Measurements of the wall pressure spectra on a full-scale experimental towed array

Turbulent wall pressure data acquired during tests of a full-scale experimental towed array over a range of tow speeds in straight tows and turns is presented. The experimental towed array contained a linear array of sensors mounted at the fluid–solid interface to measure the spectra of the wall pressure fluctuations due to the cylindrical turbulent boundary layer. The physics are dominated by the growth of a thick, high Reynolds number turbulent boundary layer at arc length Reynolds numbers as high as 9×10⁸. The measured wavenumber-frequency spectra, autospectra, cross-spectral decay and convection velocities are presented. A well-defined convective ridge exists in the wavenumber-frequency spectra obtained during straight tows and turns. Turns give rise to a complicated fluid–structure interaction problem, but do not lead to the separation of the turbulent boundary layer. As the array moves through a turn, flow-induced vibrations of the array are shown to dominate the spectra at low frequencies, with more rapid decay in the measured coherence occurring at higher frequencies. The use of tow speed as a velocity scale is shown to collapse autospectra and convection velocities.     

Landslides triggered by the 23 November 2000 rainfall event in the Imperia Province, Western Liguria, Italy

From mid-October to 22 November 2000, the western Liguria Region of Italy experienced prolonged and intense rainfall, with cumulative values exceeding 1000 mm in 45 days. The severe rainfall sequence ended on November 23 with a high-intensity storm that dumped more than 180 mm of rain in 24 h. The high-intensity event caused flooding and triggered more than 1000 soils slips and debris flows and a few large, complex landslides. Slope failures caused three fatalities and severe damage to roads, private homes, and agriculture. Large (1:13,000) and very large (1:5000) scale colour aerial photographs were taken 45 days after the event over the areas most affected by the landslides. Through the interpretation of the 334 photographs covering an area of 500 km², we prepared a landslide inventory map that shows 1204 landslides, for a total landslide area of 1.6 km². We identified the rainfall conditions that triggered landslides in the Armea valley using cumulative- and continuous-rainfall data, combined with detailed information on the time of landslide occurrence. Landslide activity initiated 8 to 10 h after the beginning of the storm, and the most abundant activity occurred in response to rainfall intensities of 8 to 10 mm per hour. For the Ceriana Municipality, an area where the landslides were numerous in November 2000, we also collected information about a historical event that occurred on 8–11 December 1910 and triggered abundant landslides resulting in severe economic damage. A comparison of the damage caused by the historical and the recent landslide events indicated that damage caused by the 1910 historical event was more diffused but less costly than the damage caused by the 2000 event.     

Comparison of theoretical and experimental wall pressure wavenumber-frequency spectra for axisymmetric and flat-plate turbulent boundary layers

The measurement and analysis of turbulent boundary layer wall pressure fluctuations using a wavenumber filter of sensors provide quantitative knowledge of turbulence physics. In addition, the sources of flow-induced noise and vibration for towed SONAR arrays can be determined. An axisymmetric turbulent boundary layer can have significantly different features than those of a comparable flat-plate boundary layer. Here, a detailed comparison of the distribution of wall pressure energy in both wavenumber and frequency between flat-plate and thick axisymmetric boundary layers is presented. The background theory of wavenumber-frequency spectra and state-of-the-art models for flat-plate boundary layers are discussed. The widely used model of Chase (1987), valid for flat-plate boundary layers over a wide range of Reynolds numbers, is used and combined with a sensor response function to allow the effects of spatial averaging to be considered. It is demonstrated that when measured boundary layer parameters for the axisymmetric case are used in the Chase flat-plate model, the results accurately predict the axisymmetric boundary layer wall pressure measurements.