Mean Velocity Measurements Research Articles

An experimental investigation of the dissipation mechanisms in the suction side boundary layer of a turbine blade

The present work is part of an extensive experimental activity carried out by the authors in recent years aimed at investigating the boundary layer transition phenomenon in turbine blades. The large scale of the cascade and the use of advanced LDV instrumentation and precision probe traversing mechanism resulted in high degree of spatial resolution and high accuracy of measurements. The main dissipation mechanism determining the profile losses in turbomachinery blades is the work of deformation of the mean motion within the boundary layer operated by both viscous and turbulent shear stresses. In the present paper, the local viscous and turbulent deformation works have been directly evaluated from the detailed measurements of boundary layer mean velocity and Reynolds shear stress. The results show the distributions and the relative importance of the viscous and turbulent contributions to the loss production, in relation with the boundary layer states occurring along the turbine profile.

High-ionic-strength electroosmotic flows in uncharged hydrophobic nanochannels

We report molecular dynamics simulation results of high-ionic-strength electroosmotic flows inside uncharged nanochannels. The possibility of this unusual electrokinetic phenomenon has been discussed by Dukhin et al. [A. Dukhin, S. Dukhin, P. Goetz, Langmuir 21 (2005) 9990]. Our computed velocity profiles clearly indicate the presence of a net flow with a maximum velocity around 2 m/s. We found the apparent zeta potential to be − 29.7 ± 6.8 mV , using the Helmholtz–Smoluchowski relation and the measured mean velocity. This value is comparable to experimentally measured values in Dukhin et al. and references therein. We also investigate the orientations of water molecules in response to an electric field by computing polarization density. Water molecules in the bulk region are oriented along the direction of the external electric field, while their near-wall orientation shows oscillations. The computation of three-dimensional density distributions of sodium and chloride ions around each individual water molecule show that chloride ions tend to concentrate near a water molecule, whereas sodium ions are diffusely distributed.

Evaluation of cerebrovascular carbon dioxide reactivity in patients with diabetes mellitus under sedative doses of propofol

The present study compared cerebrovascular CO2 reactivity in diabetic patients on different treatment modalities under sedative doses of propofol. Fifteen patients with diabetes mellitus (on three different antidiabetic treatment modalities) who required mechanical ventilation during intensive care therapy were studied, sedation during mechanical ventilation being maintained using propofol. As controls, 6 patients without diabetes were monitored. A 2.5-MHz pulsed transcranial Doppler probe was attached to the head of the patient at the right temporal window for continuous measurement of mean blood flow velocity in the middle cerebral artery (Vmca). After establishing baseline values of Vmca and cardiovascular hemodynamics, end-tidal CO2 was decreased by increasing ventilatory frequency by 5-8 breaths.min(-1). Values for absolute and relative CO2 reactivity in insulintreated patients were lower than those in the other three groups (absolute CO2 reactivity [means +/- SD]: control, 3.1 +/- 0.6 cm.s(-1).mmHg(-1), diet, 3.8 +/- 1.4 cm.s(-1) x mmHg(-1); oral antidiabetic drug 3.2 +/- 0.9 cm x s(-1) x mmHg(-1); insulin, 1.1 +/- 0.6 cm x s(-1) x mmHg(-1); P = 0.002).The present study shows that insulin-treated diabetic patients probably have lower cerebrovascular CO2 reactivity under propofol anesthesia than control patients or diabetics treated with dietary therapy or oral hypoglycemics.

Influence of spacing between buildings on wind characteristics above rural and suburban areas

A wind tunnel study has been carried out to determine the influence of spacing between buildings on wind characteristics above rural and suburban type of terrain. Experiments were performed for two types of buildings, three-floor family houses and five-floor apartment buildings. The atmospheric boundary layer (ABL) models were generated by means of the Counihan method using a castellated barrier wall, vortex generators and a fetch of roughness elements. A hot wire anemometry system was applied for measurement of mean velocity and velocity fluctuations. The mean velocity profiles are in good agreement with the power law for exponent values from α = 0.15 to α = 0.24, which is acceptable for the representation of the rural and suburban ABL, respectively. Effects of the spacing density among buildings on wind characteristics range from the ground up to 0.6δ . As the spacing becomes smaller, the mean flow is slowed down, whilst, simultaneously, the turbulence intensity and absolute values of the Reynolds stress increase due to the increased friction between the surface and the air flow. This results in a higher ventilation efficiency as the increased retardation of horizontal flow simultaneously accompanies an intensified vertical transfer of momentum.

Three-dimensional magnetic resonance velocimetry measurements of turbulence quantities in complex flow

A magnetic resonance velocimetry (MRV) experimental technique based on magnetic resonance imaging and capable of measuring the turbulent Reynolds stresses in a 3D flow domain is described. Results are presented in backward facing step flow in a square channel with a Reynolds number of 48,000 based on step height and freestream velocity at the step. MRV results are compared to particle image velocimetry (PIV) measurements in the centerplane containing the streamwise and cross-stream axes. MRV and PIV mean velocity measurements show excellent agreement. MRV measurements for Reynolds normal stresses compare to within ±20% of the PIV results while results for the turbulent shear are less accurate.

PIV Techniques for Velocity Fields of Internal Waves over a Slowly Varying Bottom Topography

This paper reviews transformation processes and nonlinear properties of internal waves over a uniform slope in a two-fluid system, attempting to reveal water particle kinematics by using particle image velocimetry (PIV). Attenuation and setup induced by wave breaking are predicted using an energy-dissipation model including the radiation stress. In the present paper, the radiation stress is associated with the time-averaged momentum flux owing to the interaction between incident and reflected internal waves. These predictions agree with experimental data obtained by an image processing technique with which the boundary plane between the upper and lower layers could be detected as a density interface. A set of halogen lamps and three high-definition digital video cameras are used to illustrate velocity vector fields during one wave period. Instantaneous velocity fields measured by the PIV system are compared with the calculated velocity distribution by the method of characteristics in combination with the first-order Stokes theory. The rundown behavior is then investigated, and its influence upon the undertow in the lower layer is explained with a simple turbulent-jet model. Furthermore, the theoretical approach for examining mass transport and undertow is discussed in relation to the measured mean velocity.

Laser Doppler Velocimetry for Joint Measurements of Acoustic and Mean Flow Velocities: LMS-Based Algorithm and CRB Calculation

This paper presents a least-mean-square (LMS) algorithm for the joint estimation of acoustic and mean flow velocities from laser Doppler velocimetry (LDV) measurements. The usual algorithms used for measuring with LDV purely acoustic velocity or mean flow velocity may not be used when the acoustic field is disturbed by a mean flow component. The LMS-based algorithm allows accurate estimations of both acoustic and mean flow velocities. The Cramer-Rao bound (CRB) of the associated problem is determined. The variance of the estimators of both acoustic and mean flow velocities is also given. Simulation results of this algorithm are compared with the CRB, and the comparison leads us to validate this estimator.

Erosion d’une vase de l’estuaire de la Loire sous l’action du courant

Erosion d’une vase de l’estuaire de la Loire sous l’action du courant

Effect of Interfacial Waves on Turbulence Structure in Stratified Duct Flows

Stratified flows are encountered in many industrial applications. The determination of the flow characteristics is essential for the prediction of pressure drop and holdup in the system. The aim of this study is to gain insight into the interaction of a gas and a liquid phase flowing in a stratified regime, with especial focus on the effect of interfacial waves on the turbulence structure of the liquid phase. Measurements of mean velocities and turbulent intensities in the liquid phase of a stratified air-water duct flow are performed. Mean velocity profiles and turbulence structure are affected differently for different wave amplitudes. The effect of small amplitude waves is restricted to the near-interface region, resembling the effect of increasing shear rate on a flat interface. On the other hand, large amplitude waves modify the flow structure throughout the whole liquid depth. The mean velocity is greatly enhanced, resulting in a higher bulk velocity. Turbulent intensities are also significantly enhanced especially in the interface region. This big difference in flow structure is not observed after the appearance of the first waves but rather when a certain critical wave amplitude is triggered, indicating that the prediction of this critical wave type turns out to be more important than the determination of the transition from a smooth to a stratified wavy regime.

The comparative effects of sevoflurane versus isoflurane on cerebrovascular carbon dioxide reactivity in patients with previous stroke

The use of volatile anesthetics is reportedly related to altered cerebrovascular carbon dioxide (CO2) reactivity. We examined the comparative effects of sevoflurane versus isoflurane on cerebrovascular CO2 reactivity in patients with previous stroke. Twenty-four patients with previous stroke and 20 patients without previous stroke (serving as controls) were studied. Anesthesia was maintained with either end-tidal 1.0 minimum alveolar concentration (MAC) sevoflurane or 1.0 MAC isoflurane in 33% oxygen and 67% nitrous oxide. A 2.5-MHz pulsed transcranial Doppler (TCD) probe was attached to the patient's head at the right or left temporal window for continuous measurement of mean blood flow velocity in the middle cerebral artery (Vmca). After establishing baseline values of Vmca and cardiovascular hemodynamics, we increased end-tidal CO2 by decreasing the ventilatory frequency by 2-5 breaths x min(-1). We found that values for absolute and relative CO2 reactivity in the sevoflurane groups were lower than those in the isoflurane groups (absolute CO2 reactivity in the sevoflurane groups: control, 3.3 +/- 0.4*; previous stroke, 3.4 +/- 0.4*; absolute CO2 reactivity in the isoflurane groups: control, 4.2 +/- 0.3; previous stroke, 4.5 +/- 0.4, cm x s(-1) x mmHg(-1); *P < 0.05 compared with isoflurane group). There were no significant differences in the values for absolute and relative CO2 reactivity between the controls and the previous-stroke patients within each of the sevoflurane and isoflurane groups. Our findings suggest that, in patients with previous stroke, cerebrovascular CO2 reactivity under sevoflurane anesthesia was lower than that under isoflurane anesthesia.

Comparative effects of propofol vs dexmedetomidine on cerebrovascular carbon dioxide reactivity in patients with septic shock

The use of sedative drugs is reportedly related to altered cerebrovascular CO2 reactivity. The present study examined the comparative effects of propofol vs dexmedetomidine on cerebrovascular CO2 reactivity in patients with septic shock. A total of 20 patients with septic shock who required mechanical ventilation were included in this study. Sedation during mechanical ventilation was maintained using either propofol or dexmedetomidine. A 2.5 MHz pulsed transcranial Doppler probe was attached to the head of the patient at the right temporal window for continuous measurement of mean blood flow velocity in the middle cerebral artery (V(mca)). After establishing baseline values of V(mca) and cardiovascular haemodynamics, end-tidal CO2 was increased by decreasing ventilatory frequency by 5-8 bpm. The absolute and relative CO2 reactivity values in patients with septic shock were lower for both propofol and dexmedetomidine than those for control groups, with significant differences between these values in the two septic shock groups (absolute CO2 reactivity in septic shock under propofol: 2.6 (sd 0.3) cm s(-1) mm Hg(-1); absolute CO2 reactivity in septic shock under dexmedetomidine: 2.0 (0.3) cm s(-1) mm Hg(-1); P<0.01). This study showed that cerebrovascular CO2 reactivity was lower under dexmedetomidine sedation than under propofol sedation during almost identical sedation in patients with septic shock.

Acoustic Velocimetry for Riverine Environments

Acoustic instruments are rapidly replacing conventional current meters for measurement of velocities in natural and man-made open channels. Greater efficiency, improved performance, and numerous safety considerations at comparable costs have provided the motivation for their broad adoption in hydrologic datacollection operations. They have no moving parts, offer relatively high spatial and temporal sampling resolution, and require fewer calibrations. Moreover, they allow measurements in field settings where conventional measurements were very difficult or costly to obtain and facilitate documentation of flow features that previously could only be documented in the laboratory Dinehart and Burau 2005 . Rapid development of acoustic velocimeters has continued since the early work on acoustic Doppler current profilers ADCPs by Christensen and Herrick 1982 , Simpson 1986 , and Gordon 1989 . Likewise, the development of acoustic Doppler velocimeters ADVs in the early 1990s Kraus et al. 1994 has revolutionized the measurement of twoand threedimensional mean velocities and turbulence characteristics in both laboratory and field studies. These technologies have profoundly changed the way that hydraulic data are collected by researchers, engineers, and technicians alike, being applied to measure velocity and thereby estimate important velocity-derived hydrodynamic quantities in support of riverine research and to complement numerical simulations and laboratory studies. Acoustic instruments are currently used for routine operations on water delivery projects, water treatment plants, stream gauging stations, and many other water resources-related projects. Given the growing interest in acoustic measurements and associated instrumentation, the Hydraulic Measurements and Experimentation Committee HME of the Environmental and Water Resources Institute EWRI decided in 2005 to produce a special issue of the Journal of Hydraulic Engineering focused on Acoustic Velocimetry for Riverine Environments. The HME is a technical committee founded in 1989 within the American Society of Civil Engineers and more recently its Environmental and Water Resources Institute. The committee consists of volunteer scientists and practitioners from academia, federal and state water

Evaluation of Mean Velocity and Turbulence Measurements with ADCPs

To test the ability of acoustic Doppler current profilers (ADCPs) to measure turbulence, profiles measured with two pulse-to-pulse coherent ADCPs in a laboratory flume were compared to profiles measured with an acoustic Doppler velocimeter, and time series measured in the acoustic beam of the ADCPs were examined. A four-beam ADCP was used at a downstream station, while a three-beam ADCP was used at a downstream station and an upstream station. At the downstream station, where the turbulence intensity was low, both ADCPs reproduced the mean velocity profile well away from the flume boundaries; errors near the boundaries were due to transducer ringing, flow disturbance, and sidelobe interference. At the upstream station, where the turbulence intensity was higher, errors in the mean velocity were large. The four-beam ADCP measured the Reynolds stress profile accurately away from the bottom boundary, and these measurements can be used to estimate shear velocity. Estimates of Reynolds stress with a three-beam ADCP and turbulent kinetic energy with both ADCPs cannot be computed without further assumptions, and they are affected by flow inhomogeneity. Neither ADCP measured integral time scales to within 60%.

First definitive observations of meteor shower particles using a high-power large-aperture radar

We present the first clear observations of meteor shower activity from meteor-head echoes detected by a high-power large-aperture radar (HPLAR). Such observations have been performed at the Jicamarca VHF radar using its interferometric capabilities allowing the discrimination of meteor shower echoes from the much more frequent sporadic meteors. Until now, HPLARs were unable to distinguish meteor shower from the much more common sporadic meteor ones. In this work we have been able to detect and characterize the η-Aquariids (ETA) as well as the Perseids (PER) showers. The shower activity is more conspicuous for the ETA than for the PER shower due to the more favorable geometry. Namely, PER meteors come from low elevation angles, experiencing more filtering due to the combined Earth–atmosphere–radar instrument. In both cases, there is an excellent agreement between the measured mean velocity of the shower echoes and their expected velocity, within a fraction of 1 km s −1. Besides the good agreement with expected visual results, HPLARs observe meteors with a variety of particles sizes and masses, not observed by any other technique. Taking into account the different viewing volumes, compare to optical observations Jicamarca observes more than 1000 times more ETA meteors. Our results indicate that Jicamarca and other HPLARs are able to detect the echoes from meteor showers, but without interferometric capabilities such populations are difficult to identify just from their velocity distributions, particularly if their velocity distributions are expected to be similar to the more dominant distributions of sporadic meteors.

Magnetic resonance velocimetry: applications of magnetic resonance imaging in the measurement of fluid motion

Magnetic resonance velocimetry (MRV) is a non-invasive technique capable of measuring the three-component mean velocity field in complex three-dimensional geometries with either steady or periodic boundary conditions. The technique is based on the phenomenon of nuclear magnetic resonance (NMR) and works in conventional magnetic resonance imaging (MRI) magnets used for clinical imaging. Velocities can be measured along single lines, in planes, or in full 3D volumes with sub-millimeter resolution. No optical access or flow markers are required so measurements can be obtained in clear or opaque MR compatible flow models and fluids. Because of its versatility and the widespread availability of MRI scanners, MRV is seeing increasing application in both biological and engineering flows. MRV measurements typically image the hydrogen protons in liquid flows due to the relatively high intrinsic signal-to-noise ratio (SNR). Nonetheless, lower SNR applications such as fluorine gas flows are beginning to appear in the literature. MRV can be used in laminar and turbulent flows, single and multiphase flows, and even non-isothermal flows. In addition to measuring mean velocity, MRI techniques can measure turbulent velocities, diffusion coefficients and tensors, and temperature. This review surveys recent developments in MRI measurement techniques primarily in turbulent liquid and gas flows. A general description of MRV provides background for a discussion of its accuracy and limitations. Techniques for decreasing scan time such as parallel imaging and partial k-space sampling are discussed. MRV applications are reviewed in the areas of physiology, biology, and engineering. Included are measurements of arterial blood flow and gas flow in human lungs. Featured engineering applications include the scanning of turbulent flows in complex geometries for CFD validation, the rapid iterative design of complex internal flow passages, velocity and phase composition measurements in multiphase flows, and the scanning of flows through porous media. Temperature measurements using MR thermometry are discussed. Finally, post-processing methods are covered to demonstrate the utility of MRV data for calculating relative pressure fields and wall shear stresses.

Aerodynamic noise simulation of the flow past an airfoil trailing-edge using a hybrid zonal RANS-LES

This paper document the evaluation of a zonal RANS-LES approach for the prediction of broadband and tonal noise generated by the flow past an airfoil trailing edge at a high Reynolds number. A multi-domain decomposition is considered, where the acoustic sources are resolved with a LES sub-domain embedded in the RANS domain. At the RANS-LES interface, a stochastic vortex method is used to generate synthetic turbulent perturbations. The simulations are performed with the general-purpose unstructured control-volume code FLUENT. The far-field noise is calculated using the aeroacoustic analogy of Ffowcs-Williams and Hawkings. The results of the simulation are compared with available acoustic and mean velocity measurements. The investigation demonstrates the ability of this approach to predict the aerodynamic and aeroacoustic properties of the flow. Two simulations are performed in order to address the sensitivity of the results to the perturbation model. The comparison clearly indicates the critical influence of the model.

High resolution PIV measurements around a model turbine blade trailing edge film-cooling breakout

Film cooling downstream of a model turbine blade trailing edge has been studied experimentally. High resolution particle image velocimetry was used to obtain spatially resolved mean velocity and turbulence measurements in the immediate vicinity of the trailing edge breakout. The mean velocity measurements imply the presence of a pair of counter-rotating longitudinal vortices shed from the sides of the breakout lands. The turbulent shear stress measurements above the breakout are significantly intensified as blowing ratio is increased. These results suggest that there is a strong mixing between the film cooling slot jets and the mainstream flow which degrades the film cooling effectiveness.

LARGE EDDY SIMULATION OF ISOTHERMAL TURBULENT SWIRLING JETS

This article investigates the application of the large eddy simulation (LES) technique to turbulent isothermal swirling flows. The aim was to improve our understanding of the flow physics and turbulence structure of unconfined swirling flows and examine the capability of LES to predict the formation of the vortex breakdown (VB) and recirculation zones. In this study, the filtered Navier-Stokes equations are closed using the Smagorinsky eddy viscosity model with the localized dynamic procedure of Piomelli and Liu (1995). The Sydney University swirl burner experiments are simulated as test cases. Three different test cases have been investigated covering a range of swirl numbers and stream wise annular velocities. The cases considered have swirl numbers ranging from 0 to 1.59 and Reynolds numbers from 32400 to 59000. The LES calculations confirm that the combination of lower swirl number and higher axial velocity of the primary annulus leads to the establishment of the downstream vortex breakdown region. For the cases considered, the LES calculations were successful in predicting observed recirculation zones, vortex breakdown and showed good agreement with experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses.

Tip Gap Effects on the Unsteady Behavior of a Tip Leakage Vortex

The effects of tip gap on the unsteady behavior of a tip leakage vortex downstream of an idealized axial compressor rotor blade have been investigated in a linear cascade wind tunnel, extending the study of Ma and Devenport. The wind tunnel features a moving end wall to simulate the relative motion between the rotor and casing, and vortex generator pairs attached to the moving end wall that produce an idealized unsteady vortical inflow. Detailed three-component mean velocity and turbulence measurements have been made just downstream of the blade trailing edges for a series of tip gaps, from 0.83 to 3.3% chord, and phase averaged with respect to the relative position of the blades and vortex generator wakes, to reveal the structure of this flow and its dependence on tip gap. Significant fluctuations in the size, strength, structure, and position of the tip leakage vortex are produced by the vortical inflow even though it is one to two orders weaker than the tip leakage vortex. Interestingly, the amplitude of these effects increases with tip gap as the tip vortex strengthens. For small tip gaps, the disturbance to the leakage vortex appears to be a consequence of direct interaction with the inflow vortices. However, for larger tip gaps it is the indirect action of the inflow vortices interfering with the shedding of circulation from the blade tip that appears to be the dominant source of unsteadiness in the leakage vortex.

Effects of nicardipine-induced hypotension on cerebrovascular carbon dioxide reactivity in patients with diabetes mellitus under sevoflurane anesthesia

The purpose of this study was to examine the effects of nicardipine-induced hypotension on cerebrovascular CO2 reactivity in patients with diabetes mellitus under sevoflurane anesthesia. Nineteen diabetic patients, and 11 nondiabetic patients (serving as controls), undergoing elective orthopedic, cardiovascular, or thoracic surgery were included in the study. The diabetic patients were divided into three groups according to the antidiabetic therapy they were receiving, i.e., diet therapy (n = 6), oral antidiabetic drugs (n = 7), and insulin (n = 6). Anesthesia was maintained with 1.0 minimum alveolar concentration of sevoflurane. Absolute and relative cerebrovascular CO2 reactivity was calculated using a 2.5-MHz pulsed transcranial Doppler (TCD) probe for the continuous measurement of mean blood flow velocity in the middle cerebral artery (Vmca). The cerebrovascular CO2 reactivity was measured both at baseline and during hypotension by increasing the ventilatory frequency by 4 to 7 breaths.min(-1). Nicardipine was used to induce hypotension. We found that values for the Bispectral index (BSI), baseline mean blood pressure, endtidal CO2 (Pet(CO2)), and Vmca were essentially identical in all patients, irrespective of the type of antidiabetic treatment being taken. Values for absolute and relative CO2 reactivity in insulin-dependent patients, at both baseline blood pressure and during hypotension, were lower than those in patients in the antidiabetic drug, diet, and control groups (during hypotension, absolute CO2 reactivity: diet group: 3.2 +/- 0.9; oral antidiabetic drug group: 3.2 +/- 0.7; insulin group: 1.5 +/- 0.6; control group: 3.4 +/- 0.8 cm.s(-1).mmHg(-1), [P < 0.05 insulin group vs the other groups]; relative CO2 reactivity: diet group, 6.3 +/- 1.0; oral antidiabetic drug group, 6.5 +/- 0.8; insulin group, 3.5 +/- 0.8; control group, 6.5 +/- 0.7%.mmHg(-1), [P < 0.05 insulin group vs the other groups]. We concluded that cerebrovascular CO2 reactivity in insulin-dependent patients is impaired during nicardipine-induced hypotension under sevoflurane anesthesia.