arXiv subjects

Chandan Bose

Publications and source records attributed to Chandan Bose.

12 recordsLinked to original sources

arXiv2026

Multi-objective Bayesian optimization of rigid and flexible nozzles for energy-efficient pulsed jet propulsion

The biomechanics of pulsed-jet propulsion in aquatic animals, including squids and jellyfish, provide valuable insights into energy-efficient locomotion. In these organisms, flexible funnel deformation enables rapid acceleration and maneuverability while minimizing energy use. Drawing inspiration from these biological systems, this study investigates performance trade-offs between rigid and flexible nozzle geometries in pulsed-jet propulsion systems. A multi-objective Bayesian optimization framework integrated with three-dimensional fluid-structure interaction (FSI) simulations identifies nozzle designs that maximize hydrodynamic impulse and minimize jet energy input. The optimization reveals fundamentally distinct performance characteristics for rigid and flexible nozzles. Rigid nozzles achieve the highest impulse amplification, up to 5 times that of a baseline cylindrical nozzle, but at substantially increased energy expenditure. In contrast, flexible nozzles yield lower peak impulse enhancement of about 2.5 times while achieving significantly greater propulsion efficiency. The maximum normalized impulse-to-energy ratio for flexible nozzles is about 1.8 times higher than that of rigid configurations, indicating more effective conversion of input energy into useful propulsive output. Analysis of the flow physics shows that optimized rigid nozzles enhance performance through geometry-induced internal entrainment, secondary vortex formation, and contraction-driven jet acceleration. This results in stronger vortex circulation and downstream convection. Flexible nozzles use traveling expansion-contraction deformation waves that promote additional entrainment during expansion and accelerate the internally entrained fluid during contraction to improve pressure recovery, reduce pressure-energy expenditure, and mitigate negative pressure impulse contributions.

physics.flu-dyn↗

arXiv2026

Computational and reduced-order modelling of elastic wave-driven impulse enhancement in pulsed jets through passively flexible nozzles

Elastic wave propagation and energy exchange in passively deforming cylindrical nozzles are investigated through three-dimensional, two-way fluid-structure interaction simulations. Flexible nozzles with varying stiffness (Eh = 75 to 500 N/m, E is Young's modulus, h is thickness) are subjected to pulsatile jet inflow at low Reynolds number (Re ~ 4400). Increased flexibility reduces deformation-wave speed following MoensKorteweg scaling, prolonging the expansion phase. This delayed expansion enhances jet entrainment and elastic energy storage while suppressing early shear-layer roll-up and vortex formation. During contraction, released elastic energy increases jet acceleration and vortex formation. For the most flexible nozzle, primary vortex-ring circulation increases by 52%, vortex convection distance by 9%, and peak outlet kinetic energy flux 4.6-fold versus a rigid nozzle, resulting in a 62% increase in total hydrodynamic impulse. A reduced-order model represents the coupled response as a lumped store-and-release oscillator, derived as a single-mode projection of the inviscid one-dimensional wave equation and closed at the exit by two terms: (i) an inertial end correction that adds the external fluid column of length Le = R accelerating with the jet, and (ii) a vortex-radiation damping term, active only during ejection, determined by the discharged-jet momentum theorem. This damping reproduces the post-overshoot velocity decay undamped closures fail to capture. The model predicts the simulated resonance frequency within 6% and momentum impulse within 4% across Eh = 75 to 500 N/m, and recovers energy histories. Outlet kinetic energy flux is predicted within 6% for the three stiffer nozzles and 14% for the most compliant.

physics.flu-dyn↗

arXiv2026

Squid-inspired soft superpropulsion

Squid span four orders of magnitude in size yet rely on pulsed jets. We show that the funnel (siphon) is a compliant nozzle whose dilation and recoil lag mantle contraction, storing and returning energy within each pulse, a mechanism we term superpropulsion. Histology reveals a collagen sheath, and chromatophore tracking in two squid species quantifies a repeatable phase lag. Engineered nozzles, 3D fluid-structure simulations, and a reduced-order mathematical model predict > 300% impulse amplification when nozzle response time matches jet acceleration (tau/T = 0.2-0.4), overlapping in vivo timing. Tuned nozzles extend jet reach, enhance plume dispersion, and improve jet-driven boat transport, with gains persisting after 40x miniaturization. Superpropulsion recasts pulsed jets as impedance matching, with a soft nozzle acting as an elastic capacitor that passively shapes impulse delivery in soft robotic thrusters and fluidic actuators.

physics.flu-dyn↗

arXiv2026

Effects of Plunging Acceleration on the Passive Morphing of Avian-Inspired Flexible Foils

This study investigates the dynamics of passively morphing foils under accelerated plunging, establishing mechanistic links between transient kinematics, structural compliance, and aerodynamic performance. Two-way coupled simulations are performed for three wing geometries: a symmetric NACA0012 foil and two bio-inspired geometries based on falcon and owl wing sections, across non-dimensional bending rigidity values, chordwise flexible segment extents from the trailing-edge (25%, 50%, and 75%), and transition speed parameters. The present findings reveal that flexible trailing-edge configurations exhibit improved aerodynamic performance relative to stiffer foils, and the aerodynamic benefit of trailing-edge compliance is strongly influenced by wing geometry. A geometry-specific optimal bending stiffness exists beyond which additional flexibility degrades performance. The extent of the chordwise flexible segment critically governs the aeroelastic response. Whilst a 25% flexible segment produces behaviour indistinguishable from a rigid wing, extending flexibility to 75% of the chord induces highly unsteady lift fluctuations, particularly for the NACA0012 foil, for which the root-mean-square lift coefficient increases sharply. The bio-inspired foils, in contrast, exhibit a moderate reduction in root-mean-square lift coefficient for the 50% and 75% cases, reflecting the stabilising influence of their cambered geometry. Increasing the transition speed parameter monotonically amplifies trailing-edge deflection, strengthens the leading- and trailing-edge vortices, and intensifies the coupling between structural deformation and instantaneous lift. These findings provide new physical insight into bio-inspired propulsion and manoeuvring strategies, with implications for the design of passively adaptive lifting surfaces in unsteady environments.

physics.flu-dyn↗

arXiv2026

Efficacy of the Weak Formulation of Sparse Nonlinear Identification in Predicting Vortex-Induced Vibrations

Vortex-induced vibrations (VIV) remain a canonical yet complex manifestation of fluid-structure interactions, where coupled nonlinear dynamics govern the motion of bluff bodies. For several years, we have relied on traditional reduced-order mathematical models derived from empirical and oscillator-based formulations; however, such models often fail to reproduce the quantitative dynamics observed in realistic flow environments. In this study, we explore a data-driven framework that leverages sparse identification of nonlinear dynamics (SINDy) and its weak formulation to uncover the governing equations of a single-degree-of-freedom cylinder undergoing VIV, using both data generated from previously developed reduced-order models and high-fidelity simulation results to assess the interpretation and efficacy of models discovered from a purely data-driven approach, particularly when the underlying dynamics are not fully known. The weak formulation (WSINDy), which replaces numerical differentiation with an integral-based representation, demonstrates marked robustness for aperiodic dynamics in particular. A complementary analysis using proper orthogonal decomposition (POD) is employed to extract the dominant spatio-temporal structures of the flow and to assess whether the temporal evolution of the wake can be represented on a reduced-dimensional manifold. The findings establish that data-driven identification can recover interpretable, quantitatively reliable models of VIV, providing a robust and computationally efficient pathway for modelling fluid-structure interactions directly from data. In particular, WSINDy is shown to be a more robust and interpretable alternative to standard SINDy for discovering VIV equations from aperiodic response dynamics, paving the way for predictive, data-informed design of fluid-structure interaction systems.

physics.flu-dyn↗

arXiv2024

Effect of sweep angle on three-dimensional vortex dynamics over plunging wings

The effects of sweep angle and reduced frequency on the leading-edge vortex (LEV) structure over flapping swept wings in the Reynolds number ($Re$) range of $\mathbf{O}(10^4)$ are yet to be completely understood. With increasing interest in designing bio-inspired micro-air-vehicles (MAVs), understanding LEV dynamics in such scenarios is imperative. This study investigates the effects of three different sweep angles ($\Lambda = 0^\circ$, $30^\circ$ and $60^\circ$) on LEV dynamics through high-fidelity improved delayed detached eddy simulation (IDDES) to analyze the underlying flow physics. Plunge ramp kinematics at two different reduced frequencies ($k = 0.05$ and $0.4$) are studied to investigate the unsteady motion effects on LEV characteristics. The leading-edge suction parameter (LESP) concept is applied to determine LEV initiation, and the results are verified against flow field visualization for swept-wing geometries. The force partitioning method (FPM) is used to investigate the spanwise lift distribution resulting from the LEV. Distinct peaks in the lift coefficient occur for the high reduced frequency case due to the impulse-like plunging acceleration. This causes the LEV to detach from the leading edge more quickly and convect faster, significantly affecting the lift generated by the wing. As reduced frequency increases, the LEV breakdown mechanism switches from vortex bursting to LEV leg-induced instabilities. These results provide insights into the complex vortex structures surrounding swept wings at $Re = 20,000$, and the impact both sweep angle and reduced frequency have on the lift contribution of these flow features.

physics.flu-dyn↗

arXiv2024

Effect of a fixed downstream cylinder on the flow-induced vibration of an elastically-supported primary cylinder

This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically-supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless centre-to-centre spacing ($L/D$, $L$ is the intermediate spacing, and $D$ is the cylinder diameter). The present two-dimensional (2D) simulations are carried out in the low Reynolds number ($Re$) regime. The primary focus of this study is to reveal the underlying flow physics behind the transition from vortex-induced vibration to galloping in the response of the primary cylinder due to the presence of another fixed downstream cylinder. Two distinct flow field regimes, namely steady flow and alternate attachment regimes, are observed for different $L/D$ and Re values. Depending on the evolution of the near-field flow structures, four different wake patterns - `2S', `2P', `2C', and `aperiodic' - are observed. The corresponding vibration response of the upstream cylinder is characterized as interference galloping and extended vortex-induced vibration. As the $L/D$ ratio increases, the lift enhancement due to flow-induced vibration is seen to be weakened. The detailed correlation between the force generation and the near-wake interactions is investigated. The present findings will augment the understanding of vibration reduction or flow-induced energy harvesting of tandem cylindrical structures.

physics.flu-dyn↗

arXiv2024

Effect of structural parameters on the synchronization characteristics in a stall-induced aeroelastic system

This study focuses on discerning the role of structural parameters on the bifurcation characteristics and the underlying synchronization mechanism in an aeroelastic system undergoing nonlinear stall behaviour. To that end, wind tunnel experiments are performed on a NACA 0012 airfoil capable of undergoing bending (plunging) and torsional (pitching) oscillations under scenarios involving nonlinear aerodynamic loads, i.e., dynamic stall conditions. Flow conditions under both deterministic/sterile flows and fluctuating/stochastic flows are fostered. The structure possesses continuous or polynomial-type stiffness nonlinearities, and therefore, is an aeroelastic experiment involving both structural and aerodynamic nonlinearities. We discern the bifurcation routes for a range of key structural parameters such as frequency ratio, static imbalance, and the extent of structural nonlinearity. In addition to interesting and atypical routes to stall-induced instabilities, we systematically demonstrate the role of modal interactions - via a synchronization analysis - over the manifestation of these instabilities. To the best of the authors' knowledge, this is perhaps the first study to document the role of multiple structural parameters on a stall-induced aeroelastic system, and in turn, cast the physical mechanism behind these dynamical transitions from the vantage of synchronization.

physics.flu-dyn↗

arXiv2024

Controlling the chaotic wake of a flapping foil by tuning its chordwise flexibility

Effects of chord-wise flexibility as an instrument to control chaotic transitions in the wake of a flexible flapping foil have been studied here using an immersed boundary method-based in-house fluid-structure-interaction solver. The ability of the flapping foil at an optimum level of flexibility to inhibit chaotic transition, otherwise encountered in a similar but rigid configuration, has been highlighted. The rigid foil manifests chaotic transition through a quasi-periodic-intermittency route at high dynamic plunge velocities; whereas, increasing the level of flexibility gradually regularises the aperiodic behaviour through a variety of interesting wake patterns. If flexibility is increased beyond an optimum level, aperiodicity sets in again and robust chaos is restored at very high flexibility levels. The mechanisms of triggering the order-to-chaos transition are different between the rigid and the high flexibility cases. Along the route to order and back to chaos, the flexible foil exhibits different flow-field behaviours, including far-wake switching, primary \& secondary vortex streets, bifurcated wakes and interactive vortices between the bifurcated wakes. The underlying interaction mechanisms of the flow-field vortices responsible for the associated dynamical signatures of the wake have been closely tracked. This study further examines the optimum propulsive performance range of the flexible flapper and investigates its connection with the periodicity/regularity of the system.

physics.flu-dyn↗

arXiv2023

Wake-induced response of vibro-impacting systems

The stability and bifurcation behaviour of a wake-induced vibro-impacting oscillator is studied. The effects of a discontinuity on the stability of the structure while it is undergoing phase-locked motions due to the surrounding fluid-structure interactions (FSI) are examined. The primary structure and the near wake dynamics are modelled as a harmonic oscillator and a Van der Pol oscillator, respectively, and are weakly coupled to each other via acceleration coupling. Qualitative changes in the dynamical behaviour of this system are investigated in the context of discontinuity-induced bifurcations (DIBs) that result from the interaction of fluid flow and non-smoothness from the primary structure. Phenomenological behaviours like the co-existence of attractors and period-adding cascades of limit cycles separated by chaotic orbits are observed. The existence of these phenomena is demonstrated via stability analysis using Floquet theory and the associated Lyapunov spectra. In addition, the behaviour of orbits in the local neighbourhood of the barrier is defined using a higher-order transverse discontinuity map. This mapping is implemented to obtain the respective Lyapunov exponents. Solutions obtained using this modified algorithm are demonstrated to accurately predict both stable and chaotic regimes, as observed from the corresponding bifurcation diagrams.

nlin.CD↗

arXiv2023

Effect of transverse gust on free-falling plates

The effects of transverse gusts on free-falling plates are investigated using two-way coupled fluid-structure interaction simulations for a Galilei number (Ga) between 10 and 50 and a density ratio (rho) between 5 and 50. We consider gust ratios (GR) of up to 5, where GR is the ratio of the free-stream velocity change to an estimate of the terminal velocity. We demonstrate that the plate experiences the gust as a transient horizontal force, which displaces it horizontally. This results in a transient reduction in the angle of attack, an increase in absolute velocity and the generation of circulation. The vertical component of the latter increases the upward aerodynamic force, slowing down the vertical descent of the plate. Furthermore, the plate's horizontal displacement with respect to its original wake results in a further transient increase in the upward aerodynamic force. The altitude gained by the plate in response to the gust is maximum for rho=15, and increases non-monotonically with Ga and GR. The non-monotonic trend is due to plate pitch: if the maximum pitch of the plate in response to the gust is close to vertical, the plate temporarily falls faster, losing some of the altitude it has gained. The present findings reveal an energy-harvesting mechanism that free-falling bodies can exploit to increase their time afloat.

physics.flu-dyn↗

arXiv2023

Porous plates at incidence

This paper investigates the effect of permeability on two-dimensional rectangular plates at incidences. The flow topology is investigated for Reynolds number ($Re$) values between 30 and 90, and the forces on the plate are discussed for $Re=30$, where the wake is found to be steady for any value of the Darcy number ($Da$) and the flow incidence ($\alpha$). At $Re=30$, for a plate normal to the stream and vanishing $Da$, the wake shows a vortex dipole with a stagnation point on the plate surface. With increasing $Da$, the separation between the vortex dipole and the plate increases; the vortex dipole shortens and is eventually annihilated at a critical $Da$. For any value of $Da$ below the critical one, the vortex dipole disappears with decreasing $\alpha$. However, at low $Da$, the two saddle-node pairs merge at the same $\alpha$, annihilating the dipole; while at high $Da$, they merge at different $\alpha$, resulting in a single recirculating region for intermediate incidences. The magnitudes of lift, drag, and torque decrease with $Da$. Nevertheless, there exists a range of $Da$ and $\alpha$, where the magnitude of the plate-wise force component increases with $Da$, driven by the shear on the plate's pressure side. Finally, the analysis of the fluid impulse suggests that the lift and drag reduction with $Da$ are associated with the weakening of the leading and trailing edge shear layer, respectively. The present findings will be directly beneficial in understanding the role of permeability on small porous wings.

physics.flu-dyn↗