Department of Civil & Infrastructure Engineering

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IIT Jodhpur

This study presents an experimental investigation of hydraulic characteristics of submerged offset jets. The instantaneous velocity was captured by an acoustic Doppler velocimeter (ADV). A total of 20 runs were carried out by varying different parameters: Reynolds number (28475-80730), offset ratio (3.16-6.2), jet Froude number (1.9-3), submergence parameter (0.65-2.82) and sluice gate openings (3, 3.5, 4, 4.5 and 5 cm). It is observed that the length of the reattachment depends on the relative offset height and the tail water depth. A mathematical formulation is developed for the submerged offset jet assuming a submerged jump in a drop structure to find the back-up depth and subcritical sequent depth by applying the momentum and continuity equations at the inlet and at tail water depth respectively. A mathematical equation is also developed for the back-up depth at the efflux section, which is experimentally verified for the range of supercritical Froude numbers from 1.1 to 10 and submergence factor. The jet trajectories (lower layer of jet, upper layer of jet and maximum jet velocity layer) are also derived applying the boundary conditions. The computed jet trajectories of the submerged offset jet show a satisfactory agreement with the experimental data.

World University root node

6th International Symposium on Hydraulic Structures: Hydraulic Structures and Water System Management, ISHS 2016

Hydraulics of submerged offset-jets

A novel hydrodynamic model for the threshold of cohesionless sediment particle motion under a steady unidirectional streamflow is presented. The hydrodynamic forces (drag and lift) acting on a solitary sediment particle resting over a closely packed bed formed by the identical sediment particles are the primary motivating forces. The drag force comprises of the form drag and form induced drag. The lift force includes the Saffman lift, Magnus lift, centrifugal lift, and turbulent lift. The points of action of the force system are appropriately obtained, for the first time, from the basics of micro-mechanics. The sediment threshold is envisioned as the rolling mode, which is the plausible mode to initiate a particle motion on the bed. The moment balance of the force system on the solitary particle about the pivoting point of rolling yields the governing equation. The conditions of sediment threshold under the hydraulically smooth, transitional, and rough flow regimes are examined. The effects of velocity fluctuations are addressed by applying the statistical theory of turbulence. This study shows that for a hindrance coefficient of 0.3, the threshold curve (threshold Shields parameter versus shear Reynolds number) has an excellent agreement with the experimental data of uniform sediments. However, most of the experimental data are bounded by the upper and lower limiting threshold curves, corresponding to the hindrance coefficients of 0.2 and 0.4, respectively. The threshold curve of this study is compared with those of previous researchers. The present model also agrees satisfactorily with the experimental data of nonuniform sediments.

Physics of Fluids

Hydrodynamics of sediment threshold

In this paper, we explore the mechanics and the turbulent structure of two-phase (fluid-solid particle) flow system, for the first time, by considering the dynamic equilibrium coupled with suspended solid particle concentration, fluid flow and energetics of the two-phase flow system. The continuity, momentum and turbulent kinetic energy (TKE) equations of the fluid and the solid phases are treated separately to derive a generalized relationship of the two-phase flow system aided by suitable closure relationships. The results obtained from the numerical solution of resulting equations show that the particle concentration and the TKE diminish with an increase in the Rouse number, while the horizontal velocity component increases. On the other hand, the TKE flux, diffusion and production rates increase with an increase in the Rouse number, while the TKE dissipation rate decreases. In the vicinity of the reference level (that is, the hypothetical level from which the particles come in suspension), the Kolmogorov number increases with an increase in the Rouse number. However, as the vertical distance increases, this behaviour becomes reverse. A close observation of the turbulent length scales reveals that the Prandtl'smixing length decreases with an increase in the Rouse number, but the Taylor microscale and the Kolmogorov length scale increase. © 2016 The Author(s) Published by the Royal Society. All rights reserved.

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

Mechanics of advection of suspended particles in turbulent flow

Carbon dioxide sequestration is considered to be an efficient method of curbing the release of carbon dioxide emissions into the atmosphere and to mitigating corresponding effects on climate change. In this regard, basaltic rocks are among the potential repositories due to their ability to trap carbon dioxide in form of carbonate minerals. A series of laboratory-scale saturation experiments was conducted on three types of basalts that were collected from different horizons of the Deccan Volcanic Province (DVP), India. The basalt cores were treated in a saturation chamber for different exposure periods under low-pressure and room-temperature conditions. A set of key mechanical, physical, and chemical properties of the pre- and post-treated samples were analyzed to identify saturation-related changes. The results show that exposure of the basalts to carbon dioxide has a strong effect on their strength. However, the extent of the effect is different for different types of basalts and is strongly controlled by the exposure time, the mineralogical composition, and rock texture. X-ray diffraction and scanning electron microscopy (SEM) show that saturation leads to dissolution of the mafic minerals, and precipitation of new carbonate minerals in the cracks and vesicles of the host rock. These changes in the mineralogical content and the development of micro-cracks in the samples are interpreted as being the primary factors that affect the loss of integrity in the rock. © 2016, Springer-Verlag Berlin Heidelberg.

Environmental Earth Sciences

Effect of carbon dioxide sequestration on the mechanical properties of Deccan basalt

The ability of a particle to uptake water and form a cloud droplet depends on its hygroscopicity. To understand its impact on cloud properties and ultimately radiative forcing, knowledge of chemically-resolved mixing state information or the one based on hygroscopic growth is crucial. Typically, global models assume either pure internal or external mixing state which might not be true for all conditions and sampling locations. To investigate into this, the current study employed an indirect approach to infer the probable mixing state. The hygroscopic parameters derived from κ-Kohler theory using size-resolved CCN measurements (κCCN) and bulk/size-resolved aerosol mass spectrometer (AMS) measurements (κAMS) were compared. The accumulation mode particles were found to be more hygroscopic (κCCN = 0.24) than Aitken mode (κCCN = 0.13), perhaps due to increased ratio of inorganic to organic mass fraction. The activation diameter calculated from size-resolved CCN activity measurements at 5 different supersaturation (SS) levels varied in the range of 115 nm–42 nm with κCCN = 0.13–0.23 (avg = 0.18 ± 0.10 (±1σ)). Further, κAMS&gt; κCCN was observed possibly due to the fact that organic and inorganic mass present in the Aitken mode was not correctly represented by bulk chemical composition and size-resolved fractional contribution of oxidized OA was not accurately accounted. Better correlation of organic fraction (forg) and κCCN at lower SS explained this behaviour. The decrease in κCCN with the time of the day was more pronounced at lower SS because of the relative mass reduction of soluble inorganic species by ∼17%. Despite the large differences between κ measured from two approaches, less over-prediction (up to 18%) between measured and predicted CCN concentration suggested lower impact of chemical composition and mixing state at higher SS. However, at lower SS, presences of externally mixed CCN-inactive aerosols lead to CCN over-prediction reflecting the significance of aerosol mixing state information. Further examination of the effect of biomass burning aerosols (∼35% in least oxidized biomass burning organic aerosol (BBOA-2 fraction) on hygroscopicity and CCN activity showed increase in the concentration of all AMS measured species (except NH4+ and SO42−), less O:C ratio, and organic mass fraction (forg) peak shift to lower diameter range caused ∼13% change in critical diameter (Da) and ∼40% change in κCCN. Increased deviation of ∼100% between κCCN and κAMS due to sudden influx of internally mixed BBOA caused suppressed hygroscopic growth. This study finally suggests the assumption of pure internally mixed aerosol does not completely hold true for this anthropogenically polluted site. © 2016

Atmospheric Environment

Deriving aerosol hygroscopic mixing state from size-resolved CCN activity and HR-ToF-AMS measurements

Droughts are expected to propagate from one type to another - meteorological to agricultural to hydrological to socio-economic. However, they do not possess a universal, straightforward temporal dependence. Rather, assessment of one type of drought (successor) from another (predecessor) is a complex problem depending on the basin's physiographic and climatic characteristics, such as, spatial extent, topography, land use, land cover and climate regime. In this paper, a wavelet decomposition based approach is proposed to model the temporal dependence between different types of droughts. The idea behind is to separate the rapidly and slowly moving components of drought indices. It is shown that the temporal dependence of predecessor (say meteorological drought) on the successor (say hydrological drought) can be better captured at its constituting components level. Such components are obtained through wavelet decomposition retaining its temporal correspondence. Thus, in the proposed approach, predictand drought index is predicted using the decomposed components of predecessor drought. Several alternative models are investigated to arrive at the best possible model structure for predicting different types of drought. The proposed approach is found to be very useful for foreseeing the agricultural or hydrological droughts knowing the meteorological drought status, offering the scope for better management of drought consequences. The mathematical framework of the proposed approach is general in nature and can be applied to different basins. However, the limitation is the requirement of region/catchment specific calibration of some parameters before using the proposed model, which is not very difficult and uncommon though. © 2016 Elsevier B.V.

Journal of Hydrology

Drought prediction using a wavelet based approach to model the temporal consequences of different types of droughts

Modeling of dam break flows is frequently required in civil and environmental engineering because of the risk associated with this catastrophic flow. Typically, model predictions are conducted using the Saint-Venant hydrostatic theory, which can lead to unrealistic predictions. The prediction of the amplitude of nonhydrostatic waves generated during dam break flows is an important engineering problem given the risk of overtopping of flow in manmade canals or the increasing of flooding areas in natural watercourses. The weakly nondispersive and fully nonlinear Serre equations are a suitable choice for modeling these flows, but there is a lack of a systematic assessment of this system of equations for dam break flow modeling reported in the literature. In this paper, the Serre equations are applied to dam break flows over horizontal rigid bottoms, whereas in the second part of this research, the nonhydrostatic dam break waves over erodible beds are considered. Here, a high resolution finite volume model is developed where a suitable time stepping scheme is systematically investigated. The impact of the vertical pressure distribution shape, nonlinear terms in the equations, and the enhancement of the linear frequency dispersion are examined in detail. The model is successfully tested against the experimental data, a solitary wave propagation test, and the three-dimensional (3D) simulations. The results obtained from finite volume method are further compared with those obtained from finite element and finite difference methods available in the literature. © 2016 American Society of Civil Engineers.

Journal of Hydraulic Engineering

Nonhydrostatic dam break flows. I: Physical equations and numerical schemes

In this paper, the scaling laws of rough turbulent flows stemming from the phenomenological theory of turbulence are reviewed. It is revealed that the scaling of the well-known empirical formulae can also be obtained by means of the phenomenological theory. The phenomenological theory thus enables to provide an insight into the physics of the flows of the problems arising from turbulence. As a new thought, the famous scaling laws of the velocity distributions, the logarithmic law and the power law of the wall, are also sought applying the same theory. © 2016 Printed in India.

Proceedings of the Indian National Science Academy

Scaling laws of rough turbulent flows from turbulence phenomenology: An overview and a new application

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal. © 2015 American Society of Civil Engineers.

Marchi's research on supercritical flow in tight bends and backwater effects

The installation of free falling jet grade control structures has become a popular choice for river bed stabilization. However, the formation and development of scour downstream of the structure may lead to failure of the structure itself. The current approaches to scour depth prediction are generally based on studies conducted with the absence of upward seepage. In the present study, the effects of upward seepage on the scour depth were investigated. A total of 78 tests without and with the application of upward seepage were carried out using three different sediment sizes, three different tailwater depths, four different flow discharges, and four different upward seepage flow discharge rates. In some tests, the three-dimensional components of the flow velocity within the scour hole were measured for both the cases with and without upward seepage. The scour depth measured for the no-seepage results compared well with the most accurate relationship found in the literature. It was found that generally the upward seepage reduced the downward velocity components near the bed, which led to a decrease in the maximum scour depth. A maximum scour depth reduction of 49% was found for a minimum tailwater depth, small sediment size, and high flow discharge. A decay of the downward velocity vector within the jet impingement was found due to the upward seepage flow velocity. The well known equation of D'Agostino and Ferro was modified to account for the effect of upward seepage, which satisfactorily predicted the experimental scour depth, with a reasonable average error of 10.7%. © 2016 Shafai-Bejestan et al.