Instantaneous Velocity Data Research Articles

Dynamic mechanism of turbulent flow in meandering channels: considerations for deflection angle

To find turbulent flow structure inside meandering channels, three physical models of river meanders representing strongly curved bend, mild bend and elongated symmetrical meander loop were tested in this paper. Instantaneous velocity data in three dimensions were measured using Micro-ADV at different cross sections of these models. Depth averaged velocity vectors, streamwise velocity, secondary currents, turbulent and mean flow kinetic energy were investigated with respect to the sediment deposition pattern. In order to gain more regarding the force acting the sediment particles, three dimensional velocity fluctuations were analyzed in detailed inside the elongated symmetrical meander loop. Occurrence frequency, transition probability and angle of attack for different events were also computed for the points close to the bed. Of the present results, the importance of sweeps and ejections on sediment deposition can be detected. Further, distribution of bursting events is presented through the water column and compared the results with the previous works. Importantly, occurrence of fluctuating velocities in three dimensions at different locations inside the river meanders in addition to the effect of mean flow and turbulent components is responsible for sediment transport. Streamwise velocity distribution through the depth is also compared with some previous mathematical models. Researchers seeking the better control over the river morphology can apply this method without sacrificing much time and cost. This study is also included some insights to be pursued by future works.

Turbulence and Suspended Sediment Measurements in an Urban Environment during the Brisbane River Flood of January 2011

In urbanized areas, flood flows constitute a hazard to populations and infrastructure, as illustrated during major floods in 2011. During the 2011 Brisbane River flood, some turbulent velocity data were collected using acoustic Doppler velocimetry in an inundated street. The field deployment showed some unusual features of flood flow in the urban environment. That is, the water elevations and velocities fluctuated with distinctive periods between 50 and 100 s linked with some local topographic effects. The instantaneous velocity data were analyzed using a triple decomposition. The velocity fluctuations included a large energy component in the slow fluctuation range, whereas the turbulent motion components were much smaller. The suspended sediment data showed some significant longitudinal flux. Altogether, the results highlighted that the triple decomposition approach originally developed for periodic flows is well-suited to complicated flows in an inundated urban environment.

Measurement Error in Estimates of Sprint Velocity from a Laser Displacement Measurement Device

This study aimed to determine the measurement error associated with estimates of velocity from a laser-based device during different phases of a maximal athletic sprint. Laser-based displacement data were obtained from 10 sprinters completing a total of 89 sprints and were fitted with a fifth-order polynomial function which was differentiated to obtain instantaneous velocity data. These velocity estimates were compared against criterion high-speed video velocities at either 1, 5, 10, 30 or 50 m using a Bland-Altman analysis to assess bias and random error. Bias was highest at 1 m (+ 0.41 m/s) and tended to decrease as the measurement distance increased, with values less than + 0.10 m/s at 30 and 50 m. Random error was more consistent between distances, and reached a minimum value (±0.11 m/s) at 10 m. Laser devices offer a potentially useful time-efficient tool for assessing between-subject or between-session performance from the mid-acceleration and maximum velocity phases (i. e., at 10 m and beyond), although only differences exceeding 0.22-0.30 m/s should be considered genuine. However, laser data should not be used during the first 5 m of a sprint, and are likely of limited use for assessing within-subject variation in performance during a single session.

Random Process Analysis on Pulse Amplitude of Open Channel Flow

This article, based on the analysis of the collected instantaneous velocity data, confirmed the randomness of water movement and met the stationary random process. The relationship between the pulse amplitude, Reynolds number and Froude number and it satisfies the trend of exponential curve, that is, with the large average pulse amplitude, Reynolds number and Froude number gets larger, and it leads to the change of the corresponding flowing structure. The pulse amplitude along the depth direction distributes according to an exponential curve, as the water depth increases, the pulse amplitude increases. The fluctuation of velocity plays an important role in the pulse velocity and incipient velocity of sediment river erosion, river bank erosion and collapse of the damaged dike. So the research of pulse amplitude is very necessary.

Physical Modeling of Unsteady Turbulence in Breaking Tidal Bores

A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under macrotidal and appropriate bathymetric conditions. This paper presents a study that physically investigates the turbulent properties of tidal bores. Results from some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds are reported. The free-surface measurements were conducted with Froude numbers of 1–1.7. Both undular and breaking bores were observed. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores are characterized. Immediately before the roller, the free-surface curves gradually upwards. The passage of the bore roller is associated with some large water elevation fluctuations; the largest free-surface fluctuations are observed during the first half of the bore roller. The turbulent velocity measurements were performed at several vertical elevations during and shortly after the passage of breaking bores. Both the instantaneous and ensemble-averaged velocity data highlight a strong flow deceleration at all elevations during the bore passage. Close to the bed, the longitudinal velocity component becomes negative immediately after the roller passage, implying the existence of a transient recirculation. The height and duration of the transient are a function of the bed roughness, with a higher and longer recirculation region above the rough bed. The vertical velocity data presented some positive, upward motion beneath the front with increasing maximum vertical velocity with increasing distance from the bed. The transverse velocity data show some large fluctuations with nonzero ensemble average after the roller passage that highlight some intense secondary motion advected behind the bore front.

E204 2×2ロッドバンドル内乱流の速度分布(OS9 熱・流動(試験))

Validation of turbulence models is not an easy task mainly due to lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured distributions of velocity and turbulence intensities of water flows in a 2x2 rod bundle by using a PIV system. The diameter and pitch of the rods were 20 and 25mm, respectively. The mean velocity ranged from 1 to 2m/s. The local time-averaged velocity and turbulent intensities were obtained from 10,000 instantaneous velocity data at each measurement point. Time resolved PIV measurements were also carried out to obtain the turbulence spectra. As a result, the following conclusions were obtained: (1) The fluctuation of velocity component normal to the rod wall is weaker in the low wave number region than that of the perpendicular component in horizontal plane in the rod gap. (2) The distribution of Reynolds stress on the centerline of the test section has the trend similar to the gradient of stream wise velocity. This implies the validity of the eddy viscosity assumption.

Vertical structure of continuous release saline and turbidity currents

This paper reports new experimental results on the mean velocity, excess fractional density, and turbulence structures of conservative and particulate density currents. Experiments were conducted in a laboratory flume consisting of a ramp followed by a horizontal bed. Instantaneous point velocity along the body of a continuous, developed underflow was measured using two 16 MHz Acoustic Doppler Velocity Probes (ADV). Mean velocity, turbulent kinetic energy (TKE) and Reynolds stress components have been derived from the instantaneous velocity data. The normalized mean velocity, Reynolds stress, and TKE profiles have been derived and plotted in different groups corresponding to saline and particulate gravity currents over sloping and horizontal beds. The Reynolds‐averaged velocity shows excellent similarity collapse in these plots, while the Reynolds stress and the TKE profiles display good similarity in the wall‐bounded shear flow region. Vertical profiles of the TKE display two peaks separated by a zone of low turbulence around the position of the maximum velocity. The ratio of the maximum to the layer‐averaged TKE is found to be approximately 1.5. The TKE at the low turbulence zone between the peaks is approximately 50% of the depth‐averaged value.

Salt Fluxes within a Very Shallow Subtropical Estuary

Abstract This article describes the transport processes and net salt flux within a shallow estuarine system, with particular reference to the Coombabah Lake–Creek system in Queensland, Australia. Observations of currents and salinity at two locations within Coombabah Lake provided a basis for assessing the relative importance of various transport processes within a very shallow (water depth <1 m) subtropical estuary. The instantaneous velocity and salinity data were decomposed into time-averaged means and time-varying components and were used to quantify the salt flux components attributed to various physical processes. In this study, advection by residual flow, which contributed 65% of the total salt flux, was identified as the dominant process in transporting salt. The advective flux also determined the direction of the net salt flux within this shallow estuarine system. This study concludes that the net salt flux varies spatially and temporarily with hydromorphological and meteorological conditions.

Unsteady Turbulence in Tidal Bores: Effects of Bed Roughness

A tidal bore is a wave propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the flood tide is confined to a narrow funneled channel. To date, theoretical and numerical studies rely upon physical experiments to validate the developments, but the experimental data are limited mostly to visual observations and sometimes free-surface measurements. Herein turbulent velocity measurements were obtained in a large-size laboratory facility with a fine spatial and temporal resolution. The instantaneous velocity measurements showed rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components were recorded beneath the bore and secondary waves. A comparison between undular (nonbreaking) and breaking bores suggested some basic differences. In an undular bore, large velocity fluctuations were recorded beneath the first wave crest and the secondary waves showing a long-lasting effect after the bore passage. In a breaking bore, some large turbulent stresses were observed next to the shear zone in a region of high velocity gradients, while some transient flow recirculation was recorded next to the bed. The effects of bed roughness were tested further. The boundary friction contributed to some wave attenuation and dispersion, and the free-surface data showed some agreement with the wave dispersion theory for intermediate gravity waves. The instantaneous velocity data showed however a significant effect of the boundary roughness on the velocity field next to the boundary (z/do<0.2) for both undular and breaking bores. Overall the findings were consistent with field observations of tidal bores and highlighted the significant impact of undular (nonbreaking) bores on natural systems.

PIV measurements of flows around the wind turbines with a flanged-diffuser shroud

The wind turbines with a flanged-diffuser shroud-so called “wind lens turbine”-are developed as one of high performance wind turbines by Ohya et al. In order to investigate the flow characteristics and flow acceleration, the paper presents the flow velocity measurements of a long-type and a compact-type wind turbines with a flanged-diffuser shroud by particle image velocimetry. In the case of the long type wind turbine, the velocity vectors of the inner flow field of the diffuser for turbine blades rotating and no blades rotating are presented at Reynolds number, 0.9×105. Furthermore the flow fields between with and without rotating are compared. Through the PIV measurement results, one can realize that the turbine blades rotating affects as suppress the disturbance and the flow separation near the inner wall of the diffuser. The time average velocity vectors are made on the average of the instantaneous velocity data. There are two large vortices in downstream region of the diffuser. One vortex behind the flange acts as suck in wind to the diffuser and raise the inlet flow velocity. Another large vortex appears in downstream. It might be act as blockage vortex of main flow. The large blockage vortex is not clear in the instantaneous velocity vectors, however it exists clearly in the time average flow field. The flow field around the wind turbine with a compact-type flanged-diffuser shroud is also investigated. The flow pattern behind the flange of the compact-type turbine is the same as the long-type one. It means that the effect of flow acceleration is caused by the unsteady vortices behind the flange. The comparison with CFD and PIV results of meridional time-average streamlines after the compact-type diffuser is also presented.

Experimental modeling of gravity underflow in a sinuous submerged channel

This paper describes the results of an experimental study involving density driven gravity underflow in a submarine sinuous channel. A single bend channel with sine generated planform was constructed inside a laboratory basin. Saline density flow was released at a constant rate at the upstream end of the channel which was submerged under fresh water. Instantaneous velocity data recorded within the body of the developed current were used to derive the vertical structures of the streamwise and cross‐stream velocity. The vertical distributions of the streamwise and lateral velocity were found to vary modestly between cross sections and also within an individual cross section. The distance of the maximum velocity position from the bed was found to vary between 0.4 and 0.6 times the local current thickness. In the vicinity of the bend apex the curvature effect led to the development of two helical flow cells with one forming on top of the other. The sense of rotation of the bottom cell was similar to that observed in curved open channel flow, while the upper circulation cell had an opposite sense of rotation. Turbulent kinetic energy profiles were derived from the instantaneous velocity data. The observed turbulent kinetic energy profiles confirm some previous observations that the location of the maximum velocity in a density underflow also corresponds to significant reduction in turbulent kinetic energy.

Hydrodynamics of the mixing chamber in RIM: PIV flow‐field characterization

AbstractThe flow‐field in a Reaction Injection Moulding, RIM, machine mixing chamber was characterized using the Particle Image Velocimetry (PIV) technique. This study covered the industrial range of Reynolds numbers, Re, for the RIM process, from 100 to 500, setting the overall trends of the flow‐field in that range, with particular focus on the flow regime transition range of 100 ≤ Re ≤ 150. The instantaneous 2D velocity vector maps were obtained with PIV in a plane containing the chamber and injectors axis. From instantaneous velocity data, the following quantities were computed: the probability density functions, the average velocity, and turbulence intensity. The present study sheds additional light into the transition of flow‐field regime that marks the onset of mixing on RIM, and also into the mixing mechanisms and respective underlying flow‐field features. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Mass Transfer in Bubble Column Reactors: Effect of Bubble Size Distribution

Mass transfer in a bubble column is analyzed in a different perspective. The experiments were performed in a bubble column reactor that was operated in a regime where the overall rate of absorption of gas into liquid was dependent on both the mass-transfer coefficient and the rate of reaction. Homogeneous catalytic oxidation of sodium sulfite was considered as a model reaction. The local instantaneous velocity data was measured using LDA in the presence of reaction and was used to yield the bubble size distribution at several locations in the column. The role of individual bubble size during mass transfer with chemical reaction was observed to vary. The findings suggest that a narrow bubble size distribution would help to achieve uniformity in the performance at bubble scale, so that an identical regime of mass transfer with reaction exists at the individual bubble level.

Flow instabilities, energy levels, and structure in stirred tanks

AbstractIn this study, the spatial distribution and significance of macro instabilities in two cylindrical tanks of different size, and one square tank, stirred with standard six bladed Rushton impellers, are investigated. The instabilities of the flow structure are investigated by searching for significant frequencies using the Lomb algorithm on instantaneous velocity data collected using laser Doppler anemometry at various locations in the tanks. The investigation of the energy levels shows that the harmonics of the blade passage differ in both magnitude and spatial distribution for different tank sizes, geometries, and energy input. Several significant dimensionless frequencies, that is, frequency Strouhal numbers, are detected and identified both in relative magnitude and spatial range. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Comprehensive experimental investigation of the hydrodynamics of large-scale, 3D, oscillating bubble plumes

An extensive study of the most important hydrodynamic characteristics of fairly large-scale bubble plumes was conducted using several measurement techniques and a variety of tools to analyze the data. Particle image velocimetry (PIV), double-tip optical probes (OP) and photographic techniques were extensively applied to measure bubble and liquid velocities, void-fraction and bubble sizes. PIV measurements in a vertical plane crossing the centre of the injector provided the instantaneous velocity fields for both phases, as well as hydrodynamic parameters, such as the movement of the axis of the plume and its instantaneous shape. Statistical studies were performed using image processing to determine the distribution of the apparent instantaneous plume diameter and centreline position. An important finding was that there is little instantaneous spreading of the bubble plume core; the spreading of the time-averaged plume width (as measured from the time-averaged void-fraction and time-averaged liquid velocity fields) is largely due to plume meandering and oscillations. The liquid-phase stress tensor distributions obtained from the instantaneous velocity data indicate that, for the continuous phase, these stresses scale linearly with the local void-fraction in the range of 0.5% < α < 2.5%. The bubbles were found to be ellipsoidal, with shape factor e ≈ 0.5.

Reynolds Stresses and Velocity Distributions in a Wave-Current Coexisting Environment

To investigate changes in the mean velocity profile owing to the interaction between waves and a current, the horizontal and vertical velocity components in the oscillating fluid were measured by a two-dimensional laser anemometer in a recirculating wave tank. The observed instantaneous velocity data concerning waves following and opposing a current were analyzed to obtain the Eulerian-mean velocity, the wave-current Reynolds stress, and the wave-current turbulent intensities. The wave-current Reynolds stress behaves differently when current direction changes. The general characteristic of the Eulerian-mean velocity is greatly affected by the wave-current Reynolds stress. The mean velocity for waves following (opposing) a current is reduced (increased) towards the free surface, when compared with the logarithmic profile. These velocity data were used to verify the results by velocity equations based on the phase-averaged Prandtl momentum-transfer theory. In addition, measurement of water-surface elevations revealed that the phase-averaged waveform for a pure wave is peaked near the crest but flatter near the trough and the variation of the water surface is well predicted by the third-order Stokes wave equation. The original pattern of the waveform varies due to the wave-current interaction, but the current direction only minimally affects the attenuation of the surface waves.

Evaluation of pressure field and fluid forces on a circular cylinder with and without rotational oscillation using velocity data from PIV measurement

An experimental method for evaluating pressure field and fluid forces on a bluff body structure is studied using instantaneous velocity data measured by particle image velocimetry (PIV). This method solves a pressure Poisson equation numerically with the experimental velocity data by PIV. The measurement of the velocity field around a circular cylinder is carried out using two CCD cameras placed side by side, which allows simultaneous measurement of velocity near and far field around the circular cylinder. The pressure and fluid forces on the stationary cylinder evaluated by this method agree closely with those in the literature, suggesting the validity of this technique. The present technique is also applied to a circular cylinder with rotational oscillation at Reynolds number 2000. It is found that the drag force on a circular cylinder is magnified at low-frequency oscillation and reduced at high-frequency oscillation. The drag coefficient at high-frequency oscillation is reduced by 30% with respect to the stationary cylinder, while the fluctuating lift is slightly increased due to the generation of synchronized vortex shedding at high-frequency oscillation.

Characteristic modes and evolution processes of shear-layer vortices in an elevated transverse jet

Characteristics and evolution processes of the traveling coherent flow structure in the shear layer of an elevated round jet in crossflow are studied experimentally in an open-loop wind tunnel. Streak pictures of the smoke flow patterns illuminated by the laser-light sheet in the median and horizontal planes are recorded with a high speed digital camera. Time histories of the instantaneous velocity of the vortical flows in the shear layer are digitized by a hot-wire anemometer through a high-speed data acquisition system. By analyzing the streak pictures of the smoke flow visualization, five characteristic flow structures, mixing-layer type vortices, backward-rolling vortices, forward-rolling vortices, swing-induced mushroom vortices, and jet-type vortices, are identified in the shear layer evolving from the up-wind edge of the jet exit. The behaviors and mechanisms of the vortical flow structure in the bent shear layer are prominently distinct in different flow regimes. The frequency characteristics, Strouhal number, power-spectrum density functions, autocorrelation coefficient, as well as the time and length scales of the coherent structure and the Lagrangian integral scales are obtained by processing the measured instantaneous velocity data. The Strouhal number is found to decay exponentially with the increase of the jet-to-crossflow momentum flux ratio. The autocorrelation coefficients provide the information for calculating the statistical time scales of the coherent structure and the integral time scales of turbulence fluctuations. The corresponding length scales of the vortical structure and the integral length scales of turbulence in the shear layer are therefore obtained and discussed.

Velocity fields in mixing-enhanced compressible shear layers

Planar velocity fields of mixing-enhanced compressible planar shear layers are measured via particle image velocimetry (PIV) in order to investigate the mechanism of mixing enhancement by sub-boundary-layer triangular disturbances. The measurements are conducted at convective Mach numbers, M c , of 0.62 and 0.24 to examine compressibility effects on effectiveness of the mixing enhancement technique. Instantaneous side- and plan-view vector maps of the shear layers are obtained, and turbulence statistical quantities are derived from the instantaneous velocity data. Schlieren and planar laser Mie scattering (PLMS) techniques are also used to measure the shear-layer thickness and growth rate as well as surveying the qualitative flow fields. The velocity fields for several disturbance configurations with different shape, size, or thickness are compared in terms of the shear-layer thickness and growth rate in order to investigate the effects of the configuration variation on the mixing enhancement strategy. Configuration parameters include thickness, the semi-vertex angle of the triangular disturbance, and the streamwise offset of the disturbance from the splitter tip

Wake-boundary layer interaction on the vaned diffuser of a centrifugal stage

An experimental investigation has been carried out by means of a two-component laser Doppler velocimeter (LDV) to survey the unsteady flow entering in the vaned diffuser of a centrifugal stage and to analyse into details the vane boundary layer development perturbed by the rotor flow. Instantaneous velocity data have been acquired in phase with the rotor blade passage and have been processed in ensemble-averaged form to separate periodic and random velocity fluctuations. Periodic incidence variations cause boundary layer transient separations near the vane leading edge, which alternate with opposition of phase on the two sides of the diffuser vane. Owing to the high turbulence energy conveyed by the wakes within the diffuser, which penetrates into the boundary layers, the periodic separation bubbles remain very short and the separated shear layers reattach early, giving rise to turbulent boundary layers capable of withstanding the adverse pressure gradients imposed by the diffuser.