Walter R. Lempert

Bio: Walter R. Lempert is an academic researcher from Princeton University. The author has contributed to research in topics: Rayleigh scattering & Laser. The author has an hindex of 19, co-authored 65 publications receiving 1834 citations. Previous affiliations of Walter R. Lempert include Ohio State University.

Laser Rayleigh scattering

Abstract: Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.

Demonstration and characterization of filtered Rayleigh scattering for planar velocity measurements

Abstract: Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

Two-dimensional measurement of density, velocity, and temperature in turbulent high-speed air flows by UV Rayleigh scattering

Abstract: Instantaneous cross-sectional images of turbulent air flows with densities on the order of one atmosphere or less can be obtained in a straightforward manner using far ultraviolet Rayleigh scattering. These images give quantitative values for the air density and show the details of turbulent structure, shock structure, and shock wave/boundary layer interactions. Two-dimensional spatial correlations taken from multiple images give the shape and extent of average turbulent structure as well as the coupling between turbulent structure and other flow features. This technique may be extended to observe velocity fields by either double pulsing the illumination source or by using a narrow linewidth atomic or molecular filter window in front of the detector array. The latter approach also yields temperature. Used in conjunction with flow marking techniques such as RELIEF, coupling between turbulent structure and velocity fluctuations can also be determined. These diagnostic techniques can be extended to combusting flows to observe instantaneous structure, mixing, flame front location, and velocity fields.

Flow tagging velocimetry in incompressible flow using photo-activated nonintrusive tracking of molecular motion (PHANTOMM)

Abstract: We report the development of a new optical flow tagging velocimetry technique for hydrodynamic flows. The method utilizes highly water-soluble caged dye Photo-Activated Fluorophores (PAF's) which serve as fluorescent tracers, with essentially indefinite lifetime. Demonstration experiments are presented in a bench-top poiseulle flow and a 5,000 gallon water channel facility. Results of experiments designed to quantify critical optical characteristics of the caged dye PAF's are also presented, as is a comparison with other, similar, optical velocimetry approaches.

Filtered Rayleigh scattering measurements in supersonic/hypersonic facilities

Abstract: Preliminary measurements are presented of flow field properties in Mach 3 and Mach 5 flows using filtered Rayleigh scattering Filter properties have been characterized by high resolution spectroscopy in order to optimize the selection of laser frequency and filter operating conditions, as well as for the development of an accurate filter modeling program An optimized filter is used the background suppression feature of this technique to image the boundary layer structure in a Mach 3 high Reynolds number facility and the shock structure in a Mach 5 overexpanded jet This had been achieved using a visible laser source By frequency scanning the laser, time-averaged velocity measurements in the Mach 3 and Mach 5 flows are made Data acquisition at 10 torr and below indicates that this approach can be extrapolated for use in hypersonic flow facilities and is applicable as an in-flight optical air data device for hypersonic vehicles

Lagrangian Properties of Particles in Turbulence

Abstract: The Lagrangian description of turbulence is characterized by a unique conceptual simplicity and by an immediate connection with the physics of dispersion and mixing. In this article, we report some motivations behind the Lagrangian description of turbulence and focus on the statistical properties of particles when advected by fully developed turbulent flows. By means of a detailed comparison between experimental and numerical results, we review the physics of particle acceleration, Lagrangian velocity structure functions, and pairs and shapes evolution. Recent results for nonideal particles are discussed, providing an outlook on future directions.

Review of temperature measurement

Abstract: A variety of techniques are available enabling both invasive measurement, where the monitoring device is installed in the medium of interest, and noninvasive measurement where the monitoring system observes the medium of interest remotely. In this article we review the general techniques available, as well as specific instruments for particular applications. The issues of measurement criteria including accuracy, thermal disturbance and calibration are described. Based on the relative merits of different techniques, a guide for their selection is provided.