Laser Doppler velocity bias in separated turbulent flows

Abstract

Abstract. Velocity bias effects on data obtained with a coincident two channel laser Doppler velocimeter in a highly turbulent separated supersonic flow are presented. Probability distributions of the fluctuating velocities were distorted by velocity bias in a manner consistent with theory and a two-dimensional velocity inverse weighting function bias correction produced reasonable appearing velocity probability distributions. The addition of an approximate correction term to account for the effects of the unmeasured third velocity component improved these results but had little effect on the velocity statistics. Experimental factors that could partially compensate or falsely add to the velocity bias, conditions for the bias to occur, and conditions for which the bias.may also be observed and corrected for are discussed. ! IntroductionThis study is part of a program investigating supersonic missile base region flows. Features of these base flows include separated regions of recirculating flow bounded by turbulent free shear layers and a recompression zone where the bounding shear layers merge and change direction. The experimental efforts of this investigation have been directed at the detailed study of simpler twodimensional model flows using laser Doppler velocimetry, Addy et al. (1983).These model experiment flowfields contain regions of high turbulence intensity and in some cases these high intensities occur where the flow is supersonic. Thus, an extreme dynamic range of velocities is possible and at the outset of experimentation it was expected that LDV velocity and fringe bias could have a substantial statistical effect on the results.An evaluation of the effect of these biases and the effectiveness and/or need for any corrective actions was felt to be essential to these LDV studies because of the controversial nature of the bias problem and the potential effects on the results. Without such an effort, systematic uncertainties and unanswered questions would exist regarding the quality and accuracy of the experimental results. The purpose of this paper is to present the results of the evaluation of velocity bias effects and the techniques used in the process. This information should be of value to those doing frequency counter based LDV measurements in highly turbulent flows. The problem of fringe bias with coincident two channel frequency shifted LDV systems was studied analytically by and Petrie et al. (1986) for the current flows and was reduced or avoided experimentally by frequency shifting.The data presented below were taken in the model experiment flowfield shown in Fig.

Field Measurements at River and Tidal Current Sites for Hydrokinetic Energy Development: Best Practices Manual

“…Many users still use this number for screening out bad data, although a generalization to some universal value is unwarranted and a close examination of the dataset is the best way to set a correlation threshold (if any) for discarding bad data points. Petrie et al (1988) and Baldwin et al (1993) first applied a velocity hodograph elliptical filtering technique to laser Doppler velocimeter measurements as an effective means of filtering LDV measured noise. In this technique, the measured data are rotated into the principal stress coordinates and an elliptic filter of size N principal stresses in the minor and major axes is applied to the 2-D probability density function (PDF).…”

Ornl Adv Post-Processing Guide and Matlab Algorithms for MHK Site Flow and Turbulence Analysis

“…Baldwin et al (1993) and Petrie et al (1988) first applied a velocity hodograph elliptical filtering technique to laser Doppler velocimeter (LDV) measurements as an effective means of filtering LDV measured noise. In this technique, the measured data are rotated into the principal stress coordinates and an elliptic filter of size N principal stresses in the minor and major axes is applied to the 2D probability density function (PDF).…”

Ornl Adcp Post-Processing Guide and Matlab Algorithms for MHK Site Flow and Turbulence Analysis

“…Optimally the user should select the smallest velocity range (in either beam or earth coordinates) for which no cut-off is observed in the histogram. Baldwin et al (1993) and Petrie et al (1988) first applied a velocity hodograph elliptical filtering technique to laser Doppler velocimeter (LDV) measurements as an effective means of filtering LDV measured noise. In this technique, the measured data are rotated into the principal stress coordinates and an elliptic filter of size N principal stresses in the minor and major axes is applied to the 2D probability density function (PDF).…”