PIV measurements of a microchannel flow

 A particle image velocimetry (PIV) system has been developed to measure velocity fields with order 1-μm spatial resolution. The technique uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution of the PIV technique is limited primarily by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring the known flow field in a 30 μm×300 μm (nominal dimension) microchannel. The resulting velocity fields have a spatial resolution, defined by the size of the first window of the interrogation spot and out of plane resolution of 13.6 μm× 0.9 μm×1.8 μm, in the streamwise, wall-normal, and out of plane directions, respectively. By overlapping the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved PIV measurements published to date. Received: 29 October 1998/Accepted: 10 March 1999     

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     

Stereoscopic micro particle image velocimetry

A stereoscopic micro-PIV (stereo-μPIV) system for the simultaneous measurement of all three components of the velocity vector in a measurement plane (2D–3C) in a closed microchannel has been developed and first test measurements were performed on the 3D laminar flow in a T-shaped micromixer. Stereomicroscopy is used to capture PIV images of the flow in a microchannel from two different angles. Stereoscopic viewing is achieved by the use of a large diameter stereo objective lens with two off-axis beam paths. Additional floating lenses in the beam paths in the microscope body allow a magnification up to 23×. The stereo-PIV images are captured simultaneously by two CCD cameras. Due to the very small confinement, a standard calibration procedure for the stereoscopic imaging by means of a calibration target is not feasible, and therefore stereo-μPIV measurements in closed microchannels require a calibration based on the self-calibration of the tracer particle images. In order to include the effects of different refractive indices (of the fluid in the microchannel, the entrance window and the surrounding air) a three-media-model is included in the triangulation procedure of the self-calibration. Test measurement in both an aligned and a tilted channel serve as an accuracy assessment of the proposed method. This shows that the stereo-μPIV results have an RMS error of less than 10% of the expected value of the in-plane velocity component. First measurements in the mixing region of a T-shaped micromixer at Re = 120 show that 3D flow in a microchannel with dimensions of 800 × 200 μm² can be measured with a spatial resolution of 44 × 44 × 15 μm³. The stationary flow in the 200 μm deep channel was scanned in multiple planes at 22 μm separation, providing a full 3D measurement of the averaged velocity distribution in the mixing region of the T-mixer. A limitation is that this approach requires a stereo-objective that typically has a low NA (0.14–0.28) and large depth-of-focus as opposed to high NA lenses (up to 0.95 without immersion) for standard μPIV.     

A microfluidic-based nanoscope

 A novel technique for noninvasively measuring the shapes of walls with resolution approaching tens of nanometers is presented. The nanoscope measures local wall position by measuring the velocity of a fluid with micron-scale spatial resolution as it flows over a surface. The location of the wall is estimated by assuming the no-slip velocity condition at the wall and extrapolating the velocity profile to zero. Nanoscope measurements were obtained in a 30 × 300-μm channel. The wall shape of the glass microchannel was determined to be flat to within a root mean square uncertainty of 62 nm. Numerical simulations show that noise in the velocity measurements contributes significantly to uncertainty in wall position. The technique can be used to measure surfaces that are immersed in liquids and in geometries that do not provide exposed surfaces, where traditional nanoscope techniques such as scanning probe microscopes (SPM) are not applicable. Received: 2 March 2001 / Accepted: 19 October 2001     

Highly spatially resolved velocity measurements of a turbulent channel flow by a fiber-optic heterodyne laser-Doppler velocity-profile sensor

Velocity measurements with a high spatial resolution are important in turbulent flow research. In this paper, we report on the development of a new fiber-optic laser-Doppler velocity-profile sensor exhibiting a spatial resolution of up to 5 μm and its application to turbulent boundary layers. The sensor developed in the present work employs a frequency-division-multiplexing technique in order to separate two measurement signals from the two fringe systems. Velocity measurements close to zero at the solid wall were realized using heterodyne technique. The use of fiber optics improved a robustness of the sensor. The measurement accuracy of the sensor was experimentally investigated with respect to the spatial resolution and velocity. Universal velocity profile of a turbulent flow was obtained in a fully developed channel flow. Mean and fluctuating velocity are presented with a high spatial resolution.     

Planar velocity measurements of a two-dimensional compressible wake

The present study describes the application of particle image velocimetry (PIV) to investigate the compressible flow in the wake of a two-dimensional blunt base at a freestream Mach number M_X=2. The first part of the study addresses specific issues related to the application of PIV to supersonic wind tunnel flows, such as the seeding particle flow-tracing fidelity and the measurement spatial resolution. The seeding particle response is assessed through a planar oblique shock wave experiment. The measurement spatial resolution is enhanced by means of an advanced image-interrogation algorithm. In the second part, the experimental results are presented. The PIV measurements yield the spatial distribution of mean velocity and turbulence. The mean velocity distribution clearly reveals the main flow features such as expansion fans, separated shear layers, flow recirculation, reattachment, recompression and wake development. The turbulence distribution shows the growth of turbulent fluctuations in the separated shear layers up to the reattachment location. Increased velocity fluctuations are also present downstream of reattachment outside of the wake due to unsteady flow reattachment and recompression. The instantaneous velocity field is analyzed seeking coherent flow structures in the redeveloping wake. The instantaneous planar velocity and vorticity measurements return evidence of large-scale turbulent structures detected as spatially coherent vorticity fluctuations. The velocity pattern consistently shows large masses of fluid in vortical motion. The overall instantaneous wake flow is organized as a double row of counter-rotating structures. The single structures show vorticity contours of roughly elliptical shape in agreement with previous studies based on spatial correlation of planar light scattering. Peak vorticity is found to be five times higher than the mean vorticity value, suggesting that wake turbulence is dominated by the activity of large-scale structures. The unsteady behavior of the reattachment phenomenon is studied. Based on the instantaneous flow topology, the reattachment is observed to fluctuate mostly in the streamwise direction suggesting that the unsteady separation is dominated by a pumping-like motion.     

Near-wall measurements of turbulence statistics in a fully developed channel flow with a novel laser Doppler velocity profile sensor

This paper reports on the measurements of the near-wall turbulence statistics in a fully developed channel flow. The flow measurements were carried out with a novel laser Doppler velocity profile sensor with a high spatial resolution. The sensor provides both the information of velocity and position of individual tracer particles inside the measurement volume. Hence, it yields the velocity profile inside the measurement volume, in principle, without the sensor being mechanically traversed. Two sensor systems were realized with different techniques. Typically the sensor has a relative accuracy of velocity measurement of 10⁻³ and the spatial resolution of a few micrometers inside the measurement volume of about 500 μm long. The streamwise velocity was measured with two independent sensor systems at three different Reynolds number conditions. The resulting turbulence statistics show a good agreement with available data of direct numerical simulations up to fourth order moment. This demonstrates the velocity profile sensor to be one of the promising techniques for turbulent flow research with the advantage of a spatial resolution more than one magnitude higher than a conventional laser Doppler technique.     

Submicron deformation field measurements: Part 1. Developing a digital scanning tunneling microscope

A new experimental method has been developed for studying deformations of micromechanical material systems at the submicron scale. To that end, a special digital scanning tunneling microscope (STM) was designed to be coupled to a mechanically deforming specimen. Operating in constant current mode, this digitally controlled STM records detailed topographies of specimen surfaces with a resolution of 10 nm in-plane and 7 nm out-of-plane over a 10 μ × 10 μ area. Three-dimensional displacement field information is extracted by comparing topographies of the same specimen area before and after deformation by way of a modified digital image correlation algorithm. The resolution of this (combined) displacement measuring method was assessed on translation and uniaxial tensile tests to be 5 nm for in-plane displacement components and 1.5 nm for out-of-plane motion over the same area. This is the first paper in a series of three in which the authors delineate the main features of this specially designed microscope and describe how it is constituted, calibrated and used with the improved version of the digital image correlation method to determine deformations in a test specimen at the nanoscale.     

Automatic Full Strain Field Moiré Interferometry Measurement with Nano-scale Resolution

Moiré Interferometry (MI) theoretically can provide real-time full strain field measurements in dynamic environment. So it’s extensively used in reliability analysis of electronic packaging. Due to the nature of specimen preparations procedure, the optical noise is usually too strong so that an accurate phase-based information processing is not possible. In this paper, a 164 nm/pixel spatial resolution Moiré Interferometer with automated full strain field calculation is proposed. Provided by two-level zooming system, the high spatial resolution increase the signal intensity and eliminate some optical noise which allows accurate full strain field map generated automatically by the combination of phase shifting technique and continuous wavelet transform (CWT). Furthermore, the calculation procedure of CWT proposed here does not require unwrapping and differentiation, which avoid the possible numerical noise introduced in these two steps. In the proposed system, pixel by pixel in-plane strain tensors will be calculate from the intensity map of interferograms using phase-based method. The resulting strain tensor can be used to model constitutive relationship or compare with finite element analysis results. A thermal experiment on BGA packaging is used to demonstrate the advantages of the proposed new design.     

Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part II: Experimental Validation for Magnifications from 200 to 10,000

A combination of drift distortion removal and spatial distortion removal are performed to correct Scanning Electron Microscope (SEM) images at both ×200 and ×10,000 magnification. Using multiple, time-spaced images and in-plane rigid body motions to extract the relative displacement field throughout the imaging process, results from numerical simulations clearly demonstrate that the correction procedures successfully remove both drift and spatial distortions with errors on the order of ±0.02 pixels. A series of 2D translation and tensile loading experiments are performed in an SEM for magnifications at ×200 and ×10,000, where both the drift and spatial distortion removal methods described above are applied to correct the digital images and improve the accuracy of measurements obtained using 2D-DIC. Results from translation and loading experiments indicate that (a) the fully corrected displacement components have nearly random variability with standard deviation of 0.02 pixels (≈25 nm at ×200 and ≈0.5 nm at ×10,000) in each displacement component and (b) the measured strain fields are unbiased and in excellent agreement with expected results, with a spatial resolution of 43 pixels (≈54 μm at ×200 and ≈1.1 μm at ×10,000) and a standard deviation on the order of 6 × 10⁻⁵ for each component.

Dual-tracer fluorescence thermometry measurements in a heated channel

The exponential growth of component density in microelectronics has renewed interest in compact and high heat flux thermal management technologies that can handle local heat fluxes exceeding 1 kW/cm². Accurate and spatially resolved thermometry techniques that can measure liquid-phase temperatures without disturbing the coolant flow are important in developing new heat exchangers employing forced-liquid and evaporative cooling. This paper describes water temperature measurements using dual-tracer fluorescence thermometry (DFT) with fluorescein and sulforhodamine B in laminar Poiseuille flow through polydimethyl siloxane-glass channels heated on one side. The major advantage of using the ratio of the signals from these two fluorophores is their temperature sensitivity of 4.0–12% per °C—a significant improvement over previous DFT studies at these spatial resolutions. For an in-plane spatial resolution of 30 μm, the average experimental uncertainties in the temperature data are estimated to be 0.3°C.     

Using digital holographic microscopy for simultaneous measurements of 3D near wall velocity and wall shear stress in a turbulent boundary layer

A digital holographic microscope is used to simultaneously measure the instantaneous 3D flow structure in the inner part of a turbulent boundary layer over a smooth wall, and the spatial distribution of wall shear stresses. The measurements are performed in a fully developed turbulent channel flow within square duct, at a moderately high Reynolds number. The sample volume size is 90 × 145 × 90 wall units, and the spatial resolution of the measurements is 3–8 wall units in streamwise and spanwise directions and one wall unit in the wall-normal direction. The paper describes the data acquisition and analysis procedures, including the particle tracking method and associated method for matching of particle pairs. The uncertainty in velocity is estimated to be better than 1 mm/s, less than 0.05% of the free stream velocity, by comparing the statistics of the normalized velocity divergence to divergence obtained by randomly adding an error of 1 mm/s to the data. Spatial distributions of wall shear stresses are approximated with the least square fit of velocity measurements in the viscous sublayer. Mean flow profiles and statistics of velocity fluctuations agree very well with expectations. Joint probability density distributions of instantaneous spanwise and streamwise wall shear stresses demonstrate the significance of near-wall coherent structures. The near wall 3D flow structures are classified into three groups, the first containing a pair of counter-rotating, quasi streamwise vortices and high streak-like shear stresses; the second group is characterized by multiple streamwise vortices and little variations in wall stress; and the third group has no buffer layer structures.     

Development and validation of echo PIV

The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase the spatial resolution of the velocity measurement to a maximum of 1.8 mm×3.1 mm. Velocity validation tests in fully developed laminar pipe flow showed good agreement with both optical PIV measurements and the expected parabolic profile. A dynamic range of 1 to 60 cm/s has been obtained to date.     

Improvements on the accuracy of derivative estimation from DPIV velocity measurements

Accuracy of out-of-plane vorticity estimation from in-plane experimental velocity measurements is investigated with particular application to digital particle image velocimetry (DPIV). Simulations of known flow fields are used to quantify errors associated with amplification of the velocity measurement noise and method bias error due to spatial sampling resolution. A novel, adaptable, hybrid estimation scheme combining implicit compact finite difference and Richardson extrapolation schemes is proposed for improved vorticity estimation. The scheme delivers higher-order truncation error with less noise amplification than an explicit second order finite difference scheme. Finally, a complete framework for predicting, a priori, the random, bias, and total error of the vorticity estimation on the basis of the error of the resolved velocities and the choice of differentiation scheme is developed and presented. A portion of this work was presented at ASME IMECE 2003 conference An erratum to this article is available at .     

Precise flow rate measurements of natural gas under high pressure with a laser Doppler velocity profile sensor

This paper reports about the first application of a laser Doppler velocity profile sensor for precise flow rate measurements of natural gas under high pressure. The profile sensor overcomes the limitations of conventional laser Doppler anemometry (LDA) namely the effect of spatial averaging and the effect of fringe spacing variation (virtual turbulence). It uses two superposed, fan-like interference fringe systems to determine the axial position of a tracer particle inside the LDA’s measurement volume. Consequently, a spatial resolution of about 1 μm can be achieved and the effect of virtual turbulence is nearly eliminated. These features predestine the profile sensor for flow rate measurements with high precision. Velocity profile measurements were performed at the German national standard for natural gas, one of the world′s leading test facilities for precision flow rate measurements. As a result, the velocity profile of the nozzle flow could be resolved more precisely than with a conventional LDA. Moreover, the measured turbulence intensity of the core flow was of 0.14% mean value and 0.07% minimum value, which is significantly lower than reference measurements with a conventional LDA. The paper describes the performed measurements, gives a discussion and shows possibilities for improvements. As the main result, the goal of 0.1% flow rate uncertainty seems possible by an application of the profile sensor.     

Analysis and reconstruction of a pulsed jet in crossflow by multi-plane snapshot POD

In this work, snapshot proper orthogonal decomposition (POD) is used to study a pulsed jet in crossflow where the velocity fields are extracted from stereoscopic particle image velocimetry (SPIV) results. The studied pulsed jet is characterized by a frequency f = 1 Hz, a Reynolds number Re _j  = 500 (based on the mean jet velocity ${\overline{U}_{j}}$  = 1.67 cm/s and a mean velocity ratio of R = 1). Pulsed jet and continuous jet are compared via mean velocity field trajectory and Q criterion. POD results of instantaneous, phase-averaged and fluctuating velocity fields are presented and compared in this paper. Snapshot POD applied on one plane allows us to distinguish an organization of the first spatial eigenmodes. A distinction between “natural modes” and “pulsed modes” is achieved with the results obtained by the pulsed and unforced jet. Secondly, the correlation tensor is established with four parallel planes (multi-plane snapshot POD) for the evaluation of volume spatial modes. These resulting modes are interpolated and the volume velocity field is reconstructed with a minimal number of modes for all the times of the pulsation period. These reconstructions are compared to orthogonal measurements to the transverse jet in order to validate the obtained three-dimensional velocity fields. Finally, this POD approach for the 3D flow field reconstruction from experimental data issued from planes parallel to the flow seems capable to extract relevant information from a complex three-dimensional flow and can be an alternative to tomo-PIV for large volume of measurement.     

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.     

Application of the PIV technique to measurements around and inside a forming drop in a liquid–liquid system

A particle image velocimetry (PIV) method has been developed to measure the velocity field inside and around a forming drop with a final diameter of 1 mm. The system, including a microscope, was used to image silicon oil drops forming in a continuous phase of water and glycerol. Fluorescent particles with a diameter of 1 μm were used as seeding particles. The oil was forced through a 200 μm diameter glass capillary into a laminar cross-flow in a rectangular channel. The velocity field was computed with a double-frame cross-correlation function down to a spatial resolution of 21 × 21 μm. The method can be used to calculate the shear stress induced at the interface by the cross-flow of the continuous phase and the main forces involved in the drop formation process.     

Measurements of velocity spectra using time-resolving Doppler global velocimetry with laser frequency modulation and a detector array

Measuring velocity spectra in turbulent flows requires methods providing a high temporal resolution and a low measurement uncertainty. Hot-wire anemometry is often used, but it is intrusive. Laser Doppler anemometry is non-intrusive, but due to the statistical arrival of individual tracers provides no constant measurement rate. We therefore propose the use of Doppler global velocimetry (DGV), which is a contactless method allowing temporally equidistant measurements of continuous signals. Additionally, 2d measurements are possible instead of single point measurements. The commonly applied slow cameras are substituted by a fibre coupled detector array consisting of 25 avalanche photo diodes, which increases temporal resolution up to 10 μs. Contrarily to conventional DGV, a sinusoidal laser frequency modulation enables omitting the reference detector array. A correction of beam splitting and image misalignment errors is thus not necessary, but disturbances due to temporal fluctuations of the scattered light can occur and have to be reduced by increasing the modulation frequency. We validate the proposed system capability of synchronously measuring velocity spectra at multiple points in turbulent flows by presenting experimental results. The acquired velocity spectra in a wind tunnel experiment show good agreement with hot-wire comparison measurements within 0.1 m/s. An uncertainty analysis is given, which allows the achievable measurement uncertainty to be estimated as a function of the desired temporal resolution. An uncertainty down to 0.2 m/s can, for example, be achieved assuming a desired temporal resolution of 1 ms. These promising results open new perspectives for turbulence and correlation studies in flows such as to investigate the turbulence characteristics behind a truncated cylinder attached to a plate or the inlet of an aircraft turbine for flow characterisation in industry.     

Construction of three-dimensional flow structure out of two-dimensional steady flow field velocity measurements

 Two-dimensional particle image velocimetry (PIV) is used to obtain a set of parallel vector maps in spanwise direction over the delta wing configuration ELAC. The out-of-plane velocity component is then constructed by application of continuity equation. This yields the whole three-dimensional separated flow field over the leeward side of the model. The spatial resolution of the measurements enables a detailed examination of the three-dimensional flow structure. The growth and the helical structure of primary vortex as well as smaller flow structures caused by secondary separation can be observed. Accuracy of the constructed velocity component is estimated with help of a numerically obtained three-dimensional dataset of the flow field around this configuration. The reconstruction procedure was applied to this data set taking the experimental uncertainty and the grid spacing of the PIV measurements into consideration. A comparison of reconstructed out-of-plane component and data of the numerical solution of Navier–Stokes equations results in a promising low error. A statistical analysis of different procedures allows interpretation of reconstruction capabilities. Received: 15 April 1998 / Accepted: 15 September 1998