Estuary Classification Revisited

This paper presents the governing equations of a tidally-averaged, width-averaged, rectangular estuary in completely nondimensionalized forms. Subsequently, we discover that the dynamics of an estuary is entirely controlled by only two variables: (i) the Estuarine Froude number, and (ii) a nondimensional number related to the Estuarine Aspect ratio and the Tidal Froude number. Motivated by this new observation, the problem of estuary classification is re-investigated. Our analysis shows that the two control variables are capable of completely determining the stratification at the estuary mouth, and therefore can specify the estuary type. The theoretical estuary classification scheme proposed in this paper is validated against real estuarine data collected from existing literature. Our classification scheme on comparison with the state-of-the-art theory shows significant improvement.Comment: 6 pages, 4 figure

On the stability of plane Couette-Poiseuille flow with uniform cross-flow

We present a detailed study of the linear stability of plane Couette-Poiseuille flow in the presence of a cross-flow. The base flow is characterised by the cross flow Reynolds number, $R_{inj}$ and the dimensionless wall velocity, $k$. Squire's transformation may be applied to the linear stability equations and we therefore consider 2D (spanwise-independent) perturbations. Corresponding to each dimensionless wall velocity, $k\in[0,1]$, two ranges of $R_{inj}$ exist where unconditional stability is observed. In the lower range of $R_{inj}$, for modest $k$ we have a stabilisation of long wavelengths leading to a cut-off $R_{inj}$. This lower cut-off results from skewing of the velocity profile away from a Poiseuille profile, shifting of the critical layers and the gradual decrease of energy production. Cross-flow stabilisation and Couette stabilisation appear to act via very similar mechanisms in this range, leading to the potential for robust compensatory design of flow stabilisation using either mechanism. As $R_{inj}$ is increased, we see first destabilisation and then stabilisation at very large $R_{inj}$. The instability is again a long wavelength mechanism. Analysis of the eigenspectrum suggests the cause of instability is due to resonant interactions of Tollmien-Schlichting waves. A linear energy analysis reveals that in this range the Reynolds stress becomes amplified, the critical layer is irrelevant and viscous dissipation is completely dominated by the energy production/negation, which approximately balances at criticality. The stabilisation at very large $R_{inj}$ appears to be due to decay in energy production, which diminishes like $R_{inj}^{-1}$. Our study is limited to two dimensional, spanwise independent perturbations.Comment: Accepted for publication in Journal of Fluid Mechanic

Modeling of high speed water jets in air

High speed water jets in air are typically used in industrial cleaning operations. Vigorous interaction between the jet and the surrounding air brings about an exchange of mass and momentum which results in the spreading of the jet. An experimental investigation of pressure distribution on a target plate placed in the jet flow field has been performed and analyzed. The main purpose of this research is to numerically simulate the flow characteristics of high speed water jets in air. Since, to the best of our knowledge, there is no numerical model to describe the flow physics of high speed jets, novel numerical models are proposed. Results obtained from the numerical simulation are compared to the existing experimental results in the literature, as well as our own experimental data. Different flow properties were analyzed to provide considerable insight into the physics of these flows

Study on the alteration of bubaline blood biochemical composition owing to slaughter

Bubaline blood biochemistry as affected by slaughter was the agenda for this work. Blood samples were collected from 30 buffaloes from abattoirs before and at slaughter. After biochemical and statistical analysis (mean was compared with t-test), it was observed that the albumin, lactate dehydrogenase and creatine kinase increased (p < 0.05), while globulin, urea nitrogen and creatinine decreased (p < 0.05) significantly. Significant (p < 0.01) increase in glucose and cholesterol, and marginal variation in concentration of total protein and biliurubin were observed. The macro-minerals of the blood were observed to have decreased significantly (p < 0.01) at slaughter. Sodium and chloride decreased, potassium was observed to have increased highly significantly (p < 0.01) due to slaughter. Slaughter induced changes in blood biochemical profile in buffaloes as reported here are of significance in commercial biomedical use of this connective tissue.Keywords: Biochemistry, blood, buffalo, slaughte

Explosive instability due to flow over a rippled bottom

In this paper, we study Bragg resonance, i.e. the triad interaction between surface and/or interfacial waves with bottom ripple, in presence of background velocity. We show that when one of the constituent waves of the triad has negative energy, the amplitudes of all the waves grow exponentially. This is very different from classic Bragg resonance in which one wave decays to cause growth of the other. The instabilities we observe are `explosive', and are different from normal mode shear instabilities since our velocity profiles are linearly stable. Our work may explain the existence of large amplitude internal waves over periodic bottom ripples in presence of tidal flow observed in oceans and estuaries

A new Lagrangian drift mechanism due to current–bathymetry interactions:applications in coastal cross-shelf transport

We show that in free surface flows, a uniform, streamwise current over small-amplitude wavy bottom topography generates cross-stream drift velocity. This drift mechanism, referred to as the current-bathymetry interaction induced drift (CBIID), is specifically understood in the context of a simplified nearshore environment consisting of a uniform alongshore current, onshore propagating surface waves, and monochromatic wavy bottom making an oblique angle with the shoreline. CBIID is found to originate from the steady, non-homogeneous solution of the governing system of equations. Similar to Stokes drift induced by surface waves, CBIID also generates a compensating Eulerian return flow to satisfy the no-flux lateral boundaries, e.g. the shoreline. CBIID increases with an increase in the particle's initial depth, bottom undulation's amplitude, and the strength of the alongshore current. Additionally, CBIID near the free (bottom) surface increases (decreases) with an increase in bottom undulation's wavelength. Maximum CBIID is obtained for long wavelength bottom topography that approximately makes $\pi/4$ angle with the shoreline. Unlike Stokes drift, particle excursions due to current-bathymetry interactions might not be small, hence analytical expressions based on the small-excursion approximation could be inaccurate. We provide an alternative $z$-bounded approximation, which leads to highly accurate expressions for drift velocity and time period of particles especially located near the free surface. Realistic parametric analysis reveals that in some nearshore environments, CBIID's contribution to the net Lagrangian drift can be as important as Stokes drift, implying that CBIID can have major implications in cross-shelf tracer transport

Predicting vortex merging and ensuing turbulence characteristics in shear layers from initial conditions

Unstable shear layers in environmental and industrial flows roll up into a series of vortices, which often form complex nonlinear merging patterns like pairs and triplets. These patterns crucially determine the subsequent turbulence, mixing and scalar transport. We show that the late-time, highly nonlinear merging patterns are predictable from the linearized initial state. The initial asymmetry between consecutive wavelengths of the vertical velocity field provides an effective measure of the strength and pattern of vortex merging. The predictions of this measure are substantiated using direct numerical simulations. We also show that this measure has significant implications in determining the route to turbulence and the ensuing turbulence characteristics