Channel Of Constant Cross Section Research Articles

Experimental study of the velocity of density currents in convergent and divergent channels

The head velocity of the density current in the convergent and divergent channel is a key parameter for evaluating the extent to which suspended material travels, and for determining the type and distribution of sediment in the water body. This study experimentally evaluated the effects of the reach degree of convergence and divergence on the head velocity of the density current. Experiments were conducted in the flume with 6.0 m long, 0.72 m width and 0.6 m height. The head velocity was measured at three convergent degrees (−8o; −12o; −26o), at three divergent degrees (8o; 12o; 26o) and two slopes (0.009, 0.016) for various discharges. The measured head velocity of the density current is compared with the head velocity of the density current in the constant cross section channel. Based on non-dimensional and statistical analysis, relations as linear multiple regression are offered for predicting head velocity of the density current in the convergent, divergent and constant cross section channel. Also the results of this research show that for the same slope and discharge, the head velocity of the density current in the convergent and divergent channel are greater and less than the head velocity of the constant cross section, respectively.

Parametric investigation of the propagation of detonation in narrow channels filled with a propane-butane-oxygen mixture

In this article, the propagation of the detonation of a propane-butane mixture with oxygen is investigated in a channel of constant cross section with a diameter smaller than the critical one for detonation propagation. The ignition of the mixture is performed in a precombustion chamber. The entering of the flame front into the narrow channel causes the development of detonation in the channel. In this work the profiles of the pressure at the entry into the channel and the average velocities of propagation of the flame front for different sections of the channel are presented. The values of the velocity correspond to the values of the Chapman-Jouguet velocity. In this work various scenarios of the propagation of detonation depending on the size of the precombustion chamber have been investigated.

Approximate calculation of channel flows under force and energy actions

Gasdynamic channel flows under force and energy actions are considered. An approximate method is proposed for solving the gasdynamic equations that describe these flows. Themethod includes the separation of an “active” flow volume in which the integral electric force and applied power, whose densities are assumed to be uniform, are concentrated and the numerical integration of the system of hydrodynamic equations over the entire channel (in laminar and turbulent variants) with the piecewise constant force and energy sources obtained. The results of experimental investigation are presented for the flow that arises after two accessories mounted on the opposite walls of the vertical rectangular channel of constant cross-section, which create a dielectric barrier discharge (DBD actuators). This flow is numerically simulated using the method developed. On the basis of the method proposed the flow characteristics are determined for a model subsonic diffuser on whose lower wall, immediately in front of the separation zone, the DBD actuator is mounted. The efficiency of this accessory in reducing the gasdynamic losses is demonstrated.

Mechanical and Microstructural Evaluation of InsituAluminium Titanium Boride Composite Processed by Severe Plastic Deformation

Nanostructured and ultrafine grained metallic materials have received great attention in recent times because of their unique mechanical, physical properties and high performance. High specific strength of the material is critical for many structural applications in automotive and aerospace design. Different techniques for producing ultra-fine-grained (UFG) materials have been introduced especially in the last decade. Severe plastic deformation (SPD) is one such process to obtain ultrafine grains wherein simple shear stress is applied to a billet during multiple passages through an angled channel of constant cross section. One of such process is equal channel angular pressing (ECAP), which is widely used than any other SPD technique to obtain fine grains. The process is capable of generating very large plastic strains that significantly refines the microstructure without altering the external dimension of the billet and therefore has attracted the attention of a number of researchers underlining the potentials to scale up for industrial applications.In this paper an attempt is made to highlight the significance of severe plastic deformation of insitualuminium titanium boride composite. The composite was processed up to 2 passes following Bc route of ECAP. The hardness, ultimate tensile strength with varied reinforcement content of the ECAP samples were found to be significantly high as compared to ascast matrix alloy. While the microstructure analysis revealed progressive refinement of grains after every pass of ECAP.

Numerical study of the structure flow of the gas-vapor mixture in a channel with injection of water droplets

Mathematical modeling algorithm for the numerical analysis of the dynamics and evaporation of the water droplet which is injected into a gas-vapor flow is offered. The algorithm was tested with the help of the problem solution about a viscous fluid flow in a plane channel of constant cross section. It was shown that the proposed mathematical model and implementation algorithm are adequate to the physics of the investigated process. It was shown that the proposed mathematical model and implementation algorithm are adequate to the physics of the investigated process. Results of numerical study of the flow structure of the gas-vapor mixture in the channel with injection of water droplet under varying the initial droplet parameters are given.

Experimental study of the velocity of density currents in convergent and divergent channels

The head velocity of the density current in the convergent and divergent channel is a key parameter for evaluating the extent to which suspended material travels, and for determining the type and distribution of sediment in the water body. This study experimentally evaluated the effects of the reach degree of convergence and divergence on the head velocity of the density current. Experiments were conducted in the flume with 6.0 m long, 0.72 m width and 0.6 m height. The head velocity was measured at three convergent degrees (−8°; −12°; −26°), at three divergent degrees (8°; 12°; 26°) and two slopes (0.009, 0.016) for various discharges. The measured head velocity of the density current is compared with the head velocity of the density current in the constant cross section channel. Based on non-dimensional and statistical analysis, relations as linear multiple regression offered for prediction head velocity of the density current in the convergent, divergent and constant cross section channel. Also the results of this research show that for the same slope and discharge, the head velocity of the density current in the convergent and divergent channel are greater and less than the head velocity of the constant cross section, respectively.

A numerical study of solidification and viscous dissipation effects on polymer melt flow in plane channels

Abstract The combined effects of solidification and viscous dissipation on the hydrodynamic and thermal behavior of polymer melt flow during the injection process in a straight plane channel of constant cross section are numerically studied by considering the shear-rate and temperature-dependent viscosity and transient-phase change behavior. A numerical finite volume method, in conjunction with a modified form of the Cross constitutive equation to account for shear rate, temperature-dependent viscosity changes and a slightly modified form of the method proposed by Voller and Prakash to account for solidification of the liquid phase, is used and a validation with an analytical solution is presented for viscous heating effects. The hydrodynamic and solidified layers growth under the influence of a transient phase-change process and viscous dissipation, are analyzed for a commercial polymer melt flow, polypropylene (PP) for different parametric conditions namely, inflow velocity, polymer injection (inflow) temperature, the channel wall temperature, and the channel height. The results demonstrate that the proposed numerical formulations, including conjugate effects of viscous heating and transient-solidification on the present thermal transport process, can provide an accurate and realistic representation of polymer melt flow behavior during the injection molding process in plane channels with less simplifying assumptions.

Thermal fluxes in a generator of low temperature plasma with a divergent channel of the outlet electrode

A short review is presented of experimental results concerning distribution of heat fluxes over the length of a generator of low temperature plasma (plasmatron). Comparative analysis is given of the operation of plasma generators with a divergent channel of the outlet electrode against plasma generators with a cylindrical channel of constant cross section. Experimental studies were performed on a specially created facility consisting of four plasma generators with sectioned and continuous outlet electrodes and automated measuring system.

Viscous–inviscid interactions in transonic flows through slender nozzles

Considering the miniaturization trend in technical applications, the need of a slender nozzle theory for such conventional, that is ideal-gas-like, fluids, which accounts for a strong boundary-layer interaction with the core region, arises in quite a natural way as the dimensions of the flow device are successively reduced. Moreover, a number of modern technological processes (e.g. organic Rankine cycles) involve fluids with high molecular complexity, some of which are expected to exhibit embedded regions with negative values of the fundamental derivative in the vapour phase commonly termed Bethe–Zel'dovich–Thompson (BZT) fluids. Linked to it, unconventional Laval nozzle geometries are needed to transform subsonic to supersonic internal flows. In the present paper, the transonic flows through nozzles of short length scales located in a channel of constant cross-section so slender that the flow in the inviscid core region is one-dimensional are considered. Rapid streamwise changes of the flow field caused by the nozzle then lead to a local breakdown of the classical hierarchical boundary-layer approach, which is overcome by the triple-deck concept. Consequently, the properties of the inviscid core and the near-wall (laminar) boundary-layer regions have to be calculated simultaneously. The resulting problem is formulated for both regular (ideal-gas-like) fluids and dense gases. Differences and similarities of the resulting flow pattern with respect to the well-known classical Laval nozzle flow are presented for perfect gases, and the regularizing influence of viscous–inviscid interactions, is examined. Furthermore, the analogous problem is considered for BZT fluids in detail as well. The results indicate that the passage through the sonic point in the inviscid core is strongly affected by the combined influence of nozzle geometry and boundary-layer displacement effects suggesting in turn an inverse Laval nozzle design in order to obtain the desired flow behaviour.

Three-dimensional features in low-Reynolds-number confined corner flows

In recent microfluidic experiments with solutions of bacteria we observed the formation of biofilms in the form of thread-like structures, called ‘streamers’, which float in the middle plane of the channel and are connected to the side walls at the inner corners. Motivated by this observation, we discuss here the pressure-driven low-Reynolds-number flow around a corner bounded by the walls of a channel with rectangular cross-section. We numerically solve the flow field in a channel of constant cross-section, which exhibits 90° sharp corners, or turns with constant curvature, or portions with slowly changing curvature along the flow direction, for finite, but small, values of the Reynolds numbers and including the limit of vanishingly small Reynolds numbers. In addition, we develop a matched asymptotic expansion solution for the flow around two boundaries intersecting at an angle α and spanning the small gap h between two horizontal plates. We illustrate the basic features of the flow in these channel geometries by describing the three-dimensional velocity field and the distribution of streamwise vorticity and helicity, and comparing the numerical solutions with predictions based on the asymptotic approach. We demonstrate that near a corner or a change in the curvature of the side wall the flow is three-dimensional and pairs of counter-rotating vortical structures are present, as identified by Balsa (J. Fluid Mech., vol. 372, 1998, p. 25). Finally, we discuss how this secondary flow depends on the significant geometric parameters, the aspect ratio of the channel cross-section, the radius of curvature of the turn and, more generally, the variation of the curvature of the channel side boundary. We believe that these three-dimensional secondary flow structures are relevant to transport problems where accumulation of material at the boundary is possible.

Study of a low temperature plasma generator with a divergent output electrode channel

Results of experimental studies of a plasma generator with a divergent output electrode channel are presented, and a comparative analysis is performed of operation of this plasma generator and plasma generator with cylindrical channel of constant cross section. Studies of sectioned output electrodes are performed along with plasma generators with solid electrodes. The influence of sectioning on the parameters of electric discharge is studied. It is shown that the optimal degree of divergence of the output electrode channel is a complex function of different parameters.

The Effect of Mixing and Dissipation on A Lock-Exchange Flow Producedin A Channel of Constant Cross-Section

Laboratory lock-exchanges of two fluids of different densities are used to examine the effects of mixing and dissipation on a dense current produced in a simple channel of constant width and height. Turbulent, small-scale mixing, induced by the flow instability at the interfacial layer, was observed above the current head. The mixing is found to affect the flow dynamics, in that the depth of the current is measured to be 0.36-0.47 H, less than a half of the full water depth H, defined as a theoretically predicted depth, and that the non-dimensional current speed is constant at 0.48 0.02, independent of all the external parameters. From these finding, we conclude that there is a loss of energy associated with turbulent dissipation and irreversible mixing and that bottom friction plays a minor role in the propagation speed of the current.

Flow-free transport of cells in microchannels by frequency-modulated ultrasound

We demonstrate flow-free transport of cells and particles by the use of frequency-modulated ultrasonic actuation of a microfluidic chip. Two different modulation schemes are combined: A rapid (1 kHz) linear frequency sweep around approximately 6.9 MHz is used for two-dimensional spatial stabilization of the force field over a 5 mm long inlet channel of constant cross section, and a slow (0.2-0.7 Hz) linear frequency sweep around approximately 2.6 MHz is used for flow-free ultrasonic transport and positioning of cells or particles. The method is used for controlling the motion and position of cells monitored with high-resolution optical microscopy, but can also be used more generally for improving the robustness and performance of ultrasonic manipulation micro-devices.

Mixing by random stirring in confined mixtures

We study the relaxation of initially segregated scalar mixtures in randomly stirred media, aiming to describe the overall concentration distribution of the mixture, its shape and rate of deformation as it evolves towards uniformity. A stirred scalar mixture can be viewed as a collection of stretched sheets, possibly interacting with each other. We consider a situation in which the interaction between the sheets is enforced by confinement and is the key factor ruling its evolution. It consists of following a mixture relaxing towards uniformity around a fixed average concentration while flowing along a constant cross-section channel. The interaction between the sheets is found to be of a random addition nature in concentration space, leading to concentration distributions that are stable by self-convolution. The resulting scalar field is naturally coarsened at a scale much larger than the dissipation scale. Consequences on the mixture entropy and scalar rate of dissipation are also examined.

Control of separation phenomena in a high-speed flow by means of the surface electric discharge

The results of an experimental and theoretical investigation of the interaction between a surface electric discharge and a supersonic air flow in a constant cross-section channel are given. The features of the generation of the surface discharge in the flow are described. A model of the interaction is proposed. The regime of gasdynamic screening of a mechanical obstacle on the channel wall is investigated. Data on the change in the main flow parameters as a result of the generation of a surface discharge are given. The experimental results are compared with the results of calculations based on a simplified model of the interaction.

Mixing Layers in Sink Flow: Effect of Length of Flight on Mixing in a Channel Downstream

We have experimentally and analytically studied transport of a passive scalar in the wake of a thin flat plate located at the centerline of a planar contraction with flat walls. The constant Launder parameter in the contraction, K = 6.25 ×10 − 6, was twice the value required for a turbulent boundary layer to relaminarize. In addition to the mixing analysis inside the contraction, layer mixing is also investigated downstream, where the flow continues inside a constant cross-section channel. In order to generate the passive scalar, the airflow above the plate was heated and the temperature stratification in the wake was traced by measuring the temperature field using constant current anemometry. Using different plate lengths, we found that the degree of mixing, obtained at a given position in the straight channel, is a function of the distance from the plate trailing edge to the contraction outlet. For a plate which does not protrude into the straight channel, we demonstrate the existence of an optimal trailing edge-contraction outlet distance that results in the lowest possible degree of mixing at a given downstream position in the straight channel. This finding is also supported by a semi-empirical relationship based on our developed self-similar solution for mixing layers in planar contractions.

Nonlinear superposition of unidirectional and plane flows of viscoelastic Lodge fluids

The paper deals with steady fully developed flows through straight channels of constant cross-section under the influence of an axial pressure gradient and under the action of a moving wall. Those flow fields are characterized by Cartesian velocity components of the type u(x,y), v(x,y) and w(x,y) and thus may be considered to be the superposition of a longitudinal and a transverse plane part. They possess some striking properties, not only in case of a Newtonian fluid, but also for quasi-linear viscoelastic fluids with single-integral constitutive equations. We find a one-sided coupling between the axial component w and the transverse components u and v. Accordingly, the transverse flow does not depend on the axial boundary conditions and can be treated first as a two-dimensional problem. Afterwards, a linear integro-differential boundary value problem concerning w(x,y) results, from which some superposition theorems follow, especially with respect to integral quantities like the axial volume flux and the power input. This has some general implications regarding the pumping characteristics of the combined pressure-drag flow, and that at arbitrary values of the Reynolds number and of the Deborah number.

Nonisothermal Flow of Polymer Solutions and Melts in Channels of Constant Cross Section

Under the assumption that the temperature of a polymer solution or melt is constant over a channel cross section, the nonisothermal flow in various channels of constant longitudinal geometry was investigated in the cases where the polymeric liquid viscosity varies with temperature exponentially or hyperbolically. It was demonstrated that solving the problem of the flow of polymeric liquids under nonisothermal conditions is reduced to solving identical equations for different channels, and the pressure difference in channels and the power consumption are determined from the corresponding isothermal solutions with allowance made for a certain factor, which is constant for different channels.

Simple waves on a shear gas flow in a channel of constant cross section

The plane-parallel unsteady-state shear gas flow in a narrow channel of constant cross section is considered. The existence theorem of solutions in the form of simple waves of a set of equations of motion is proved for a class of isentropic flows with a monotone velocity profile over the channel depth. The exact solution described by incomplete beta-functions is found for a polytropic equation of state in a class of isentropic flows.

Interaction of fluctuating river flow with a barotropic tide: A demonstration of wavelet tidal analysis methods

Wavelet transforms provide a valuable new tool for analysis of tidal processes that deviate markedly from an assumption of exact periodicity inherent in traditional harmonic analysis. A wavelet basis adapted to nonstationary tidal problems is constructed and employed to analyze the modulation of the external tide in a river by variations in streamflow. Interaction of a surface tide with river flow is the best available demonstration of the continuous wavelet transform (CWT) methods developed. It is the simplest and perhaps the only nonstationary tidal process for which both sufficient data and dynamical understanding exist to allow detailed comparisons between CWT analyses and analytical predictions of the response of tides to nontidal forcing. Variations at upriver locations of low‐frequency elevation (river stage ZR) and three tidal species are deduced from cross‐sectionally integrated equations. For landward propagation in a channel of constant cross section with quadratic friction, the log of the amplitude of the diurnal (D1), semidiurnal (D2) and quarterdiurnal (D4) elevations should vary at far upriver locations with the square root of the river flow (QR;), and river stage (ZR) should depend on the square of river flow. Convergent geometry and species‐species frictional interactions modify these predictions somewhat. CWT analyses show that the predicted amplitude behavior for the tidal species is approximately correct. Best results are obtained for the dominant, dynamically simplest processes (ZR and D2). In the past, further progress in understanding river tides has been limited by a lack of data analysis tools. Data analysis tools are now clearly better than the available analytical solutions.