A. Lecuona

Data validation, false vectors correction and derived magnitudes calculation on PIV data

Due to the particular features that appear in the vector maps delivered by the PIV method, there are postprocessing steps that can substantially enhance its performance. These steps include: detection of false vectors, correction of these vectors and the calculation of derived flow magnitudes. Many derived magnitudes can be of interest but this work focuses on the calculation of the first spatial derivative, component of flow divergence or vorticity, on a two-dimensional flow configuration. New algorithms, developed for each step, are described, with the aim of jointly applying them in a coherent way. Where applicable, an analytical tool for filter analysis and design is explained. Application to synthetic and real PIV data is presented as well as a performance contrast with other conventional algorithms, in terms of accuracy, frequency response and error propagation, among others.

Local field correction PIV, implemented by means of simple algorithms, and multigrid versions

Local field correction particle image velocimetry (LFCPIV), which was first presented in 1997, is the only correlation PIV method able to resolve flow structures smaller than the interrogation window. It presents advantages over conventional systems and thus offers an alternative in the field of super-resolution methods. Improvements of the initial version are likely to promote its application even further. The issues defining some of these possible improvements were already indicated in the paper that originally introduced LFCPIV, but not developed. This work presents refinements and also simplifications of the technique, so that it can be applied using current algorithms of advanced correlation PIV systems. Furthermore, these refinements reduce the measurement error and enlarge the range of application of LFCPIV. In particular, the application of the system is no longer constrained to images with mean distances between particles larger than 4 pixels. Besides that, the use of interrogation windows smaller than in its previous version is evaluated. This allows multigrid LFCPIV implementations. The results show how multigrid LFCPIV can obtain better measurements than can the usual multigrid PIV, but still the refined version of the LFCPIV technique performs even better, at the expense of a larger computing time. The performance of these methods is evaluated for synthetic and real images. This includes examples in which the ability to cope with gradients in velocity, gradients in seeding density and the presence of boundaries is highlighted.

Volumetric characterization of dispersed two-phase flows by digital image analysis

A digital-based image analysis system has been developed for the experimental determination of size, spatial distribution and two components of velocity for particles suspended in any clear fluid flow. A volumetric measurement directly provides number density and consequently particle flow. A peculiarity of the developed system is its suitability for relatively large particles (some mm in diameter), which can hardly be measured with other systems. Double exposed particle images, in the same frame, provide the starting point for image processing. The system includes a procedure for the classification of the particles in the image, the separation of the partially overlapped particles and the selection of those that are found in the control volume. These parameters serve as a basis for the velocity determination through a particle-tracking algorithm (PTV), which is based on iteratively estimating the match probability as a measure of the likelihood of pairing. The depth of the control volume is obtained with a new technique, based on the loss of focus of the particles in the image, combined by diffuse illumination, provided by twin short pulse argon spark discharge lamps. The system has been extensively checked using calibrated micro-balls and sprays that simulate rain.

Compressors driven by thermal solar energy: entropy generated, exergy destroyed and exergetic efficiency

This work is devoted to the study of the entropy generated, the exergy destroyed and the exergetic efficiency of lithium-bromide absorption thermal compressors of single and double effect, driven by the heat supplied by a field of solar thermal collectors. Two different applications have been considered and compared: air-cooled and water-cooled units. Water-cooled compressors work with temperatures and pressures lower than air-cooled compressors considering, in both cases, the same suction temperature, equal to 5°C. While the absorption temperature in water-cooled compressors can reach 40°C, in air-cooled systems it can vary between 30°C and more than 50°C. Under these conditions, the discharge temperature (boiling temperature within the desorber) of a single effect air-cooled unit lies between 65 and 110°C, the maximum discharge pressure being around 0.12 bar. The discharge temperatures (boiling temperature within the high pressure desorber) of the double effect air-cooled thermal compressor lies between 110°C for a final absorption temperature of 30°C, and 180°C for a final absorption temperature of 50°C. Discharge pressures can reach values of 0.3 and 1.5 bar, respectively. The lithium-bromide air-cooled thermal compressors of double-effect can be viable with absorption temperatures around 50°C, when the temperature difference between the lithium-bromide solution and the outside air is about 8°C. The double effect thermal compressor generates less entropy and destroys less exergy than the single effect unit, leading to a higher exergetic efficiency. In both cases, the compression process of the cooling fluid occurs with entropy reduction.

Analysis and alternatives in two-dimensional multigrid particle image velocimetry methods: application of a dedicated weighting function and symmetric direct correlation

Multigrid particle image velocimetry (PIV) is an open path in the search for high-resolution PIV methods. It is based on an iterative scheme that uses the information of initial processing to adapt the method parameters in order to improve the measurements. This is mainly performed by reducing the size of the interrogation windows and shifting them. In multigrid PIV, two sources of error can significantly affect the final measurement quality: (1) the error coming from the amplitude response of the initial large interrogation windows to spatial frequencies; (2) the error originating from the truncation of particles at the borders of the final small interrogation windows. By applying weighting functions and using symmetric direct correlation both errors can be reduced, respectively. These techniques have been separately tested in the past, but a joint implementation has not yet been analysed. This task is fulfilled and both sources of error are further clarified. For this purpose, a one-dimensional single wavelength displacement field is used. This gives us the opportunity to analyse the non-linear behaviour of PIV, together with the influence of basic parameters on it. In addition to this, the multigrid method, so far described, is enhanced by compensation of the particle pattern deformation. The metrological performance of this advanced method is tested using synthetic images and the results are compared with those delivered by established PIV methods. Coherence between these results and those obtained in a real image is also detailed.

PIV evaluation algorithms for industrial applications

Particle image velocimetry (PIV) is now applied with confidence in industrial facilities such as large wind tunnels, yielding data not possible before. Despite the difficulties that arise in such environments, the conventional PIV methods can provide high quality data, especially when dealing with spatial and temporal slowly varying values of the flow magnitudes. Obtaining highly spatially resolved velocity fields is still challenging due to the inherent difficulties in these industrial facilities, such as large velocity gradients, background light and reflections. This work has focused on: (i) the behaviour of conventional PIV when the conventional limits of the processing algorithm are approached or surpassed and (ii) which kind of advanced methods can reduce the main sources of error that are characterized in this paper. The focus is on the description of vortex flows. Group locking, a major source of error, is introduced, modelled and metrologically characterized. The part of the study devoted to advanced methods deals with one that has already shown to be of profit in images from industrial facilities, local field correction PIV (LFC-PIV). This is a robust and an accurate method, able to obtain a high yield of measurements in these environments, without the need of external adjustment to each particular situation. To illustrate this point, some examples of processing of real images are given. The conclusions of this work suggest guidelines about the error figures when measuring flows with embedded vortex using conventional and advanced methods in industrial facilities.

Multiple Δt strategy for particle image velocimetry (PIV) error correction, applied to a hot propulsive jet

PIV (particle image velocimetry) is a measurement technique with growing application to the study of complex flows with relevance to industry. This work is focused on the assessment of some significant PIV measurement errors. In particular, procedures are proposed for estimating, and sometimes correcting, errors coming from the sensor geometry and performance, namely peak-locking and contemporary CCD camera read-out errors. Although the procedures are of general application to PIV, they are applied to a particular real case, giving an example of the methodology steps and the improvement in results that can be obtained. This real case corresponds to an ensemble of hot high-speed coaxial jets, representative of the civil transport aircraft propulsion system using turbofan engines. Errors of ~0.1 pixels displacements have been assessed. This means 10% of the measured magnitude at many points. These results allow the uncertainty interval associated with the measurement to be provided and, under some circumstances, the correction of some of the bias components of the errors. The detection of conditions where the peak-locking error has a period of 2 pixels instead of the classical 1 pixel has been made possible using these procedures. In addition to the increased worth of the measurement, the uncertainty assessment is of interest for the validation of CFD codes.

Simulation of Particle Trajectories in a Vortex-Induced Flow: Application to Seed-Dependent Flow Measurement Techniques

The trajectories of heavy particles (ρparticle/ρfluid>>1) are simulated in a two-dimensional free vortex flow. The results show that heavy particles, even with small diameters, cannot properly trace the fluid and develop a centrifugal motion. This behaviour leads to a rapid depletion of particles in the vortex core, which, in seed-dependent measurements such as particle image velocimetry (PIV) or laser Doppler velocimetry, produces a marked increase in measurement errors. Some examples are given and the evolution of the local concentration of particles is simulated. Synthetic images have been generated using the information about its motion in a two-dimensional free vortex flow. A wing-tip vortex is simulated, showing good agreement with experimental results present in the literature. Standard PIV measurements have been performed over the synthetic images, showing the effect of core depletion of particles on the incidence of erroneous measurements.

Analysis of the Vortex Street Generatedat the Core-Bypass Lip of a Jet-Engine Nozzle

The reduction of the noise generated by jet-engine aircrafts is of growing concern in the present society. A better understanding of the aircraft noise production and the development of predictive tools is of great interest. Within this framework, the CoJeN (Coaxial Jet Noise) European Project includes the measurement of the flow field and the noise generated by typical turbofan jet-engine nozzles. One of the many aspects of interest is the occasional presence of acoustic tones of a defined frequency, symptomatic of the presence of quasiperiodic coherent structures within the flow. This chapter analyzes the characteristics of a vortex street in the core-bypass lip of one of the nozzles under study. The measurements were made by means of advanced PIV techniques within the above-mentioned project.

Near field vortex dynamics in axially forced, laminar, co-flowing jets : a descriptive study of the flow configurations

Abstract An experimental study is presented of the vortex structures that appear in the shear layer of laminar, co-flowing air jets subjected to strong axial forcing. A set of flow visualisation experiments has been performed to elucidate the nature of the different structures and the mechanisms leading to their appearance and further interactions. The axial forcing sets the axisymmetric instability to prescribed values of amplitude and frequency (and thus wavelength) and produces a strong effect in the lateral spreading of the inner jet. It is shown that the near field development of the flow can be explained via inviscid vortex dynamics arguments, involving three vortex structures. Due to the strong axial forcing, all these vortices already appear as developed concentrations of vorticity in the surroundings of the nozzle exit. An azimuthal perturbation is added to the flow in the form of a lobed nozzle exit, in order to lock the azimuthal organisation of the vortices. The results are discussed and some representative configurations are examined. Each configuration appears for a given range of the forcing parameters. A tentative model of the near-field vortex dynamics is developed, but quantitative measurements are still necessary.