Department of Civil & Infrastructure Engineering

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Bed particle entrainment by turbulent wall-shear flow is a key topic of interest in hydrodynamics because it plays a major role to govern the planetary morphodynamics. In this paper, the state-of-the-art review of the essential mechanisms governing the bed particle entrainment by turbulent wall-shear flow and their mathematical modeling is presented. The paper starts with the appraisal of the earlier multifaceted ideas in modeling the particle entrainment highlighting the rolling, sliding, and lifting modes of entrainment. Then, various modeling approaches of bed particle entrainment, such as deterministic, stochastic, and spatiotemporal approaches, are critically analyzed. The modeling criteria of particle entrainment are distinguished for hydraulically smooth, transitional, and rough flow regimes. In this context, the responses of particle size, particle exposure, and packing condition to the near-bed turbulent flow that shears the particles to entrain are discussed. From the modern experimental outcomes, the conceptual mechanism of particle entrainment from the viewpoint of near-bed turbulent coherent structures is delineated. As the latest advancement of the subject, the paper sheds light on the origin of the primitive empirical formulations of bed particle entrainment deriving the scaling laws of threshold flow velocity of bed particle motion from the perspective of the phenomenological theory of turbulence. Besides, a model framework that provides a new look on the bed particle entrainment phenomenon stemming from the stochastic-cum-spatiotemporal approach is introduced. Finally, the future scope of research is articulated with open questions. © 2018 Author(s).

World University root node

Physics of Fluids

Review Article: Advances in modeling of bed particle entrainment sheared by turbulent flow

An analytical solution of the standard k-ε model is proposed to study the turbulent characteristics of nonuniform flow. Initially, the most suitable functional forms are assumed for turbulent kinetic energy, k, turbulence dissipation rate, e, and streamwise velocity, u, and the coefficients of the power series of all three variables are determined by solving Reynolds equation and the standard k-ε equation for nonuniform flow. Additionally, to investigate the effect of the damping function on velocity and length scale, an analytical solution is further formulated by incorporating the damping function into the basic equations. The appropriateness of the analytical solution is verified with experimental data for the most accelerated flow and the most decelerated flow. The profile of turbulent energy dissipation increased near the free surface when the damping function was included in the analytical solution. Moreover, depending on the nature of the nonuniform flow, the turbulent energy is dissipated more for accelerated flow than for decelerated flow. The eddy viscosity reduced near the free surface due to the pronounced effect of the damping function. Turbulent mixing is also damped in the accelerated flow and amplified in the decelerated flow. Finally, the effective distributions of k, ε, u, and eddy viscosity for nonuniform flow ensured the desirability of the functional form presumed for turbulent kinetic energy, turbulence dissipation rate, and velocity. © 2018 American Society of Civil Engineers.

Journal of Hydraulic Engineering

Analytical solution of k-ε model for nonuniform flows

In this study, macro-rough flows over beds with different permeability values are simulated using the large-eddy simulation, and the results are analysed by applying the double-averaging (DA) methodology. Spheres of different sizes and arrangements were used to form the beds, which are deemed to be permeable granular beds. The influence of bed permeability on the turbulence dynamics and structure is investigated. It was observed that the scales of the spanwise vortical structures over more permeable beds are larger than those over less permeable beds. This is attributed to large-scale spanwise-alternate strips of varying Reynolds shear stress (RSS), emerging from the surface of macro-rough elements for the permeable beds. The DA stress balance suggests that the time-averaged spanwise vortical structure leads to a damping in DA RSS and an unusual peak of the form-induced stress in the main flow. In the streamwise direction, both large turbulent structures that originate from the Kelvin-Helmholtz-type instability and small turbulent structures that are associated with the turbulent transport across the gaps of the roughness elements are more prevalent over highly permeable beds. Near the bed, the relative magnitude of turbulent events shows a transition from a ejections-dominating to sweeps-dominating zone with vertical distance. Further, several hydrodynamic characteristics normalized by inner scales (kinematic viscosity to shear velocity ratio) show a greater dependency on permeability Reynolds number than those normalized by sediment size. The study provides an insight into the mechanism of mass transfer near the fluid-particle interface, which is vital to benthic and aquatic ecology. © 2018 Cambridge University Press.

Journal of Fluid Mechanics

Influence of permeable beds on hydraulically macro-rough flow

Gasoline direct injection (GDI) vehicles have recently been identified as a significant source of carbonaceous aerosol, of both primary and secondary origin. Here we investigated primary emissions and secondary organic aerosol (SOA) from four GDI vehicles, two of which were also retrofitted with a prototype gasoline particulate filter (GPF). We studied two driving test cycles under cold- and hot-engine conditions. Emissions were characterized by proton transfer reaction time-of-flight mass spectrometry (gaseous non-methane organic compounds, NMOCs), aerosol mass spectrometry (sub-micron non-refractory particles) and light attenuation measurements (equivalent black carbon (eBC) determination using Aethalometers) together with supporting instrumentation. Atmospheric processing was simulated using the PSI mobile smog chamber (SC) and the potential aerosol mass oxidation flow reactor (OFR). Overall, primary and secondary particulate matter (PM) and NMOC emissions were dominated by the engine cold start, i.e., before thermal activation of the catalytic after-treatment system. Trends in the SOA oxygen to carbon ratio (O : C) for OFR and SC were related to different OH exposures, but divergences in the H : C remained unexplained. SOA yields agreed within experimental variability between the two systems, with a tendency for higher values in the OFR than in the SC (or, vice versa, lower values in the SC). A few aromatic compounds dominated the NMOC emissions, primarily benzene, toluene, xylene isomers/ethylbenzene and C3-benzene. A significant fraction of the SOA was explained by those compounds, based on comparison of effective SOA yield curves with those of toluene, o-xylene and 1,2,4-trimethylbenzene determined in our OFR, as well as others from literature. Remaining discrepancies, which were smaller in the SC and larger in the OFR, were up to a factor of 2 and may have resulted from diverse reasons including unaccounted precursors and matrix effects. GPF retrofitting significantly reduced primary PM through removal of refractory eBC and partially removed the minor POA fraction. At cold-started conditions it did not affect hydrocarbon emission factors, relative chemical composition of NMOCs or SOA formation, and likewise SOA yields and bulk composition remained unaffected. GPF-induced effects at hot-engine conditions deserve attention in further studies.. © 2018 Copernicus GmbH. All rights reserved.

Fulltext

Atmospheric Chemistry and Physics

Gas-phase composition and secondary organic aerosol formation from standard and particle filter-retrofitted gasoline direct injection vehicles investigated in a batch and flow reactor

Pile foundations in liquefiable soils need to be checked for the bending moments due to the liquefaction induced stresses to arrest the plastic hinge formation. Recent studies reveal that the bending moments in the pile are significantly higher during the partial liquefaction stage (excess pore water pressure ratio <1.0) than at the fully liquefied condition. This study proposes a method to estimate the pile bending moments during partial liquefaction. Advanced multi-stage cyclic triaxial tests are conducted on sandy soil to understand the partial liquefaction behavior. The test results are then utilized to model the soil pile interaction in partially liquefiable soils following the beams on nonlinear Winkler foundation (BNWF) approach. The method is based on mobilized strength design concept. The developed numerical model is analyzed using the traditional lateral soil springs along with the proposed springs. It is understood from this study that the bending response of pile foundations in partially liquefiable soils can be effectively estimated with the proposed methodology compared to the existing models. © ASCE.

Geotechnical Special Publication

Lateral behavior of pile foundations during partial liquefaction

This article presents a framework based on the direct stiffness method for nonlinear thermo-mechanical analysis of reinforced concrete plane frames subjected to fire. It accounts for geometric nonlinearity, material nonlinearity, and nonlinear thermal gradients and incorporates two-way coupling between thermal and structural analyses. Force deformation relations are derived from classical Euler–Bernoulli beam theory and are expressed in terms of temperature-dependent stability and bowing functions. This is one of the unique features of proposed framework and allows a coarser spatial discretization to be used as opposed to full finite element–based approaches (such as SAFIR [registered trademark of the software SAFIR developed at the University of Liege]). The cross sections of the structural members are discretized with two-dimensional meshes for thermal analysis while structural analysis utilizes a line element based on direct stiffness method. Equivalent bending and axial rigidities of this line element are computed using several fibers along the length of the member, passing through the nodes of the two-dimensional mesh used for thermal analysis. The total strain at each fiber is decomposed into mechanical, thermal, creep, and transient thermal components. A discrete damage parameter is introduced at fiber level to ensure irreversibility of crushing and cracking in accordance with relevant constitutive laws. Five numerical examples are presented to demonstrate the accuracy and efficacy of the developed framework with respect to theoretical solutions, experimental observations, and some of the existing macro- and micro-finite element–based approaches. It is found that the developed framework can predict the response of reinforced concrete structures very well. © 2017, © The Author(s) 2017.

Advances in Structural Engineering

Nonlinear analysis of reinforced concrete plane frames exposed to fire using direct stiffness method

International Conference on Water (ICW)-Pollution to Purification

In-situ synthesis of iron-sulfide within the porous media for immobilization of arsenic

Historical observations suggest that severe drought frequency has been increasing in the recent decades over India. In the future climate, significant variability and change in monsoon rainfall and temperature would further affect the future drought conditions. Here, we analysed data from five Global Climate Models (GCMs) that participated in the coupled model intercomparison project-5 (CMIP5) to estimate the changes in drought conditions under the warming climate in India. Our analysis show more frequent droughts in the warming climate in near (2011-2040), mid (2041-2070) and end (2071-2100) period using standardized precipitation evapotranspiration index (SPEI) whereas the drought projections based on just precipitation (Standardized Precipitation Index: SPI) show a decrease in the drought frequency in the 21st century. The change in drought frequency is projected to increase by more than two-three severe droughts per decade over the majority of India using SPEI in the end period. The increased warming resulted in an increased atmospheric water demand (potential evapotranspiration: PET) in the region, which is reflected in the increased drought frequency. The change in area under severe drought is projected to increase by 150% at the end of the 21st century using SPEI against the reference period 1971-2000. Overall, we conclude that the drought frequency of severe drought is projected to increase in warmer and wetter future climate in India and the increased frequency of droughts can largely attributed to increased PET due to anthropogenic warming. This chapter highlights that despite the projected increase in the monsoon season precipitation, the risk of frequent droughts in India is higher under the warming climate, which will have implications to water management in the future.

Impact of Climate Change on Drought Frequency over India

This state-of-the-art review describes the progress in analytical modeling of suspended sediment transport by turbulent wall shear flow. The mechanics of the suspended sediment transport by turbulent flow represented by various concepts, such as the diffusion concept, the energy concept and the stochastic concept, is appraised. The effects of sediment suspension on velocity distribution, von Kármán constant value and turbulence characteristics are highlighted. As the latest development of the subject, this paper explains the scaling law of suspended sediment concentration within the wall shear layer stemming from the energetics of turbulent eddies. This scaling law elucidates that within the wall shear layer, the suspended sediment concentration obeys a unique scaling law with the vertical distance from the bed. Finally, the modeling challenges are outlined as a future scope of research. © 2018 International Association for Hydro-environment Engineering and Research, Asia Pacific Division

Journal of Hydro-Environment Research

Advances in analytical modeling of suspended sediment transport

The study deals with the characteristics of scour downstream of a culvert. The main objective is to study the scour characteristics including the influence of different parameters (tailwater depth, width of the culvert, sediment size, velocity of water through the culvert and drop height) on maximum scour depth. Dimensional analysis has been performed to get the maximum scour depth as a function of the considered parameter. From the experimental investigation it was found that with the increase of densimetric Froude number the maximum equilibrium scour depth increases. The variation of maximum scour depth with tailwater was sagging in nature and a nonlinear relationship exists between maximum scour depth and tailwater depth. On the other hand the variation of maximum scour depth with the grain size was hogging in nature and with an increase in grain size, the maximum scour depth decreases. In this study two polynomials were considered for the scour and dune profile. The expressions were developed to find the coefficients of these two polynomials from the boundary conditions. It was shown that the profiles computed from these equations (for 1 h and 10 h) have a satisfactory agreement with the experimental data. The volume per unit width of the scour hole and the sediment deposition as dune was expressed in terms of the polynomial equations and concluded that 16% of sediment by volume is washed out by the flow. © Proceeding of the 21st LAHR-APD Congress 2018. All rights reserved.

Proceedings - International Association for Hydro-Environment Engineering and Research (IAHR)-Asia Pacific Division (APD) Congress: Multi-Perspective Water for Sustainable Development, IAHR-APD 2018