M. L. Riethmuller

Pattern recognition analysis of the turbulent flow past a backward facing step

A pattern recognition technique for the investigation of large-scale coherent structures, is applied to analyze the turbulentseparated flow over a backward facing step (BFS) at a Reynolds number Re h =5.0×10 3 . The instantaneous two-dimensional velocity distribution is obtained by means of digital particle imagevelocimetry (D-PIV) measurements. High spatial resolution (Δr/h=1/25) is achieved with the application of an iterative window refinement image processing algorithm. The measurement plane is oriented in order to investigate spanwise aligned vortices footprints. The detection algorithm is based on velocity pattern spatial cross correlation. An additional isotropy condition is imposed to improve the detection of vortices and shear layer. The structure of the shear layer emanating from the step edge is examined emphasizing the role of coherent fluctuations with a length scale d ranging from 0.12 h to 0.44 h. A characteristic statistical spatial occurrence is found for the educed spanwise-aligned rollers: a quasi-linear spreading region extends from x/h=0.8 up to x/h=3.5. Within the same region the production of turbulent kinetic energy exhibits a maximum. At smaller scale, the vortices show a significant presence of counter-rotating structures inside the free shear layer suggesting that the spanwise rollers undergo early three dimensional instability and breakdown within a few step units. Conditional data averaging is also applied to the results and structural properties (coherent velocity, vorticity and turbulence production) are highlighted: close to the step edge the coherent vorticity distribution is strongly distorted showing an intense interaction between the rollers and the shear layer. A roughly circular pattern is recovered downstream x/h=4.

Extension of PIV to super resolution using PTV

This work constitutes the study and the development of an integrated PIV-PTV method, which combines the advantages of the two approaches: the robustness of PIV and the spatial resolution of PTV. The technique is a hybrid algorithm that starts by statistical evaluation of a tracers displacement (PIV analysis) and further refines the resolution of measurement with the tracking of individual particles (PTV). This strategy is referred to as the super resolution analysis. This work presents and critically discusses the principles of this processing. Key points of the method are emphasized. This work focuses mainly on the particle extraction and on the particle tracking steps aiming at investigating monophasic flows. Efforts to face imaging conditions for highly seeded flows and to improve the robustness of the technique are made. Tests on real images are presented and discussed.

Nonintrusive measurements of temperature and size of single falling raindrops

A nonintrusive laser technique, based on the detection of a rainbow, is presented that permits one to determine simultaneously the temperature and size of droplets. Therefore the Airy theory for a rainbow and a calibration rainbow pattern at isothermal conditions are applied. Rainbow patterns coming from droplets in the millimeter range have been recorded on a linear CCD array. It has been found that the sphericity of the droplets plays an important role for this rainbow-based technique.

Rainbow phenomena applied to the measurement of droplet size and velocity and to the detection of nonsphericity

An experimental method is presented that detects whether a droplet is spherical. The method is based on a comparison between two droplet diameters deduced from two different optical interference patterns observed in a rainbow that is created by a droplet scattering laser light. Experimental validation has been carried out with a CCD camera. Once a rainbow pattern has been identified as coming from a spherical droplet, we can derive a reliable droplet velocity and diameter from the same interference patterns, using theories for the rainbow that are valid only for spherical droplets. Preliminary experiments have been carried out with a laser beam and a photomultiplier.

Global rainbow thermometry for droplet-temperature measurement

Standard rainbow thermometry connects the scattering angle of the main rainbow maximum, generated by a single droplet, to the droplets refractive index and thus to its temperature. Droplet nonsphericity influences the rainbow position and therefore degrades the quality of the droplet-temperature measurement. We propose global rainbow thermometry, which measures the average rainbow position that is created by multiple droplets and from which a mean temperature can be derived. The new technique aims at eliminating the nonsphericity effect. The principle of this method is presented, and a typical recorded image is discussed.

An adaptive sampling and windowing interrogation method in PIV

This study proposes a cross-correlation based PIV image interrogation algorithm that adapts the number of interrogation windows and their size to the image properties and to the flow conditions. The proposed methodology releases the constraint of uniform sampling rate (Cartesian mesh) and spatial resolution (uniform window size) commonly adopted in PIV interrogation. Especially in non-optimal experimental conditions where the flow seeding is inhomogeneous, this leads either to loss of robustness (too few particles per window) or measurement precision (too large or coarsely spaced interrogation windows). Two criteria are investigated, namely adaptation to the local signal content in the image and adaptation to local flow conditions. The implementation of the adaptive criteria within a recursive interrogation method is described. The location and size of the interrogation windows are locally adapted to the image signal (i.e., seeding density). Also the local window spacing (commonly set by the overlap factor) is put in relation with the spatial variation of the velocity field. The viability of the method is illustrated over two experimental cases where the limitation of a uniform interrogation approach appears clearly: a shock-wave–boundary layer interaction and an aircraft vortex wake. The examples show that the spatial sampling rate can be adapted to the actual flow features and that the interrogation window size can be arranged so as to follow the spatial distribution of seeding particle images and flow velocity fluctuations. In comparison with the uniform interrogation technique, the spatial resolution is locally enhanced while in poorly seeded regions the level of robustness of the analysis (signal-to-noise ratio) is kept almost constant.

Experimental investigation of the flow in the vaneless diffuser of a centrifugal pump by particle image displacement velocimetry

This work presents the application of particle image displacement velocimetry to the measurement of fluid velocities in a centrifugal pump diffuser. Measurements are taken at different operating points and allow to define the variation of radial and tangential velocity components along a pitch. They are further processed to determine the relative velocity and vorticity fields. Results are also compared with laser Doppler measurements taken in the same facility.

Global Rainbow Thermometry for Mean Temperature and Size Measurement of Spray Droplets

Global rainbow thermometry is a new technique for measuring the average size and temperature of spray droplets. For data inversion a global rainbow pattern is employed, which is formed by constructive interference of laser light scattered by an ensemble of spherical droplets. The non-spherical droplets and liquid ligaments provide a uniform background and hence do not influence the interference pattern from which average size and temperature are derived. This is a large improvement with respect to standard rainbow thermometry, investigated since 1988, which is strongly influenced by particle shape. Moreover, the technique is applicable to smaller droplets than the standard technique because the global pattern is not spoiled by a ripple structure. Data inversion schemes based on inflection points, minima and maxima are discussed with respect to spray dispersion and droplet flux. The temperature derivation from inflection points appears to be independent of spray dispersion. Preliminary measurements in a heated water spray are reported. The mean diameter obtained from the rainbow pattern is smaller than the arithmetic mean diameter measured by phase-Doppler anemometry. The accuracy of the temperature measurement by global rainbow thermometry is shown to be a few degrees Celsius.

Vortex ring evolution in an impulsively started jet using digital particle image velocimetry and continuous wavelet analysis

Digital particle image velocimetry and wavelet analysis are combined in this work in order to study the characteristics of the leading vortex ring generated in an impulsively starting jet flow. The wavelet analysis allows one, by virtue of its properties of selectivity in space and scale, to detect coherent structures and to compute their position and size and, through further processing, their convection velocity and circulation. It has been observed that the energy of the leading vortex ring continuously increases even after it has pinched off from the jet shear layer generating it.

Global rainbow thermometry: improvements in the data inversion algorithm and validation technique in liquid-liquid suspension

Improvements on the validation of a nonintrusive laser-based measurement technique are presented. This new technique, called global rainbow thermometry (GRT), is capable of determining the temperature and the size distributions of liquid droplets dispersed in a liquid or gaseous bulk. We propose a new data inversion algorithm that takes into account the whole rainbow pattern. Experimental validation of the GRT technique is performed for a liquid-liquid suspension. We performed the validation by comparing the measurements obtained with the GRT technique for the mean droplet temperature and size with the results obtained with alternative techniques.