arXiv subjects

Muhammad Saif Ullah Khalid

Publications and source records attributed to Muhammad Saif Ullah Khalid.

At least 19 recordsLinked to original sources

arXiv2026

A Fully Parallel Dual-Grid Immersed-Boundary Framework for Flow-Induced Sound from Complex Moving and Deforming Bodies

Predicting flow-induced sound from moving and deforming bodies is computationally demanding because the near-field hydrodynamics and the far-field acoustics require substantially different spatial resolutions and domain extents. A fully parallel hybrid framework is developed to address this disparity by coupling an incompressible Navier-Stokes solver to an acoustic perturbation equation (APE) solver on independently generated, non-conforming Cartesian grids. A sharp-interface ghost-cell immersed boundary method, with radial-basis-function reconstruction, imposes the boundary conditions for complex moving geometries on both grids. The converged flow field supplies the acoustic source through a one-way, precomputed parallel interpolation operator. This arrangement confines the flow grid to the body and wake while allowing the acoustic grid to extend independently into the far field. The framework is validated for Gaussian-pulse propagation, pulse scattering by a rigid cylinder, tonal sound from flow past a cylinder, and radiation from a traveling wavy foil. The predicted waveforms, wavelengths, pressure amplitudes, and radiation patterns agree closely with analytical solutions and published reference data. Applications to eel and Jack fish locomotion, a four-eel school, a manta ray, and a harbor seal further demonstrate the treatment of realistic three-dimensional morphologies, large boundary deformation, and multiple interacting swimmers. The results resolve morphology-dependent acoustic signatures and interference-driven changes in far-field directivity without requiring the flow grid to span the acoustic far field.

physics.flu-dyn↗

arXiv2026

How do flapping avian wings exhibit superior aerodynamic performance?

This work investigates the unsteady aerodynamic performance and vortex dynamics of avian-inspired flapping wings using our in-house sharp-interface immersed-boundary solver, VorteXdyn. A falcon-inspired body-wing model based on NACA 4312 profile is employed to examine aerodynamic force generation and vortex evolution during steady forward flapping flight at Reynolds numbers of 2,500, 5000, and 10000 and Strouhal numbers of 0.18, 0.225, and 0.27. The influence of physiologies is examined using three wing configurations: a simplified wing without distinct feather structures, a wing incorporating feather-like structures (serrations) along the trailing edge, and a geometrically detailed wing incorporating multiple feather layers consisting of primary, secondary, and median feathers over its span. The aerodynamic performance of these configurations is quantified using the temporal profiles and time-averaged characteristics of the lift and drag coefficients and the lift-to-drag ratio. The associated vortex dynamics are characterized through the formation and evolution of multiple leading-edge vortices (LEVs), their spanwise coherence, circulation, characteristic size, and persistence over the wings' surfaces. Particular emphasis is placed on the spanwise development of the LEVs from the root to the wingtip, their interactions with the tip vortices, and the resulting wake evolution during the downstroke. Our results demonstrate that increasing geometric fidelity modifies aerodynamic force production, the formation and evolution of multiple LEV structures, vortex-vortex and votex-wing interactions, and wake topology. These findings provide insight into the aerodynamic role of feather morphology in three-dimensional flapping flight for the aerodynamic design of efficient bio-inspired flapping-wing micro air vehicles.

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arXiv2026

Wing-Rotor Aerodynamic Interactions in Small UAVs During Hover and Cruise

A compact vertical take-off and landing aircraft requires the same tilt-rotor configuration to perform two fundamentally different aerodynamic tasks: sustain hover and deliver efficient cruise. This work investigates the underlying wing-rotor interactions in both operating regimes using a validated unsteady Reynolds-averaged Navier-Stokes equations-based computational framework. For cruise, the advance ratio governs the balance between thrust production, propulsive efficiency, and wake coherence. Lower advance ratios produce a tightly wound slipstream that undergoes strong vortex interactions, leapfrogging, and early wake bifurcation. At higher advance ratios, the slipstream retains a narrower and more coherent jet-like structure, improving propulsive efficiency while reducing both thrust of the propellor and lift of the wing. The flow impingement at the wing is characterized through the approaching, interaction, and convection phases, revealing the combined influence of vortex stretching, wake bifurcation, blockage, image-induced velocity, and streamwise momentum convection on the downstream wake. In hover, the propeller's rotational speed governs the overall aerodynamic performance more strongly than the wing's placement. Although the position of the wing with respect to the propellor modifies the local wake interactions and flow impingement on its leading edge, its influence on the integrated thrust coefficient and figure of merit remains limited. We also explain the vortex and wake dynamics around the propellor and the wing responsible for governing these aerodynamic performance.

physics.flu-dyn↗

arXiv2026

Optimized Fish Locomotion using Design-by-Morphing and Bayesian Optimization

Nature has always inspired scientists and engineers to understand the underlying mechanism leading to optimal design in bio-inspired dynamics. This study presents a computational framework for optimizing undulatory swimming profiles using a combination of Design-by-Morphing and Bayesian optimization strategies. The swimming profile are expressed by morphing five baseline bio-inspired profiles using Design-by-Morphing to create an exploratory design space. The optimization objective is to find the optimal swimming profile, wavelength and undulation frequency to maximize propulsive efficiency. The optimized swimming profiles demonstrate a marked improvement in propulsive efficiency relative to the reference anguilliform and carangiform modes. The best-performing optimized cases achieve peak efficiencies in the range of 49-57\% over a broad range of kinematic conditions, representing an overall enhancement of 16-35\% compared to reference anguilliform and carangiform modes. The improved performance is attributed to favorable surface stress distributions and enhanced energy recovery mechanisms. A detailed force decomposition reveals that the optimal swimmer minimizes resistive drag and maximizes constructive work contributions, particularly in the anterior and posterior body regions. Spatial and temporal work decomposition indicates a strategic redistribution of input and recovered energy, enhancing performance while reducing energetic cost relative to propulsive force. These findings demonstrate that morphing-based parametric design, when guided by surrogate-assisted optimization, offers a powerful framework for discovering energetically efficient swimming gaits, with significant implications for the design of autonomous underwater propulsion systems and the broader field of bio-inspired locomotion.

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arXiv2026

Design Implications of Chord Length and Number of Blades on Self-Starting Process in Vertical-Axis Wind Turbines

Self-starting remains a key limitation of lift-driven vertical-axis wind turbines and is strongly influenced by geometric design choices that also govern steady-state performance. This work quantifies the roles of chord length and blade number on startup dynamics and the attained steady tip-speed ratio using two-dimensional URANS simulations of freely rotating Darrieus-type rotors. Two configuration families are examined, an equal-chord set in which three and five bladed turbines share the same chord length, and an equal-solidity set in which the chord length is reduced for the five blade turbines to match solidity with the three blade counterparts. Results are analyzed using the time evolution of tip-speed ratio, reduced-frequency measures to identify sustained unsteady intervals, vorticity-field diagnostics of dynamic stall vortex formation and detachment, and a torque decomposition into pressure and viscous moments. The results show that increasing number of blades can enhance early stage acceleration but generally lowers the steady tip-speed ratio by intensifying blade-vortex interaction in the downstream half cycle. Increasing chord length promotes self-starting by strengthening unsteady loading during the transition out of the low-speed regime, but also increases viscous losses and wake interaction, leading to lower the steady tip-speed ratio for self-starting high chord configurations. The role of viscous moments is also analyzed to quantify their contribution to self-starting behavior and to assess their influence on limiting the attainable operating state after self-starting. These findings provide design-relevant guidance on the startup--performance trade-off associated with the chord length and number of blades in freely accelerating vertical-axis wind turbines.

physics.flu-dyn↗

arXiv2026

Dynamical Characteristics of the Body-Caudal Fin Joint of a Carangiform Swimmer and its Influence on Hydrodynamics

The hydrodynamics of fish swimming depend on the interaction between the undulation of the body and the flapping of the caudal fin. This study develops a computational framework of a Jackfish-inspired swimmer with an independently mounted caudal fin that pitches passively under fluid forces and a nonlinear torsional spring. The fin synchronizes with the body when damping and stiffness parameters are tuned correctly, producing passive pitching that closely resembles to the displacement of the actively pitching tail. At Re = 3000, synchronized passive pitching generates coherent hairpin and ring vortices that reinforce streamwise momentum and contribute to thrust, whereas larger phase differences lead to wake spread in lateral direction and drag-dominated behavior. These results reveal that nonlinear peduncle mechanics naturally regulate amplitude, phase, and recoil, offering a biologically inspired pathway toward underwater robotic design using passive kinematics.

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arXiv2025

Vortex Dynamics from Burst-and-Coast Motion of Anguilliform and Carangiform Swimmers

Fish perform various propulsive maneuvers while swimming by generating traveling waves along their bodies and producing thrust through tail strokes. Anguilliform swimmers spread motion along the body, while carangiform swimmers' motion is more prominent near their tails. Many species also switch between continuous undulation and intermittent swimming, such as burst-and-coast maneuver, which can save energy but can also change the wake structure and hydrodynamic forces. Our current study aims at explaining} how duty cycle (DC), undulatory gaits, and Strouhal number (St), shape the near-body vortices, overall wakes, and the hydrodynamic forces. We carry out three-dimensional simulations at Re = 3000 for flows around an eel (anguilliform) and a Jack Fish (carangiform) for DC = 0.2-1.0 and St = 0.30 and 0.40. Our results reveal that the burst-and-coast motion for both swimmer produce bow-shaped wakes, the two rows of which on the sides approach each other to form a more coherent wake as DC is increased to 1.0 that corresponds to the wake of continuously undulating swimmers. It is also found that the intermittent motion at a higher Strouhal number produces more drag, contrary to the continuous undulatory kinematics. We further investigate this behavior by quantifying the strengths of vortices produced around the two swimmers and their instantaneous kinematic metrics. A detailed analysis for the role of different body sections in the production of unsteady streamwise forces is also presented. These insights provide important connections between the swimmers' physiologies, their kinematics, and the governing vortex dynamics to attain certain hydrodynamic metrics for designing next-generation autonomous bio-inspired underwater robots.

physics.flu-dyn↗

arXiv2025

Comparative Investigations on Active and Passive Tails of Undulating Swimmers

Fish display remarkable swimming capabilities through the coordinated interaction of the body and caudal fin, yet the potential role of a passively pitching tail in enhancing hydrodynamic performance remains unresolved. In this work, we evaluate the performance of a carangiform swimmer equipped with either an actively pitching tail or a passively pitching tail. Fluid-structure interactions-based simulations are employed to asses how variations in joint stiffness, damping, and inertia influence thrust generation, power demand, and overall stability at two representative Reynolds numbers of 500 and 5000. The results reveal that actively pitching tails tend to generate greater thrust, while passively pitching tails deliver improved outcomes in terms of the power demand at the lower Reynolds number. Larger pitching amplitudes contribute positively only when associated with higher swimming frequency, when produced by reduced inertia for more flexible joints, they lead to unfavorable effects. At the higher Reynolds number, active tails consistently outperform passive ones, although a small subset of passive cases still achieve favorable performance. Across all cases, a recurring balance emerges, with thrust production and power expenditure varying inversely. These findings clarify the hydrodynamic consequences of passive versus active tail motion and establish design principles for bio-inspired underwater vehicles, where smaller swimmers may benefit from passive tail pitching, while larger swimmers are better served by active control

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arXiv2025

Characterizing the Role of Hind Flippers in Hydrodynamics of A Harbor Seal

In this paper, we investigate the hydrodynamic characteristics of harbor seal locomotion, focusing on the role of hind flippers in thrust generation and wake dynamics. Through three-dimensional numerical simulations using an immersed boundary method at Reynolds number of 3000, we analyze the impact of varying Strouhal number (St = 0.2-0.35) and propulsive wavelength ($λ^\ast = 1.0-1.2$) on swimming performance. Our findings reveal two distinct wake patterns: a single-row structure at lower Strouhal numbers ($St \leq 0.25$) and a double-row configuration at higher St ($St \geq 0.3$). Increasing wavelength generally enhances thrust production by reducing both pressure and friction of drag components. Additionally, we identify critical vortex interactions between the front and hind flippers, with destructive interference occurring at lower St and constructive patterns emerging at higher St. Circulation analysis confirms stronger vortex formation at higher St and $λ^\ast$}, particularly during the left stroke phase. These results provide novel insights into the hydrodynamic mechanisms underlying seal locomotion and contribute to our understanding of efficient aquatic propulsion systems.

physics.flu-dyn↗

arXiv2025

On the role of morphology and kinematics of biological swimmers to spread and suppress their odors in the wake

Understanding the interplay between hydrodynamics and chemical sensing in aquatic environments is crucial for unraveling biological swimmers' navigation, foraging, and communication strategies. This study investigates the role of kinematics and morphologies of fish in dispersion and suppression of odor cues in their wake. We employ high-fidelity three-dimensional computational fluid dynamics simulations, integrating a sharp-interface immersed-boundary method with an odor transport model. Using carangiform and anguilliform kinematics for a jackfish and an eel, we analyze the transport of chemical cues in the wake of undulatory swimmers at a Reynolds number of 3000 and Strouhal numbers of 0.25 and 0.4. Our findings reveal that odor plumes closely align with vortex structures, emphasizing a strong coupling between hydrodynamics and chemical dispersion. We demonstrate that kinematics, rather than morphology, predominantly govern odor transport, with anguilliform motion generating broader, more persistent odor trails. Increasing the amplitude of undulation improves the effectiveness of the odor, driven primarily by convection, while diffusion plays a secondary role. These insights provide a deeper understanding of underwater sensing mechanisms and inform the design of bio-inspired robotic systems with improved navigation and chemical detection capabilities.

physics.flu-dyn↗

arXiv2024

Enhancing Thrust in Flapping Airfoils Through Wake Interactions with Oscillating Cylinder

Inspired by the natural motion of insects, fish, and other animals, flapping airfoils have gained significant importance due to their applications in fields such as ship propulsion, micro aerial vehicles, and autonomous underwater vehicles. Over the past two decades, extensive research has focused on understanding the dynamics of these airfoils, their thrust production capabilities, and methods to enhance this thrust in unsteady flows. This study investigates how the presence of a cylinder oscillating due to incoming flow affects the thrust performance of the flapping airfoil. The results indicate that the flapping airfoil generates increased thrust when placed in the wake of the oscillating cylinder compared to a scenario without the cylinder's wake. A direct relationship has been found between the strouhal number ($St$) of the flapping airfoil and its pitching amplitude, which significantly influences the airfoil's performance. This analysis highlights the potential for optimizing flapping airfoil efficiency through strategic selection of flapping parameters and the placement of an oscillating cylinder.

physics.flu-dyn↗

arXiv2024

How does vortex dynamics help undulating bodies spread odor?

In this paper, we examine the coupling between odor dynamics and vortex dynamics around undulating bodies, with a focus on bio-inspired propulsion mechanisms. Utilizing computational fluid dynamics (CFD) simulations with an in-house Immersed-Boundary Method (IBM) solver, we investigate how different waveform patterns, specifically carangiform and anguilliform, influence the dispersion of chemical cues in both water and air environments. Our findings reveal that vortex dynamics significantly impact the overall trajectory of odor spots, although the alignment between odor spots and coherent flow structures is not always precise. We also evaluate the relative contributions of diffusion and convection in odor transport, showing that convection dominates in water, driven by higher Schmidt numbers, while diffusion plays a more prominent role alongside convection in air. Additionally, the anguilliform waveform generally produces stronger and farther-reaching chemical cues compared to carangiform swimmers. The critical roles of Strouhal number and Reynolds number in determining the efficiency of odor dispersion are also explained, offering insights that could enhance the design of more efficient, adaptive, and intelligent autonomous underwater vehicles (AUVs) by integrating sensory and hydrodynamic principles inspired by fish locomotion.

physics.flu-dyn↗

arXiv2023

Physics-Informed Scaling Laws for the Performance of Pitching Foils in Schooling Configurations

This study introduces novel physics-based scaling laws to estimate the propulsive performance of synchronously pitching foils in various schooling configurations at Re=4000. These relations are derived from quasi-steady lift-based and added mass forces. Hydrodynamic interactions among the schooling foils are considered through vortex-induced velocities imposed on them, constituting the ground effect. Generalized scaling equations are formulated for cycle-averaged coefficients of thrust and power. These equations encompass both the pure-pitching and induced velocity terms, capturing their combined effects. The equations are compared to computational results obtained from two-foils systems, exhibiting foil arrangements over a wide range of parameter space, including Strouhal number (0.15 \leq St \leq 0.4), pitching amplitude (5 deg \leq θ_0 \leq 14 deg), and phase difference (0 deg \leq ϕ\leq 180 deg). The individual contributions of pure-pitching and induced velocity terms to propulsive performance elucidate that solely relying on the pure-pitching terms leads to inadequate estimation, emphasizing the significance of the induced velocity terms. Validity of the approach is further assessed by testing it with a three-foil configuration, which displays a collapse. This indicates that the scaling laws are not only applicable to two-foils systems but also extend their effectiveness to multi-foil arrangements.

physics.flu-dyn↗

arXiv2023

On the Association of Kinematics, Spanwise Instability and Growth of Secondary Vortex Structures in the Wake of Oscillating Foils

Three-dimensional wake of an oscillating foil with combined heaving and pitching motion is numerically evaluated at a range of chord-based Strouhal number (0.32 \le Stc \le 0.56) and phase offset (90 deg \le ϕ\le 70 deg) at Re = 8000. The changes in ϕand Stc reflect a unique route of transition in mechanisms that govern the origin of spanwise instabilities and growth of secondary wake structures. At lower Stc, heave dominated kinematics demonstrates a strong secondary leading edge vortex (LEV ) as the source of growing spanwise instability on the primary LEV , followed by an outflux of streamwise vorticity filaments from the secondary LEV . With increasing heave domination, the origin of stronger spanwise instability is governed by a counter-rotating trailing edge vortex (TEV ) and LEV that leads to growth of streamwise secondary structures. A decreasing heave domination ultimately coincides with an absence of strong LEV undulations and secondary structures. The consistent transition routes are represented on a phase-space map, where a progression of spanwise instability and growth of secondary structures becomes evident within regimes of decreased heave domination. The increasing strength of circulation for the primary LEV , with increasing Stc, provides a crucial reasoning for this newly identified progression.

physics.flu-dyn↗

arXiv2023

On the association of secondary hairpin growth and surface pressure gradient for oscillating foils

The correspondence of secondary spanwise structures and pressure gradient is numerically evaluated for a foil, performing heaving and pitching motion, at a range of phase offsets (90$^\circ$ $\le ϕ\le$ 270$^\circ$) and reduced frequency (0.32 $\le St_c \le$ 0.56). The Reynolds number is $Re =$ 8000. The wake is shown to be dominated by secondary hairpin-like structures that are formed due to an elliptic instability prompted by the paired primary and secondary leading edge vortex ($LEV$). The weaker secondary $LEV$ undergoes a core deformation, resulting in streamwise vorticity outflux across the span of the foil, and hence, the growth of hairpin-like structures. Evaluating pressure gradients on the surface of the foil reveals a unique fundamental measure to quantitatively characterize the growth of these coherent structures. Their dominant presence can be directly linked to the growth of the secondary $LEV$ formed due to the large-scale interactions under localized adverse pressure gradients. These promote a streamwise flow compression in neighboring regions of the primary $LEV$. This association also presents a vivid consistency across a range of kinematics. Therefore, this correspondence provides a novel procedure to investigate the mechanisms involved in the formation of secondary structures in the wake of an oscillating foil.

physics.flu-dyn↗

arXiv2021

Classification of vortex patterns of oscillating foils in side-by-side configurations

The unsteady hydrodynamics of two in-phase pitching foils arranged in side-by-side (parallel) configurations is examined for a range of Strouhal number and separation distance. Three distinct vortex patterns are identified in the Strohual number-separation distance phase maps, which include separated wake, merged wake, and transitional-merged wake. Furthermore, a novel model is introduced based on fundamental flow variables including velocity, location, and circulation of dipole structures to quantitatively distinguish vortex patterns in the wake. The physical mechanism of wake merging process is also elucidated. When an oscillating foil experiences the jet deflection phenomenon, secondary structures shed from the primary street traverse in the other direction by making an angle with its parent vortex street. For parallel foils, secondary structures from the vortex street of the lower foil interact with the primary vortex street of the upper foil under certain kinematic conditions. This interaction triggers the wake merging process by influencing circulation of coherent structures in the upper part of the wake. It is unveiled that merging of the wakes leads to enhancements in propulsive efficiency by increasing thrust generation without a significant alteration in power requirements. These are attributed to the formation of a high-momentum jet by the merged vortex street, which possesses significantly larger circulation due to the amalgamation of the vortices, and major alterations in the evolution of leading edge vortices. Thus, flow physics that are thoroughly explored here are crucial in providing novel insights for future development of flow control techniques for efficient designs of bio-inspired underwater propulsors.

physics.flu-dyn↗

arXiv2021

On the Aerodynamics of Multistage Co-Axial Vertical-Axis Wind Turbines

This study explored the aerodynamics of a new multi-stage co-axial vertical-axis wind turbine based on bio-inspiration from natural swimming habit of fish. The turbine was formed from a conventional straight-bladed vertical-axis turbine (VAWT) with a small inner rotor, also of three blades. The azimuthal and radial locations of the inner rotor were varied. Using numerical simulations, performance of the proposed new design was evaluated over a range of tip-speed ratios. The preliminary results identified a 600% increase in power output for multi-stage VAWTs at tip-speed rations TSR = 0 - 3, and a substantial drop in power coefficient at TSR > 3.0. The wake dynamics analyses revealed that the increase was due to interactions between the blades of one rotor and the other. This reduced the unsteady separation from the outer rotor which produced most of the power. A detailed parametric study was also completed, which showed the implications of geometric and kinematic details on the performance of the proposed multistage VAWT.

physics.flu-dyn↗

arXiv2020

How does altering synchronization of pitching parallel foils change their wake dynamics?

This study was inspired from the swimming habits of red nose tetra fish that prefer side-by-side configurations and exhibit changes to their synchronization mid-swimming. Using numerical simulations, alterations to the unsteady wake dynamics imposed by abrupt changes in the phase angle between two pitching side-by-side foils were examined at Reynolds number of 4;000 and Strouhal number of 0:50. Four hybrid modes were considered in thisstudy with two modes representing an abrupt phase change by pi during the 20th cycle. The other two modes represented a simplified case of burst-and-coast swimming, in which there was a brief (2 oscillation periods) suspension of oscillations before imposing a change in phase angle. In all cases, the foils initially performed out-of-phase pitching, and then they started their in-phase motion by either performing the upstroke or down-stroke first. This kinematic change resulted in the formation and growth of a secondary vortex street in between two primary streets, which enabled and maintained a split wake configuration. Furthermore, the phase switching altered the pressure levels on the top and bottom surfaces of both foils to almost similar levels, which attributed to a reduction in the side-force. The growth rate of the secondary vortex street remained consistent for all four hybrid modes.

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