Douglas Neal

Collaborative framework for PIV uncertainty quantification: comparative assessment of methods

A posteriori uncertainty quantification of particle image velocimetry (PIV) data is essential to obtain accurate estimates of the uncertainty associated with a given experiment. This is particularly relevant when measurements are used to validate computational models or in design and decision processes. In spite of the importance of the subject, the first PIV uncertainty quantification (PIV-UQ) methods have been developed only in the last three years. The present work is a comparative assessment of four approaches recently proposed in the literature: the uncertainty surface method (Timmins et al 2012), the particle disparity approach (Sciacchitano et al 2013), the peak ratio criterion (Charonko and Vlachos 2013) and the correlation statistics method (Wieneke 2015). The analysis is based upon experiments conducted for this specific purpose, where several measurement techniques are employed simultaneously. The performances of the above approaches are surveyed across different measurement conditions and flow regimes.

Collaborative framework for PIV uncertainty quantification: the experimental database

The uncertainty quantification of particle image velocimetry (PIV) measurements has recently become a topic of great interest as shown by the recent appearance of several different methods within the past few years. These approaches have different working principles, merits and limitations, which have been speculated upon in subsequent studies. This paper reports a unique experiment that has been performed specifically to test the efficacy of PIV uncertainty methods. The case of a rectangular jet, as previously studied by Timmins et al (2012) and Wilson and Smith (2013b), is used. The novel aspect of the experiment is simultaneous velocity measurements using two different time-resolved PIV systems and a hot-wire anemometry (HWA) system. The first PIV system, called the PIV measurement system (?PIV-MS?), is intended for nominal measurements of which the uncertainty is to be evaluated. It is based on a single camera and features a dynamic velocity range (DVR) representative of typical PIV experiments. The second PIV system, called the ?PIV-HDR? (high dynamic range) system, features a significantly higher DVR obtained with a higher digital imaging resolution. The hot-wire is placed in close proximity to the PIV measurement domain. The three measurement systems were carefully set to simultaneously measure the flow velocity at the same time and location. The comparison between the PIV-HDR system and the HWA provides an estimate of the measurement precision of the reference velocity for evaluation of the instantaneous error in the measurement system. The discrepancy between the PIV-MS and the reference data provides the measurement error, which is later used to assess the different uncertainty quantification methods proposed in the literature. A detailed comparison of the uncertainty estimation methods based on the present datasets is presented in a second paper from Sciacchitano et al (2015). Furthermore, this database offers the potential to be used for comparison of the measurement accuracy of existing or newly developed PIV interrogation algorithms. The database is publicly available on the website www.piv.de/uncertainty.

Hot-Wire Measurements Around a Controlled Diffusion Airfoil in an Open-Jet Anechoic Wind Tunnel

The aeroacoustic measurements in the large anechoic wind tunnel of Ecole Centrale de Lyon, which previously focused on the wall pressure distribution and the far-field acoustic pressure, are extended to velocity measurements by hot-wire probes in the nozzle jet and in the vicinity of a Controlled Diffusion airfoil. The present work focuses on the flow conditions corresponding to a Reynolds number based on the airfoil chord length Re c = 1.6×10 5 and a geometric angle of attack α g of 8°. Midspan measurements were achieved at the exit plane of the wind tunnel nozzle upstream of the test airfoil and in a large eddy simulation domain that was embedded in the potential core around the airfoil mockup. The inlet measurements by a single hot-wire probe provided insight into the free-stream turbulence intensity upstream of the profile. The X-probe measurements on the upper and lower computational boundaries show the overall deflection of the jet potential core by the cambered airfoil. These are compared to previous Reynolds averaged Navier-Stokes predictions. The X-probe measurements in the airfoil wake provide information on the development of the airfoil boundary layer and the resulting wake after separation

Peak-locking reduction for particle image velocimetry

A parametric study of the factors contributing to peak-locking, a known bias error source in particle image velocimetry (PIV), is conducted using synthetic data that are processed with a state-of-the-art PIV algorithm. The investigated parameters include: particle image diameter, image interpolation techniques, the effect of asymmetric versus symmetric window deformation, number of passes and the interrogation window size. Some of these parameters are found to have a profound effect on the magnitude of the peak-locking error. The effects for specific PIV cameras are also studied experimentally using a precision turntable to generate a known rotating velocity field. Image time series recorded using this experiment show a linear range of pixel and sub-pixel shifts ranging from 0 to ±4 pixels. Deviations in the constant vorticity field (ω z ) reveal how peak-locking can be affected systematically both by varying parameters of the detection system such as the focal distance and f-number, and also by varying the settings of the PIV analysis. A new a priori technique for reducing the bias errors associated with peak-locking in PIV is introduced using an optical diffuser to avoid undersampled particle images during the recording of the raw images. This technique is evaluated against other a priori approaches using experimental data and is shown to perform favorably. Finally, a new a posteriori anti peak-locking filter (APLF) is developed and investigated, which shows promising results for both synthetic data and real measurements for very small particle image sizes.

Hot Wire Measurements Around a CD Airfoil in an Open-Jet Anechoic Wind Tunnel

The aeroacoustic measurements in the ECL large anechoic wind tunnel, which originally focused on the wall pressure distribution and the far field acoustic pressure are extended to velocity measurements by hot-wire probes in the nozzle jet and in the vicinity of the subject airfoil. The present work focuses on the exit plane of the wind tunnel nozzle upstream of the test airfoil and in the LES computational domain that is embedded in the potential core around the airfoil mock-up at mid span. The inlet measurements by a single hot-wire probe provide some insight on free-stream turbulence intensity upstream of the profile. The X-array measurements on the upper and lower LES boundaries show the overall deflection of the jet potential core by the cambered airfoil. These are compared to the Fluent RANS predictions. The X-array measurements in the airfoil wake provide information on the development of the airfoil boundary layer and the resulting wake after separation. The measured wake velocity defect has been compared with both the RANS and LES predictions.Copyright