shear stress fluctuations

Measurements of velocity and wall shear stress Uo fluctuations were made in an external turbulent boundary layer developed over a towed surface-piercing flat plate. An array of eight flush-mounted wall shear stress sensors _ was used to compute the space-time correlation function. _ A methodology for in situ calibration of the sensors for ship hydrodynamic applications is presented. The inten- 6 sity of the wall shear stress fluctuations, Trras/Tavg, was 3" measured as 0.25 and 0.36 for Ro=3,150 and 2,160 re- 0 spectively. The probability density is shown to exhibit v positive skewness, and lack of flow reversals at the wall. g Correlations between velocity and wall shear stress 2c fluctuations are shown to collapse with outer boundary zw layer length and velocity scales, verifying the existence of Tavg large-scale coherent structures which convect and decay Zrm~ along the wall at an angle of inclination varying from 10 Offo) to 13 ~ over the range of Reynolds numbers investigated. The wall shear stress convection velocity determined from narrow band correlation measurements is shown to scale with outer variables. The space-time correlation of the wall shear is shown to exhibit a well-defined convective ridge, and to decay 80% over approximately 3(5 for R0=3,150.