New insights into particle image velocimetry data using fuzzy-logic-based correlation/particle tracking processing

Digital particle image velocimetry (DPIV) data processing has been developed to the point where DPIV image data are processed via auto- or cross-correlation techniques in near real time and the results are displayed on screen as they are processed. Correlation techniques are highly desirable, since they provide velocity measurements on a regular grid, which are readily comparable to CFD predictions of the flow field. In high-speed flows, particle lag effects are always of concern; however, the correlation operation does not provide any means for minimization or elimination of systematic errors in the recorded particle image data. In this paper, we present a combined correlation processing/particle tracking technique providing “super-resolution” velocity measurements. Fuzzy-logic principles are employed to maximize the information recovery in the correlation operation and to determine the correct particle pairings in the tracking operation. The combined correlation/particle tracking technique is applied to DPIV data obtained in the diffuser region of a high-speed centrifugal compressor producing velocity vector maps with an average density of 6 vectors/mm². Inspection of the particle tracking results revealed large particles that were not following the flow. Using preknowledge of the flow field, the biased velocity estimates arising from large particles in the flow were removed, thereby improving the accuracy of the measurements. Received: 21 October 1999/Accepted: 19 August 2000     

Turbulent structure during transition to self-similarity in a round jet

 The developing turbulent region of a round jet was investigated using an improved implementation of digital particle image velocimetry (DPIV). The two-dimensional flow field in planes normal and parallel to the axial velocity was measured at locations between 15 and 30 diameters downstream, for two Reynolds numbers of 5500 and 16,000. The study consisted of instantaneous snapshots of the velocity and vorticity fields as well as measurements of velocity correlations up to third order. In this regime, the Reynolds number had a significant effect on both the instantaneous flow structure and the profiles of mean velocity across the jet. Coherent streamwise structures were present in the jet core for the lower Reynolds number. Additional structures whose evolution was governed by time scales two orders of magnitude larger than the convective scale inside the jet were observed in the entrainment field. The velocity correlations provided further support for the validity of DPIV turbulence measurements. The data was consistent with the equations of motion and momentum was conserved. DPIV measurements of turbulent kinetic energy components agreed with the hot-wire measurements of previous studies. Received: 27 November 1996/Accepted: 14 July 1997     

Measured turbulent entropy production with large eddy particle image velocimetry

In this paper, statistical post-processing of measured velocity, dissipation rate and turbulence data is performed to establish whole-field distributions of entropy production within a channel. Thermal irreversibilities arising from temperature variations were not included in the study, as the experiments were conducted between unheated plexiglass plates in an essentially isothermal water tunnel. Unlike velocity or temperature, the measurement of entropy cannot be performed directly, so entropy production is measured indirectly through spatial differencing of measured velocities in large eddy PIV. In contrast to single-point methods of anemometry, large eddy PIV enables whole-field, time-varying measurements of the velocity field, which can be post-processed to yield entire spatial variations of the entropy production rate. An uncertainty analysis is performed to estimate measurement uncertainties with the new experimental technique. The uncertainties are decomposed into systematic and random components, including a propagated uncertainty, due to spatial differencing of the velocity field. Close comparisons between measured results of turbulence dissipation and direct numerical simulations provide useful verification of the formulation, before post-processed results of dissipation rates are used to determine entropy production within a channel.     

Use of simultaneous IR temperature measurements and DPIV to investigate thermal plumes in a thick layer cooled from above

 Simultaneous measurements of surface temperature and the underlying velocity field are presented for a thick horizontal layer evaporatively cooled from above. Previous studies have focused on either the temperature field at the cooled surface or, in a small number of cases, on point velocity measurements in the flow. The current investigation is, to the knowledge of the authors, the first to simultaneously and non-intrusively document both the surface temperature and underlying velocity field in this type of flow. An infrared (IR) sensing array was used to capture the instantaneous free surface temperature field while two-dimensional velocity measurements in planes either perpendicular or parallel to the free surface were acquired using digital particle image velocimetry (DPIV). Data from two cases are discussed. Received: 19 January 1998/Accepted: 22 October 1998     

Non-intrusive temperature measurement using microscale visualization techniques

μPIV is a widely accepted tool for making accurate measurements in microscale flows. The particles that are used to seed the flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion introduces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the literature (Hohreiter et al. in Meas Sci Technol 13(7):1072–1078, 2002), making the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature of a moving fluid. A standard epi-fluorescence μPIV system was used for all the measurements. The experiments were conducted using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were measured. This method allows simultaneous non-intrusive temperature and velocity measurements in integrated cooling systems and lab-on-a-chip devices.     

Visualization and two-color DPIV measurements of flows in circular and square coaxial nozzles

 High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle image velocimetry (DPIV) and thermal anemometry measurements in flows generated by equivalent coaxial circular and square jets are presented. Visualization results were obtained for three square, coaxial configurations, and a reference circular coaxial nozzle, at two Reynolds numbers of the outer jet (19,000 and 29,000) and for inner-to-outer jet velocity ratios of 0.15, 0.22, and 0.3. These indicated that the internal unmixed region diminished with decreasing velocity ratio. Strong evidence of unsteady recirculation and back-flow was observed at the end of the core of the inner jet, for the low velocity ratios. Comparisons between circular and square jet configurations indicated considerable mixing enhancement when square nozzles were used. Low-coherence, organized large-scale structure was evident from the visualizations and DPIV measurements near the origin of the inner mixing-region shear layers, and more so in the core region of the near field. These observations were confirmed by velocity spectra, which displayed peaks corresponding to a free shear-layer instability mode in the inner mixing-region shear layers, and a wake-type mode in the core region where the mean flow has a wake-like character. Although some large-scale structure was observed in the outer mixing layer during the visualizations, this was found to be incoherent on the basis of the DPIV measurements and the velocity spectra. It is noted that no axis-switching phenomena were observed in the square nozzle flows examined here. This is attributed to the absence of an organized structure in the outer shear layer, which was initially highly turbulent, and the weakly coherent nature of the organized structure observed in the inner mixing-region near field. Received: 2 November 1998/Accepted: 8 September 2000     

Digital particle velocimetry technique for free-surface boundary layer measurements: Application to vortex pair interactions

A variation of the digital particle image velocimetry (DPIV) technique was developed for the measurement of velocity at a free surface for low Froude number flows. The two-step process involves first determining the location of the free surface in the digital images of the seeded flow using the fast Fourier transform-based method of surface elevation mapping (SEM), which takes advantage of total internal reflection at the interface. The boundary-fitted DPIV code positions the interrogation windows below the computed location of the interface to allow for extrapolation of interfacial velocities. This technique was designed specifically to handle large surface-parallel vorticity which can occur when the Reynolds number is large and surface-active materials are present. The SEM technique was verified on capillary-gravity waves and the full boundary-fitted DPIV technique was applied to the interaction of vortex pairs with a free surface covered by an insoluble monolayer. The local rise and fall of the free surface as well as the passage and return of a contamination front was clearly observed in the DPIV data. Received: 20 June 1999/Accepted: 27 November 2000     

A hybrid digital particle tracking velocimetry technique

A novel approach to digital particle tracking velocimetry (DPTV) based on cross-correlation digital particle image velocimetry (DPIV) is presented that eliminates the need to interpolate the randomly located velocity vectors (typical of tracking techniques) and results in significantly improved resolution and accuracy. In particular, this approach allows for the direct measurement of mean squared fluctuating gradients, and thus several important components of the turbulent dissipation. The effect of various parameters (seeding density, particle diameter, dynamic range, out-of-plane motion, and gradient strength) on accuracy for both DPTV and DPIV are investigated using a Monte Carlo simulation and optimal values are reported. Validation results are presented from the comparison of measurements by the DPTV technique in a turbulent flat plate boundary layer to laser Doppler anemometer (LDA) measurements in the same flow as well as direct numerical simulation (DNS) data. The DPIV analysis of the images used for the DPTV validation is included for comparison. Received: 29 August 1994/Accepted: 31 May 1996     

Accuracy limits for planar measurements of flow field velocity, temperature and pressure using Filtered Rayleigh Scattering

 We present experimental results using Filtered Rayleigh Scattering to make planar measurements of velocity, temperature and pressure in ambient air and in a Mach 2 free jet. The ambient air measurements are used to identify and calibrate experimental uncertainties. The Mach 2 measurements demonstrate the usefulness of the technique for making accurate planar measurements in a high speed flow. The measured values for velocity, temperature and pressure in the Mach 2 jet ranged, through a shock system, from 205 to 235 m/s, 150 to 170 K and 700 to 1000 torr, with estimated uncertainties of ±5.4 m/s, ±3.2 K and ±38 torr (±2 to 3%, ±2% and ±4–5%, respectively). Received: 10 December 1996/Accepted: 14 July 1997     

Tracking of coherent thermal structures on a heated wall by means of infrared thermography

This paper deals with measurements of convective velocity of large-scale thermal structures, using the thin foil technique and infrared thermography to visualize the thermal pattern on the wall. An image correlation method is proposed to track the displacement of the observed thermal pattern. The idea of the method is similar to that of particle image velocimetry, but the thermal patterns on the heated wall are used, rather than tracing particles. On this basis, the thermal patterns created by the coherent structures of turbulent channel flow are examined. Particular attention is paid to the determination of the optimal parameters of image acquisition, including spatial and temporal separation. An attempt is made to relate momentum and scalar transport analyses by considering the propagation velocity of large-scale temperature structures. The proposed technique appears to be an attractive alternative for non-intrusive analysis of turbulent flow, especially, where opaqueness of channel walls excludes the use of optical methods. Received: 18 January 2000/Accepted: 20 May 2000     

Experimental study of heat flux in mixed convective flow over solid waves

In this experimental study, we address transport processes in a mixed convective flow over a heated wavy surface. Therefore, we combine digital particle image velocimetry (DPIV) and two-color planar laser induced fluorescence (PLIF) to simultaneously measure the velocity and temperature field. For this, we propose to use the dye combination Rhodamine B and Rhodamine 110, both excited with the Nd:YAG laser also used for the PIV measurements. We investigate the influence of mixed convection over a wavy surface on the velocity field, turbulence statistics, the temperature field and the heat flux. By computing these quantities we find a correlation between the maximum in the Reynolds stress profiles and the components of the heat flux vector, thus regions of maximum momentum and scalar transport coincide. In addition, we apply a proper orthogonal decomposition (POD) to extract the most dominant flow structures in a measurement plane above the wavy surface. This first POD mode is identified as streamwise-oriented, counter-rotating vortices whose spanwise scaling is also correlated with the maximum of heat flux.     

Measurements of the gas-particle convective heat transfer coefficient in a packed bed for high-temperature energy storage

Experimental measurements of the forced convection gas-particle heat transfer coefficient in a packed bed, high-temperature, thermal energy storage system were performed using a custom-made experimental facility. Special attention was paid to the application of uncertainty analysis (a very important concept in experimentation). General and detailed uncertainty analyses were carried out, which identified the choices that were made in the experimental planning and procedure to ensure reliable final results. The experimental data reduction program used the governing equations and the results of the uncertainty analysis while making allowance for media property variations with temperature. Results were correlated in terms of Nusselt number, Prandtl number and Reynolds number and comparisons were made with existing correlations developed with similar storage media. The maximum temperature for the bed was about 1000°C (1830°F) with flue gas as the operating fluid in the storage mode and atmospheric air in the recovery mode. Because most related previous studies were not necessarily focused on high-temperature applications, the published gas-particle heat transfer correlations were obtained at relatively low temperature ranges, generally at room temperature or at temperatures slightly above room temperature. Moreover, only a few of the previously reported correlations associated the results with the corresponding uncertainty margins. The results from this study give a convective gas-particle heat transfer correlation for high-temperature thermal energy storage applications. Also, due to substantial uncertainties normally associated with the measurements of this heat transfer coefficient, it is significant to note that no firm conclusions can be reached on the validity or non-validity of previously reported related correlations for which the uncertainty margins were not reported.     

Temperature sensing with thermochromic liquid crystals

A review of the most recent developments in the application of thermochromic liquid crystals to fluid flow temperature measurement is presented. The experimental aspects including application, illumination, recording, and calibration of liquid crystals on solid surfaces, as well as in fluid suspensions, are discussed. Because of the anisotropic optical properties of liquid crystals, on-axis lighting/viewing arrangements, combined with in-situ calibration techniques, generally provide the most accurate temperature assessments. However, where on-axis viewing is not possible, calibration techniques can be employed, which reduce the uncertainty associated with off-axis viewing and lighting arrangements. It has been determined that the use of hue definitions that display a linear trend across the color spectrum yield the most accurate correlation with temperature. The uncertainty of both wide-band and narrow-band thermochromic liquid crystal calibration techniques can be increased due to hysteresis effects, which occur when the temperature of the liquid crystals exceeds their maximum activation temperature. Although liquid crystals are commonly used to provide time-mean temperature measurements, techniques are available which allow the monitoring of temporal changes. Selected examples illustrating the use of thermochromic liquid crystals are shown, and a survey of reported temperature measurement uncertainties is presented. Received: 3 February 1999/Accepted: 30 March 2000     

A comparative analysis of the uncertainty of astigmatism-μPTV,stereo-μPIV,and μPIV

Astigmatism or wavefront deformation, microscopic particle tracking velocimetry (A-μPTV) (Chen et al. in Exp Fluids 47:849–863, 2009; Cierpka et al. in Meas Sci Technol 21:045401, 2010b) is a method to determine the complete 3D3C velocity field in micro-fluidic devices with a single camera. By using an intrinsic calibration procedure that enables a robust and precise calibration on the basis of the measured data itself (Cierpka et al. in Meas Sci Technol 22:015401, doi:, 2011), accurate results without errors due to spatial averaging or bias due to the depth of correlation can be obtained. This method takes all image aberrations into account, allows for the use of the whole CCD sensor, and is easy to apply without expert knowledge. In this paper, a comparative study is presented to assess the uncertainties of two state-of-the-art methods for 3C3D velocity field measurements in microscopic flows: stereoscopic micro-particle image velocimetry (S-μPIV) and astigmatism micro-particle tracking velocimetry (A-μPTV). First, the main parameters affecting all methods’ measurement uncertainty are identified, described, and quantified. Second, the test case of the flow over a backward-facing step is analyzed using all methods. For comparison, standard 2D2C μPIV measurements and numerical flow simulations are shown as well. Advantages and disadvantages of both methods are discussed.     

Influence of the optical configuration on temperature measurements with fluid-dispersed TLCs

An accurate temperature calibration of fluid-dispersed thermochromic liquid crystal (TLC) particles is an important prerequisite for quantitative liquid crystal thermometry (LCT) measurements in flows. Encapsulated TLCs are subjected to uniform and linear temperature fields and are illuminated with a sheet of white light. A digital camera records the color distribution reflected by the particles. For the first time, a telecentric objective is used to eliminate the angular dependence of the color within the image plane. The paper systematically assesses how the temperature calibration is affected by the angle between the camera axis and the light-sheet plane, and by the properties of the working fluid. The obtained results provide design criteria for quantitative LCT measurements in situations where small spatial variations of the fluid temperature need to be resolved, namely for turbulent heat transfer problems in wall-bounded flows. Received: 22 January 2001/Accepted: 16 October 2001     

High-accuracy particle recognition using an extended phase-Doppler anemometer

Simultaneous measurements of diameter, velocity and concentration of particles are carried out in a flow with two disperse phases in a liquid. For high-accuracy particle recognition by different light-scattering mechanisms, an extended phase-Doppler anemometer utilizing the sign of the signal phase shift is used. The possibility of distinguishing between two disperse phases is verified with water as the continuous phase, and air bubbles and glass particles as the disperse phases in a specially developed three-phase flow channel. The technique is demonstrated for three-phase flows with different loadings of bubbles and glass beads. Received: 9 May 2000/Accepted: 14 August 2000     

Simultaneous velocity and temperature measurements in a premixed flame

Procedures which allow the correlation of velocity signals from a laser anemometer and temperature signals from a compensated, small-diameter thermocouple are described together with the error sources associated with the use of the technique in premixed flames. The digital compensation procedure includes the effect of velocity and temperature on the time constant of the thermocouple and the influence of its exposure to the solid particles required by the laser anemometer are quantified and shown to be able to cause large differences in the measured probability-density-distribution of the reaction progress variable. The technique has been used to measure the probability-density-distribution of temperatures, conditioned by the arrival of velocity signals and velocity conditioned by the temperature signal and sample results are presented to help quantify the accuracy of the measurements.     

In situ calibration of hot wires close to highly heat-conducting walls

A calibration procedure has been derived that permits reliable hot-wire measurements close to walls. When hot wires are calibrated in a free flow and subsequently used for near-wall velocity measurements, erroneous velocity information results because of additional heat losses to the wall. On the other hand, laser-Doppler anemometry (LDA) measurements of local time mean velocities are very little affected by the presence of the wall and this readily suggests in situ calibration of hot wires located just behind the LDA measuring volume and at the same distance from the wall. Calibrations of this kind are described for highly heat-conducting walls and the results show good agreement with corresponding data obtained through numerical investigations. The present investigations permit a generally applicable correction curve to be suggested for hot-wire velocity measurements close to walls of high thermal conductivity. Received: 3 May 2000/Accepted: 24 November 2000     

On the uncertainty of CFD in sail aerodynamics

A verification and validation procedure for yacht sail aerodynamics is presented. Guidelines and an example of application are provided. The grid uncertainty for the aerodynamic lift, drag and pressure distributions for the sails is computed. The pressures are validated against experimental measurements, showing that the validation procedure may allow the identification of modelling errors. Lift, drag and L₂ norm of the pressures were computed with uncertainties of the order of 1%. Convergence uncertainty and round‐off uncertainty are several orders of magnitude smaller than the grid uncertainty. The uncertainty due to the dimension of the computational domain is computed for a flat plate at incidence and is found to be significant compared with the other uncertainties. Finally, it is shown how the probability that the ranking between different geometries is correct can be estimated knowing the uncertainty in the computation of the value used to rank. Copyright © 2013 John Wiley & Sons, Ltd.     

Ultrasonic Doppler velocimetry in liquid gallium

For the first time, flow velocity is measured in a vortex of liquid gallium, using the pulsed Doppler shift ultrasonic method. At the top of a copper cylinder filled with liquid gallium, we spin a disk and create a turbulent vortex with a dominant nearly axisymmetric velocity field with little variation in the axial direction. The velocity profiles are shown to be well resolved and in quantitative agreement with earlier observations. Reliable velocity measurements in liquid gallium could be obtained only after serious problems due to the formation of oxides were solved. This work opens the way to performing accurate velocity measurements in other liquid metals; preliminary results for liquid sodium are shown. Received: 14 January 2000 / Accepted: 12 January 2001