Department of Civil & Infrastructure Engineering

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Going beyond the classical Gaussian approximation of Einstein's fluctuation theory, Ruppeiner gave it a Riemannian geometric structure with an entropic metric. This yielded a fundamental quantity, the Riemannian curvature, which was used to extract information on the nature of interactions between molecules in fluids, ideal gases, and other open systems. In this article, we examine the implications of this curvature in a nonequilibrium thermodynamic system where relaxation is sufficiently slow so as not to invalidate the local equilibrium hypothesis. The nonequilibrium system comprises a rubbery polymer undergoing strain induced crystallization. The curvature is found to impart information on a spurious isochoric energy arising from the conformational stretching of already crystallized segments. This unphysical component perhaps arises as the crystallized manifold is considered Euclidean with the stretch measures defined via the Euclidean metric. The thermodynamic state associated with curvature is the key to determine the isochoric stretch and hence the spurious energy. We determine this stretch and propose a form for the spurious free energy that must be removed from the total energy in order for the correct stresses to be recovered. © 2022 American Physical Society.

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Physical Review E

Geometric thermodynamics of strain-induced crystallization in polymers

The aim of this experimental study is to investigate the interaction between turbulent flow and a gravel-bed that mimics the actual roughness structures of a natural bed and its implication on sediment transport. In particular, the response of the Reynolds stresses and the role of intermittency to the bed roughness is the primary focus of the current study. To this end, the flow field, measured with an Acoustic–Doppler Velocimeter (ADV), is thoroughly examined, considering the conditional Reynolds shear stresses and the high-order structure functions of velocity. The study results show that the features and the roughness structure of the water-worked gravel-bed (WWGB) have a strong impact on bursting events. The reason for this is attributed to the flow layer above the crest in the WWGBs being primarily affected by sweep events rather than ejection events, although the latter events are prevalent in this flow layer in an immobile, man-made, gravel-bed. These coherent structures, which are primarily responsible for sediment transport, also are the key cause of the presence of an intermittency excess that breaks the Kolmogorov self-similarity hypothesis, leading to multi-fractal behavior of the velocity structure functions. © 2021 International Research and Training Centre on Erosion and Sedimentation / the World Association for Sedimentation and Erosion Research

International Journal of Sediment Research

Response of Reynolds stresses and scaling behavior of high-order structure functions to a water-worked gravel-bed surface and its implication on sediment transport

International Journal of Environmental Science and Technology

A review on the fate and transport behavior of engineered nanoparticles: possibility of becoming an emerging contaminant in the groundwater

In this paper, the linear stability of sand waves sheared by a turbulent flow is analyzed. The velocity distribution in the streamwise direction is considered to follow the logarithmic law. The modified pressure distribution owing to the streamline curvature induced by the sand waves is employed in the formulation. The pressure distribution is derived using the Boussinesq approximation for the variation of streamline curvature over the flow depth. The flow model is coupled with the sediment transport model to study the stability of sand waves. Both the modes of sediment transport as bedload and suspended load are considered. The linear stability analysis reveals the favorable region for the formation of dunes and antidunes on the plane formed by the Froude number and the dimensionless wavenumber. The region of instability increases with an increase in particle parameter, while it decreases with an increase in relative roughness. The analysis reveals that the sand waves form beyond a limiting Froude number corresponding to the threshold Shields number. The theoretical results compare well with the available experimental data. © 2021, The Author(s), under exclusive licence to Springer Nature B.V.

Environmental Fluid Mechanics

Linear stability of sand waves sheared by a turbulent flow

In this article, we explore the submarine channel formation driven by the interaction of turbidity currents with an erodible bed. The theoretical analysis considers the three-dimensional continuity and momentum equations of the fluid phase, and the advection-diffusion and Exner equations of the solid phase. The governing equations are linearized by imposing periodic perturbations on the base flow. We study the response of both the base flow (profiles of velocity and suspended sediment concentration) and perturbations (growth rate and perturbation fields) to changes in key parameters related to the flow and sediment transport. The growth rate and the critical wavenumber are examined for a given quintet formed by the gravitational parameter, longitudinal bed slope, sediment concentration at the edge of the driving layer, Rouse number and erosion coefficient. The critical wavenumber reduces with an increase in gravitational parameter, longitudinal bed slope, sediment concentration at the edge of the driving layer and erosion coefficient, while it increases with the Rouse number. For the submarine channel formation, we identify the upper threshold values for the gravitational parameter, longitudinal bed slope, sediment concentration at the edge of the driving layer and erosion coefficient and the lower threshold value for the Rouse number. © 2022 The Authors.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

Submarine channels formation driven by turbidity currents interacting with an erodible bed

The turbulence characteristics in open-channel flow owing to a sudden change from smooth (upstream) to rough (downstream) bed are investigated experimentally using a Particle Image Velocimetry system. The upstream flow had a shear Reynolds number of 4.86, characterizing the hydraulically smooth flow, while the downstream flow had a shear Reynolds number greater than 70, characterizing the hydraulically rough flow. The Reynolds stress results reveal that for a given vertical distance, both the Reynolds shear and normal stresses in the downstream bed increase with the streamwise distance as compared to their upstream values. Their peaks appear at a distance of one-fifth of the flow depth from the bed. In addition, the bed shear stress enhances with the streamwise distance. The stress contours corroborate that the formation of a roughness-induced layer over the downstream bed thickens with the streamwise distance. The third-order correlations reveal that an arrival of slowly moving fluid streaks associated with an outward Reynolds stress diffusion prevails in the flow on the upstream bed, while an inrush of rapidly moving fluid streaks associated with an inward Reynolds stress diffusion governs the near-bed flow zone in the downstream bed. These results are in conformity with those obtained from the turbulent kinetic energy (TKE) fluxes and the bursting events. With regard to the TKE budget, the peaks of TKE production and dissipation rates appear near the downstream bed and are greater than those in the upstream bed. However, in the downstream bed, an enhanced negative TKE diffusion prevails near the bed. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.

Response of open-channel flow to a sudden change from smooth to rough bed

We report an unprecedented existence of the zeroth law of helicity spectrum (i.e., the helicity spectrum becomes independent of the wavenumber) in the transition from production range to inertial range, herein termed the pre-inertial range, of wall turbulence. The zeroth law is explained by the superposition effect of the forward joint cascade of energy and helicity caused by twisting and stretching of wall-attached superstructures in an equilibrium layer. The phenomenological model perfectly predicts the zeroth law in the pre-inertial range. Experimental data support the existence of the zeroth law. © 2022 Author(s).

Physics of Fluids

Discovery of the zeroth law of helicity spectrum in the pre-inertial range of wall turbulence

With the goal to explore the effects of natural bed roughness on bedload transport, numerical simulations of flow and particle saltation are carried out with varying bed roughness and particle spatial density. A combination of Eulerian and Lagrangian point-particle methods is applied to solve the equations of motion of the fluid and the particles within the large-eddy simulation framework. Flows over smooth and rough beds with four particle densities are considered. As the bed roughness increases, there is a leftward shift of the double-averaged streamwise velocity profiles against dimensionless vertical distance, which is scaled with the bed roughness height, an upward shift of the peak values of double-averaged Reynolds stresses, and a fragmentation and disappearance of coherent structures in the form of high-speed and low-speed near-bed streaks. These observations are consistent with those of previous studies. As the bed roughness increases, the mean resting time of saltating particles increases, however the particles' saltation length, velocity, and angular velocity decrease, while their saltation height remains almost unchanged. Saltation height, saltation length, particle angular velocity, and resting time exhibit linear, gamma, normal, and exponential distributions, respectively. Further, as the bed roughness increases, the kurtosis and skewness of some particle parameters vary, and the particle velocity shifts from a symmetrical normal to an asymmetrical gamma distribution. © 2022. American Geophysical Union. All Rights Reserved.

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Water Resources Research

Response of Flow and Saltating Particle Characteristics to Bed Roughness and Particle Spatial Density

In this Perspective article, we seek the origin of the scaling laws of developing turbulent boundary layers over a flat plate from the perspective of the phenomenological theory of turbulence. The scaling laws of the boundary-layer thickness and the boundary shear stress in rough and smooth boundary-layer flows are established. In a rough boundary-layer flow, the boundary-layer thickness (scaled with the boundary roughness) and the boundary shear stress (scaled with the dynamic pressure) obey the "2/(1-σ)"and "(1+σ)/(1-σ)"scaling laws, respectively, with the streamwise distance (scaled with the boundary roughness). Here, σ is the spectral exponent. In a smooth boundary-layer flow, the boundary-layer thickness (scaled with the viscous length scale) and the boundary shear stress (scaled with the dynamic pressure) obey the "8/(5 - 3σ)"and "3(1+σ)/(5 - 3σ)"scaling laws, respectively, with the Reynolds number characterized by the streamwise distance. © 2022 Author(s).

Origin of the scaling laws of developing turbulent boundary layers

Submerged flexible vegetation occupies the core of aquatic ecosystem research. Hydrodynamics of submerged flexible vegetation and its interaction with the flow in an open channel are of great significance in studying the mass and momentum transports in the flow. In this study, a numerical model for highly flexible vegetation based on large eddy simulation and the immersed boundary method was used to simulate the flow–vegetation interaction. It is recognised that alternate vortices with opposite sense rotations appear at the flow–vegetation interface. These vortices prompt the vegetation canopy to have wave-like coherent waving motion, commonly called the monami phenomena. The spatial scale and the spreading velocity in the streamwise direction of these vortices determine the wavelength, frequency and amplitude of the vegetation coherent waving motion. In this study, the fast Fourier transform method was applied to analyse the factors affecting the characteristics of the vegetation coherent waving motion. It is revealed that, as the flow velocity increases, the wavelength of the coherent waving motion decreases, while the frequency and amplitude increase. Besides, as the vegetation spacing increases, the wavelength and amplitude of the coherent waving motion increase, but the frequency decreases. Furthermore, an increase in the relative density of vegetation magnifies the amplitude of coherent waving motion without affecting the wavelength and frequency. © The Author(s), 2022. Published by Cambridge University Press.