Electrogasdynamic spectral anemometer

Description

A device which may complement the hot-wire anemometer and the laser velocimeter is the electrogasdynamic (EGD) spectral anemometer. The conception of this instrument and some preliminary measurements represent the body of this report. The EGD anemometer consists of an injector of unipolar charged particles fed into the flow to the probed, an equilibration region, and a collector for the charges. The particles are many times heavier than the molecules in the gas and they interact with the flow through drag forces. If these particles are tightly coupled to the flow, then the collecting probe receives information in electrical form about the fluid velocity. Besides being a direct velocity measurement, the probe can be of streamlined shape and very rugged, and it is insensitive to temperature differences in the flow. The charged particles can be chosen so as to give the desired range of frequency response. The instrument is sensitive to impurities in the flow but in principle the effect may be calibrated out. An EGD spectral anemometer has been tested using water droplets charged as they condense through a corona discharge. The anemometer consists of a short-circuited EGD generator; this is the first time that such an application has been reported and a Navy patent has been awarded for this device. The feasibility of the anemometer depends on the magnitude of the charged particle slip. The coupling between the flow field and the electric field depends on the frequency range of the velocity fluctuations as well as on the charge, size, and mass of the particles. The work treats the mass as a first order perturbation, in the particle momentum equation. The unsteady flow studied is a region in the wake of a cylinder where charged water droplets are injected and the observed frequency response of the anemometer tested. The EGD spectral anemometer data are compared to hot-wire data taken at the same location; some results compare within (+/_-) 2 dbs up to a frequency of 630 Hz whereas others are within (+/-) 6 dbs up to 20 KHz. The parameters which varied are the steam pressure and temperature, and the corona characteristics. Predictions of frequency response as a function of charged particle size are given. (Author)

Subjects