Effect of measuring volume length on two-component laser velocimeter measurements in a turbulent boundary layer

Abstract

The effects of finite measuring volume length on laser velocimetry measurements of turbulent boundary layers were studied. Four different effective measuring volume lengths, ranging in spanwise extent from 7 to 44 viscous units, were used in a low Reynolds number (Re o = 1440) turbulent boundary layer with high data density. Reynolds shear stress profiles in the near-wall region show that <u v) strongly depends on the measuring volume length; at a given y-position, <u v) decreases with increasing measuring volume length. This dependence was attributed to simultaneous validations on the U and V channels of Doppler bursts coming from different particles within the measuring volume. Moments of the streamwise velocity showed a slight dependence on measuring volume length, indicating that spatial averaging effects well known for hot-films and hot-wires can occur in laser velocimetry measurements when the data density is high. <> U~ V dp /eft" l + y+ 2 .+ U + U P /2' V' Reo Ruv R12 (0, 0, z) T~ List of symbols time-averaged quantity wall friction velocity, = (%/Q)1/2 kinematic viscosity pinhole diameter spanwise extent of LDV measuring volume viewed by photomultiplier non-dimensional length of measuring volume, =-l~e f u~/v non-dimensional coordinate in spanwise direction, =_ y u Jr non-dimensional coordinate in spanwise direction, Z ~T/V non-dimensional mean velocity, =-<U)/u~ instantaneous streamwise velocity fluctuation, =_u-<u) instantaneous normal velocity fluctuation, _= V-<V> RMS streamwise velocity fluctuation, -<u2> x/2 RMS normal velocity fluctuation, -= <vZ) lj~ Reynolds number based on momentum thickness, -U~ 0/v cross-correlation coefficient, =-<u v ) /u ' v' two point correlation between u and v with z-separation, -= <u(0, 0, 0) v(0, 0, z)>/<u(O, O, O) v(O, O, 0)> rate at which bursts are validated by counter processor Taylor time microscale, ~-u' ( < du/ dt ) 2)-t/2

Influence of PIV interrogation area on turbulent statistics up to 4th order moments in smooth and rough wall turbulent flows

“…Johansson and Alfredsson [10] studied the effects of (spanwise) length of hot-wire on the fluctuating components of the streamwise velocity. Unlike the results of Johnson and Barlow [8], their measurements revealed a 10% reduction in u þ max as l þ z increased from 14 to 32. Above, y + = 15, their u + profiles from the various probes collapsed onto one curve but profiles of S u and F u showed more sensitivity to l þ z than observed in the LDA results reported by Johnson and Barlow [8].…”

“…Unlike the results of Johnson and Barlow [8], their measurements revealed a 10% reduction in u þ max as l þ z increased from 14 to 32. Above, y + = 15, their u + profiles from the various probes collapsed onto one curve but profiles of S u and F u showed more sensitivity to l þ z than observed in the LDA results reported by Johnson and Barlow [8]. Johansson and Alfredsson [10] also compiled data of u þ max obtained by various researchers with varying l þ z values and concluded that u þ max decreased from 2.9 to 2.1 as l þ z increased from about 3 to 100.…”

“…The observation that S v and F v did not converge even for N = 2500 is not all that surprising because prior LDA studies (Johnson and Barlow [8]) using N = 20000 samples revealed that u, S u and F u as well as Àuv converged but v, S v and F v did not. Similar Table 1 Summary of interrogation areas sizes IA in pixels: pixels · pixels · overlap IA in mm (l x · l y ) IA side in wall units ðl þ y Þ l y /g or better convergence were obtained for the other tests, and all the quantities reported subsequently were those computed from N = 2500 images.…”

“…A number of studies have been conducted in the past to quantify the effects of probe volume dimension on the mean velocity and higher order moments. Johnson and Barlow [8] reviewed prior studies related to spatial resolution of LDA, and then studied the effect of probe volume size of two-component LDA in a turbulent boundary layer. Their Reynolds number based on the boundary layer momentum thickness, h, was Re h = 1440.…”

On the development of a turbulent jet subjected to aerodynamic excitation in the helical mode

“…The resulting uncertainties for the statistical data derived from the LDA measurements presented in this paper are defined for the 95% confidence interval, based on the bootstrap re-sampling procedure [55] due to the unknown probability distributions of the velocity signals. This methods have been employed by many studies [56,57]. The analysis shows that the uncertainty of the data varied with the location of the measurement point, with the highest uncertainty being more than 3% for points at the edge of jet and less than 0.04% in the centre 3.…”