Rough Open Channel Flow Research Articles

NUMERICAL INVESTIGATION OF RECTANGULAR RIB ROUGHNESS EFFECTS ON FLOW RESISTANCE IN OPEN-CHANNEL FLOW

A series of numerical simulations are undertaken to investigate the flow resistance of a rough open-channel flow. The roughness is generated by periodically spaced bars and transversely glued on the channel bottom. The free surface flow is modeled by using the finite volume method and the Reynolds stress model coupled to the two-phase model. The results obtained show that for a given bar thickness, the maximum flow resistance is obtained when the ratio of cavity length to roughness height is around 12.5. For several longitudinal thicknesses of the bars, it is found that the flow resistance and the generated roughness are higher when the longitudinal thickness-to-height ratio of the bars is less than unity. For given streamwise spacing bars, the friction factor and the generated roughness are inversely proportional to the longitudinal thickness of the bars. By including the geometric dimensions of the roughness elements and their spacing, a relationship between the friction factor and the generated roughness is derived. A good agreement was found with the existing expressions in the relevant literature. It is postulated that the extension of the proposed formula to the large-scale roughness reproduces acceptable results.

Application of the sampling moiré method to shallow open-channel flows with circular roughness elements

This research presents an application of the sampling moiré method, a novel technique originally developed for measuring slight deformation of a static object such as a plate, to fluctuating free water surface in rough open channel flows. The method was used to capture the two-dimensional water surface deformation at a high sampling rate by dyeing the color of water white. The target flow was a very shallow subcritical flow with a depth of 1.5 times the height of the roughness. As a result, a series of instantaneous water surface profiles with non-uniform rough planes were successfully measured at a sampling rate of 100 Hz.It has become clear that the effect of dispersive gravity waves traveling on the water surface is greater than that of non-dispersive waves driven by the turbulence, and that this feature becomes more pronounced as the Froude number increases in such shallow flow conditions. Furthermore, it was experimentally and theoretically verified that the intensity of water surface fluctuation increases with the square of the Froude number. The decomposition of surface fluctuations due to gravity waves and subsurface turbulence using the wavenumber frequency spectrum revealed that the effects of subsurface turbulence are about 20%. This is much less than the contribution from gravity waves in very shallow flows as in this experiment.

Effects of aspect ratio on higher-order moments, conditional statistics, TKE budget and anisotropy in narrow open channel flow

The purpose of the present study is to investigate the influence of aspect ratio on the higher- order statistics of velocity fluctuations, conditional statistics, turbulent kinetic energy (TKE) fluxes, TKE budget, and Reynolds stress anisotropy in hydraulically rough narrow open channel flow (OCF). In the experiments, the aspect ratios were maintained low and varied between 2.5 and 4. The velocities were measured with an acoustic Doppler velocimeter (ADV). The higher-order moments, quadrant analysis, and TKE budget from the present experiments were compared with available literature data in wide and narrow OCFs to elucidate the effect of the aspect ratio. In this analysis, the third-order moments of velocity fluctuations were found to be sensitive to the aspect ratio in the outer region. The fractional contributions of all quadrant events are approximately equal in magnitude in lower aspect ratio flows, whereas ejections and sweeps are the dominant events as the aspect ratio increases. The upward transfer of TKE flux increases in the outer layer with increase in aspect ratio. In the inner layer, the TKE production and rate of dissipation are found to be increasing with decreasing aspect ratio. Therefore, the production and dissipation of TKE are dependent on the aspect ratio of the flow. The analysis of Reynolds stress AIM reveals that for low aspect ratio flows turbulence tends to attain rod-like axisymmetric turbulence only in the intermediate layer whereas for higher aspect ratio, turbulence attains rod-like axisymmetric turbulence throughout the depth.

A new analytical model for dip modified velocity distribution in fully developed turbulent open channel flow

This paper presents a new analytical model to predict the streamwise time-averaged velocity profile affected by the dip phenomenon in open channel flows. The novel approach of the present study is that the Finley wake law has been used instead of Coles’ wake law for the outer layer. To validate the new analytical model, six high quality experiments were conducted in a hydraulically rough bed open channel flow by considering variations of aspect ratio, defined as the ratio of the width of the channel to the depth of flow, from 2 to 4. In these controlled experiments, the time-averaged velocities were measured using a Nortek Vectrino-plus acoustic Doppler velocimeter. In addition, 14 sets of available experimental data, including five field experiments conducted across the globe were also used to test the performance of the proposed model. The proposed model, the Finley-dip-modified-log-wake law (FDMLWL), was used to develop a semiempirical equation to compute the dip position as a function of the dip correction factor and the wake parameter. In addition, using the experimental data and FDMLWL, an empirical equation was developed to compute the dip correction factor for hydraulically smooth open channel flows. The comparison of the FDMLWL model with the experimental data belonging to hydraulically smooth, transition, and rough regimes has consistently indicated better representation of the velocity dip phenomenon. The FDMLWL model has also been compared with other analytical models available in the literature and the superior performance of the proposed model is further observed. Finally, based on the satisfactory validation between experimental data and FDMLWL, it is inferred that the proposed model is better suited for modeling zero velocity gradient at the boundary layer edge, as in open channel flows with dip phenomenon.

Effect of aspect ratio on developing and developed narrow open channel flow with rough bed

This study attempts to unravel the effect of aspect ratio on the turbulence characteristics in developing and fully developed narrow open channel flows. In this regard, experiments were conducted in a rough bed open channel flow. Instantaneous 3D velocities were acquired using an acoustic Doppler velocimeter at various locations along the centerline of the flume. The variables of interest include the mean components of the flow velocity, turbulence intensity, wall normal Reynolds shear stress, correlation coefficient, turbulence kinetic energy, and anisotropy. A new correlation between the equivalent roughness and velocity shift from the smooth wall logarithmic equation as a function of aspect ratio is proposed. Aspect ratio was found to influence the velocity characteristics throughout the depth in the developing flow region, while the effects are confined to the outer layer for fully developed flows. New equations to describe the variation of turbulence intensities and turbulent kinetic energy are proposed for narrow open channel flows. Reynolds stress anisotropy analysis reveals that level of anisotropy in narrow open channel flow is less than in wide open channel flows. Finally, a linear regression model is proposed to predict flow development length in narrow open channel flows with a rough bed.

Effect of aspect ratio on higher order moments of velocity fluctuations in hydraulically rough open channel flow

The effect of aspect ratio on the higher order statistics of velocity fluctuations in a hydraulically rough turbulent open channel flow is investigated. In this regard, an experiment was conducted in a rough bed narrow open channel flow of aspect ratio equal to three and the instantaneous flow velocities were measured using a Nortek Vectrino+ Acoustic Doppler Velocimeter. To understand the effect of aspect ratio, the results obtained from the present study are compared with the literature data of approximately same Reynolds number and bed roughness in a wide open channel flow for turbulence intensities and higher order statistics of velocity fluctuations. Comparison of turbulence intensities between Narrow OCF and Wide OCF shows occurrence of higher streamwise and vertical turbulence intensities in the outer region of Narrow OCF. The results of third order moments of velocity fluctuations are sensitive to aspect ratio in the outer region.

Turbulence structure of non-uniform rough open channel flow

This paper focuses on the turbulence structure in a non-uniform, gradually varied, sub-critical open channel flow (OCF) on a rough bed. The flow field is analysed under accelerating, near-uniform and decelerating conditions. Information for the flow and turbulence parameters was obtained at multiple sections and planes using two different techniques: two-component laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Different outer region velocity scaling methods were explored for evaluation of the local friction velocity. Analysis of the mean velocity profiles showed that the overlap layer exists for all flow cases. The outer layer of the decelerated velocity profile was strongly affected by the pressure gradient, where a large wake was noted. Due to the prevailing nature of the experimental setup it was found that the time-averaged flow quantities do not attained equilibrium conditions and the flow is spatially heterogeneous. The roughness generally increases the friction velocity and its effect was stronger than the effect of the pressure gradient. It was found that for the decelerated flow section over a rough bed, the mean flow and turbulence intensities were affected throughout the flow depth. The flow features presented in this study can be used to develop a model for simulating flow over a block ramp. The effect of the non-uniformity and roughness on turbulence intensities and Reynolds shear stresses was further investigated.

An Innovative Preston Tube for Determination of Shear Stress on Smooth and Rough Beds

Preston tube is a shear stress measuring technique which is widely used due to its simple operation and construction. Although application of this technique in smooth bed is convenient and reliable, such application in rough bed is problematic due to the appearance of additional parameters such as roughness length scale and zero-plane displacement. In present study, a different sort of Preston tube technique is developed which is suitable for bed shear stress measurement in both smooth and rough bed open channel flows. The new instrument works according to an equation derived from wall-similarity approach and double averaging method. Unlike the conventional Preston tube, this instrument consisted of three Pitot tubes and one Prandtl static tube. Other available methods for shear stress estimation [i.e. logarithmic law, Reynolds shear stress profile, energy slope and conventional Preston tube (the later just in the case of smooth bed)] were used for to control the accuracy of the data collected by new instrument. The results comparison shows that this instrument is capable of measuring bed shear stress with accuracy up to ±15.0 and ±24% respectively in smooth and rough bed conditions. These accuracy ranges assure applicability of this device in future open-channel flow studies.

Prediction of the Clogging Profile Using the Apparent Porosity and Momentum Impulse

AbstractThis paper advances prediction of the clogging profile using the apparent porosity concept. The clogging profile is the vertical distribution of fine sand particles within a gravel-bed substrate that is overlain by hydraulically rough open channel flow. The author introduces the apparent porosity to represent the conveyance of fluid momentum through porous media, which is therefore a function of both the media and fluid, i.e., an apparent property. The author parameterizes the apparent porosity considering that coherent structures inject fluid momentum and sand into a clean gravel bed and the flow becomes laminar at its limit when the gravel is fully packed with sand. The author couples the apparent porosity with the previously published momentum-impulse model to provide a process-based prediction of the clogging profile, and the results show good agreement with experimental data. Results analyzed in dimensionless form via the dimensionless clogging depth show that the gravel porosity and roughnes...

Clogging of Fine Sediment within Gravel Substrates: Dimensional Analysis and Macroanalysis of Experiments in Hydraulic Flumes

AbstractAn understanding of the clogging of fine sediments within gravel substrates is advanced through use of dimensional analysis and macroanalysis of previously conducted clogging experiments in hydraulic flumes. Dimensional analysis via the Buckingham pi theorem is used to suggest that the dimensionless clogging depth can be potentially collapsed using the original and adjusted bed-to-grain ratios, i.e., ratio of substrate diameter to fine sediment diameter, substrate porosity, roughness Reynolds number, and Peclet number. Macroanalysis and statistical analysis is performed using 10 previously published studies that include a total of 146 different test conditions reporting noncohesive, fine sediment clogging, or deposition in porous gravel-beds with hydraulically rough turbulent open channel flow. Results suggest that the adjusted bed-to-grain ratio is a reliable predictor of the initiation of clogging with clogging occurring below 27 for the fine fluvial sediment clogging in a gravel bed substrate f...

Universal Velocity Distribution for Smooth and Rough Open Channel Flows

The Prandtl second kind of secondary current occurs in any narrow channel flow causing velocity dip in the flow velocity distribution by introducing the anisotropic turbulence into the flow. Here, a study was conducted to explain the occurrence of the secondary current in the outer region of flow velocity distribution using a universal expression. Started from the basic Navier-Stokes equation, the velocity profile derivation was accomplished in a universal way for both smooth and rough open channel flows. However, the outcome of the derived theoretical equation shows that the smooth and rough bed flows give different boundary conditions due to the different formation of log law for smooth and rough bed cases in the inner region of velocity distribution. Detailed comparison with a wide range of different measurement results from literatures (from smooth, rough and field measured data) evidences the capability of the proposed law to represent flow under all bed roughness conditions.

Spatial and temporal scales of force and torque acting on wall-mounted spherical particles in open channel flow

Data from direct numerical simulation of open channel flow over a geometrically rough wall at a bulk Reynolds number of Reb = 2900, generated by Chan-Braun et al. [“Force and torque acting on particles in a transitionally rough open-channel flow,” J. Fluid Mech. 684, 441–474 (2011)]10.1017/jfm.2011.311 are further analysed with respect to the time and length scales of force and torque acting on the wall-mounted spheres. For the two sizes of spheres in a square arrangement (11 and 49 wall units in diameter, yielding hydraulically smooth and transitionally rough flow, respectively), the spatial structure of drag, lift, and spanwise torque is investigated. The auto-correlation and spectra in time as well as the space-time correlation and convection velocities are presented and discussed. It is found that the statistics of spanwise particle torque are similar to those of shear stress at a smooth wall. Particle drag and lift are shown to differ from spanwise particle torque, exhibiting considerably smaller time and length scales; the convection velocities of drag and lift are somewhat larger than those of spanwise torque. Furthermore, correlations between the flow field and particle-related quantities are presented. The spatial structure of the correlation between streamwise velocity and drag/spanwise torque features elongated shapes reminiscent of buffer-layer streaks. The correlation between the pressure field and the particle drag exhibits two opposite-signed bulges on the upstream and downstream sides of a particle.

Vortices and Large-Scale Structures in a Rough Open-Channel Flow Subjected to Bed Suction and Injection

The effect of seepage (injection and suction) on the dynamics of vortices and large-scale structures in a rough open channel flow was examined experimentally. The velocity measurements in the open channel flow were carried out using particle image velocimetry. A swirling strength-based vortex identification algorithm was used to extract the location, size, and strength of vortices in the flow. Proper orthogonal decomposition (POD) was used to analyze the dynamics of large-scale structures in the flow. The results indicate that suction induces a greater change compared with an equivalent amount of injection on the open channel flow. Suction dampens the size and strength of the vortices compared to the no-seepage case. POD analysis indicates that seepage tends to introduce more small-scale structures in the flow and destroys the large-scale organized motion present otherwise in the flow. DOI: 10.1061/(ASCE)EM.1943-7889.0000354. © 2012 American Society of Civil Engineers. CE Database subject headings: Seepage; Open channel flow; Vortices; Decomposition. Author keywords: Seepage; Open channel flow; Vortices; Large-scale-structures; Swirling strength; Proper orthogonal decomposition.

Force and torque acting on particles in a transitionally rough open-channel flow

AbstractDirect numerical simulation of open channel flow over a geometrically rough wall has been performed at a bulk Reynolds number of ${\mathit{Re}}_{b} \approx 2900$. The wall consisted of a layer of spheres in a square arrangement. Two cases have been considered. In the first case the spheres are small (with diameter equivalent to $10. 7$ wall units) and the limit of the hydraulically smooth flow regime is approached. In the second case the spheres are more than three times larger ($49. 3$ wall units) and the flow is in the transitionally rough flow regime. Special emphasis is given to the characterisation of the force and torque acting on a particle due to the turbulent flow. It is found that in both cases the mean drag, lift and spanwise torque are to a large extent produced at the top region of the particle surface. The intensity of the particle force fluctuations is significantly larger in the large-sphere case, while the trend differs for the fluctuations of the individual components of the torque. A simplified model is used to show that the torque fluctuations might be explained by the spheres acting as a filter with respect to the size of the flow scales which can effectively generate torque fluctuations. Fluctuations of both force and torque are found to exhibit strongly non-Gaussian probability density functions with particularly long tails, an effect which is more pronounced in the small-sphere case. Some implications of the present results for sediment erosion are briefly discussed.

Outer Scaling for Open Channel Flow over a Gravel Bed

Similarity analysis is performed for hydraulically rough open channel flow over a gravel bed to provide mixed outer scaling of the mean-velocity profile. The analysis is based on equilibrium turbulent boundary-layer theory derived using the asymptotic invariance principle. Outer scaling based on the similarity theory is validated with velocity measurements from the laboratory and field, having a Reynolds number range that includes 1× 104 , 1× 105 , and 1× 106 and a Froude number range from 0.26 to 0.83. The results show that the free-stream velocity is an appropriate outer scale for gravel-bed river flows at moderate and bankfull stage. The results agree well with the velocity measurements and collapse laboratory and field data, which allow an important connection between open channel research in the laboratory and the applications for which the research is performed in the field. The results show that the R/a D84 roughness parameter is consistent with the mixed scale used in boundary-layer velocity scali...

Peculiar mean velocity profiles within a porous bed of an open channel

We present the velocity profile measured within a porous bed below rough turbulent open channel flow. Mean streamwise velocity has a minimum in the top pore, just below the bed surface, and then increases towards a constant value in the deeper pores. We argue that the velocity minimum in the top pore is due to enhanced turbulence generated within the external flow. This produces a flow regime similar to rough turbulent pipe flow, with very efficient momentum extraction. In the deeper pores turbulence intensities are damped so that momentum extraction is less efficient, allowing higher mean velocities to develop.

The relationship between stream periphyton dynamics and near-bed turbulence in rough open-channel flow

The main goal of the present work is to characterize, at the laboratory scale, the physical relationships between periphyton biomass and water velocity. Experiments, conducted in a rough open-channel, and numerical simulations, have been performed for a large range of values of the main hydrodynamic parameters. The major role of the shear stress exerted by the flow on the bed, in the periphyton dynamics, is clearly proved. We have developed a model of periphyton biomass dynamics taking the friction velocity, as external physical variable. We fitted the model to our laboratory data and, despite their poorness, the model has shown a good fit, clearly showing that local physical processes are the main factors controlling the dynamics in running waters.

EFFECTS OF WALL ROUGHNESS ON VELOCITY DISTRIBUTION AND TURBULENCE MODULATION IN SUSPENDED SEDIMENT OPEN-CHANNEL FLOWS

Suspended sediment in open-channel flows has been paid great attention in river engineering. It has been pointed out that flow resistance is changed by the existence of sand particles. Moreover, knowledge of interaction between sand-particles (solid-phase) and water (fluid-phase) is necessary to improve sediment transport rate formula. However, the mechanism of particle-fluid interactions is not fully understood. In this study, simultaneous measurements were conducted with discriminator PTV in rough open-channel flows with suspended sediment. The roughness elements were set up on the channel-bed. Consequently, the relative velocity between particles and fluid and the turbulence modulation were investigated by changing the particle concentration systematically.

Friction velocity and power law velocity profile in smooth and rough shallow open channel flows

This paper examines the mean velocity profiles in shallow, turbulent open channel flows. Velocity measurements were carried out in flows over smooth and rough beds using a laser-Doppler anemometer. One set of profiles, composed of 29 velocity distributions, was obtained in flows over a polished smooth aluminum plate. Three sets of profiles were obtained in flows over rough surfaces. The rough surfaces were formed by two sizes of sand grains and a wire mesh. The flow conditions over the rough surface are in the transitional roughness state. The measurements were obtained along the centerline of the flume at three different Froude numbers (Fr ~ 0.3, 0.8, 1.0). The lowest Froude number was selected to obtain data in the range of most other open channel testing programs and to represent a low subcritical Froude number. For each surface, the Reynolds number based on the boundary layer momentum thickness was varied from about 600 to 3000. In view of the recent questions concerning the applicability of the log-law and the debate regarding log-law versus power law, the turbulent inner region of the boundary layer is inspected. The fit of one type of power law for shallow flows over a smooth surface is considered. The appropriateness of extending this law to flows over rough surfaces is also examined. Alternate methods for determining the friction velocity of flows over smooth and rough surfaces are considered and compared with standard methods currently in use.Key words: power law, open channel flow, velocity profile, surface roughness.

Interplay between turbulence and periphyton in rough open-channel flow

During the low flow period, important development of periphyton occur in rivers. The understanding of the mutual 'periphyton-flow' influences have induced, through recent studies, conflicting conclusions which underlined the particular complexity of these interactions. An experimental flume has been build to show that the near-bed regime (which depends on the water discharge, the nature and form of the substrate, the presence or absence of blowing,...) and the development of the periphyton are strongly dependent on each other. Indeed, different experiments show that the periphytic characteristics differ depending on the hydrodynamic conditions and that the flow properties are modified by the presence of the periphytic matrix. The periphyton induces a smoothing of the roughness of the bottom which is significant depending on the thickness of the periphytic matrix. This thickness has been taken into account in terms of a gap of the y-axis in the interpretation of measurements of the hydrodynamic parameters, contrary to recent papers.