Turbulence Intensity Components Research Articles

Scour holes downstream of bed sills in low-gradient channels

An experimental study on long local scouring downstream of bed sills in a monogranular sand bed was carried out in the hydraulic laboratory flume at the Mediterranean Agronomic Institute of Bari (Italy). The main objectives of this study were to determine scour hole dimension, with its maximum scour depth as a function of time and at the equilibrium stage, the scour hole shapes and the investigation of the influence of sills on the distribution of the three-velocity components through the scour hole at the same stage. Four experimental configurations were tested, the main difference between them being the distance between sills. Based on experimental data, the classical dimensional analysis of the variables that influence the development of the scour hole has been carried out in order to obtain two empirical formulas predicting the maximum scour depth and the length of the scour hole at the equilibrium stage. Moreover, it was observed that the distance between sills influences the scour hole dimension and shape. The three-velocity components of the flow, measured with an acoustic Doppler velocimeter, show that in the scour hole, at the equilibrium stage, the three components of the flow turbulence intensities are very high. Near-bed flow vortexes in addition to secondary currents are also observed.

The effect of standing biomass on flow velocity and turbulence in Spartina alterniflora canopies

Plant-flow interactions on the surface of tidal wetlands result in flow characteristics that are profoundly different from non-vegetated flows. Reductions in mean flow velocity and turbulence, especially the vertical components, limit vertical mixing and may impact a wide range of processes including geochemical exchanges at the sediment water interface, larval recruitment and dispersion, and sediment deposition and retention. The goal of this paper is to quantify horizontal and vertical components of velocity, turbulence intensity and total turbulent kinetic energy in Spartina alterniflora canopies in southeastern North Carolina and to relate flow characteristics to particulate transport on the marsh surface. Another aim of this paper is to assess the extent to which the distribution of standing biomass affects mean flow and turbulence by comparing S. alterniflora data to other canopy types and through a series of canopy manipulations which altered canopy height and stem densities. The results of this study indicate that flow velocity, turbulence intensity, and total turbulent kinetic energy (TKE) are significantly reduced within the vegetated canopy and that this reduction is inversely related to the amount of biomass present in the water column. Within the canopy, approximately 50% of the initial mean velocity and TKE is reduced within 5 m of the canopy edge. Within the canopy, mean velocity and TKE horiz usually exceeded vertical velocity or TKE vert and the vertical components of flow were attenuated more strongly than the horizontal. These results suggest that within the vegetation, turbulence contributes more to lateral advection than to vertical mixing. As a result, total suspended solid concentrations were shown to decrease logarithmically with distance from the canopy edge and to decrease at a faster rate in more densely vegetated regions of the canopy (i.e. lower TKE vert) as compared to areas of sparser vegetation (i.e. higher TKE vert).

Response of velocity and turbulence in submerged wall jets to abrupt changes from smooth to rough beds and its application to scour downstream of an apron

This paper addresses how the turbulent flow field in submerged wall jets responds to an abrupt change from smooth to rough beds. Experiments were conducted for submerged wall jets having different submergence factors and jet Froude numbers. The bed configurations investigated consisted of different combinations of the lengths of smooth beds and the roughness of rough beds. The vertical profiles of time-averaged velocity components, turbulence intensity components and Reynolds stress were detected by an acoustic Doppler velocimeter at different streamwise distances; and the horizontal distributions of bed shear stress were estimated from the Reynolds stress profiles. The flow field displays the decay of jet velocity due to abrupt changes from smooth to rough beds. The boundary layer grows more quickly with increase in roughness of rough beds. The change in bed roughness induces an increased depression of the free surface over the smooth bed. The Reynolds and bed shear stresses are also computed by solving the Navier–Stokes equations. The response of the turbulent flow characteristics of submerged wall jets to abrupt changes from smooth to rough beds is analysed from the point of view of similarity, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The significant observation is that the flow in the fully developed zone is plausibly self-preserving on both smooth and rough beds. Also, the use of a common length scale makes it possible to collapse all the flow data onto a single band; and there is a gradual variation of flow at the junction of the smooth and rough beds.The equilibrium scour profiles downstream of a smooth apron due to submerged wall jets are computed from the threshold condition of the sediment particles on the scoured bed. Use of the modified bed shear stress for the downstream variation of scoured bed permits the computation of the equilibrium scour profiles. The time-variation of maximum scour depth is computed from the bed shear stress with a modification for the time dependence. The agreement between the results obtained from the model and the experimental data is satisfactory.

Effects of Topographical Slope Angle and Atmospheric Stratification on Surface-Layer Turbulence

The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ w /U) is the smallest over the downslope surface whereas that of the u component (σu /U) has a minimum over the upslope surface. Consequently, σw /u* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ u /u* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface.

3D flow field in a scour hole at a wing-wall abutment

Experiment was conducted in a laboratory flume to measure the 3D turbulent flow field by an acoustic Doppler velocimeter in a scour hole at a 45? wing-wall abutment under a clear water regime. The profiles of time-averaged velocity components, turbulence intensity components, turbulent kinetic energy and Reynolds stresses at different azimuthal planes are presented. In the upstream, vector plots of flow field at azimuthal and horizontal planes show the presence of a large primary vortex associated with a downflow within the scour hole. In the downstream, the flow is upward. The bed shear stresses within the scour hole are estimated from the Reynolds stresses and the velocity gradients. The data presented in this study would be useful to the researchers for the future development and validation of theoretical models of flow field in a scour hole at bridge abutments

Velocity and turbulence in a scour hole at a vertical-wall abutment

An experiment was conducted in a laboratory flume to measure the three-dimensional turbulent flow field, using an acoustic Doppler velocimeter, in a scour hole at a vertical-wall abutment under a clear water scour condition. The distributions of time-averaged velocity components, turbulence intensity components and Reynolds stresses at different vertical sections are presented. In the upstream, a large primary vortex associated with the downflow exists inside the scour hole. On the other hand, in the downstream, the flow field is chaotic due to the vortex shedding. The data presented in this paper would be useful for the development and validation of a flow field model, which can be used for the estimation of scour depth at bridge abutments.

Turbulent flow field in a scour hole at a semicircular abutment

The three-dimensional turbulent flow field in a scour hole at a semicircular abutment under a clear water regime was experimentally measured in a laboratory flume using an acoustic Doppler velocimeter. The distributions of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy, and Reynolds stresses at different azimuthal planes are presented. Upstream, presentation of flow field through vector plots at azimuthal and horizontal planes shows the existence of a large primary vortex associated with the downflow inside the scour hole. On the other hand, downstream, the flow field is irregular. The bed shear stresses are determined from the Reynolds stresses and velocity gradients. The data presented in this paper would be useful for the development and validation of flow field models, which can be used to determine the strength of the primary vortex that is used to estimate scour depth at bridge abutments.Key words: bridge abutments, fluid flow, three-dimensional flow, turbulent flow, open channel flow, scour, sediment transport, hydraulic engineering.

Turbulent Boundary Layer Distorted by a Longitudinal Vortex Pair Produced by a Delta-Wing with an Attack Angle (Reynolds Stress Profiles in a Symmetrical Plane)

An experimental investigation has been made on the Reynolds stress field in the interaction process between boundary layer and longitudinal vortex. Two types longitudinal vortex pair, namely common-flow up and common-flow down, were introduced into developed turbulent boundary layer from the free stream. Effect of the extra rate of strain, ∂W/∂z and ∂V/∂y, was examined in the profiles of three turbulent intensity components and Reynolds shear stress profiles. The boundary layer along symmetrical plane is classified into “cross flow” in case of cfd and “identifiable streamwise vortices” in case of cfu. The extra rates of strain modify magnitude of turbulent intensity and Reynolds shear stress in the inner layer in case of cfd. Otherwise, close to the wall in the case of cfu, local adverse pressure gradient is deduced from the fact that magnitude of transverse divergence is grater than that of spanwise convergence.

Turbulent flow measurement by the ADV in the vicinity of a rectangular cross-section cylinder placed at a channel sidewall

This paper presents the findings of an experimental study on the three-dimensional turbulent flow field in the vicinity of a rectangular cross-section cylinder placed at the sidewall of a rectangular open channel. Uniform sand of median diameter 0.52 mm was glued over the channel bottom to make it rough. Detailed measurements of the time-averaged velocity components, turbulent intensity components, turbulent kinetic energy and Reynolds stresses were taken by the acoustic doppler velocimeter (ADV) at different vertical sections around the cylinder. In the upstream, presentation of flow fields through vectors at vertical and horizontal sections show a primary vortex associated with the downflow near the cylinder. On the other hand, in the downstream, the flow field shows a three-dimensional separated flow with vortex shedding. Using the Reynolds stresses and velocity gradients, the bed shear stresses are determined. In front of the cylinder, the velocity and the bed shear stress are found to be approximately 1.6 and 3.3 times of their magnitudes at free flow upstream zone, respectively. The data presented in this paper would be useful to the investigators for the development and verification of mathematical models of flow field in the vicinity of vertical-wall abutments.

Velocity and turbulence at a wing-wall abutment

Experimental investigation of the 3D turbulent flow field around a 45‡ wing-wall abutment, resting on a rough rigid bed, is reported. The experiment was conducted in a laboratory flume using the Acoustic Doppler Velocimeter (ADV). Profiles of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy and Reynolds stresses at different azimuthal planes are presented. Vector plots of flow fields at azimuthal and horizontal planes show the presence of a primary vortex associated with the downflow in the upstream side of the abutment and a wake vortex on the downstream side. The shear stresses acting on the bed around the abutment are estimated from the Reynolds stresses and velocity gradients. The data presented in this study would be useful to researchers for future development and comparison of theoretical models of flow fields around bridge abutments.

Measurement of turbulent flow field at a vertical semicircular cylinder attached to the sidewall of a rectangular channel

Measurement of three-dimensional turbulent flow field at a vertical semicircular cylinder, attached to the sidewall of a rectangular channel, was taken in the laboratory using an Acoustic Doppler Velocimeter (ADV). The distributions of time-averaged velocity components, turbulent intensity components, turbulent kinetic energy and Reynolds stresses at different azimuthal planes are presented. The vector plots of flow field at azimuthal and horizontal planes show the presence of a primary vortex upstream of the cylinder. The bed shear stresses in the vicinity of the cylinder are determined from the Reynolds stresses and velocity gradients.

DNS of Turbulent Channel Flow Subjected to the Acceleration

The reverse transition of accelerated turbulent flow is investigated using a direct numerical simulation (DNS). The tendency of the results is consistent with those of the previous experiments. Our major finding is that the response of streaks and quasistreamwise vortices to the acceleration does not coincide qualitatively. The streamwise component of turbulence intensity near the wall is maintained, even in the accelerated flow field. However, the other components of turbulence intensities and the streamwise vorticity are reduced in the whole region of the accelerated flow field because of the termination of the energy redistribution. Furthermore, it is found that the stretching of main flow reduces the regeneration of quasi-streamwise vortices through the decrease in distortion.

Video‐imaged surf zone wave and roller structures and flow fields

The measurement of mean water levels, roller geometries, and phase ensemble‐averaged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two‐dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble‐averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time‐averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone.

Stagnation Line Heat Transfer Augmentation Due to Freestream Vortical Structures and Vorticity

Abstract An experimental study has been performed to investigate the effect of freestream vortical structures and vorticity on stagnation region heat transfer. A heat transfer model with a cylindrical leading edge was tested in a wind tunnel at Reynolds numbers ranging from 67,750 to 142,250 based on leading edge diameter of the model. Grids of parallel rods were placed at several locations upstream of the heat transfer model in orientations where the rods were perpendicular and parallel to the stagnation line to generate freestream turbulence with distinct vortical structures. All three components of turbulence intensity, integral length scale and the spanwise and transverse vorticity were measured to characterize the freestream turbulence. The measured heat transfer data and freestream turbulence characteristics were compared with existing empirical models for the stagnation line heat transfer. A new correlation for the stagnation line heat transfer has been developed that includes the spanwise fluctuating vorticity components.

Water Air Bubble Grid Turbulence

The interaction of a swarm of bubbles with the isotropic grid turbulence generated in a vertical water channel has been studied experimentally. Measurements of local void fraction, mean velocity and all three turbulence intensity components are presented at a distance from the grid where isotropic turbulence prevails in single-phase flow. At low gas flow rate the interaction is rather mild with minor redistribution of the void but significant increase of the turbulence intensity. High gas flow rates result in significant modifications of the flow structure. Void distribution presents maxima at locations between the center of the channel and the wall reminiscing of the Segre-Sielberberg effect. Similar maxima are observed in mean velocity measurements at values much higher than the corresponding single-phase flow. Turbulence intensity components increase is following different trends in longitudinal and transverse direction and the isotropy of the flow is destroyed.

Direct Numerical Simulation of Wind-Driven Turbulent Flow.

In this study, Direct numerical simulation (DNS) of wind-driven turbulent flow at the Reynolds number is about 3 300 based on the gas layer height and the free stream velocity, was conducted by the direct numerical solution procedure (MARS method) for a coupled gas-liquid flow. As the results, completely mass conservation was kept during the all calculation time and the capacity of MARS method as the method for DNS of multiphase turbulent flow indicated. In wind-driven turbulent flow, local friction coefficient in gas side was 15% decreased compared with the wall turbulent flow. Near free surface, large horizontal scale shaped the water surface wave motion supplied the velocity fluctuations to the fluid motion. But, vertical component of turbulent intensity was constrained by the water surface existence as well as turbulent open-channel flow at low Froude number. In both gas- and water-side, high and low speed streaky structures were observed and these structures and surface wave motion was interacted each other. It seems that relationships between wave height and turbulent boundary layer thickness was one of the important factors in wind-driven turbulent structure.

1349 鉛直管内気泡乱流に関する研究

Sufficient knowledge of bubble-induced turbulence (B.I.T.) is indispensable to develop an accurate turbulence model in a gas-liquid two-phase bubbly flow. In this study, we measured turbulence intensities and Reynolds stresses in air-water bubbly flow in a vertical pipe under the condition of a low liquid Reynolds number (Re=900). To enable the LDV measurement of Reynolds stresses, we could confirm that (1) turbulent kinetic energy of B.I.T. is proportional to local void fraction, (2) the ratio among axial, radial and azimuthal components of turbulence intensity is 2 : 1 : 1 and (3) the Reynolds shear stress of B. I. T. is well correlated with the void fraction gradient.

UNSTEADINESS EFFECTS ON VON KARMAN CONSTANT IN SMOOTH OPEN-CHANNEL FLOWS

The inner and outer layers of unsteady smooth open channel flows were measured by making use of two-sets of two-component laser Doppler anemometers (LDA) under the condition that the unsteadiness parameter is very high. The water depth variations at three sections were measured by using of three-sets of supersonic water depth gauges. It was concluded that the von Karman constant is regarded to be a universal constant if the unsteadiness parameter a is smaller than 0.002. The spanwise velocity in unsteady open-channel flows was first measured in this study. It was found that the magnitude of each turbulent intensity components are not affected by the unsteadiness. It was suggested that not only large-scale eddies but also small-scale eddies were generated when the peak depth is achieved from wavelet analysis.

Laser-Doppler Velocimetry Measurements Inside a Backward Curved Centrifugal Fan

Laser-Doppler velocimetry (LDV) measurements are presented of relative mean velocity and turbulence intensity components inside the impeller passage of a centrifugal fan with twelve backward curved blades at design, under-design, and over-design flow rates. Additional LDV measurements were also performed at the volute outlet to examine the uniformity of the outlet flow for the three selected flow rates. Complementary flow visualization results in the tongue region are further presented. It is found that the number of characteristic flow regions and the average turbulence level increase with decreasing air flow rate. For the case of under-design flow rate, there are a through-flow region on the suction side, a reverse flow region on the pressure side, and a shear layer region in between. The corresponding average turbulence intensity is as high as 9.1% of blade tip velocity.

Flow Measurements in Enlarged-Bore Seal Chambers

Laser doppler velocimetry (LDV) and high speed video photography were used to measure flow velocity and visualize solid particle motion within enlarged bore seal chambers. The measurements were obtained at two shaft rotational speeds for an impeller with and without balance holes. The respective flow Taylor numbers for the two shaft speeds are 3.37 × 109 and 1.6 × 109; the Reynolds numbers are 3.52 × 105 and 2.43 × 105. A Taylor vortex extending along the seal chamber length was measured at these Taylor numbers. The presence of balance holes in the impeller causes higher velocities and turbulence intensities compared with the same operating conditions when balance holes are plugged. For the impeller with balance holes, the effect of the Taylor vortex in mixing the chamber flow is measurable. The flow at a lower Taylor number resembles that at the higher Taylor number, but with lower magnitudes of circumferential velocity and turbulence intensity components. Solid particles with a mean diameter of approximately 40 μm or less follow the Taylor vortex flow. Larger particles are trapped near the gland plate and cause erosion. Protrusions in the chamber drastically reduce the solid particle concentration. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998