arXiv subjects

Ashoke De

Publications and source records attributed to Ashoke De.

69 records · Page 4Linked to original sources

arXiv2021

Passive control of the onset of vortex shedding in flow past a circular cylinder using Slit

The present study investigates the passive flow control phenomena over a two-dimensional circular cylinder using numerical simulations in the laminar regime. The aim is to explore one of the passive control techniques, which involves the introduction of a slit to the geometry of the cylinder. The two parameters, slit width ratio S/D (slit width/diameter) and slit angle (measured with respect to the incoming flow direction), play an essential role in determining the trend of critical Reynolds number (Rec). Most of the analysis invokes flow visualization and saturation amplitude methods to obtain the critical Reynolds number (indicative of the onset of vortex shedding) for different cases. Further, Hopf bifurcation analysis using Stuart-Landau equation and global stability analysis confirm the accuracy and consistency in the predicted solutions. The additional amount of flow through slit increases the pressure downstream of the cylinder, which consequently leads to an increase in Rec of the modified cylinder. The critical Reynolds number increases with S/D of the modified cylinder at 0 deg slit angle (as an additional amount of flow grows with S/D). The critical Reynolds number shows an increasing trend with the slit angle in the range of S/D (0.05-0.15) as the fluctuation intensity reduces with slit angle in this range. For S/D= 0.15-0.25, the extra amount of flow through slit induces the instability in wake which causes a decrease in Rec with slit angle. A correlation is obtained that estimates the critical Reynolds number for a given S/D and slit angle.

physics.flu-dyn↗

arXiv2021

Investigation of asymmetrically pitching airfoil at high reduced frequency

The expanding application in Micro-Air Vehicles has encouraged many researchers to understand the unsteady flow around a flapping foil at a low Reynolds number. We numerically investigate an incompressible unsteady flow around a two-dimensional pitching airfoil (SD7003) at high reduced frequency in the laminar regime. This study interrogates the effect of different unsteady parameters, namely amplitude (A), reduced frequency (k), Reynolds number (Re), and asymmetry parameter (S) for pitching motion on the force coefficients. The inviscid theoretical model is utilized to calculate the lift coefficient for sinusoidal motion in the viscous regime, and a comparison is made with the numerical results. The theoretical analysis identifies the influence of the non-circulatory lift over circulatory lift at a high reduced frequency. Further, the results indicate that the reduced frequency (k) and asymmetry parameter (S) have a significant impact on the instantaneous and time-averaged force coefficients as well as on the vortex structure in the wake. Finally, the Fast Fourier Transformation (FFT) analysis is carried out over a simulated case with fixed amplitude and Reynolds number for distinct k and S values. The findings confirm that the dominant frequency in the flow (k*) has a direct correlation to the airfoil pitching frequency (k).

physics.flu-dyn↗

arXiv2021

Modal Decomposition of Turbulent Supersonic Cavity

Self-sustained oscillation in Mach 3 supersonic cavity with a length-to-depth ratio of 3 is investigated using wall modeled Large Eddy Simulation (LES) methodology for ReD = 3.39*10^5. The unsteady data obtained through computation is utilized to investigate the spatial and temporal evolution of the flow field, especially second invariant of velocity tensor; while the phase averaged data is analyzed over a feedback cycle to study the spatial structures. This analysis is accompanied by the Proper Orthogonal Decomposition (POD) data, which reveals the presence of discrete vortices along the shear layer. The POD analysis is performed in both the spanwise and streamwise planes to extract the coherence in flow structures. Finally, Dynamic Mode Decomposition (DMD) is performed on the data sequence to get the dynamic information and deeper insight into the self-sustained mechanism.

physics.flu-dyn↗

arXiv2021

Investigation of Shock Wave Interactions involving Stationary and Moving Wedges

The present study investigates the shock wave interactions involving stationary and moving wedges using a sharp interface immersed boundary method combined with a fifth order weighted essentially non oscillatory (WENO) scheme. Inspired by Schardins problem, which involves moving shock interaction with a finite triangular wedge, we study influences of incident shock Mach number and corner angle on the resulting flow physics in both stationary and moving conditions. The present study involves three incident shock Mach numbers (1.3, 1.9, 2.5) and three corner angles (60deg, 90deg, 120deg), while its impact on the vorticity production is investigated using vorticity transport equation, circulation, and rate of circulation production. Further, the results yield that the generation of the vorticity due to the viscous effects are quite dominant compared to baroclinic or compressibility effects. The moving cases presented involve shock driven wedge problem. The fluid and wedge structure dynamics are coupled using the Newtonian equation. These shock driven wedge cases show that wedge acceleration due to the shock results in a change in reflected wave configuration from Single Mach Reflection (SMR) to Double Mach Reflection (DMR). The intermediary state between them, the Transition Mach Reflection (TMR), is also observed in the process. The effect of shock Mach number and corner angle on Triple Point (TP) trajectory, as well as on the drag coefficient, is analyzed in this study.

physics.flu-dyn↗

arXiv2021

Investigation of Mixing Characteristics in Strut Injectors using Modal Decomposition

Effect of large-scale vortical structure on mixing and spreading of shear layer is numerically investigated. Two strut configurations namely Straight & Tapered strut at two convective Mach numbers (Mc = 1.4 & 0.37) for two jet heights (0.6 & 1mm) are investigated. Hydrogen jet is injected through a two-dimensional slot in oncoming coflow at Mach 2. Excellent agreement between simulated and experimental data is witnessed, whereas the instantaneous data reveal the presence of various large-scale structures in the flow field. From the instantaneous field, it becomes apparent that both the geometries have different vortical breakdown locations. It is also noticed that an early onset of vortex breakdown manifests itself into the mixing layer thickness enhancement, effect of which is reflected in overall mixing characteristics. It becomes evident that the shear strength plays an important role in the near field mixing. The higher shear strength promotes the generation of large vortices. The analysis shows that SS-0.6 case offers highest mixing efficiency being dominated by relatively large-scale structures. Eigenmodes obtained through Proper Orthogonal Decomposition (POD) confirm the presence of dominating structures and shed light into the series of events involved in vortex pairing/merging and breakdown. Dynamic modal decomposition (DMD) also strengthens the observation made through the POD.

physics.flu-dyn↗

arXiv2021

Role of Jet Spacing and Strut Geometry on the formation of Large Scale Structures & Mixing Characteristics

The present study primarily focuses on the effect of jet spacing and strut geometry on the evolution and structure of the large-scale vortices which play a key role in mixing characteristics in turbulent supersonic flows. Numerically simulated results corresponding to varying parameters such as strut geometry and jet spacing (Xn=nDj such that n=2, 3 & 5) for square jet of height Dj=0.6 mm is presented in the current study, while the work also investigates the presence of the local quasi-two-dimensionality for the X2(2Dj) jet spacing; however, the same is not true for higher jet spacing. Further, the tapered strut (TS) section is modified to the straight strut (SS) for investigation, where the remarkable difference in flow physics is unfolded between the two configurations for similar jet spacing (X2: 2Dj). The instantaneous density and vorticity contours reveal the structures of varying scales undergoing different evolution for the different configurations. The effect of local spanwise rollers is clearly manifested in the mixing efficiency and the jet spreading rate. SS configuration exhibits excellent near field mixing behavior amongst all the arrangements. However, in case of TS cases, only X2(2Dj) configuration performs better due to the presence of local spanwise rollers. The qualitative and quantitative analysis reveals that near-field mixing is strongly affected by the two-dimensional rollers, while the early onset of wake mode is another crucial parameter to have improved mixing. Modal decomposition performed for SS arrangement sheds light into the spatial and temporal coherence of the structures, where the most dominant structures are found to be the Von-Karman street vortices in the wake region.

physics.flu-dyn↗

arXiv2021

Vortex-induced Vibrations of a Confined Circular Cylinder for Efficient Flow Power Extraction

A simple method to increase the flow power extraction efficiency of a circular cylinder, undergoing vortex-induced vibration (VIV), by confining it between two parallel plates is proposed. A two-dimensional numerical study was performed on VIV of a circular cylinder inside a parallel plate channel of height H at Reynolds number 150 to quantify the improvement. The cylinder is elastically mounted with a spring such that it is only free to vibrate in the direction transverse to the channel flow and has a fixed mass ratio (m*) of 10. The energy extraction process is modelled as a damper, with spatially constant damping ration ((), attached to the cylinder. The simulations are performed by varying the reduced velocity for a set of fixed mass-damping (α = m*() values ranging between 0 to 1. The blockage ratio (b = D/H) is varied from 0.25 to 0.5 by changing the channel height. The quasi-periodic initial branch found for the unconfined cylinder shrinks with the increasing blockage. The extracted power is found to increase rapidly with the blockage. For maximum blockage (b = 0.2), the maximum flow power extracted by the cylinder is an order of magnitude larger as compared to what it would extract in an open domain with free stream velocity equal to the channel mean velocity. The optimal mass-damping (αc ) for extracting maximum power is found to lie between 0.2 to 0.3. An expression is derived to predict the maximum extracted power from the undamped response of a confined/unconfined cylinder. With the assumption αc = 0.25, the derived expression can predict the maximum power extraction within +-20% of the actual values obtained from present and previous numerical and experimental studies.

physics.flu-dyn↗

arXiv2021

Numerical simulation of Delft-jet-in-hot-coflow (DJHC) flames using the Eddy Dissipation Concept model for turbulence-chemistry interaction

In this paper we report results of a numerical investigation of turbulent natural gas combustion for jet in a coflow of lean combustion products in the Delft-Jet-in-Hot-Coflow (DJHC) burner which emulates MILD (Moderate and Intense Low Oxygen Dilution) combustion behavior. The focus is on assessing the performance of the Eddy Dissipation Concept (EDC) model in combination with two-equation turbulence models and chemical kinetic schemes for about 20 species (Correa mechanism and DRM19 mechanism) by comparing predictions with experimental measurements. We study two different flame conditions corresponding to two different oxygen levels (7.6% and 10.9% by mass) in the hot coflow, and for two jet Reynolds number (Re=4100 and Re=8800). The mean velocity and turbulent kinetic energy predicted by different turbulence models are in good agreement with data without exhibiting large differences among the model predictions. The realizable k-e model exhibits better performance in the prediction of entrainment. The EDC combustion model predicts too early ignition leading to a peak in the radial mean temperature profile at too low axial distance. However the model correctly predicts the experimentally observed decreasing trend of lift-off height with jet Reynolds number. A detailed analysis of the mean reaction rate of the EDC model is made and as possible cause for the deviations between model predictions and experiments a low turbulent Reynolds number effect is identified. Using modified EDC model constants prediction of too early ignition can be avoided. The results are weakly sensitive to the sub-model for laminar viscosity and laminar diffusion fluxes.

physics.flu-dyn↗

arXiv2021

Assessment of turbulence-chemistry interaction models in MILD combustion regime

The present paper reports on the assessment of different turbulence-chemistry interaction closures for modeling turbulent combustion in the Moderate and Intense Low oxygen Dilution combustion regime. 2D RANS simulations have been carried out to model flames issuing from two burners DJHC burner and Adelaide JHC burner which imitate the MILD combustion. In order to model these flames, two different approaches of turbulence-chemistry interaction models are considered; while in the PDF based modeling, two different variants are invoked to understand the applicability of the PDF based models in the MILD regime: one is based on presumed shape PDF approach and the other one is transported PDF approach. A comprehensive study has been carried out by comparing the results obtained from these different models. For the DJHC burner, the computations are carried out for a jet speed corresponding to Reynolds numbers of Re=4100, whereas the Adelaide JHC burner computations are performed for a jet speed corresponding to Reynolds number of Re=10000. The effects of molecular diffusion on the flame characteristics are also studied by using different micro-mixing models. In the case of DJHC burner, it has been observed that the mean axial velocity and the turbulent kinetic energy profiles are in good agreement with the measurements. However, the temperature profiles are over-predicted in the downstream region by both EDC and the PDF based models. In the context of Adelaide JHC burner, the profiles of the temperature and the mass fraction of major species (CH4, H2, O2, H2O, CO2) obtained using LPDF approach are in better agreement with the measurements compared to those obtained using EDC model; although, both the solution approaches fail to capture CO and OH radical profiles.

physics.flu-dyn↗

arXiv2021

Numerical Study of Trailing and Leading Vortex Dynamics in a Forced Jet with Coflow

The interaction of trailing and leading vortex structures in low Reynolds number forced (varicose) jet with coflow is studied using Direct Numerical Simulation (DNS), where we report the dynamics of these vortices for a range of varying parameters. In a circular forced jet with Strouhal number 0.2, continuous formation of these vortices is observed which undergo pairing, tearing and disintegration. The forced jet is analyzed for varying frequency of forced perturbations, coflow temperature, turbulence intensity, momentum thickness, coflow intensity and Reynolds number at a constant forcing amplitude. The observation reveals that any reduction in momentum thickness increases the circulation as well as energy capacity of the leading and the trailing vortices, thereby enhancing the vortex pairing time. However, the hot coflow increases the strength of the leading vortex to such an extent that the generated trailing vortex is weak and dissipates even before it undergoes vortex pairing, while an incomplete disintegration of the leading vortex occurs in this case. Moreover, the strength of leading and trailing vortices is found to decrease with an increase in coflow power, thereby leading to a weaker vortex pairing. The decrease in Reynolds number also reduces the intensity of trailing vortices; whereas, at a lower Reynolds Number (Re) the trailing vortices dissipate soon after they form. The findings are achieved based on the analysis done by Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD), fast Fourier transform (FFT), Q-criterion and vortex tracking method to understand the dynamic interaction and behavior of the vortices.

physics.flu-dyn↗

arXiv2021

A Novel Sharp Interface Immersed Boundary Framework for Viscous Flow Simulations at Arbitrary Mach Number Involving Complex and Moving Boundaries

This work presents a robust and efficient sharp interface immersed boundary (IBM) framework, which is applicable for all-speed flow regimes and is capable of handling arbitrarily complex bodies (stationary or moving). The work deploys an in-house, parallel, multi-block structured finite volume flow solver, which employs a 3D unsteady Favre averaged Navier Stokes equations in a generalized curvilinear coordinate system; while we employ a combination of HCIB (Hybrid Cartesian Immersed boundary) method and GC(Ghost-cell) for solution reconstruction near immersed boundary interface. A significant difficulty for these sharp interface approaches is of handling sharp features/edges of complex geometries. In this study, we observe that apart from the need for robust node classification strategy and higher order boundary formulations, the direction in which the reconstruction procedures are performed plays an important role in handling sharp edges. Taking this into account we present a versatile interface tracking procedure based on ray tracing algorithm and a novel three step solution reconstruction procedure that computes pseudo-normals in the regions where the normal is not well-defined and reconstructs the flow field along those directions. We demonstrate that this procedure enables solver to efficiently handle and accurately represent sharp-edged regions. A fifth-order weighted essentially non-oscillatory (WENO) scheme is used for capturing shock-induced discontinuities and complex fluid-solid interactions with high resolution. The developed IBM framework is applied to a wide range of flow phenomena encompassing all-speed regimes (M=0.001 to M = 2.0). A total of seven benchmark cases (three stationary and four moving bodies) are presented involving various geometries (cylinder, airfoil, wedge) and the predictions are found to be in excellent agreement with the published results.

physics.comp-ph↗

arXiv2021

Large Eddy Simulation of a Premixed Bunsen flame using a modified Thickened-Flame model at two Reynolds number

A modified Thickened Flame (TF) model based on Large Eddy Simulation methodology is used to investigate premixed combustion and the model predictions are evaluated by comparing with the piloted premixed stoichiometric methane-air flame data (Chen et al., 1996) for Reynolds numbers Re = 24,000 (flame F3) and Re=52,000 (flame F1). The basic idea of Thickened-Flame approach is that the flame front is artificially thickened to resolve on the computational LES grid while keeping the laminar flame speed ( ) constant. The artificially thickening of the flame front is obtained by enhancing the molecular diffusion and decreasing the pre-exponential factor of the Arrhenius law. Since the flame front is artificially thickened, the response of the thickened flame to turbulence is affected and taken care of by incorporating an efficiency function (E) in the governing equations. The efficiency function (E) in the modified TF model is proposed based on the direct numerical simulations (DNS) data set of flame-vortex interactions (Colin et al., 2000). The predicted simulation results are compared with the experimental data and with computations reported using RANS based probability distribution function (PDF) modeling approach (Lindstedt, R. P. and Vaos, E. M., 2006, Transported PDF modeling of high-Reynolds-number premixed turbulent flames, Combustion and Flame, 145, 495) and RANS based G-equation approach (Herrmann, M., 2006). It is shown that the results with the modified TF model are generally in good agreement with the data, with the TF predictions consistently comparable to the PDF model predictions and superior to the results with the G-equation approach.

physics.flu-dyn↗

arXiv2021

Large Eddy Simulation of MILD combustion using PDF based turbulence-chemistry interaction models

The present work reports on LES of turbulent reacting flow of the Delft-Jet-in-Hot-Coflow (DJHC) burner, emulating MILD combustion, with transported PDF based combustion models using ANSYS FLUENT 13.0. Two different eddy viscosity models for LES (Dynamic Smagorinsky and Kinetic Energy Transport) along with two solution approaches for PDF transport equation, i.e. Eulerian and Lagrangian, have been used in the present study. Moreover, the effects of chemical kinetics and the micro-mixing models have also been investigated for two different fuel jet Reynolds number (Re = 4,100 and Re = 8,800). The mean velocity and turbulent kinetic energy predicted by the different models are in good agreement with experimental data. Both the composition PDF models predict an early ignition resulting in higher radial mean temperature predictions at burner exit. The models, however, correctly predict the formation mechanism of ignition kernels and the decreasing trend of the lift-off height with increasing jet Reynolds number, as observed experimentally.

physics.flu-dyn↗

arXiv2021

Numerical Investigation of Coaxial GCH4/LOx Combustion at Supercritical Pressures

This article aims to numerically investigate the combustion phenomenon of coaxial gaseous CH4 LOx at supercritical pressures. The choice of turbulence model, real gas model, and chemical kinetics model are the critical parameters in numerical simulations of cryogenic combustion at high pressure. At this supercritical operating pressure, the ideal gas law does not remain valid for such cases. Therefore, we have systematically carried out a comparative study to analyze the importance of real gas models, turbulence parameters, and chemical kinetics at such conditions. The comparison of real gas models with the NIST database reveals better conformity of SRK (Soave Redlich Kwong Equation of State (EoS)) model predictions with the database. Further, the computed results indicate that the Standard k-e turbulence model with modified constant captures the better flame shape and temperature peak position compared to other RANS based turbulence models while invoking the non-premixed steady b-PDF flamelet model for simulating the combustion process. Furthermore, a comparative study comparing two different chemical kinetics models indicates that the reduced Jones Lindstedt mechanism can accurately predict the flame characteristics with the least computational cost. Finally, we have studied the effect of chamber pressure and LOx inlet temperature on the flame characteristics. The flame characteristics exhibit a strong sensitivity towards the chamber pressure due to the weakening of the pseudo-boiling effect with an increase in pressure. As a consequence of lower turbulent rates of energy and mass transfer through the transcritical mixing layer, the flame spreading becomes narrower at elevated pressure and temperature, thereby yielding an increased flame length at transcritical conditions.

physics.flu-dyn↗

arXiv2020

Characteristics of shock tube generated compressible vortex rings at very high shock Mach numbers

Compressible vortex rings, usually formed at the open end of a shock tube, often show interesting phenomena during their formation, evolution, and propagation depending on the shock Mach number (Ms) and exit flow conditions. The Mach number of the translating compressible vortex rings (Mv) investigated so far in the literature is subsonic as, the shock tube pressure ratio (PR) considered is relatively low. In this numerical study we focus on low to high vortex ring Mach numbers (0.31 < Mv < 1.08) cases with a particular focus on very high Mv cases that are not been reported in experiments as, it is difficult to obtain in laboratory. Using hydrogen as a driver section gas inside the shock tube, a supersonic compressible vortex ring (Mv > 1) is obtained for first time. It is established that the SST k-ω based DES turbulent model replicates the experimental observation better than the previously published results at different stages of development of the vortex ring. DES, which is an inbuilt hybrid of LES and RANS approaches is evoked that can automatically switch to the sub-grid scale (SGS) model in the LES regions (i.e. with different scale vortical structures) and to a RANS model in the rest of the region (i.e. where the grid spacing is greater than the turbulent length scale). The DES model can predict characteristics of the shear layer vortices as well as counter-rotating vortex rings (CRVRs) as reported in the experimental measurements. Formation of multiple triple points and the corresponding slip-stream shear layers and thus multiple CRVRs behind the primary vortex ring at different radial locations, in addition to the usual CRVRs, appears to be a unique characteristic for high Mach number vortex rings. For high PR, H2, case during formation stage, a vortex layer of reverse circulation (that of primary vortex ring) is formed (contd...)

physics.flu-dyn↗