BITS Faculty Publications

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Analysis of Hypersonic Rarefied Flow past a Flat Plate Using dsmcFoam+ Solver
(IEEE, 2021-06) Aneesh, A.M.
Accurate estimation of surface heat flux at rarefied operating conditions is indispensable for the design of a reliable thermal protection system for Reusable Launch Vehicles. At rarefied atmospheric conditions, conventional continuum-based numerical methods fail to predict the flow field properties, Direct Simulation Monte Carlo (DSMC) method is recommended for the flow field predictions. In the present work, hypersonic flow over a flat plate was numerically simulated using dsmcFoam+ solver for near-continuum flow regime. The solver is validated for hypersonic flow over a flat plate case with respect to the literature and the results are analysed in detail to understand viscous interaction. Further, a parametric study was conducted by varying Mach number from 4 to 6 and the contours of pressure, density, and temperature distribution over a flat plate at supersonic to hypersonic flow conditions were analysed. Local pressure coefficient, surface heat flux, and heat transfer coefficient for the given operating conditions are also estimated and the results are discussed. The extension of this study for wedge and Orion geometries is under progress.
Analysis of Hypersonic Rarefied Reactive Flow Over RAM-C II Forebody Using hy2Foam Solver
(Begell House, 2021) Aneesh, A.M.
The re-entry of space vehicle to the rarefied atmosphere at hypervelocity results in formation of a strong bow shock in front of the vehicle. This causes a sudden rise in gas temperatures, excitation in the different energy modes of the gas molecules, and the onset of various chemical reactions. In this paper, numerical simulations on the hypersonic rarefied reactive flow over the nose cap region of the RAM-C II forebody have been proposed. For the simulations, hy2Foam - an open-source hybrid CFD-DSMC code has been used. We considered dissociation, exchange, and recombination types of chemical reactions with five species, N2, O2, NO, N, and O. The hy2Foam results have been validated with the literature. Further, the solver has been used to study the effects of variation of Mach number and Knudsen number on non-equilibrium hypersonic flows. The maximum fluid temperature increased with an increase in Mach number and decrease in Knudsen number. Also, the maximum wall temperature and wall heat flux increase with an increase in Mach number; however, the maximum wall temperature increases, and the wall heat flux decreases with an increase in Knudsen number.
Bypassing traditional molecular dynamics with artificial neural networks
(AIP, 2023-05) Aneesh, A.M.
An attempt has been made to speed up molecular dynamics simulations using machine learning. LAMMPS package was used to generate data for training the ML model which was programmed in PyTorch. The fidelity of the data generated by LAMMPS was first validated by simulating the evaporation of an Argon droplet in its own vapor. Results from simulations were compared with the D2 Law of droplet evaporation and a reasonably good agreement between theory and simulation was observed. Training and testing datasets consisted of per-timestep snapshots from 6 simulations of equilibration of up to 100 atoms in a periodic box. These were converted to images of dimension (10,10,6), such that 100 pixels of dimension (1,1,6) stored the coordinates and velocity components (x,y,z,vx,vy,vz) of up to 100 atoms. A Symplectic Recurrent Convolutional Hamiltonian Neural Network (SRCHNN) was proposed in which a conserved scalar analogous to the Hamiltonian of a system of interacting atoms was modeled using a Convolutional Neural Network. Using Hamilton's equations of motion, time derivatives of positions and velocities were obtained by taking the symplectic gradient of the Hamiltonian, calculated using backpropagation. Symplectic time integration with the Leapfrog algorithm was employed for predicting trajectories using the calculated time derivatives. The model was trained in a recurrent manner with sequences of particles’ positions and velocities. The performance of SRCHNN was tested against the length of the sequence used for training, ranging from 1 to 6. The mean square error (L2 loss) between the true and predicted output states did not decrease significantly with larger training sequence lengths. The percentage error between the predicted and true number of droplet particles was least for the smallest sequence length of 1; while the percentage errors between the droplet and ambient temperatures were roughly the same for all training sequence lengths. The SRCHNN was able to predict 15 future states in sequence within acceptable degrees of accuracy and a 3.1x speedup over LAMMPS was observed.
CFD Study on Thermal Hydraulic Performance of A Wavy Channel Based PCHE Model
(Springer, 2016-09) Aneesh, A.M.
Three dimensional CFD study is done here—using a commercial software- to propose an efficient PCHE (Printed Circuit Heat Exchanger) model; used as a recuperator in International Thermonuclear Experimental Reactor (ITER). The present work is aimed to study a wavy channel based PCHE model, with certain modifications in design to demonstrate better thermal and hydraulic performance. The waviness for the hot as compared to cold channel is in anti-phase. The study is done for various angle of bend (0° (straight), 5°, 10° and 15°) and Reynolds number (350, 700, 1400 and 2100). The inlet temperature of the hot and cold channel is taken as 1173 and 813 K, respectively; and the operating pressure of the PCHE is taken as 3 MPa. Thermal hydraulic performance parameters are presented for the various periodic sections of the wavy-channel. Power density as well as pressure drop increases with increasing Reynolds number and angle of bend. Wavy as compared to plane channel based PCHE is demonstrated here to give better thermal-hydraulic performance.
Development and Testing of Computational Model for the Thermal Analysis of a CubeSat Nanosatellite
(IEEE, 2021) Aneesh, A.M.
Due to extremely low cost, compact form factor and ease of manufacturing, CubeSats have been increasingly used for academic research. The thermal management of CubeSat is very essential to ensure proper functioning however it is difficult due to its compact form factor. A three dimensional numerical model of a 3U CubeSat on MATLABTM is proposed for the thermal analysis and developed. Governing equations for conduction and radiation heat transfer are considered in the model and the corresponding algebraic equations are formulated using Finite Difference Method. The model is tested against simple test cases and the results predicted by this model for simpler geometries are verified using ANSYS® WorkbenchTM . Using an in-house orbit propagator the irradiation due to solar radiation, earth albedo and earth IR is estimated. The numerical model is extended to determine the temperature distribution in a CubeSat in orbit and the results are analysed and verified with COMSOL Multiphysics ®
Effects of wavy channel configurations on thermal-hydraulic characteristics of Printed Circuit Heat Exchanger (PCHE)
(Elsevier, 2018-03) Aneesh, A.M.
Printed Circuit Heat Exchanger (PCHE) is a widely chosen plate type compact heat exchanger for high pressure applications. The present work mainly focuses on two high pressure applications; firstly, Helium Cooling System (HCS) of Test Blanket Module (TBM) in International Thermonuclear Experimental Reactor (ITER) and secondly, Intermediate Heat exchangers (IHX) in Very High Temperature Reactors (VHTR). In this work, a reduced numerical model for a single banked PCHE core working in He-He counter flow circuit has been numerically modelled and verified against the results available in the literature. The same model is then extended for studying the effect of three wavy-channel configurations viz. triangular, sinusoidal and trapezoidal in a single banked PCHE core made of Alloy-617. The nature of local flow and heat transfer in the periodic channels has been studied by visualizing the vortex core using iso-normalized helicity surfaces. Thereafter, the thermo-hydraulic performances of these models are compared with straight channel PCHEs. Among the various models studied, the trapezoidal PCHE model is found to offer highest heat transfer with the largest pressure drop compared to sinusoidal, triangular and straight channel based PCHE models. A maximum of 41% increase in the heat transfer rate is predicted for the trapezoidal wavy channel compared to the straight channel PCHEs, for the tested operating conditions. For the sinusoidal and triangular wavy-channel PCHE configurations, the corresponding heat transfer advantages are predicted to be 33% and 28% respectively. The optimal thermo-hydraulic performance is also assessed, considering the thermal performance factor (TPF) obtained for all the three channels. The highest values of TPF are predicted for trapezoidal wavy channels (3.5) which is followed by sinusoidal (2.5) and triangular (1.5) wavy channels.
Modeling hemodynamics in unruptured intracranial aneurysms under varied blood pressure conditions: an in silico study
(AIP, 2025-03) Aneesh, A.M.
Chronic hypertension is a significant risk factor for intracranial aneurysm (IA) formation, growth, and rupture. This work aims to predict the hemodynamics in unruptured IAs and investigate the impact of aneurysm necking and bulging, along with hypotension and hypertension on the same. We simplified a patient-specific IA geometry from the literature and employed two patient-specific velocity profiles in our three-dimensional unsteady computational fluid dynamics simulations. The blood is modeled as Newtonian, and the blood vessels are non-deforming. The fast Fourier transform study reveals that the frequency in the parent artery has decreased by a factor of 40 within the aneurysm geometry. Velocity waveforms with higher pulsatility indexes, common among young adults, pose more risk factors, such as high frequency and higher wall shear stress (WSS), and are aggravated more by hypertension in patients suffering from aneurysms. This computational study lays the groundwork for improving risk assessment and treatment planning for patients with unruptured aneurysms in varying systemic pressure conditions. Hypertension increases WSS and flow dynamics frequency, raising risks of aneurysm rupture, while hypotension promotes stagnation zones, raising risks of thrombus formation. Aneurysm necking and bulging significantly alter flow patterns, correlating geometry with unique vortex modes and maximum WSS. The developed flow regime map aids in diagnosing and treating aneurysms under varying conditions. High pulsatility indices, particularly in younger individuals, amplify risks, highlighting the need for tailored management strategies.
Modeling of Phase Change in Nanoconfinement Using Moment Methods
(ASME, 2023-01) Rana, Anirudh; Aneesh, A.M.
Accurate prediction of liquid–vapor phase change phenomena is critical in the design of thin vapor chambers and microheat pipes for the thermal management of miniaturized electronic systems. In view of this, we have considered the heat and mass transfer between two-liquid meniscuses separated by a thin gap of its own vapor. Assuming the heat and mass flow are to be steady and one-dimensional, analytic solutions are obtained to the linearized equations from the regularized 26-moment framework. Our analytic solutions provide excellent predictions for the effective heat conductivity of a dilute gas with those from the molecular dynamics (MD) and Boltzmann equation where Fourier's law fails. We also verified that the predicted heat and mass flow rates over the whole range of the Knudsen number are consistent with the kinetic theory of gases. Further, the model has been used to predict the effect of evaporation and accommodation coefficients on the heat and mass transfer between the liquid layers
Thermal hydraulic performance and characteristics of a microchannel heat exchanger: experimental and numerical investigations
(ACM Digital Library, 2025-02) Aneesh, A.M.
This paper presents extensive fluid flow and Heat Transfer studies conducted through an experimental setup followed by a detailed three-dimensional (3D) numerical analysis of the same setup using a commercial package for computational fluid dynamics (CFD), known as cfd-ace® for additive-manufactured counterflow AlSi10 Mg microchannel heat exchangers (MCHEs). A detailed 3D computational model of the experimentally tested MCHEs was built and analyzed using the commercial software cfd-ace® for the same experimentally tested operating conditions. The computational model results are in good agreement with experimental data of tested MCHE within +2% to +7% and ∼0% to −13.5% variation for cold and hot fluids for the entire set of design of experiments (DoEs). This percentage disagreement may be due to various factors, such as manufacturing deviation within tolerance, longitudinal conduction, variation in the thermal conductivity of the material after heat treatment, variation in environmental temperature, sensor deviation, and surface roughness of internal channels. Instead of Stainless steel (SST), AlSi10 Mg was used because of its lower manufacturing cost because AlSi10 Mg was lighter than SST, though its thermal conductivity is almost ∼8–10 times more than that of SST. A higher thermal conductivity is not good for MCHEs because it leads to higher longitudinal conduction, which eventually degrades the performance of MCHEs in terms of effectiveness. MCHE effectiveness is also reduced by ∼12% to 18% owing to longitudinal conduction from ideal effectiveness.
Thermal hydraulic performance evaluation of an additively manufactured minichannel heat exchanger using a combined experimental and multivariate regression model-based approach
(Elsevier, 2025-09) Aneesh, A.M.
This study investigates the thermal–hydraulic performance of an additively manufactured heat exchanger (AMHE) operating in a nitrogen-nitrogen counter flow open loop. The AMHE, consisting of ten semicircular mini channels with diverging inlets and converging outlet headers for both hot and cold fluids, was 3D printed using the Selective Laser Melting (SLM) technique with AlSi10Mg. The rough surface of its internal channels is characterized by using a cut sample with Field Emission Scanning Electron Microscopy (FESEM) images and a surface profilometer. An open-loop experimental test facility was developed to evaluate AMHE performance. Experiments are conducted by varying balanced mass flow rates (1.11 to 4.44 kg/h) and hot inlet temperatures (324.9 to 353.0 K). Balanced mass flow rate, temperature, and pressure measurements were recorded at steady state, and heat transfer rates and channel pressure drops were calculated. AMHE achieved a maximum power density of about 125.4 kW/m3 at a low log mean temperature difference (LMTD) of 6.5 K in a counter-flow arrangement. The experimental results were compared with standard ∊-NTU correlations available in the literature and showed agreement within 1 %. We noted that the effectiveness and entropy generation increase, and axial conduction decreases with an increase in balanced flow rates. A multivariable regression model was developed to predict the experimentally obtained heat transfer rate and pressure drops within a 2 % error limit and used to predict the effect of various operating conditions. Parametric results showed that increasing the balanced flow rate and hot inlet temperature enhanced the heat transfer rate by a factor of about 5, with the corresponding pressure drop rising by up to a factor of 10. This novel combined experimental and multivariable regression approach provides practical predictive correlations for gas-to-gas mini-channel heat exchangers, compensates for input variations, and enables reliable performance estimation under varied operating conditions, offering a valuable contribution for future design and optimization.
Thermal-hydraulic characteristics and performance of 3D straight channel based printed circuit heat exchanger
(Elsevier, 2016-04) Aneesh, A.M.
CFD study is done to propose an efficient PCHE (Printed Circuit Heat Exchanger) model; used as a recuperator in International Thermonuclear Experimental Reactor (ITER). 3D steady-state conjugate heat-transfer simulations are done with helium as a working fluid and alloy 617 as the solid substrate. Effect of variation of thermo-physical properties, operating conditions and three different design modifications are studied. Thermal hydraulic performance is found better for single as compared to double banking and is the same for aligned as compared to the staggered arrangement of the hot and cold channels. PCHE models with hemispherical dimples are found to give better thermal hydraulic performance. The performance is presented for the variation of the heat transfer density (for a PCHE model) and the pressure drop (in the hot and cold channel). Various types of flow patterns are presented to analyze the thermal-hydraulic characteristics − leading to the heat transfer enhancement by the dimples.
Thermal-hydraulic characteristics and performance of 3D wavy channel based printed circuit heat exchanger
(Elsevier, 2015-08) Aneesh, A.M.
CFD study is done here to propose an efficient PCHE (Printed Circuit Heat Exchanger) model; used as a recuperator in International Thermonuclear Experimental Reactor (ITER). 3D steady state conjugate heat-transfer numerical simulations are done; considering the variation of thermo-physical properties as a function of temperature. Helium is used as a working fluid and alloy 617 as solid substrate. The study is done for various angle of bend (θ = 0°(straight), 5°, 10° and 15°) and Reynolds number (Re = 350, 700, 1400 and 2100). Various types of flow patterns, within one wavy-section, are presented to analyze thermal-hydraulic characteristics. Thermal hydraulic performance parameters are presented for the various wavy-sections as well as within a section; and for the complete PCHE model. Heat transfer enhancement as compared to pressure penalty is higher for the wavy channel; and increases with increasing Re and θ. Wavy as compared to plane channel based PCHE is demonstrated here to give better thermal-hydraulic performance. A detailed characteristics as well as performance-parameters for thermal hydraulics in a 3D wavy channel based PCHE model − not found in the literature − is presented here.
Thermo-Hydraulic Performance of Zigzag, Wavy, and Serpentine Channel Based PCHEs
(Springer, 2016-09) Aneesh, A.M.
Printed Circuit Heat Exchanger (PCHE) is a plate type compact heat exchanger that usually finds application in high pressure systems. The scope of the present work is restricted to two high pressure applications, viz. (i) Test Blanket Module (TBM) cooling circuit in International Thermonuclear Experimental Reactor (ITER) and (ii) Intermediate Heat exchangers (IHX) in Very High Temperature Reactors (VHTR). A counter flow PCHE, made of Alloy-617, working in He–He circuit has been considered as the reference for numerical simulations performed on ANSYS Fluent 13. A reduced model for single banked straight channel PCHE model has been proposed and is validated with literature [1]. The scientific and engineering aspects of flow and heat transfer characteristics in straight channel PCHE have been carefully examined for constant as well as varying thermo-physical properties and are compared with each other. Three different channel configurations viz. zigzag, sinusoidal and swept-zigzag (trapezoidal) have been numerically modeled and tested in a single banked PCHE core and the respective thermo-hydraulic performances are compared with straight channel PCHEs.
Vortex dynamics around a fluctuating beam
(AIP, 2023-05) Aneesh, A.M.
Fluid Structure Interaction (FSI) analysis for fluctuating beam in viscous fluids has been carried out for the preliminary design of propeller-less underwater drones. The Immersed Boundary Method has been used to address a large variety of Fluid Structure Interaction problems such as insect flight, locomotion of fishes and batoid pieces etc.IB2d is an open-source MATLAB code developed by Battista, which can be used for solving two-dimensional Fluid Structure Interaction problems using the Immersed Boundary Method. The settings of IB2d solver have been verified by reproducing a flapping beam problem. The parameters for this problem have been changed to study the flow generated around a beam tethered at both ends and subjected to a perturbation of the beam in static fluid. The beam has been modelled as a setof Lagrangian points for a given length and stiffness value and the fluid around the beam is modelled as a Eulerian regionof certain viscosity and density. The beam has been perturbed from its equilibrium position by an ellipsoidal arc and the dynamics of vortices around the beams are analyzed both qualitatively and quantitatively. The simulations have been repeated by varying viscosities of the Eulerian region, varying length of the beam and varying stiffness values betweenthe Lagrangian points. The parametric study shows that the magnitude of vorticity around the beam reduces with increasein viscosity and stiffness It is also found that for the same curvature, the motion speed reduces with reduction in the beamlength.