By summarizing the historical trajectory of immersed boundary methods and addressing some frequently asked questions about these methods, this article attempts to empower researchers to innovate in ways that advance the state-of-the-art in these methods.

We present a multi–physics computational model of the human heart accounting for the electrophysiology, elasto-mechanics, and hemodynamics, including their complex interactions. The model is accurate and computationally efficient and, thanks to the implementation on GPU architectures, it allows cardiovascular simulations of physiologic and pathologic configurations within a time–to–solution compatible with clinical practice. Results are shown for healthy conditions and for myocardial infarction with the aim of assessing the reliability and predictive capabilities of the model which can be used to anticipate the outcome of surgical procedures or support clinical decisions.

Elastic hoops can jump on water by harnessing the reaction force from water’s form drag, much like fishing spiders. These artificial jumpers allow us to mathematically understand the drag-based water jumps, which can achieve greater velocities than the surface-tension-based jumps seen in water striders and springtails.

Elastic turbulence is a chaotic flow state observed in dilute polymer solutions in the absence of inertia. In this paper we show that an infinitesimal amount of polymer diffusion gives raise to a new wall-mode instability. This instability leads to chaotic dynamics in inertialess Couette flow. This new instability might provide a generic transition pathway to elastic and elasto-inertial turbulence.

Rigid particles in red blood cell (RBC) flows are forced to migrate to the near-wall region, through a phenomenon known as margination. In this work, we use a three-dimensional particle tracking method to provide an experimental perspective into the intricate dynamics of synthetic particles within RBC flows and make a significant departure from the simulation-driven research landscape in microcirculation studies, where experimental data has been notably scarce. The experimental results show the critical role of the physical interactions between synthetic particles and RBCs in triggering margination: A phenomenon characterized by speed, irreversibility, and unpredictability.

In this work, we model acoustic and thermoacoustic pressure and velocity oscillations from synthetic data. The synthetic data captures the rich nonlinear behavior of thermoacoustic oscillations observed in propulsion and power generation. We develop acoustic neural networks, in which prior physical knowledge is embedded as both soft and hard constraints. We predict and extrapolate in time thermoacoustic oscillations, reconstruct pressure and velocity over the entire domain from pressure sensors only, and obtain a model that is robust to noise and generalizable to unseen scenarios.

The evolution of inertio-elastic shear-layer and jet-column instabilities of submerged jets of viscoelastic polymer solutions is studied by direct numerical simulation. At high Reynolds numbers, viscoelasticity has a destabilizing effect in the linear region of perturbation growth, but stabilizes the jet perturbations at the nonlinear regime. Two competing instability mechanisms are identified and analyzed in order to explain the observed results for the different regimes.

Penetrative convection is the interaction between a lower turbulent convective layer of fluid and an upper stably-stratified one. It plays a key role in the atmosphere’s dynamics and in stellar interiors. In this study, we built an experiment of penetrative convection in gases which is closer to the geophysical applications. The internal gravity waves field is studied, as well as the growth of the convective layer. In our setup, the convective layer grows quadratically in time which is strikingly different from the 1/2 exponent obtained with classical models and salted water experiments.

We simulate numerically convection in a rectangular cell filled with an ideal gas rotating about an axis perpendicular to the direction of gravity, corresponding to an experiment with a convection cell placed in a rapidly rotating centrifuge in which the centrifugal force plays the role of gravity. The compressibility of the gas in combination with the rotation of the cell leads to a drifting mode at the onset of convection even in the presence of sidewalls. The temporal variation of this mode is rapid enough to cause the anelastic approximation to fail at parameters typical of realizable laboratory experiments. We also investigate the heat transport in such a convection cell.

Revealing insights into liquid metal magnetoconvection: Our study delves into the dynamics of Rayleigh-Baposenard convection within liquid gallium under the influence of a vertical magnetic field. Employing a combination of laboratory experiments and numerical simulations, we elucidate the transitions and heat transfer phenomena within this complex system. This investigation not only enhances our understanding of magnetoconvection from convective onset to supercriticality, but also provides a close comparison with liquid metal rotating convection.

We report the observation of asymmetrical fingering instabilities unfolding across an upward-moving dissolution interface while downward fingers also evolve. We tackled complexities characterized by large viscosity ratios (M) and Rayleigh (Ra) numbers - phenomena often challenging to replicate experimentally and simulate numerically. We present our Rayleigh-Taylor mixing experiments conducted at a substantial viscosity ratio (M$\approx 5\times {10}^5$) and high Rayleigh numbers (Ra~${10}^5-3\times {10}^6$). Our experiments have confirmed the emergence of asymmetric growth in fingering instabilities along an upward-moving interface, accompanied by significant downward finger evolution.

We identify the limit behaviors of fully compressible convection going beyond the anelastic and Boussinesq approximations. This is done in terms of the governing parameters, the dissipation number and the superadiabaticity. We demonstrate the asymmetry between the top and bottom boundary layers with respect to these parameters; other results include the common characteristics of compressible convection despite seemingly different physical conditions. Finally we show that Mach number alone is inadequate to elucidate the complex system and more importantly, the strongly stratified case has features that resemble solar surface convection.

Exploring droplet dynamics: This study sheds light on the behavior of droplets sliding down vertical fibers, a key issue in microfluidics and fog harvesting. Using real-time tracking, the research covers single and multi-fiber systems. A standout finding is that multiple vertical fibers increase droplet speed but also lead to greater liquid loss in grooves. This discovery is captured in a detailed theoretical model, offering valuable insights for optimizing droplet-based technologies.

Before a droplet can contact a solid surface, it must drain the air beneath it. In our daily experience, the air doesn’t alter the global outcome of contact formation in the blink of an eye. In this work, we find that the lubricating air film formed beneath an impacting droplet can resist rupture for droplets impacting at a velocity exceeding 1/2 a meter per second; however, beyond a critical impact velocity, contact always occurs from the liquid’s closest approach to the solid surface through the air, at a distance of 20 - 30 nanometers. Below the critical velocity, contact is suppressed for impact upon atomically smooth, cleaved mica, shedding light on the critical phase of liquid-solid contact.

This paper focuses on secondary instabilities in the shear layer of a supersonic jet over a convex wall. The methods of conditional averaging and dynamic mode decomposition analysis are involved to quantitatively investigate the roll-modes and secondary instability modes. Both sinuous- and varicose-type secondary instabilities and their competitions exist in the shear layer. The mechanism of the instability is due to the velocity fluctuations and Reynolds stress parallel to the local mean flow gradient. The local inflection point instability caused by the mean flow gradient is also the source of turbulent energy that sustains the instabilities.

Methods for reducing turbulent skin-friction drag are seldom employed in the context of laminar flows, where the level of frictional drag is already low. This work explores the possibility that such technologies may affect a laminar flow during transition. Here we consider a parallel shear flow. Its temporal stability is studied when a standing wave of spanwise wall velocity alters the base flow, making it three-dimensional. It is found that the spanwise forcing has a profound and positive effect on the stability of the flow, with a significant reduction of the nonmodal instability.

Aeroacoustic effects are commonly neglected in the design of high-performance turbomachinery. This study concentrates on the role of acoustic feedback mechanisms for compressible flow through a single-blade configuration. It uses global and adjoint stability tools, together with impulse responses and structural sensitivities, to quantify the impact of propagating sound waves on the overall flow dynamics.

This study generalizes the previous investigation (Part I) to multiple-blade passages, focusing in particular on synchronization and phase-locking effects across several neighboring blades. Arising large-scale low-frequency structures, spanning multiple identical flow units, are described and analyzed, and their role within the overall stability framework is determined.

The dynamic response of viscous droplets to a sudden change of interfacial tension with the external phase is systematically examined in microchannels. A two-step hydrodynamic focusing section is employed to continuously generate high-viscosity oil droplets in immiscible alcohols at the first junction and inject droplets into miscible alcohol phases at the second junction. Upon entering stratifications, droplets are seen to strongly elongate depending on fluid properties and flow conditions. Functional relationships are developed to characterize droplet dynamics in a variety of solvents and examine out-of-equilibrium behavior of ternary systems at short-time scales.

The settling of rods is ubiquitous in nature and industry. In inertial regimes, the body’s orientation is coupled to the translational equation of motion which makes the problem unsteady and very hard to solve analytically. In this article, we focus on two regimes: the very short-time and long-time dynamic of an arbitrarily oriented cylinder settling under gravity.

High-fidelity multiphase fluid dynamics data are scarce and expensive. We propose a Bayesian data-driven method to predict the drop size distribution of a liquid jet from sparse measurements. The proposed method provides uncertainty estimation, and it can be used for the optimal design of experiments.

The Lyapunov field measures the sensitivity to small perturbations of the fluctuating flow behind an accelerated plate. Using a robot, we repeat this experiment 42 times and measure the difference between the flows as they turn turbulent. The Lyapunov field of a single experiment (left) mirrors the (logarithmic) difference between its 42 repetitions (right). It is an expression of ergodicity, and a vivid illustration of the butterfly effect.

Velocity fields induced by small-scale shear layers and vortex tubes in turbulence are investigated with the triple decomposition of a velocity gradient tensor. The velocity fields are reconstructed with the Biot-Savart law applied to the vorticity vectors of shear and rigid-body rotation. The key image shows a mean flow pattern of small-scale shear layers in isotropic turbulence. The velocity associated with the shear layers dominates the turbulent kinetic energy budget, scalar transport, and energy cascade in isotropic turbulence and planar jets.

This study discusses the necessity of anisotropic subgrid-scale (SGS) stress separated from the energy transfer in large-eddy simulations (LESs). We investigate the budget equation for grid-scale (GS) Reynolds stress in turbulent channel flows. The anisotropic stress has a large and nondissipative contribution to the streamwise and spanwise components of GS Reynolds stress when the filter size is large. The positive contribution is prominent at a scale consistent with the spacing of streaks in the near-wall region. Therefore, we infer that anisotropic stress contributes to the generation mechanism of coherent structures, which is key to further improving SGS models.

In this paper, we present a model for large eddy simulations (LES) which reconstructs the unresolved subgrid-scale velocity, by representing it with wavelet basis functions. Exploiting the compact support of the wavelet basis, accurate statistically inhomogeneous and anisotropic subgrid-scale velocity statistics can be generated. We apply the model to LES of single-phase and particle-laden turbulent flows. The generated subgrid-scale velocity possesses the correct characteristic features of turbulence, such as the correct spatial correlations, and improves the description of the behavior of particle clustering and the particle pair dispersion, which are poorly predicted in classical LES.

In this work, we investigate the dynamics of very long wind turbine wakes (up to 215 rotor diameters), which includes the entire wake recovery process, for three ground surface roughness lengths. The focus is on how the flow structures of different scales vary as they pass through a wind turbine and travel to further downwind locations. Particularly, we examine the energy density in scale space and its evolution in the streamwise direction, providing insights into the range of scales influenced by a wind turbine and its wake.

Motivated by the potential impact of unsteady kinematics on small turbomachines and propellers, we examine a revolving wing subjected to pitch perturbations. Through experiments and numerical analysis, we find that the wing’s performance during perturbations is mainly influenced by the pitch-rotation velocity and mean angle of attack. Beyond a certain perturbation amplitude, the stable leading-edge vortex that forms on a revolving wing becomes unstable, significantly altering wing-surface pressures. Our study demonstrates that rotational effects dominate the resulting wing performance when the flow structures around the wing are destabilized.

Understanding the process of wave generation by the wind remains a formidable challenge for scientists. While the majority of research has concentrated on the air-water interface, where viscous effects are minimal, our study focuses on the influence of liquid viscosity on the development of mechanically generated waves. Using Synthetic-Schlieren measurements of wind-generated waves over silicon oils 20 and 50 times more viscous than water, we show that Miles’ model, conventionally applied to water waves, also provides an accurate description of wave growth in more viscous liquids. We further explore the dependence of the maximum growth rate and critical wind velocity with the liquid viscosity.

The peculiar reflection of internal waves in rotating stratified fluids enables the concentration of energy on a limit cycle, called an attractor. In this work, the existence of internal gravity wave attractors for rotating stratified fluids is predicted in three-dimensional axisymmetric geometry. This information is used to design experiments using a truncated conical shaped tank in order to form an inertia-gravity waves attractor. We highlight an important difference in the nonlinear regime between the stratification-only and the rotation-only cases.