A method for the measurement and the statistical analysis of atmospheric turbulence

The instantaneous values of output voltages representing the wind velocity vector and the temperature at different elevations of the 250-foot meteorological tower located at NASA Wallops Flight Center are provided with the three dimensional split-film TSI Model 1080 anemometer system. The output voltages are sampled at a rate of one every 5 milliseconds, digitized and stored on digital magnetic tapes for a time period of approximately 40 minutes, with the use of a specially designed data acqusition system. A new calibration procedure permits the conversion of the digital voltages to the respective values of the temperature and the velocity components in a Cartesian coordinate system connected with the TSI probe with considerable accuracy. Power, cross, coincidence and quadrature spectra of the wind components and the temperature are obtained with the use of the fast Fourier transform. The cosine taper data window and ensemble and frequency smoothing techniques are used to provide smooth estimates of the spectral functions

Coherent structures in uniformly sheared turbulent flow

Uniformly sheared turbulent flow has been generated in a water tunnel and its instantaneous structure has been examined using flow visualization and particle image velocimetry. The shear-rate parameter was approximately equal to 13 and the streamwise turbulence Reynolds number was approximately 150. The flow was found to consist of regions with nearly uniform velocity, which were separated by regions of high shear containing large vortices. The concentration of vortices and the distributions of their directions of rotation, strengths, sizes and shapes have been determined. These results demonstrate that horseshoe/hairpin-shaped vortices were prevalent, even though wall effects were negligible in this flow. Both ‘upright’ and ‘inverted’ vortices have been observed, in contrast to turbulent boundary layers, in which only ‘upright’ vortices can be found, suggesting that the presence of the wall may suppress the development of ‘inverted’ structures. Our observations demonstrate that the dominant coherent structures of fully developed uniformly sheared flow are very different from the structures observed in the flow exiting the shear-generating apparatus, which points to an insensitivity of the former to initial effect

Measurements of turbulent diffusion in uniformly sheared flow

The diffusion of a plume of dye in uniformly sheared turbulent flow in a water tunnel was investigated using simultaneous stereoscopic particle image velocimetry (SPIV) and planar laser-induced fluorescence (PLIF). Maps of the mean concentration and the turbulent concentration fluxes in planes normal to the plume axis were constructed, from which all components of the second-order turbulent diffusivity tensor were determined for the first time. Good agreement between the corresponding apparent and true diffusivities was observed. The turbulent diffusivity tensor was found to have strong off-diagonal components, whereas the streamwise component appeared to be counter-gradient. The different terms in the advection–diffusion equation were estimated from the measurements and their relative significance was discussed. All observed phenomena were explained by physical arguments and the results were compared to previous one

Self-sustained oscillations in homogeneous shear flow

Generation of the large-scale coherent vortical structurs in homogeneous shear flow couples dynamical processes of energy and enstrophy production. In the large rate of strain limit, the simple estimates of the contributions to the energy and enstrophy equations result in a dynamical system, describing experimentally and numerically observed self-sustained non-linear oscillations of energy and enstrophy. It is shown that the period of these oscilaltions is independent upon the box size and the energy and enstrophy fluctuations are strongly correlated.Comment: 10 pages 6 figure

Relative dispersion of a passive scalar plume in turbulent shear flow

Relative dispersion of a passive scalar plume was investigated in uniformly sheared, nearly homogeneous, turbulent flow with Re??150 using planar laser-induced fluorescence. Mean concentration maps were determined both in the laboratory frame and in a frame attached to the instantaneous center of mass of the plume cross section. The distance-neighbor function had a shape that was compatible with Richardson's expression. The mean square particle separation, two estimates of which were found to be nearly identical, had a streamwise evolution that was consistent with Richardson-Obukhov scaling with a Richardson's constant of g=0.35. Batchelor scaling was also consistent with a wide range of the result

Landslide susceptibility mapping using the Rock Engineering System approach and GIS technique: an example from southwest Arcadia (Greece)

The purpose of this study is to prepare a susceptibility map in a landslide-prone area in Greece using Rock Engineering System (RES) and a geoprocessing tool called Model Builder. The implementation of RES is achieved through an interaction matrix, where ten parameters were selected as controlling factors for the landslide occurence. The validation of the developed model was achieved by using field-verified data, showing excellent correlation between the expected and existing landslide susceptibility level. In conjunction with Model Builder, which can overlay different layers and produce landslide susceptibility maps, RES can act as a tool for calculating the instability index and getting a prognosis of a potential slope failure in relation to sustainable development planning processes in landslide susceptible areas

Heat transfer from a flat plate in inhomogeneous regions of grid-generated turbulence

Experiments on the convective heat transfer from a flat plate, vertically mounted and parallel to the flow in a wind tunnel, were carried out via Infra-Red thermography and hot-wire anemometry. The Reynolds number based on the inflow velocity and on the length of the plate was about 5×1055×105. A step near the leading edge of the plate was used to promote transition to turbulence, with tripping effects on the heat transfer coefficients shown to be negligible for more than 90% of the plate’s length. Different types of grids, all with same blockage ratio σg=28%σg=28%, were placed upstream of the plate to investigate their potential to enhance the turbulent heat transfer. These grids were of three classes: regular square-mesh grids (RGs), single-square grids (SSGs) and multi-scale inhomogeneous grids (MIGs). The heat transfer coefficients at the mid-length of the plate were correlated with the mean velocity and the turbulence intensity of the flow at a distance from the plate at which the ratio of the standard deviations of the streamwise and wall-normal velocity fluctuations began to increase. However, the heat transfer was shown to be insensitive to the turbulence intensity of the incoming flow in close proximity of the tripping step. Furthermore, the integral length scale of the streamwise turbulent fluctuations was found not to affect the heat transfer results, both near the tripping step and in the well-developed region on the plate. For the smallest plate-to-grid distance, the strongest heat transfer enhancement (by roughly 30%) with respect to the no-grid case was achieved with one of the SSGs. For the largest plate-to-grid distance, the only grid producing an appreciable increase (by approximately 10%) of the heat transfer was one of the MIGs. The present results demonstrate that MIG design can be optimised to maximise the overall heat transfer from the plate. A MIG that produces a uniform transverse mean shear, which is approximately preserved over significant downstream distances from the grid and with a velocity decreasing with distance from the plate, allows a sustained heat transfer enhancement, in contrast to all other grid designs tested here. The most efficient configuration for a MIG is one for which the section of the grid that has lower blockage and thicker bars is adjacent to the plate