Scalar dispersion in high Reynolds number turbulent boundary layers

Well-resolved turbulent velocity and concentration measurements were obtained in the atmospheric surface layer (ASL) that flows over the salt flats of Utah's western desert. All experiments were performed under conditions of near neutral thermal stability. The Reynolds number based on momentum thickness was estimated as R_&thetas; ≈ 2 × 10 6. Data to date indicate that under the proper conditions, the flow over the salt flats mimics the flow that would be obtained in an extremely large, low speed wind tunnel.; The initial set of field experiments interrogated the streamwise velocity, u, field in the viscous sublayer and buffer layer regions of the ASL. The present salt flats data, in comparison with those acquired at R_&thetas; = 2000, indicate a logarithmic Reynolds number dependence in the peak of the inner normalized root mean square streamwise velocity. This has been attributed to an increased contribution from low frequency motions at high Reynolds number. Reynolds number trends were also observed in the near-wall profiles of the skewness of u and spanwise vorticity, ω _z, as well as in the u and ω _z spectra.; A second set of field experiments was performed to investigate scalar dispersion in the near-field of an elevated point source. Macroscale concentration, c, measurements were obtained from an array of sensors mounted on a planar grid oriented perpendicular to the mean wind direction. Ramp-cliff structures in the time series of c, near the mean plume centerline, were identified with the large-scale plume meander. Microscale measurements were obtained with a custom scalar transport probe (STP). The development and characterization of the STP is described in some detail. The STP was used primarily to compare the magnitudes of the terms in the scalar variance equation. Turbulent transport of scalar variance due to vertical velocity fluctuations dominated over dissipation, production, and advection. Probability density functions revealed the highly intermittent nature of the vertical transport term. Furthermore, motions associated with the vertical transport displayed a characteristic wavelength on the order of the Taylor microscale, which was two orders of magnitude smaller than the integral scale of either the velocity or scalar.