Showing papers on "Open-channel flow published in 2017"

Pre‐Darcy Flow in Porous Media

Abstract: Fluid flow in porous media is very important in a wide range of science and engineering applications. The entire establishment of fluid flow application in porous media is based on the use of an experimental law proposed by Darcy (1856). There are evidences in the literature that the flow of a fluid in consolidated and unconsolidated porous media does not follow Darcy law at very low fluxes, which is called pre-Darcy flow. In this paper, the unsteady flow regimes of a slightly compressible fluid under the linear and radial pre-Darcy flow conditions are modeled and the corresponding highly nonlinear diffusivity equations are solved analytically by aid of a generalized Boltzmann transformation technique. The influence of pre-Darcy flow on the pressure diffusion for homogeneous porous media is studied in terms of the nonlinear exponent and the threshold pressure gradient. In addition, the pressure gradient, flux, and cumulative production per unit area are compared with the classical solution of the diffusivity equation based on Darcy flow. The presented results advance our understanding of fluid flow in low-permeability media such as shale and tight formations, where pre-Darcy is the dominant flow regime.

Formation of sediment patterns in channel flow: minimal unstable systems and their temporal evolution

Abstract: The phenomenon of sediment pattern formation in a channel flow is numerically investigated by performing simulations which resolve all the relevant length and time scales of the problem. The numerical approach employed and the flow configuration considered is identical to our previous study (Kidanemariam & Uhlmann J. Fluid Mech., vol. 750, 2014, R2), the only difference being the length of the computational domain. The latter was systematically varied in order to investigate its influence on the initiation and evolution aspects. By successively reducing the streamwise length, the minimum box dimension which accommodates an unstable sediment bed is revealed, thus determining the lower threshold of the unstable modes. For the considered parameter point, the cutoff length for pattern formation lies in the range 75–100 times the particle diameter (3–4 times the clear fluid height). We also simulate the flow in a very long streamwise box with a size of 48 times the clear fluid height (featuring well over one million particles), accommodating approximately 11 initial ripple units with a wavelength in the range of 100–110 particle diameters. The evolution of the amplitude of the patterns exhibits two regimes of growth: an initial exponential regime, with a growth rate independent of the chosen domain size, and a subsequent nonlinear regime which is strongly constrained by the domain length. In the small domain cases, after the initial exponential regime, the ripples evolve steadily, maintaining their shape and migration velocity, at a mean wavelength equal to the length of the domain. The asymmetric ripple shape is characterized by a spectrum which exhibits a power-law decay over the first few dominant non-dispersive modes propagating at the mean dune migration velocity. The rate of particle transport and the mean interface shear stress exhibited an increase with increasing ripple dimensions. Nevertheless, the relationship between the two was observed to be approximately described by the empirical power-law formula for sediment transport by Wong & Parker (J. Hydraul. Engng, vol. 132, 2006, pp. 1159–1168).

Very large scale motions in rough-bed open-channel flow

Abstract: Long-duration particle image velocimetry measurements in rough-bed open-channel flows (OCFs) reveal that the pre-multiplied spectra of the streamwise velocity have a bimodal distribution due to the presence of large- and very-large-scale motions (LSMs and VLSMs, respectively). The existence of VLSMs in boundary layers, pipes and closed channels has been acknowledged for some time, but strong supporting evidence for their presence in OCF has been lacking. The data reported in this paper fill this gap. Length scales of the LSMs and VLSMs in OCF exhibit different scaling properties; whereas the streamwise length of the LSM scales with the flow depth, the VLSM streamwise length does not scale purely with flow depth and may additionally depend on other scales such as the channel width, roughness height or viscous length. The transverse extent of the LSMs was found to increase with increasing elevation, but the VLSM transverse scale is anchored around two flow depths. The origin and nature of LSMs and VLSMs are still to be resolved, but differences in their scaling suggest that VLSMs in rough-bed OCFs form independently rather than as a spatial alignment of LSMs.

The variation of flow and turbulence across the sediment-water interface

Abstract: A basic framework characterising the interaction between aquatic flows and permeable sediment beds is presented here. Through the permeability Reynolds number (ReK = Ku/ν, where K is the sediment permeability, uis the shear velocity and ν is the fluid viscosity), the framework unifies two classical flow typologies, namely impermeable boundary layer flows (Rek ≪1) and highly permeable canopy flows (Rek ≫ 1) . Within this range, the sediment-water interface (SWI) is identified as a transitional region, with Rek in aquatic systems typically O(0.001-10). As the sediments obstruct conventional measurement techniques, experimental observations of interfacial hydrodynamics remain extremely rare. The use of refractive index matching here allows measurement of the mean and turbulent flow across the SWI and thus direct validation of the proposed framework. This study demonstrates a strong relationship between the structure of the mean and turbulent flow at the SWI and Rek. Hydrodynamic characteristics, such as the interfacial turbulent shear stress, velocity, turbulence intensities and turbulence anisotropy tend towards those observed in flows over impermeable boundaries as ReK → 0 and towards those seen in flows over highly permeable boundaries as Rek →. A value of Rek ≈ 1-2 is seen to be an important threshold, above which the turbulent stress starts to dominate the fluid shear stress at the SWI, the penetration depths of turbulence and the mean flow into the sediment bed are comparable and similarity relationships developed for highly permeable boundaries hold. These results are used to provide a new perspective on the development of interfacial transport models at the SWI.

Theory to predict particle migration and margination in the pressure-driven channel flow of blood

Abstract: A coarse-grained theory quickly and accurately estimates the cross-flow transport of red blood cells and platelets in pressure-driven channel flow. The deformability-induced cell migration and shear-induced diffusion are the keys to understanding blood suspensions at low Reynolds number.

Experimental study on the multimodal dynamics of the turbulent horseshoe vortex system around a circular cylinder

Abstract: A turbulent horseshoe vortex (HV) system is generated around a wall-normal cylinder when the approaching boundary layer separates from the wall. This study investigates the dynamics of the turbulent HV system around a circular cylinder in open channel flows with cylinder Reynolds numbers ranging from 8600 to 13900. The velocity fields in the upstream symmetry plane of the cylinder are measured using time-resolved particle image velocimetry. The joint probability density function of the streamwise and vertical velocities in the HV system region is found to exhibit three peaks, indicating that three major types of flow events are induced by the turbulent HV system. The conditional averaged velocity fields based on the characteristic velocity vectors of these events are obtained by using the method of linear stochastic estimation. The estimated flow fields reveal that the turbulent HV system interplays mainly among the back-flow, intermediate, and zero-flow modes. These modes are present for the smallest, mo...

On the flow and coherent structures generated by a circular array of rigid emerged cylinders placed in an open channel with flat and deformed bed

Abstract: The flow and the turbulence structure generated by a circular porous cylinder of diameter containing solid cylinders of diameter placed in an open channel of depth are investigated using eddy-resolving simulations which resolve the wakes past the individual solid cylinders in the array. The solid cylinders extend from the bed through the water surface. This geometrical set-up is directly relevant to understand the physics of flow past an emerged patch of aquatic vegetation developing in a river channel or over its floodplain. Simulations are conducted with different solid volume fractions (SVFs) of the porous cylinder ( ), relative diameters of the solid cylinders ( and 0.06) and with flat and equilibrium scour bathymetry corresponding to the start and respectively the end of the erosion and deposition process. Comparison with the limiting case of a solid cylinder ( ) is also discussed. The bed shear stress distributions and the turbulent flow fields are used to explain the sediment erosion mechanisms inside and around the porous cylinder. Simulations of the flat-bed cases reveal that for sufficiently large SVF values ( ), necklace vortices form around the upstream face of the cylinder, the downflow penetrates partially inside the porous cylinder and a region of strong flow acceleration forms on the sides of the porous cylinder. These flow features are used to explain the development of scour around high-SVF porous cylinders. The effects of the SVF and on generating ‘corridors’ of strong flow acceleration in between the solid cylinders and energetic eddies in the wake of these cylinders are discussed, as these flow features control the amplification of the bed shear stress inside the porous cylinder. Simulations results are also used to quantify the time-averaged drag forces on the cylinders in the array, to identify the regions where these forces are comparable to those induced on an isolated cylinder and the percentage of cylinders in the array subject to relatively large mean drag forces. A logarithmic decrease of the mean time-averaged streamwise drag coefficient of the solid cylinders, , with increasing non-dimensional frontal area per unit volume of the porous cylinder, , is observed. Behind the cylinder, the eddies shed in the separated shear layers (SSLs) of the porous cylinder, and, for sufficiently large SVFs, the von Karman wake billows are the main coherent structures responsible for the amplification of the bed shear stress and sediment entrainment. This paper also analyses the vertical non-uniformity of the mean flow and turbulent kinetic energy, and discusses how the SVF and bathymetry affect the spatial extent of the wake region (e.g. length of the SSLs and steady wake, total wake length) and other relevant variables (e.g. strength of the bleeding flow, dominant wake frequencies, turbulence amplification in the near wake). For the relatively shallow flow conditions ( ) considered, the simulation results show that the antisymmetric (von Karman) shedding of wake billows behind the porous cylinder is greatly weakened once equilibrium scour conditions are approached. Comparison with data from laboratory experiments and from 3-D and 2-D simulations conducted for long porous cylinders (no bed) is also discussed.

Destabilizing turbulence in pipe flow

Abstract: Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities, so far no methods exist to altogether eliminate turbulence if velocities are sufficiently large. We demonstrate for pipe flow that appropriate distortions to the velocity profile lead to a complete collapse of turbulence and subsequently friction losses are reduced by as much as 95%. Counterintuitively, the return to laminar motion is accomplished by initially increasing turbulence intensities or by transiently amplifying wall shear. The usual measures of turbulence levels, such as the Reynolds number (Re) or shear stresses, do not account for the subsequent relaminarization. Instead an amplification mechanism measuring the interaction between eddies and the mean shear is found to set a threshold below which turbulence is suppressed beyond recovery.

Simulations of natural transition in viscoelastic channel flow

Abstract: Orderly, or natural, transition to turbulence in dilute polymeric channel flow is studied using direct numerical simulations of a FENE-P fluid. Three Weissenberg numbers are simulated and contrasted to a reference Newtonian configuration. The computations start from infinitesimally small Tollmien–Schlichting (TS) waves and track the development of the instability from the early linear stages through nonlinear amplification, secondary instability and full breakdown to turbulence. At the lowest elasticity, the primary TS wave is more unstable than the Newtonian counterpart, and its secondary instability involves the generation of -structures which are narrower in the span. These subsequently lead to the formation of hairpin packets and ultimately breakdown to turbulence. Despite the destabilizing influence of weak elasticity, and the resulting early transition to turbulence, the final state is a drag-reduced turbulent flow. At the intermediate elasticity, the growth rate of the primary TS wave matches the Newtonian value. However, unlike the Newtonian instability mode which reaches a saturated equilibrium condition, the instability in the polymeric flow reaches a periodic state where its energy undergoes cyclical amplification and decay. The spanwise size of the secondary instability in this case is commensurate with the Newtonian -structures, and the extent of drag reduction in the final turbulent state is enhanced relative to the lower elasticity condition. At the highest elasticity, the exponential growth rate of the TS wave is weaker than the Newtonian flow and, as a result, the early linear stage is prolonged. In addition, the magnitude of the saturated TS wave is appreciably lower than the other conditions. The secondary instability is also much wider in the span, with weaker ejection and without hairpin packets. Instead, streamwise-elongated streaks are formed and break down to turbulence via secondary instability. The final state is a high-drag-reduction flow, which approaches the Virk asymptote.

Direct numerical simulations of supersonic turbulent channel flows of dense gases

Abstract: The influence of dense-gas effects on compressible wall-bounded turbulence is investigated by means of direct numerical simulations of supersonic turbulent channel flows Results are obtained for PP11, a heavy fluorocarbon representative of dense gases, the thermophysics properties of which are described by using a fifth-order virial equation of state and advanced models for the transport properties In the dense-gas regime, the speed of sound varies non-monotonically in small perturbations and the dependency of the transport properties on the fluid density (in addition to the temperature) is no longer negligible A parametric study is carried out by varying the bulk Mach and Reynolds numbers, and results are compared to those obtained for a perfect gas, namely air Dense-gas flow exhibits almost negligible friction heating effects, since the high specific heat of the fluids leads to a loose coupling between thermal and kinetic fields, even at high Mach numbers Despite negligible temperature variations across the channel, the mean viscosity tends to decrease from the channel walls to the centreline (liquid-like behaviour), due to its complex dependency on fluid density On the other hand, strong density fluctuations are present, but due to the non-standard sound speed variation (opposite to the mean density evolution across the channel), the amplitude is maximal close to the channel wall, ie in the viscous sublayer instead of the buffer layer like in perfect gases As a consequence, these fluctuations do not alter the turbulence structure significantly, and Morkovin’s hypothesis is well respected at any Mach number considered in the study The preceding features make high Mach wall-bounded flows of dense gases similar to incompressible flows with variable properties, despite the significant fluctuations of density and speed of sound Indeed, the semi-local scaling of Patel et al (Phys Fluids, vol 27 (9), 2015, 095101) or Trettel & Larsson (Phys Fluids, vol 28 (2), 2016, 026102) is shown to be well adapted to compare results from existing surveys and with the well-documented incompressible limit Additionally, for a dense gas the isothermal channel flow is also almost adiabatic, and the Van Driest transformation also performs reasonably well The present observations open the way to the development of suitable models for dense-gas turbulent flows

Magnetic field effect on Poiseuille flow and heat transfer of carbon nanotubes along a vertical channel filled with Casson fluid

Abstract: Applications of carbon nanotubes, single walls carbon nanotubes (SWCNTs) and multiple walls carbon nanotubes (MWCNTs) in thermal engineering have recently attracted significant attention. However, most of the studies on CNTs are either experimental or numerical and the lack of analytical studies limits further developments in CNTs research particularly in channel flows. In this work, an analytical investigation is performed on heat transfer analysis of SWCNTs and MWCNTs for mixed convection Poiseuille flow of a Casson fluid along a vertical channel. These CNTs are suspended in three different types of base fluids (Water, Kerosene and engine Oil). Xue [Phys. B Condens. Matter 368, 302–307 (2005)] model has been used for effective thermal conductivity of CNTs. A uniform magnetic field is applied in a transverse direction to the flow as magnetic field induces enhancement in the thermal conductivity of nanofluid. The problem is modelled by using the constitutive equations of Casson fluid in order to characterize the non-Newtonian fluid behavior. Using appropriate non-dimensional variables, the governing equations are transformed into the non-dimensional form, and the perturbation method is utilized to solve the governing equations with some physical conditions. Velocity and temperature solutions are obtained and discussed graphically. Expressions for skin friction and Nusselt number are also evaluated in tabular form. Effects of different parameters such as Casson parameter, radiation parameter and volume fraction are observed on the velocity and temperature profiles. It is found that velocity is reduced under influence of the exterior magnetic field. The temperature of single wall CNTs is found greater than MWCNTs for all the three base fluids. Increase in volume fraction leads to a decrease in velocity of the fluid as the nanofluid become more viscous by adding CNTs.

How vegetation in flows modifies the turbulent mixing and spreading of jets

Abstract: While studies on vegetated channel flows have been developed in many research centers, studies on jets interacting with vegetation are still rare. This study presents and analyzes turbulent jets issued into an obstructed cross-flow, with emergent vegetation simulated with a regular array of cylinders. The paper presents estimates of the turbulence diffusion coefficients and the main turbulence variables of jets issued into a vegetated channel flow. The experimental results are compared with jets issued into unobstructed cross-flow. In the presence of the cylinder array, the turbulence length-scales in the streamwise and transverse directions were reduced, relative to the unobstructed crossflow. This contributed to a reduction in streamwise turbulent diffusion, relative to the unobstructed conditions. In contrast, the transverse turbulent diffusion was enhanced, despite the reduction in length-scale, due to enhanced turbulent intensity and the transverse deflection of flow around individual cylinders. Importantly, in the obstructed condition, the streamwise and transverse turbulent diffusion coefficients are of the same order of magnitude.

Study of surface wettability effect on cavitation inception by implementation of the lattice Boltzmann method

Abstract: Cavitating flow through the orifice is numerically solved by implementation of the lattice Boltzmann method. The pseudo-potential single-component multiphase Shan-Chen model is used to resolve inter-particle interactions and phase change between the liquid and its vapor. The effect of surface wettability on the cavity formation and shape is studied by imposing an appropriate wall boundary condition for the contact angle between the liquid-vapor interface and the solid surface. Efficiency of the numerical approach presented is examined by computing the cavitation inception, growth, and collapse for internal cavitating flows over a sack-wall obstacle placed inside a channel and through a convergent-divergent nozzle section. The results obtained demonstrate that hydrophobic walls act as surface nuclei and contribute to the process of cavitation inception even at high cavitation numbers. In contrast, the solid wall with hydrophilic properties shows no contribution to the onset of cavitation in the geometries ...

Spectral analysis of near-wall turbulence in channel flow at Re τ =4200 with emphasis on the attached-eddy hypothesis

Abstract: Results from a large direct numerical simulation of channel flow with a friction Reynolds number of 4200 are used to examine the structural and spectral properties of near-wall turbulence.

How Different Cross-Sectional Shapes Influence the Separation Zone of an Open-Channel Confluence

Abstract: A key feature of a schematized open-channel confluence is the separation zone that is present when the tributary flow detaches from the downstream corner of the confluence. This zone of rec...

Grout Spread and Injection Period of Silica Solution and Cement Mix in Rock Fractures

Abstract: A systematic presentation of the analytic relations of grout spread to the time period is established They are divided following the nature of the flow, the property of the mix and the driving process This includes channel flow between parallel plates and radial flow between parallel discs, nonlinear Newtonian fluids like silica solution, polyurethane and epoxy, and Bingham material like cement-based grout, and three grouting processes at a constant flow rate, constant pressure and constant energy The analytic relations for the constant energy process are new and complete the relations of the constant flow rate and constant pressure processes The well-known statement that refusal cannot be obtained during finite time for any injected material at a constant flow rate or constant injection pressure is extended to include the energy process The term refusal pressure or energy cannot be supported for stop criteria Stop criteria have to be defined considering confirmed relation of the spread to the time period and of the flow rate to the pressure and spread It is shown that it is always possible to select a grouting process along which the work will exceed any predefined energy, the consequence of which is that jacking is related to the applied forces and not to the injected energy Furthermore, a clarification is undertaken concerning the radial flow rate of a Bingham material since there are two different formulations Their difference is explained and quantified Finally, it is shown that the applied Lugeon theory is not supported by the analytic relations and needs to be substantially modified

Insight into the boundary layer flow of non-Newtonian Eyring-Powell fluid due to catalytic surface reaction on an upper horizontal surface of a paraboloid of revolution

Abstract: Bonnet of a car, the upper surface of a pointed bullet and upper surface of the pointed part of an aircraft are typical examples of an upper horizontal surface of a paraboloid of revolution (uhspr) However, the flow of some fluids past these kinds of objects fit the description of Eyring-Powell fluid flow Theoretical investigation of two-dimensional Eyring-Powell fluid flow over such object which is neither cone/wedge nor horizontal/vertical is investigated It is assumed that the flow of Eyring-Powell fluid is induced by catalytic surface reaction and stretching fluid layers at the free stream The numerical solutions of the governing equation are obtained using classical fourth order Runge-Kutta scheme together with shooting techniques The impacts of the most important parameters on the flow are presented It is concluded that the maximum velocity of the flow is ascertained when the flow is characterized as Newtonian fluid flow On the surface of uhspr, rapid increase and suppress in the temperature distribution and concentration with an increase in the magnitude of one of the Eyring-Powell fluid parameters are guaranteed A significant fall in the local skin friction coefficients is ascertained due to rise in the magnitude of thickness parameter

Unsteady Flow in Open Channels

Abstract: 6.1.1 Introduction. In considering unsteady flow in channels it is necessary to add time as a variable to the other combinations of hydraulic variables considered primarily in the first three chapters. By virtue of its free surface, unsteady channel flow is essentially non-uniform flow and it can therefore be conveniently considered in two categories, gradually varied and rapidly varied unsteady flow. Such a classification is made on the same basis as that used for steady non-uniform flow. The first category will have almost parallel streamlines, gradual changes in depth, vertical accelerations will play a minor part and channel friction forces a large one. The second category will be characterised by very pronounced curvature of the streamlines, rapid changes in depth and sometimes discontinuous water surface profiles, large vertical accelerations and channel friction forces of secondary importance.

Wake interactions in a fluid flow past a pair of side-by-side square cylinders in presence of mixed convection

Abstract: In this research article, we investigated the phenomena of a buoyancy-driven cross-flow impinging on a bulk flow from the inlet for a flow past a pair of side-by-side square cylinders in a confined channel wall (which are kept in an adiabatic condition), a special case of an “internal flow” type problem. The density difference in the flow was achieved through a subtle temperature difference between the ambient fluid and the solid walls present in the domain. The study has been carried out at the Reynolds number Re = 1–40 for a transverse gap ratio s/d = 0.7–10 and the Richardson number Ri = 0–1 at a constant value of the Prandtl number Pr = 50. During a rigorous parametric study, we found that the mixed convection not only brings an early unsteadiness in the flow but also fetches an early formation of different flow regimes at Re = 40. An effort has been made to identify the precise near-wake formations leading to the vortex shedding processes in a mixed convection flow for a various range of s/d values. ...