Nieves Andrés

Digital speckle pattern interferometry as a holographic velocimetry technique

In this paper digital speckle pattern interferometry (DSPI) as a digital image plane holography (DIPH) technique is presented and its potential for fluid velocimetry are discussed. The recording is carried out with a spatial phase shifting (SPS) DSPI set-up, which can also be viewed as an off-axis DIPH set-up. A theoretical study of both SPS–DSPI analysis using a Fourier transform method and DIPH analysis is presented for a set-up with only one illuminated plane. From the DIPH analysis, a way to extend the SPS–DSPI set-up to simultaneously record but independently reconstruct several fluid planes is inferred. Some preliminary results from a convective flow illustrate the feasibility of the quasi 3D recording.

DIGITAL SPECKLE-PATTERN INTERFEROMETRY AS A FULL-FIELD FLUID-VELOCIMETRY TECHNIQUE

We present a novel fluid-velocimetry technique based on speckle interferometry. The light scattered from an illuminated plane is recorded with a CCD camera at the same time as a speckled reference beam. Substraction of two nonsimultaneous frames provides information about the velocity field for an out-of-plane component. An application to a Rayleigh-Bénard convective flow is given.

Application of DSPI to detect inhomogeneous heating on superconducting ceramics

This paper presents the first application of digital speckle pattern interferometry (DSPI) to detect inhomogeneous heat generation on a superconducting ceramic at cryogenic temperatures. The light scattered by the object is recorded with a CCD camera at the same time as a smooth reference beam. Comparison of two non-simultaneous frames provides information about the out-of-plane deformation field. Spatial phase shifting is used in order to get a good quality fringe pattern. The technique has been applied as a non-destructive evaluation of the performance of ceramic high temperature superconducting materials. DSPI allows the determination of the point where a hot spot will be generated with heating levels that do not deteriorate the sample properties. An excellent agreement between DSPI hot spot location and the position of the melting point that appeared in a destructive experiment has been obtained.

Holographic interferometry versus stereoscopic PIV for measuring out-of-plane velocity fields in confined flows

This paper describes and compares holographic interferometry and stereoscopic PIV as techniques for measuring out-of-plane velocity fields. Most relevant aspects in both techniques are revised. Special attention is paid to the specific problems encountered in confined flows. The presence of walls limiting the flow prevents the use of some optical configurations for the photographic and holographic recordings. Several optical configurations have been analysed. Experiments have been carried out in a small Rayleigh-Benard convective cell. Results obtained for the out-of-plane component with both techniques are compared and discussed.

Velocity measurements in a convective flow by holographic interferometry

A holographic interferometry technique has been developed that can be used to measure the three components of the velocity field in a whole plane of a fluid flow simultaneously. The light scattered from an illuminated fluid plane is recorded on a hologram. Several interferograms are obtained in the reconstruction of the hologram. Each interferogram is automatically analyzed and produces quantitative information about one velocity component. Parameters that affect the quality of the interferograms are analyzed. The technique is demonstrated in a Rayleigh-Bénard convective flow. Holographic interferometry and particle image velocimetry techniques are compared.

Shifted knife-edge aperture digital in-line holography for fluid velocimetry

We describe a digital holography technique that, with the simplicity of an in-line configuration, produces holograms where the real and virtual images are completely separated, as in an off-axis configuration. An in-line setup, in which the object is imaged near the sensor, is modified by placing a shifted knife-edge aperture that blocks half the frequency spectrum at the focal plane of the imaging lens. This simple modification of the in-line holographic configuration allows discriminating the virtual and real images. As a fluid velocimetry technique, the use of this aperture removes the minimum defocusing distance requisite and reduces the out-of-plane velocity measurement errors of classical in-line holography. Results with different test objects are shown.

Microstructure origin of hot spots in textured laser zone melting Bi-2212 monoliths

Hot spots are one of the main limitations in the development of large-scale high-power applications with superconducting materials. The application of digital speckle interferometry to detect inhomogeneous heating on ceramic superconductors allows the determining of a hot spot location in these materials before any damage is caused to the material. The technique detects deformations that are induced in the material due to dilatation, attaining a resolution of 0.45 µm /fringe. In this paper this technique has been applied to analyse the heating generation in Bi-2212 superconducting monoliths at room temperature and in operation conditions. In the first case a homogeneous heating is obtained, leading to a parallel fringe pattern. In the second case, a situation with an inhomogeneous heating origin has been detected. Once the position of this hot spot is determined, microstructure studies have been performed to determine which defects are responsible for hot spot generation.

The development of full field interferometric methods for fluid velocimetry

Abstract The development of full field interferometry as a velocimetry technique in experimental fluid mechanics from its inception in 1977 to the present is discussed. In 1977, holographic interferometry (HI) was applied for the first time to measure the velocity field in a liquid flow. It was not until 1998 that the first application to a gaseous flow was reported. The only kind of speckle interferometry that has been applied to measure velocity fields so far is digital speckle pattern interferometry (DSPI). It was in 1999 that DSPI was demonstrated both in a liquid and a gaseous flow.

Fast quantitative processing of particle image velocimetry photographs by a whole-field filtering technique

A fast quantitative processing of particle image velocimetry photographs by a whole-field spatial filtering technique is described. Photographs are observed through a conventional filtering setup. This produces fringe patterned images with each fringe corresponding to a fixed value of one velocity component. These images are acquired with a CCD camera and digitally processed to retrieve the fringe centerline positions. The interpolation of these data provides the velocity value on a grid of regularly spaced points.Photographs taken from a Rayleigh-Bénard convective flow have been processed with this technique and with a previously reported point-by-point technique. Results from both techniques compare well.

Two-dimensional quantification of the corrosion process in metal surfaces using digital speckle pattern interferometry

The applicability of digital speckle pattern interferometry (DSPI) to the analysis of surface corrosion processes has been evaluated by studying the evolution of an Fe surface immersed in sulfuric acid. This work describes the analysis process required to obtain quantitative information about the corrosion process. It has been possible to evaluate the corrosion rate, and the results agree with those derived from the weight loss method. In addition, a two-dimensional analysis has been applied, showing that DSPI measurements can be used to extract information about the corrosion rate at any region of the surface.