Anthony E. Smart

Measurement of thin liquid films by a fluorescence technique

Abstract A method is described for measuring the thickness of surface films of substances which are fluorescent, either naturally or by doping. Preliminary results are presented for oil film behaviour on a spinning cylinder. These observations have important implications for oil starvation in roller bearings. The range of thickness which can be measured depends on the optical absorption characteristics and degree of fluorescence of the film. For a light lubricating oil it is typically 0.1 μm to 1 mm. The technique needs optical access with small aperture from one side only and has a fast response with good spatial resolution. Contact with the surface film is not required.

Multiple-scattering suppression by cross correlation

We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.

Gradient-driven fluctuations experiment: fluid fluctuations in microgravity

We describe an experimental breadboard developed for the investigation of nonequilibrium fluctuations induced by macroscopic temperature and concentration gradients under microgravity conditions. Under these conditions the amplitude of the fluctuations diverges strongly for long wavelengths. The setup was developed at the University of Milan and at the University of California at Santa Barbara within the gradient-driven fluctuations experiment (GRADFLEX) project of the European Space Agency, in collaboration with the National Aeronautics and Space Administration. The apparatus uses a quantitative shadowgraph technique for characterization of the static power spectrum of the fluctuations S(q) and the measurement of their dynamics. We present preliminary experimental results for S(q) obtained in the presence of gravity for gradient-driven fluctuations for two cases, those induced in a liquid mixture with a concentration gradient produced by the Soret effect and those induced in a single-component fluid by a temperature gradient.

Physics of Hard Spheres Experiment: a general-purpose light-scattering instrument

A general-purpose, multifunction light-scattering instrument has been developed at the NASA Lewis Research Center for Space Shuttle and Space Station colloid crystallization and other microgravity experiments. For a single sample, the instrument can measure two-dimensional Bragg scattering from 0.5 degrees to 60 degrees , dynamic and static light scattering from 10 degrees to 170 degrees , the shear modulus of samples before and after crystallization, and digital color images of the sample. A carousel positions any one of eight 3-ml samples into the test position for separate experiments. Program challenges and flight results from the STS-83 Space Shuttle mission are discussed.

Aero-engine applications of laser anemometry

The measurement of flow inside an aero-engine or component rig is made difficult by the hot, high-pressure environment, problem of access, and the requirement not to disturb the flow. Laser anemometry offers an almost ideal solution to several of these problems and has been used under laboratory conditions for several years. However, because it is extremely difficult to add particles evenly to the very high mass flow involved, methods based on the naturally occurring particles in the flow have been developed and are described here. The information available from the light signal scattered from particles in a flow is examined and a real fringe system with fringe counting or photon correlation is chosen as the most useful processing method for laser anemometry in this situation. Measurements obtained from engine component rigs using a counting system are presented together with measurements from several aero-engines obtained using photon correlation. All results were obtained using a 15-mW HeNe laser.

Practical considerations in photon correlation experiments

Obtaining true and beautiful data from any photon correlation experiment demands serious attention to optimizing both the measuring system and experimental conditions. The laser must have sufficient power, be stable under all likely conditions, and usually be restricted to a single transverse mode. The beam-delivery optics must be carefully designed, built, and verified. The scattering medium must contain a proper concentration of suitably sized scatterers with appropriate characteristics. Surfaces surrounding the point of measurement must not introduce optical noise. Flare reduction in the receiver optics may be improved with ghost analysis, spatial and spectral filtering, and careful choice of stops, baffles, and surface coatings. The photon detector must have adequate speed and sensitivity with suitably low internal correlations and noise. The choice of correlator is crucial. Sometimes the equipment must be small to reach inaccessible places. Performance may be compromised by thermal, mechanical, or electrical instabilities caused by exposure to environmental excesses. Errors may even be introduced by preprocessing hardware and software before proper information is extracted. With so many conditions and potential problems, how does one obtain beautiful data, leading to correct results and enlightening information? That is the focus of our work.

Optical velocity sensor for air data applications

An optical velocity sensor, based on the sheet-pair transit-time technology, was designed, built to flightworthy standards, and test flown on an F-16B to 50,000 ft and Mach 1.2, as part of an optoavionic air data system. Brief descriptions of the work leading to 24 flights in January and February 1990 are given, with examples of data and discussions of experiences. Compared with conventional pneumatic sensors the system offers advantages that include potential improvements in accuracy, latency, calibration, dynamic range of speed and attitude, robustness, and possibly life, cost, and range of application, without modification of the vehicle skin contour. Measurements corresponding well with the aircraft system were obtained under all conditions except heavy cloud, which demands small design changes for future systems. The importance of modeling for software and hardware design optimization is stressed and measurements are presented.

Quantitative simulation of errors in correlation analysis

Bounding the errors of measurements derived from correlation functions of light scattered from some physical systems is typically complicated by the ill conditioning of the data inversion. Parameter values are estimated from fitting well-chosen models to measurements taken for long enough to look acceptable, or at least to yield convergence to some reasonable result. We show some simple numerical simulations that indicate the possibility of substantial and unanticipated errors even in comparatively simple experiments. We further show quantitative evidence for the effectiveness of a number of ad hoc aspects of the art of performing good light-scattering experiments and recovering useful measurements from them. Separating data-inversion properties from experimental inconsistencies may lead to a better understanding and better bounding of some errors, giving new ways to improve overall experimental accuracy.

Retrieval of flow statistics derived from laser anemometry by photon correlation

A method of retrieving velocity probability distributions from photon correlograms of laser anemometer signals using a fast Fourier transform is described. Mean velocity, turbulence and higher-order moments may be obtained with sufficient accuracy for most engineering applications.

Suppression of Multiple Scattering Using a Single Beam Cross-Correlation Method

We present a simple, single beam, laser light scattering technique which discriminates against multiple scattering in turbid media using cross-correlation of the scattered intensity at slightly different spatial positions. Experimental results obtained at a scattering angle of 90° for colloidal suspensions of various concentrations show that the technique yields information on particle diameter, even for samples which are visually opaque.