Xingkui Wang

Velocity profile of sediment suspensions and comparison of log-law and wake-law

The log-law and wake-law of velocity profile of open channel flow of sediment suspensions are discussed and compared in the paper. Data from 9 literatures are employed for comparison of the two laws and regression analyses are conducted on the main factors affecting velocity profile. Empirical formulas are obtained for estimation of the factors from the flow conditions. The elevation of the maximum velocity and the deviation of velocity from the logarithmic formula at the water surface are functions of the aspect ratio of the channel. The log-law is developed into Eq. (20) applicable to the whole flow including the region near the water surface for various boundary conditions. The wake law describes the velocity distribution below the maximum velocity point. The relative error of wake-law (11%) is larger than that of log-law (6%). Moreover, the wake coefficient must be determined by using the measured velocity profile because there is no reliable formula to estimate its value from the flow conditions.

Three-dimensional unstructured-mesh eutrophication model and its application to the Xiangxi River, China.

The Xiangxi River is one of the main tributaries in the Three Gorges reservoir, with the shortest distance to the Three Gorges Project Dam. Severe and frequent algal bloom events have occurred frequently in the Xiangxi River in recent years. Therefore, the current study develops a three-dimensional unstructured-mesh model to investigate the dynamic process of algal bloom. The developed model comprises three modules, namely, hydrodynamics, nutrient cycles, and phytoplankton ecological dynamics. A number of factors, including hydrodynamic condition, nutrient concentration, temperature, and light illumination, that would affect the evolution of phytoplankton were considered. Moreover, the wave equation was used to solve the free surface fluctuations and vertical Z-coordinates with adjustable layered thicknesses. These values, in turn, are suitable for solving the algal bloom problems that occurred in the river style reservoir that has a complex boundary and dramatically changing hydrodynamic conditions. The comparisons between the modeling results and field data of years 2007 and 2008 indicate that the developed model is capable of simulating the algal bloom process in the Xiangxi River with reasonable accuracy. However, hydrodynamic force and external pollution loads affect the concentrations of nutrients, which, along with the underwater light intensity, could consequently affect phytoplankton evolution. Thus, flow velocity cannot be ignored in the analysis of river algal bloom. Based on the modeling results, building an impounding reservoir and increasing the releasing discharge at appropriate times are effective ways for controlling algal bloom.

Coherent structures and their interactions in smooth open channel flows

The study presents experimental results of coherent structures and their interactions in a smooth open channel flow based on measurement of instantaneous two-dimensional velocity vectors with particle image velocimetry. The sampled data were analyzed through techniques of ensemble average, vortex extraction, and proper orthogonal decomposition (POD). Redistribution of turbulent kinetic energy is observed in the near-surface region. The spanwise vortices, which are closely related to hairpin vortices, exhibit a clear dependence on Reynolds number of the flow. Hairpin vortex packets and long streamwise vortices are identified as typical large-scale and super-scale coherent structures, respectively, and their interaction is revealed by examining the relationship between the population density of spanwise vortices and the coefficient functions of the first POD mode. Interactions between large-scale and super-scale structures have been recognized to support the hypothesis of closed-loop feedback cycle.

An improved swirling-strength criterion for identifying spanwise vortices in wall turbulence

Swirling strength, λci, is an effective vortex indicator in wall turbulence, and existing vortex extraction methods introduce normalisation of λci with its root mean square as the basis for defining a universal threshold. This study presents an improved criterion by normalising the vortex indicator for prograde and retrograde vortices, respectively, with its conditional mean. Finally, a single, universal threshold is selected for extracting prograde and retrograde vortices simultaneously. The improved method was compared with the previous method based on open channel flow data measured with particle image velocimetry. Results show that while the two methods are basically equivalent in extracting prograde vortices, the improved method extracts a higher fraction of retrograde vortices. The approach of separately normalising vortex indicators associated with prograde and retrograde vortices can also be applied to other vortex extraction criteria.

Experimental study on three dimensional movements of particles I Effects of particle diameter on velocity and concentration distributions

Based on the three Dimensional Particle Tracking Velocimetry (3D PTV) system, the characteristics of motion of particles with four different diameters were investigated under the steady flow conditions. The longitudinal average velocity profiles of these particles were in accordance with Log-law, while the vertical and transverse velocities remained very low with minimal fluctuation. The time-average velocity of particles in the bed load layer was 8.50u*, close to Bagnolds assumption Un − ω. The vertical concentration distribution of particles in the suspension region agreed with the Rouse equation. When the diameter of particles was relatively large, there existed an evident concentration gradient in the bed load layer.

Comparison of vortex identification criteria for planar velocity fields in wall turbulence

This study derives and compares vortex identification methods for detecting vortices in planar velocity fields. Two-dimensional (2D) forms of the commonly used Δ, Q, λci, and λ2 criteria are derived in detail based on the 2D counterpart of the full velocity gradient tensor. These four criteria are compared mathematically and experimentally in the case of using zero thresholds. The results show that while all methods are capable of extracting strong vortices, their efficiencies in identifying weaker vortices are not necessarily the same. The Δ and λci criteria impose the least requirements on the identified structures and extract the most number of vortices, and the λ2 criterion is the most restrictive one and tends to discard the weakest vortices. However, non-zero thresholds are generally necessary for applying vortex identification criteria in real turbulent flows, and normalizing the vortex indicators with their root mean squares is needed to enable the selection of universal threshold for vortices res...

Experimental study on the role of spanwise vorticity and vortex filaments in the outer region of open-channel flow

ABSTRACT The dynamic importance of spanwise vorticity and vortex filaments has been assessed in steady, uniform open-channel flows by means of particle image velocimetry. By expressing the net force due to Reynolds’ turbulent shear stress, , in terms of two velocity–vorticity correlations, and , the results show that both spanwise vorticity and the portion of it that is due to spanwise filaments make important contributions to the net force and hence the shape of the mean flow profile. Using the swirling strength to identify spanwise vortex filaments, it is found that they account for about 45% of , the remainder coming from non-filamentary spanwise vorticity, i.e. shear. The mechanism underlying this contribution is the movement of vortex filaments away from the wall. The contribution of spanwise vortex filaments to the Reynolds stress is small because they occupy a small fraction of the flow. The contribution of the induced motion of the spanwise vortex filaments is significant.

Large-scale particle tracking velocimetry with multi-channel CCD cameras

Abstract This paper presents a large-scale particle tracking velocimetry (LSPTV) system for measuring surface velocity in vast unsteady flows. The system consists of a flexible number of one-computer-six- camera working units connected via local network or the internet. Multiple zero-tilting cameras are used to increases the field of view with minimum perspective distortions. External synchronization of video outputs of all cameras facilitates uninterrupted image acquisition within the system. Effectiveness and efficiency of the multi-channel LSPTV are enhanced by optimizing illumination, flow seeding, image acquisition and correction, tracer identification, particle matching, and post-processing. The capacity of the LSPTV was demonstrated in the measurement of physical model flows for the Three Gorges Project.

Statistical analysis of turbulent super‐streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow

Long streamwise-elongated high- and low-speed streaks are repeatedly observed near the free surface in open channel flows in natural rivers and lab experiments. Super-streamwise vortex model has been proposed to explain this widespread phenomenon for quite some time. However, statistical evidence of the existence of the super-streamwise vortices as one type of coherent structures is still insufficient. Correlation and proper orthogonal decomposition (POD) analysis based on PIV experimental data in the streamwise-spanwise plane near the free surface in a smooth open channel flow are employed to investigate this topic. Correlation analysis revealed that the streaky structures appear frequently near the free surface and their occurrence probability at any spanwise position is equal. The spanwise velocity fluctuation usually flows from low-speed streaks toward high-speed streaks. The average spanwise width and spacing between neighboring low (or high) speed streaks are approximately h and 2h respectively. POD analysis reveals that there are streaks with different spanwise width in the instantaneous flow fields. Typical streamwise rotational movement can be sketched out directly based on the results from statistical analyses. Point-by-point analysis indicates that this pattern is consistent everywhere in the measurement window and is without any inhomogeneity in the spanwise direction, which reveals the essential difference between coherent structures and secondary flow cells. The pattern found by statistical analysis is consistent with the notion that the super-streamwise vortices exist universally as one type of coherent structure in open channel flows.

Experimental study on three dimensional movements of particles II Effects of particle diameter on turbulence characteristics

Abstract Based on the 3D PTV (Particle Tracking Velocimetry) measuring system, the 3D movement characteristics of particles with four different diameters were investigated. Under specific flow conditions, the impact of particle diameters on 3D motion of particles was studied, and the turbulence characteristics were analyzed by different statistical methods. The results showed that the turbulence intensity of coarse particle decreased as the diameter increased. In near wall region, the probability density distributions of longitudinal and vertical fluctuation velocities both deviated from the normal distribution; while in the outer region, the probability density distribution of vertical fluctuation velocity approximately agreed with the normal distribution.