761 research outputs found
Prediction of overtopping dike failure: sediment transport and dynamic granular bed deformation model
No full textEarth dike failure due to overtopping flow produces a significant displacement of bed-sediment mass. While sediment dynamics typically prevails as a nonequilibrium condition, the bed deformation is resulted from the simultaneous bed erosion and dike body displacement. However, most of the existing shallow water erosion models do not resolve the dynamic bed deformation. In this study, a depth-averaged nonequilibrium sediment transport model is developed and coupled with a set of Savage-Hutter type equations to characterize the dynamic bed deformation, leading to an innovative approach to tackle flows over erodible/deformable beds. The one-sided first-order upwind finite-volume method was adopted for the solution of the system of conservation laws of flow and bed deformation (granular mass transport). A static resistance condition for granular mass was thoroughly defined to preserve the numerical stability of the equations representing the bed deformation component of the model. The model was tested against the experimental data sets for dike overtopping flow, dam-break flow of dry granular mass on a slope, and the analytical solutions. The proposed model enhances the prediction capability by the existing shallow water equations-based equations due to the dynamic bed deformation modeling.</p
Maximum scour depth at piers in armor-beds
No full textThe paper presents a design method to determine the maximum equilibrium scour depth at piers embedded in a sand-bed overlain by a thin armor-layer of gravels using the experimental data of Dey and Raikar and those of Ettema. The proposed equation is in terms of empirical relationships, termed K-factors, which account for the effects of flow depth, pier shape, flow intensity, bed sediment size and armor gravel size on scour depth for individual cases of scour holes as identified by Dey and Raikar. These K-factors are determined by fitting envelope curves to the experimental data. The estimated largest possible scour depths that can occur at circular and square piers with an armor-layer are 3.15 and 3.47 times the pier width, respectively
Controls on grain-size distribution in an ancient sand sea
No full textThe authors would like to thank Subhasish Dey for editing, Claiton Scherer and an anonymous reviewer for their constructive reviews. We thank Bernardo Peixoto for reviewing the manuscript and palaeocoordinate conversion code debug. We thank several colleagues from the all over the world that joined us during field-trips in Brazil, Uruguay and Namibia.Peer reviewe
Temporal scales for live-bed scour at abutments
No full textLive-bed scour at a vertical-wall abutment is experimentally investigated with specific attention paid to the conceptual issues concerning the temporal development of local scour phenomenon. First explored are the time scales for the initial rising phase of the time variation of scour depth. An appropriate identification of such scales and of their normalizing parameters makes it possible to recognize a quantitative dependency of nondimensional time scales on flow intensity. Second, the time scales for the subsequent fluctuations around a mean equilibrium value are considered. Experimental results indicate that the quasiperiodical fluctuations of scour depths do not always correspond to those of bed forms. A conceptual model is outlined to explain this aspect
Discharge prediction in compound channels by end depth method
No full textExperimental observations showed that in a compound channel, the free surface elevation transverse to the flow direction in the main channel is different to that in the floodplain in the vicinity of the end section. The non-horizontal transverse free surface profile (sagging at the main channel zone) is due to the differential accelerated flow velocities in the main channel and the floodplains. In this paper, free overfall in compound channels is theoretically modeled, considering vertical subdivisions of flow area into three zones: main channel, left floodplain, and right floodplain. The Froude number in a compound channel should be estimated on the basis of the flow in subdivided zones rather than for the whole cross-section. The finding that the critical depth in the main channel and the floodplains do not coincide has important implications for the determination of the Froude number in compound channels. Applying a momentum equation based on the Boussinesq approximation, equations of end depth that are related to the discharge are developed separately for the subdivided zones. A method to estimate the discharge from the end depths in the main channel and the floodplain is presented through computational steps. The streamwise flow profiles in the main channel and the floodplain, upstream of the end section, are computed using the streamline curvature at free surface. Experiments were conducted in three different shaped symmetrical compound channels to verify the model. The results, obtained using the model, agree satisfactorily with the experimental data.Subhasish Dey & Martin F. Lamber
Bed shear in equilibrium scour around a circular cylinder embedded in a loose bed
No full textThe bed shear stress for a horseshoe vortex in the equilibrium scour hole around the base of a circular cylinder, embedded in a loose bed with a uniform approach flow under a clear water regime, is computed from turbulent boundary layer analysis by integrating the Navier-Stokes equations. Inside the thin boundary layer, which is developed by the reverse flow at the bed due to swirl motion, Pohlhausen's method is applied with an arbitrary 1/7th power distribution of velocity, as was assumed by Weber in solving turbulent boundary layer. Outside the boundary layer, the expressions of 3-D flow field given by Dey are used. Finally the above technique produces two unknowns, namely bed shear stress and boundary layer thickness. The expression of wall shear stress given by Blasius for turbulent flow in pipes as well as over flat plates is assumed for solving boundary layer thickness, and subsequently bed shear stress. The computational results of bed shear stress and boundary layer thickness are presented
Free overfall from circular channels with flat base
No full textThis paper presents a comparative study between the results obtained from the mathematical model and the experimental data of free overfall in circular channels (with and without flat base), following statistical and error analyses of data to ascertain the adequacy of the mathematical model. For the discharge predictions, the mathematical model tuned by an adjustable parameter is found to be accurate one, while the regression (empirical) formula fails to estimate discharge precisely
Secondary boundary layer and wall shear for fully developed flow in curved pipes
No full textThe secondary motion of incompressible fluid for a fully developed turbulent flow in horizontally laid curved pipes is theoretically analysed with the aid of the turbulent boundary-layer approach, integrating the Navier-Stokes equations and satisfying the continuity equation, having mean-time-average parameters. Outside the boundary layer that is developed due to the secondary flow, the inviscid motion of fluid is treated using the Euler and continuity equations. Inside the thin boundary layer, the Pohlhausen method is applied considering a one-seventh power distribution of velocity for solving the integrals of the Navier-Stokes equations. The expression of shear given by Blasius for the turbulent flow in pipes is incorporated into the integrals of the Navier-Stokes equations as an apparent shear to derive an implicit equation that is numerically solved to determine the boundary-layer thickness and subsequently the wall shear stress. The computational results of the boundary-layer thickness and the wall shear stresses are presented in non-dimensional (graphical) form for different Reynolds numbers and curvature ratios. The model is applicable over a wide range of Dean numbers (D) 57 ≤ D ≤ × 105
Hecalus shanayai Nikoshe & Meshram & Dey 2020, sp. nov.
No full textHecalus shanayai Nikoshe & Meshram, sp. nov. (Figs. 3, 8, 13, 34–40) Male (Figs. 3, 8) Color yellowish green to green. Anterior margin of head with transverse submarginal fuscous line, face (Fig. 13) brown, gena black, frontoclypeus transverse black striae invisible in middle. Compound eyes black, ocelli yellow. Forewings subhyaline, with diffuse fuscous transverse band across costal margin. Bases of tibial macrosetae and tarsi light brown. Head including eyes as long as width of pronotum. Head produced in front, crown length 0.7x shorter than width between eyes. Face as long as wide; anterior margin of pronotum not extending beyond eyes, more or less parallel with eyes. Frontoclypeus longer than wide; frontal suture, terminating laterad of ocelli. Ocelli near anterior margin, very closely oppressed to eyes. Pronotum length 0.2x as long as wide and 0.9x length of scutellum. Male genitalia. Pygofer (Fig. 34) longer than wide, posterior margin triangular, with posterior ½ densely setose. Style (Fig. 36) with preapical lobe obtusely angulate, with few microsetae, apophysis short, 0.25 of the total length. Connective (Fig. 40) with stem 2x shorter than arms. Aedeagal shaft narrowed apically, broad medially in lateral view, foliate in apical 0.2 with sparsely serrated margin, dorsal margin with short subapical tooth, with pair of slender apical processes half as long as shaft extended anterolaterad, gonopore subapical on ventral margin (Figs. 37–39). Measurements (mm). Male 4.63 long, 1.3 wide across eyes, 1.01 wide across hind margin of pronotum. Type material. Holotype ³, INDIA: Maharashtra: Chikhaldara (21.4030° N, 77.3268° E), 22.ix.2015, Sweep net Coll. Akash Nikoshe (NPC). Paratypes, 6 ³ with same data as holotype. Etymology. This species is named in honour of “Miss Shanaya” (daughter of the corresponding author) for her support to the author to pursue his passion in leafhopper taxonomy. Remarks. Hecalus shanayai sp. nov. externally resembles H. lutescens (Distant) (Figs. 2, 7, 12, 27–33) but differs in having the aedeagal shaft foliate in the apical 0.2 with a sparsely serrated margin, the dorsal margin with a short subapical tooth, and lacking mid-dorsal lateral expansions which are more distinct in the latter. From H. ghauri Rao and Ramakrishnan, (Figs. 19, 20 & 21) it can be distinguished by the aedeagal shaft foliate in the apical 0.2 with a sparsely serrated margin, the dorsal margin with a short subapical tooth, and lacking mid-dorsal lateral expansions more distinct in the former.Published as part of Nikoshe, Akash P., Meshram, Naresh M., Stuti & Dey, Debjani, 2020, Indian Hecalina (Hemiptera: Cicadellidae: Deltocephalinae: Hecalini) with description of three new species, pp. 573-585 in Zootaxa 4881 (3) on pages 576-578, DOI: 10.11646/zootaxa.4881.3.9, http://zenodo.org/record/428398
Double-averaging turbulence characteristics in seeping rough-bed streams
No full textResults of an experimental study on the effects of bed injection (upward seepage from the permeable bed) and suction (downward seepage) on the double-averaged (DA) turbulent flow characteristics over and within the interface of flow gravel beds are presented. The DA Reynolds shear stress within the interfacial sublayer undergoes a higher damping with suction and a lesser damping with injection, as compared to that with no seepage. The form‐induced stress within the interfacial sublayer has a decelerating effect with no seepage and suction and an accelerating effect with injection. The quadrant analysis suggests that within the interfacial sublayer the sweep events in flows with injection and the ejection events with suction are the primary contributors toward the Reynolds shear stress production, while both the events in flows with no seepage are equally prevalent. In the roughness sublayer (form-induced and interfacial sublayers together), the mean duration of bursting increases with suction and decreases with injection, but the bursting events become less frequent with suction and more frequent with injection. The analysis of DA higher-order moments confirms that the seepage affects the Reynolds normal stress flux, the turbulent kinetic energy (TKE) flux, and the TKE budget. A downward-upstream flux of TKE is developed within the roughness sublayer with injection; while an upward-downstream flux of TKE is prevalent over the entire flow depth with suction. The significance of this study lies on the hydrodynamic process in such a physical system that may result in modifying the flow resistance, sediment entrainment, and morphological characteristics of a streambed
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