Showing papers on "Open-channel flow published in 1985"

Features of a reattaching turbulent shear layer in divergent channel flow

Abstract: Experimental data have been obtained in an incompressible turbulent flow over a rearward-facing step in a diverging channel flow. Mean velocities, Reynolds stresses, and triple products that were measured by a laser Doppler velocimeter are presented for two cases of tunnel wall divergence. Eddy viscosities, production, convection, turbulent diffusion, and dissipation (balance of kinetic energy equation) terms are extracted from the data. These data are compared with various eddy-viscosity turbulence models. Numerical calculations incorporating the k-epsilon and algebraic-stress turbulence models are compared with the data. When determining quantities of engineering interest, the modified algebraic-stress model (ASM) is a significant improvement over the unmodified ASM and the unmodified k-epsilon model; however, like the others, it dramatically overpredicts the experimentally determined dissipation rate.

Open Channel Hydraulics

Abstract: Concepts of fluid flow energy principle the momentum principle development of uniform flow concepts computation of uniform flow theory and analysis of gradually and spatially varied flow design of channels flow measurement rapidly varied flow in nonprismatic channels turbulent diffusion and dispersion in steady open-channel flow turbulent, buoyant, surface jets and associated phenomena gradually varied, unsteady flow rapidly varied, unsteady flow hydraulic models.

Compressible fluid flow

Abstract: 1. Fundamental Concepts and Definitions. 2. Equation of Flow. 3. Isentropic Flow. 4. Normal Shock Waves. 5. Adiabatic Frictional Flow in a Constant-Area Duct. 6. Flow with Heat Interaction and Generalized Flow. 7. Two-Dimensional Waves. 8. Linearized Flow. 9. Method of Characteristics. 10. Computational Fluid Dynamics. 11. Methods of Experimental Measurements. Appendix: Tables and Figures. General References. Index.

Fluid flow and weld penetration in stationary arc welds

Abstract: Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.

The structure of the vorticity field in turbulent channel flow. I - Analysis of instantaneous fields and statistical correlations

Abstract: An investigation into the existence of hairpin vortices in turbulent channel flow is conducted using a database generated by the large-eddy simulation technique. It is shown that away from the wall the distribution of the inclination angle of vorticity vector gains its maximum at about 45° to the wall. Two-point correlations of velocity and vorticity fluctuations strongly support a flow model consisting of vortical structures inclined at 45° to the wall. The instantaneous vorticity vectors plotted in planes inclined at 45° show that the flow contains an appreciable number of hairpins. Vortex lines are used to display the three-dimensional structure of hairpins, which are shown to be generated from deformation (or roll-up) of sheets of transverse vorticity.

Recirculation in swirling flow - A manifestation of vortex breakdown

Abstract: The addition of a sufficiently high degree of swirl to flow going into a circular pipe produces a limited region of reversed flow. Such a vortex breakdown, as it is termed, represents a zone of transition from a supercritical to a subcritical flow state. If the flow remains subcritical, an unavoidable consequence is that the geometry and conditions downstream directly affect the upstream flow up to, and including, the breakdown region. Laser Dopper anemometer measurements of the swirl and axial velocity components, as well as the corresponding streamline patterns, are presented for water flow in a model based upon the geometry of a swirl combustor. It is shown that a strong exit contraction (55 percent of the diameter) has practically no influence on a flow which reverts to supercritical, whereas even a weak contraction (15 percent of the diameter) has a significant influence on a flow which remains subcritical. It is argued that a cold flow is likely to be totally unrepresentative of a reacting flow through the same geometry, and, also, that great care has to be taken over the boundary conditions to be imposed for the numerical computation of subcritical flows. 20 references.

Overbank Flow with Vegetatively Roughened Flood Plains

Abstract: An experimental and theoretical investigation into the flow characteristics of channels with complex cross sections was undertaken. In this study, particular attention was given to the problem of non‐submerged flood‐plain roughnesses. This differs from previous studies in which compound‐channel flow was primarily investigated for more or less uniform boundary roughnesses. In order to achieve more conformity with natural rivers, the flood plain and the main channel were separated in the model by a sloping bank. Two cross sections with varying aspect ratios and flood‐plain roughnesses were investigated. The necessary measurements were carried out by applying LDV and Preston‐tube techniques. On the basis of simple turbulence assumptions methods are presented by which the flow resistance in vegetatively roughened flood plains and main channels can be properly predicted as a function of independent and directly determinable basic flow parameters. A first verification of this model was accomplished using field ...

Stokes flow in arbitrary two-dimensional domains: shear flow over ridges and cavities

Abstract: A method is described for solving the integral equations governing Stokes flow in arbitrary two-dimensional domains. It is demonstrated that the boundary-integral method provides an accurate, efficient and easy-to-implement strategy for the solution of Stokes-flow problems. Calculations are presented for simple shear flow in a variety of geometries including cylindrical and rectangular, ridges and cavities. A full description of the flow field is presented including streamline patterns, velocity profiles and shear-stress distributions along the solid surfaces. The results are discussed with special relevance to convective transport processes in low-Reynolds-number flows.

On the structure of jets in a crossflow

Abstract: Spectral analysis and flow visualization are presented for various velocity ratios and Reynolds numbers of a jet issuing perpendicularly from a developing pipe flow into a crossflow. The results are complete with conditional averages of various turbulent quantities for one jet-to-cross-flow velocity ratio R of 0.5. A unique conditional-sampling technique separated the contributions from the turbulent jet flow, the irrotational jet flow, the turbulent crossflow and the irrotational crossflow by using two conditioning functions simultaneously. The intermittency factor profiles indicate that irrotational cross-flow intrudes into the pipe but does not contribute to the average turbulent quantities, while the jet-pipe irrotational flow contributes significantly to them in the region above the exit where the interaction between the boundary-layer eddies and those of the pipe starts to take place. Further downstream, the contributions of the oncoming boundary-layer eddies to the statistical averages reduce significantly. The downstream development depends mainly on the average relative eddy sizes of the interacting turbulent fields.

Observation of waves during oscillatory channel flow

Abstract: We have observed steady and oscillatory flow through a two-dimensional channel expansion. The experimental results are supported by numerical solutions of the unsteady Navier–Stokes equations. This work was prompted by the recent discovery of vortex waves during steady flow past a moving indentation in a channel wall. Our work deals with both asymmetric channels, in which we show that vortex waves are observed during oscillatory flow with rigid walls, and with symmetric channels, in which a vortex street is observed. We believe that the vortex street is not a vortex wave, but the result of a shear-layer instability.

Flow along a channel with a time-dependent indentation in one wall: the generation of vorticity waves

Abstract: We describe flow-visualization experiments and theory on the two-dimensional unsteady flow of an incompressible fluid in a channel with a time-dependent indentation in one wall. There is steady Poiseuille flow far upstream, and the indentation moves in and out sinusoidally, its retracted position being flush with the wall. The governing parameters are Reynolds number Re, Strouhal number (frequency parameter) St and amplitude parameter e (the maximum fraction of the channel width occupied by the indentation); most of the experiments were performed with e ≈ 0.4. For St ≤ 0.005 the flow is quasi-steady throughout the observed range of Re (360 0.005 a propagating train of waves appears, during every cycle, in the core flow downstream of the indentation, and closed eddies form in the separated flow regions on the walls beneath their crests and above their troughs. Later in the cycle, a second, corotating eddy develops upstream of the first in the same separated-flow region (‘eddy doubling’), and, later still, three-dimensional disturbances appear, before being swept away downstream to leave undisturbed parallel flow at the end of the cycle. The longitudinal positions of the wave crests and troughs and of the vortex cores are measured as functions of time for many values of the parameters; they vary with St but not with Re. Our inviscid, long-wavelength, small-amplitude theory predicts the formation of a wavetrain during each cycle, in which the displacement of a core-flow streamline satisfies the linearized Kortewegde Vries equation downstream of the indentation. The waves owe their existence to the non-zero vorticity gradient in the oncoming flow. Eddy formation and doubling are not described by the theory. The predicted positions of the wave crests and troughs agree well with experiment for the larger values of St used (up to 0.077), but less well for small values. Analysis of the viscous boundary layers indicates that the inviscid theory is self-consistent for sufficiently small time, the time of validity increasing as St increases (for fixed e).

Wall-layer structure and drag reduction

Abstract: When drag-reducing additives are confined entirely to the linear sublayer of a turbulent channel flow of water, both the spanwise spacing and bursting rate of the wall-layer structure are the same as those for a water flow and there is no evidence of drag reduction. Drag reduction is measured downstream of the location where the additives injected into the sublayer begin to mix in significant quantities with the buffer region (10 y + The superscript + denotes a dimensionless quantity scaled with the kinematic viscosity ν and the wall shear velocity v * = (τ w /ρ) ½ . of the channel flow. At streamwise locations where drag reduction does occur and where the injected fluid is not yet uniformly mixed with the channel flow, the dimensionless spanwise streak spacing increases and the average bursting rate decreases. The decrease in bursting rate is larger than the corresponding increase in streak spacing. The wall-layer structure is like the structure in the flow of a homogeneous, uniformly mixed, drag-reducing solution. Thus, the additives have a direct effect on the flow processes in the buffer region and the linear sublayer appears to have a passive role in the interaction of the inner and outer portions of a turbulent wall layer.

Large Eddy Simulation of Turbulent Channel Flow by One-Equation Modeling

Abstract: Turbulent plane channel flow is numerically studied using Large Eddy Simulation(LES). Cyclic boundary conditions are imposed on velocity and pressure in the downstream and spanwise directions. The noslip boundary condition is imposed on the walls. Both Smagorinsky model and 1-equation model are applied, and the comparison is made. The importance of the diffusion term in subgrid scale (SGS) turbulent energy balance is pointed out.

On the Determination of Ripple Geometry

Abstract: Laboratory measurements were carried out on the mobile bed covered by ripples generated by an open channel flow. The bed materials used were cohesionless and reasonably uniform; the steady-state subcritical flume flow was in equilibrium and it was nearly two-dimensional. In addition to the conventional runs conducted with water, special runs were conducted with a water and glycerine mixture. Using the results of these measurements, as well as the data of other reliable sources, a series of experimental curves were determined which can be used to predict the length and height of ripples. The analysis of the data indicates that the dimensionless quantities related to the geometry of ripples are functions of two dimensionless variables. One of them must be a combination reflecting the intensity of sediment-transporting flow, the other must be an arrangement of parameters characterizing the physical nature of the liquid and solid phases involved.

Force- and loss-free transitions between flow states

Abstract: Force- and loss-free transitions between flow states in channels and vortex tubes are investigated. It is shown that in general a force-free transition appears either as a single transition which produces an entropy increase and connects an upstream supercritical flow state to a downstream subcritical flow state, or it appears as a two-stage transition, in which an isentropic transition leading to a second supercritical flow state is followed by a transition which produces an entropy increase and leads to a subcritical flow state. Depending on the downstream boundary condition either the single or the two-stage transition appears. For a certain regime of downstream boundary conditions both types of transitions are possible. Which of the two appears depends upon the startup of the flow. The result is a hysteresis. It is suggested that a particular kind of vortex breakdown can be understood in terms of the two transition types considered. Moreover, an apparent paradox is resolved which is associated with the fact that a vortex flow in a diffuser approaches the critical state. The analysis is based on an extension of a variational principle proposed by Benjamin [1]. Complete numerical results are presented which show loss-free transitions (including their internal structure) in a Rankine vortex.

Source–sink flow inside a rotating cylindrical cavity

Abstract: The axisymmetric flow inside a rotating cavity with radial outflow or inflow of fluid is discussed. The basic theoretical model of Hide (1968) is extended, using the integralmomentum techniques of von Karman (1921), to include laminar and turbulent flows; both linear and nonlinear equations are considered. The size of the source region is estimated using a ‘free disk’ model for the outflow case and a free vortex for the inflow case. In both cases, the estimates are in good agreement with available experimental data. Theoretical values of the tangential component of the velocity outside the Ekman layers on the disks, obtained from solutions of the laminar and turbulent integral equations, are compared with experimental values. The experiments were conducted in a number of rotating-cavity rigs, with a radial outflow or inflow of air, and laser-Doppler anemometry was used to measure the velocity in the ‘interior core’ between the Ekman layers. The measurements provide good support for the theoretical models over a wide range of flow rates, rotational speeds and radial locations. Although only isothermal flow is considered in this paper, the methods can be readily extended to non-isothermal flow and heat transfer.

LDA Measurements in Open Channel

Abstract: This paper presents mean velocity and some turbulence measurements for uniform subcritical flow in a smooth rectangular channel for three aspect ratios equal to 5.08, 7.83, and 12.3 obtained with a...

The dynamics of waves at the interface between a viscoelastic coating and a fluid flow

Abstract: The dynamics of two-dimensional uniform wavetrains on the interface between a viscoelastic compliant coating and a boundary-layer flow are explored theoretically. The coating is treated as a single-layer isotropic Voigt material of finite thickness that is bonded to a rigid half-space. The flow is modelled first by potential theory and then modified to incorporate pressure phase shifts and magnitudes found in boundary-layer flow over wavy walls. The consideration of viscoelastic effects has led to an important dimensionless damping parameter γt = Ct τt/d (where τt is the strain relaxation time, Ct is the elastic shear-wave speed and d is the layer depth) that seems to have been overlooked by experimentalists. The flow and the damping are found to have dramatic effects on wave propagation. Using flow pressure and material-damping parameters typical of experiments, the results show that both upstream- and downstream-propagating waves exist at low flow speeds. At higher flow speeds, shorter waves can no longer propagate upstream. At still higher velocities, two instabilities, ‘static divergence’ and ‘flutter’, are found. Static divergence occurs for flow speeds above 2.86Ct and consists of slow waves moving with speeds of about 0.02Ct. These results compare fairly well with published experimental data. Static divergence is found to be a damping instability for these coating systems. When the flow speed is increased further, the flutter instability appears consisting of waves with phase speeds about equal to Ct.

Mass transfer characteristics in a channel with symmetric wavy wall for steady flow.

Abstract: Mass transfer characteristics in a channel with symmetric wavy wall were investigated by the Leveque theory and the electrochemical method. The channel used has a geometry similar to that of the Oxford membrane blood oxygenator. The flow regime covered ranged from laminar to turbulent flow. The local Sherwood number distributions indicate that the reversed-flow region significantly differs from the forward-flow region in mass transfer characteristics. For laminar flow, mass transfer enhancement of the wavy channel is scarcely expected as compared with the corresponding straight channel, but is found to be remarkable for turbulent flow.

Fluid and Sediment Interaction over a Plane Bed

Abstract: A mathematical model is developed for the fluid and sediment interaction in an erodible channel The model is based on the conservation of mass and momentum for the fluid and sediment The interaction between the fluid and sediment, as well as the interaction of sediment particles moving in the vicinity of the bed is described in the model An analysis is performed to derive the vertical distributions of the fluid velocity and sediment concentration over a plane bed in a wide open channel The analysis yields the friction factor due to the combined effects of grain roughness and sediment movement, as well as the rate of sediment transport The developed model agrees with existing data for plane beds The predicted rate of transport of coarse sediment agrees with the Meyer-Peter and Mullen formula However, detailed measurements of the fluid velocity and sediment concentration over a plane bed are required for a rigorous verification of the developed model

Relaxation effects observed for turbulent flow over a wavy surface

Abstract: Measurements are presented for different flow rates of the time-averaged wall shear stress and of the root-mean-square value of the turbulent fluctuations along a small-amplitude sinusoidally shaped solid surface. The stresses are found to have a variation along the wave surface which is also sinusoidal. The influence of flow rate and of wavelength on the amplitude and phase angle can be correlated by using a wave-number α+ made dimensionless with wall parameters.It is found that for α+ > 10−2 a frozen-turbulence assumption can be made whereby the influence of the wave-induced variation of the mixing length can be ignored. For α+ < 10−4 the flow can be described by assuming the Reynolds stresses are given by an equilibrium assumption. The relaxation from this equilibrium condition is characterized by a sharp change in the phase angle for 6 × 10−4 < α+ < 10−3.This relaxation is associated with physical processes in the viscous wall region which are not yet understood. It is argued that these are principally related to the wave-induced variation of the pressure gradient.The wave-induced variation of the turbulent fluctuations in the wall shear stress also indicate a relaxation in that the maximum turbulence intensity is located in a region of favourable pressure gradient.

Viscous flow through a rotating square channel

Abstract: Fully developed flow of an incompressible Newtonian fluid driven by a pressure gradient through a square channel that rotates about an axis perpendicular to the channel roof is analyzed here with the aid of the penalty/Galerkin/finite element method Coriolis force throws fast‐moving fluid in the channel core in the direction of the cross product of the mean fluid velocity with the channel’s angular velocity Two vortex cells form when convective inertial force is weak Asymptotic limits of rectilinear flow and geostrophic plug flow are approached when viscous force or Coriolis force dominates, respectively A flow structure with an ageostrophic, virtually inviscid core is uncovered when Coriolis and convective inertial forces are both strong This ageostrophic two‐vortex structure becomes unstable when the strength of convective inertial force increases past a critical value The two‐vortex family of solutions metamorphoses into a family of four‐vortex solutions at an imperfect bifurcation composed of a pair of turning points

Critical flow condition in open channel hydraulics

Abstract: Transition between sub- and supercritical flows in open channel occurs at the so-called critical point, for which critical flow conditions appear. This particular flow state has been originally introduced for flows with parallel streamlines. However, as streamlines are considerably sloped and inclined in the vicinity of the critical point, these effects have to be accounted for. The present investigation explores in detail these effects. In particular, present investigations include i) first order analysis expressing discharge in terms of upstream head and channel geometry (calibration of discharge measurement structures) and ii) determination of surface profiles for transitional flow states by accounting for the effective pressure and velocity distributions. Analysis is based on a first order model with restriction to typical channel bottom and sidewall geometry. Results are compared with observations, and a fair agreement between the two is noted.