An experimental study was undertaken to investigate the effects of roughness on the structure of turbulent boundary layers in open channels. The study was carried out using a laser Doppler anemometer in shallow flows for three different types of rough surface, as well as a hydraulically smooth surface. The flow Reynolds number based on the boundary layer momentum thickness ranged from 1400 to 4000. The boundary layer thickness was comparable with the depth of flow and the turbulence intensity in the channel flow varied from 2 to 4 percent. The defect profile was correlated using an approach which allowed both the skin friction and wake strength to vary. The wake parameter was observed to vary significantly with the type of surface roughness in contradiction to the wall similarity hypothesis. Wall roughness also led to higher turbulence levels in the outer region of the boundary layer. The profound effect of surface roughness on the outer region as well as the effect of channel turbulence on the main flow indicates a strong interaction, which must be accounted for in turbulence models

Rough wall turbulent boundary layers in shallow open channel flow

The near wake of a vertical, circular cylinder in shallow water is investigated using a combination of visualization marker and a technique of high-image-density particle image velocimetry. The formation of a large-scale Karman vortex involves upward ejection of fluid through its center, which eventual leads to a horizontal vortex that induces significant distortion of the free surface. The strength of this secondary vortex is, however, an order of magnitude smaller than that of the large-scale Karman vortex. Using global, instantaneous imaging, the relationship between patterns of streamline topology and vorticity is established for successive phases of the oscillation cycle, and in horizontal planes at and above the bed. The sequence of phase-referenced states shows that it is possible to identify topological characteristics, including the location of a specific critical (saddle) point that are consistent at elevations on and above the bed. In turn, the sequential development of topological streamline and vorticity patterns is related to time-averaged representations. At the bed, the time-averaged streamline topology downstream of the base of the cylinder takes on a form known as an owl face of the first kind, which was originally defined for a completely different exterior flow. Immediately adjacent to the base of the cylinder, an additional system of saddle points is located at either end of a nodal line. At locations above the bed, one of the two principal saddle points of the owl face of the first kind disappears and the principal foci are transformed from a stable to an unstable state. Furthermore, at progressive elevations above the bed, the degree of concentration of vorticity of the major, large-scale vortex successively decreases. In turn, the foregoing features are related to the patterns of Reynolds stress and velocity fluctuation at and above the bed.

Vortex formation from a cylinder in shallow water

The two-dimensional local scour occurring downstream from a submerged sluice gate for various tailwater depths was studied using a video imaging technique and a laser-Doppler anemometer. The flow field was found to be dynamic (i.e., varied with time). Previously reported equilibrium scour profiles for similar test conditions were not attained even after 96 h of laboratory testing. The scaling laws suggested in the literature were found to be unsuitable. New scaling laws are presented for the condition when a stable equilibrium state has not yet been attained. Velocity profiles are presented at various stations during digging and refilling conditions. Key words: local scour, time scale, video imaging, scour profile, velocity profile, sluice gate, cohesionless material, tailwater effect.

The effect of tailwater depth on the dynamics of local scour

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

Experimental and numerical modelling of sedimentation in a rectangular shallow basin

The present study deals with the noninvasive measurement of both velocity and concentration in the near region of shallow turbulent wakes using a laser-Doppler anemometer and a video-imaging technique. A 40 mm wide flat plate placed normal to the flow is used as the wake generator. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. Tests were conducted at two depths of flow (h =20 and 40 mm) and the boundary layer thickness of the approaching flow is comparable to the depth. The Reynolds number of the flow based on the approaching freestream boundary layer momentum thickness varies from 180 to 400, while, the Reynolds number based on the test body width was maintained nearly constant (4000). Measurements were carried out at three axial stations (2.5, 5, and 10 plate widths) downstream of the bluff body. At each axial station, the velocity measurements were carried at distances of h/4, h/2, and 3h/4 from the channel bottom and spanning the cross section of the wake. Appropriate length and velocity scales are identified to characterize the wake

Characteristics of shallow turbulent near wakes at low Reynolds numbers

The results of an experimental investigation of turbulent boundary layers in shallow open channel flows at low Reynolds numbers are presented. The study was aimed at extending the database toward lower values of Reynolds number. The data presented are primarily concerned with the longitudinal mean velocity, turbulent-velocity fluctuations, boundary layer shape parameter and skin friction coefficient for Reynolds numbers based on the momentum thickness (Reθ ) ranging from 180 to 480. In this range, the results of the present investigation in shallow open channel flows indicate a lack of dependence of the von Karman constant κ on Reynolds number. The extent to which the mean velocity data overlaps with the log-law decreases with decreasing Reθ . The variation of the strength of the wake with Reθ is different from the trend proposed earlier by Coles.

Shyam Ramachandran

Papers

Characteristics of shallow turbulent near wakes at low Reynolds numbers

Turbulent Boundary Layers in Low Reynolds Number Shallow Open Channel Flows

Skin Friction Correlation in Open Channel Boundary Layers