Recent efforts to study discrete large-amplitude gust encounters have often focused on one of three canonical problems: transverse, vortex, or streamwise gust encounters. A selection of this work is highlighted, concentrating on two-dimensional problems where the length scale of the gust flow is similar to the wing chord, and the amplitude of the gust flow is similar to the freestream. Current and outstanding questions are outlined for future work, including the role of low-order models and the limitations of using canonical problems to represent real-world gust encounters.

Wall-bounded turbulence is sometimes described as consisting of hierarchies of similar structures. This is embodied in the Attached Eddy Model, and at high Reynolds numbers this model predicts a logarithmic variation of the turbulence intensity with distance from the wall. The high Reynolds number experiments described here confirm this behavior, but they also reveal new aspects such as the slower than expected rise in the near-wall peak, and the unexpected appearance of an outer layer peak. Turbulence remains an elusive phenomenon.

Technical communication is a critical component of scientific life but is an uncomfortable area for many. Fortunately, integrating regular communication training and practice into a research lifestyle is not difficult. Activities, exercises, and practices to help both individuals and research groups improve their technical communication skills are presented.

A perspective highlights the recent progress in our understanding of how the interplay between viscous, capillary, and frictional forces shape the interactions of multiphase flow with deformable granular media, with particular attention to the central role of wettability. Beyond their intrinsic interest as processes that give rise to spectacular pattern formation, these coupled phenomena in granular media can control continental-scale fluxes, such as methane venting from the seafloor, and geohazards, such as earthquakes and landslides.

Advanced GPU-optimized algorithms coupled with a multiresolution short-time sampling paradigm can make extreme-scale turbulence simulations focused on small-scale physics much more accessible. One example is in the study of scaling exponents of the locally averaged energy dissipation rate at high Reynolds number.

Understanding how turbulence leads to the enhanced irreversible transport of heat and other scalars in density-stratified fluids is a fundamental research challenge in geophysical and environmental fluid dynamics, although there are still leading-order open questions. One useful approach to addressing these questions is to consider carefully chosen idealized flow geometries. It is then possible to analyze in detail the energetic pathways and the subtle interplay between various characteristic time and length scales. Such analyses can lead to insight into at least some of the apparently mysterious emergent properties of turbulent stratified mixing.

The influence of suspended particles on Taylor-Couette (TC) flow transitions is a new direction in the fluid mechanics of suspensions. The microstructure and properties of suspensions are reviewed to support discussion of this and other bulk behavior. Tracer particle tracks in a 10% solid suspension in TC
 flow in the circular Couette flow, spiral vortex flow, and Taylor vortex flow (left to right, respectively) show some of the flows examined.

Hydrodynamic interactions in multiphase flows result in rich multiscale physics, such as clustering and pseudoturbulence, with important practical implications. Alternative theoretical formulations that overcome current challenges, and particle-resolved direct numerical simulations to build accurate closure models for unclosed terms that arise in these statistical theories, are outlined as promising future research directions. The study provides a perspective on the importance of integrating theoretical, modeling, computational, and experimental efforts at different scales.

Theoretical and computational analyses show that a shear-thinning viscosity alone can cause a substantial enhancement to the propulsion of helical microswimmers.

Theory and experiment are combined to investigate how two liquid metal drops resting on a surface first coalesce and then jump off the surface.

Using a combination of singular perturbation analysis and conformal mapping techniques, the self-propulsion velocity is found in the limit of fast reaction for a Janus particle with half of the boundary active and the other half inert.

How will liquids flow out of a glass tube when the tube is tipped? There is a lot of science to it. Here, we study and elucidate the effects of the width, shape, and wettability of the glass tube opening on the liquid release.

Large particles disturb a flow more than small particles. Using Magnetic Resonance Imaging (MRI) and pressure drop measurements we find that large spherical and cubic particles in pipe flow promote subcritical turbulent-like flow disturbances (as seen in the radial mean velocity and rms profiles at Re=700). These disturbances lead to significant changes in the laminar-turbulent transition at low particle concentrations ($<5$% per volume). We suggest these observations can be explained by the relative magnitude of (i) particle interactions, (ii) flow disturbances introduced by the particles and (iii) viscous dissipation in the system.

Flame-flame interactions play an important role in altering flame surface area and fuel consumption rate but have received comparatively little attention so far. An investigation into the statistics of flame-flame interaction events in the progress variable space aims to quantify the relative frequency of occurrence of these events and looks at the topology of each type of interaction. Understanding the statistics and the topology can help to identify the overall influence of flame-flame interactions on flame properties and, in future, help to model these effects.

A numerical study of an elastoviscoplastic (EVP) drop in Newtonian shear flow is presented. The three-dimensional Navier-Stokes equations are combined with the level-set method and the Saramito EVP model to study the yielding process under various nondimensional parameters. A regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes. It is shown that yielding is suppressed by a higher Bingham number but, surprisingly, also by a higher Weissenberg number.

Filament stretching using a flow-focusing microfluidic device coupled with image analysis has emerged as a promising method to resolve extensional properties of low viscosity, low molecular weight solutions. This method was used to calculate extensional properties of aqueous methylcellulose (MC) solutions at varying NaCl concentrations, where transient changes in filament diameter were used to calculate the flow-driven, apparent extensional viscosity of each solution. The increase in apparent extensional viscosity at room temperature as the NaCl concentration increases is attributed to the presence of a fibrillar MC network formed in solution.

Experimental evidence is reported of a new merge-split transition (MST) to elastoinertial turbulence (EIT) in Taylor-Couette flows of viscoelastic polymer solutions, caused by merging and splitting of base Taylor vortices when crossed by elastic axial waves. Vortex merging and splitting are random in nature and increase in frequency with Reynolds number. When superimposed on a RSW flow state, they cause abrupt changes in the axial spatial wavelength, leading to the transition from a RSW to the EIT state. MST is thus identified as an inertial feature solely triggered by elasticity and independent of any shear-thinning behavior.

Compressibility effects on pressure fluctuations in wall-bounded turbulent flows at various Mach numbers are investigated by deriving a pressure Poisson equation that allows splitting pressure fluctuations into rapid and slow terms, as in incompressible flows, and into additional mass-flux and viscous terms. The rapid and slow terms are further split into their solenoidal and dilatational components by using Helmholtz decomposition. The solenoidal components are found to be independent of Mach number and similar to incompressible flows, whereas the dilatational components increase quadratically with Mach number.

A technique to compute vibrationally resolved transport collision integrals for atom-diatom systems directly from ab inito potential energy surfaces (PES) is presented. These calculations are performed for the oxygen systems employing the Varga et al. set of PES, and Guyta-Yos style fits to the data are provided. It is found that simple empirical models are often unable to capture the dependence of these collision integrals on the vibrational state of the molecule. Differences of up to 80% are observed between the model predictions and the values computed directly from the PES.

Shock waves are very efficient at compressing fluid. Ideally, the maximum mass compression ratio is $f+1$, where $f$ accounts for molecular degrees of freedom. Sufficiently strong shocks, those that induce very high temperatures downstream, may alter $f$ by adding vibrational modes or promoting molecular dissociation. In addition, the nonadiabaticity of these effects, and others like ionization and/or radiation, also affect the mass compression ratio and other shock properties. These changes ultimately modify the D’yakov-Kontorovich limits associated with the acoustic stability of planar isolated shocks.

Rotating convection under both strong thermal forcing and strong rotation serves as an essential baseline for understanding geophysical and astrophysical flows. Here, heat transfer and temperature profile measurements in rotating convection with unprecedentedly strong thermal forcing and rotational influence are reported. Changes in the vertical temperature gradient are found to separate various flow phenomenologies. Additionally, a novel flow regime is described where the temperature gradient scaling with thermal forcing and the bounding parameter ranges distinguish it from previously known regimes.

The thermal boundary layer is identified and studied using numerical simulations of Rayleigh-Bénard convection. Different methods of defining the thermal boundary layer are investigated when applied to fixed temperature or fixed heat-flux boundary conditions. The crossover in advective and conductive heat flux is a robust way to define the thermal boundary layer.

A theoretical model of thermal convection induced by dielectric heating is developed via further exploitation of the analogy between gravity-driven thermal convection and convection driven by an electrohydrodynamic effect. The model is applied to a horizontal layer of dielectric fluid to determine the conditions of convection generation by the linear stability theory. The competition between gravity and the electrohydrodynamic effect under dielectric heating gives rise to different flow patterns depending on the Rayleigh number.

Axial vortices are ubiquitous in nature and in engineering, and the breakdown of these vortices can have far-reaching consequences. In the presence of a temperature gradient, topological behavior dramatically changes depending on the various flow parameters. Quantities such as heat transport, scalar mixing, and turbulent correlations are influenced immensely by the interplay between rotation and convection.

When CO${}_2$ dissolves in a solution of calcium ions, a precipitation mineralization reaction can take place, producing solid calcium carbonate particles that sink to the bottom of the host phase. Convective motions present both above and below the reaction front favor the entrainment of CO${}_2$ toward the bottom, which is favorable for the security of carbon sequestration techniques. An experimental study of the influence of such a precipitation reaction on convective dissolution of CO${}_2$ is conducted.

Reservoir computing models are one possible architecture of recurrent neural networks. Here, a reservoir computing model is applied to reproduce the low-order statistics of a two-dimensional turbulent Rayleigh-Bénard flow without solving the underlying Boussinesq equations.

A volume-of-fluid-based numerical study on bouncing and coalescence between a spinning droplet and a nonspinning droplet undergoing a head-on collision is presented. It is found that the spinning droplet can induce significant nonaxisymmetric flow features. The nonaxisymmetric bouncing is caused by conversion of the spinning angular momentum into orbital angular momentum. The spinning motion can also enhance interface oscillation and internal-flow-induced viscous dissipation, leading to a delayed separation after temporary coalescence.

The effect of turbulence on the evaporation of diethyl ether droplets is investigated by means of digital in-line holography. Lagrangian tracking of thousands of droplets evidences an increase of the mean evaporation rate related not only to the mean relative velocity viewed by the droplets but also to the fluctuations of this velocity.

Droplet dynamics in microconfined environments influence the behavior of liquid-liquid emulsions and multiphase flows. Here, a microfluidic hydrodynamic trap is used to study the shape relaxation of confined droplets following a perturbation of their spherical shape, to better understand droplet deformation events with varying viscosity ratios and droplet sizes.

When Leidenfrost drops are confined on a spherical surface, they can oscillate in the form of stars; drops in this configuration are popularly known as Leidenfrost stars. Here, emission of sound in the form of periodic beats from the Leidenfrost stars is studied. It is shown that the vapors escaping from the drop are responsible for sound emission, and a theoretical framework is developed to establish that the frequencies of the sounds depend on the size of the drop, in the same way that frequencies of acoustic modes depend on the length of wind musical instruments.

Direct numerical simulation is employed to study the behaviors of droplets impinging a surface with a wettability difference. Unlike the four stages of a droplet passing through on homogeneous surfaces, results illustrate that the droplet undergoes asymmetric spreading, retracting, detaching, and self-migrating phases when it impacts the surface with nonuniform decorations. This happens because of the unbalanced Young’s force acting on the three-phase contact line around the drop periphery, which points toward the direction of increasing wettability, or decreasing contact angle.

Recent microfluidic experiments have been capable of designing highly ordered multiple emulsions. Understanding the fluid-structure interaction occurring in these systems is crucial for their manufacturing in devices of interest in material science and biotechnology. Here it is shown that, under shear flow, multiple emulsions exhibit a variety of nonequilibrium states, whose formation essentially depends on internal vorticity and multibody collisions among droplets.

The Richtmyer-Meshkov instability of a cylindrical light-heavy density interface is investigated in the framework of a two-fluid plasma model. Interfacial perturbations are enhanced by the induced Lorentz force. The Biermann battery effect is an important source of the self-generated magnetic field.

Dynamo action is studied using numerical simulations of planar Boussinesq convection at rapid rotation, focusing on dynamo action below convective onset. Subcritical dynamo action is successfully sustained in numerous cases, with an extension of the subcritical regime at more rapid rotation. The presence of a subcritical dynamo is dependent on a large-scale mean field with optimum values of the magnetic Prandtl number and magnetic Reynolds number.

We investigate the modal and nonmodal linear stability of a stably stratified Blasius boundary-layer flow, composed of a velocity and a thermal boundary layer. The nonmodal analysis, based on optimization of a weighted sum of the kinetic and potential energies, provides oblique optimal structures close to the wall at short target times and spanwise-homogeneous rolls at the freestream for long target times. The latter disappear when variation of stratification strength with height is accurately accounted for in the norm definition, underlining the importance of the choice of a meaningful norm for thermal boundary layer optimization.

A subcritical route to turbulence is found in a quasi-two-dimensional exponential boundary layer, driven by the action of the Orr mechanism on a Tollmien-Schlichting (TS) wave. A finite band of initial disturbance energies triggers a nonlinear contortion of the TS wave, producing sufficient nonlinear growth for a transition to turbulence. Remarkably though, the resulting turbulent state is never indefinitely sustained. Larger initial energies disrupted the transition process, leading to rapid relaminarization, while the growth generated by disturbances with smaller initial energies is insufficient to reach the turbulent attractor.

A convolutional neural network (CNN) is proposed to model laminar-turbulent transition. It takes velocity profiles as input and extracts scalars from the profiles without using the shape parameter directly. The CNN-extracted scalar correlates very well with the shape parameter for Falkner-Skan boundary layers.

When a plate is swiftly pulled out of a liquid bath, a thin liquid film is dragged out by the moving surface. High-speed imaging reveals surprising structure of such a seemingly mundane process. A new technique allows precise measurement of the structure, and a systematic comparison with the reverse process of plunging a plate is carried out

MHz-frequency surface acoustic waves (SAWs) in a solid substrate will stabilize neighboring micron- and submicron-thick liquid films against film breakup and substrate de-wetting at large ratios of acoustic to capillary stresses in the films. Low such ratios actively de-stabilize oil films, yet continue to stabilize water films. The difference in the response of water and oil films to the SAW appears to be connected to the presence of excess pressure from the electrical double layer force in water films.

Three types of depinning behaviors of a drop on a heterogeneous surface are numerically investigated. Depinning is found to occur due to two processes: slow- and fast-moving. A dynamic equilibrium results from competition between gravity and the capillary force in the whole drop. At the microscopic scale, based on measurement of the real-time contact angle, the local force balance in the contact line region is maintained by the unbalanced Young’s force and the substrate resistance in the slow-moving stage, while the unbalanced Young’s force provides the driving force for contact line motion in the fast-moving stage.

The dispensing of liquids in tubing often involves repeated, intermittent flows that leave a thin liquid layer on the inner wall of the tube. The entrainment in the coating film of particles present in the liquid can lead to the contamination of the tube. A study demonstrates the conditions under which particles dispersed in a liquid are deposited on the wall of the tube when the liquid is displaced by air.

Large waves forming on the surface of a liquid film flowing down an inclined plate are accompanied by a steady train of capillary waves. A study shows that the wavelength and spatial attenuation of these waves are consistent with the properties of linear viscocapillary waves having the same phase velocity as the large wave.

A study shows that during oblique impact of a particle onto a liquid surface, nonaxisymmetric wetting of the particle causes a nonaxisymmetric distribution of fluid pressure and shear stress along the particle surface. This leads to a deviation of the dominant forces at different impact stages from the particle direction of motion. The particle then follows a different trajectory from that of the impact direction while a viscous moment is generated that rotates the particle.

Even when a liquid is perfectly wetting on a smooth substrate (zero Young’s angle), it can manifest finite contact angles if evaporating. These angles are of a dynamic, evaporation-induced nature, a compromise between the tendencies to spread and to evaporate. They form as a result of the resolution of a macroscopic evaporation-flux singularity at a microscopic scale in a tiny vicinity of the contact line referred to as the microregion. A critical evaluation generalizes and unifies the various models thereof, for nonpolar liquids undergoing diffusion-limited evaporation into air, and compares them with experiment.

Finite-sized particles in complex flow fields often experience migration and trapping. Using three-dimensional particle tracking and numerical simulations, it is shown that even density-matched microparticles can experience migration and trapping because of their interaction with boundaries. This effect is particularly sound when particles are being advected in the complex three-dimensional flow generated by oscillating microbubbles confined in a microchannel.

Dynamic similarity, while commonly applied in fluid systems, has recently been extended to locomotion problems in flat beds of cohesionless grains by assuming a frictional continuum model. Here, expanded scaling relations are derived for beds that are sloped or composed of cohesive grains. The proposed scalings are validated by discrete element method simulations using rotating “wheels” of various shape families, which suggests the usage of these scalings as potential design tools for off-road vehicles and extraplanetary rovers, and as an analysis tool for biolocomotion in sands and soils.

An experimental study of a solid ball impacting heated granular beds is presented. The result shows that temperature plays a greater role in granular splashing than previously suspected. These observations are interpreted in terms of two physical effects: the influence of the gas viscosity on the ejecta drag force and an enhanced static strength within the bed caused by thermal expansion of the granules.

Uniformly distributed particles in inertial flows migrate across the streamlines to form nonuniform distributions in the flow cross sections. Here, an experimental study of the influence of inertial migration of particles on flow transitions of a suspension in Taylor-Couette geometry is presented. It is shown that, relative to uniform concentration, the particle distribution following inertial migration either stabilizes or destabilizes the flow depending on the underlying flow structure and flow Reynolds number.

Settling of inertial particles in basally heated fluids is a topic of great significance in nature and, in particular, for the study of how magma cools and solidifies. The residence time of particles in Rayleigh-Bénard convection is computed for a broad range of flow and particle parameters, and a general analytic formula is designed that captures the results. It is found that particles tend to settle rapidly compared with the characteristic solidification timescale of magmatic systems. In addition, the horizontal distribution of settling events shows a surprising pattern: Heavy particles settle preferentially below clusters of upwelling plumes.

A hybrid approach is developed to simulate the transport and breakup of droplets in a turbulent round jet. The inflow size distribution is obtained from a one-dimensional parcel model that is coupled to a coarse large-eddy simulation (LES) of a turbulent jet. The LES results are compared to experimental data and show good agreement. Additionally, LES allows us to quantify the distributions of the mean and variability of key quantities of the polydisperse distribution.

The mobility of bidisperse segregated beds is studied numerically with a coupled fluid discrete element method model in the bedload regime. The transport is observed to be higher when small particles are buried below large ones. This is interpreted as a granular process and a new simple explanation is given in the framework of the $\mu (I)$ rheology. A predictive model for the increase of transport is proposed based on granular rheological arguments.

Direct numerical simulations are used to study preferential concentration of heavy inertial particles in the particle-induced convective instability. This process and the resulting significant particle concentration enhancement are investigated using the two-fluid equations. Motivated by dominant balance arguments, the scaling of the particle concentration enhancement over the mean is found; the maximum scales as $u_{\text{rms}}^2{\tau }_p/{\kappa }_p$, and the typical as $(u_{\text{rms}}^2{\tau }_p/{\kappa }_p{)}^{1/2}$, where $u_{\text{rms}}$ is the rms of the fluid velocity, ${\tau }_p$ is the particle stopping time, and ${\kappa }_p$ is the assumed particle diffusivity.

Nonspherical particles such as fibers are commonly dispersed by turbulent boundary layer flows encountered in industry as well as our daily living environment. Fiber length has an effect on the orientation, translation, and rotation of fibers. In this experimental study it was found that irrespective of fiber length, mean squared fiber tumbling rates strongly increased in the buffer layer, but remained constant in the log-layer and the wake region. A clear length effect was observed as longer fibers tumbled at higher rates than shorter ones.

In spark plasma discharges, vortex rings are shown to control the expansion and convective cooling of the hot gas kernel. The rates of both processes increase with the electrical energy deposited. This has implications on momentum transport and passive scalar mixing in plasma-based flow and combustion control applications.

The injection of long-chain polymer additives and the water-repellent (or superhydrophobic) texturing of submerged solid walls, have both evolved independently over the years into effective, stand-alone methods for frictional drag reduction in wall-bounded turbulent flows. Experiments performed in turbulent Taylor-Couette flow demonstrate that the two techniques, when combined carefully, result in an additive effect, producing significant enhancements in the overall level of drag reduction achieved.

Large turbulent flow simulations can lead to information bottlenecks as data is generated faster than it can be processed and saved. Innovative in-situ data compression techniques are needed. Here we examine a deep learning approach to in-situ compression using a novel autoencoder architecture customized for three-dimensional turbulent flows. We compare it to a randomized single-pass singular value decomposition method and demonstrate improved compression and reconstruction, particularly with respect to important statistical quantities such as turbulent kinetic energy, enstrophy, and Reynolds stresses, at lower computational cost.

The convergence of the Karunen-Loève (KL) decomposition in high-Reynolds-number boundary layers is investigated using large-eddy simulations. It is found that the KL dimension, namely, the number of KL modes necessary to represent 90% of the turbulent kinetic energy, is up to 3 orders of magnitude higher than values commonly reported in earlier studies. This indicates that more caution should be exercised when considering the convergence of a proper orthogonal decomposition basis in high-Reynolds-number boundary layers, and it illustrates once more the challenges associated with representing turbulence in a low-dimensional basis.

The pressure-Hessian tensor (PHT) is an important factor governing the Lagrangian evolution of velocity gradients in turbulent flows. To help develop a physically consistent model for PHT, we train a tensor basis neural network (TBNN) using velocity gradient information as input. Our trained model is found to be superior to existing models, not just in terms of root-mean-squared error, but also for capturing PHT physical attributes. Our model retrieves almost identical alignment statistics between the pressure-Hessian and the strain-rate eigenvectors when compared with well established direct numerical simulation (DNS) datasets.

Three-dimensional particle tracking velocimetry is used to characterize the time-resolved turbulent flow in an asymmetric diffuser. The measurements cover the full diffuser and show that the separated flow consists of large-scale structures with a Strouhal number smaller than 0.03. Three large vortices are identified; two vortices originate from the wall with the largest diverging angle and the third vortex from the neighboring diverging wall.

Energy transfer between scales in the turbulent cascade requires geometric alignment between scale-dependent turbulent stresses and strain rates. However, each of these tensors evolves dynamically and differently, leading to nontrivial temporal behavior of this alignment. A numerical simulation of turbulence is used to study this process and, by formulating the energy flux in a fully Lagrangian way, the important role played by deformation in the energy cascade is highlighted.

In the flow over a smooth wall, the statistics of velocity and temperature, properly normalized, collapse (Reynolds’ analogy); when the wall is rough, this analogy fails. Its failure has long been known to be due to the pressure gradient, which is absent in the scalar transport equation. A study examines how the geometrical features of the roughness affect the failure of the Reynolds’ analogy, by focusing on the transport equations for temperature variance and turbulent kinetic energy.

From an energy budget perspective, the decomposition of mean friction drag in adverse-pressure-gradient turbulent boundary layers is conducted, obtaining contributions associated with dissipation, production, and convection across the boundary layer. In the wall-normal distributions of the decomposed constituents, positions of inner peaks are well scaled in viscous units, whereas outer peaks scale in outer units, regardless of Reynolds number and pressure-gradient magnitude.

“We propose a dynamic subgrid scale (SGS) scalar flux model based on the exact turbulent flux production rate. The model incorporates a tensor diffusivity explicitly dependent on subgrid stresses and the resolved velocity gradient. Representing diffusivity by a tensor allows the scalar flux vector to be misaligned with the filtered temperature gradient. Since modeling the subgrid scales represents their influence on resolved scales as an average of small scales, we show that extending RANS type closures to an LES framework is a promising route to cost effective and robust subgrid modeling.

A study of the combined effect of outer-layer large-scale superposition, amplitude modulation, and distortions on the near-wall dynamics of turbulent wall-bounded flows is presented. A novel approach to detect the amplitude modulation is introduced that more clearly reveals this phenomenon compared to existing techniques. The study also shows that scale separation by the empirical mode decomposition approach and by conventional spectral filtering yields similar conclusions about the interaction mechanisms.

The significance of local phase shifts in the experimental realization of soliton and breather waves in finite water depth as well as in deep water is explored. When suppressing the corresponding phase shift, the coherence of the wave envelope disintegrates by forming distinct soliton patterns. All experimental results are in excellent agreement with the predictions based on the nonlinear Schrödinger equation framework, suggesting the universality of observed dip and extreme wave pattern dynamics.

Internal gravity waves in stratified turbulence are searched for using spatiotemporal Fourier transforms of the 3D velocity and density perturbation fields obtained from direct numerical simulation. Waves at high frequency up to $\omega =N$ are uncovered and the Doppler shift imparted on them by the horizontal mean flow is used to develop a method for estimating their energy content. The results highlight a variation of the wave energy with buoyancy Reynolds number. The wave signal is concentrated at the largest scales and is much less discernible over the majority of other scales, containing low-frequency nonlinear and anisotropic motions.

The behavior of a melting sphere, sinking in a linearly stratified fluid, is carefully investigated using an experimental setup. The melting of a sphere induces a large enhancement of the drag coefficient. A stratification drag enhancement for high Reynolds numbers is also observed. Internal gravity waves generated by wake turbulence redistribute the kinetic energy into the stratified fluid over a long period.