Analysis of intermittency and probe data in a supersonic flow with injection

 Probe measurements were performed in the flow field produced by injection of helium or air into a supersonic airstream. The injectant was seeded with water and Rayleigh scattering was used to image the injectant plume. The region of the flow containing injectant–air mixture is seen to be highly unsteady, leading to the intermittent presence of injectant in certain regions. The intermittency is inferred. It is shown that bias errors can occur when the probe data is analyzed by techniques which assume steady flow. A technique for relatively bias-free analysis utilizing the intermittency measurements is presented and the bias errors are estimated. The gas-sampling probe is shown to measure the mass-weighted-mean mass fraction of helium, which is significantly less than the simple mean. A new measure of mixing efficiency obtained from the combined probe and intermittency measurements is discussed. Received: 2 February 1996/Accepted: 2 December 1996     

台阶式溢洪道掺气特性的试验研究

通过试验研究了台阶式溢洪道上的掺气现象以及掺气区域的划分，试验表明，台阶式溢洪道沿程掺气可以分为非掺气区、掺气发展区和掺气充分发展区等三个区域；断面掺气分为台阶影响的含气区、楔形清水区、水中气泡区和空中水滴区。试验还测量了台阶式溢洪道上的断面掺气浓度及其沿程分布，计算了断面含水率，提出了初始掺气点的计算方法。与光滑溢洪道相比较，台阶式溢洪道具有初始掺气点前移，掺气发生早，发展快，掺气浓度大等特点。     

Green water void fraction due to breaking wave impinging and overtopping

The present study uses laboratory measurements to investigate the void fraction of an overtopping flow on a structure. The overtopping flow, also called green water, was generated by the impingement of a plunging breaking wave on the structure following the Froude similarity of an extreme hurricane wave and a simplified offshore structure. The flow is multi-phased and turbulent with significant aeration. A fiber optic reflectometer (FOR) and bubble image velocimetry (BIV) were employed to measure the void fraction and velocity in the flow, respectively, and to determine the water level on the deck. Mean properties of void fraction and velocity were obtained by ensemble-averaging and time-averaging the repeated instantaneous measurements. The temporal and spatial distributions of void fraction reveal that the flow is very highly aerated near the front of green water and has relatively low aeration near the deck surface. The mean void fraction and velocity distributions were also depth-averaged for simplicity and potential use in engineering applications. Using the measured data, similarity profiles for depth-averaged void fraction, depth-averaged velocity, and water level were found. The study suggests that using only the velocity data is insufficient if the flow momentum or the flow rate is to be determined. The accuracy of the void fraction measurements was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume based on the measured velocity and void fraction.     

Measurement of laminar,transitional and turbulent pipe flow using Stereoscopic-PIV

Stereoscopic particle image velocimetry (SPIV) is applied to measure the instantaneous three component velocity field of pipe flow over the full circular cross-section of the pipe. The light sheet is oriented perpendicular to the main flow direction, and therefore the flow structures are advected through the measurement plane by the mean flow. Applying Taylor’s hypothesis, the 3D flow field is reconstructed from the sequence of recorded vector fields. The large out-of-plane motion in this configuration puts a strong constraint on the recorded particle displacements, which limits the measurement accuracy. The light sheet thickness becomes an important parameter that determines the balance between the spatial resolution and signal to noise ratio. It is further demonstrated that so-called registration errors, which result from a small misalignment between the laser light sheet and the calibration target, easily become the predominant error in SPIV measurements. Measurements in laminar and turbulent pipe flow are compared to well established direct numerical simulations, and the accuracy of the instantaneous velocity vectors is found to be better than 1% of the mean axial velocity. This is sufficient to resolve the secondary flow patterns in transitional pipe flow, which are an order of magnitude smaller than the mean flow.     

Self-entrainment of air on stepped spillways

The pseudo-bottom-inception point related to air entrainment is located further upstream on stepped spillways than on smooth spillways, for otherwise identical conditions. Its position is relevant concerning cavitation aspects, flow losses and flow depths. This Paper presents and discusses visual observations made with a high-speed camera and air concentration measurements in the vicinity of the pseudo-bottom air inception point on a stepped model spillway. Insight into the bottom aeration processes is provided, pointing at the effects of dynamic and turbulent air-phase surface troughs instantaneously protruding to the pseudo-bottom. In addition, the measured data were analyzed with regard to the extensions of these surface troughs. The trough bases were found to reach approximately 70–80% of the mixture flow depth upstream of the inception point, to 60–70% at the inception point and to 40–50% at the equilibrium flow region downstream of the inception point. The highly-turbulent character of developed flow is described and the general air transport process specified on the basis of air concentrations and related parameters.     

High speed rotor wake total temperature measurements using a hot-film anemometer

 To develop a quantitative understanding of unsteady and interacting turbomachine flow fields, it is necessary to quantify the instantaneous efficiency of high speed turbomachines. This requires the measurement of both the unsteady velocity and total temperature variation in the exit flow of a high speed rotor. In this paper, techniques to utilize a single slant-film anemometer to measure unsteady total temperature are developed and evaluated. Then a series of preliminary experiments are performed in a high speed axial fan facility to quantify the instantaneous rotor efficiency. This is accomplished by utilizing these single slant-film methods to measure the total temperature in the rotor wakes. Results show that measurements at multiple overheats and several probe orientations are required. The simplest method proves to be useful for determining parameters used in other methods. An analysis based on King’s law gives good results even when measurements are outside the calibration range. Within the calibration range, a polynomial representation of the wire response to mass flux and total temperature yields good total temperature fluctuation results. A model analysis technique is also assessed. Received: 13 November 1997/Accepted: 16 February 1998     

Measurement of the acoustic velocity field of nonlinear standing waves using the synchronized PIV technique

The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases.     

A particle image velocimetry system for microfluidics

 A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles, and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm. In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles. The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements. The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder, with a bulk velocity of approximately 50 μm s^-1. Received: 26 November 1997/Accepted: 26 February 1998     

Two-component hot-film probe for measurements of wall shear stress

A two-component, hot-film probe has been developed, to measure the wall shear stress as a vector quantity in time-dependent flows. The probe has been applied for the measurements of the bottom shear stress in a flow generated by combined waves and current in a water basin where the magnitude and the direction of the bottom shear stress varied periodically with respect to time. The probe has also been used to measure the bottom shear stress around a vertical cylinder exposed to water waves generated in a wave flume.     

Measurement of instantaneous Eulerian acceleration fields by particle image accelerometry: method and accuracy

Acceleration is a fundamental quantity in fluid mechanics because it reflects the sum of all forces (pressure and viscous) present within the flow. However, measurements of acceleration have been difficult to achieve relative to the ease with which fluid velocity can be measured. A particle image accelerometer (PIA) has been developed to measure Eulerian acceleration fields by time-differencing successive measurements of the Eulerian velocity field as measured by particle image velocimetry (PIV). The measurements can also be made in uniformly translating frames. With current video camera technology, it is often not possible to measure the two velocity fields with a time separation sufficiently small enough to permit accurate finite difference approximation of the time derivative. A two-CCD-camera system has been developed to alleviate this limitation. Polarization filtering is utilized to separate the particle images viewed by each camera. The polarization filtering is achieved using cross-polarized light-sheets and a polarization filter just upstream of the imaging optics of the cameras. In this manner, PIV measurements can be achieved easily at time delays several orders of magnitude smaller than the shutter-time of the CCD cameras. The accuracy of the acceleration measurements is determined by numerical finite differencing errors and random noise and bias errors associated with the measurement of velocity. These errors, and methods of compensating for them, are studied.     

Detecting coherent patterns in a flume by using PIV and IR imaging techniques

We investigated the flow field in a turbulent boundary layer in a flume, by using Particle Image Velocimetry (PIV) and Hot-Foil Infrared Imaging (HFIRI) techniques. Coherent patterns in the flow were identified and characterized by using instantaneous velocity and temperature fields. The velocity fields in the streamwise–spanwise plane were measured in parallel to the temperature distribution of the flume bottom. The identified patterns are represented by means of their spatial characteristics – a non-dimensional spatial separation between streamwise patterns, ⁺.     

Estimating void fraction in a hydraulic jump by measurements of pixel intensity

A hydraulic jump is a sudden transition from supercritical to subcritical flow. It is characterized by a highly turbulent roller region with a bubbly two-phase flow structure. The present study aims to estimate the void fraction in a hydraulic jump using a flow visualization technique. The assumption that the void fraction in a hydraulic jump could be estimated based on images’ pixel intensity was first proposed by Mossa and Tolve (J Fluids Eng 120:160–165, 1998). While Mossa and Tolve (J Fluids Eng 120:160–165, 1998) obtained vertically averaged air concentration values along the hydraulic jump, herein we propose a new visualization technique that provides air concentration values in a vertical 2-D matrix covering the whole area of the jump roller. The results obtained are found to be consistent with new measurements using a dual-tip conductivity probe and show that the image processing procedure (IPP) can be a powerful tool to complement intrusive probe measurements. Advantages of the new IPP include the ability to determine instantaneous and average void fractions simultaneously at different locations along the hydraulic jump without perturbing the flow, although it is acknowledged that the results are likely to be more representative in the vicinity of sidewall than at the center of the flume.     

Electrical impedance string probes for two-phase void and velocity measurements

An instrumentation system was developed to measure two-phase flow velocity and void fraction. The principle of operation of this system was based on the measurement of the electrical impedance of two-phase mixtures. Two-phase velocity is estimated by time-of-flight analysis of signals from two spatially separated sensors. A technique involving measurement of both the capacitance and the conductance of the mixture was used to determine void fraction and correct for the effect of liquid distribution. The string probe instrumentation proved to be durable in air/water and steam/water flows and demonstrated an ability to measure a wide range of flow velocities (1–17 m/s) and void fractions (0.25?0.99+).     

Experiments on unsteady cavitation

 The unsteady behaviour of cloud cavitation is obviously influenced by its internal flow pattern. The main purpose of this work is to investigate such a two phase flow during a cavitation cycle. The tests are carried out with a convergent divergent nozzle. Observations are made by using a classical video set in combination with a stroboscopic light sheet. The use of a double optical probe enables void fraction and velocity to be measured inside the two phase flow structure. Data acquisition is governed by a pressure signal measured near the cavity closures to follow their evolution during the shedding process. Special care has been taken in validating the experimental techniques because they have not been used in such flows. The measurements show an extended reversed flow occurring along the solid surface. It plays a significant function in the vapour cloud shedding process. Received: 11 September 1995 / Accepted: 28 June 1996     

Measurements in the flow around a marine propeller at the stern of an axisymmetric body

An experimental study of the flow around and behind an axisymmetric body driven by a marine propeller is reported. Experiments were performed in a wind tunnel to document this complex, unsteady, three-dimensional, turbulent shear flow. Measurements were made in the boundary layer and wake of the bare body with a fixed dummy hub for the propeller, with the dummy hub rotating, and finally, with the propeller in operation. A five-hole yaw probe was employed for the mean-flow measurements, and two- and threesensor hotwires were used to obtained the mean and turbulent velocity fields. Part 1 of this two-part paper describes the experimental arrangement and circumferentially-averaged results which clarify certain overall aspects of the flow when it is viewed as a rotationally-symmetric flow. These are of special interest in marine hydrodynamics. In Part 2, the triple-sensor hotwire data are analyzed using phase-averaging techniques to reconstruct the instantaneous velocity and Reynolds-stress fields downstream of the propeller to show the evolution of the wakes of individual blades, blade-tip vortices, and the complex flow associated with vortices generated at hub-blade junctions.     

Particle imaging techniques for microfabricated fluidic systems

This paper presents the design and implementation of velocimetry techniques applicable to the analysis of microfluidic systems. The application of both micron-resolution particle image velocimetry (micro-PIV) and particle tracking velocimetry (PTV) to the measurement of velocity fields within micromachined fluidic channels is presented. The particle tracking system uses epifluorescent microscopy, CCD imaging, and specialized image interrogation algorithms to provide microscale velocity measurement resolution. The flow field in a straight channel section is measured using cross-correlation micro-PIV and compared to the analytical solution for a measured mass flow rate. Velocity field measurements of the flow at the intersection of a cross-channel are also presented and compared with simulations from a commercially available flow solver, CFD-ACE+. Discussions regarding flow seeding, imaging optics, and the flow setup for measuring flows in microfabricated fluidic devices are presented. A simple process for estimating measurement uncertainty of the in-plane velocity measurements caused by three-dimensional Brownian motion is described. A definition for the measurement depth for PTV measurements is proposed. The agreement between measured and predicted values lends further support to the argument that liquid microflows with characteristic dimensions of order 50-μm dimension channels follow macroscale flow theory.     

Infrared micro-particle image velocimetry in silicon-based microdevices

A non-intrusive diagnostic technique, infrared micro-particle image velocimetry (IR-PIV), is developed for measuring flow fields within micro-electromechanical system (MEMS) devices with micron-scale resolution. This technique capitalizes on the transparency of silicon in the infrared region, and overcomes the limitation posed by the lack of optical access with visible light to sub-surface flow in silicon-based microstructures. Experiments with laminar flow of water in a circular microcapillary tube of hydraulic diameter 255 m demonstrate the efficacy of this technique. The experimental measurements agree very well with velocity profiles predicted from laminar theory. Cross-correlation and auto-correlation algorithms are employed to measure very low and moderate to high velocities, respectively; the former approach is suitable for biomedical applications while the latter would be needed for measurements in electronics cooling. The results indicate that the IR-PIV technique effectively extends the application of regular micro-PIV techniques, and has great potential for flow measurements in silicon-based microdevices.     

3D Flow reconstruction using ultrasound PIV

Ultrasound particle image velocimetry (PIV) can be used to obtain velocity fields in non-transparent geometries and/or fluids. In the current study, we use this technique to document the flow in a curved tube, using ultrasound contrast bubbles as flow tracer particles. The performance of the technique is first tested in a straight tube, with both steady laminar and pulsatile flows. Both experiments confirm that the technique is capable of reliable measurements. A number of adaptations are introduced that improve the accuracy and applicability of ultrasound PIV. Firstly, due to the method of ultrasound image acquisition, a correction is required for the estimation of velocities from tracer displacements. This correction accounts for the fact that columns in the image are recorded at slightly different instances. The second improvement uses a slice-by-slice scanning approach to obtain three-dimensional velocity data. This approach is here demonstrated in a strongly curved tube. The resulting flow profiles and wall shear stress distribution shows a distinct asymmetry. To meaningfully interpret these three-dimensional results, knowledge of the measurement thickness is required. Our third contribution is a method to determine this quantity, using the correlation peak heights. The latter method can also provide the third (out-of-plane) component if the measurement thickness is known, so that all three velocity components are available using a single probe.