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computer. That is, one could do this whole thing on the computer and immediately process the data so it would be a matter of fractions of a second after you have taken the number of scans desired that the size distribution would appear.

SINNOTT: Another technique very similar to this is to set up an interference pattern in the scattering region and then watch the particle scattered light fluctuations.

that.

CHABAY: Right. I think Dr. Yanta is going to be talking about

SINNOTT: techniques?

Do you have any remarks on the relationship of these two

CHABAY: Maybe it's appropriate to wait until Dr. Yanta's talk. I think generally the techniques are very similar. I know of no other study which has put together both the intensity and the falling velocity information to get the size distribution. You can certainly get the velocity of the droplet by the laser Doppler velocimeter measurements. The basic physics of the technique is the same. In our case, we do not image a real fringe field in space and look at the particle moving through the light and dark regions to determine the rate at which it goes through the fringes. The process of heterodyning gives the same information on velocity, though.

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412
Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements
Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

MEASUREMENTS OF AEROSOL SIZE DISTRIBUTIONS WITH A
LASER DOPPLER VELOCIMETER (LDV)*

William J. Yanta

Naval Ordnance Laboratory
Silver Spring, MD 20910

ABSTRACT

A miniature wind tunnel has been built which together with the Laser Doppler Velocimeter (LDV) has been used to determine aerosol size distributions. In principle the LDV was used to measure the particle lag of individual aerosol particles as they were accelerated through a small supersonic nozzle. The measured velocity lag was then used in conjunction with numerical predictions to determine the particle size. An optical owl was used to determine the mean of the size distributions. The LDV measurements were in good agreement with the owl measurements. Key words: aerosol sizing; aerosol spectrometer; aerosol sprays; Doppler measurements of particle size; droplet sizing; interferometer; laser light scattering by aerosols; particle size measurements; particle velocity measurements.

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INTRODUCTION

The Laser Doppler Velocimeter has been used for many years in measuring flow velocities. The primary requirement for the LDV is that micrometer size particles be present in the flow. The LDV then measures the velocity of these particles. If one assumes the particles are moving at the same velocity as the fluid (either gas or liquid), then the fluid velocity is inferred directly from the particle velocity. However, in air, particles greater than one micrometer may not follow the flow precisely, that is, the particle may lag the flow. the flow. This is especially true in supersonic flow [3]. It is demonstrated in reference 3 that the particle lag can be predicted if the particle size is known. Conversely, one can predict particle size if the particle lag is known. This particle lag can be determined from velocimeter results (particle speed) in a calibrated or known flow field from which the particle size can be deduced. The particle lag can be induced by placing a particle rapidly accelerating or decelerating flow field. A convenient method of generating this type of flow field is with supersonic nozzle. Using the computational procedure described in reference 3, one can readily predict a particle's velocity in a supersonic nozzle.

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*This paper is a unification of references [1] and [2].

THEORY

A particle placed in an airstream will experience a force

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ticle;

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where p is the gas density; A, the cross-sectional area of the parthe velocity of the particle; Uq, the velocity of the airand Cp the drag coefficient. If we assume: one-dimensional a spherical particle and ignore potential force fields, then the force is given by the mass times the acceleration

stream; motion,

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Thus, one can completely predict the particle behavior in a flow field by simply integrating the above ordinary differential equation.

ease

The

of integration will depend on the complexity of the drag

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the drag

then a closed-form solution may be obtained. However, in order to take into account the effects of Mach number and Knudsen number, the coefficient given by the following equations [4] was used:

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Where CD,C, CD, FM, A and n are given functions of M in table 1. The use of the above drag equation in eq (3) necessitates a numerical integration of eq (3). (3). Thus, given the flow distribution through a particular nozzle configuration, one can calculate the particle's velocity along the nozzle. This can be done for a range of particle

sizes.

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EXPERIMENT

In order determine the size distributions of aerosols an LDV electronic system was used in conjunction with a Mach 5 wind tunnel.

The optical arrangement used in this study is shown schematically in figure 1. This system is commonly referred to as the dual scatter or fringe system. The laser beam is split into two beams of equal intensity and then focused to a common volume (the measurement volume). At the location where the two beams intersect constructive and destructive interference takes place thus forming about 50 to 100 fringes (fig. 2). As particles pass through these fringes, the particles will alternately scatter and not scatter light depending on

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