Showing papers on "Open-channel flow published in 1990"

A dynamic subgrid-scale eddy viscosity model

Abstract: One major drawback of the eddy viscosity subgrid-scale stress models used in large-eddy simulations is their inability to represent correctly with a single universal constant different turbulent field in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work, a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity (Germano 1990) between the subgrid-scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid-scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near-wall region of a turbulent boundary layer. The results of large-eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

Pressure drop in horizontal wells and its effect on production performance

Abstract: Fluid flow in horizontal pipes becomes turbulent at Reynolds numbers large than 2,000. In practical situations, turbulent flow in horizontal wells may occur at rates of thousands of cubic feet per day (for a 24.4-cm (9 5/8-in.) cased well). Wells flowing in a turbulent regime experience a flow resistance many orders of magnitude higher than that for laminar flow. Therefore, along-hole well-pressure gradients generally cannot be neglected, and a proper description of horizontal well flow needs to be included in the design of these wells (and in reservoir simulators). This is particularly imperative in situations of very low drawdowns (to avoid gas or water cresting). This paper presents a simple analytical method that links single-phase turbulent well flow to stabilized reservoir flow. The resulting second-order differential equation is solved numerically for the appropriate boundary conditions. Results are presented in dimensionless form for general applicability.

On the space‐time characteristics of wall‐pressure fluctuations

Abstract: A database obtained by direct numerical simulation of turbulent channel flow was used to compute the three‐dimensional frequency/wave‐number spectrum of wall‐pressure fluctuations. The spectrum was used to deduce scaling laws for pressure fluctuations and to evaluate the similarity form for the power spectrum. The convection velocity as a function of frequency, wave number, and spatial and temporal separations was calculated and compared with the experimental data. The problem of artificial ‘‘acoustics’’ in numerical simulation of incompressible flows is discussed.

Turbulence measurements in a shear layer region of a compound channel

Abstract: The paper describes some turbulence measurements carried out in the SERC Flood Channel Facility at Hydraulics Research Ltd., Wallingford, U.K. The facility represents a large scale model of a river system with floodplains, and is designed to produce fully developed boundary layer flows with transverse shear. This article presents some of the open channel flow data, including measurements of the primary velocity, Ū/u ∗, the distribution of turbulent intensities, u1/u∗ , v1/u∗ , and w1/u∗ , the kinetic energy, k/u2∗ , and the Reynolds stresses τzx and τyx in the region of strong lateral shear induced by transverse variation in depth. Attention is focussed on the non linear nature of the Reynolds stresses in the shear layer, flow structures and the lateral variations in eddy viscosity and local friction factor.

Stability of the laminar flow in a rectangular duct

Abstract: The stability of the laminar flow in a rectangular duct of an arbitrary aspect ratio is investigated numerically by expanding the flow fields of both the main flow and the disturbance into series of Legendre polynomials and solving the eigenvalue problem of the resulting matrix equation. The stability of the flow is found to depend upon the aspect ratio of the duct and the mode of the disturbance. The flow is unstable to two of the four possible modes of different parity and stable to the other two.

A centre manifold description of containment dispersion in channels with varying flow properties

Abstract: High-order asymptotic approximations to the equation governing the longitudinal dispersion of a passive contaminant in Poiseuille channel flow are derived, and their validity discussed. The derivation uses centre manifold theory, which provides a systematic and near rigorous approach to calculating each successive approximation. It also enables the derivation of the correct initial conditions for the Taylor model of shear dispersion. Approximations that are valid when the channel is of a varying cross section are systematically derived via this approach, and the effects of time-dependent flow and variable diffusivity are also investigated. The resultant modifications to the effective advection velocity and the effective dispersion coefficient are calculated and some general trends indicated.

Internal flow field studies in a simulated cylindrical port rocket chamber

Abstract: The objective of these studies is to experimentally characterize the mean and fluctuating flow field that develops along the length of a simulated cylindrical port rocket chamber. Flow simulation was accomplished by injecting ambient temperature nitrogen uniformly along the walls of 10.2-cm (4-in.) diam, porous-tube chambers connected to a choked sonic nozzle. Experiments were conducted with chamber L/D ratios of 9.5 and 14.3, at injection Mach numbers and Reynolds numbers typical of rocket motor values. Maximum Reynolds numbers based on injection and centerline velocities were, respectively, 1.8 x 10 and 1.6 x 10. Mean and fluctuating speed and turbulent shear stresses were measured in the principle coordinate directions using three-element hot-wire anemometers. The data show that noticeable velocity fluctuations in the head-end region generally decrease in intensity, relative to centerline speed, over the first five port diameters. At this point, regular velocity oscillations appear near the wall, just prior to the transition to turbulent flow. The oscillation frequency characteristics suggest the occurrence of vortical disturbances which exhibit pairing as they move away from the wall. The downstream turbulence development is characterized by a slow spreading toward the centerline: peak values of turbulence intensity and shear stress occur a few tenths of a port radius from the wall and remain relatively constant. Mean velocity profiles prior to transition show fair agreement with those derived for a rotational inviscid flow injected normal to the surface. A slow transition from these profiles occurs downstream in the turbulent region. Two surprising features of the flow were the occurrence of both buoyant flow influences and flow spinning in forward regions of the chamber.

On the large‐eddy simulation of transitional wall‐bounded flows

Abstract: The structure of the subgrid scale fields in plane channel flow has been studied at various stages of the transition process to turbulence. The residual stress and subgrid scale dissipation calculated using velocity fields generated by direct numerical simulations of the Navier-Stokes equations are significantly different from their counterparts in turbulent flows. The subgrid scale dissipation changes sign over extended areas of the channel, indicating energy flow from the small scales to the large scales. This reversed energy cascade becomes less pronounced at the later stages of transition. Standard residual stress models of the Smagorinsky type are excessively dissipative. Rescaling the model constant improves the prediction of the total (integrated) subgrid scale dissipation, but not that of the local one. Despite the somewhat excessive dissipation of the rescaled Smagorinsky model, the results of a large eddy simulation of transition on a flat-plate boundary layer compare quite well with those of a direct simulation, and require only a small fraction of the computational effort. The inclusion of non-dissipative models, which could lead to further improvements, is proposed.

2‐D Depth‐Averaged Flow Computation near Groyne

Abstract: The depth-averaged velocity and bottom shear stress distributions in a rectangular channel near a groyne are computed by using a 2-D depth averaged model. The model uses a hybrid finite difference scheme and an iterative method to solve the governing equations of flow and turbulence transport. Due to streamline curvature effects in the region near the groyne tip, a correction factor is incorporated into the \Ik\N=ϵ\N turbulence model that significantly improves the agreement between the computed and experimental data of the velocities and of the streamline pattern compared to previous numerical methods. In this region the bottom shear stress is found to be largely influenced by the 3-D effects. A 3-D correction factor is introduced which considerably improves the computed bottom shear stresses. Sensitivity analysis is made on the \Ik\N=ϵ\N model coefficients and on the correction factors of the streamline curvature and the 3-D effects. The experimental errors in the velocity and bottom shear stress measurements are analyzed. The average errors between the computed and previous experimental results are presented with confidence intervals.

Topographic Influences on Wind-Driven, Stratified Flow in a β-Plane Channel: An Idealized Model for the Antarctic Circumpolar Current

Abstract: Topographic influences are examined in an eddy-resolving model of oceanic channel flow forced by steady zonal winds. With small explicit lateral friction, transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer. In the flat-bottom case, bottom friction is the only efficient sink of eastward momentum. When bottom topography is present, the topographic form stress can replace the bottom friction sink in the momentum budget, and a large decrease of the zonal transport results. Large wale topography (of the scale of the forcing) provides the largest form stress. Topographic effects decay with height as suggested by the Prandit scaling, and therefore only topographic scales larger than the Rossby radius can affect the whole water column. In that case, the interfaces are deformed by standing eddies on topographic length scales, and standing eddies replace transient eddies in transferring momentum downward. The bottom-layer mean streamfunc...

The different capillary flow regimes of entangled polydimethylsiloxane polymers : macroscopic slip at the wall, hysteresis and cork flow

Abstract: Polymers, melted at room temperature, have been extruded through capillary dies using either a controlled pressure system or an automatic capillary rheometer in which the rate of flow is controlled. When the polymer is highly entangled, macroscopic slip is observed at the wall. However, flow curves differ: for controlled pressure conditions, slip appears simultaneously with a sudden increase in the rate of flow, an dflow curves exhibit a hysteretic regime. For controlled flow conditions, slip is accompanied by oscillations of the rate of flow and pressure head around a mean value, as a result of polymer compressibility. The succession and evolution of the different flow defects are clearly identified. As the flow regime increases, scratches appear first. Then, beyond a critical value of wall shear stress in the exit region, cracks are formed just at the exit of the capillary die. These cracks are accompanied by the formation of “rings”, which are more easily observed as the molecular weight increases. At high flow rates, when macroscopic slip appears at the wall, the aspect of the extrudate depends on the system used. For controlled pressure conditions, the polymer is ejected in the form of an opaque, irregular jet, where swelling is quasi-non-existent. For controlled flow conditions, cork flow is observed. At higher flow regimes the extrudate becomes chaotic. The existence of a master curve was also shown for variations in average velocity vs. wall stress, during flow with macroscopic slip at the wall. It is important to note that this curve represents the sliding friction properties of the polymer under consideration.

Flow reversal and heat transfer of fully developed mixed convection in vertical channels

Abstract: The present analysis is concerned with flow reversal phenomena and heat transfer characteristics of the fully developed laminar combined free and forced convection in the heated vertical channels. Three fundamental combinations of thermal boundary conditions on the respective wall surface (namely isoflux-isoflux, isofluxisothermal, and isothermal-isothermal) are considered separately so as to investigate extensively their distinct influence on the flow pattern. Results of the velocity distribution and temperature distribution as well as the Nusselt number in terms of bulk mean temperature are carried out. Based on the analytical solutions obtained, flow reversal adjacent to the relatively colder wall is found to exist within the channel as Re/Gr is below than a threshold value, which is related to the thermal boundary conditions. Parameter zones for the occurrence of reversed flow are presented. Comparisons and verification are made using the existing numerical solutions at locations far downstream of developing flow.

On sublayer streaks

Abstract: Turbulent sublayer streaks are studied with the aid of a simplified theoretical model. In this the nonlinear activity is assumed to be intermittent and to act locally in space during a very short initial time so as to set up the initial conditions for the subsequent linear and inviscid evolution of the resulting three-dimensional flow disturbance. The mean shear flow is taken as a parallel one and a correction for the long-tern effects of viscosity is applied. A model for the initial nonlinear phase is chosen to represent the local Reynolds stresses that would be produced by a patch of local inflectional instability. The streamwise dimension of the resulting eddy is found to grow linearly with time in accordance with the algebraic instability mechanism (Landahl 1980). The associated Reynolds shear stress is expressible in a simple manner in terms of the liftup of the fluid elements and is suggestive of an algebraic-type Reynolds stress model similar to, but not identical to, that of Prandtl's (1925) mixing-length theory.

Plane waves and structures in turbulent channel flow

Abstract: A direct simulation of turbulent flow in a channel is analyzed by the method of empirical eigenfunctions (Karhunen-Loeve procedure, proper orthogonal decomposition). This analysis reveals the presence of propagating plane waves in the turbulent flow. The velocity of propagation is determined by the flow velocity at the location of maximal Reynolds stress. The analysis further suggests that the interaction of these waves appears to be essential to the local production of turbulence via bursting or sweeping events in the turbulent boundary layer, with the additional suggestion that the fast acting plane waves act as triggers.

Measurements of turbulent flow in a channel at low Reynolds numbers

Abstract: Normal and streamwise components of the velocity fields of turbulent flow in a channel at low Reynolds numbers have been measured with laser-Doppler techniques. The experiments duplicate the conditions used in current direct numerical simulations of channel flow, and good, but not exact, agreement is found for single-point moments through fourth order. In order to eliminate LDV velocity bias and to measure velocity spectra, the mean time interval between LDV signals was adjusted to be much smaller than the smallest turbulence time scale. Spectra of the streamwise and normal components of velocity at locations spanning the channel are presented.

Flow resistance in compound channels

Abstract: Preliminary results from the Flood Channel Facility sponsored by the British Science and Engineering Research Council and Hydraulics Research Ltd. are presented. The Facility takes the form of a large compound channel and the first series of experiments have been analysed to assess flow resistance characteristics of simple and compound channels having smooth boundaries. The effects of momentum transfer from main channel to flood plains on the capacity of the compound and main channel sections have been demonstrated. Flow resistance relationships for the compound section, main channel and flood plains have been presented in terms of Manning's and Darcy-Weisbach resistance coefficients, and compared with simple channel shapes. This has emphasised the complexity of compound channel flow resistance, while also pointing out the likely errors involved in applying simple channel methods of analysis and design to rivers and channels of compound geometry.

Flow patterns and pressure drop in isothermal gas-liquid concurrent flow in a horizontal capillary tube.

Abstract: A gas-liquid two-phase flow in a capillary tube or a narrow passage is encountered in many applications, for example, the flow of coolant in evaporators of many kinds of heat exchangers. Sufficlent information on fundamental parameters, such as void fraction and pressure drop, however, are not available in designing such devices. In the present investigation, we made a detailed observation of the flow pattern of an air-water two-phase flow in a horizontal capillary tube with an inner diameter of 1.0mm to 4.9mm, and attempted to correlate and explain the behavior of the pressure drop changing with the flow rates of both phases by introducing a physical model.

Separation and reattachment of non-newtonian fluid flows in a sudden expansion pipe

Abstract: In the current flow visualization studies, the role of non-Newtonian characteristics (such as shear-rate-dependent viscosity and viscoelasticity) on flow behavior across the sudden expansion step in a circular pipe is investigated over a wide range of Reynolds numbers including the turbulent flow. The expansion ratios tested are 2.000 and 2.667 and the range of the Reynolds number covered in the current flow visualization tests are 10–35 000 based on the inlet diameter. The reattachment lengths for the viscoelastic fluids in the laminar flow regime are found to be much shorter than those for the Newtonian fluid. In addition they decrease significantly with increasingly concentration of viscoelastic fluid at the same Reynolds number. However, in the turbulent flow regime, the reattachment length for the viscoelastic fluids is two or three times longer than those for water, and gradually increases with increasing concentration of viscoelastic solutions, resulting in 25 and 28 step-height distances for 500 ppm and 1000 ppm polyacrylamide solutions respectively. This may be because the elasticity in polyacrylamide solutions suppresses the eddy motion and controls separation and reattachment behavior in the sudden expansion pipe flow. The reattachment lengths for the purely viscous non-Newtonian fluids are found to be almost the same as those for water.

Turbulent structure in a channel flow with polymer injection at the wall

Abstract: Two-component laser velocimeter measurements in a fully developed turbulent water channel flow with polymer injection were used to examine the effect of polymer injection on the Reynolds stresses and the production terms in the Reynolds stress transport equations. These measurements show that while the root-mean-square fluctuation level of the streamwise velocity was increased, the r.m.s. of the wall-normal velocity and the Reynolds shear stress were reduced.

On transition to chaos in two-dimensional channel flow symmetrically driven by accelerating walls

Abstract: We study theoretically the flow of a viscous incompressible fluid in a parallel-walled channel: the flow is driven by uniform steady suction through the porous and accelerating walls of the channel. Previous authors have discussed special cases of such flows, confining attention to flows which are symmetric, steady and two-dimensional; a similarity form of solution is assumed, as used by Berman and originally due to Hiemenz, to reduce the Navier–Stokes equations to a nonlinear ordinary differential equation. We generalize their work by considering asymmetric flows, unsteady flows and three-dimensional perturbations. By use of numerical calculations, matched asymptotic expansions for large values of the Reynolds number, both positive and negative, and the theory of dynamical systems we find many more exact solutions of the Navier–Stokes equations, examine their stability and interpret them; although much of the theory is for the general case, most of the numerical calculations are for the case of zero suction. In particular we show that most of the previously found steady solutions are unstable to antisymmetric two-dimensional disturbances. This leads to a pitchfork bifurcation, stable asymmetric steady solutions, a Hopf bifurcation, stable time-periodic solutions, stable quasi-periodic solutions and chaos in succession as the Reynolds number increases from zero, and a pitchfork bifurcation, stable asymmetric steady solutions, a Hopf bifurcation, periodic solutions, chaos via period doubling, other periodic solutions and chaos in succession as the Reynolds number decreases from zero.

Curvature- and rotation-induced instabilities in channel flow

Abstract: In a curved channel streamwise vortices, often called Dean vortices, may develop above a critical Reynolds number owing to centrifugal effects. Similar vortices can occur in a rotating plane channel due to Coriolis effects if the axis of rotation is normal to the mean flow velocity and parallel to the walls. In this paper the flow in a curved rotating channel is considered. It is shown from linear stability theory that there is a region for which centrifugal effects and Coriolis effects almost cancel each other, which increases the critical Reynolds number substantially. The flow visualization experiments carried out show that a complete cancellation of Dean vortices can be obtained for low Reynolds number. The rotation rate for which this occurs is in close agreement with predictions from linear stability theory. For curved channel flow a secondary instability of travelling wave type is found at a Reynolds number about three times higher than the critical one for the primary instability. It is shown that rotation can completely cancel the secondary instability.

Laminar flow in twisted pipes

Abstract: The effect of torsion on fully developed laminar flow in a twisted pipe is considered in the low-Reynolds-number regime. The flow in a pipe of elliptical cross-section whose axis is straight but which is twisted about that axis is examined and the secondary flow pattern found. The helical pipe of circular cross-section is also revisited; in this case, the first-order torsional effect on the streamtubes is verified, and previous conclusions that such an effect would be of higher order are explained.

Experimental and Computational Study of Turbulent Flows in a Channel With Two Pairs of Turbulence Promoters in Tandem

Abstract: Measurements and computations are presented of mean velocity and turbulence intensity for an arrangement of two pairs of turbulence promoters mounted in tandem in developing channel flow. The Reynolds number (ReD) and the pitch ratio (PR) were varied in the range of 1.2 x 10E4 to 1.2 x 10E5 and 1 to 100, respectively. The three pitch ratios 5, 10, 15 were found to provide three characteristic flows which are a useful test of the computational models. The effects of PR on the reattachment lengths and the pressure loss as well as the influence of ReD on the reattachment length were documented in detail. It was found that PR=10 was preferable to PR=5 and PR=15 from the standpoint of heat transfer enhancement.

Spectral element—Fourier methods for incompressible turbulent flows

Abstract: A mixed spectral element-(Fourier) spectral method is proposed for solution of the incompressible Navier-Stokes equations in general, curvilinear domains. The formulation is appropriate for simulations of turbulent flows in complex geometries with only one homogeneous flow direction. The governing equations are written in a form suitable for both direct (DNS) and large-eddy (LES) simulations allowing a unified implementation. The method is based on skew-symmetric convective operators that induce minimal aliasing errors and fast Helmholtz solvers that employ efficient iterative algorithms (e.g. multigrid). Direct numerical simulations of channel flow verified that the proposed method can sustain turbulent fluctuations even at ‘marginal’ Reynolds numbers. The flexibility of the method to efficiently simulate complex-geometry flows is demonstrated through an example of transitional flow in a grooved channel and an example of transitional-turbulent flow over rough wall surfaces.

Dividing Flow in Open Channels

Abstract: Junctions are of considerable importance in the study of openchannel flows. For dividing flow at rightangled junctions of rectangular open channels, an estimate of the discharge ratio is obtained i...

Response of a turbulent boundary layer to sinusoidal free-stream unsteadiness

Abstract: In this paper, selected findings of a detailed experimental investigation are reported concerning the effects of forced free-stream unsteadiness on a turbulent boundary layer. The forced unsteadiness was sinusoidal and was superimposed locally on an otherwise-steady mainstream, beyond a turbulent boundary layer which had developed under constant-pressure conditions. Within the region over which free-stream unsteadiness was induced, the sinusoidal variation in pressure gradient was between extremes of zero and a positive value, with a positive average level. The local response of the boundary layer to these free-stream effects was studied through simultaneous measurements of the u- and v-components of the velocity fieldAlthough extensive studies of unsteady, turbulent, fully-developed pipe and channel flow have been carried out, the problem of a developing turbulent boundary layer and its response to forced free-stream unsteadiness has received comparatively little attention. The present study is intended to redress this imbalance and, when contrasted with other studies of unsteady turbulent boundary layers, is unique in that: (i) it features an appreciable amplitude of mainstream modulation at a large number of frequencies of forced unsteadiness, (ii) its measurements are both detailed and of high spatial resolution, so that the near-wall behaviour of the flow can be discerned, and (iii) it allows local modulation of the mainstream beyond a turbulent boundary layer which has developed under the well-known conditions of steady, two-dimensional, constant-pressure flowResults are reported which allow comparison of the behaviour of boundary layers under the same mean external conditions, but with different time dependence in their free-stream velocities. These time dependences correspond to: (i) steady flow, (ii) quasi-steadily varying flow, and (iii) unsteady flow at different frequencies of mainstream unsteadiness. Experimental results focus upon the time-averaged nature of the flow; they indicate that the mean structure of the turbulent boundary layer is sufficiently robust that the imposition of free-stream unsteadiness results only in minor differences relative to the mean character of the steady flow, even at frequencies for which the momentary condition of the flow departs substantially from its quasi-steady state. Mean levels of turbulence production are likewise unaffected by free-stream unsteadiness and temporal production of turbulence appears to result only from modulation of the motions which contribute to turbulence production as a time-averaged measure.