| A new sensor is described for measuring the fluctuating component of the wall shear stress in the atmospheric surface layer over relatively smooth uniform terrain. The sensor was tested at the SLTEST site on the western salt-flats of Utah, giving the first ever direct measurements of this quantity in an atmospheric scale flow. The device consists of a lightweight floating element whose position is detected using a spherical mirror that deflects a laser beam onto a duo-lateral position sensing photodiode. The sensor has a frequency response of 25 Hz, and a circular sensing area, of 50 mm diameter, making it suitable for atmospheric scale measurements.; Measurements were made under near-neutral buoyant conditions at SLTEST using the new sensor and a rake of five sonic anemometers, positioned at wall-normal positions from z = 0.62 m to 2.5 m. Cross-correlations of the wall-shear stress and velocity measurements indicate a structure-inclination angle of 14°, which is consistent with analogous laboratory-scale turbulent boundary layer measurements at three orders of magnitude lower Reynolds number. Other quantities were also found to agree well with canonical laboratory scale results. This includes the RMS of the fluctuating wall-shear stress and the power spectra. The structure of the near-wall turbulence is further studied by looking at the cross-correlations of the two components of fluctuating wall-shear stress with the three components of velocity at the various wall-normal positions, and using VITA analysis. |
