Measurements Of Velocity Fluctuations Research Articles

Unsteady vortex structures in the wake of nonaxisymmetric bumps using spiral MRV

The temporal characteristics of the wake structure behind a nonaxisymmetric bump are studied using spiral magnetic resonance velocimetry (MRV) to measure time-series of all three velocity components in y–z-planes for bump orientations of $$5^\circ$$ , $$10^\circ$$ , $$15^\circ$$ , and $$20^\circ$$ relative to the freestream flow. The mean wake structure was shown previously to be highly sensitive to bump orientation (Ching et al. Exp Fluids, 2018). The spiral MRV method is validated by comparison to the established MRV techniques in a flow with forced periodicity. Measured velocity fluctuations in the bump wake decrease significantly as the bump angle is increased. A quasi-periodic shedding cycle is reconstructed using spectral proper orthogonal decomposition. The reconstructed cycles of each velocity component are matched through a divergence minimization method to examine the structure of the shed vortices. The vortex shed from the trailing edge side of the bump weakens as the bump angle is increased, whereas the vortex shed from the leading edge side maintains its strength. The alternating vortices move across the centerline as they are advected downstream, explaining the common-down vortex pair observed in the far wake mean fields.

Observation of geodesic acoustic mode in EAST using Doppler backscattering system

This paper presents an investigation of the geodesic acoustic mode (GAM) using two poloidally separated Doppler backscattering systems in Experimental Advanced Superconducting Tokamak. Each system allows for the simultaneous measurement of turbulence rotation velocity and density fluctuations with high precision. With the Doppler backscattering systems, the GAM frequency, the symmetric feature of poloidal flow fluctuations, and the interaction between GAM and turbulence are surveyed. The results of bispectral analysis show a clear interaction of GAM with the ambient turbulence. A clear measurement of GAM in the envelope of plasma density fluctuations using Doppler backscatter system is shown, and the modulations of the turbulence perpendicular velocity on the density turbulence are correlated at two different poloidal position.

Effect of inclined transverse jets on trailing-edge noise generation

In this work, the effect of multiple transverse jets on the turbulent boundary layer developing over a flat plate is experimentally investigated for aeroacoustic purposes. A single line of jet nozzles with different spanwise spacings is located parallel to the trailing-edge of the plate, at approximately 30 jet diameters upstream of the trailing-edge. The axes of the jet nozzles have an inclination of 15° with respect to the streamwise direction. Two values of the jet velocity ratio (r = ujet/u∞) are considered, r = 1 and r = 2. The simultaneous measurement of streamwise velocity and surface pressure fluctuations is performed with hot-wire anemometry and flush-mounted microphones, respectively. The mean velocity profiles show that the low inclination angle of the multiple jets prevents the formation of adverse pressure gradients, and therefore, the multiple jets injection does not lead to flow separation, at least at the range of downstream locations under investigation. From the velocity measurements, the jets merge downstream of the jet nozzles and form a layer of jet fluid characterized by a low energy content. The estimates of the far-field noise show that jets injection at a velocity ratio of r = 1 leads to noise attenuation over the whole range of frequencies under analysis. At a velocity ratio of r = 2, jets injection enables to gain a larger noise reduction than at r = 1 at low frequencies, but the estimated far-field noise is expected to increase at high frequencies.

Measurements of surface velocity and level fluctuation in an actual continuous wide slab casting mold

In the current work, a kind of nailboard made of three rows of nails was designed to simultaneously measure the surface velocities and level fluctuations of the loose side, the center plane and the fixed side in an actual continuous wide slab casting mold. Seven nailboard dipping experiments were conducted to obtain both the transient and time-averaged behaviors near the meniscus. The distribution of disturbed oscillation marks (OMs) on the surface of corresponding slabs (in common with nailboard experiments strand) was systematically studied. In order to investigate the relationship between disturbed OMs and entrapment of inclusions or slag to the solidified shell, samples were taken at 10 locations on one relevant slab, and they were polished 2 mm each time. Inclusions larger than 10 µm were analyzed using the optical microscope. The level fluctuates with the continuous casting processing. The level positions on the fixed side and loose side may not be consistent. Surface level near the submerged entry nozzle (SEN) and the narrow face fluctuates on a higher position, while on the region between the SEN and the narrow face, the level fluctuates on a lower position. Time-history of the level fluctuation on the loose side and the centerline is synchronous, while the flow fluctuates in another pace on the fixed side. Root-mean-square (RMS) value of level fluctuation varies along the mold width. There are peak values at the locations near the SEN and narrow face, where slag entrapment may be induced. The OMs are more likely to be disturbed at the middle and 1/4 location in mold width. There is a big difference in entrapment of inclusions at the solidified shell between loose side and fixed side. On the fixed side, the distribution of number density of inclusions is in agreement with number distribution of disturbed OMs on meniscus, while more inclusions were entrapped at narrow faces on the loose side.

Cold wire constant voltage anemometry to measure temperature fluctuations and its application in a thermoacoustic system.

The knowledge of temperature fluctuations is essential for most thermoacoustic systems. In the present paper, cold wire constant-voltage anemometry (CVA) to measure temperature fluctuations is presented. Corrections for the thermal inertia and for the end losses of the wire are applied during the post-processing. The correction for the thermal inertia of the cold wire is achieved by applying a time dependent thermal lag as proposed originally for a constant-current anemometry (CCA) system. This thermal lag is measured in parallel by a hot wire. The thermal end losses of the wires to their supports are also considered and approximate corrections are proposed. The procedure for the cold wire CVA is validated in the acoustic field of an acoustic resonator with wires of different lengths. A comparison between a CVA and a CCA measurement also confirms the CVA measurement. Furthermore, the proposed measurement procedure is applied close to the stack of a thermoacoustic refrigerator. Supposing a two-dimensional flow, the simultaneous measurement of velocity and temperature fluctuations is possible. This allows a detailed examination of the acoustic field close to the stack, including the study of the correlation between temperature and velocity.

Measurements of scalar released from point sources in a turbulent boundary layer

Measurements of velocity and concentration fluctuations for a horizontal plume released at several wall-normal locations in a turbulent boundary layer (TBL) are discussed in this paper. The primary objective of this study is to establish a systematic procedure to acquire accurate single-point concentration measurements for a substantially long time so as to obtain converged statistics of long tails of probability density functions of concentration. Details of the calibration procedure implemented for long measurements are presented, which include sensor drift compensation to eliminate the increase in average background concentration with time. While most previous studies reported measurements where the source height is limited to, , where sz is the wall-normal source height and δ is the boundary layer thickness, here results of concentration fluctuations when the plume is released in the outer layer are emphasised. Results of mean and root-mean-square (r.m.s.) profiles of concentration for elevated sources agree with the well-accepted reflected Gaussian model (Fackrell and Robins 1982 J. Fluid. Mech. 117). However, there is clear deviation from the reflected Gaussian model for source in the intermittent region of TBL particularly at locations higher than the source itself. Further, we find that the plume half-widths are different for the mean and r.m.s. concentration profiles. Long sampling times enabled us to calculate converged probability density functions at high concentrations and these are found to exhibit exponential distribution.

High sampling-rate measurement of turbulence velocity fluctuations in Mach 1.8 Laval jet using interferometric Rayleigh scattering**Project supported by the National Natural Science Foundation of China (Grant No. 11272337).

Interferometric Rayleigh scattering diagnostic technique for the time-resolved measurement of flow velocity is studied. Theoretically, this systematic velocity-measured accuracy can reach up to 1.23 m/s. Measurement accuracy is then evaluated by comparing with hot wire anemometry results. Moreover, the distributions of velocity and turbulence intensity in a supersonic free jet from a Laval nozzle with a Mach number of 1.8 are also obtained quantitatively. The sampling rate in this measurement is determined to be approximately 10 kHz.

High resolution experiments of velocity and concentration fluctuations in a jet flow

In this paper we present a novel experimental technique aimed at the simultaneous measurement of velocity and scalar fluctuations with high spatial and time resolution. Measurements have been performed using wire-mesh sensors in combination with Stereo Particle Image Velocimetry (SPIV) to achieve synchronized measurements of both velocity and scalar fluctuations, and therefore of the turbulent fluxes uj′φ′‾. The proof-of-concept of this measurement technique was obtained on a jet flow experimental facility HiRJet (High Resolution Jet) developed and built in the Experimental and Computational Multiphase Flow (ECMF) laboratory at the University of Michigan. Acquired experimental data indicates a high temporal resolution of proposed technique and capabilities of achieving a high level of synchronization which is confirmed by cross-correlation between wire-mesh and SPIV signals.

Acoustic measurements of vertical velocity and temperature fluctuations of a hydrothermal plume with comparisons to large eddy simulations

The acoustic scintillation method is used to study the hydrothermal plume of Dante within the Main Endeavour vent field (MEF) at the Endeavour segment of the Juan de Fuca Ridge. Forty days of vertical velocity and temperature fluctuations were obtained from the rising plume above the Dante edifice in an environment where the flow is dominated by strong (5cm/s) semi-diurnal tidal currents and a northerly mean residual current (3cm/s). These acoustic measurements provide a window on deep-sea hydrothermal plume dynamics in strong oscillatory cross flows. A large eddy simulation, parameterized with anisotropic mixing coefficients, taking into account ambient stratification and time-dependent background flows and calibrated by the acoustic measurements, yields insight into turbulent processes, entrainment, plume bending, rise height, and, inferentially, mound heat flux. The turbulent dissipation rates for kinetic energy (ε) and thermal variance (εθ) is approximated by computing the Reynolds averaged sub-grid scale turbulent production from shear and buoyancy (ε = P-B) and from temperature fluxes (εθ = Pθ), respectively, which are needed to compare to the acoustic derived turbulence levels. A new cabled observatory reciprocal acoustic scintillation system on the NEPTUNE observatory that will allow real time measurements of mean and turbulent flow of a hydrothermal plume will also be introduced.

Incipient motion of coarse uniform gravel

ABSTRACTThe influence of several parameters on the incipient motion of gravels has been carefully examined from theoretical and experimental standpoints. The effect of the longitudinal bed slope is significant and well described with existing methods. This analysis shows that the effects of the grain Reynolds number and relative submergence are negligible for turbulent flows over hydraulically rough boundaries. Laboratory experiments for homogeneous gravels on a plane bed provided detailed measurements of both mean flow velocity and velocity fluctuations for the analysis of the lift and drag coefficients. This analysis of the lift and drag coefficients enables a better evaluation of the Shields parameter for incipient motion as a function of the angle of repose . The theoretical developments presented in this paper are corroborated by the results of our laboratory experiments as well as many others available in the literature.

ESCOAMENTO NA ESTEIRA DE TURBINAS EÓLICAS: ANÁLISE ESPECTRAL DA TURBULÊNCIA MEDIANTE TESTES EM TÚNEL DE VENTO

The interaction between the incident wind and wind turbines in a wind farm causes mean velocity deficit and increased levels of turbulence in the wake. The turbulent flow is characterized by the superposition of the wind turbine wakes. In this work, the technique of turbulence spectral evaluation to reduce scale models in a boundary layer wind tunnel is presented, and different measurements of velocity fluctuations are analyzed. The results allow evaluating the spectrum configuration at different frequency ranges and the differences of the spectral behavior between the incident wind and the turbine wake flow.

Experimental quantification of turbulence and its applications in the study of multiphase flotation pulps

Turbulence is common in daily activities, both naturally and industrially. However, it is not well understood on the microscopic level because there is a lack of suitable quantification techniques especially for multi-phase particulate systems such as flotation pulps. In this paper, the different quantification techniques which have been used to characterise turbulence in the literature have been reviewed in terms of their basic principles, system structure, the range of application and limitations. Optical techniques such as Laser Doppler Anemometry and Particle Image Velocimetry can produce accurate measurement results with high spatial and temporal resolution, but their applications are limited to transparent flows. Hot wire probes have high spatial and temporal resolution in turbulence quantification, but they are susceptible to environmental factors. Conductivity probe, Electrical Resistance Tomography (ERT) and piezoelectric sensor are techniques that can quantify turbulence in multiphase flows, but their spatial resolutions are limited. Particle Tracking Velocimetry can reveal particle trajectories in multiphase flows; it can also measure velocity fluctuation. Positron Emission Particle Tracking cannot be used to quantify turbulence although it can also be used to determine the Lagrangian trajectory of particles. For turbulence quantification in three phase flows, piezoelectric sensor, ERT and conductivity probe require further development.

Velocity fluctuations and population distribution in clusters of settling particles at low Reynolds number

A study on the spatial organization and velocity fluctuations of non-Brownian spherical particles settling at low Reynolds number in a vertical Hele-Shaw cell is reported. The particle volume fraction ranged from 0.005 to 0.05, while the distance between cell plates ranged from 5 to 15 times the particle radius. Particle tracking revealed that particles were not uniformly distributed in space but assembled in transient settling clusters. The population distribution of these clusters followed an exponential law. The measured velocity fluctuations are in agreement with that predicted theoretically for spherical clusters, from the balance between the apparent weight and the drag force. This result suggests that particle clustering, more than a spatial distribution of particles derived from random and independent events, is at the origin of the velocity fluctuations.

Impact of Compressor Surge on Performance and Emissions of a Marine Diesel Engine

The operation during compressor surge of a medium speed marine diesel engine was examined on a test bed. The compressor of the engine's turbocharger was forced to operate beyond the surge line, by injecting compressed air at the engine intake manifold, downstream of the compressor during steady-state engine operation. While the compressor was surging, detailed measurements of turbocharger and engine performance parameters were conducted. The measurements included the use of constant temperature anemometry for the accurate measurement of air velocity fluctuations at the compressor inlet during the surge cycles. Measurements also covered engine performance parameters such as in-cylinder pressure and the impact of compressor surge on the composition of the exhaust gas emitted from the engine. The measurements describe in detail the response of a marine diesel engine to variations caused by compressor surge. The results show that both turbocharger and engine performance are affected by compressor surge and fast Fourier transform (FFT) analysis proved that they oscillate at the same main frequency. Also, prolonged steady-state operation of the engine with this form of compressor surge led to a non-negligible increase of NOx emissions.

Krypton tagging velocimetry in a turbulent Mach 2.7 boundary layer

The krypton tagging velocimetry (KTV) technique is applied to the turbulent boundary layer on the wall of the “Mach 3 Calibration Tunnel” at Arnold Engineering Development Complex (AEDC) White Oak. Profiles of velocity were measured with KTV and Pitot-pressure probes in the Mach 2.7 turbulent boundary layer comprised of 99 % $$\hbox {N}_{2}$$ /1 % Kr at momentum-thickness Reynolds numbers of $${Re}_{\varTheta }= 800, 1400$$ , and 2400. Agreement between the KTV- and Pitot-derived velocity profiles is excellent. The KTV and Pitot velocity data follow the law of the wall in the logarithmic region with application of the Van Driest I transformation. The velocity data are analyzed in the outer region of the boundary layer with the law of the wake and a velocity-defect law. KTV-derived streamwise velocity fluctuation measurements are reported and are consistent with data from the literature. To enable near-wall measurement with KTV ( $$y/\delta \approx $$ 0.1–0.2), an 800-nm longpass filter was used to block the 760.2-nm read-laser pulse. With the longpass filter, the 819.0-nm emission from the re-excited Kr can be imaged to track the displacement of the metastable tracer without imaging the reflection and scatter from the read-laser off of solid surfaces. To operate the Mach 3 AEDC Calibration Tunnel at several discrete unit Reynolds numbers, a modification was required and is described herein.

Measurement of velocity fluctuations in microfluidics with simultaneously ultrahigh spatial and temporal resolution

Although unsteady and electrokinetic flows are widely used in microfluidics, there is unfortunately no velocimeter today that can measure the random velocity fluctuation at high temporal and spatial resolution simultaneously in microfluidics. Here we, for the first time, theoretically study the temporal resolution of laser induced fluorescence photobleaching anemometer (LIFPA) and experimentally verify that LIFPA can have simultaneously ultrahigh temporal \(({\sim } 4\,\upmu \hbox {s})\) and spatial \(({\sim }203\,\hbox {nm})\) resolution and can measure velocity fluctuation up to at least 2 kHz, whose corresponding wave number is about \(6\times 10^6\,{/}\hbox {m}\) in an electrokinetically forced unsteady flow in microfluidics.

Turbulence Locality and Granularlike Fluid Shear Viscosity in Collisional Suspensions.

We reanalyze previous experimental measurements of solid volume fraction, mean velocity, and velocity fluctuations in collisional suspensions of plastic cylinders and water flowing over inclined, erodible beds. We show that the particle pressure scales with the granular temperature, as predicted by kinetic theory of granular gases. The assumption that the particle shear stress is also well predicted by kinetic theory permits us to determine the fluid shear stress and the effective fluid viscosity from the experiments. The fluid viscosity can be decomposed into turbulent and granularlike components: the turbulent viscosity can be modeled using a mixing length, which is a decreasing function of the local volume fraction and does not depend upon the distance from the bed; the granularlike viscosity, associated with the transfer of momentum due to the conjugate motion of the fluid mass added to the particles, can be modeled by replacing the particle density with the density of the added fluid mass in the viscosity of kinetic theory.

Simulation of plume dispersion of multiple releases in Fusion Field Trial-07 experiment

Abstract For an efficient source term estimation, it is important to use an accurate dispersion model with appropriate dispersion parameters. This is examined by simulating the dispersion of plumes resulted from the available multiple releases conducted at Fusion Field Trials, Dugway Proving Ground, Utah. The simulation is carried out 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 discussed separately in both stable and unstable conditions in light of (i) plume behavior of observed and predicted concentrations in the form of isopleths, (ii) peak/maximum concentrations and (iii) overall concentration distribution. Simulated results from IIT model are compared with those obtained using AERMOD. Both, IIT model and AERMOD, predicted peak concentrations within a factor of two in all the releases and tracer transport is mostly along the mean wind direction. With IIT model, the higher concentrations are predicted close to observations in all the trials of stable conditions and with in a factor of two in the trials of unstable conditions. However, the relatively smaller concentrations are under-predicted severely in stable conditions and over-predicted in unstable conditions. The AERMOD exhibits the similar prediction of concentrations as in IIT model except slightly over-prediction in stable conditions and under-prediction in unstable conditions. The statistical measures for both the models are found good in agreement with the observations and a quantitative analysis based on F-test shows that the performance from both the models are found to be similar at 5% significance level.

Turbulence Statistics for Jet Noise Source Modeling from Filtered PIV Measurements

Time-resolved, particle image velocimetry was used to measure the velocity fluctuation field at three locations along the lipline of two subsonic jets. The time history data within these fields were filtered in the time domain using empirical mode decomposition to produce two signals from the velocity fluctuation measurements. One signal was for the low-frequency, large-scale turbulent eddy component of the velocity fluctuations and represented the computed results of a large-eddy simulation. The other signal was for the high-frequency, small-scale turbulent eddy component of the velocity fluctuations. This represented the missing scales of a large-eddy simulation. The effect of filtering on the statistical properties of the velocity fluctuations relevant to jet noise prediction modeling were studied. Two-point, space-time, cross correlations were computed for the axial velocity fluctuations. Comparisons were made for the second- and fourth-order two-point correlations, the integral phase velocities, length scales, and time scales, and the frequency dependent phase velocities, length scales, and time scales between the total, large-scale, and small-scale velocity fluctuations.

AC plasma anemometer—characteristics and design

The characteristics and design of a high-bandwidth flow sensor that uses an AC glow discharge (plasma) as the sensing element is presented. The plasma forms in the air gap between two protruding low profile electrodes attached to a probe body. The output from the anemometer is an amplitude modulated version of the AC voltage input that contains information about the mean and fluctuating velocity components. The anemometer circuitry includes resistance and capacitance elements that simulate a dielectric-barrier to maintain a diffuse plasma, and a constant-current feedback control that maintains operation within the desired glow discharge regime over an extended range of air velocities. Mean velocity calibrations are demonstrated over a range from 0 to 140 m s−1. Over this velocity range, the mean output voltage varied linearly with air velocity, providing a constant static sensitivity. The effect of the electrode gap and input AC carrier frequency on the anemometer static sensitivity and dynamic response are investigated. Experiments are performed to compare measurements obtained with a plasma sensor operating at two AC carrier frequencies against that of a constant-temperature hot-wire. All three sensors were calibrated against the same known velocity reference. An uncertainty based on the standard deviation of the velocity calibration fit was applied to the mean and fluctuating velocity measurements of the three sensors. The motivation is not to replace hot-wires as a general measurement tool, but rather as an alternative to hot-wires in harsh environments or at high Mach numbers where they either have difficulty in surviving or lack the necessary frequency response.