arXiv subjects

Ashoke De

Publications and source records attributed to Ashoke De.

At least 37 records · Page 2Linked to original sources

arXiv2022

Three-Dimensionality in the flow of an elastically mounted circular cylinder with two-degree-of-freedom vortex-induced-vibrations

The study numerically investigates the three-dimensionality in the flow and two-degree-of-freedom (2 DOF) vortex-induced-vibrations (VIV) characteristics of an elastically mounted circular cylinder. The cylinder is allowed to vibrate in both streamwise and transverse directions. A low value of mass-ratio with the zero damping coefficient is taken for the simulations. The primary aim is to understand the vortex shedding behind the cylinder and the transition characteristics of the wake-flow from two-dimensional (2D) to three-dimensional (3D). The Reynolds number (Re) is varied from 150 (fully 2D flow) to 1000 (fully 3D flow), which lies inside the laminar range. The reduced velocity is varied which covers all three major VIV branches (Initial Branch (IB), Upper Branch (UB), and the Lower Branch (LB)). The oscillating cylinder sweeps the figure-eight trajectory. Two branches (IB, LB) and three branches (IB, UB, LB) amplitude responses are obtained for the low and high Re values, respectively. The wake behind the cylinder with 2-DOF VIV undergoes the mode-C transition of 2D to 3D flow as opposed to the direct mode-B transition observed for transverse only VIV in the literature. The critical Re range of the 2D to 3D transition for the 2-DOF VIV cylinder at a reduced velocity of 6 is around 250, less than the 1-DOF VIV. Also, this range varies with the variation in and the streamwise to transverse oscillation frequency ratio. A map is proposed for the 2-DOF VIV, highlighting the different modes of transition obtained for combinations of reduced frequency and Re.

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arXiv2022

Numerical analysis of dilute methanol spray flames in vitiated coflow using extended Flamelet Generated Manifold model

The present work focuses on the large eddy simulation (LES) and the study of turbulent dilute methanol spray flames in vitiated coflow using the secondary oxidizer Flamelet Generated Model (FGM). The modified FGM model uses an additional secondary oxidizer parameter in addition to the three other parameters previously used for spray flames progress variable, mixture fraction, and enthalpy. The results for gas phase and droplet properties are validated against the dilute methanol spray flame database for varying fuel injection amounts. The droplet statistics and the lift-off flame heights are accurately captured for all the cases. A proper orthogonal decomposition (POD) of the scalar fluctuating hydroxyl radical (OH) field and velocity-temperature field capture the flame structures in the downstream region of ignition kernels. The detailed POD analysis reveals that the base frequency of the dominant OH field equals that of the dominant vortical structure of 67.3 Hz. The flame propagation happens around these dominant vortical structures because of the less-strained fluid mixing.

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arXiv2022

Dynamics of Vortex-Induced-Vibrations of a Slit-Offset Circular Cylinder for Energy Harvesting at Low Reynolds Number

Vortex-Induced Vibrations (VIV) offer a safe, renewable, and environmentally friendly energy source for energy harvesting. To enhance the energy harvesting capability of the circular cylinder-based devices, the authors explore the placement of the normal slit by determining the most effective slit offset location from the cylinder's center. Using the open-source Computational Fluid Dynamics (CFD) program OpenFOAM, a series of numerical simulations are conducted to determine the utility of the slit placement on the circular cylinder and how it influences (positively/negatively) the harvester's performance for a 1-degree-of-freedom (1-DOF) VIV system. The study shows that the slit-cylinder displays the three-branch VIV response (i.e. initial branch (IB), upper branch (UB), and lower branch (LB)). At a Reynolds number of 150, the cylinder with no slit exhibits the two-branch VIV response (i.e. IB and LB) but lacks the upper branch. This is the first time the Upper Branch has been acquired for such low Re flows. The results indicate that adding a normal slit to the middle of the cylinder improves the alternating suction and blowing phenomena. Offsetting it from the center towards the back stagnation point suppresses the VIV and makes it unsuitable for energy harvesting applications. While positioning the slit toward the front stagnation point improves aerodynamic lift, and the cylinder sheds vortex closer to each other. It enhances the amplitude of transverse oscillation and, consequently, the power extraction. In addition, the peak energy transfer ratio for these scenarios is comparable to that of the no-slit case but with a larger range of peak energy transfer ratio values. It makes it suitable for energy harvesting applications.

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arXiv2022

Flow Characteristics of Elastically Mounted Slit Cylinder at Sub-critical Reynolds Number

The present work numerically investigates vortex-induced vibrations (VIV) of a two-dimensional circular cylinder with an axisymmetric slit at Reynolds number 500. The study examines the effects of slit shape (i.e., converging, diverging, and parallel slits), the effect of slit-area ratios, slit-width, and its angle with the freestream velocity on aerodynamic forces, vibration response, and associated flow characteristics. The results demonstrate that the addition of the slit assists the VIV suppression by adding an extra amount of flow to the main flow. It results in the stabilized wake with the pressure recovery downstream of the cylinder and causes a reduction in the lift force over the cylinder. Also, there exist different shedding patterns associated with different slit shapes. Proper Orthogonal Decomposition (POD) of the flow field suggests that among all the three slits, the parallel slit heavily modifies the flow behind the cylinder by distributing the amount of energy to a large number of modes as compared to other slits (converging slit and diverging slit), where the most of the energy is contained in a couple of modes. Further, increasing the slit angle (for parallel slit) with respect to the freestream increases the slit effectiveness up to a specific value of slit-angle and beyond that starts to affect the VIV suppression adversely. Observations for the effect of slit-width are also reported from the perspective of suppression of VIV.

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arXiv2022

Thermal Cooling Enhancements in a Heated Channel Using Flow-Induced Motion

The paper presents the comparative study of the vortex-induced cooling of a heated channel for the four different cross-sections of the rigid cylinder, i.e., circular, square, semi-circular, and triangular, with or without the rigid/flexible splitter plate at the Reynolds number (based on the hydraulic diameter) of 200. The study presents a comprehensive analysis of the flow and thermal performance for all the cases. For flexible plate cases, a partitioned approach is invoked to solve the coupled fluid-structure-convection problem. The simulations show the reduction in the thermal boundary layer thickness at the locations of the vortices resulting in the improved Nusselt number. Further, the thin plate's flow-induced motion significantly increases the vorticity field inside the channel, resulting in improved mixing and cooling. It is observed that the plate-motion amplitude is maximum when the plate is attached to the cylinder with the triangular cross-section. The power requirement analysis shows that the flexible plate reduces the power required to pump the channel's cold fluid. Thus, based on the observations of the present study, the authors recommend using the flexible plate attached to the cylinder for improved convective cooling.

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arXiv2021

Leading Edge Vortex Dynamics of Airfoils, Pitching Continuously at High Amplitudes

Airfoils pitching in the stalled regime have been of keen interest in recent years due to their desirable aerodynamic force characteristics. In this study, we are numerically investigating the unsteady flow past a NACA 0012 airfoil under sinusoidally pitching motion, using a finite volume based sharp interface immersed boundary solver. The flow is investigated in the low Reynolds number regime (Re=3000) for reduced frequencies of 0.1 and 0.5, at three different pivot locations (c/3, c/2 and 2c/3 from the leading edge). The airfoil is subjected to sinusoidal oscillations with its incidence angle varying from 15o to 45o. Leading edge vortices (LEVs) that are formed during the pitching motion dictate the transient aerodynamic characteristics. The flow field data is used to identify individual LEVs in the flow field. The spatio-temporal evolution of LEVs in terms of their strengths are traced throughout the pitching cycle to obtain a quantitative estimate of its evolution. The effect of pivot location and pitching frequency on the vortex evolution and aerodynamic forces is investigated. Change in pitching frequency is found to have a drastic effect on the vortex dynamics. LEV formation is delayed to higher fractional times and LEVs are found to interact and merge into each other at higher frequencies. The maximum lift and drag are also found to increase with pitching frequency. Pivot location is found to have a higher influence on the magnitude and phase of aerodynamic coefficients at higher frequencies. However, at both frequencies, moving the pivot axis aftward is found to delay LEV evolution and cause a phase lag on the aerodynamic forces.

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arXiv2021

Acoustic characteristics of supersonic planar impinging jets

The present work aims to study the tonal and broadband noise associated with a supersonic planar jet impinging on a flat plate. Five different cases are considered corresponding to different plate distance and angle of impingement. The near-field noise is analyzed using Large Eddy Simulation (LES). For this, we employ a low dispersive and dissipative Finite Volume solver using AUSM for inviscid fluxes and third order Runge-Kutta method for temporal discretization. The far-field noise is calculated using a hybrid approach in which the acoustic sources and mean flow are provided by the fluid solver while the far-field acoustic pressure is calculated by an acoustic solver. It solves Acoustic Perturbation Equations using a spectral/hp element method with a Discontinuous Galerkin projection. The present work highlights many important aspects of impinging jets. Firstly, the far-field sound with the effect of non-uniform base-fields is obtained which is important as the far-field acoustic pressure in most of the literature is reported either without any basefields or by the application of uniform base-fields. Secondly, a correlation between the different modes of oscillations of the impinging jet and the production of impinging tones is studied using Modal Decomposition techniques. This analysis also sheds some light on the number of concurrent cycles for different nozzle to plate distance. Lastly, the relation between the phase lag term and the distance between the plate and the nozzle exit is highlighted using a vortex tracking method. This observation can serve to provide a better understanding of the sound generation process.

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arXiv2021

Suppression of vortex shedding using a slit through the circular cylinder at low Reynolds number

The present article aims to study the suppression of vortex shedding using a passive flow control technique (slit through a circular cylinder) in the laminar regime (Re=100-500). The slit width ratio S/D (slit width/diameter) on the modified cylinder plays an essential role to control the vortex shedding. The additional flow through the slit leads to the suppression of the global instability and vortex shedding, whereas a large amount of flow through the slit drastically alters the behavior of vortex shedding. The nature of vortex shedding remains periodic for all S/D, and the root mean square (rms) value of the lift coefficient decreases (in turn, vortex shedding suppression) with S/D up to Re = 300. For the range Re > 300, the root mean square (rms) value of the lift coefficient decreases up to S/D < 0.15, and the flow exhibits periodic vortex shedding, while the root mean square (rms) increases beyond S/D > 0.15 due to irregular vortex shedding downstream of cylinder. The variation of the Re for the S/D=0.20 shows bifurcation points where the flow changes its behavior from symmetric to asymmetric solution at Re=232 and again becomes symmetric at Re=304. The unsteady flow analysis over the modified cylinder also indicates the suppression in the vortex shedding; however, the analysis provides the qualitative property of suppression. The reduced order modeling, i.e. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD), is utilized to quantify suppression and investigate the dominant vortical structure for slit through the cylinder.

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arXiv2021

Effect of vortex and entropy sources in sound generation for compressible cavity flow

The present study reports the effect of different source terms on the near and far-field acoustic characteristics of compressible flow over a rectangular cavity using hybrid computational aeroacoustics methodology. We use a low dispersive and dissipative compressible fluid flow solver in conjunction with an acoustic perturbation equation solver based on the spectral/hp element method. The hybrid approach involves calculating the base fields and the acoustic sources from a fluid simulation in the first step. In the next step, the acoustic solver utilizes the variables to predict the acoustic propagation due to the given sources. The validation of the methodology against benchmark cases provides quite accurate results while compared against the existing literature. The study is then extended to assess the importance of the entropy source term for the flow over a rectangular cavity. The predictions of hybrid simulations with vortex and entropy source terms reproduce the perturbation pressure values very close to the existing direct numerical simulation results. Moreover, the results suggest that the use of just the vortex source terms over-predicts the perturbation pressure near the source region. Finally, we have carried out detailed simulations with all the source terms to investigate the noise sources for compressible flow over the cavity for different Mach number ranges (M = 0.4; 0.5; 0.6; 0.7; 1.5). The obtained acoustic spectra and the sound directivity are in close agreement with the reference experiment.

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arXiv2021

Investigation of passive flow control over an airfoil using leading edge tubercles

The present paper numerically investigates flow control over NACA0021 (National Advisory Committee for Aeronautics) airfoil by applying a leading-edge tubercle as a passive flow control device at transitional Reynolds number (Re) regime. The study includes simulations over the unmodified airfoil and modified airfoil operating at Re=120000 using kT-kL-ω based Delayed Detached Eddy Simulation (DDES) model. The calculations include both the pre-stall and post-stall regions, while the performance of the mean flow aerodynamic properties like pressure, lift, skin-friction, and drag coefficients are also investigated in the presence of tubercles. The spanwise flow over the modified airfoil interacts with the primary flow. The turbulent separation gets delayed compared to the unmodified airfoil, thereby significantly improving the stall behavior and flow behavior in the post-stall region. The modified airfoil provides the gradual stall behavior against the steep stall behavior for the unmodified airfoil. To optimize tubercle configuration, the study considers two modified airfoils with different combinations of amplitude and wavelength. The unsteady analysis, including the reduced-order modeling, i.e. 3D Proper orthogonal decomposition (POD), characterizes the dominant vortical structure of the flow to evaluate the improvement in the aerodynamic performance of the modified airfoil. The model decomposition for flow over airfoil helps to provide a deeper understanding of the flow control phenomenon.

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arXiv2021

Role of corner flow separation in unsteady dynamics of hypersonic flow over a double wedge geometry

This study investigates the origin and sustenance of self induced oscillations of shock structures in a hypersonic flow over a double wedge configuration. Previously, various researchers have considered the double wedge flow configuration for inviscid flow with variations of different inflows as well as geometric parameters such as inflow Mach number , wedge angles, and wedge lengths. Few recently published articles reveal an unsteady flow physics involved with the hypersonic viscous flow for double wedge configuration with large second wedge angles. However, the reason for such self sustained flow oscillations is not completely clear. The present work seeks out to investigate the origin of such oscillations in a low enthalpy hypersonic flow with different aft wedge angles and wedge length ratios.In the current study, viscous flow over a double wedge at Mach 7 and fore wedge angle of 30° is considered. An improved version of rhoCentralFoam solver in OpenFOAM is used to investigate the double wedge flow over different grid resolutions in the separation region and shock shock interaction region. This study corroborates the observation from the previous literature with an improvement in the range of parameters which results in a self sustained periodic oscillation. The present study also suggests that the unsteadiness becomes possible when the incidence shock is in the proximity of the aft wedge expansion corner as a consequence of different wedge length ratios(L1/L2) or aft wedge angles (θ2). Flow can still be steady at a large aft wedge angle if the incidence shock is far ahead of the aft wedge expansion corner.

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arXiv2021

Numerical Study of Bifurcating Flow through Sudden Expansions: Effect of divergence and geometric asymmetry

A numerical study of laminar flow through symmetric and slightly asymmetric sudden expansion, of expansion ratio 1:3, in channels with increasing cross section, is carried out using two different approaches - Conventional CFD and Lattice Boltzmann Method. The effect of divergence of walls of the channel, after a sudden expansion, on the symmetry of flow and recirculation is studied for various Reynolds numbers. It is seen that the angles of the walls play an active role in disrupting the symmetry of flow. For non-parallel walls, the symmetry breaking bifurcation phenomenon no longer exists and the loss of symmetry is a gradual process. The effect of asymmetry of geometry on flow is also studied by considering two types of asymmetry - First type is at the plane of expansion where the steps on either side are of unequal heights, while the second one deals the walls of the channel are at a different inclination with the direction of inflow. The present study reveals that small asymmetry impedes the sharp transition and thus smoothens the pitchfork bifurcation. Increased asymmetry finally leads to the unique solution of the governing Equations, indicating complete loss of bifurcation pattern.

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arXiv2021

An Experimental and Computational Study of a Swirl-Stabilized Premixed Flame

An unconfined strongly swirled flow is investigated for different Reynolds numbers using particle image velocimetry (PIV) and Large Eddy Simulation (LES) with a Thickened Flame (TF) model. Both reacting and non-reacting flow results are presented. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the experimental data. Also the predicted RMS fluctuations exhibit a double-peak profile with one peak in the burnt and the other in the un-burnt region. The measured and predicted heat release distributions are in qualitative agreement with each other and exhibit the highest values along the inner edge of the shear layer. The precessing vortex core (PVC) is clearly observed in both the non-reacting and reacting cases. However, it appears more axially-elongated for the reacting cases and the oscillations in the PVC are damped with reactions.

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arXiv2021

Assessment of Soot Formation Models in Lifted Ethylene/Air Turbulent Diffusion Flame

In the present study, soot formation in the turbulent lifted diffusion flame, consisting of ethylene-air is numerically investigated using three different soot modeling approaches and is comprehensively reported. For turbulence-chemistry interaction, Flamelet generated manifold (FGM) model is used. A detailed kinetics is used which is represented through POLIMI mechanism (Ranzi et al. 2012). Soot formation is modeled using two different approaches, semi-empirical two-equation approach and Quadrature methods of moments approach, where both the approaches consider various sub-processes such as nucleation, coagulation, surface growth and oxidation. The radiation heat transfer is taken into account considering four fictitious gasses in conjunction with the weighted-sum-of-gray gas (WSSGM) approach for modeling absorption coefficient. The experimental and earlier published numerical data from Köhler et al. (2012) and Blacha et al. (2011) are used for assessment of different soot modeling approaches. The discrepancies between numerical and experimental data are observed due to under-prediction of OH radicals concentration and poor fuel-air mixing ratios in the vicinity of the fuel jet region leading to early soot formation and the trends are unaffected after invoking radiation.

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arXiv2021

Simulation of Coupled Heat and Mass Transport With Reaction in PEM Fuel Cell Cathode using Lattice Boltzmann Method

Fluid, heat and species transport and oxygen reduction in the cathode of a PEM fuel cell are simulated using multi relaxation lattice Boltzmann method. Heat generation due to oxygen reduction and its effects on transport and reaction are considered. Simulations for various cell operating voltages, temperatures and flow rates with various values of porous media properties, namely, permeability, porosity, and effective porous media diffusion coefficient, are performed to study transport and operating characteristics in the electrode. It is seen that maximum output power density achievable is limited by the mass transport rate. A small increase in current density is obtained by increasing the operating temperature. However, this results in an increase in the rate of heat generation. Permeability and porosity of the gas diffusion layer do not show a significant impact on the performance in the range of values presently simulated. Higher permeability, in turn, resulted in enhancement of thermal gradients in the porous layer. A significant increase in the maximum current density obtainable is observed with increase in flow rates. The higher convection associated with high flow rate facilitates better transport of species and heat at the catalyst layer resulting in larger current density with a lesser chance of hotspot formation. Increased species diffusion coefficient also resulted in increasing the power output substantially. In addition, the fuel utilization is also improved at high diffusion rates in the porous media. The study analyses and shows the impact of various operating and material parameters affecting the performance of a PEM fuel cell with special attention on enhancing the maximum power density attainable.

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arXiv2021

Investigation of NOx in piloted stabilized methane-air diffusion flames using Finite-rate and Infinitely-fast chemistry based combustion models

The present work reports on the numerical investigation of NOx in three turbulent piloted diffusion flames of different levels of extinction. The study involves two-dimensional axisymmetric modeling of combustion in these flames with fairly detailed chemistry, i.e. GRI 3.0 mechanism. The main focus of the study is to analyze the effects of the two different combustion model approaches, such as infinitely fast chemistry based unsteady flamelet and finite rate chemistry based EDC, in predicting the NOx formation in three piloted methane jet flames (Sandia D, E, and F). The EDC approach is able to predict the passive scalar quantities but shows over-prediction in the reactive scalar quantities and NO prediction, while the unsteady flamelet modeling is found to be essential in predicting the accurate formation of slow kinetic species like NOx. The inability of flamelet and EDC approach in capturing localized flame extinction is observed, which lead to an over-prediction of NOx at larger downstream locations. Further, the dominance of NOx formation pathways is investigated in all three flames.

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arXiv2021

Investigation of strut-ramp injector in a Scramjet combustor: effect of strut geometry, fuel and jet diameter on mixing characteristics

The strut based injector has been found to be one of the most promising injector designs for supersonic combustor, offering en-hanced mixing of fuel and air. The mixing and flow field characteristics of the straight (SS) & tapered strut (TS), with fixed ramp an-gle and height at freestream Mach number 2 in conjunction with fuel injection at Mach 2.3 have been investigated numerically and reported. In the present investigation, hydrogen (H2) and ethylene (C2H4) are injected in oncoming supersonic flow from the back of the strut, where jet to freestream momentum ratio is maintained at 0.79 and 0.69 for H2 & C2H4, respectively. The predicted wall static pressure and species mole fractions at various downstream locations are compared with the experimental data for TS case with 0.6 mm jet diameter and found to be in good agreement. Further, the effect of jet diameter and strut geometry on the near field mixing in strut ramp configuration is discussed for both the fuel. The numerical results are assessed based on various parameters for the performance evaluation of different strut ramp configurations. The SS configuration for both the injectant is found to be an optimum candidate, also it is observed that for higher jet diameter larger combustor length is required to achieve satisfactory near field mixing.

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arXiv2021

Prediction of Separation Induced Transition on Thick Airfoil Using Nonlinear URANS Based Turbulence Model

Most of the turbulence models in practice are based on the assumption of a linear relation between Reynolds stresses and mean flow strain rates which generally provides a good approximation in case of attached and fully turbulent flows. A two dimensional numerical study has been carried out over NACA 0021 with k-ωSST model with non-linear correction at Re = 120,000 for various angles of attack which experiences the formation of a laminar separation bubble (LSB). A correct prediction of LSB requires an accurate resolution of anisotropy in Reynolds stresses. For comparison with other linear models, the simulations are also performed with k-kl-ω, k-ωSST and Spalart Allmaras. The performance of these models is assessed through aerodynamic lift, drag, pressure and friction coefficients. It is found that the non-linear k-ωSST and k-kl-ωtransition model provide comparable quality of prediction in lift and drag coefficients (in spite of the fact that non-linear k-ωSST involves solving less number of transport equation than the transition model) as observed in the experiments whereas k-ωSST and SA models under predict the drag coefficient value at low angle of attack due to inability to capture the separation induced transition. It is also observed that the location of laminar separation bubble is captured accurately when nonlinear or transition model is used as opposed to the SA or linear SST models, which lack in the ability to predict the same.

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