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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.

Three-dimensional and oscillatory thermo-capillary flow in a liquid bridge of 5 cSt silicone oil (Pr = 67) has been numerically investigated by performing dynamic mode decomposition (DMD) analysis applied to the time-resolved thermal data generated by the ‘method of snapshots’. For various heat transfer conditions defined on the free surface of the curved liquid bridge, a series of dynamic modes has been extracted to unfold the underlying physical mechanism that governs the considered dynamical system. Unstable and neutral modes primarily accountable for the transition to the supercritical state established in the liquid bridge have been identified by decomposing the acquired temperature disturbances into spatial and temporal coherent structures. Under microgravity conditions, unsteady and three-dimensional computations are carried out in a liquid bridge of different volume ratios (VR = 0.9 and 1.1) using the finite volume method (FVM) to capture the dynamical evolution of temperature disturbances at the onset of oscillatory instability. The present numerical investigation is mainly aimed at providing broader insight into the complex fluid dynamic processes associated with the oscillatory thermal convection by determining the growth/decay rate and frequency of each characteristic mode with the help of DMD spectra. For each oscillation pattern, the dominant behavior of oscillatory thermo-capillary flow is revealed by its coherent structures. The characteristic modes having more energy content (dominant modes) are also analyzed by the norm of modes || ϕ || by demonstrating the frequency spectra of DMD analysis.

Present work describes a numerical investigation concerning the response of oscillatory thermo-capillary convection to free surface heat loss in a cylindrical half floating zone of high Prandtl number liquid ( Pr =67) under microgravity conditions. Unsteady and three-dimensional computations have been carried out in the liquid domain using the finite volume method (FVM) to reveal the oscillatory thermo-fluid dynamic regimes at the onset of instability. The influence of different free surface heat loss conditions on the bifurcation from the basic steady axisymmetric state has been discussed in terms of critical azimuthal wavenumber, m. The dynamic evolution of the temperature disturbances at supercritical conditions are characterized by ‘pulsating’ and ‘rotating’ modes of spatio-temporal convection. The development of counter-propagating hydrothermal waves in azimuthal direction is found to be followed by an even mode of convection characterised by m = 2. The liquid bridge system exhibiting the well-known transition from periodic ( m =2) to quasi-periodic behaviour ( m =1) with an increase in Biot number, Bi has been analysed from both spatial and temporal points of view. The existence of both symmetric and asymmetric modes of the supercritical state for different Bi are presented. The dominant fluid dynamic information of 3D oscillatory state are extracted from a specific set of disturbance temperature data by employing a data-driven computational technique ‘dynamic mode decomposition’ (DMD). Various neutral, stable and unstable dynamic modes responsible for bifurcation to the 3D supercritical state are depicted with the help of DMD spectrum and the frequency spectrum.

In the present work, three-dimensional numerical investigations are carried out to study the influence of top wall confinement on unsteady flow and heat transfer characteristics of sinusoidal wavy ...

Membrane-based liquid desiccant air conditioning (MLAC) system is one of the energy-efficient alternatives to the conventional vapor compression based air conditioning systems. However, due to inherent resistance for heat and mass transfer in the membrane and heat of absorption generated during the mass transfer, the desired room temperature and specific humidity are not achieved in the MLAC system. Present study proposes a novel MLAC system with an additional air cooling heat exchanger for the efficient control of both temperature and specific humidity of the supply air. The influences of different liquid desiccants, ambient conditions, and design and operating parameters are studied. The performance of MLAC system is presented in terms of room temperature and specific humidity, latent effectiveness of dehumidifier and regenerator, coefficient of performance and exergy efficiency. The results of the present study prove that the proposed MLAC system is capable of achieving better indoor conditions than the conventional MLAC system. The effects of the mass flow rate of desiccant and the effectiveness of the desiccant to desiccant heat exchanger on the performance of the proposed MLAC system are found to be significant.

A novel membrane dehumidifier is proposed for hybrid air conditioning system for which investigations are carried out for hot and humid climatic conditions. With separate cooling arrangements for air and desiccant, efficient control of both temperature and specific humidity is possible. In the present study, the effects of flow configurations, and operating parameters of air, cooling water and desiccant on the performance of the dehumidifier are investigated. The performance is presented in terms of moisture removal rate, and sensible, latent and total effectiveness. The configuration with cooling water flow in parallel and counter to air and desiccant respectively is found to be optimum for the dehumidifier. It is also found that the sensible effectiveness of the dehumidifier is independent of the changes in the operating parameters of desiccant.

Experimental studies have been conducted to depict the flow patterns in a small rectangular ejector by using schlieren flow visualisation technique. A transparent ejector is designed, fabricated, and operated using air as working fluid. Geometry of the ejector is in form of two-dimensional extrusion with two suction inlets: one on top and other at bottom of the convergent-divergent nozzle. Ejector performance has been analysed by measuring the mass flow rates at primary and secondary inlets. The results of flow visualisation studies have been compared with those obtained using numerical computations. Variation of entrainment ratio with change in primary stream pressure has been presented. Moreover, the shock wave patterns in constant area mixing chamber and diffuser for set of inlet and outlet conditions have been visualised and presented.

Effect of side wall boundary conditions and aspect ratio in very small boxes on patterns formed during Raleigh-Benard (RB) convection is studied using an indigenously developed flow solver based on the finite difference method. While keeping the bottom (hot) and top (cold) walls isothermal, the side wall thermal conditions are kept either constant heat flux or isothermal type. For constant heat flux side wall condition, the considered value of heat flux lies between 0 and maximum corresponding to conductive heat flux between top and bottom walls. For isothermal side wall condition, the chosen value of temperature is between the top and bottom wall temperatures. The computational domain is a cube or a square prism with aspect ratio in longitudinal direction (ARx) varied between 1.2 and 2 in steps of 0.2. For fixed value of ARx = 1.6, aspect ratio in transverse direction (ARz) has been varied for adiabatic side wall conditions. Effect of side wall condition on orientation of roll axis and the number of rolls has been investigated with observations well supported by stream traces and temperature contours. Moreover, for selected parameters, based on nature of pattern formation for steady flow, onset of oscillatory convection is explained.

Present work is an extensive numerical investigation of interfacial flow in 5 cSt silicone oil that develops under the action of thermo-capillary force and interaction of this flow with the surrounding air motion using the commercial software ANSYS Fluent 17.2. The physical model is a half floating zone of high Prandtl number fluid (Pr = 56) co-axially surrounded by ambient air under microgravity conditions. The surface of a slender liquid bridge (VR = 0.9) is subjected to parallel air stream which is co-directed and counter-directed with the thermo-capillary flow along the interface. This forced air motion influences the basic flow in the system via shear stress and heat transfer at the interface. The effect of ambient air velocity and the volume ratio of the liquid bridge on the evolution of flow states is examined by performing axisymmetric computations in both liquid and air domains. The considered values of air velocity are comparable or larger than the Marangoni velocity to account for the shear action employed by the parallel airflow. The results show that the heat loss at the interface characterized in terms of local Biot number is significantly higher in case of counter-directed airflow as compared to that of co-directed airflow. By varying the volume ratio of the liquid bridge (0.90

Two-dimensional numerical simulations have been carried out on flow past two inline circular cylinders with rotating downstream cylinder. Computations are performed for fixed Reynolds number equal to 150 such that the resulting flow field remains laminar and two-dimensional. The inter-cylinder spacing has been chosen equal to 5d ('d' being diameter of cylinder) such that the wake flow is predominantly unsteady. Rotational speed of the downstream cylinder has been varied to investigate its effect on transition in characteristics of temporal wake. This has been achieved by performing Hilbert-Huang transformation (HHT) on time series signals of drag and lift coefficients for the rotating cylinder. Unsteady periodic, unsteady non-periodic and steady transitions in flow behavior have been observed with an increase of rotational speed. Results are presented in the form of vorticity contours, Hilbert spectra and marginal spectra. Degree of stationarity of the signals as measure of nonlinearity has also been quantified. Comparisons are drawn against results from Fourier analysis and it has been shown that HHT is better suited to capture inter-wave and intra-wave modulations indicating nonlinear interactions in the wake.

Three dimensional computations have been carried out using Lattice Boltzmann Method (LBM) and Finite Volume Method (FVM) for the case of flow past a stationary sphere. The Reynolds number of the mean flow have been varied from 50 to 300 during which flow transitions from steady state to unsteady state. The LBM simulations were carried out using the open source solver Palabos developed in the University of Geneva while the FVM computations were performed using the commercial solver ANSYS Fluent 17.2. Qualitative results like streamlines and three dimensional vortex structures in the wake of the sphere, obtained from the two different techniques have been compared alongside results reported in literature. Quantitative results such as, variation of velocity in the sphere wake and length of wake bubble have also been compared. The present study also tabulates the computational time required to complete the flow simulation for the both the solvers while utilizing the same computational resource. The results from the present study highlights the advantages of LBM solver for simulation of unsteady three dimensional flow but also indicate that FVM technique has definitive benefits when it comes to steady flows.

Present study is a three-dimensional numerical investigation of flow past cube with wings under the influence of wind gust. Gust effect is incorporated at inlet using user-defined function (UDF). Simulations are carried out using commercial software ANSYS Fluent 18.1 at two Reynolds numbers (Re = 100, 200) at a fixed frequency of 2 Hz for the wind gust of varying intensity. The mean drag coefficient decreases, but the amplitude of drag coefficient increases as the intensity of wing gust is increased. High intensity of wind gust is likely to enhance wake unsteadiness.

Three-dimensional numerical investigations have been carried out for flow past surface mounted finite-height square cylinder using Open Source Field Operation and Manipulation (OpenFOAM). Unsteady wake behaviour has been studies for different values of angle of incidence (α) varying from 0o to 45o at fixed Reynolds number equal to 150. Uniform and non-uniform velocity profiles have been considered at inlet. Unsteady wake oscillations have been examined using Hilbert Huang transformation. Transverse velocity signal in the wake has been decomposed into component signals and frequency variation of each decomposed signals has been presented in Hilbert spectra. Appearance of frequency modulations in the spectra has been related with nonlinear oscillations in the wake. Frequency distribution corresponding to each value of α has been presented in marginal spectra. Wake nonlinearity has been quantified in terms of degree of stationarity. Variation in mean drag coefficient with change in values of α has also been reported.

The present study investigates thermo-capillary convective flow phenomena in a half floating zone of high Prandtl number liquid (\(Pr\) = 39.3) under microgravity conditions. The spatiotemporal flow structures inside the half floating zone are studied by performing unsteady and three-dimensional computations in a domain consisting of a flat liquid bridge of 5 cSt silicone oil. The liquid zone is held in its position by thermo-capillary forces (surface tension) between two circular, co-axial and differentially heated rods of the similar diameter. Thermal convection equations which govern the convective flow are solved using a commercial software ANSYS Fluent 18.1 which is based on the finite volume method. The well-known transition to oscillatory and three-dimensional flow occurs when the applied thermo-capillary force (temperature difference between the rods) reaches a critical value for the onset of instability. The observed oscillatory thermo-capillary convection in the present case is described by a standing wave of critical azimuthal wave number (m) equal to 2.

Direct numerical simulations (DNS) have been performed using open source computational fluid dynamics (CFD) package openfoam to investigate the unsteady flow and heat transfer characteristics of primary and secondary corrugated wavy channels for fixed values of wavelength and amplitude by varying Reynolds number (Re). Computations are carried out by considering only one module of the wavy channel and applying periodic boundary conditions to reduce the time for computations. Numerical method has been validated thoroughly against the numerical and experimental results reported in the literature. Steady as well as unsteady flow characteristics in the primary and secondary corrugated channels have been elaborated and explained with the help of streamlines, velocity contours, isosurfaces of velocity, and Q value. Temperature and Nusselt number contours are presented to illustrate the heat transfer characteristics of wavy channels.

Three-dimensional computations have been performed to study the effect of variation of cylinder size on flow characteristics of two cylinders in side-by-side arrangement. The RANS simulations are performed using the commercial solver, STAR-CCM+®. Effect of variation of diameter ratio (DR) between two cylinders at fixed gap ratio and moderate Reynolds numbers (Re=2000) on hydrodynamic characteristics, such as drag and lift force coefficients as well as wake characteristics, such as Strouhal number (St) has been presented. The coupled interactions between the cylinders in side- by-side arrangements are studied by means of cross-recurrence plots.

Wavy channels are known to be one of the passive methods for heat transfer enhancement and are widely employed in different heat exchanging devices. Heat transfer augmentation in the wavy channel can be achieved by interruption of thermal boundary layer growth due to generation of vortices. The present study deals with the modification of open-source computational fluid dynamics solver OpenFOAM to simulate fully developed flow and heat transfer in wavy channels with constant wall temperature boundary condition. Validation of numerical method has been confirmed by simulating the flow and heat transfer in plane and wavy channel and are compared with the existing literature.

Three-dimensional computations have been carried out to investigate the effect of aspect ratio on wake characteristics of a prolate spheroid. Open source LBM solver (Palabos) has been used for the computations after obtaining necessary validations. Three different aspect ratios, namely, 1:2, 1:4 and 1:6 have been considered in present study for a fixed Reynolds number (Re) of 200. Wake remains unsteady for all aspect ratios at this Re. The coherent structures in the wake (vortices) have been identified using instantaneous plots of Q-surface, vorticity contours and line integral convolution. These results indicate transition of wake from steady three-dimensional to unsteady three-dimensional with the increase in aspect ratio.

Flow past cube with wings under the influence of gusty flow is investigated in this paper. Gusty nature is incorporated at inlet by using User Defined Functions (UDF). Three-dimensional numerical simulations have been carried out using commercial software ANSYS Fluent 18.1. Flow field behavior has been presented using vorticity contours at fixed Reynolds number equal to 300 and varying frequency of gust at inlet. Wake oscillation frequency has been reported using time-varying signal of lift coefficient with respect to inlet flow frequency. The mean drag coefficient is found to decrease with increase in inlet flow frequency. At high frequency (say 4 Hz) lift forces on geometry are vanishing.

Three-dimensional computations have been carried out to investigate flow past a sphere performing rotation and rotary oscillation about streamwise axis. The effect of streamwise rotation has been analysed for fixed Reynolds number of 300, with non-dimensional rotational rate taking values 0, 0.5, 1, 1.5 and 2. The transitions in the near wake have been depicted with the help of time-averaged path lines, iso- λ 2 surfaces and vortex core transformation. This study is further extended by considering forced rotational oscillations of the sphere for two different amplitudes of oscillation and reduced oscillation frequency ranging from 0.5 to 2. Effect of these parameters on flow structure has been described using λ 2 -surfaces and streamlines at different locations in the flow field. The response of mean hydrodynamic forces to variation in amplitude and frequency of oscillation has also been reported. Temporal behaviour of wake has been disclosed through Hilbert spectral analysis of time dependent force coefficient signals. The growth and evolution of wake nonlinearities have been elaborated with the help of marginal spectra, Hilbert spectra and distribution of non-stationarity.

Heat transfer performance of fin-tube heat exchangers can be enhanced by introducing longitudinal vortices generated using vortex generators (VGs). Location of VGs with respect to tubes plays major role in deciding the enhancement in heat transfer. Three-dimensional numerical investigations have been carried out to study the effect of winglet location on heat transfer characteristics of fin-tube heat exchangers for inline tube arrangement. In this study, rectangular winglet pair (RWP) type VGs having common flow down configuration are considered. Initially, effect of different possible locations of RWP corresponding to tube centre have been investigated to analyse the heat transfer and flow field characteristics. Further, effects of angle of attack (β) ranging from 15° to 60° and Reynolds number (Re) in the range from 2000 to 4000 have also been studied for the location associated with maximum heat transfer corresponding to mean values of β and Re in the considered range. To illustrate the interesting flow features of vortices generated due to placement of RWPs and how those affect heat transfer, streamlines and temperature contours have been investigated. Enhancement in heat transfer has been quantified using Nusselt number while pressure loss is estimated with the help of friction factor. Role of secondary flow induced by RWPs is quantified in terms of secondary flow intensity. Thermal performance factor (JF) is used to evaluate the combined effect of enhancement in heat transfer and increase in pressure drop. For upstream located RWPs, the value of JF is found to be higher due to lower pressure loss as compared to RWPs mounted in other regions. The value of JF is maximum for RWP located at ΔX = −2.0, ΔY = ±1.25 and correspondingly 37.6% enhancement in heat transfer is observed as compared to the case for the arrangement in absence of RWPs.

Purpose Growing application of micro aerial vehicle (MAV) sets in demand for accurate computations of low Reynolds number flows past their wings. The purpose of this study is to investigate the effect of unsteady freestream velocity or wind gust on a harmonically plunging symmetric NACA0012 airfoil at Re = 1,000. The influence of unsteady parameters, such as reduced frequency of plunging motion (0.25 < k < 1.5), non-dimensional plunging amplitude (ho = 0.2) and non-dimensional amplitude of wind gust (0.1 = λ = 0.4) has been studied. Design/methodology/approach Computations have been carried out using commercial software ANSYS Fluent 16.0. To incorporate the plunging motion, the entire reference frame is oscillating, and thereby, a source term is added in the Navier–Stokes equation. Findings The results have been presented in the form of streamlines, vorticity contours, lift and drag signals and their spectra. It is observed that the ratio of plunging frequency to gust frequency (f/fg) has strong influence on periodic characteristics of unsteady wake. It has also been observed that for a fixed plunging amplitude, an increase in value of k results into a change from positive drag to thrust. Practical implications The research has implications in the development of MAV. Originality/value This study is intended to get a better understanding of unsteady parameters associated with gusty flow in flapping wing applications and possible ways to alleviate its adverse effect on it.

Two-dimensional numerical investigations have been carried out to study the temporal wake characteristics of laminar flow past two identical inline square cylinders performing transverse oscillations. Both the cylinders are forced to perform harmonic oscillations of same frequency and amplitude but with a phase difference. Computations are carried out using commercial software ANSYS Fluent 16.1 on a dynamically sliding mesh for fixed Reynolds number equal to 100. The oscillation frequency is varied from 0.4 to 1.6 times the frequency of vortex shedding behind a single stationary square cylinder. The amplitude of transverse oscillation is kept fixed equal to 0.4D (D = side of the cylinder). In addition, the effect of variation of inter-cylinder spacing has been investigated on wake interference which influences the modes of vortex shedding and resulting dynamic effects on the cylinders. Temporal signals as well as mean characteristics of lift and drag coefficients have been presented for different values of inter-cylinder spacing, phase difference between the two cylinders and frequency of oscillation.

Flow and heat transfer characteristics of curved rectangular winglet vortex generators (RWVGs) are investigated numerically. Three-dimensional numerical computations have been carried out for flow through a channel with curved RWVG mounted on its bottom wall. Effect of curvature of RWVG having concave and convex shapes with respect to the flow facing surface, has been addressed in terms of arc angle which is varied in the range from 15° to 120° for fixed arc length. Effect of curvature in concave and convex RWVG on flow and heat transfer characteristics has been compared with that of plane RWVG. Temperature and flow field characteristics near the plate surface have been presented and discussed with the help of temperature contours and streamline plots. Enhancement in heat transfer and pressure loss are examined by using Nusselt number and friction factor respectively. Maximum enhancement in heat transfer is found to be 22% for concave shape RWVG having arc angle equal to 75° as compared to channel in absence of RWVGs. The mechanism of heat transfer enhancement is explored with the help of secondary flow intensity and field synergy principle. Thermodynamic performance of various RWVGs has been analysed by calculating entropy generation rate caused by heat transfer and friction. Finally, overall thermo-hydraulic performance of various curved RWVGs has been reported and compared with that of plane RWVG.

Humidity control of indoor space using the conventional air conditioning system is energy intensive. The liquid desiccant dehumidifier, which operates on low grade energy sources, is one of the energy efficient alternatives for humidity control. Membrane dehumidifiers avoid the desiccant carryover and hence are preferred over the packed bed dehumidifiers. However, their performance is lower due to the additional resistance in the membrane. Internal cooling is one way to improve the performance of the membrane dehumidifier and the present study experimentally investigates such a dehumidifier. The operating parameters considered are specific humidity, mass flow rate, temperature and of inlet air. The performances of the adiabatic and internally cooled dehumidifiers are presented in terms of moisture removal rate and latent effectiveness. It is found that these are higher by 60 and 50%, respectively, for the internally cooled dehumidifier.

Three-dimensional numerical investigations have been carried out to study effect of skewness in wavy channels on flow and heat transfer characteristics. Computations are performed using ANSYS Fluent 16.1 for different values of wave amplitude, skewness angle, Reynolds number (Re) and fixed values of channel width, wavelength. Streamline plots are presented in various planes for different values of skewness angle to understand the flow field characteristics of skewed wavy channels. Volume-averaged as well as span-averaged variation of secondary flow intensity for different skewness angles have been shown to elucidate the strength of induced secondary flow. Effect of secondary flow on heat transfer in skewed wavy channels has been quantified with the help of field synergy angle. Finally, effect of skewness on thermodynamic as well as overall thermo-hydraulic performance of the channel has been presented. Introduction of skewness in the wavy channel induces stronger secondary flow which makes the flow three-dimensional. The stronger secondary flow influences both thermodynamic as well as overall thermo-hydraulic performance of the channel.

The present study focuses on thermocapillary convection in liquid bridges of high-Prandtl-number fluid (5 cSt silicone oil) under microgravity environment and varying ambient air conditions. The ge...

The performance and flow structure in a three-dimensional rectangular ejector have been numerically investigated by using air as the working fluid. Commercial software ANSYS Fluent 15.5 has been used for numerical simulation of supersonic, compressible, turbulent flow ejectors. In order to confirm the appropriate turbulence model, the computed results are validated against indigenous experimental data. The study aims to optimize the convergent-divergent (C-D) nozzle position for each operating condition and to bring out the influence of reflected shock waves and boundary layer in the constant area mixing chamber on the performance of the ejector. Also, the variation of entrainment ratio (ER) for a range of primary, secondary and exit pressure has been investigated. Both the on-design and the off-design characteristics and local flow structure of a rectangular ejector have been considered.

The liquid desiccant system is one of the energy efficient alternatives to the conventional air conditioning systems for addressing the impact of latent load. Membrane-based liquid desiccant system is used to avoid desiccant carryover with the process air, which is commonly encountered in direct contact desiccant systems. The performance of such a desiccant system depends on the type of membrane used as well as the nature of desiccant. In the present study, a numerical model is developed to investigate the effect of operating parameters in the membrane-based dehumidifier using different desiccants, such as LiCl, LiBr and CaCl 2 , for hot and humid climatic conditions prevailing in Chennai. The operating parameters of air side include inlet specific humidity, temperature and mass flow rate while those of desiccant side are inlet concentration, temperature and mass flow rate. Performance of the three desiccants has been compared by analysing moisture removal rate as well as sensible and latent effectiveness. Results show that the membrane-based dehumidifier can achieve a better moisture removal rate at high air flow rates and also with LiCl desiccant.

In the present study, numerical investigations have been performed to study the flow transition mechanism in wavy channels using finite volume-based open source field operation and manipulation. Two different wavy channel configurations are chosen, which represent two different flow destabilization mechanisms, viz., Kelvin–Helmholtz and centrifugal instabilities. Sinusoidal walls with out-of-phase and in-phase channel configurations have been considered in the present study. Steady to chaotic flow transition in two different channel configurations are investigated by varying Reynolds number. A detailed flow regime map is presented for the two different wavy channel configurations. Unsteady flow features have been illustrated with the help of instantaneous streamlines, velocity contours, vorticity contours, and iso-Q surfaces. For the out-of-phase configuration, the flow changes from two-dimensional steady to two-dimensional unsteady in the Re range of 175–185, and then three-dimensional unsteady flow is observed for the Re varying from 250 to 260. On the contrary, for the in-phase configuration, the transition happens directly from steady two-dimensional flow (Re 102) in a very narrow range of Re. Transitions in the two different wavy channels have been examined in detail using Hilbert–Huang transformation, phase-space reconstruction, Poincare section, recurrence plot, and dynamic mode decomposition. Frequency, growth rate, and vortex structures of the dominant modes are illustrated corresponding to each value of Re for the considered channel configurations.

Present three-dimensional numerical study aims to investigate the effect of mounting rectangular winglet pair (RWP) on heat transfer enhancement in flow over a flat plate. Computations for incompre...

A first attempt is made for identifying the wake characteristics of circular cylinder confined by a wavy wall at laminar flow regime. Numerical study of flow characteristics past circular cylinder with wavy-wall confinement perpendicular to cylinder axis has been carried out in the range of Reynolds number 20–100. The finite volume-based CFD solver Ansys Fluent (Version 15.0) is used for computations. The results are presented in the form of streamline plots, mean drag co-efficient, flow separation angle and recirculation length. Wavy-wall confinement leads to highly significant changes in the cylinder wake such as the evolution of strong x-plane vortices, enhanced fluid mixing, wake suppression near the crest region and vortex stretching near the trough region on the downstream of the cylinder has been observed. Flow separation angle varies significantly along the axis of the cylinder. Increased wall shear stress on rear surface of the cylinder has also been observed. The part of vorticity magnitude as compared to strain rate has been distinguished and identified using vortex identification methods such as Q-criterion and Lambda-2 criterion.

The effect of external pulsation on a pair of stationary Lamb–Oseen vortices of equal strength has been analyzed to investigate kinematic behavior of a fluid particle. The assumption of vortices being treated stationary or fixed vortex filaments is valid in a reference frame attached to the vortex system with axes along and perpendicular to the line of their centers. Also, it is assumed that change in core shape and size is much small, with least possibility of core merger. In such situations, periodic particle paths are observed and superposition of pulsation becomes beneficial. In the present work, motion of a representative fluid particle is modeled as a non-linear dynamical system by varying both amplitude and frequency of external pulsation. Effect of external pulsation has been brought out with the help of quantification of deviation from periodic paths by using the concept of total average deviation. Results are presented in terms of particle paths, velocity phase plots, velocity signals and their spectra for varying amplitude and frequency of external pulsation.

Two dimensional flow past circular cylinder confined by walls with local waviness near the cylinder has been studied. The aim of the present study is to identify the ability of the waviness to control vortex shedding, for which two different waviness configurations such as in-phase configurations (IPC) and out-phase configurations (OPC) are considered. Further, the effect of location of the local waviness with respect to the cylinder has also been studied. Air is the working fluid and the flow is assumed to be laminar and incompressible at Re=200. The finite volume based CFD solver Ansys Fluent (Version 15.0) is used for the computations. Flow characteristics such as drag, lift and Strouhal number are computed. Interesting shedding characteristics and drag reduction are observed due to the presence of local waviness. However, the significant factor is the location of waviness in the confining walls that leads to complete suppression of shedding. Among various locations and configurations of waviness studied, waviness in downstream with OPC3 suppresses the vortex shedding completely with reduced drag.

Numerical investigations have been carried out to study aerodynamics of non-flapping characteristics of bird flight. Commercial software ANSYS Fluent 16.1 has been used for three-dimensional computations by considering four types of wing models. At first, square prisms with built-in side flaps are considered and the effects of Reynolds number (Re), angle of attack and dihedral angle on flow characteristics have been investigated. Thereafter, flow past actual bird wing has been considered for wings of owl, seagull, teal and bat. Role of shear layer formation at mid-span of wings has been illustrated with the help of streamline plots for different values of Re. In addition, pulsating flow is imposed on inlet velocity and the responses are obtained in form of force coefficients, phase portraits, time-dependent signals and their spectra.