arXiv subjects

Mohammad-Reza Alam

Publications and source records attributed to Mohammad-Reza Alam.

At least 19 recordsLinked to original sources

arXiv2026

The underwater Brachistochrone

The brachistochrone, the curve of fastest descent under gravity, is a cycloid when friction is absent. Underwater, however, buoyancy, viscous drag, and the added mass of entrained fluid fundamentally alter the problem. We formulate and solve the brachistochrone for a body moving through a dense fluid, incorporating all three effects together with a Reynolds-number-dependent drag coefficient. The classical cycloid becomes increasingly suboptimal as the body density approaches the fluid density, and below a critical density ratio it fails to reach the endpoint altogether. Near the critical Reynolds number for the drag crisis, the optimal trajectory is acutely sensitive to the density ratio and object size; constant-drag approximations can yield qualitatively incorrect paths. A decomposition of physical effects shows that neglecting drag and added mass together yields a predicted transit time roughly half the realised minimum, and that omitting added mass alone underestimates the transit time by approximately 20%. We extend the formulation to a three-point brachistochrone in which the trajectory must pass through an intermediate waypoint, revealing a finite reachable domain that is absent in the classical problem. The underwater brachistochrone as presented here provides a simple planning tool for short-range trajectories of buoyancy-driven underwater vehicles.

physics.flu-dyn↗

arXiv2026

World without Viscosity

Viscosity, the internal friction of fluids, is among the most consequential yet underappreciated properties in physics. This paper explores what would happen if viscosity vanished from all fluids while other material properties remained unchanged. The consequences are catastrophic and universal. Aircraft cannot generate lift because circulation around wings requires viscous action. Rotating machinery seizes without lubricating fluid films. Cardiovascular systems lose the resistance necessary for pressure regulation. Rivers become violent torrents, aquifers drain in hours, and storms persist indefinitely without frictional dissipation. The pedagogical value lies in illuminating viscosity's role providing resistance, damping, and control across all scales - from cellular interiors to planetary atmospheres. Evolution, engineering, and climate have exploited viscous dissipation for billions of years; its absence would render complex life impossible and Earth uninhabitable. By imagining a world without viscosity, we better understand the viscous world we inhabit.

physics.pop-ph↗

arXiv2022

Deep water wave cloaking using a multi-layered buoyant plate

Trajectory of surface gravity waves in potential flow regime is affected by the gravitational acceleration, water density, and seabed depth. While the gravitational acceleration and water density are approximately constant, the effect of water depth on surface gravity waves exponentially decreases as the water depth increases. In shallow water, cloaking an object from surface waves by varying the seabed topography is possible, however, as the water depth increases, cloaking becomes a challenge since there is no physical parameter to be engineered and subsequently affects the wave propagation. In order to create an omnidirectional cylindrical cloaking device for finite-depth/deep-water waves, we propose a multi-layered elastic plate that floats on the surface around a to-be-cloaked cylinder. The buoyant elastic plate is made of axisymmetric, homogeneous, and isotropic layers which provides adjustable degrees of freedom to engineer and affect the wave propagation. We first develop a pseudo-spectral method to efficiently determine the wave solution for a multi-layered buoyant plate. Next, we optimize the physical parameters of the buoyant plate (i.e. elasticity and mass of every layer) using a real-coded evolutionary algorithm to minimize the energy of scattered-waves from the object. We show that the optimized cloak reduces the energy of scattered-waves as high as 99.2% for the target wave number. We also quantify the wave drift force exerted on the structures and show that the buoyant plate reduces the exerted force by 99.9%.

physics.flu-dyn↗

arXiv2020

Rapid Phase-Resolved Prediction of Nonlinear Dispersive Waves Using Machine Learning

In this paper, we show that a revised convolutional recurrent neural network (CRNN) can decrease, by orders of magnitude, the time needed for the phase-resolved prediction of waves in a spatiotemporal domain of a nonlinear dispersive wave field. The problem of predicting such waves suffers from two major challenges that have so far hindered analytical or direct computational solutions in real time or faster: (i) the reconstruction problem, that is, how one can calculate from measurable wave amplitude data the state of the wave field (wave components, nonlinear couplings, etc.), and (ii) if such a reconstruction is in hand, how to integrate equations fast enough to be able to predict an upcoming rouge wave in a timely manner. Here, we demonstrate that these two challenges can be overcome at once through advanced machine learning techniques based on spatiotemporal patches of the time history of wave height data in the domain. Specifically, as a benchmark here we consider equations that govern the evolution of weakly nonlinear surface gravity waves such as those propagating on the surface of the oceans. For the case of oceanic surface waves considered here, we demonstrate that the proposed methodology, while maintaining a high accuracy, can make phase-resolved predictions more than two orders of magnitude faster than numerically integrating governing equations.

physics.flu-dyn↗

arXiv2020

Active Cloaking in Stokes Flows via Reinforcement Learning

Hydrodynamic signatures at the Stokes regime, pertinent to motility of micro-swimmers, have a long-range nature. This implies that movements of an object in such a viscosity-dominated regime, can be felt tens of body-lengths away and significantly alter dynamics of the surrounding environment. Here, we devise a systematic methodology to actively cloak swimming objects within any arbitrarily crowded suspension of micro-swimmers. Specifically, our approach is to conceal the target swimmer throughout its motion using cooperative flocks of swimming agents equipped with adaptive decision-making intelligence. Through a reinforcement learning algorithm, the cloaking agents experientially learn optimal adaptive behavioral policy in the presence of flow-mediated interactions. This artificial intelligence enables them to dynamically adjust their swimming actions, so as to optimally form and robustly retain any desired arrangement around the moving object without disturbing it from its original path. Therefore, the presented active cloaking approach not only is robust against disturbances, but also is non-invasive to motion of the cloaked object. We then further generalize the proposed approach and demonstrate how our cloaking agents can be readily used, in any region of interest, to realize hydrodynamic invisibility cloaks around any number of arbitrary intruders.

physics.flu-dyn↗

arXiv2020

Stealthy Movements and Concealed Swarms of Swimming micro-Robots

Here we show that micro-swimmers can form a concealed swarm through synergistic cooperation in suppressing one another's disturbing flows. We then demonstrate how such a concealed swarm can actively gather around a favorite spot, point toward a target, or track a desired trajectory in space, while minimally disturbing the ambient fluid. Our findings provide a clear road map to control and lead flocks of swimming micro-robots in stealth versus fast modes, tuned through their active collaboration in minimally disturbing the host medium.

physics.flu-dyn↗

arXiv2020

Landslide Tsunamis in Lakes

Landslides plunging into lakes and reservoirs can result in extreme wave runup at shores. This phenomenon has claimed lives and caused damage to near-shore properties. Landslide tsunamis in lakes are different from typical earthquake tsunamis in the open ocean in that (i) the affected areas are usually within the near-field of the source, (ii) the highest runup occurs within the time period of the geophysical event, and (iii) the enclosed geometry of a lake does not let the tsunami energy escape. To address the problem of transient landslide tsunami runup and to predict the resulting inundation, we utilize a nonlinear model equation in the Lagrangian frame of reference. The motivation for using such a scheme lies in the fact that the runup on an inclined boundary is directly and readily computed in the Lagrangian framework without the need to resort to approximations. In this work, we investigate the inundation patterns due to landslide tsunamis in a lake. We show by numerical computations that Airy's approximation of an irrotational theory using Lagrangian coordinates can legitimately predict runup of large amplitude. We also demonstrate that in a lake of finite size the highest runup may be magnified by constructive interference between edge-waves that are trapped along the shore and multiple reflections of outgoing waves from opposite shores, and may occur somewhat later after the first inundation.

physics.flu-dyn↗

arXiv2020

Fabry-Perot resonance of water waves

We show that significant water wave amplification is obtained in a water resonator consisting of two spatially separated patches of small-amplitude sinusoidal corrugations on an otherwise flat seabed. The corrugations reflect the incident waves according to the so-called Bragg reflection mechanism, and the distance between the two sets controls whether the trapped reflected waves experience constructive or destructive interference within the resonator. The resulting amplification or suppression is enhanced with increasing number of ripples, and is most effective for specific resonator lengths and at the Bragg frequency, which is determined by the corrugation period. Our analysis draws on the analogous mechanism that occurs between two partially reflecting mirrors in optics, a phenomenon named after its discoverers Charles Fabry and Alfred Perot.

physics.flu-dyn↗

arXiv2019

Frequency-dependent higher-order Stokes singularities near a planar elastic boundary: implications for the hydrodynamics of an active microswimmer near an elastic interface

The emerging field of self-driven active particles in fluid environments has recently created significant interest in the biophysics and bioengineering communities owing to their promising future biomedical and technological applications. These microswimmers move autonomously through aqueous media where under realistic situations they encounter a plethora of external stimuli and confining surfaces with peculiar elastic properties. Based on a far-field hydrodynamic model, we present an analytical theory to describe the physical interaction and hydrodynamic couplings between a self-propelled active microswimmer and an elastic interface that features resistance toward shear and bending. We model the active agent as a superposition of higher-order Stokes singularities and elucidate the associated translational and rotational velocities induced by the nearby elastic boundary. Our results show that the velocities can be decomposed in shear and bending related contributions which approach the velocities of active agents close to a no-slip rigid wall in the steady limit. The transient dynamics predict that contributions to the velocities of the microswimmer due to bending resistance are generally more pronounced than to shear resistance. Our results provide insight into the control and guidance of artificial and synthetic self-propelling active microswimmers near elastic confinements.

physics.flu-dyn↗

arXiv2019

Broadband Wall-less Waveguide for Shallow Water Waves

We present a broadband waveguide for water waves obtained through mere manipulation of seabed properties and without any need for sidewalls. Specifically, we show that a viscoelastic seabed results in a modified effective gravity term in the governing equations of water waves, which provides a generic broadband mechanism to control oceanic wave energy and enables confining surface waves inside a long narrow path without sidewalls. Our findings have promising applications in guiding and steering waves for oceanic wave energy farms or protecting shorelines.

physics.flu-dyn↗

arXiv2019

The Experimental Realization of an Artificial Low-Reynolds-Number Swimmer with Three-Dimensional Maneuverability

The motion of biological micro-robots -- similar to that of swimming microorganisms such as bacteria or spermatozoa -- is governed by different physical rules than what we experience in our daily life. This is particularly due to the low-Reynolds-number condition of swimmers in micron scales. The Quadroar swimmer, with three-dimensional maneuverability, has been introduced for moving in these extreme cases: either as a bio-medical micro-robot swimming in biological fluids or a mm-scale robot performing inspection missions in highly viscous fluid reservoirs. Our previous studies address the theoretical modeling of this type of swimmer system. In this work, we present the mechatronic design, fabrication, and experimental study of a mm-scale Quadroar swimmer. We describe the design methodology and component selection of the system based on the required performance. A supervisory control scheme is presented to achieve an accurate trajectory tracking for all the actuators used in the swimmer. Finally, we have conducted experiments in silicone oil (with 5000 cP viscosity) where two primary modes of swimming - forward translation and planar reorientation - have been tested and compared with the theoretical model.

physics.app-ph↗

arXiv2019

Propulsion and Mixing Generated by the Digitized Gait of Caenorhabditis elegans

Nematodes have evolved to swim in highly viscous environments. Artificial mechanisms that mimic the locomotory functions of nematodes can be efficient viscous pumps. We experimentally simulate the motion of the head segment of Caenorhabditis elegans by introducing a reciprocating and rocking blade. We show that the bio-inspired blade's motion not only induces a flow structure similar to that of the worm, but also mixes the surrounding fluid by generating a circulatory flow. When confined between two parallel walls, the blade causes a steady Poiseuille flow through closed circuits. The pumping efficiency is comparable with the swimming efficiency of the worm. If implanted in a sealed chamber and actuated remotely, the blade can provide pumping and mixing functions for microprocessors cooled by polymeric flows and microfluidic devices.

physics.flu-dyn↗

arXiv2018

Flow Characteristics of Chlamydomonas result in Purely Hydrodynamic Scattering

It has long been believed that swimming eukaryotes feel solid boundaries through direct ciliary contact. Specifically, based on observations of behavior of green alga Chlamydomonas reinhardtii it has been reported that it is their "flagella [that] prevent the cell body from touching the surface" [Kantsler et al. PNAS, 2013]. Here, via investigation of a model swimmer whose flow field closely resembles that of C. reinhardtii, we show that the scattering from a wall can be purely hydrodynamic and that no mechanical/flagellar force is needed for sensing and escaping the boundary.

physics.flu-dyn↗

arXiv2018

Shape Optimization of Wave Energy Converters for Broadband Directional Incident Waves

Here, through a systematic methodology and the use of high performance computing, we calculate the optimum shape for a wave energy converter under the action of incident waves of (i) monochromatic unidirectional, (ii) monochromatic directional, (iii) polychromatic unidirectional and (iv) polychromatic directional (with both directional symmetry and asymmetry). As a benchmark for our study, without loss of generality, we consider a submerged planar pressure differential wave energy converter, and use Genetic Algorithm to search through a wide range of shapes. A new parametric description of absorber shape based on Fourier decomposition of geometrical shapes is introduced, and for each shape hydrodynamic coefficients are calculated, optimum power take-o? parameters are obtained, and overall efficiency is determined. We show that an optimum geometry of the absorber plate can absorb a significantly higher energy (in some cases a few times higher) when compared to a circular shape of the same area. Specifically, for a unidirectional incident wave, the optimum shape, as expected, is obtained to be the most elongated shape. For directional incident waves, a butterfly-shape is the optimum geometry whose details depend on not only the amplitude and direction of incident wave components, but also the relative phases of those components. For the latter effect, we find an optimally averaged profile through a statistical analysis. Keywords: Wave energy conversion, Shape optimization

physics.flu-dyn↗

arXiv2017

Dominant Resonance in Parametric Subharmonic Instability of Internal Waves

Parametric Subharmonic Instability (PSI) is one of the most important mechanisms that transfer energy from tidally-generated long internal waves to short steep waves. Breaking of these short waves results in diapycnal mixing through which oceanic abyssal stratification is maintained. It has long been believed that PSI is strongest between a primary internal wave and perturbative waves of half the frequency of the primary wave. Here, we rigorously show that this is not the case. Specifically, we show that neither the initial growth rate nor the maximum long-term amplification occur at the half frequency, and demonstrate that the dominant subharmonic waves have much longer wavelengths than previously thought.

physics.flu-dyn↗

arXiv2017

Oblique internal-wave chain resonance over seabed corrugations

We show that monochromatic long-crested corrugations on an otherwise flat seafloor can coherently scatter the energy of an oblique incident internal wave to multiple multi-directional higher-mode waves via a series of resonant interactions. We demonstrate that a resonance between seabed corrugations and a normally or slightly oblique incident internal wave results in a series of follow-up resonant interactions, which take place between the same corrugations and successively resonated shorter waves. A chain resonance of internal waves that carries energy to small scales is thus obtained, and we find that the Richardson number decreases by several orders of magnitude over the corrugated patch. If the incidence angle is large, and the incident wave perfectly satisfies a resonance condition with the topography, it turns out that not many higher-mode resonance or near-resonance conditions can be satisfied, such that energy stays confined within the first few modes. Nevertheless, if the incident waves are sufficiently detuned from satisfying a perfect resonance condition with the seabed corrugations, then we show that this frequency detuning may balance off the large detuning due to oblique incidence, leading to a chain resonance that again carries energy to small scales. The evolution of the incident and resonated wave amplitudes is predicted from the envelope equation for internal waves over resonant seabed topography in a three-dimensional rotating fluid, which we derive considering the Boussinesq and $f$-plane approximations with $f$ the Coriolis frequency, linear density stratification and small-amplitude corrugations. Our results suggest that topographic features on the ocean floor with a dominant wavenumber vector, through the chain resonance mechanism elucidated here, may play a more important role than previously thought in the enhancement of diapycnal mixing and energy dissipation.

physics.flu-dyn↗

arXiv2017

Rogue Wave Morphology in Broadband Nonbreaking Seas

Here, we show through a statistical analysis that rogue waves in broadband non-breaking seas are spatially asymmetric. In addition to the top-down asymmetry due to nonlinear effects, we show that the two troughs adjacent to the rogue wave crest are generally of different depths, which is unlike the conventional picture of rogue waves with symmetric fore and aft troughs often obtained from model equations. The rogue-wave trough asymmetry is demonstrated for sea states 4 to 6 on Douglas Sea Scale. Considering the deepest trough leads to approximately 10% increase in the calculation of the mean rogue-wave height compared to previous results for rogue waves reported with symmetric troughs. This implies that estimates of rogue-wave trough-crest amplitudes based on model equations should be re-assessed upward for all realistic oceanic conditions.

physics.flu-dyn↗

arXiv2016

Broadband cloaking for flexural waves

The governing equation for elastic waves in flexural plates is not form invariant, and hence designing a cloak for such waves faces a major challenge. Here, we present the design of a perfect broadband cloak for flexural waves through the use of a nonlinear transformation, and by matching term-by-term the original and transformed equations. For a readily achievable flexural cloak in a physical setting, we further present an approximate adoption of our perfect cloak under more restrictive physical constraints. Through direct simulation of the governing equations, we show that this cloak, as well, maintains a consistently high cloaking efficiency over a broad range of frequencies. The methodology developed here may be used for steering waves and designing cloaks in other physical systems with non form-invariant governing equations.

physics.class-ph↗