Measurements Of Velocity Fluctuations Research Articles

Optical Coherence Tomography Velocimetry with Complex Fluids

We present recent results obtained with an Optical Coherence Tomography Velocimetry technique. An optical interferometer measures the velocity of a sheared fluid at specific depths of the sample using the coherence length of the light source. The technique allows the dynamics of 3 pico liter volumes to be probed inside opaque complex fluids. In a study of opaque starch suspensions, classical bulk rheology experiments show non-linear shear thickening, whereas observations of the velocity profiles as a function of distance across the gap show Newtonian behavior. The ability of the technique to measure velocity fluctuations is also discussed for the case of polyacrylamide samples which were observed to display shear banding behavior. A relationship between the viscoelasticity of the sample and the size of the apparent fluctuations is observed.

Corrections on LIFPA velocity measurements in microchannel with moderate velocity fluctuations

Laser-induced fluorescence photobleaching anemometer (LIFPA) has been developed in order to measure velocity fluctuations of the unsteady micro electrokinetic turbulent flows in microfluidics. The statistical errors of LIFPA measurement, because of 3-D flows and Taylor’s hypothesis (compared with local Taylor’s hypothesis Pinton and Labbe in J Phys II 4:1461–1468, 1994), are theoretically estimated and compared to hot-wire anemometer (HWA) measurement that is used for conventional turbulence measurement. The correction factor in the direction parallel to the laser beam is estimated, and the influence of directional correction factors of LIFPA is also investigated. It is found that in our investigation, the error due to Taylor’s hypothesis is negligible. The influence of 3-D flows on the first derivative variance of velocity fluctuations in LIFPA is smaller than that in HWA measurement.

Analysis of hot-wire measurements accuracy in turbulent boundary layer

AbstractThis paper discusses the issue of measuring velocity fluctuations of turbulent boundary layer using hot-wire probes. The study highlights the problem of spatial resolution, which is essential when measuring small-scales in wall-bounded flows. Additionally, attention was paid to the inconsistency in streamwise fluctuation measurements using single- and X-wire probes. To clarify this problem, the energy spectra using wavelet transformation were calculated. The analysis was performed for turbulent boundary layer flow, which was characterized by Reynolds number based on the friction velocity equal Re

S0510405 周期的擾乱を与えた水平二平板の後流特性および後流渦構造に関する研究

The purpose of this study is to understand the wake characteristic and wake vortex structures of two parallel flat plates with periodic disturbance. Separated shear layer which form wake without reverse-flow region is unstable and rolls up into the vortex with various structures. We have made two parallel flat plates in order to make the wake with wide width and without reverse-flow region. Wake characteristics and wake vortex structures in the wake of the parallel plates with periodic disturbance were investigated by measurement of velocity fluctuations and by the flow visualization, respectively. As the result, it was found that there are various significant difference in waveform of velocity fluctuations and spectra, and the wake characteristics have various phenomena. Flow patterns in the vortex roll-up process on various disturbance frequency were showed.

Wake-induced vibration of tandem cylinders of different diameters

The wake-induced vibration (WIV) of the downstream cylinder of a tandem pair is investigated for different diameter ratios of D1/D2=1/1, 1/2 and 1/3, where D1 and D2 refer to the upstream and downstream cylinders, respectively. The streamwise separation between the cylinders was L/D1=3.5, 7.0 and 6.5, respectively, measured from the centre of the upstream cylinder to the forward stagnation point of the downstream cylinder. Experiments with low mass-damping cylinders have been conducted in a water channel at around Re=25000. The dynamic response showed that the downstream cylinder experienced WIV for all diameter ratios investigated, with displacement amplitudes reaching more than 1.5 diameters for higher reduced velocities beyond the vortex resonance range. The frequency response showed a similar behaviour for all three configurations, giving hints that a type of wake-stiffness mechanism might be governing the frequency of oscillation for all diameter ratios. The response was found to be dependent on both D1/D2 and L/D1. In all cases, the static upstream cylinder was found to shed vortices as an isolated cylinder, not influenced by the presence or movement of the downstream body. Lift and drag coefficients as well as measurements of velocity fluctuations in both wakes are presented for all cases.

Fabrication and Characterization of a Novel Nanoscale Thermal Anemometry Probe

The development, fabrication, and characterization of a novel nanoscale thermal anemometry probe (NSTAP) for measuring velocity fluctuations in turbulent flows are described. This miniature MEMS anemometer consists of a freestanding 30 or 60 × 1 × 0.1 μm platinum filament with electrically conductive pads and a silicon structure. A novel deep reactive ion etching lag-based process for fabricating a 3D silicon support structure is described together with other microfabrication steps. The sensors behave similarly to conventional hotwire anemometers, with an order of magnitude better spatial and temporal resolution. Batch fabrication allows a relatively low-cost high-yield process, which together with its superior frequency response and size, make it an attractive thermal anemometry sensor for velocity measurements in turbulent flows.

1103 ターボ形遠心送風機の乱流騒音源の探索と騒音発生メカニズムに関する研究

Centrifugal Turbo-Blower is able to produce high pressure, but operation sound of it is bigger than that of other blowers. The mechanism of the noise generation has to be elucidated in order to decrease that noise. In this study, we focus on lap area (gap between inlet pipe and impeller) which affects blower noise, and investigate the noise at lap area by measurement of noise and velocity fluctuation and numerical analysis. As a result, we found that noise have relation to a re-entrant flow, and the interaction between the re-entrant flow and inlet flow increases velocity fluctuation which causes the noise at lap.

Non-contact Measurement Techniques for Structural Health Monitoring

The application of 3D scanning laser vibrometry for visualising stress wave propagation in a specimen with a non-surface penetrating defect is presented. The results support the need for a good understanding of the 3D velocity fluctuation measurements to provide a good insight of the propagation of the stress wave over a given region. It will be shown that a full knowledge of the velocity field will facilitate an understanding on the interaction of the various wave modes with the defect in the structure. For the purpose of this paper, a flat aluminium test plate with non-surface penetrating defect will be used to illustrate the application of this form of non-contact measurement technique for this stress wave visualisation work. It will be shown that the modal content and the time-development of the incident and the scattered wave modes can be evaluated with this type of measurement technique.

Effect of Mesh Grids on the Turbulent Mixing Layer of an Axisymmetric Jet

This article focuses on the effect that two different mesh grids have on the structure of the mixing layer of an axisymmetric jet. Detailed measurements of mean velocity and turbulent velocity fluctuations are made with an X hot-wire probe in the range 0.5 ≤ x/d ≤ 10, where x is the longitudinal distance from the nozzle exit plane and d is the nozzle diameter. The grids are introduced at two locations—one location just downstream of the nozzle exit plane and the other location upstream of the nozzle exit plane in order to perturb the nozzle exit boundary layer. One mesh completely covers the nozzle (full mesh or FM) and the other mesh covers the central, high-speed zone (disk mesh or DM). With reference to the undisturbed jet, FM yields a significant reduction in the turbulence intensity and width of the shear layer, whereas DM enhances the turbulence intensity and increases the width of the shear layer. Both grids suppress the formation of the Kelvin–Helmholtz instability in the mixing layer. Results are presented, mainly at x/d = 5 and 6 in both the spectral domain and physical space. In the latter context, second- and third-order structure functions associated with u (the longitudinal velocity fluctuation) and v (the lateral or radial velocity fluctuation) are presented only for the flow perturbed by placing the mesh outside the nozzle. All mesh geometries have a more significant effect on the second-order structure function of u than on that of v. The third-order energy transfer term is affected in such a way that, relative to the undisturbed jet, its peak location is shifted to a smaller scale when FM is used and to a larger scale with DM. This is consistent with our observations that FM reduces the turbulence in the shear layer while DM enhances it. It is suggested that the large-scale vortices that are formed at the edge of the grids play a significant role in the transfer of energy.

Simulation of plume dispersion from single release in Fusion Field Trial-07 experiment

Accurate description of source–receptor relationship is required for an efficient source reconstruction. This is examined by simulating the dispersion of plumes resulted from the available ten trials of single releases conducted at Fusion Field Trials, Dugway Proving Ground, Utah. The simulation is addressed with an earlier developed IIT (Indian Institute of Technology) dispersion model using the dispersion parameters in terms of measurements of turbulent velocity fluctuations. Simulation is described separately in both stable and unstable conditions, characterizing the peak as well as overall observed concentration distribution. Simulated results are compared with those obtained using AERMOD.With IIT model, peak concentrations are predicted within a factor of two in all the trials. The higher concentrations (>5 × 10−4 g m−3) are well predicted in stable condition and under-predicted (within a factor of two) in unstable condition whereas relatively smaller concentrations (<5 × 10−4 g m−3) are severely under-predicted in stable conditions and over-predicted in unstable conditions.The AERMOD exhibits the similar prediction of concentrations as shown by IIT model in most of the trials. Overall, both the models predict 70–80% concentrations in stable conditions and 85–95% concentrations in unstable conditions within a factor of six. The statistical measures for both the models are found well in agreement with the observations.

Variable-viscosity mixing in the very near field of a round jet

One of the persistent challenges in turbulent mixing is understanding of the phenomenology associated with heterogeneous mixtures of gases. The turbulence in single, homogeneous fluid has drawn much attention. However, heterogeneous mixing of gases is prevalent in real flows, whether nonreactive or reactive. This paper is dedicated to advancing our understanding of the turbulent mixing phenomenology in a round jet flow of propane issuing in an oxidizer. The ratio of the propane/oxidizer kinematic viscosities is 1/5.5, whereas their densities are nearly equal. Therefore, the main physical property that differentiates the two fluids is their viscosity. The focus of this paper is on the role played by the viscosity gradients on the turbulent flow, over the first several diameters. The addressed questions concern a comparison between the present flow, which is a variable-viscosity flow (hereafter, VVF), and an air jet flow issuing in the air (constant-viscosity flow, hereafter CVF). The comparison is made at the same initial conditions, i.e. the same jet momentum per surface unit, M0 = ρ0U20 = 130 and 360 kg m−1 s−2 (U0 is the injection velocity and ρ0 is the fluid density). For doing so, an experiment has been designed and both the velocity and the scalar fields have been measured by using a technique based on hot-wire anemometry and Rayleigh light scattering, as well as laser Doppler velocimetry which provides two velocity components. The comparison criteria between the VVF and CVF are the instantaneous aspect of the turbulent velocity field, the axial mean velocity () and the rms (root mean squares) of the two measured velocity fluctuations. The one-point kinetic energy budget, which involves the mean energy dissipation rate, is developed and the expression of the mean energy dissipation rate is revisited. The latter involves additional terms reflecting the velocity–viscosity gradient correlation. This contribution is positive, thus enhancing the real mean energy dissipation rate in VVF. We show that, in comparison with the CVF, the VVF evolves much faster toward a self-similar regime, the decay of the axial mean velocity is enhanced and the mean energy dissipation rate is more important. We discuss, finally, a possible mechanism for the accelerated turbulence decay observed in VVF. Firstly, an external viscous fluid is brought inwards of the jet core by the jet frontier instabilities (nozzle-wake instabilities, Kelvin–Helmholtz instabilities and instabilities associated with the variable-viscosity interface). Secondly, these viscous-fluid blobs lead to enhanced local velocity gradients, to greater local dissipation and to increased production of lateral velocity fluctuations behind these irregular obstacles. While this scenario is speculative, it is likely to explain the observed statistics of the velocity field.

Causality correlation analysis on a cold jet by means of simultaneous particle image velocimetry and microphone measurements

The aeroacoustic sound generation processes of a cold jet were investigated by means of simultaneous Particle-Image-Velocimetry (PIV) in the near-field and microphone measurements in the far field. The measurements were conducted in a synchronized manner so as to enable the calculation of the cross correlation coefficient between the acoustic pressure and flow quantities derived from the measured velocity fluctuations. In this manner the regularities in the near-field fluctuations which are related to the radiated sound field could be identified. The tests were run for different Mach numbers M=0.5, 0.7 and 0.9 in air. Five microphones arranged at angles of 24°<θ<88° to the jet centre line were used. The correlation with the axial velocity was highest. The spatial as well as the temporal distribution of the coefficient was observed to be dominated by coherent structures and showed a strong dependency on M and θ. For the M=0.9 jet the correlation coefficients fell below the confidence interval for θ=67°.

Field Measurements of Wind Turbine Wakes with Lidars

AbstractField measurements of the wake flow produced from a 2-MW Enercon E-70 wind turbine were performed using three scanning Doppler wind lidars. A GPS-based technique was used to determine the position of the wind turbine and the wind lidar locations, as well as the direction of the laser beams. The lidars used in this study are characterized by a high spatial resolution of 18 m, which allows the detailed characterization of the wind turbine wake. Two-dimensional measurements of wind speed were carried out by scanning a single lidar over the vertical symmetry plane of the wake. The mean axial velocity field was then retrieved by averaging 2D scans performed consecutively. To investigate wake turbulence, single lidar measurements were performed by staring the laser beam at fixed directions and using the maximum sampling frequency. From these tests, peaks in the velocity variance are detected within the wake in correspondence of the turbine top tip height; this enhanced turbulence could represent a source of dangerous fatigue loads for downstream turbines. The spectral density of the measured velocity fluctuations shows a clear inertial-range scaling behavior. Then, simultaneous measurements with two lidars were performed in order to characterize both the axial and the vertical velocity components. For this setup, the two velocity components were retrieved only for measurement points for which the two laser beams crossed nearly at a right angle. Statistics were computed over the sample set for both velocity components, and they showed strong flow fluctuations in the near-wake region at turbine top tip height, with a turbulence intensity of about 30%.

High-Reynolds-Number Flow past a Pipe

The present aim is to reveal the flow past a pipe which is immersed parallel to the mainstream at high Reynolds numbers. In a wind tunnel, we carry out (1) base-pressure measurements, (2) velocity-fluctuation measurements using a hot-wire anemometer and (3) flow visualisations by a smoke-wire method with PIV analyses, where we take consecutive picutures using a high-speed camcorder to obtain quantitative flow-field information such as velocity vector and vorticity. The tested parameter ranges are as follows: Re = 2.0×103 - 1.3×104, d/t = 4.0 - 10.0 and l/t = 1.0 - 10.0, where Re, d, t and l are the Reynolds number, mean diameter, thickness and length of the pipe, respectively. As a result, the Re effects are negligible. The base-suction coefficient -Cpb monotonically decreases with decreasing d/t, or with increasing l/t. We propose a unified formula to predict -Cpb, which are consistent with both a two-dimensional prism and a rod for l/t < 4.0 in addition to a ring. In contrast, the Strouhal number St almost coincides with that for a two-dimensional prism at any l/t, if we can detected any dominant frequencies. In addition, we conduct flow visualisations, and reveal the effects upon axisymmetry of wake. Finally, we classify the flow into three modes based on both periodicity and axisymmetry. Such a modal classification reveals that the enhancement of flow's irregularity corresponds to the decrease of -Cpb.

Features of Internal Waves in a Shoaling Thermocline

Observations and numeric modeling of internal wave generation and transformation in the shelf zone of sea show that the main part of tidal energy is transported to shores in form of internal gravitational waves. Long-term measurements of temperature and current velocity fluctuations at many levels in the near-bottom thermocline were carried out during the periods when stable seasonal thermocline was present. Analysis of the measurements permits us to understand mechanisms of internal wave destruction with turbulent motion generation and corresponding rebuilding of velocity and density mean fields in the stratified near-bottom layer. Spectral analysis of temperature fluctuations shows that in shoaling internal waves the low-frequency maxima disappear, maxima at higher frequencies appear, and the spectra slope in the high frequency range changes with depth. Taking into account the concurrent analysis of near-bottom pressure fluctuations and current velocity fluctuations from surface till bottom we come to the conclusion that breaking internal waves in a near-bottom thermocline generate not only small-scale three-dimensional turbulence, but also quasi-horizontal turbulence of larger scales, which considerably contributes into mixing and sediments, alluvium, and nutrients transport in the shelf zone of sea.

1108 円形ノズル端に取り付けたパンチングメタルから発生する空力音(OS11-2 流体関連振動・騒音,OS11 流体関連振動・騒音)

This paper deals with the aerodynamic sound generated by the perforated metal. Perforated metal is produced by stamp out the sheet metal using a press die. In this study, the perforated metal is attached at the tip of circular nozzle and it experiments by wind velocity 50 m/s. Aerodynamic sound measurement experiments show that the characteristics of the generated aerodynamic sound depend on the perforated metal thickness, hole diameter and its attachment orientation. Moreover, the measurement of velocity fluctuation of air flow flowing out from perforated metal is carried out. The result shows the perforated metal is effectivity for reduce the low frequency noise. The distribution of band overall of velocity fluctuation indicates the mechanism of reducing the low frequency noise generate from the jet.

Coupled dynamics of the co‐evolution of gravel bed topography, flow turbulence and sediment transport in an experimental channel

A series of flume experiments were conducted in a large experimental channel at the St. Anthony Falls Laboratory to understand the coupled dynamics of flow and bed forms above the sediment‐water interface. Simultaneous high resolution measurements of velocity fluctuations, bed elevations and sediment flux at the downstream end of the channel, were made for a range of discharges. The probability density functions (pdfs) of bed elevation increments and instantaneous Reynolds stress reveal a power law tail behavior and a wavelet cross‐correlation analysis depicts a strong dependence of these series across a range of scales, indicating a feedback between bed form dynamics and near‐bed turbulence. These results complement our previous findings in which the signature of bed form evolution on the near‐bed velocity fluctuations was confirmed via the presence of a spectral gap and two distinct power law scaling regimes in the spectral density of velocity fluctuations. We report herein a strong asymmetry in the probability distribution of bed elevation increments and instantaneous Reynolds stresses, the latter being further analyzed and interpreted via a quadrant analysis of velocity fluctuations in the longitudinal and vertical directions. We also report the presence of intermittency (multifractality) in bed elevation increments and interpret it, in view of the asymmetric nature of the pdfs, as the result of scale coupling. In other words, the geometric asymmetry at the bed form scale gets transferred down to a probabilistic asymmetry at all smaller scales indicating a local anisotropy in the energy transfer. Finally, we propose a predictive relationship between bed form averaged sediment transport rates and bed form averaged instantaneous Reynolds stress and validate it using our experimental data.

Digital particle flow visualization methods as applied to characterization of turbulent boundary layer

In the present paper, we discuss various approaches to measurement of velocity fields and velocity fluctuations in incompressible turbulent boundary layer by digital particle flow visualization methods. In particular, performing measurements in the vicinity of streamlined surface is considered. A description of the optical measuring system and methods to seed the flow with tracer particles is presented. Measured data for flow velocity fields and turbulent velocity pulsations in boundary layer on a flat impermeable plate are analyzed. Comparative data illustrating the efficiency of various algorithms for treating PTV images are presented.

Wall-pressure fluctuations of separated and reattaching flow over blunt plate with chord-to-thickness ratio c/d = 9.0

The separated and reattaching turbulent flow over a finite blunt plate with the chord-to-thickness ratio c/d=9.0 was experimentally studied; the Reynolds number based on the plate’s thickness (d) was Red=1.58×104. Wall-pressure fluctuations on the plate surface were taken into extensive consideration, which are closely related to unsteady flow behaviors buried in the separated and reattaching flow and the unsteady wake. Toward this end, synchronized measurements of wall-pressure fluctuations and velocity fluctuations were performed by using a microphone array and a split-fiber film probe, respectively. Characteristics of the separated and reattaching flow were discussed in terms of time-averaged streamwise velocity and its fluctuation intensity, reverse-flow intermittency, wall-pressure fluctuation coefficient, cross-correlation and coherence of wall pressure and velocity. The results showed that the peaked wall-pressure fluctuation coefficient (x/d=4.25) appears 1d behind the time-mean reattachment point (x/d=3.25), which is due to the intensified impingement fluid induced by the growing large-scale vortical structures in the reattachment zone. Two characteristic frequencies fd/U0=0.118 and 0.162 were determined, which correspond to the shedding large-scale vortical structures and the unsteady wake, respectively. Cross-correlation of the wall-pressure field demonstrated rapid decay of the large-scale vortical structures beyond the reattachment zone (x/d>4.75), which might be due to strong interference with the unsteady wake at higher frequency fd/U0=0.162. The wall-pressure auto-spectra showed that wall-pressure fluctuations near the trailing edge are overwhelmingly dominated by the unsteady wake at fd/U0=0.162. Coherence of the wall-pressure field and the streamwise velocity at x/d⩾7.75 elucidated that unsteady behaviors of the redeveloping boundary layer at both frequencies fd/U0=0.118 and 0.162 play important role in wall-pressure fluctuations in large upstream area of the surface.

G405 MTVによる超音速マイクロ噴流の速度変動測定法に関する研究(GS4-1 計測技術,一般セッション)

G405 MTVによる超音速マイクロ噴流の速度変動測定法に関する研究(GS4-1 計測技術,一般セッション)