. These coherent structures give a higher spectral energy to small frequency modes in EIT characterized by LDV measurements of the velocity fluctuations at the jet centerline. Finally, our LDV measurements reveal a frequency spectrum characterized by a −3 power-law exponent, different from the well-known −5/3 power-law exponent characteristic of Newtonian turbulence.

We study the interaction-induced migration of bubbles in shear flow and observe that bubbles suspended in elastoviscoplastic emulsions organize into chains aligned in the flow direction, similarly to particles in viscoelastic fluids. To investigate the driving mechanism, we perform experiments and simulations on bubble pairs, using suspending fluids with different rheological properties. First, we notice that, for all fluids, the interaction type depends on the relative position of the bubbles. If they are aligned in the vorticity direction, then they repel, if not, then they attract each other. The simulations show a similar behavior in Newtonian fluids as in viscoelastic and elastoviscoplastic fluids, as long as the capillary number is sufficiently large. This shows that the interaction-related migration of the bubbles is strongly affected by the bubble deformation. We suggest that the cause of migration is the interaction between the heterogeneous pressure fields around the deformed bubbles, due to capillary pressure.

We study the behavior of a thin liquid film flowing over a compliant substrate that is thin enough. The breakdown of time-reversal symmetry includes the odd component of the Cauchy stress tensor, resulting in some significant effects that were not previously explored. We utilize the long-wave theory to obtain equations coupling the thickness of the film and the compliant substrate. These equations consider inertia, damping effects, wall tension, and odd viscosity. Through linear stability analysis, we establish that the compliant substrate destabilizes the system while odd viscosity significantly stabilizes. We also apply a weakly nonlinear approach to explore the system's dynamics. The coupled long-wave equations lead to the derivation of the Kuramoto-Sivashinsky equation. Notably, incorporating the odd viscosity tensor component can avert chaotic behavior in compliant substrates within the weakly nonlinear limit. Numerical simulations reveal that the odd viscosity produces remarkable effects on substrate deflection. Furthermore, we demonstrate the consistency of the linear and weakly nonlinear theories with numerical results by performing a numerical investigation of the coupled long-wave equations.

The shear-induced diffusivity of non-Brownian and neutrally buoyant spheres suspended in a viscous fluid was measured. The monodisperse spheres were roughened using a mechanical process and the diffusions of the original and roughened particles were measured and compared. The rougher particles were found to have a lower diffusivity for volume fractions larger than 0.25, despite expectations that roughness increases diffusion. Simulations that incorporate a contact force predict two regimes of dependence of the diffusion coefficient on roughness: below a volume fraction of 0.2, rougher particles were found to have a larger diffusivity, but a lower diffusivity for volume fractions higher than 0.25. The simulations reveal that increased roughness promotes organization of the concentrated particles into layered structures aligned in the flow direction at high volume fractions. This organization results in fewer particle collisions and a corresponding decrease in the diffusivity. Simulations with and without bounding walls predict similar trends for the diffusion coefficient as a function of volume fraction. Using a sufficiently polydisperse distribution of particle sizes disrupts the organization of the particles and consequently the effect of roughness on diffusivity.

High-fidelity large-eddy simulations of the flow around two rectangular obstacles are carried out at a Reynolds number of $10000$ based on the freestream velocity and the obstacle height. The incoming flow is a developed turbulent boundary layer. Mean-velocity components, turbulence fluctuations, and the terms of the turbulent-kinetic-energy budget are analyzed for three flow regimes: skimming flow, wake interference, and isolated roughness. Three regions are identified where the flow undergoes the most significant changes: the first obstacle's wake, the region in front of the second obstacle, and the region around the second obstacle. In the skimming-flow case, turbulence activity in the cavity between the obstacles is limited and mainly occurs in a small region in front of the second obstacle. In the wake-interference case, there is a strong interaction between the freestream flow that penetrates the cavity and the wake of the first obstacle. This interaction results in more intense turbulent fluctuations between the obstacles. In the isolated-roughness case, the wake of the first obstacle is in good agreement with that of an isolated obstacle. Separation bubbles with strong turbulent fluctuations appear around the second obstacle.

Turbulent pipe flows exhibit organizational states (OSs) that are labeled by discrete azimuthal wave number modes and are reminiscent of the traveling wave solutions of low Reynolds number regimes. The discretized time evolution of the OSs, obtained through stereoscopic particle image velocimetry, is shown to be non-Markovian for data acquisition carried out at a structure-resolved sampling rate. In particular, properly defined time-correlation functions for the OS transitions are observed to decay as intriguing power laws, up to a large-eddy time horizon, beyond which they decorrelate at much faster rates. We are able to establish, relying upon a probabilistic description of the creation and annihilation of streamwise streaks, a lower-level Markovian model for the OS transitions, which reproduces their time-correlated behavior with meaningful accuracy. These findings indicate that the OSs are distributed along the pipe as statistically correlated packets of quasistreamwise vortical structures.

Coherent flows of self-propelled particles are characterized by vortices and jets that sustain chaotic flows, referred to as active turbulence. Here, we reveal a crossover between defect-free active turbulence and active turbulence laden with topological defects. Interestingly, we show that concurrent to the crossover from defect-free to defect-laden active turbulence is the restoration of the previously broken $\mathrm{SO}\left(2\right)$ symmetry signaled by the fast decay of the two-point correlations. By stability analyses of the topological charge density field, we provide theoretical insights on the criterion for the crossover to the defect-laden active turbulent state. Despite the distinct symmetry features between these two active turbulence regimes, the flow fluctuations exhibit universal statistical scaling behaviors at large scales, while the spectrum of polarity fluctuations decays exponentially at small length scales compared to the active energy injection length. These findings reveal a dynamical crossover between distinct spatiotemporal organization patterns in polar active matter.

A filter membrane may be frequently used during its lifetime, with filtration and drying processes occurring in the porous medium for several cycles. During these cycles, the concentration and distribution of molecules or contaminants as well as the medium morphology evolve. As a consequence, the filter performance ultimately deteriorates after several cycles. In this work, we formulate a coupled mathematical model for the filtration and drying dynamics in a porous medium occurring consecutively. Our model accounts for the porous medium internal morphology (internal structure, porosity, etc.), the contaminant deposition, and the evolution of dry-fluid interfaces due to evaporation. An asymptotic model is derived based on the small aspect ratio of the thin filter membrane. The reduced model provides insights to the overall porous medium evolution over cycles of filtration and drying processes and predicts the timeline to discard the filter based on its optimum performance. Given the complexity of fluid boundary movements due to the filtration and drying processes, the reduced model still acts as an efficient prediction tool offering a tremendous reduction in computational costs.

We study the propagation and wall scattering of two-dimensional thermodynamic disturbances in a gas at noncontinuum conditions. Initial system perturbations are modeled as arbitrary (local) small-amplitude density or temperature inhomogeneities, prescribed in the vicinity of an impermeable specular or diffuse (isothermal) wall. The problem is analyzed in the free-molecular and ideal-flow limits, complemented by direct simulation Monte Carlo (DSMC) calculations at arbitrary rarefaction rates. Closed-form results are obtained for the collisionless and ideal-flow gas responses to impulse excitation, followed by comparisons between DSMC and limit-case predictions for the case of Gaussian perturbations. While the acoustic signal is subject only to a geometric two-dimensional decay and carries to large distances in the inviscid limit, it decays rapidly at high rarefaction rates due to the mechanism of molecular dispersion. Different from the identical gas responses to local compression and heating in the inviscid regime, qualitative differences are highlighted and analyzed in the free-molecular limit. The results additionally indicate lower pressure levels in an isothermal- compared with a specular-wall system, due to the exchange of gas-surface energy taking place in the former. Finally, the acoustic force acting on the solid surface is examined. Apart from an early-time repelling impact, peculiar late-time attraction is found in the case of gas heating excitation at high Knudsen numbers.

Identifying and tracking transient fluid responses has long been challenging due to the inherent and correlated complexities in both spatial and temporal domains. Kernel mode decomposition (KMD) is a newly proposed method capable of capturing the transitioning amplitude and frequency locally. In this study we extend the KMD with a sparsification network that groups the response modes based on their spectral and spatial proximity using an energylike index to instigate systems with transitional behaviors. A synthetic problem setup is used to demonstrate how the proposed method can identify essential modes with changing both amplitude and frequencies. A two-sided oscillating lid-driven cavity flow problem demonstrates that the KMD network can further track the transition of modes even when spatial distributions vary over time. Finally, we inspect the formation process of a laminar separation bubble with the proposed method and isolate multiple competing mechanisms involved in the process. These results reveal that KMD can extend the application of modal analysis methods to identify transitioning spatial structures associated with varying frequency in an interpretable fashion, a necessary step for their use for understanding broad dynamic systems.

We seek to develop a low-dimensional model for the interactions between horizontally adjacent turbulent convection rolls. This was tested in Rayleigh-Bénard convection experiments with two adjacent cubic cells with a partial wall in between. Observed stable states include both counterrotating and corotating states for Rayleigh number $7.6\times {10}^7<$ Ra $<3.5\times {10}^9$ and Prandtl number 6.41. The stability of each of these states and their dynamics can be modeled low-dimensionally by stochastic ordinary differential equations of motion in terms of the orientation, amplitude, and mean temperature of each convection roll. The form of the interaction terms is predicted based on an effective turbulent diffusion of temperature between the adjacent rolls, which is projected onto the neighboring rolls with sinusoidal temperature profiles. With measurements of a constant coefficient for effective thermal turbulent diffusion, quantitative predictions are made for the nine forcing terms which affect stable fixed points of the corotating and counterrotating states for $5.5\times {10}^8<$ Ra $<3.5\times {10}^9$. Predictions are found to be accurate within a factor of 3 of experiments. This suggests that the same turbulent thermal diffusivity that describes macroscopically averaged heat transport also controls the interactions between neighboring convection rolls. The surprising stability of corotating states is due to the temperature difference between the neighboring rolls becoming large enough that the heat flux between the rolls stabilizes the temperature profile of aligned corotating states. This temperature difference can be driven with an asymmetry, for example, by heating the plates of the two cells to different mean temperatures. When such an asymmetry is introduced, it also shifts the orientations of the rolls of counterrotating states in opposite directions away from their preferred orientation, which is otherwise due to the geometry of the cell. As the temperature difference between the plates of the different cells is increased, the shift in orientation increases until the counterrotating states become unstable and only corotating states are stable. At very large temperature differences between cells, both the counterrotating and predicted corotating states become unstable; instead we observe a corotating state with much larger temperature difference between the rolls that cannot be explained by turbulent thermal diffusion. Spontaneous switching between corotating and counterrotating states is also observed, including in nominally symmetric systems. Switching to counterrotating states occurs mainly due to cessation (a significant weakening of a convection roll), which reduces damping on changes in orientation, allowing the orientation to change rapidly due to diffusive fluctuations. Switching to corotating states is mainly driven by smaller diffusive fluctuations in the orientation, amplitude, and mean temperature of rolls, which have a positive feedback that destabilizes the counterrotating state.

We investigate the individual and collective dynamics of torque-driven particles, called microrollers, near fluid-fluid interfaces. We find that the viscosity ratio across the interface controls the speed and direction of the particles, their relative motion, the growth of a fingering instability, and the self-assembled motile structures that emerge from it. By combining theory and large scale numerical simulations, we show how the viscosity ratio across the interface governs the long-range hydrodynamic interactions between particles and thus their collective behavior.