An Experimental Verification of Laser-Velocimeter Sampling Bias and Its Correction

Johnson, Dennis A.; Modarress, D.; Owen, F. K.

doi:10.1115/1.3242405

Cited by 31 publications

(15 citation statements)

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“…The statistical prediction and the corrected results are in good agreement up to 40% intensity levels. The low speed data of Johnson et al (1982) are similar. The turbulence intensity based on the local mean velocity magnitude was 30%-35% at y* =0, the middle of the mixing layer.…”

Section: Resultssupporting

confidence: 55%

“…The reduction of the bias between free running and processor controlled sampling has been observed experimentally, see Johnson et al (1982), Craig et al (1984), and Stevenson et al (1982), and it appears that if the sampling rate is on the order of 50 times less than the validated particle data rate, the data is unbiased. Conversely, the mean validated particle data rate must be significantly smaller than the processor sampling rate for the data to be completely biased.…”

Section: Resultsmentioning

confidence: 80%

“…Since the geometry factor, a/b, is typically smaller than 1/20, the z component term will only be significant when Ui and Vi are small. Johnson et al (1982) argued that the simple two-dimensional velocity inverse weight should be sufficiently accurate because the z term contribution is always small but no examination of the effect of including the z term has been made. An appraisal has been made below.…”

Section: Veloci O Inverse Weightingmentioning

confidence: 99%

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Laser Doppler velocity bias in separated turbulent flows

Petrie

Samimy

Addy

2004

Experiments in Fluids

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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.

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Section: Resultssupporting

confidence: 55%

Section: Resultsmentioning

confidence: 80%

Section: Veloci O Inverse Weightingmentioning

confidence: 99%

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Laser Doppler velocity bias in separated turbulent flows

Petrie

Samimy

Addy

2004

Experiments in Fluids

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“…This validation rate was high enough to permit eliminating velocity bias by inhibiting the counter processors for a fixed time interval between samples, approximating equal time sampling. This method has previously been described by Stevenson et al 11 and its effectiveness confirmed in subsequent studies, including the work by Gould et al, 9 Durrett et al, 10 Johnson et al, 12 and Gouldet al 13 …”

Section: Flow Systemmentioning

confidence: 66%

Investigation of turbulent transport in an axisymmetric sudden expansion

1990

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Simultaneous two-component laser velocimeter measurements were made in the incompressible turbulent flowfield following an axisymmetric sudden expansion. Mean velocities, Reynolds stresses, and triple products were measured and are presented at axial positions ranging from x/H = 0.2-14. A balance of the turbulent kinetic energy in the flow was performed. The production, convection, and diffusion of turbulent kinetic energy were computed directly from the experimental data using central differencing. A specially designed correction lens was employed to correct for optical aberrations introduced by the circular tube. This lens system allowed the accurate simultaneous measurement of axial and radial velocities in the test section. The experimental measurements were compared to predictions generated by a code that employed the k-e turbulence model. Agreement was good for mean axial velocities, turbulent kinetic energy, and turbulent shear stresses. However, the modeled turbulent normal stresses where in poor agreement with the measured values. The modeled diffusion of turbulent kinetic energy was underpredicted in the region between the shear layer and the centerline of the flow giving lower values of turbulent kinetic energy downstream of the potential core than measured. NomenclatureC D = turbulence constant, =0.09 H -step height, 38.1 mm _ _ __ k = turbulent kinetic energy, = (uu + vv + ww)/2 / = mixing length, mm P = time-averaged mean static pressure, N/m 2 p = pressure fluctuation about time-averaged mean, N/m 2 RI = inlet radius of sudden expansion, 38.1 mm R 2 = outlet radius of sudden expansion, 76.2 mm r = radial coordinate direction, ( = 0 at centerline) U = time-averaged mean axial velocity, m/s UQ = inlet reference velocity, 22 m/s u_ -fluctuating axial velocity, m/s up = time-averaged axial velocity-ressure correlation, __ N/m-s uju -time-averaged axial turbulent normal stress, m 2 /s 2 uv = time-averaged turbulent shear stress, m 2 /s 2 uuu = turbulent triple product, mVs 3 uuv -turbulent triple product, mVs 3 uvv = turbulent triple product, mVs 3 V -time-averaged mean radial velocity (positive toward wall), m/s v_ = fluctuating radial velocity, m/s vp = time-averaged radial velocity-pressure correlation, N/ _ m-s vv = time-averaged radial turbulent normal stress, m 2 /s 2 vvv = turbulent triple product, mVs 3 w = fluctuating tangential velocity, m/s ww = time-averaged tangential turbulent normal stress, mVs 2 x = axial coordinate direction y = radial coordinate direction, ( = 0 at centerline) e = dissipation of turbulent kinetic energy, m 2 /s 3 fji = dynamic viscosity, Kg/m-s JLI, = turbulent viscosity, Kg/m-s /Xeff = effective viscosity, Kg/m-s p = fluid density, kg/m 3 o = standard deviation

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“…This ''bias'' was eliminated from the measurements by a post-processing step, proposed by Pandey et al (2004), where multiple recordings by the same particle were discarded. The second kind of ''velocity bias'' associated with LDV measurements is associated with long measurement times (Johnson et al, 1983). To minimize this bias, the velocity measurements were done for a short span.…”

Section: Resultsmentioning

confidence: 99%

Evaluation of a Backscatter Imaging LDV System and its Application to a Pilot-Scale Circulating Fluidized Bed

Pandey

Turton

Yue

et al. 2006

Particulate Science and Technology

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This study focuses on the evaluation and application of a backscatter imaging LDV (laser Doppler velocimeter) system (BILS) built by TSI Inc. (Shoreview, Minn.). This nonintrusive instrument has the ability to record particle images and velocity data simultaneously. A series of validation tests was conducted prior to measuring particle velocity in a turbulent system. A set of experiments was conducted to measure terminal velocity and particle size for five different-size cuts of near-spherical, Nu-Pareil particles, and the results were compared with the standard drag curve, which showed good agreement. The imaging data were used to estimate the sphericity (U) of the particles. The data confirmed that U was close to 1 (0.98 < U < 0.99). The system was then used to measure particle velocity in a cold flow circulating fluidized bed at various operating conditions. The effect of operating conditions, such as superficial gas velocity and solids circulation rate, on the solids velocity was studied. The mean solids velocity near the wall was found to increase with increasing superficial gas velocity and decreasing solids circulation rate. The granular temperature was also estimated and was found to increase with increasing distance from the wall. The granular temperature was found to lie in the range of 0.1-1.0 m 2 =s 2 for the operating conditions studied. The granular temperature calculations also showed that the flow of particles moving downwards was more ordered than the flow moving up.

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An Experimental Verification of Laser-Velocimeter Sampling Bias and Its Correction

Cited by 31 publications

References 0 publications

Laser Doppler velocity bias in separated turbulent flows

Laser Doppler velocity bias in separated turbulent flows

Investigation of turbulent transport in an axisymmetric sudden expansion

Evaluation of a Backscatter Imaging LDV System and its Application to a Pilot-Scale Circulating Fluidized Bed

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