The unsteady behavior of an Edney type-IV shock-dominated hypersonic separated flows over a double-wedge geometry for different gas compositions are studied using the time-accurate direct-simulation Monte Carlo (DSMC) and window proper orthogonal decomposition (WPOD) methods. Near steady state, we find that the POD modes can be correlated with the global modes predicted by linear stability theory. The WPOD analyses show that the first mode for each flow quantity outlines the corresponding steady-state solution and that higher spatial POD modes are pronounced in the bow shock, separation, and transmitted shocks and shear layers, thereby coupling these regions. The temporal modes corresponding to each macroscopic flow quantities are observed to have the same decay rates. The effects of three types of gas compositions—molecular nitrogen, nonreacting air consisting of molecular nitrogen and oxygen, and reacting air with oxygen dissociation and the ${\mathrm{N}}_2$ + O exchange reactions—on flow stability are considered. Nonequilibrium thermochemical effects are found to change the shock structures, the size of the separation region, and the time required to reach steady state. The decay rate of the least damped eigenmode for the chemically reacting air case is found to be smaller in comparison to the nonreacting air case since the translational temperatures downstream of the bow shock are lower due to endothermic chemical reactions. The simulated heat fluxes and shock standoff distance are found to be in qualitative agreement with recent experiments where the freestream density is eight times higher than the current DSMC computations.

A bi-frequency open-loop control strategy aiming to combine both high- and low-frequency forcing effects is used to experimentally reduce the drag of a simplified car model at a slight yaw angle of $5^{\circ }$. The unforced mean wake features a lateral asymmetry which induces a low base pressure footprint close to the leeward side and increases drag compared to the aligned model. Forcing is performed with pulsed jets along the windward trailing edge. High-frequency forcing acts as a time-invariant flap. The fluidic flap effect deviates the windward shear layer towards the leeward side and reduces the wake bluffness, but the lateral asymmetry of the near wake is still observed. The drag reduction related to this high-frequency forcing is about 6% with a high actuation efficiency. A modulation of the high-frequency forcing with a low-frequency component is then introduced in order to modify the mass and momentum exchange in the separating shear layer at the windward trailing edge. We find that the modulated forcing provides the ability to manipulate the mean wake orientation while maintaining the fluidic flap effect. Among all wake orientations, those reducing drag are the ones having a mean symmetric wake. The bi-frequency control strategy leads to a maximum drag reduction of 7% for the best choice of frequencies. Importantly, the bi-frequency control is more efficient than the single high-frequency forcing, the actuator requiring only half the actuation energy and presenting an actuation efficiency multiplied by 3. Finally, the physical mechanisms related to drag reduction are carefully analyzed. In particular, we show that the wake symmetrization reduces the global production of turbulent kinetic energy in the shear layers. These results open up opportunities for closed-loop control of wake asymmetries.

The aerodynamic loads on a static airfoil are modified below its stall margin by deliberate formation and regulation of trapped vorticity on the suction and pressure surfaces near the trailing edge. Vorticity accumulation and shedding are effected using a hybrid actuator comprising a miniature two-dimensional passive protuberance having a cross-stream scale that is nominally commensurate with the local boundary layer thickness, and the vorticity flux is regulated using integrated spanwise arrays of equally spaced high aspect ratio synthetic jet actuators. The aerodynamic loads are varied by independent differential control of flux of opposite sense vorticity from both sides of the airfoil that are characterized using measurements of the force/torque, surface pressure, and particle image velocimetry over the suction surface and in the airfoil's near wake. The induced changes in circulation and lift and in the pitching moment are traced to complex transitory changes in vorticity flux associated with the onset and termination of the actuation. These changes are characterized by transitory variations in the balance of flux and shedding of clockwise and counterclockwise vorticities that are triggered, respectively, by actuation on the pressure or suction surfaces. These investigations indicate that hybrid flow control by trapped vorticity, which has the potential to manipulate prestall aerodynamic loads without mechanical control surfaces, may enable novel wing designs.

When transported in confined geometries rigid fibers show interesting transport dynamics induced by friction with the top and bottom walls. Fiber flexibility causes an additional coupling between fiber deformation and transport and is expected to lead to more complex dynamics. A first crucial step for their understanding is the characterization of the deformed fiber shape. Here we characterize this shape for a fiber transported in a confined plug flow perpendicular to the flow direction using a combination of microfluidic experiments and numerical simulations. In the experiments, size, initial orientation, and mechanical properties of the fibers are controlled using microfabrication techniques and in situ characterization methods. The numerical simulations use modified Brinkman equations as well as full three-dimensional simulations. We show that the bending of a perpendicular fiber results from the force distribution acting on the elongated object and is proportional to the elasto-viscous number, which compares viscous to elastic forces. We quantitatively characterize the influence of the confinement on the fiber deformation. The precise understanding of the deformation of a flexible fiber in a confined geometry can also be used in future to understand the deformation and transport of more complex deformable particles in confined flows, such as vesicles or red blood cells.

Experiments were conducted with three-dimensional printed disks and rods to study the orientation dynamics of nonspherical particles under surface gravity waves. Trials were run with both neutrally buoyant and slightly negatively buoyant particles, which were large enough that inertia due to their finite size was important. Although the particles had a broad distribution of initial orientations, over time the waves were observed to focus these orientations toward a preferred angle that agreed with theory. Negatively buoyant particles additionally exhibited a tendency to adopt an orientation that maximized vertical drag. The overall orientation can be described as the result of a competition between the orientation favored by waves and the orientation favored by settling. The spread about the mean orientation was also observed to increase with wave strength. Finally, the stabilization of out-of-plane orientations of disk-shaped particles was observed due to their finite size.

We present an experimental analysis of the linear and nonlinear regimes of an attractor of inertial waves in a trapezoidal cavity under rotation. Varying the rotation rate and the forcing amplitude and wavelength, we identify the scaling laws followed by the attractor amplitude and wavelength in both regimes. In particular, we show that the nonlinear scaling laws can be well described by replacing the fluid viscosity in the linear model by a turbulent viscosity, a result that could help in extrapolating attractor theory to geophysically and astrophysically relevant situations. We further study the triadic resonance instability of the attractor which is at the origin of the turbulent viscosity. We show that the typical frequencies of the subharmonic waves produced by the instability behave very differently from previously reported numerical results and from the prediction of the theory of triadic resonance. This behavior might be related to the deviation from the horizontal invariance of the attractor in our experiment in relation to the presence of vertical walls of the cavity, an effect that should be at play in all practical situations.

To date, axisymmetric internal wave fields, which have relevance to atmospheric internal wave fields generated by storm cells and oceanic near-inertial wave fields produced by surface perturbations, have been experimentally realized using an oscillating sphere or torus as the source. Here we use a wave generator configuration capable of exciting axisymmetric internal wave fields of arbitrary radial form to generate axisymmetric internal wave modes. After establishing the theoretical background for axisymmetric mode propagation, taking into account lateral and vertical confinement, and also accounting for the effects of weak viscosity, we study modes of different order. We characterize the efficiency of the wave generator through careful measurement of the wave amplitude based upon group velocity arguments, and then consider the effect of vertical confinement to induce resonance, identifying a series of experimental resonant peaks that agree well with theoretical predictions. In the vicinity of resonance, the wave fields undergo a transition to nonlinear behavior that is initiated on the central axis of the domain and proceeds to erode the wave field throughout the domain.

Sorting cells based on their intrinsic properties is a highly desirable objective, since changes in cell deformability are often associated with various stress conditions and diseases. Deterministic lateral displacement (DLD) devices offer high precision for rigid spherical particles, while their success in sorting deformable particles remains limited due to the complexity of cell traversal in DLD devices. We employ mesoscopic hydrodynamics simulations and demonstrate prominent advantages of sharp-edged DLD obstacles for probing deformability properties of red blood cells (RBCs). By consecutive sharpening of the pillar shape from circular to diamond to triangular cross-section, a pronounced cell bending around an edge is achieved, serving as a deformability sensor. Bending around the edge is the primary mechanism which governs the traversal of RBCs through such a DLD device. This strategy requires an appropriate degree of cell bending by fluid stresses, which can be controlled by the flow rate, and exhibits good sensitivity to moderate changes in cell deformability. We expect that similar mechanisms should be applicable for the development of novel DLD devices that target intrinsic properties of many other cells.

Particle image velocimetry and particle tracking velocimetry have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved direct numerical simulations (DNSs) have also been performed with the immersed boundary method at the same experimental conditions, bulk Reynolds number ${\text{Re}}_{2H}=5600$, duct height to particle-size ratio $2H/d_p=14.5$, particle volume fraction $\mathrm{\Phi }=1\%$, and particle to fluid density ratio ${\rho }_p/{\rho }_f=1.0035$. Good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution, and turbulent statistics of the two phases. Additional experimental results considering two particle sizes $2H/d_p=14.5$ and 9 and multiple $\mathrm{\Phi }=1\%$, 2%, 3%, 4%, and 5% are reported at the same ${\text{Re}}_{2H}$. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture in terms of fluid velocity statistics differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable detail, even in regions where the local volume fraction is relatively high.

Numerical simulations were carried out to examine a ${24}^{\circ }$ compression ramp interaction in which the confinement effect of sidewalls was very strong. The ratio of turbulent boundary-layer thickness to domain width at the start of the interaction was $\delta /w=0.12$. Inflow conditions of $M_{\infty }=2.25$ and ${\mathrm{Re}}_{\theta i}=1800$ were chosen to match previous simulations. Comparing to the limited data available from similar computations, the centerline separation length scale $L_s/\delta$ was seen to vary with the flow confinement parameter $\delta /w$, as previously recognized in reflected shock interactions. Significant differences in flow structure were observed between these computations with confining sidewalls and previous computations with periodic boundary conditions at the lateral boundaries of the domain. The most intense turbulent fluctuations occurred in the corners of the domain. Symmetric and antisymmetric motions of the separation region were identified in the instantaneous flow using conditional averages and spatial correlations. The presence of the sidewalls was found to influence both the mean and unsteady flow. Accurately capturing the sidewall flows is judged to be essential in realistic modeling of confined flows.

Historically, there has been little faith in particle tracking velocimetry (PTV) as a tool to make quantitative measurements of thermal counterflow in He II, since tracer particle motion is complicated by influences from the normal fluid, superfluid, and quantized vortex lines, or a combination thereof. Recently, we introduced a scheme for differentiating particles trapped on vortices (G1) from particles entrained by the normal fluid (G2). In this paper, we apply this scheme to demonstrate the utility of PTV for quantitative measurements of vortex dynamics in He II counterflow. We estimate $\ell$, the mean vortex line spacing, using G2 velocity data, and $c_2$, a parameter related to the mean curvature radius of vortices and energy dissipation in quantum turbulence, using G1 velocity data. We find that both estimations show good agreement with existing measurements that were obtained using traditional experimental methods. This is of particular consequence since these parameters likely vary in space, and PTV offers the advantage of spatial resolution. We also show a direct link between power-law tails in transverse particle velocity probability density functions (PDFs) and reconnection of vortex lines on which G1 particles are trapped.

Existing methods to identify the interfaces separating different regions in turbulent flows, such as turbulent/nonturbulent interfaces, typically rely on subjectively chosen thresholds, often including visual verification that the resulting surface meaningfully separates the different regions. Since machine learning tools are known to help automate such classification tasks, we here propose to use an unsupervised self-organizing map (SOM) machine learning algorithm as an automatic classifier. We use it to separate a boundary layer undergoing bypass transition into two distinct spatial regions, the turbulent boundary layer (TBL) and non-TBL regions, the latter including the laminar portion prior to transition and the outer flow which possibly contains weak free-stream turbulence. Both regions are separated by the turbulent boundary layer interface (TBLI). The data used in this study are from a direct numerical simulation and are available on an open database system. In our analysis of one snapshot in time, every spatial point is characterized by a 16-dimensional vector containing the magnitudes of the components of total and fluctuating velocity, magnitudes of the velocity gradient tensor elements, and the streamwise and wall-normal coordinates, all normalized by their global standard deviation. In an unsupervised fashion, the SOM classifier separates the points into TBL and non-TBL regions, thus identifying the TBLI without the need for user-specified thresholds. Remarkably, it avoids including vortical streaky structures that exist in the laminar portion prior to transition as well as the weak free-stream turbulence in the turbulent boundary layer region. The approach is compared quantitatively with existing methods to determine the TBLI (vorticity magnitude, cross-stream velocity fluctuation). Also, the SOM classifier is cast as a linear hyperplane that separates the two clusters of data points, and the method is tested by finding the TBLI of other snapshots in the transitional boundary layer data set, as well as in a fully turbulent boundary layer with similar levels of free-stream turbulence. Variants in which the approach failed are also summarized.

The motion of wall-bounded drops moving on solid substrates driven by a fully turbulent channel flow is experimentally investigated for four substrates of different wetting properties. An appropriate scaling has been found to describe the dimensionless drop velocity (capillary number) in terms of a dimensionless flow attack velocity, taking into account the surface wetting properties. The derived model agrees very well with the experimental observations.

Flapping insects are remarkably agile fliers, adapted to a highly turbulent environment. We present a series of high-resolution numerical simulations of a bumblebee interacting with turbulent inflow. We consider both tethered and free flight, the latter with all six degrees of freedom coupled to the Navier-Stokes equations. To this end, we vary the characteristics of the turbulent inflow, either changing the turbulence intensity or the spectral distribution of turbulent kinetic energy. Active control is excluded in order to quantify the passive response real animals exhibit during their reaction time delay, before the wing beat can be adapted. Modifying the turbulence intensity shows no significant impact on the cycle-averaged aerodynamical forces, moments, and power, compared to laminar inflow conditions. The fluctuations of aerodynamic observables, however, significantly grow with increasing turbulence intensity. Changing the integral scale of turbulent perturbations, while keeping the turbulence intensity fixed, shows that the fluctuation level of forces and moments is significantly reduced if the integral scale is smaller than the wing length. Our study shows that the scale-dependent energy distribution in the surrounding turbulent flow is a relevant factor conditioning how flying insects control their body orientation.

We present high-resolution simulations of the instability and subsequent breakdown of standing waves, or seiches, in a fluid continuously stratified in the vertical direction. It is well known that such waves can evolve to form nonlinear, dispersive wave trains. When the initial dimensionless amplitude is large, it is possible that a stratified shear instability develops, possibly at the same time as dispersive wave trains. While both dispersion and shear instability serve to move energy from large to small scales, they are fundamentally different. The development into wave trains is nondissipative in nature, and in the asymptotic limit of small but finite amplitude seiches may be described by variants of the Korteweg–de Vries (KdV) equation. Shear instability, on the other hand, yields Kelvin-Helmholtz billows, which in turn provide one of the basic archetypes of transition to turbulence, with greatly increased rates of mixing and viscous dissipation. We discuss how the two phenomena vary as the aspect ratio of the tank and the height of the interface between lighter and denser fluids are changed, highlighting cases where the two phenomena coexist. A quantitative accounting for the evolution of the horizontal modewise decomposition of the kinetic energy of the system in addition to the creation of a semianalytical model of the evolution of the fundamental mode is presented. Finally, the mixing is characterized during the evolution of the seiche, illustrating a fundamental transition that occurs as the aspect ratio is decreased.

A one-dimensional (1D) reduced-order model (ROM) is developed for a 3D Rayleigh-Bénard convection system in the turbulent regime with Rayleigh number $\mathrm{Ra}={10}^6$. The state vector of the 1D ROM is horizontally averaged temperature. Using the Green's function (GRF) method, which involves applying many localized weak forcings to the system one at a time and calculating the responses using long-time averaged direct numerical simulations (DNSs), the system's linear response function (LRF) is computed. Another matrix, called the eddy flux matrix (EFM), which relates changes in the divergence of vertical eddy heat fluxes to changes in the state vector, is also calculated. Using various tests, it is shown that the LRF and EFM can accurately predict the time-mean responses of temperature and eddy heat flux to external forcings and that the LRF can predict well the forcing needed to change the mean flow in a specified way (inverse problem). The non-normality of the LRF is discussed and its eigenvectors or singular vectors are compared with the leading proper orthogonal decomposition modes of the DNS data. Furthermore, it is shown that if the LRF and EFM are simply scaled by the square root of the Rayleigh number, they perform equally well for flows at other $\mathrm{Ra}$, at least in the investigated range of $5\times {10}^5\le \mathrm{Ra}\le 1.25\times {10}^6$. The GRF method can be applied to develop 1D or 3D ROMs for any turbulent flow, and the calculated LRF and EFM can help with better analyzing and controlling the nonlinear system.

We show that Leidenfrost droplets made of an aqueous solution of surfactant undergo a violent explosion for a wide range of initial volumes and concentrations. This unexpected behavior turns out to be triggered by the formation of a gel-like shell during the evaporation when the surface concentration of surfactant reaches some critical value. Shortly thereafter, the temperature sharply increases above the normal boiling point, leading to fast bubble growth, shell stretching, and explosion. However, most of the droplet life is characterized by a self-similar evolution of the radial surfactant distribution during which surface and mean concentrations grow in proportion, independently of the initial conditions. The temperature rise (attributed to boiling point elevation with surface concentration) and nucleation followed by growth of vapor bubbles inside the shell are key features leading to the explosion, differing from the implosion (buckling) scenario reported by other authors.

Ferromagnetic microparticles energized by an alternating magnetic field exhibit fascinating collective behavior ranging from the emergent self-assembled spinners to a variety of self-organized rolling states. Despite their simplicity, quantifying their essentially multibody collective behavior remains elusive due to a multitude of relevant interactions, from short-range collisions to long-range magnetic and hydrodynamic forces. Here we develop a high-performance computational algorithm based on smoothed particle hydrodynamics to quantify the role of individual interactions in the emergent collective state. The computational model provides insight into the role of hydrodynamic interaction on the onset of collective behavior and allows characterization of dynamic regimes that are hard to access experimentally. Comparison with high-resolution experimental data allows validation of the algorithm. Our work expands the scope of modern computational tools for predictive modeling of microscopic active systems and provides insight into the intricate role of hydrodynamic interactions on the onset of collective behavior in living and synthetic active matter.

The dynamic frictional force between solid surfaces in relative motion differs from the static force needed to initiate motion, but this distinction is not usually thought to occur for liquid drops moving on a solid. Recent experiments [Gao et al., Nat. Phys. 14, 191 (2018)] have challenged this view and claim to observe an analog of solid-on-solid friction for sliding drops. We use molecular dynamics simulations to investigate the forces that moving liquids exert on solids in several situations. In contrast to the indirect techniques required in laboratory experiments, the forces involved in friction are directly accessible in these calculations. We find that, aside from possible inertial effects due to the abrupt initiation of motion and aging effects for unconfined drops, the frictional forces are constant in time.

A parametric study of the magnetic dipole behavior in resistive incompressible magnetohydrodynamics inside a rotating sphere is performed, using direct numerical simulations and considering Reynolds and Ekman numbers as controlling parameters. The tendency is to obtain geodynamolike magnetic dipole reversal regimes for sufficiently small Ekman and large Reynolds numbers. The typical dipole latitude obtained in the reversal regime is around ${40}^{\circ }$ (with respect to the rotation axis of the sphere). A statistical analysis of waiting times between dipole reversals is also performed, obtaining a non-Poissonian distribution of waiting times, indicating long-term memory effects. We also report the presence of a $1/f$ frequency power spectrum in the magnetic dipole time series, which also shows a tendency to grow toward lower frequencies as the Ekman number is decreased.