arXiv subjects

Indranil Saha Dalal

Publications and source records attributed to Indranil Saha Dalal.

12 recordsLinked to original sources

arXiv2026

Benchmarking Pedestrian Dynamics Models for Common Scenarios: An Evaluation of Force-Based Models

Extensive research in pedestrian dynamics has primarily focused on crowded conditions and associated phenomena, such as lane formation, evacuation, etc. Several force-based models have been developed to predict the behavior in these situations. In contrast, there is a notable gap in terms of investigations of the moderate-to-low density situations. These scenarios are extremely commonplace across the world, including the highly populated nations like India. Additionally, the details of force-based models are expected to show significant effects at these densities, whereas the crowded, nearly packed, conditions may be expected to be governed largely by contact forces. In this study, we address this gap and comprehensively evaluate the performance of different force-based models in some common scenarios. Towards this, we perform controlled experiments in four situations: avoiding a stationary obstacle, position-swapping by walking toward each other, overtaking to reach a common goal, and navigating through a maze of obstacles. The performance evaluation consists of two stages and six evaluating parameters - successful trajectories, overlapping proportion, oscillation strength, path smoothness, speed deviation, and travel time. Firstly, models must meet an eligibility criterion of at least 80\% successful trajectories and secondly, the models are scored based on the cutoff values established from the experimental data. We evaluated five force-based models where the best one scored 57.14\%. Thus, our findings reveal significant shortcomings in the ability of these models to yield accurate predictions of pedestrian dynamics in these common situations.

physics.soc-ph↗

arXiv2025

Effects of Geometric Modelling and Blood Rheology in Patient-Specific Arterial Blood Flow Simulations with Speed-Accuracy Trade-Off Analysis

This study investigates the effects of geometric model reduction on blood flow simulations in the patient-specific descending aorta, followed by speed-accuracy trade-off analysis using 3D simulations. We demonstrate how wall shear stresses (WSS) can be reliably estimated for such realistic arteries using significantly faster simulations of highly idealized equivalent geometries, for any blood rheology model. CFD simulations (3D) are performed at two levels of geometry reduction employing realistic pulsatile inflow and pressure outlet boundary conditions and utilizing both Newtonian and non-Newtonian blood rheology models, including the one developed recently by Apostolidis and Beris. The first level of reduction does not retain effects due to local asymmetry but can approximate various flow parameters and patterns, while showing a significant computational speedup. However, further simplification to an idealized smooth geometry loses all information about the vortex structures and flow circulation. The non-Newtonian models retain more accuracy than the Newtonian models in geometry reductions, as quantified by correlations defined in this study. The idealized smooth geometry, combined with area correction, yields WSS estimates that closely approximate those of the actual artery. This study is expected to be applicable in geometric reductions (and speed enhancements) for more complex patient-specific 3D simulations while maintaining accuracy.

physics.flu-dyn↗

arXiv2025

Analysis of Hematocrit-Plasma Separation in a Trifurcated Microchannel by a Diffusive Flux Model

Platelet-enriched plasma and red blood cells (RBC) are needed in the treatment of blood-related diseases, including anaemia and blood cancer. These essential components must be separated from blood in well-designed experimental setups. If active techniques are used, the blood components are likely to be damaged or contaminated while handling. Passive techniques for component separation are preferred, and their design for effectiveness before manufacturing is the subject of this article. Specifically, the performance of a design consisting of a trifurcated microchannel is examined in the framework of 3D numerical simulation, following similar design ideas in recent experimental studies. The influence of geometrical parameters of the channel, such as width and separation arm angle, inlet extension, flow constriction, and flow parameters, including flow rates, hematocrit concentration, and temperature, is studied. The present study utilizes the diffusive flux model (DFM) to model the shear-driven migration of red blood cells (RBC) in a microchannel along with an appropriate rheology model. The physical mechanism driving separation is the formation of the cell-free layer near the walls, using which the separation efficiency and device effectiveness are quantified. It is found that a microchannel with a smaller width and an extended inlet, along with diluted blood samples of lower hematocrit, is effective for greater separation, while the device performance is less sensitive to the flow rates, flow constriction, and the separator angle.

physics.flu-dyn↗

arXiv2025

Variable Goal Approach (VGA) Enhancing Pedestrian Dynamics Modeling

Pedestrian dynamics models have provided valuable insights into pedestrian interactions, collision avoidance, and self-organized crowd behavior using mathematical, computational, AI-based, and heuristic approaches. However, existing models often fail to capture fundamental aspects of human decision-making, particularly the tendency to adopt indirect routes by sequentially selecting intermediate goals within the line of sight. In this study, we propose a novel Variable Goal Approach (VGA) that integrates human intelligence into pedestrian dynamics models by introducing multiple intermediate goals, termed variable goals, which guide pedestrians toward their final destination. These variable goals function as an adaptive guidance mechanism, enabling smoother transitions and dynamic navigation. VGA also enhances the efficiency of a model while minimizing interactions and disruptions. By strategically positioning variable goals, VGA introduces an element of stochasticity. This allows the model to simulate varied pedestrian paths under identical conditions, reflecting the diversity in human decision-making. In addition to its effectiveness in simple scenarios, VGA demonstrates strong performance in replicating high-density scenarios, such as lane formation, providing results that closely match real-world data.

physics.soc-ph↗

arXiv2025

Automatic Estimation of Pedestrian Gait Features using a single camera recording: Algorithm and Statistical Analysis for Gender Difference and Obstacle Interactions

The pedestrian gait features - body sway frequency, amplitude, stride length, and speed, along with pedestrian personal space and directional bias, are important parameters to be used in different pedestrian dynamics studies. Gait feature measurements are paramount for wide-ranging applications, varying from the medical field to the design of bridges. Personal space and choice of direction (directional bias) play important roles during crowd simulations. In this study, we formulate an automatic algorithm for calculating the gait features of a trajectory extracted from video recorded using a single camera attached to the roof of a building. Our findings indicate that females have 28.64% smaller sway amplitudes, 8.68% smaller stride lengths, and 8.14% slower speeds compared to males, with no significant difference in frequency. However, according to further investigation, our study reveals that the body parameters are the main variables that dominate gait features rather than gender. We have conducted three experiments in which the volunteers are walking towards the destination a) without any obstruction, b) with a stationary non-living obstacle present in the middle of the path, and c) with a human being standing in the middle of the path. From a comprehensive statistical analysis, key observations include no significant difference in gait features with respect to gender, no significant difference in gait features in the absence or presence of an obstacle, pedestrians treating stationary human beings and stationary obstacles the same given that the gender is same to match the comfort level, and a directional bias towards the left direction, likely influenced by left-hand traffic rule in India.

physics.soc-ph↗

arXiv2024

Tunability of Dissipative Particle Dynamics simulations for Excluded Volume and Hydrodynamic Interactions in polymer solutions and Rheological predictions

Even though the Dissipative Particle Dynamics (DPD) has shown its worth in a variety of research areas, it has been rarely used for polymer dynamics, particularly in dilute and semi-dilute conditions and under imposed flow fields. For such applications, the most popular technique has been Brownian dynamics (BD), even though the formulation of the same may be complicated for flow in complex geometries, which is straightforward for DPD. This is partly due to the flexibility of BD simulations to mimic any dynamic regime for polymer solutions by independently tuning hydrodynamic interactions (HI) and excluded volume (EV). In this study, we reveal that DPD also offers a similar flexibility and the regimes with respect to dominant EV and HI can be selected as conveniently as BD. This flexibility is achieved by tuning the repulsive interaction parameter of polymer beads and the spring length (which determines the chain resolution). Our results show that the former sets the chain size (and thus, EV) while the latter can be used to set the HI, nearly independently of each other. Thus, any rheological regime of certain level of EV and HI can be attained by appropriately tuning only these two parameters, providing a flexibility of similar levels as BD simulations. We further indicate the suitability of DPD by comparing rheological predictions with equivalent models in BD. For this, we imposed startup uniaxial extensional flows and steady shear flows on the system. Our results indicate the consistency of DPD with BD simulations, which is known to agree well with experiments.

cond-mat.soft↗

arXiv2022

Petascale Brownian dynamics simulations of highly resolved polymer chains with hydrodynamic interactions using modern GPUs

Brownian dynamics simulations of fairly long, highly detailed polymer chains, at the resolution of a single Kuhn step, remains computationally prohibitive even on the modern processors. This is especially true when the beads on the chain experience hydrodynamic interactions (HI), which requires the usage of methods like Cholesky decomposition of large matrices at every timestep. In this study, we perform Petascale BD simulations, with HI, of fairly long and highly resolved polymer chains on modern GPUs. Our results clearly highlight the inadequacies of the typical models that use beads connected by springs. In this manuscript, firstly, we present the details of a highly scalable, parallel hybrid code implemented on a GPU for BD simulations of chains resolved to a single Kuhn step. In this hybrid code using CUDA and MPI, we have incorporated HI using the Cholesky decomposition method. Next, we validate the GPU implementation extensively with theoretical expectations for polymer chains at equilibrium and in flow with results in the absence of HI. Further, our results in flow with HI show significantly different temporal variations of stretch, in both startup extensional and shear flows, relative to the conventional bead-spring models. In all cases investigated, the ensemble averaged chain stretch is much lower than bead-spring predictions. Also, quite remarkably, our GPU implementation shows a scaling of $\sim$$N^{1.2}$ and $\sim$$N^{2.2}$ of the computational times for shorter and longer chains in the most modern available GPU, respectively, which is significantly lower than the theoretically expected $\sim$$N^{3}$. We expect our methods and results to pave the way for further analysis of polymer physics in flow fields, with long and highly detailed chain models.

cond-mat.soft↗

arXiv2022

Formulation of an equivalent GNF model as an efficient approximation for flow of polymer solutions described by FENE-P

The molecular constitutive models, like FENE-P for polymer solutions, are known to have convergence issues at relatively larger flow rates. In this study, we investigate the possibility of a numerically efficient GNF-based approximation of FENE-P, which would closely approximate the flow field. Firstly, we compare the flow fields predicted by FENE-P and an equivalent GNF model. For these studies, we considered the flow around a sphere and selected the Carreau-Yasuda model as the representative GNF. This is made equivalent to the FENE-P by selecting parameters to equalize the viscosity-shear rate dependence. Our results show severe deficiencies of the GNF model, owing to its inability to account for chain stretching, particularly near the stagnation points. Next, the effect of extensional components on the local viscosity was added by formulating an equivalent GNF-X [Journal of Rheology 64, 493 (2020)] model. Even this failed to capture the asymmetry in the stress and flow profiles and predicted very large stresses at both stagnation points, relative to FENE-P. Hence, we proposed a novel modified formalism (denoted as GNF-XM) that was able to capture all trends successfully. The drag coefficients from GNF-XM agreed well with FENE-P predictions for all flow rates considered. Significantly, the computational times required to solve the flow field with GNF-XM is about an order of magnitude lower than that of FENE-P, especially at higher flow rates. Thus, we have successfully formulated a highly efficient GNF-based approximation to the FENE-P, whose formalism can be extended to other similarly complicated constitutive models.

physics.flu-dyn↗

arXiv2022

Effects of chain resolution on the configurational and rheological predictions from Brownian dynamics simulations of an isolated polymer chain in flow

A reasonably accurate representation of a polymer chain is provided by beads connected with rods, or stiff, inextensible springs that mimic a single Kuhn step. Due to high computational cost, coarse-grained bead-spring models are used in typical applications, where each spring is supposed to replace several Kuhn steps. Earlier investigations indicate that the BD simulation predictions of the steady state in different flows, with these different levels of discretization, are largely qualitatively similar. However, subtle quantitative differences exist even for the steady states. In this study, we perform a detailed analysis of the behavioral differences arising out of the varying degrees of chain discretization, ranging from one to several hundred Kuhn steps. We compare the transient and steady behavior of both configuration and rheological properties for a single chain in uniaxial extension and steady shear flow. Our analysis highlights differences, particularly in the stress and viscosity values, obtained at intermediate and high flow rates, between the bead-rod and bead-spring models. Such a thorough understanding helps to provide an estimate of the best possible bead-spring representation for an underlying polymer chain in a given application. Additionally, we also investigate the limit of break-down of the spring laws i.e. the minimum number of Kuhn steps that a spring can mimic faithfully.

cond-mat.soft↗

arXiv2021

Universal Nucleation Behaviour of Sheared Systems

Using molecular simulations and a modified Classical Nucleation Theory, we study the nucleation, under flow, of a variety of liquids: different water models, Lennard-Jones and hard sphere colloids. Our approach enables us to analyze a wide range of shear rates inaccessible to brute-force simulations. Our results reveal that the variation of the nucleation rate with shear is universal. A simplified version of the theory successfully captures the non-monotonic temperature dependence of the nucleation behavior, which is shown to originate from the violation of the Stokes-Einstein relation.

cond-mat.soft↗

arXiv2020

Seeding Method for Ice Nucleation under Shear

Hydrodynamic flow can have complex and far-reaching consequences on the rate of homogenous nucleation. We present a general formalism for calculating the nucleation rates of simply sheared systems. We have derived an extension to the conventional Classical Nucleation Theory, explicitly embodying the shear rate. Seeded Molecular Dynamics simulations form the backbone of our approach. The framework can be used for moderate supercoolings, at which temperatures brute-force methods are practically infeasible. The competing energetic and kinetic effects of shear arise naturally from the equations. We show how the theory can be used to identify shear regimes of ice nucleation behaviour for the mW water model, unifying disparate trends reported in the literature. At each temperature, we define a crossover shear rate in the limit of $1000-10,000 \ s^{-1}$, beyond which the nucleation rate increases steadily upto a maximum, at the optimal shear rate. For $235$, $240$, $255$ and $260 \ K$, the optimal shear rates are in the range of $\approx 10^6-10^7 \ s^{-1}$. For very high shear rates beyond $10^8 \ s^{-1}$, nucleation is strongly inhibited. Our results indicate that the shear-dependent nucleation rate curves have a non-monotonic dependence on temperature.

physics.comp-ph↗

arXiv2019

Dissipative particle dynamics simulations of a single isolated polymer chain in a dilute solution

In this study, we investigate the suitability of dissipative particle dynamics (DPD) simulations to predict the dynamics of polymer chains in dilute polymer solutions, where the chain is represented by a set of beads connected by almost inextensible springs. In terms of behaviour, these springs closely mimic rods that serve as representations of Kuhn steps. We find that the predictions depend on the value of the repulsive parameter for bead-bead pairwise interactions used in the DPD simulations ($a_{ij}$). For all systems, the chain sizes and the relaxation time spectrum are analyzed. For $a_{ij} = 0$, theta solvent behaviour is obtained for the chain size, whereas the dynamics at equilibrium agrees well with the predictions of the Zimm model. For higher values of $a_{ij}$, the static properties of the chain show good solvent behaviour. However, the scaling laws for the chain dynamics at equilibrium show wide variations, with consistent results obtained only at an intermediate value of $a_{ij} = 25$. At higher values of the repulsive parameter ($a_{ij} \geq 25$), our simulations are also able to predict the abrupt cut-off in the relaxation spectrum, which has been observed earlier in experiments of dilute solutions. The cut-off reached an extent that, for chain lengths of 10 Kuhn steps, the spectrum consists of a single time scale. This agrees remarkably well with earlier experiments and MD simulations. To verify further, we also studied the chain dynamics in shear flow using DPD simulations. Specifically, we analysed the variation of the chain stretch and end-over-end tumbling with shear rates. Overall, the trends obtained from DPD simulations agree well with those observed in earlier BD simulations.

cond-mat.soft↗