Ambient Fluid Research Articles

Squirming in density-stratified fluids

In this work, we study the axisymmetric motion of a prototypical swimming organism, a spherical squirmer, in a linearly density-stratified fluid. We assume the inertia is negligible, the stratification is weak, and the swimmer is oriented either vertically upward or downward. The swimmer acts as a settling particle or a neutrally buoyant organism depending on the relative magnitude of its density and the ambient fluid density. While the stratification reduces the speed of a settling particle, it increases (respectively, reduces) the speed of a neutrally buoyant puller (respectively, pusher). The stratification only affects the flow far from the swimmer and this far-field flow due to a settling particle (respectively, neutrally buoyant organism) at low advection of density is same as the flow due to a point-force (respectively, force-dipole) placed in a stratified fluid. The swimmer mixes the surrounding fluid. The mixing caused by a settling particle scales as the ratio of the particle size to the stratification length scale but the mixing due to a neutrally buoyant organism scales as the ratio of the buoyancy to the viscous forces. The latter mixing estimate is much larger than the previous estimate based on the force-dipole model of an organism but still negligible compared to the oceanic mixing. These results are useful to understand the stratification effects on the motility of the organism and the induced mixing.

Experimental verification of theoretical approaches for radial gravity currents draining from an edge

We present an experimental study of inertial gravity currents (GCs) propagating in a cylindrical wedge under different drainage directions (inward/outward), lock-release (full/partial gate width) and geometry (annulus/full cylinder). We investigate the following combinations representative of operational conditions for dam-break flows: (i) inward drainage, annular reservoir, full gate; (ii) outward drainage, full reservoir, full gate; and (iii) outward drainage, full reservoir, partial gate. A single-layer shallow-water (SW) model is used for modelling the first two cases, while a box model interprets the third case; the results of these approximations are referred to as “theoretical”. We performed a first series of experiments with water as ambient fluid and brine as intruding fluid, measuring the time evolution of the volume in the reservoir and the velocity profiles in several sections; in a second series, air was the ambient and water was the intruding fluid. Careful measurements, accompanied by comparisons with the theoretical predictions, were performed for the behaviour of the interface, radial velocity and, most important, the volume decay {mathcal {V}}(t)/{mathcal {V}}(0). In general, there is good agreement: the theoretical volume decay is more rapid than the measured one, but the discrepancies are a few percent and the agreement improves as the Reynolds number increases. Velocity measurements show a trend correctly reproduced by the SW model, although often a delay is observed and an over- or under-estimation of the peak values. Some experiments were conducted to verify the role of inconsistencies between experimental set-up and model assumptions, considering, for example, the presence or absence of a top lid, wedge angle much less than 2pi , suppression of the viscous corner at the centre, reduction of disturbances in the dynamics of the ambient fluid: all these effects resulted in negligible impacts on the overall error. These experiments provide corroboration to the simple models used for capturing radial drainage flows, and also elucidate some effects (like oscillations of the radial flux) that are beyond the resolution of the models. This holds also for partial width lock-release, where axial symmetry is lost.

Euler–Lagrange framework for deformation of granular media coupled with the ambient fluid flow

A full-scale continuum framework for modeling fluid interaction with granular media is computationally exhaustive due to the diverse range of length and time scales. Given the nature of these processes, the computational burden aggravates the problem of real-time simulation for large-scale scenarios. In the current study, the interaction between Newtonian fluid and deformable granular media is modeled through a coupled framework comprising of a two-dimensional divergence-free Incompressible Smoothed Particle Hydrodynamics and a depth-integrated Finite Volume Method, respectively. The coupling is based on the interaction force pair tailored for each module appropriately. The formulation has been validated against a variety of test-cases comprising of a diverse configuration of initial conditions for both fluid and granular modules. The proposed framework is in consonance with experimental data and may be extended to real-time scenarios owing to lower computational overhead.

Persistent fluid flows defined by active matter boundaries

Biological systems control ambient fluids through the self-organization of active protein structures, including flagella, cilia, and cytoskeletal networks. Self-organization of protein components enables the control and modulation of fluid flow fields on micron scales, however, the physical principles underlying the organization and control of active-matter-driven fluid flows are poorly understood. Here, we use an optically-controlled active-matter system composed of microtubule filaments and light-switchable kinesin motor proteins to analyze the emergence of persistent flow fields. Using light, we form contractile microtubule networks of varying size and shape, and demonstrate that the geometry of microtubule flux at the corners of contracting microtubule networks predicts the architecture of fluid flow fields across network geometries through a simple point force model. Our work provides a foundation for programming microscopic fluid flows with controllable active matter and could enable the engineering of versatile and dynamic microfluidic devices.

Multiscale roughness influence on hydrodynamic heat transfer in a single fracture

This study investigates the multiscale roughness effects on hydrodynamic heat transfer (or heat transfer associated with water flow) in a single fracture, based on direct numerical simulations of fluid flow and heat transfer in fracture-matrix systems with or without secondary roughness considered. The simulation results show that the primary roughness controls the low velocity regions and the overall flow direction in the fracture, while the secondary roughness enhances the complexity of the flow near the local sharp cornered asperities and promotes the generation of eddies. The changes of flow behavior could in turn profoundly affect the water temperature distribution and local heat transfer coefficient in the fracture-matrix system. In addition, two competing mechanisms impacting the overall heat transfer coefficient (h) due to the multiscale roughness are revealed. On one hand, the increase in flow velocity induced by primary and secondary roughness increases h. On the other hand, the eddy zone induced by the secondary roughness can trap the high-temperature fluid and inhibit the heat transfer from the fracture surface to the ambient fluid, which could decrease h. The macroscopic effect of surface roughness on h is the result of the joint action of these two competing influence mechanisms.

Nonlinear dynamics of fully developed swirling jets

This paper characterises the steady and time-periodic behaviour of swirling jets using numerical bifurcation analysis. Its objective is to elucidate the dynamics of fully developed, unconfined, laminar swirling jets under variations in the Reynolds number $\textit {Re}$ and swirl ratio $S$ . Within the $(0,0)\leq (\textit {Re},S)\leq (300,3)$ range, the steady, axisymmetric flow exhibits several distinct patterns ranging from a quasi-columnar jet along the central axis at low $S$ to a radial jet attached to the containing wall at high $S$ with various forms of vortex breakdown in between. A cusp bifurcation appears in the steady solution manifold which triggers bistable behaviour due to a competition between inner and outer low pressure regions associated with vortex breakdown and entrainment of the ambient fluid, respectively. Instability of the steady flow is linked to eigenmodes which are singly ( $|m|=1$ ) or doubly ( $|m|=2$ ) azimuthally periodic, although additional instabilities with other azimuthal wavenumbers occur at $(\textit {Re},S)$ -values beyond the leading neutral curves. The various branches of limit cycle solutions stemming from these neutral curves are associated with both super- and sub-critical Hopf bifurcations. The resulting unsteady flow fields exhibit a wide array of rotating, three-dimensional flow structures, and comparisons between the time-averaged and steady flow patterns highlight the role of these unsteady nonlinear interactions on the overall behaviour of swirling jets. Similarities and differences between this laterally unconfined jet and broader classes of swirling flows, including confined swirling jets and unconfined vortex models, are also discussed.

On the challenges of precise velocity measurements in vertical convective wall jets

For the transition of our energy supply towards a higher share of renewables, thermal energy storage (TES) systems are, besides electric batteries and chemical energy storage systems, one promising solution to overcome the volatile nature of renewable energy sources. For the most efficient operation, the liquid storage material in the tank should be stratified by its temperature-dependent density. As a result, the cold fluid remains at the bottom, and the heated fluid rises to the top (Alva et al. (2018)). Typically steel tanks are used for TES, and thus, the wall material has a thermal diffusivity that is one to two orders of magnitudehigher than that of the storage fluid. Consequently, the tank’s sidewalls work as a thermal bridge between the stratified layers. In recent studies, the authors have shown that the resulting heat flux induces two counterdirected, convective wall jets near the sidewalls of the tank, which increase mixing of the stratification and thus lowers the exergy content and the storage efficiency (Otto et al. (2019, 2020)). Using a model experiment of a TES, the entire vertical extent of the detected wall jets is investigated. Hence, the typical flow structures of vertical, natural convection under the influence of non-zero temperature gradients in the ambient fluid can be analyzed, which can help to improve storage tanks in the future. The velocity in the region of the wall jets is measured via 2d particle-image velocimetry (PIV) in a rectangular model experiment of 750mm height on a base area of 375mm×375mm made from polycarbonate. The jets evolve on the surface of an aluminum plate simulating the storage tank’s sidewall. The measuring system consists of four cameras with a resolution of 2160×2560 pixels combined with objective lenses with 100mm focal length capturing the raw images in a plane perpendicular to the aluminum wall. A Nd:YAG laser with a wavelength of 532nm illuminates the measuring plane. Simultaneously using up to four cameras adjacent to each other and stitching their resulting vector fields, the vertical extent of the field of view increases from 38mm up to 140mm. Despite this, the field of view is still much smaller than the vertical extent of the model experiment, so that seven consecutive runs are performed to cover the entire height. Disturbing reflections of the laser light sheet on the aluminum wall are eliminated using optical filters for the cameras that are opaque for the green laser light in combination with fluorescently (Rhodamine B) dyed PMMA tracer particles with a diameter between 1–20μm. The particles emit light at a wavelength of 610nm (orange light) and can therefore be detected through the cameras’ filters. During four separate measuring periods, where each lasts for two minutes, double frame images are captured with a time difference of 19.981 ms (maximum possible value) at a measuring frequency of 7 Hz. Figure 1 shows a schematic of the camera setup next to the model experiment and the measurement and evaluation procedure to finally receive one time-averaged velocity field per measuring period of the full height of the experiment. The raw data evaluation process starts with calculating the vector fields of all cameras used at a certain measuring position and stitching them to one flow field of this position. Since the wall jets’ horizontal extents are with 2–7mm relatively small and they show high velocity gradients, the raw images are evaluated in both single-frame and double-frame mode. With a velocity threshold that corresponds to a pixel displacement of 1/4 of the interrogation window size and the time difference of the single-frames, the resulting vector fields are masked and merged into one final vector field. This vector field consists of high velocities evaluated in double-frame mode and low velocities evaluated in single-frame mode (see Figure 2) thus minimizing the relative error. The algorithm used in this work is similar to the multi-frame PIV approach introduced by Hain and K¨ahler (2007). Figure 3 shows the time-averaged results of the first measuring period for each of the seven measuring positions in height.

Influence of thermal stratification on vertical natural convection—Experimental investigations on the example of thermal energy storage systems

Stratified thermal energy storages (TESs) are a promising solution for the large-scale energy storage problem of surplus renewable energy. Recent studies have shown parasitic convection occurring in near-wall regions inside such storage tanks, decreasing the working fluid's thermal stratification and reducing their exergy efficiency. This paper presents an experimental investigation of vertical convective flows in thermally stratified environments to complement the theoretical studies in this field. Specifically, we consider natural convection within a stratified laminar flow driven not by active heating but by the temperature gradient along a vertical wall, as is the case in real TES systems. The insights gained into the fundamental physical mechanisms of stratified vertical convection can promote efficiency improvements in TES systems. Therefore, we combine multiple particle image velocimetry and temperature measurements at different heights and thus obtain high-resolution vector fields of the entire wall jet flow and vertical temperature profiles for a TES model experiment. We appropriately modify scaling arguments found in the literature to develop a theory specifically suited to the experimental setup. The experimental data agree well with the modified theory. The results show two laminar counter-directed jets next to the vertical sidewall. In regions with high temperature gradients, the wall jets slow down, and flow reversals occur next to them. Moreover, the wall jets are asymmetric due to temperature-dependent fluid properties in conjunction with the ambient fluid stratification. In the stratification's upper, hot part, the wall jet is thinner and faster than the bottom jet in the cold region.

Flow pumping by external periodic shear applied to a soft interface

Flow pumping in viscous fluids is of prime importance in micro-fluidic applications. Here we show that a single colloidal particle in front of a soft wall, manipulated by external means like an optical tweezer, can pump the ambient viscous fluid. The particle, moving back and forth parallel to the soft wall, can produce an averaged net flow in a direction perpendicular to the wall. Using a perturbative scheme, we present the results. Analysis show that this flow in terms of capillary number, scales as {text {Ca}}^2.

Fog Formation Related to Gravity Currents Interacting with Coastal Topography

An interesting mixing-fog event was identified during the C-FOG field campaign, where a cold-frontal airmass arriving from the north-east collided with The Downs peninsula in Ferryland, Newfoundland, Canada, to produce misty/foggy conditions. A comprehensive set of field observations suggests that this collision caused turbulent mixing of nearly saturated ambient air with an almost saturated cold-frontal airmass, creating conditions for mixing fog. To delve into the physical processes underlying this phenomenon, laboratory experiments were performed on the interaction of lock-exchange-induced gravity currents with a rectangular obstacle. Instantaneous velocity and density fields were obtained using particle image velocimetry and planar laser-induced fluorescence. The observations suggest that the obstacle starts affecting the approaching gravity-current propagation at an upstream distance of 2H and, upon collision, the mixing occurs over a length of 0.83H, where H is the depth of the ambient fluid layer. The time for larger-scale turbulent stirring to permeate to the smallest scales of turbulence and activate the condensation nuclei is estimated as \(3t^*\), where \(t^*=\sqrt{H/g'}\) is the intrinsic time scale of the gravity current, and \(g'\) is the reduced gravity. Extrapolation of laboratory results to field conditions shows a good agreement with observations.

Visualization of the flow morphology of submerged jets in subcritical and supercritical water

Direct observation of flow behaviors is helpful for design of supercritical water reactor, so an in situ observation platform was built. Water jets under sub- and supercritical conditions (300–550 °C, 23 MPa) were observed by high-speed camera, which can be divided into four typical morphologies: mushroom momentum jets, amorphous plumes, banded plumes and cylindrical momentum jets. The time-average velocity field of each morphology was obtained through the image correlation method to quantitatively describe the flow and diffusion characteristics of jets. The influencing factors on the jet morphology, including the initial flow state of the jet, the driving force of the jet and the turbulence intensity of the ambient fluid, are analysed. Several dimensionless quantities are introduced to describe these factors quantitatively. The results show that the driving force of the jet is the decisive factor to appear to be a plume or momentum jet. The jet initial state has a large influence on the formation of the mushroom vortex. Turbulence of ambient fluid can lead to irregular deformation of the jet shape.

Plunging criterion for particle-laden flows over sloping bottoms: Three-dimensional turbulence-resolving simulations

Hyperpycnal flows are observed when the density of a fluid entering into a quiescent environment is greater than that of the ambient fluid. This difference can be due to salinity, temperature, concentration, turbidity, or a combination of them. Over a sloping bottom, the inflowing momentum decreases progressively until a critical stage is reached where the inflow plunges underneath the ambient and flows adjacent to the bed as an underflow density current. In the present work, a new equation is proposed in order to predict the critical depth for plunging, i.e., the plunging criterion. It differs from previous studies since it includes the role of the settling velocity and the bed slope. The high spatiotemporal resolution from twelve original numerical simulations allows us to validate the initial hypotheses established, in addition to numerical and experimental data available in the literature, and good agreement is found between them. A negative value for the mixing coefficient was observed for the first time for the hyperpycnal flow in a tilted channel. This indicates that if the settling velocity of the suspended material is high enough, the submerged flow may lose fluid to the environment (detrainment), instead of incorporating it. The proposed plunging criterion may assist in the design of future experimental or numerical works.

Influence of the surface roughness on the collision behavior of fine particles in ambient fluids

Particle-wall collisions appear in many different processes of particle technology. During a collision, different micro processes, such as friction, plastic deformation and adhesion can take place in the contact and lead to an energy dissipation. The surrounding fluid between particle and wall can also decrease the impact velocity due to viscous forces. This study focuses on the influence of the surface roughness on particle-wall collisions in ambient air and water. The single particle impact tests were performed with spherical zirconium oxide particles on a polished and an untreated titanium surfaces to investigate the energy dissipation during collision in a wide range of Stokes numbers (20–8900) depending on the surface roughness. A 3D high-speed camera system was used to obtain the particle impact and rebound behavior. The particle and wall roughness were measured with confocal microscopy and scanning probe microscopy, from which an equivalent roughness parameter was derived. The experiments showed that with an increase of the roughness, the coefficient of restitution decreases in air, while increasing during collisions in water. An elastohydrodynamic model of particle collision on the wall covered by a liquid layer was used to predict the coefficient of restitution for the contact in ambient fluids while considering measured roughness parameters.

Fully implicit spectral boundary integral computation of red blood cell flow

This paper is on an implicit time integration scheme for simulation of red blood cell (RBC) flow in an ambient fluid. The intra- and extracellular plasmas are modeled as Stokes flows and represented by boundary integral equations (BIE) written in a weakly singular form. The cell membrane is modeled as a thin elastic shell. Expressed in this way, the RBC flow model constitutes an implicit ordinary differential equation (IODE) in the cell shape. The cell shape and velocity field are discretized spatially by a spectral approach using spherical harmonic basis functions. It is then convenient to express the BIE in the Galerkin form with the spherical harmonics themselves as test functions. The key aspect in this paper is the recognition of the IODE structure of the RBC flow model and consequent application of a multi-step implicit solver for time integration. As with any implicit solver, a nonlinear equation in the cell shape is solved at each time step, for which Newton's method is applied. This requires the Jacobian of the IODE, or equivalently computation of Jacobian-vector products. An important contribution is the formulation of such Jacobian-vector products as evaluating a second BIE. The original weakly singular form is crucial in facilitating this formulation. The implicit solver employs variable order and adaptive time stepping controlled by truncation error and convergence of Newton iterations. Numerical examples show that larger time steps are possible and that the number of matrix-vector products is comparable to explicit methods. Source code is provided in the online supplementary material.

Flow Rate along the Length of the Swirling Vortex Axis at an Intake

In this study, the characteristics of the flow in the region of swirling vortex are examined. The potential flow model based on the summing infinite number of spherical sinks along the vortex core is introduced to predict the flow field and the flow rate along the vortex axis. The flow towards the swirling vortex core has considerable effects on the radial velocity distribution within the ambient fluid flow region near the intake. The agreement between available test data relating to the radial velocity and the method introduced in this study is found to be satisfactory.

Viscoplastic toroidal drop in compressional Stokes flow

We report the dynamics of a viscoplastic (Bingham) torus suspended in an unbounded Newtonian medium. In this study, the immiscible ambient fluid is subjected to an axisymmetric compressional (biaxial-extensional) flow. Numerical simulations applying the boundary integral formulation to the Stokes flow are performed for the torus drop having initially a circular cross section. The quasi-stationary dynamic simulation reveals that depending on the initial conditions and the capillary number (Ca), the Bingham number (Bn), and the viscosity ratio (λ), three different scenarios of the drop evolution are obtained for a range of viscosity ratio and Bingham number: collapsing toward the axis of symmetry, expanding infinitely, and having a stationary toroidal shape.

Nonlinear pulsations of horizontal jets

A shallow-water model with horizontally nonuniform density is used to study the dynamics of jet flows that arise under the influence of buoyancy and the Coriolis force. Within this approach, the jet is described by a self-similar compactly-localized solution and interpreted as a band of shear flow having a temperature contrast with the ambient fluid. In addition to stationary states, the dynamics of such jets admit cyclonic rotation with a constant angular velocity and transverse nonlinear pulsations. The phase portrait corresponding to this model shows that regimes with pulsating jets develop along closed trajectories bounded by the separatrix loop. The theory predicts that the period for warm jet pulsations is longer than the inertial oscillation period caused by the Earth’s rotation, while for cold jet pulsations, it is shorter. Thus, only warm jets can have a noticeable effect on the atmospheric dynamics in the synoptic range. In particular, they may well be responsible for additional spectral peaks that appear in this range of wind speed fluctuations.

Relative impacts of permeability heterogeneity and viscosity contrast on solute mixing

Solute transport and mixing in porous media are affected by the heterogeneous structure of the permeability field and the viscosity contrast between the dissolved solute and the ambient fluid. We provide a quantitative separation of the impacts of fingering and heterogeneity and we parameterize the concentration probability distribution function.

Layered Chiral Active Matter: Beyond Odd Elasticity.

In equilibrium liquid crystals, chirality leads to a variety of spectacular three-dimensional structures, but chiral and achiral phases with the same broken continuous symmetries have identical long-time, large-scale dynamics. In this Letter, starting from active model H^{*}, the general hydrodynamics of a pseudoscalar in a momentum-conserving fluid, we demonstrate that chirality qualitatively modifies the dynamics of layered liquid crystals in active systems in both two and three dimensions due to an active "odder" elasticity. In three dimensions, we demonstrate that the hydrodynamics of active cholesterics differs fundamentally from smectic-A liquid crystals, unlike their equilibrium counterpart. This distinction can be used to engineer a columnar array of vortices, with an antiferromagnetic vorticity alignment, that can be switched on and off by external strain. A two-dimensional chiral layered state-an array of lines on an incompressible, freestanding film of chiral active fluid with a preferred normal direction-is generically unstable. However, this instability can be tuned in easily realizable experimental settings when the film is either on a substrate or in an ambient fluid.

Three-Dimensional Numerical Investigation of Hypersonic Projectile Launched by Railgun on Transitional Ballistics

Supersonic accelerators use sabots to suppress bore balloting. However, sabot separation induces flight deviation in projectiles owing to their interactions with fluids and shock waves. A seamless calculation of the acceleration phase in the tube and the full separation process using 3-D computational fluid dynamics coupled with rigid body dynamics is presented in this study to investigate the shock-wave interactions around a railgun-launched projectile. The railgun acceleration induced a normal shock wave propagating ahead of the projectile; when the projectile reached the tube end, a substantial expansion at the muzzle generated two shock waves, namely, a spherical precursor shock wave and a Mach disk, outside the tube. Aerodynamic forces on the projectile/sabot decreased to almost zero just after the tube exit, with the ambient fluid around the tube being faster than the projectile by the substantial expansion. After exiting, the projectile penetrated the precursor shock wave at ; this increased the aerodynamic forces acting on the projectile, initiated sabot separation, and generated multiple shock-wave interactions, which induce unsteady aerodynamic loads on the projectile.