Results for a Two-Component Doppler Global Velocimeter

Abstract

A two-component Doppler global velocimeter (DGV) system is described, and velocity measurements obtained to quantify its accuracy are presented. Molecular iodine vapor cells are used as frequency discriminating lters to determine the Doppler shift of laser light that is scattered off of seed particles in a ow, from which velocity is determined. Results are presented for velocity distributions over the surface of a rotating wheel, a fully developed pipe ow, and a freejet. For the rotating wheel DGV results, rms noise levels display standard deviations of § § 1 m/s, whereas total velocity range errors are between § § 1 and 2 m/s out of 59 m/s. The rms error is dominated by residual errors in the at-eld correction, and the 8-bit camera resolution, whereas velocity range errors are largely in uenced by iodine cell calibration accuracy. Measurements for fully developed turbulent pipe ow and axisymmetric jet ow show good agreement with pitot-static probe data. A random velocity offset error due to iodine cell side arm temperature variations of about 4-5 m/s has been found to be the dominant error source in the present DGV system. A reference tab has been used to record the zero velocity signals in the pipe and jet ows. IntroductionT HE accuracy of Doppler global velocimetry (DGV), a nonintrusive, planar imaging, Doppler-basedvelocimetry technique, is studiedin the currentwork. DGV uses an iodinevapor cell absorption line lter (ALF) to determine Doppler shift, and hence velocity, of small seed particles in a ow eld, as these particles pass through a two-dimensionalsheet of laser light. The same portion of the light sheet is viewed through a beam splitter by a pair of video cameras, with the ALF placed in the optical path of one of the cameras. Laser wavelength and ALF absorption band are matched such that the range of ow velocities of interest yields Doppler shifted frequencies that lie in the linear portion of the absorption band of the ALF. As a result, the ratio of the light intensities seen by the two detectorsat a point in the ow yields a signal proportionalto the particle velocity. Use of conventional charged-coupled device (CCD) cameras to view the light sheet yields velocity data at an effective resolution of at least 100 pixels by 100 lines, at framing rates of up to 30 frames/s, at a typical velocity accuracy of on the order of about 5% of full scale. This reduced accuracy relative to conventional laser velocimetry (LV) is primarily due to camera noise and pixel registration errors, as well as laser speckle noise for systems using pulsed Nd:YAG lasers. 1 , 2 A two-component point Doppler velocimeter (PDV) system has been previously developed by Kuhlman et al. 3 A two-component, planar imaging DGV system using CCD cameras has also recently been developed,as initially describedby Naylor and Kuhlman. 4 The accuracy limits of both systems are being systematically explored, through a series of measurementsin relativelysimple ows. A rotating wheel is also being used as a velocitystandard.The present paper describest...

Characteristics of the structure of the phase inhomogeneity and of nonequilibrium of the active medium beyond a three-dimensional expansion nozzle array of a gasdynamic laser

“…Planar Doppler velocimetry (PDV)-also referred to as Doppler global velocimetry (DGV)-has subsequently been refined by several research groups. Systems based on a pulsed laser have been developed by Meyers and Komine [2], Elliott et al [3], Beutner et al [4] and McKenzie [5], while systems designed around a CW laser have been developed by Naylor and Kulhman [6] and Roehle et al [7]. In spite of the apparent differences, these systems share the fundamental concept introduced by Komine of employing a molecular/atomic absorption filter to determine the frequency of the scattered laser light.…”

The application of laser measurement techniques to aerospace flows

“…This type of system has been applied by Naylor and Kuhlman [177] to record axial and radial velocity data from a turbulent-free jet. This two-view arrangement overcomes the standard three-dimensional DGV requirement for three orthogonal views or three light sheet directions.…”