Shyama Prasad Das

This paper reports the experimental investigations of the wake of a rotating and translating sphere in the close proximity of a plane wall at moderate Reynolds numbers (Re = 450–2020, defined based on velocity of translation and diameter of the sphere) and for a non-dimensional rotation rate, Ω* (ratio of surface velocity to the translation velocity of the sphere), of 1. For the current gap (h)-to-diameter (d) ratio of the sphere (h/d = 1/16), at Re = 1000, hairpin vortices shed in the wake where looping of small-scale vortices increases with the increase in Re. At Re > 1000, the transition from hairpin type to shear layer vortex shedding occurs. At a relatively high Reynolds number (Re = 1130), the separated vortex sheets from both sides of the sphere exhibit Kelvin–Helmholtz instability, and the wake shows multiple vortical structures. At Re = 2020, a strong interaction of vortex rings in the wake of the sphere and boundary layer at plane wall results in the breakdown of larger vortical structures into smaller scales. The separation angle changes from the equatorial region to the pole of the sphere because of differential rotational speeds at other planes. At midplane, the separation angle increases with Re, and it decreases again when hairpin–shear layer vortex shedding transition occurs. The Strouhal number as a function of Re indicates the presence of dual frequency beyond this transition.

This paper deals with computational investigation of a simplest pulsating heat pipe (PHP) configuration and interdependence of some factors that determine the likeliness of oscillations to sustain or decay. The focus of this paper is on some extremities leading to the oscillations which always decay. The major thrust of study has been given to the effect of "operational triad" on the sustainability of oscillations, where operational triad has been taken as a combination of the hot and cold section temperatures of PHP and the fluid reservoir pressure. Various combinations of the triad elements have been explored and it is observed that certain combinations stringently promote the oscillations to decay. For different extremities within the triad, the mechanisms and factors responsible for decay of oscillations have been studied in detail. The effect of adiabatic length on oscillation sustainability has also been discussed along with the corresponding decay mechanism.

This paper presents the numerical investigation of the effect of thermal buoyancy in flow past three circular cylinders in staggered configuration in a vertical duct. The heat transfer and flow characteristics are studied by varying the Richardson number Ri, cylinder spacing S, and the Reynolds number Re. The study reveals that for a certain Richardson number (Ri = 0.4) vortex shedding from the rear cylinders is suppressed by the accelerating flow caused by aiding buoyancy and modified flow patterns around the cylinders. The critical values of Ri were found to vary for different spacings indicating the influence of the channel walls on stabilizing the flow. The streamlines, vortex structures, and isotherm patterns for different S (0.75, 1, 1.25), Ri (–4 to +4), and Re (100, 180) are presented and discussed. Study also reveals that the Nusselt number Nu and the drag coefficient CD are strong functions of S and Re. At Re = 100 and S = 1, with a decreasing strength of opposing buoyancy, vortex shedding behind the side cylinders starts getting suppressed and CD reduces from a value of 8 to 3.2 for Ri changing from –4 to 4. The coefficient CD reduces with decreasing strength in opposing buoyancy (CD = 4.2 at Ri = –4) and increases with aiding buoyancy (CD = 4.8 at Ri = 4) due to the change in the recirculation zone behind the middle cylinder. The surface-averaged Nu increases monotonically from 4.9 to 7.45 with Ri for the middle cylinder. There is a monotonic increase in Nu with aiding buoyancy but only for the side cylinders heat transfer is reduced with decrease in the opposing buoyancy. The coefficient CD is a weak function of S. A change in CD with a change in Re is significant in aiding buoyancy compared to the opposing buoyancy.

Cusp singularities in fluids have been experimentally demonstrated in the past only at a low Reynolds number, Re ≪ 1, and large capillary number, Ca ≫ 1, in Newtonian or non-Newtonian fluids. Here, we show that the collapse of a free surface wave depression cavity can lead to inertial-viscous cusp formation at local Re > 1 and Ca > 1, which gives rise to extreme events, i.e., very high-velocity surface jets. The cavities are generated in a cylindrical container (2R = 10 cm), partially filled with glycerine-water solution, by parametrically forcing the axi-symmetric wave mode beyond the breaking limit. By varying the forcing amplitude and the fluid viscosity, parabolic or cusp singularities manifest, depending on the last stable wave amplitude b that determines the cavity shape. Cusp formation in collapse without bubble pinch-off, leading to very high-velocity surface jets, is obtained when b is close to the singular wave amplitude bs and Ca > 1. The free surface shape is self-similar, changing from an inertial to a viscous regime when the singularity is approached. At cusp singularity, the cavity shape takes the form of (z - Z0)/R ∼-(r/R)2/3, where Z0 is the final cavity depth. Cavity collapse with bubble pinch-off, which occurs when b > bs, also exhibits a cusp singularity when bs < b ≤ 1.14 bs and Ca > 1, but surface jet velocities are much less because about half of the wave energy is lost.

Flow control for performance enhancement over airfoils has become an increasingly important topic. This work details the characteristics of flow control using synthetic jets over a NACA0015 airfoil at a Reynolds number of 896,000 based on the chord length and free stream velocity, and at 20° angle of attack wherein the flow is separated. Numerical simulations were performed to help understand the behaviour of the controlled flow for a range of synthetic jet parameters. Analysis of key flow parameters such as phase averaged pressure and streamline profiles indicate that the synthetic jet is efficient in increasing the lift coefficient; more so for larger jet amplitudes and at smaller angles of jet injection. Behaviour of the flow characteristics for controlled cases has been analysed from the flow structures obtained from the same. This work serves as a platform to qualitatively and quantitatively understand the effects of the jet parameters on the separated flow over the airfoil.

In the present study, the flow past two spheres placed in a tandem arrangement is investigated numerically using open source field operation and manipulation (OpenFOAM) at a fixed Reynolds number (Re) of 300, where the Reynolds number is defined based on the diameter (D) of the downstream sphere (DS) and the freestream velocity at the inlet. Keeping size of the DS constant, the diameter of the upstream sphere (US) has been varied, so that the diameter ratio, DR (ratio of the diameters upstream and downstream spheres), takes the values of 0.4, 0.6, 0.8, 1.0, and 1.5. The spacing between the spheres (S) is also varied from 1D to 5D. Iso-Q surfaces show that both upstream and downstream wakes undergo various transitions with changes in the values of DR and S. For US, transition from steady symmetric to planar symmetric occurs at DR = 0.6 and S = 1, which corresponds to a local Re of 180. For DR = 0.8, steady to unsteady transition occurs at S = 2, whereas for all other values of S, the wake remains steady. For DR > 0.8 and S > 1, both upstream and downstream wakes are found to be unsteady. Hilbert analysis revealed that unsteady wakes are periodic for DR values of 0.4, 0.8, and 1.0 and are aperiodic for DR = 1.5. In addition, the extent of wake nonlinearity has also been quantified in terms of degree of stationarity. The drag force on both the spheres increases with an increase in spacing between the spheres.

The main goal of modern axial compressor development is to increase the power to weight ratio with higher efficiency. In the present investigation, highly loaded single stage axial compressor with tandem stator vanes is used. Tandem vanes help in attaining the compact compressor stage along with high pressure loading. It is designed for a stage pressure ratio of 2, mass flow rate of 9.02 kg/s operating at 30800 rpm resulting in transonic flow field. The aerodynamic performance of this compressor detoriates due to the tip leakage and secondary flows. Steady-state numerical investigation is carried out to study the flow structures near the tip region of transonic rotor and how different tip gaps influence the overall performance of the compressor. Further the effects of tip leakage flow variation on the performance of tandem vanes are also highlighted. Transonic fan stage with baseline tip gap of 0.5mm is analyzed along with different tip clearance values ranging from 0 % to 3 % of axial chord. Three-dimensional viscous Reynolds Averaged Navier Stokes (RANS) equations are solved using SST k-ω turbulence model. Computational domain discretized with high quality hexahedral elements (Y+ < 2) in AUTOGRID, Numeca. The numerical procedure is verified against the experimental results of Rotor37 transonic rotor test case. Tip leakage losses contribute a substantial amount to the total loss of stage. Overall performance and the stall characteristics for the compressor stage has been evaluated for different tip gap variations.. Further, the topological properties are exploited to visualize the critical points and separation lines on rotor and tandem vanes. Increase in rotor total pressure loss coefficient is observed with increasing tip gap. In contrary, overall total pressure loss coefficient improves for smaller tip gap values and then detoriates. It is observed optimum tip gap height lies close to the 1.125mm, 2% of baseline design value.

Instabilities arising in unsteady boundary layers with reverse flow have been investigated experimentally. Experiments are conducted in a piston driven unsteady water tunnel with a shallow angle diffuser placed in the test section. The ratio of temporal (Πt) to spatial (Πx) component of the pressure gradient can be varied by a controlled motion of the piston. In all the experiments, the piston velocity variation with time is trapezoidal consisting of three phases: constant acceleration from rest, constant velocity and constant deceleration to rest. The adverse pressure gradient (and reverse flow) are due to a combination of spatial deceleration of the free stream in the diffuser and temporal deceleration of the free stream caused by the piston deceleration. The instability is usually initiated with the formation of one or more vortices. The onset of reverse flow in the boundary layer, location and time of formation of the first vortex and the subsequent flow evolution are studied for various values of the ratio Πx/(Πx+Πt) for the bottom and the top walls. Instability is due to the inflectional velocity profiles of the unsteady boundary layer. The instability is localized and spreads to the other regions at later times. At higher Reynolds numbers growth rate of instability is higher and localized transition to turbulence is observed. Scalings have been proposed for initial vortex formation time and wavelength of the instability vortices. Initial vortex formation time scales with convective time, δ/ΔU, where δ is the boundary layer thickness and ΔU is the difference of maximum and minimum velocities in the boundary layer. Non-dimensional vortex formation time based on convective time scale for the bottom and the top walls are found to be 23 and 30 respectively. Wavelength of instability vortices scales with the time averaged boundary layer thickness.

The unsteady wake of a rotating and translating sphere is experimentally investigated at Reynolds numbers (Re) ranging from 115 to 550 for two rotational speeds. Non-dimensional rotational speed, Ω∗, is defined as the ratio of the maximum azimuthal speed of the sphere and the speed of translation. Motion of the sphere is generated using a specially designed mechanism in a channel, and its wake has been visualized using fluorescent dye. Unlike the steady flow past a stationary sphere where the limiting Reynolds number is 270, the flow becomes unsteady when sufficient rotation rate is introduced at much lower Reynolds number (Re = 115 , Ω∗= 0.375). The transition from one-sided to double-sided hairpin vortex occurs at one critical Re for a given rotational speed, and it switches back to one-sided hairpin vortex shedding at another critical Re. Particle image velocimetry results demonstrate the vorticity distribution in the wake and the variation of the wake width with axial distance. At very low Re, velocity profiles in the wake are nearly self-similar. The strength of the vortex rings weakens in the far wake. The decay of centerline velocity in axial direction confirms the dissipation of the vorticity in the far wake. In this unsteady flow, the time-averaged value of the separation angle is higher than that of the steady flow on the advancing side due to enhanced adverse pressure gradient. The Strouhal number calculated from the dye visualization in this range of Ω∗ and Re varies from 0.175 to 0.25, a clear increasing trend both with increasing Re and Ω∗. The coefficient of drag evaluated at the center plane remains constant at 0.94 for the present investigation up to Re = 550. The wavelength of the unstable hairpin vortices is found to scale inversely with the translational velocity of the sphere.

Parametrically forced gravity waves in axisymmetric mode in a circular cylinder filled with FC-72 with large liquid depth have been studied numerically. The instability threshold and wave breaking thresholds are plotted from the simulated results which show good agreement with the reported experimental and theoretical results. A notable observation is the presence of different time scales of wave amplitude modulations at different regimes. The wave amplitude response exhibits amplitude modulations, period tripling and period quadrupling without breaking of waves. Inertial collapse of the wave trough causes a high velocity jet ejection has also been observed when forcing amplitude crosses the breaking limit.