Flow Structure In Channel Research Articles

An Alternative Symmetry for Large-Scale Structures in Channel Flow

An Alternative Symmetry for Large-Scale Structures in Channel Flow

Flow structure of partly vegetated open-channel flows with eelgrass

Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegetation in order to manage fluvial processes. Experiments in an open-channel flume with natural vegetation were carried out to study the influence of vegetation on the flows. In a half channel with two different densities of vegetation, the flow velocity, Reynolds stresses, and turbulence intensities were measured using an Acoustic Doppler Velocimeter (ADV). We obtained velocity profiles in the lateral direction, Reynolds stresses in the vertical direction, and the flow transition between the vegetated and non-vegetated zones in different flow regimes. The results show that the streamwise velocity in the vegetated zone with higher density is almost entirely blocked. Reynolds stress distribution distinguishes with two different regions: inside and above the vegetation canopies. The turbulence intensities increase with increasing Reynolds number. The coherent vortices dominate the vertical transport of momentum and are advected clockwise between the vegetated zone and non-vegetated zone by secondary currents (a relatively minor flow superimposed on the primary flow, with significantly different speed and direction), generated by the anisotropy of the turbulence.

Bifurcation Problem of Coherent Structures in Channel Flow

Bifurcation Problem of Coherent Structures in Channel Flow

NONLINEAR EVOLUTION OF TURBULENT COHERENT STRUCTURES IN CHANNEL FLOWS

The generation and the development of turbulent coherent structures in channel flows are investigated by using numerical simulation of Navier-Stokes equation and the theoretic model of turbulent coherent structures built up by the flow stability theories. The three-dimensional coupling compact difference scheme with high accuracy and resolution developed can be applied to the calculative region including points near the boundary. The results computed show nonlinear evolution process and characteristics of Reynolds stress, stream-wise vortices and span-wise vorticities, especially the nonlinear interactions between different coherent structures.

Interaction between a large-scale structure and near-wall structures in channel flow

Direct numerical simulation of a turbulent channel flow in a periodic domain of relatively wide spanwise extent, but minimal streamwise length, is carried out at Reynolds numbers and 349. The large-scale structures previously observed in studies of turbulent channel flow using huge computational domains are also shown to exist even in the streamwise-minimal channels of the present study. Moreover, the limitation of the streamwise length of the domain enforces the interaction between large-scale structures and near-wall structures, which consequently makes it tractable to extract a simple cycle of processes sustaining the structures in the present channel flow. It is shown that the large-scale structures are generated by the collective behaviour of near-wall structures and that the generation of the latter is in turn enhanced by the large-scale structures. Hence, near-wall and large-scale structures interact in a co-supporting cycle.

Structure of channel flows with surface mounted ribs

The flow inside a channel with surface mounted ribs is studied by numerical simulation and experimental flow visualization. Both the geometry with one rib and the geometry with two ribs, placed perpendicular to the flow, are studied, with particular emphasis on the effect of the variation of the distance between the ribs on the structure of the flow. Results show that, for Reynolds number flows of 400 and 1000 (based on the uniform incoming velocity and the height of the ribs), the flow reattaches between the ribs when they are separated by distances of 12h (h being the height of the ribs) and 6h, respectively. In this respect, however, the numerical results for the Reynolds number flow of 1000 are in contradiction with published results.

EXPERIMENTAL STUDY OF FLOW IN CURVED RECTANGULAR OPEN CHANNEL WITH SIDE CONCAVITY

This study aims to clarify the flow structure in curved rectangular open channel with a side concavity. Surface flow velocity profile is measured using PIV technique and bottom flow direction is obtained by a new method using image analysis of bottom streamlines visualized by dye injection method. In curved channel flow, developing process of secondary flow is examined. The structure of channel flow with side concavity in each side is investigated and their differences are clarified. The results show that water level is lager in the case the side concavity is installed in the outer bank. On the other hand, surface fluctuation is greater in the inner bank case. The aspect ratio of channel width to water depth changed the structure of the flow around side concavity.

Probing structures in channel flow through SO(3) and SO(2) decomposition

SO(3) and SO(2) decompositions of numerical channel flow turbulence are performed. The decompositions are used to probe, characterize, and quantify anisotropic structures in the flow. Close to the wall, the anisotropic modes are dominant and reveal the flow structures. The dominance of the (j, m) = (2, 1) mode of the SO(3) decomposition in the buffer layer is associated with hairpin vortices. The SO(2) decomposition in planes parallel to the walls allows us also to access the regions very close to the wall. In those regions we have found that the strong enhancement of intermittency can be explained in terms of streaklike structures and their signatures in the m = 2 and m = 4 modes of the SO(2) decomposition.

Exact coherent structures in channel flow

Exact coherent states in no-slip plane Poiseuille flow are calculated by homotopy from free-slip to no-slip boundary conditions. These coherent states are unstable travelling waves. They consist of wavy low-speed streaks flanked by staggered streamwise vortices closely resembling the asymmetric coherent structures observed in the near-wall region of turbulent flows. The travelling waves arise from a saddle-node bifurcation at a sub-turbulent Reynolds number with wall-normal, spanwise and streamwise dimensions smaller than but comparable to 50+, 100+ and 250+, respectively. These coherent solutions come in pairs with distinct structure and instabilities. There is a three-dimensional continuum of such exact coherent states.

Flow structure in depth‐limited, vegetated flow

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open‐channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.

On the Development and Application of the Fast-Response Aerodynamic Probe System in Turbomachines—Part 2: Flow, Surge, and Stall in a Centrifugal Compressor

The present paper, Part 2 of a trilogy, is primarily focussed on demonstrating the capabilities of a fast-response aerodynamic probe system configuration based on the simplest type of fast-response probe. A single cylindrical probe equipped with a single pressure sensor is used to measure absolute pressure and both velocity components in an essentially two-dimensional flow field. The probe is used in the pseudo-three-sensor mode (see Part 1). It is demonstrated that such a one-sensor probe is able to measure high-frequency rotor-governed systematic fluctuations (like blade-to-blade phenomena) alone or in combination with flow-governed low-frequency fluctuations as rotating stall (RS) and mild surge (MS). However, three-sensor probes would be needed to measure stochastic (turbulence-related) or other aperiodic velocity transients. The data shown refer to the impeller exit and the vaned diffuser of a single-stage high-subsonic centrifugal compressor. Wall-to-wall probe traverses were performed at the impeller exit and different positions along the vaned diffuser for different running conditions. The centrifugal compressor was operated under stable as well as unstable (pulsating or stalled) running conditions. The turbomachinery-oriented interpretation of these unsteady flow data is a second focus of the paper. A refined analysis of the time-resolved data will be performed in Part 3, where different spatial/temporal averaging methods are compared. Two different averaging methods were used for the data evaluation: impeller-based ensemble-averaging for blade-to-blade systematic fluctuations (with constant period length at a constant shaft speed), and flow-based class averaging for the relatively slow MS and RS with slightly variable period length. Due to the ability of fast-response probes to simultaneously measure velocity components and total and static pressure, interesting insights can be obtained into impeller and diffuser channel flow structures as well as into the time behavior of such large-domain phenomena as RS and MS. [S0889-504X(00)01103-X]

Turbulent Structure in Channel Flow through an Expanded Region.

Effects of impulsive change in pressure gradient in a two-dimensional turbulent channel flow have been experimentally investigated. The rapid change of pressure gradient was accomplished by passing an expanded region. Reynolds shear stress and the statistical properties, i.e., integral length scale, skewness/flatness factors and turbulent diffusions, were investigated in the relaxing region behind the expanded region. An organized structure was examined using conditional sampling techniques. Mean frequency of the bursting was obtained using the VITA Technique. The contributions to uv in the four quadrants on the u-v plane were examined by means of the quadrant analysis method. Reynolds shear stress decreases in the near-wall region, and increases in the outer region. The frequency of the bursting decreases with the pressure gradient. The contribution of the ejection to Reynolds shear stress decreases in the near-wall region, and increases in the outer region due to pressure gradient.

Study of turbulent boundary layer structure using the invariants of the velocity gradient tensor

The geometry of the velocity field in a numerically simulated incompressible turbulent boundary layer over a flat plate at Re θ = 670 is studied with the use of the invariants of the velocity gradient tensor ( P, Q, and R). These invariants are computed at every grid point in the flow and used to form the discriminant ( D = 27 R 2/4 + Q 3). Of primary interest are those regions in the flow where the discriminant is positive—regions where, according to the characteristic equation, the eigenvalues of the velocity gradient tensor are complex. An observer moving with a frame of reference that is attached to a fluid particle lying within such a region would see a local flow pattern of the type stable-focus-stretching or unstable-focus-compressing. When the flow is visualized in this way, continuous, connected, large-scale structures are revealed that extend from just below the buffer layer (y + ≈ 1.0) out into the beginning of the wake region. These structures are aligned with the mean shear close to the wall and arch in the cross-stream direction away from the wall. In some cases, the structures observed are very similar to the hairpin eddy vision of boundary-layer structure proposed by Theodorsen [1]. Recently, this conceptual picture has been used by Perry et al. [2] to develop a successful model of the turbulent boundary layer based on an extension of the attached eddy hypothesis first proposed by Townsend [3]. That the structure of the flow is revealed more effectively by the discriminant rather than by the vorticity is important and adds support to the recent analysis of channel flow structure by Blackburn et al. [4]. Of particular importance (and also in contrast with the use of the vorticity) is the fact that the procedure does not require the use of an arbitrary threshold in the discriminant. Further analysis using computer flow visualization shows a high degree of spatial correlation between the regions of positive discriminant, extreme negative pressure fluctuations, and large instantaneous values of Reynolds shear stress ( u′ v′).

Vortical Structures in Channel Flows with a Backward-Facing Step

Vortical Structures in Channel Flows with a Backward-Facing Step

Autogeneration of near-wall vortical structures in channel flow

The evolution of a symmetric pair of quasistreamwise vortical structures extracted from the two-point correlation tensor of turbulent channel flow data by a linear stochastic estimation procedure is studied through direct numerical simulation. It is observed that an Ω-shaped hairpin vortex is formed quickly from this initial structure. Sufficiently strong hairpin vortices are observed to generate a hierarchy of secondary hairpin vortices, and the mechanism of their creation closely resembles the formation of the primary hairpin vortex. New streamwise vortices are also generated, and this process provides a means for a few vortical disturbances of adequate strength to reproduce themselves and eventually populate the near-wall layer.

Turbulent Structure in Channel Flow with Injection.

An experimental study was carried out on a fully developed turbulent channel flow with uniform injection through a porous wall at five injection rates. The turbulence intensities of velocity fluctuations and Reynolds shear stress were measured by means of a hot-wire anemometer. The statistical properties, i. e., power spectrum, probability density function and skewness/flatness factors, were investigated. Turbulence intensities and Reynolds shear stress increase with increasing injection rate. The correlation coefficient depends slightly on injection rate and is constant across the channel. The skewness factor increases with increasing injection rate in the near-wall region. The result reveals that sweep motion is strengthened by injection.

Comprehensive study of heat transfer enhancement in tubular heat exchangers

E. K. Kalinin G. A. Dreitser N. V. Paramonov A. S. Myakochin A. I. Tikhonov S. G. Zakirov E. S. Levin L. S. Yanovsky Moscow Aviation Institute, Moscow, USSR Tashkent Polytechnical Institute, Tashkent, USSR Research and Production Association at Central Boiler-Turbine Institute, Leningrad, USSR IIAt the Moscow Aviation Institute a highly effective method was developed to enhance heat transfer in tubular heat exchangers, and comprehensive studies were made of its effectiveness when applied to gas and liquid flow in tubes, circular channels, and in-flow tube bundles over a wide range of performance parameters as well as to boiling and condensation of heat carriers. The essence of the method is as follows. Equidistant annular grooves are rolled on the outer surface of a heat transfer tube. In this case, annular smooth-configuration diaphragms are formed on the inner tube surface. Annular grooves and diaphragms turbulize the flow in a wall layer and provide heat transfer enhancement outside and inside the tubes. Experimental investigations were conducted at the Moscow Aviation Institute, Moscow Aviation Technology Institute, Tashkent Polytechnical Institute, Research and Production Association at the Central Boiler-Turbine Institute, and elsewhere. The results of introducing this method in various types of heat exchange apparatuses are also presented. The theoretical analysis of a turbulent flow structure in channels and in a separation zone and the experimental studies in channels of various configurations make it possible to reveal a hitherto unknown law for heat transfer on the channel wall in the presence of discrete flow turbulization at forced convection: At a certain ratio of turbulizer sizes and locations, heat transfer increases faster than hydraulic resistance, as compared to a smooth channel. New investigations conducted on 20 types of tubes with turbulizers of various shapes have shown that the use of smooth-shape turbulizers provides, at the same Nu/Nusm , a 30-35% reduction of ~/~sm (Nu is the Nusselt number, ~ is the hydraulic resistance coefficient, subscript s m refers to a smooth channel) and enable one to extend the region of advancing growth of heat transfer. A study was conducted of heat transfer enhancement in a heated tube in which a supercritical-pressure kerosene and oil flowed under the conditions of carbon deposit. Heat transfer increased more than fivefold and hydraulic resistance, 3.75-fold. Compared to smooth tubes, there was a substantial decrease in deposits in turbulizer-equipped tubes. This method proved to be very effective when applied to boiling of heat carriers. At disperse and slug film boiling inside tubes, heat transfer increases 3-8-fold, and at surface boiling of water and water-glycerine mixtures, by 30-40%. Heat transfer enhancement at film and drop condensation of vapors of water, benzene, acetone, and their mixtures on the outer tube surface as well as at vapor condensation from vapor-air mixtures was investigated. Decreasing groove pitch, making a projecting part of the tube convex, and making a smooth junction between groove surfaces and projecting tube parts (Author's Certificate USSR. No 1307211) enable one to augment film condensation heat transfer by factors of 1.7-2.8 on vertical tubes and 2 .5-3 on horizontal tubes due to effective separation of the condensate film or its running down into grooves. Annular grooves and diaphragms decrease salt deposits outside and inside the tubes and make heat exchangers serviceable at carbonate hardness of water up to 20 mg-eq/liter. The experimental data outlined in the present article were supported by tests of commercial heat exchange apparatuses. In single-phase medium apparatuses,