Mark F. Tachie

Rough Wall Turbulent Boundary Layers in Shallow Open Channel Flow

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

Velocity measurement of flow through a model three-dimensional porous medium

The paper reports an experimental investigation of flow through model porous medium adjacent to open flow in a two-dimensional channel. The model consists of circular cylindrical rods installed vertically on the bottom wall of the channel in regular square arrays. The channel height was kept constant but the ratio of rod height to channel height was varied to simulate different filling fractions. Various combinations of rod diameter and rod spacing were chosen to achieve solid volume fractions (ϕ) in the range 0.01⩽ϕ⩽0.50. A viscous fluid having a refractive index similar to that of the rods was selected as the working fluid. Particle image velocimetry was used to conduct detailed velocity measurements between the rods and in the open flow between the top edges of the rods and the top wall of the channel. From these measurements, values of the slip velocity at the interface between the rods and the open flow were determined. It was found that values of the slip velocity normalized by the maximum velocity ...

PIV Study of Separated and Reattached Open Channel Flow Over Surface Mounted Blocks

A particle image velocimetry is used to study the mean and turbulent fields of separated and redeveloping flow over square, rectangular, and semicircular blocks fixed to the bottom wall of an open channel. The open channel flow is characterized by high background turbulence level, and the ratio of the upstream boundary layer thickness to block height is considerably higher than in prior experiments. The variation of the Reynolds stresses along the dividing streamlines is discussed within the context of vortex stretching, longitudinal strain rate, and wall damping. It appears that wall damping is a more dominant mechanism in the vicinity of reattachment. In the recirculation and reattachment regions, profiles of the mean velocity, turbulent quantities, and transport terms are used to document the salient features of block geometry on the flow. The flow characteristics in these regions strongly depend on block geometry. Downstream of reattachment, a new shear layer is formed, and the redevelopment of the shear layer toward the upstream open channel boundary layer is studied using the boundary layer parameters and Reynolds stresses. The results show that the mean flow rapidly redeveloped so that the Clauser parameter recovered to its upstream value at 90 step heights downstream of reattachment. However, the rate of development close to reattachment strongly depends on block geometry.

PIV measurements of flow through a model porous medium with varying boundary conditions

This paper reports an experimental investigation of pressure-driven flow through models of porous media. Each model porous medium is a square array of circular acrylic rods oriented across the flow in a rectangular channel. The solid volume fraction φ of the arrays ranged from 0.01 to 0.49. Three boundary conditions were studied. In the first boundary condition, the model porous medium was installed on the lower wall of the channel only and was bounded by a free zone. In the second and third boundary conditions, porous media of equal and unequal φ were arranged on the lower and upper channel walls so that the two media touched (second boundary condition), and did not touch (third boundary condition). Using water as the working fluid, the Reynolds number was kept low so that inertia was not a factor. Particle image velocimetry was used to obtain detailed velocity measurements in the streamwise-transverse plane of the test section. The velocity data were used to study the effects of φ and the different boundary conditions on the flow through and over the porous medium, and at the interface. For the first boundary condition, it was observed that at φ = 0.22, flow inside the porous medium was essentially zero, and the slip velocity at the porous medium and free zone interface decayed with permeability. In the second and third boundary conditions, flow communication between the porous media was observed to be dependent on the combinations of φ used, and the trends of the slip velocities at the interface between the two porous media obtained for that boundary condition were indicative of complicated interfacial flow.

Skin friction correlation for smooth and rough wall turbulent boundary layers

In this paper, we propose a novel skin friction correlation for a zero pressure gradient turbulent boundary layer over surfaces with different roughness characteristics. The experimental data sets were obtained on a hydraulically smooth and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. The flow Reynolds number based on momentum thickness ranged from 3730 to 13,550. We propose a correlation that relates the skin friction, Cf, to the ratio of the displacement and boundary layer thicknesses, δ*∕δ, which is valid for both smooth and rough wall flows. The results indicate that the ratio Cf1∕2∕(δ*∕δ) is approximately constant, irrespective of the Reynolds number and surface condition.

Characteristics of Shallow Turbulent Near Wakes at Low Reynolds Numbers

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

Open channel boundary layer relaxation behind a forward facing step at low Reynolds numbers

We report laser-Doppler anemometer measurements of mean velocity and turbulence statistics upstream and downstream of a 3-mm forward facing step in a shallow open channel flow. The Reynolds numbers based on the momentum thickness (θ) of the approaching upstream flow and step height (h) are in the range 1010≤Re θ ≤2240 and 960≤Re h ≤1890, respectively. Measurements are obtained at 50 step heights upstream of the leading edge of the step and 1≤x/h≤162 downstream of the step

Characteristics of Turbulent Three-Dimensional Offset Jets

Three-dimensional turbulent offset jets were investigated using a particle image velocimetry technique. Three jet exit Reynolds numbers, Re j = 5000, 10,000, and 20,000, and four offset heights, h/d=0.5, 1.0, 2.0, and 4.0, were studied. The mean flow and turbulence statistics were studied over larger downstream distances than in previous studies. The decay and spread rates were found to be nearly independent of Reynolds number and offset height at certain exit diameters (x = 73d) downstream and h/d ≤ 2. The decay rates of 1.18 ± 0.03 and spread rates of 0.055 ± 0.001 and 0.250 ± 0.005 in the wall-normal and lateral directions were obtained, respectively. The reattachment lengths are also independent of Re j but increase with offset height. The locations of the maximum mean velocities increased linearly with streamwise distance in the self-similar region. It was observed that profiles of the mean velocities, turbulence intensities, and Reynolds shears stresses are nearly independent of Re j and h/d far downstream. The triple products in the symmetry plane indicated turbulence transport from the outer region of the jet towards the wall region.

Particle image velocimetry study of turbulent flow over transverse square ribs in an asymmetric diffuser

The objective of this paper is to study the combined effects of rib roughness and adverse pressure gradient produced in an asymmetric diffuser on turbulent flows. The two-dimensional asymmetric diffuser was comprised of a straight flat floor and a curved roof. The diffuser section was preceded and followed by straight parallel walls. The complete test conditions were comprised of a reference smooth floor and repeated arrays of transverse square ribs glued onto the floor to produce three pitch-to-height ratios, p∕k=3, 6, and 8. The curved roof was kept smooth in all the experiments. For each of the four test conditions, a particle image velocimetry was used to conduct detailed velocity measurements within the diverging section and also at locations upstream and downstream of the diverging section. From these measurements, the mean streamlines, mean velocities, turbulent intensities, Reynolds shear stress, and production terms in the transport equations for the turbulent kinetic energy and Reynolds stresses...

Characteristics of Turbulent Three-Dimensional Wall Jets

Three-dimensional turbulent wall jet was investigated using a particle image velocimetry technique. Three Reynolds numbers based on the jet exit velocity and diameter of 5000, 10,000, and 20,000 were studied. Profiles of the mean velocities, turbulence intensities, and Reynolds shear stresses as well as two-point velocity correlations and proper orthogonal decomposition analyses were used to document the salient features of the wall jets. The decay and spread rates are independent of Reynolds numbers in the self-similar region. The estimated values of 1.15, 0.054, and 0.255 for the decay rate, wall-normal spread rate, and lateral spread rate, respectively, are within the range of values reported in the literature. The two-point correlation analysis showed that the inclination of the streamwise velocity correlation contours in the inner layer is 11 ± 3 deg in the wall region, which is similar to those of canonical turbulent boundary layers. The results from the proper orthogonal decomposition indicate that low-order modes contribute more to the turbulence statistics in the self-similar region than in the developing region. The Reynolds shear stresses are the biggest benefactors of the low-order mode contribution while the wall-normal turbulence intensities are the least.