Spatial resolution and measurement of turbulence in the viscous sublayer using subminiature hot-wire probes
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Measurements in the viscous sublayer of a flat-plate turbulent boundary layer in air, using single hot-wire sensors with lengths from 1-60 viscous length scales show that, at a given distance from the surface, the turbulence intensity, flatness factor, and skewness factor of the longitudinal velocity fluctuation are nearly independent of wire length when the latter is less than 20-25 times the viscous length scale (i.e. 20-25 "wall units"), and decrease significantly and abruptly for larger wire lengths. This conclusion is consistent with other workers' probability density functions of streak spacing: the lateral spacing of "streaks" in the viscous sublayer is 80-100wall units on average with minimum spacing of 20-25 wall units, which implies that signals would be strongly attenuated by wires whose length exceeds 20-25 wall units. To achieve wire lengths of less than 20-25 wall units, subminiature hot wire probes like those described by Ligrani and Bradshaw (1987), having lengths as small as 150 ~tm, are necessary for sublayer measurements in typical laboratory wind tunnels. As well as the measurements mentioned above, dissipation spectra are presented, to show the effect of spanwise averaging on the high-frequency motions, which is necessarily more severe than the effect on overall intensities.
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