ICTS

A simplified two-fold pattern of convection in the Earth’s core is often used toexplain the non-axisymmetric magnetic flux concentrations in the present daygeomagnetic field. For large thermal anomalies in the lower-mantle, however,a substantial east-west dichotomy in core convection may be expected. In thisstudy, the effect of a large lateral variation at the outer boundary is examinedin (a) cylindrical annulus experiments that achieve approximate geostrophyof the convection and (b) a rapidly rotating spherical shell dynamo model.The pattern of convection in the dynamo simulations closely follows thatin the annulus experiments, which suggests that the lateral buoyancy at theequator essentially determines the structure of core convection. In particular,an isolated downwelling forms only beneath the Siberian region in stronglydriven convection. This can explain the relative instability of the magneticflux patches observed in the Western hemisphere.

Certain non-Newtonian fluids like hen’s egg, condensed milk, polymer melts when subjected to a partially immersed rotating rod exhibit ‘rod-climbing’. This spectacular phenomenon known as the Weissenberg effect has been so far attributed to the dominance of the normal stress differences over the rotation generated centrifugal forces. In this study, we delve into the question of whether these rod-climbing fluids continue their ascent on the rod in presence of a slippery interfacial layer (rod-fluid interface)? To execute this, immersed portion of the rod is covered with an oil layer (silicone and castor oils). Polyethylene oxide suspensions in deionized water are used as a model polymeric fluid in this study. In conjunction with this, the strength of first normal stress difference is also modulated to investigate if it affects the capacity of the polymer in sustaining the rod-climbing effect. This variation is brought about altering the polymer concentration in the aqueous suspension. Surprisingly, three states of positive Weissenberg (fluid-meniscus rise or classical rod climbing), negative Weissenberg (fluid-meniscus dip) and absence of Weissenberg effect are observed. The primary centrifugal force induced shear deformation of the meniscus height is evaluated to characterize these states. A consolidated phase diagram based on the experimental results is proposed demarcating these states as a function of the polymer concentration, stirrer rotational speed (or applied shear) and type of oil layer. However, the rod-climbing effect is also known to be accompanied by the secondary flows. Dye visualization experiments are performed to qualitatively map the secondary flows that show a characteristic flow direction depending on the state (positive/negative) exhibited by the fluid.

Multiple jets are found in several applications such as fuel injection, smoke stacks, jets enginesof aerospace vehicles, etc. Several researchers have experimentally studied the flow fielddevelopment of twin round jets. Okamoto et al. [1] carried out one of the earliest experimentalinvestigation of the interaction of parallel, twin turbulent round jets. The mean velocity and pressureprofiles were reported, but, turbulence quantities were not. They observed the shift in position ofmaximum velocity from the individual jet axes to the plane of symmetry of the jets. Rathakrishann,et al [2] also explored experimentally the flow field development of jets issuing from two circularnozzles set at a common end wall, with different nozzle separation distances. Their experimentshowed that the nozzle separation distance has a considerable influence on the development of thevelocity profiles up to downstream distances of 30d. Even a pair of round jets can have adistinctive development because, after the inevitable merging, there can be axis switching. LES ofparallel, twin jets reveal the structure and mechanisms of the flow-field. The LES is by an explicitfiltering method [3]. The twin round jets are at a Reynolds number Re of 230000, based ondiameter and mean velocity at exit. Distance between jet centers was 5 diameters. Closequantitative agreement with experiment was found on the development of mean profiles andspreading. Velocity fluctuations between the jets are weaker than those on the outer boundaries.Axis switching was observed.

In aquatic habitat, many species use pulse jet propulsion to obtain the thrust force. Thiskind of propulsion can be seen in a jellyfish where the contraction of the subumbrellar cavityresults in the production of thrust producing jet. The aim of the present study is to understandthe fluid dynamics of an impulsive force generation in the aquatic medium. The simplifiedexperimental model has been constructed using two thin flat plates interconnected with atorsion spring. The spring has held in the tension initially such that the angle between twoplates would be 60 degrees. Due to the sudden release of tension, both plates perform clappingaction in which the inter-plate angle starts reducing. During the clapping action of plates,fluid present in the inter-plate cavity will be ejected out and the body experiences an impulsivethrust force. Experiments have been conducted in the quiescent water medium.

The computational study of clapping propulsion has been performed using ‘ANSYS19’.The experimentally measured clapping body kinematics used as input to the computation. Thechallenges involved in the computational modeling of clapping action such as large displacementand contraction of the interplate cavity have been carefully handled using the ‘Overset Mesh’technique. The complicated 3D flow field has been studied computationally, which can bedifficult to analyze experimentally. The future study includes the investigation of the roleof added mass in such unsteady fluid dynamics. Based on the results of this computationalstudy, the wake of the clapping body, sufficiently downstream, can be approximately modeledas an ‘axisymmetric vortex’ ring, which is in the agreement with experiments. The involvedmomentum and energy balance in clapping propulsion may be studied using an ‘axisymmetricvortex ring model’.

An experimental investigation of the effect of elimination of the horseshoe vortex at a simplified wing-body junction, using an asymmetric nose-fairing is carried out. The wing used has a NACA0018 airfoil as cross-section and is mounted on a flat plate. Measurements are carried out at a low freestream velocity of 31m/s. A simple procedure is devised to design an asymmetric nose-fairing that could prevent saddle point separation at the wing-plate junction, for the wing operating at 4 o incidence. The asymmetric nose-fairing designed for this wing-plate junction is found effective in eliminating the horseshoe vortex and improving the flow quality at the junction. Despite the fairing geometry resulting an increase of wing surface area (by 9.5%) in the wing root region, there was nearly a 6% reduction in the momentum deficit in the plate boundary-layer at x/c of 1.04. However, with the elimination of the horseshoe vortex, the low momentum fluid penetrated into the junction region. This lead to an increase in the boundary-layer shape factor H in the near junction region as compared to the unfaired configuration.

Observations show that galaxies like our Milky Way are surrounded by a multiphase (spanning a range of temperatures) “circumglactic medium” (CGM) with the hot (~10​6K) phase confining the cooler (~104K) phase having a much smaller volume filling factor. The structure of this multi phase medium is governed by the inter play of shear driven mixing and cooling which tends to produce distinct temperature phases. Cooling occurs due to atomic transitions in different ion species whose abundance is governed by ionization and recombination processes. Cooling and ionization length / time scales are much smaller compared to the natural scales of this system, making it extremely difficult to simulate the multiphase CGM. There are clear analogies with turbulent combustion and cloud physics which I wish to highlight. This is one of the most exciting recent developments in galaxy formation which deeply involves the physics of turbulent multiphase flows.

The study of particles suspended in turbulent flows is a topic of active research due to its wide applicabilityin industrial flows, geophysical flows, ans atmospheric flows. Particles suspended in turbulent flows are knowto preferentially sample the flow with spending most of the time in straining regions than in the vorticalregions. Another striking feature of tracers particles suspended in incompressible turbulent flows are known togain energy slowly and loose energy abruptly. This phenomenon, termed as “flight-crash" events [1], canprovide a quantitative measurement of the irreversibility in turbulent flows. The irreversibility was shown toincrease with Taylor-Reynolds number Reλ [1] and Stokes number St [2]. We studied the effect of a constantbackground rotation on these “flight-crash" effects in three-dimensional turbulent flows using direct numericalsimulations (DNS). We observed that rotation generates weak but spatially extended vortical structures in theflow. As a result, in presence of rotation, the particles spends almost equivalent time in straining and vorticalregions. We showed that rotation suppresses the “flight-crash” events in turbulent flows. We quantify the time-irreversibility (Ir = <p3>/ε3) in the flow by computing the third moment of power (p) normalized by the meanenergy dissipation rate (ε). We showed that the irreversibility (Ir) decreases with decreasing Rossby numberRo (increasing rotation rate). The scenario is, however, non-monotonic for heavy particles (St ≠ 0). In case ofheavy particles, the irreversibility, depends on both St and Ro.

Under an increasing applied shear stress, viscosity of many dense particulate suspensions increases drastically beyond a stress onset, a phenomenon known as discontinuous shear-thickening (DST). Recent studies point out that some suspensions can transform into a stress induced solid-like shear jammed (SJ) state at high particle volume fractions. SJ state develops a finite yield stress and hence is distinct from a shear-thickened state. Here, we study the steady state shear-thickening behaviour of dense suspensions formed by dispersingcolloidal polystyrene particles in polyethylene glycol. We find that for small values the viscosity of the suspensions as a function of can be well described by Krieger-Dougherty (KD) relation. However, for higher values of applied stress (much larger than the stress onset for shear thickening), KD relation systematically overestimates the measured viscosity,particularly for higher volume fractions.This systematic deviation can be rationalized by the weakening of the sample due to flow induced fractures and failures when the sample is in a solid like SJ state. Using Wyart-Cates model, we propose a method to predict the SJ onset from the steady state rheology measurements. Our results are further supported by in-situ optical imaging of the sample boundary under shear

We study experimentally the stokesian sedimentation (Re∼10−4) of a one dimensional latticeof discs in a quasi-two-dimensional geometry with the trajectory of the centres of the disks lying ina plane. We induce initial positional perturbations over a configuration in which the disks are uni-formly spaced with their separation vectors and normals aligned, and perpendicular to gravity. Forvarious perturbation wavenumbers and interparticle separations, we find two classes of behaviour:(i)a transient wave of orientations coupled with number-density fluctuations and (ii) a clumping in-stability resembling that of spheres [J.M. Crowley, J.Fluid Mech. 45, 151 (1971)], decorated withorientations. We construct the equations of motion for displacements and orientations using pair-wise addition of forces and torques [R. Chajwa et al. PRL 122, 224501 (2019)]. Linear stabilityanalysis demarcates a phase boundary between neutrally stable and unstable regimes in the planeof wavenumber and lattice spacing, consistent with our experiments. We predict non-modal growthin this plane, with a critical density of the lattice below which all wavenumbers are asymptoticallystable, showing that orientable particles need not be subject to the inevitable clumping instabilityof spheres.

We study the dynamics of a drop immersed in an ambient fluid subject to a generallinear flow. The main motivation is to study the transport of heat or mass in such dispersemultiphase systems. Such studies have important practical applications ranging from flowswith suspended drops undergoing chemical reactions, vaporisation of atomised fuel jets in aturbulent ambient, which are industrially relevant, to natural phenomena like cloud formation,uptake of nutrients by living cells. We perform Boundary Element simulations (BEM) of asingle drop in a simple shear flow, and compare it to existing theoretical predictions, with theemphasis being on the alteration of the streamline topology around the drop. The simulationsare done over a wide range of viscosity ratios (λ) and the Capillary number (Ca), in theabsence of inertia (Re= 0 ); the Capillary numberCais a dimensionless measure of therelative magnitudes of viscous and interfacial tension forces. For all cases examined, we finda non-zero flux in the flow-gradient plane, and an associated outflux along the vorticity axis.The strength of this flux scales with the Capillary number, and implies a qualitative alterationof the streamline topology due to drop deformation, hitherto unobserved in experiments andunnoticed in earlier computational studies. Our findings have profound implications for theconvective transport from a drop; up until now, the deformation of drop has always beenthought to have only an infinitesimal effect on the transport. An analogous phenomenon ofinertia qualitatively altering the streamline topology, and thereby, enhancing transport rateshad been previously established. The flux associated with the streamlines corresponding tonon-zeroCais found to be opposite to that of the aforementioned inertial case, and the drift issignificantly faster, suggesting deformation-induced convective transport to be the dominantmechanism for large drops in shearing flows.