Sarath Kossery Prakasan

2025

Accurate prediction of laminar-to-turbulent flow transition is challenging because of its complex nature. In this study, we use a combination of linear stability theory (LST) and direct numerical simulations (DNSs) to examine perturbation growth on ogive-cylinder forebodies, relevant to modern hypersonic vehicles, focusing on the effect of nose bluntness. The spatial evolution of disturbances is first examined through the solution of a linear eigenvalue problem. As the ogive nose bluntness is increased, the first mode, which was unstable downstream for the sharp nose, moves upstream, and its oblique component displays higher instability. The LST predictions are used to inform DNS to examine the effect of amplitude on perturbation growth and modal characteristics. For small 2D perturbations, second-mode growth with DNS yields similar results as LST for all bluntness cases examined, suggesting that the streamwise gradients inherent to ogive forebodies have relatively little effect on this behavior. When the forcing is azimuthally localized, the oblique nature associated with low-frequency perturbations becomes apparent, analogous to the first mode. These are, however, inhibited when the forcing is azimuthally coherent. Furthermore, at higher amplitudes, nonlinear interactions appear among unstable modes, providing insights into the different interactions governing transition in such nose geometries.

2025

Hypersonic transition studies on systems sustaining multimodal dynamics are critical to understanding aerothermal loading on flight-relevant configurations. The present work evaluates transition mechanisms in hypersonic boundary layers over a cone–cylinder–flare geometry, and its sensitivity to free stream disturbance amplitudes, using a global linear stability approach and direct numerical simulations (DNS). Under relatively quiet conditions, the flow field resembles the laminar solution, consisting of a large separation zone over the cylinder–flare junction. Linear analysis identifies multiple convective instabilities including, oblique first modes and two-dimensional second modes over the cone segment, and shear layer instabilities over the separation zone.

2024

Semi-circular enclosures have an inherent advantage of decreasing unmelt volume on top with an increase in melt height when heated from bottom. Hence the melt dynamics differs from conventional rectangular enclosures of similar orientation and realizes faster thermal response during the entire regimes of melting. Experimental and numerical studies on Rayleigh Benard convection in a semi-circular enclosure having straight, corrugated, and wavy bottom surface with constant heat flux conditions are presented. Melt front evolution, thermal convection, and unsteady surface heat transfer characteristics of a phase changing material contained in a semicircular confinement are presented in this study. Enthalpy-porosity method based finite volume solver is used to simulate the unsteady thermal convection of PCM in semi-circular enclosure. Experimental thermal response at various locations inside the confinement and shadowgraph images are used to verify the numerical results quantitatively and qualitatively.

2023

Experimental and numerical investigations of thermal convection in the vicinity of a straight and wavy horizontal heat transfer fluid passage (HTF)s placed in a rectangular enclosure filled with phase change material (PCM) are presented. The enthalpy-porosity-based finite volume solver is used to simulate the spatiotemporal evolution of the thermal convection, which is well-complemented by experimental visualization and thermocouple measurements. Heat transfer from the horizontal HTF exhibits asymmetrical melting. Heat diffusion is the dominating energy transfer mechanism on the bottom part of the HTF passage and is compared with the Stefan problem. The temporal evolution of thermal convection on the top part of HTF passage involves three distinct phases of melting, viz. initial conduction phase, Rayleigh–Bénard (R-B) convection with linear convection, and vortex coalescence effects. The final stage of the melting process is influenced by the topography of a peninsula of unmelt PCM, which is influenced by specific top-boundary conditions. Thermal convection in PCM above the wavy HTF passage exhibits restricted plume development leading to an improved melting rate and a higher average melt height.

2023

Experimental and numerical investigations of thermal convection in the vicinity of a straight and wavy horizontal heat transfer fluid passage (HTF)s placed in a rectangular enclosure filled with phase change material (PCM) are presented. The enthalpy-porosity-based finite volume solver is used to simulate the spatiotemporal evolution of the thermal convection, which is well-complemented by experimental visualization and thermocouple measurements. Heat transfer from the horizontal HTF exhibits asymmetrical melting. Heat diffusion is the dominating energy transfer mechanism on the bottom part of the HTF passage and is compared with the Stefan problem. The temporal evolution of thermal convection on the top part of HTF passage involves three distinct phases of melting, viz. initial conduction phase, Rayleigh–Bénard (R-B) convection with linear convection, and vortex coalescence effects. The final stage of the melting process is influenced by the topography of a peninsula of unmelt PCM, which is influenced by specific top-boundary conditions. Thermal convection in PCM above the wavy HTF passage exhibits restricted plume development leading to an improved melting rate and a higher average melt height.

2023

At the outset, though the phase-changing process in latent heat storage systems appears simple, a critical observation unveils the complex evolution of various coupled fluid-thermal transfer processes. An astute examination of such phenomena is essential to effectively figure out the role of geometry, orientation, process variables and their dimensionless groups, boundary conditions, material property, etc., in developing accurate models and correlations. The presence of vortical flow structures and gradients of velocity and temperature lead to local variations in heat transfer coefficients in boundaries separating the heat transfer fluid (HTF) passages and phase-changing material (PCM). Gravity influences the Rayleigh–Bénard convection phase-changing process, which offers a higher energy transfer rate than heat diffusion-dominated melting/solidification. A wide variety of test cases are surveyed and conclusively figured out the role of geometry and orientation-dependent convection in the phase-changing process.

2023

This work investigates the stability and transition to turbulence in a diverging channel subjected to a time-varying trapezoidal-shaped inflow boundary condition. Numerical simulations are performed for different deceleration rates and Reynolds numbers while maintaining a constant acceleration rate. The flow transition begins with two-dimensional primary instability with the formation of inflectional velocity profiles, followed by local separation and the emergence of an array of shear layer vortices. We divide simulation cases systematically into three categories based on the onset of secondary instability and the generation of streamwise vorticity. At low and medium Reynolds numbers (type I), the spanwise vortex rolls formed by inflectional instability remain two-dimensional and diffuse at the channel centre without exhibiting further instabilities. At high Reynolds numbers and deceleration rates (type II), the rolled shear layer exhibits secondary instability during the zero mean inflow phase, followed by local incipient turbulent structure formation. The streamwise vorticity that develops over the shear layer structures causes oscillations with a spanwise wavelength similar to those associated with the elliptic instability in a counter-rotating vortex pair. Using the Lamb–Oseen approximation of vortices in conjunction with the dynamic mode decomposition algorithm of the three-dimensional flow field, we captured successfully the characteristics of the secondary instability. The third category (type III) is characterized by periodic unsteady separation, secondary instability, and merging of shear layer vortices, which occurs when Reynolds numbers are high and deceleration rates are low.

2022

This work explores the three-dimensional vortex-dynamics past a wall-attached bluff body kept in a variable velocity field using direct numerical simulations. A trapezoidal pulse of mean velocity, consisting of acceleration phase from rest followed by constant velocity phase and deceleration phase to rest, is imposed at the inlet of the computational domain. The flow evolution starts with the formation of a primary vortex followed by a two-dimensional circular array of spanwise vortex tubes by inflectional shear-layer instability. At a sufficiently high Reynolds number, the shear layer vortices originated from two-dimensional fluctuations deformed by three-dimensional instabilities, giving fragmented streamwise vorticity. The distinct flow features, including mode shape, frequency, and growth rate associated with the shear-layer instability, are identified using the dynamic mode decomposition (DMD) algorithm. Using the maximum growth rate criteria, the DMD technique successfully separates the coherent shear layer modes associated with two-dimensional shear layer instability from the flow field.