21. Wake-induced response of vibro-impacting systems

Rohit Chawla, Aasifa Rounak, Chandan Bose, Vikram Pakrashi

https://doi.org/10.1063/5.0236147

This study examines what happens when a vibrating structure interacting with a fluid repeatedly collides with a rigid barrier. The motion of the structure and the swirling wake it generates are represented using simple mathematical oscillator models. When impacts occur, the system’s behaviour changes in unexpected ways: stable vibration patterns can suddenly shift, multiple motion states can coexist, and the dynamics can alternate between regular cycles and chaotic motion. By developing a specialised mathematical mapping technique, we accurately describe how motion evolves near the impact events and predict when the system remains stable or becomes chaotic. In practical terms, the work helps explain and anticipate complex vibrations in engineering systems where fluid forces and intermittent contact occur together, such as flexible components near walls or constraints.

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

Alex Cavanagh, Chandan Bose , Kiran Ramesh

https://doi.org/10.1063/5.0227012

This study explores how the shape of a wing and the speed of its flapping motion influence the formation of swirling air structures known as leading-edge vortices (LEVs), which play a key role in lift generation. Using detailed computer simulations, we analyze wings with different sweep angles and flapping speeds similar to those of small flying animals and micro-air vehicles. The results show that faster flapping creates sharp bursts of lift but also causes the vortices to detach and move away more quickly, altering how lift is produced. We also find that the way these vortices break down changes with motion speed, revealing different instability mechanisms. In simple terms, the study clarifies how wing geometry and flapping rhythm work together to control aerodynamic performance, offering useful guidance for designing more efficient bio-inspired flying devices.

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

Junlei Wang, Shenfang Li, Daniil Yurchenko, Hongjun Zhu, Chandan Bose

https://doi.org/10.1063/5.0207136

This study examines how placing a small fixed cylinder behind a vibrating cylinder changes the way fluid flow affects its motion. When fluid moves past a flexible structure, swirling vortices can cause regular oscillations known as vortex-induced vibration. However, the presence of a nearby downstream cylinder can significantly alter the flow, sometimes triggering a different, larger-amplitude motion called galloping. Using computer simulations, we show that the spacing between the cylinders controls how the wake patterns form and interact, which in turn determines whether vibrations are amplified, suppressed, or become irregular. In simple terms, the work explains how nearby structures reshape fluid forces and vibrations, offering insights useful for reducing unwanted oscillations or improving energy-harvesting devices in engineering systems such as offshore components, heat exchangers, and slender structural elements.

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

Chhote Lal Shah, Dipanjan Majumdar, Chandan Bose , Sunetra Sarkar

https://doi.org/10.1016/j.jfluidstructs.2024.104134

This study investigates how making a flapping foil slightly flexible can stabilise the complex flow patterns that form behind it. A rigid foil flapping at high speeds tends to produce irregular, chaotic wakes, leading to unpredictable forces and reduced efficiency. By introducing flexibility, the foil can passively adapt its shape, which smooths the wake and restores more regular, repeating flow structures. Interestingly, there is an optimal level of flexibility: too little flexibility allows chaos to persist, while too much flexibility brings chaotic behaviour back through different physical mechanisms. The results show that flexibility can act as a natural flow-control strategy, improving stability and propulsion performance. In simple terms, a well-tuned flexible flapper behaves more like a self-regulating system, offering useful design insights for bio-inspired propulsion, energy harvesters, and soft robotic swimmers.

17. Passive Suppression of Vortex-Induced Vibrations Using a Nonlinear Energy Sink - Numerical and Analytical Perspective

Abraham Thomas Chirathalattu*; Santhosh B; Chandan Bose , Rony Philip; Bipin Balaram

https://doi.org/10.1016/j.ymssp.2022.109556

This study investigates the suppression mechanism of instabilities induced by fluid-structure interactions (FSI) using passive vibration absorption devices, such as nonlinear energy sink (NES). The present FSI framework comprises a low-order phe- nomenological model, wherein the wake effect is modelled using the classical Van der Pol oscillator. The structure is represented as a cylindrical bluff body with degree-of- freedom along the cross-flow direction. The response of the NES-augmented struc- ture exhibits specific relaxation type oscillations, referred to as strongly modulated response (SMR), passively suppressing the high amplitude vortex-induced vibrations (VIV). The underlying mechanism of SMR is studied using an analytical approach based on the Complexification-Averaging (CXA) technique.

16. Chordwise flexible aft-tail suppresses jet-switching by reinstating wake periodicity in a flapping foil

Chhote Lal Shah, Dipanjan Majumdar, Chandan Bose , Sunetra Sarkar

https://doi.org/10.1017/jfm.2022.591

This study explores how adding a flexible rear section to a flapping wing changes the flow patterns it produces. A wing with a fully rigid tail tends to generate unstable wakes, where the jet of air behind it repeatedly switches direction and can even become chaotic. When a flexible tail is introduced, the wing can passively adapt its shape during motion, which stabilises the wake and suppresses this jet-switching behaviour. With sufficient flexibility, the wake becomes more symmetric and regular, indicating smoother, more predictable aerodynamic forces. The results show that structural flexibility can act as a natural stabilising mechanism, helping control complex flow behaviour. In simple terms, a well-designed flexible tail allows the wing to “self-adjust,” improving flow stability and offering useful guidance for designing efficient bio-inspired flyers and fluid-driven propulsion systems.

15. Transition boundaries and an order-to-chaos map for the flow-field past a flapping foil

Dipanjan Majumdar, Chandan Bose , Sunetra Sarkar

https://doi.org/10.1017/jfm.2022.385

This study develops a dynamical transition framework for the unsteady wake of a flapping foil at low Reynolds numbers, presenting an order-to-chaos map derived from a broad kinematic parameter space. The analysis shows that conventional nondimensional groups fail to consistently characterise wake transitions across varying motion parameters. To address this limitation, two physically grounded measures—the maximum effective angle of attack and a leading-edge amplitude-based Strouhal number—are introduced. These parameters successfully collapse diverse datasets and enable generalised transition boundaries separating periodic, quasi-periodic, and chaotic regimes. Validation against published results demonstrates strong agreement, supporting the robustness and predictive utility of the proposed transition map for flapping-foil dynamics.

14. Numerical investigation of discontinuity-induced bifurcations in an aeroelastic system with coupled non-smooth nonlinearities

Sai Vishal Gali*, Ashwad Raaj*, Chandan Bose , Venkatramani Jagadish, Grigorios Dimitriadis

https://link.springer.com/article/10.1007/s11071-022-07352-3

The present study focuses on investigating the bifurcation characteristics of a pitch-plunge aeroelastic system possessing coupled non-smooth nonlinearities, both in structural and aerodynamic fronts. At low airspeeds, the dynamical transitions occur predominantly due to the structural freeplay nonlinearity while the flow remains attached to the surface of the wing. However, beyond a critical value of airspeed, the system response is dominated by high amplitude pitch-dominated limit-cycle oscillations, which can be attributed to stall flutter. It is demonstrated that the freeplay gap plays a key role in combining the effects of structural and aerodynamic nonlinearities. At higher values of the freeplay gap, interesting discontinuity-induced bifurcation scenarios, such as grazing and boundary equilibrium bifurcations arise due to coupled nonlinear interactions, which can significantly impact the safety of the aeroelastic system.

13. Experimental Investigation on the Synchronization Characteristics of a Pitch-Plunge Aeroelastic System Exhibiting Stall Flutter

Dheeraj Tripathi*, R. Shreenivas*, Chandan Bose , Sirshendu Mondal, J Venkatramani

https://doi.org/10.1063/5.0096213

This study focuses on characterizing the bifurcation scenario and the underlying synchrony behaviour in a nonlinear aeroelastic system under deterministic as well as stochastic inflow conditions. Wind tunnel experiments arecarried out for a canonical pitch-plunge aeroelastic system subjected to dynamic stall conditions. We observe intermittent phase synchronization between pitch and plunge modes near the fold point; whereas, synchronization via phase trappingis observed near the Hopf point. Repeating the experiments under stochastic inflow conditions, we observe two different aeroelastic responses; low amplitude noise-induced random oscillations (NIROs) and high amplitude random LCOs (RLCOs) during stall flutter. The present study shows asynchrony between pitch and plunge modesin the NIRO regime. At the onset of RLCOs, asynchrony persists even though the relative phase distribution changes. With the further increase in flow velocity, we observe intermittent phase synchronization in the flutter regime.

12. Stall-induced fatigue damage analysis of nonlinear aeroelastic systems subjected to randomly fluctuating inflow

Dheeraj Tripathi*, Sai Vishal*, Chandan Bose , J Venkatramani

https://doi.org/10.1016/j.ijnonlinmec.2022.104003

This study focuses on characterizing the fatigue damage accumulated in nonlinear aeroelastic systems subjected to stochastic input flows. The response dynamics and the associated fatigue damage of the aeroelastic system, possessing different sources of nonlinearities, are systematically investigated under isolated cases of deterministic and stochastic input flows. It is observed that different time scales and intensities of the oncoming flow fluctuation play a pivotal role in dictating the fatigue damage in aeroelastic systems. Fatigue damage is observed to be significantly higher for torsionally dominant oscillations in the dynamical stall regime compared to the oscillations at the attached flow regime.

11. Routes to Synchronization in a pitch-plunge aeroelastic system with coupled structural and aerodynamic nonlinearities

Sai Vishal Gali*, Ashwad Raaj*, Chandan Bose , Venkatramani Jagadish

https://doi.org/10.1016/j.ijnonlinmec.2021.103766

In this study, we investigate the response characteristics of a pitch-plunge aeroelastic system possessing coupled non-smooth nonlinearities, namely free-play in the structure and dynamic stall in the flow. We demonstrate the need to present a one-one relationship between the dynamic stall behavior with respect to the output aeroelastic response. We systematically demonstrate the different routes to synchronization in the nonlinear aeroelastic problem and use the same to heuristically demarcate coupled mode flutter against stall flutter.

10. Enhancing Piezoelectric Energy Harvesting from the Flow-Induced Vibration of a Circular Cylinder Using Dual Splitters

Junlei Wang; Shanghao Gu; Abdessattar Abdelkefi; Chandan Bose

https://doi.org/10.1088/1361-665X/abefb5

This study is focused on enhancing the piezoelectric energy harvesting from the flow-induced vibration of a circular cylinder by using two symmetric splitters in different relative angular positions with respect to the oncoming uniform flow. Both wind tunnel experiments and numerical simulations are carried out to study the effect of different installation angles of the dual splitters on the energy harvesting efficiency with the increasing flow velocity. This study systematically carries out the performance analysis of the vortex-induced vibration-based energy harvester with multiple splitters and directly contributes to the optimized design of an innovative wind energy harvester.

9. Dynamic interlinking between near and far-field wakes behind a pitching-plunging airfoil

Chandan Bose ; Sayan Gupta; Sunetra Sarkar

https://doi.org/10.1063/5.0024084

This study explores the transitional wake dynamics of a simultaneously pitching–heaving airfoil at low Reynolds numbers, focusing on the largely unexplored loss of stability of deflected reverse Kármán wakes at high dynamic heave velocities (κh). While classical transitions from Kármán to reverse Kármán patterns occur within periodic regimes, the work shows that further increases in κh induce quasi-periodic behaviour in the near-field, accompanied by intermittent aperiodic windows. These instabilities trigger far-field jet deflection switching through vortex-couple flipping, ultimately leading to fully chaotic wake dynamics via rapid aperiodic jet-switching. The analysis establishes a dynamic link between near-field vortex interactions, leading-edge vortex behaviour, and far-field wake transitions, using nonlinear dynamical diagnostics to reveal the mechanisms governing wake deflection and chaos onset.

8. Effect of gusty inflow on the jet-switching characteristics of a plunging foil

Dipanjan Majumdar; Chandan Bose ; Sunetra Sarkar

https://doi.org/10.1063/5.0024084

This study investigates how stochastic inflow fluctuations influence jet-switching dynamics in the wake of a harmonically plunging elliptic foil at low Reynolds numbers. The inflow disturbances are modelled using an Ornstein–Uhlenbeck process, enabling controlled variation of fluctuation intensity and timescales. While jet-switching in the deterministic case arises from quasi-periodic vortex motion at higher dynamic plunge velocities, the introduction of fluctuations disrupts the organized vortex street and triggers earlier switching even at lower κh. The analysis further shows increased switching frequency and larger wake deflection angles, with inflow correlation length governing the switching characteristics. Combined qualitative flow visualisation and quantitative diagnostics clarify the mechanisms by which stochastic forcing modifies vortex interactions and wake dynamics.

7. Capturing the Dynamical Transitions in the Flow-field of a Flapping Foil using Immersed Boundary Method

Dipanjan Majumdar; Chandan Bose ; Sunetra Sarkar

https://doi.org/10.1016/j.jfluidstructs.2020.102999

This study examines the flow-physics mechanisms governing the transition from periodic to chaotic wake dynamics around a plunging foil as the nondimensional plunge velocity increases. High-resolution simulations using an in-house discrete-forcing immersed boundary method (IBM) reveal a quasi-periodic route to chaos, with detailed analysis across distinct dynamical regimes. The IBM predictions are systematically benchmarked against a well-validated body-fitted arbitrary Lagrangian–Eulerian (ALE) formulation, demonstrating strong qualitative and quantitative agreement. The results highlight the capability of non-conformal mesh approaches to capture complex, aperiodic flow topologies, while showing that distributed momentum forcing and mass/source–sink treatments within the IBM framework accurately reproduce the unsteady vortex interactions responsible for triggering and sustaining aperiodicity.

6. Transition to chaos in the flow-induced vibration of a pitching-plunging airfoil at low Reynolds numbers : Ruelle-Takens-Newhouse scenario

Chandan Bose ; Sayan Gupta; Sunetra Sarkar

DOI: 10.1016/j.ijnonlinmec.2018.11.012

This work numerically investigates the transition from periodic to chaotic behaviour in the coupled fluid–elastic response of a two-degree-of-freedom flexibly mounted airfoil with chordwise rigidity. A high-fidelity Navier–Stokes solver is weakly coupled to a geometrically nonlinear structural model incorporating cubic stiffness terms, representing lightweight aeroelastic systems at low Reynolds numbers and very low added-mass ratios typical of micro air vehicles. Through bifurcation analysis using wind velocity as the control parameter, the study identifies the Ruelle–Takens–Newhouse quasi-periodic route to chaos—reported here for the first time in a flexible pitch–plunge configuration. Nonlinear time-series diagnostics are employed to rigorously characterise the evolving dynamical states and underlying aeroelastic mechanisms.

5. Investigating chaotic wake dynamics past a flapping airfoil and the role of vortex interactions behind the chaotic transition

Chandan Bose ; Sunetra Sarkar

DOI: 10.1063/1.5019442

This study examines how the wake of a symmetric NACA0012 airfoil evolves under coupled pitching–plunging motion at low Reynolds numbers, revealing a clear transition from periodic to chaotic dynamics as plunge amplitude increases. The work identifies leading-edge separation as the initial trigger for aperiodicity, followed by nonlinear vortex processes—shredding, merging, splitting, and collision—that generate spontaneous vortex pairs and sustain chaos. By interpreting the wake as a nonlinear dynamical system and employing both classical metrics and recurrence-based time-series tools, the study provides a mechanistic, quantitative framework linking vortex interactions to flow transition, demonstrating the effectiveness of non-classical dynamical analysis for flapping-flight aerodynamics.

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4. Dynamical behaviour of unsteady flow-field of an elastically mounted flapping airfoil

Chandan Bose ; Sayan Gupta; Sunetra Sarkar

DOI: 10.2514/1.J056664

This study characterises the emergence of quasi-periodic wake dynamics behind a spanwise flexible flapping airfoil, demonstrating that beyond a critical wind velocity the trailing-edge wake transitions from periodic to quasi-periodic behaviour in a regime of comparable structural and fluid added mass. The quasi-periodicity is established using complementary diagnostics, including vorticity-field correlations, reconstructed phase space, frequency spectra, Poincaré sections, and recurrence analysis of aerodynamic loads. The loss of strict periodicity is shown to induce pronounced variability in unsteady forces, highlighting the dynamical consequences of fluid–structure coupling and indicating that feedback control strategies may be necessary to ensure stable operation in such regimes.

3. Transient and stable chaos in dipteran flight inspired flapping flight

Chandan Bose ; Vikas Reddy; Sayan Gupta; Sunetra Sarkar

DOI: 10.1115/1.4038447

This paper deals with the nonlinear fluid structure interaction (FSI) dynamics of a Dipteran flight motor inspired flapping system in an inviscid fluid. In the present study, the FSI effects are incorporated to an existing forced Duffing oscillator model to gain a clear understanding of the nonlinear dynamical behavior of the system in the presence of aerodynamic loads. The present FSI framework employs a potential flow solver to determine the aerodynamic loads. Interesting dynamical behavior including period doubling, chaotic transients, periodic windows, and finally an intermittent transition to stable chaotic attractor have been observed in the response with an increase in the bifurcation parameter.

2. Investigations on a vortex induced vibration based energy harvester

S Krishna Kumar; Chandan Bose ; Shaikh Faruque Ali; Sayan Gupta; Sunetra Sarkar

DOI: 10.1063/1.5001863

This study investigates energy harvesting from vortex induced vibrations of a flexible cantilevered flapper placed in the wake of a rigid circular cylinder. The effect of the gap between the cylinder and the flapper on the energy harvested is investigated through wind tunnel experiments and numerical simulations. As the flow speed is varied, a transition in the flapper dynamics is observed, which in turn affects the power extracted by the harvester. Numerical investigations reveal that the flapper dynamics is different depending on whether the vortices are shed ahead or behind the flapper. This study concludes that the choice of the gap influences the energy harvesting potential of such harvesters.

1. Identifying the route to chaos in the flow past a flapping airfoil

Sandeep Badrinath; Chandan Bose; Sunetra Sarkar

DOI: 10.1016/j.euromechflu.2017.05.012

This work investigates the emergence of chaotic wake dynamics behind flapping airfoils, focusing on the transition mechanisms at high plunge velocities. Using an incompressible Navier–Stokes framework and bifurcation analysis with plunge amplitude as the control parameter, the study shows that initially periodic vortex shedding is disrupted by intermittent chaotic bursts whose frequency grows with amplitude, ultimately leading to fully developed chaos. The transition is rigorously characterised using nonlinear time-series tools, including phase-space reconstruction and recurrence plots, which establish a Type I intermittency route to chaos—an observation not previously reported for flapping-airfoil flows. The analysis further links intermittency-driven vortex interactions to variations in thrust production, while recurrence-based quantitative measures reveal early signatures of the impending chaotic regime.