This paper is associated with a video winner of a 2022 American Physical Societys Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0076

Droplet impact onto solid substrates is not only an interesting natural phenomenon, but also has applications in various fields. Recent studies in the literature reported that following the first peak of the force exerted on a superhydrophobic substrate by the droplet at the moment of impact, the droplet retraction and jump-off led to the emergence of a second peak of this force, which scales with the inertia-dominated impact force. In this paper, we have found this result to fail in the case of droplets with a broadly varying viscosity. New scaling models based on the observation of flow focusing are proposed to express the relevant timescale and the magnitude of the second force peak.

It is shown that the rotational motion of a cylindrical rotor immersed in an electroconducting medium at rest can produce a dynamo effect, provided that the electrical conductivity or magnetic permeability of the rotor is anisotropic. Results are reported for three pairs of rotor materials: copper/kapton, iron/tin, and $\mu$-metal/tin. The value of using such an anisotropic rotor in liquid metal such as sodium or galinstan for a laboratory dynamo experiment is clearly demonstrated.

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluids Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0040

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0036

This paper is associated with a poster winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.P0030

This paper is associated with a poster winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.P0005

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0085

This paper is associated with a video winner of a 2022 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0115

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0008

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion (GFM) Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0037

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0070

This paper is associated with a poster winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.P0027

Jets-in-supersonic-crossflow have many influential areas of technical applications, amongst them the field of separation control when implemented as air-jet vortex generators (AJVGs). The jet velocity profile at the exit of the jet orifice is affected by the jet-injection-pipe length. In this study, we utilize a combined experimental-numerical approach to characterize the effect of jet-velocity profile on the jet-induced structures and their direct impact on AJVG separation-control effectiveness. First, we conducted large-eddy simulations (LESs) of two single spanwise-inclined jet-in-crossflow configurations with different injection-pipe lengths. The case with a short jet-pipe length exhibits an underdeveloped flow in the pipe with unstable growth modes. The longer jet-pipe length, on the other hand, allows for a fully established velocity profile and contributes to stronger and more coherent jet-induced structures. Equipped with this knowledge, we conducted detailed experiments applying AJVGs with both pipe lengths to control shock-induced flow separation and assessing the respective control effectiveness. The experiments were conducted at similar crossflow and jet-plenum conditions as the LESs for both jet-pipe lengths. The control effectiveness of longer-pipe jets is higher, with a $24\%$ reduction in flow-separation length compared with only $17\%$ reduction for the shorter-pipe case with underdeveloped jet flow. Boundary-layer velocity measurements downstream of the jets show the expected higher velocities close to the wall for the longer-pipe case, which are related to the stronger jet-induced streamwise vortices that are responsible for the improved control effectiveness.

The universal velocity log law proposed by von Kármán in wall-bounded turbulent flows is one of the cornerstones of turbulence theory. When buoyancy effects are important, the universal velocity log law is typically believed to break down according to Monin-Obukhov similarity theory (MOST), which has been used in almost all global weather and climate models to describe the dependence of the mean velocity profiles on buoyancy near the earth's surface and to characterize the surface-atmosphere exchange of momentum, heat, water vapor, and carbon dioxide. In contrast to MOST, we propose logarithmic profiles of near-wall mean velocity in the stably stratified atmospheric boundary layers based on direct numerical simulations and field observations across a wide range of buoyancy effects. We find that buoyancy does not seem to change the logarithmic nature of velocity profiles but instead modifies the slope of the log law in stably stratified conditions. This paper provides a perspective on wall turbulence and can be applied to numerical simulations of turbulence, weather, and climate.

The propulsion of mammalian spermatozoa relies on the spontaneous periodic oscillation of their flagella. These oscillations are driven internally by the coordinated action of ATP-powered dynein motors that exert sliding forces between microtubule doublets, resulting in bending waves that propagate along the flagellum and enable locomotion. We present an integrated chemomechanical model of a freely swimming spermatozoon that uses a sliding-control model of the axoneme capturing the two-way feedback between motor kinetics and elastic deformations while accounting for detailed fluid mechanics around the moving cell. We develop a robust computational framework that solves a boundary integral equation for the passive sperm head alongside the slender-body equation for the deforming flagellum described as a geometrically nonlinear internally actuated Euler-Bernoulli beam, and captures full hydrodynamic interactions. Nonlinear simulations are shown to produce spontaneous oscillations with realistic beating patterns and trajectories, which we analyze as a function of sperm number and motor activity. Our results indicate that the swimming velocity does not vary monotonically with dynein activity, but instead displays two maxima corresponding to distinct modes of swimming, each characterized by qualitatively different wave forms and trajectories. Our model also provides an estimate for the efficiency of swimming, which peaks at low sperm number.

The vortex dynamics of laminar flow past a rectangular cavity is investigated using numerical simulations and microfluidic experiments. The flow is inherently three-dimensional and is characterized by a large, dominant vortex structure that fills most of the cavity at moderate Reynolds numbers with a weak, yet significant, flow in the axial direction along the vortex core. Classical bubble-type vortex breakdown is observed within the cavity above a certain critical Reynolds number, which is a function of the channel width. The critical Reynolds number for the onset of breakdown is determined as a function of the channel width, and the evolution and dynamical transitions of the breakdown regions are investigated as functions of the channel width and Reynolds number. At large cavity widths, two vortex breakdown bubbles emerge near the sidewalls symmetric about the center plane, which grow and eventually merge as the Reynolds number increases. For large-enough widths, the vortex breakdown regions remain well separated, and their structures become independent of the cavity width. The stability and bifurcations of the stagnation points and their transitions to stable and unstable limit cycles are analyzed. At the intermediate width regime, a single vortex breakdown bubble appears above the critical Reynolds number. In the narrow width regime, the flow exhibits more complicated modes. An additional vortex breakdown mode with reversed flow patterns is observed in this width regime, along with multiple shifts in the stability of stagnation points. The experimental and numerical results also demonstrate the sensitivity of the flow to the inlet conditions, such that relatively small asymmetries upstream can result in significant changes to the vortex breakdown behavior in the cavity.

In this study, we leverage geometry-induced asymmetries to rectify oscillatory flows and induce net directional transport of fluid in a looped network. We demonstrate this valveless rectification phenomenon in an experimental apparatus that consists of an acrylic flow chamber with a deformable wall connected to a syringe pump via a T-junction. Here, oscillatory flows were forced by the syringe pump. We found that the asymmetries that existed in the geometric configuration of the looped channel could be exploited to generate net directional flow and that this geometric configuration is key in determining the directionality. By varying the pumping parameters in the system, we determined a relationship between the frequency and stroke length of the forcing syringe pump and the resulting directional flow in the loop. Using an analytical model and flow computations, we posit how asymmetric flow separation occurring at the T-junction could explain the observed flows.

We investigate experimentally coherent structures of low-Stokes-number particles in the thermocapillary liquid bridge as a closed system. Such coherent structures in thermocapillary liquid bridges have been known as particle accumulation structures (PASs) realized in traveling-wave-type convection. We discover the PAS in the flow of $m=1$ in an azimuthal wave number in the liquid bridge of $O\left({10}^{-3}\mathrm{m}\right)$, whose spatial structure is different from those hitherto indicated in previous research, with the particles heavier than the test liquid. The coherent structures by the particles suspended in the liquid bridge are illustrated in the laboratory frame and in the rotating frame of reference. Individual particles of Stokes number $\mathrm{St}=O\left({10}^{-5}\right)$ are tracked to indicate their spatial-temporal behaviors forming the coherent structures. We unveil that two major coherent structures are simultaneously emanated inside the liquid bridge of $\mathrm{\Gamma }=1.6$ in aspect ratio. Their Poincaré sections are illustrated, which well reproduce qualitatively the predictions of Barmark et al. [Phys. Rev. Fluids 6, 084301 (2021)].