Aero-acoustic oscillations inside large deep cavities

This investigation focuses on the pressure amplitude response, within two deep cavities characterized by their length over depth ratios (L/H = 0.2 and 0.41), under varying free stream velocity in a large wind tunnel. Experiments have shown that for deep rectangular cavities at low Mach number, oscillations of discrete frequencies can be produced. These oscillations appear when the free stream velocity becomes higher than a minimum value. In addition, as flow velocity is increased, upward jumps in oscillation frequency are observed in the two cavity configurations. Convection velocity of instabilities along the shear layer was measured using velocity cross-correlations. This study shows that the hydrodynamic modes of the cavity shear layer are correctly predicted by the Rossiter model when the convection velocity is determined and the empirical time delay is neglected. For L/H = 0.2 the first oscillation mode is observed on the spectrogram. For L/H = 0.41, both the first and the second mode have approximately the same amplitude. Time-resolved Particle image velocimetry measurements were performed to obtain the vorticity distribution during the oscillation cycle along the cavity shear layer. It is found that the number of structures in the cavity shear layer depends on the mode order of cavity oscillation

Control of deep cavity tones using a spanwise cylinder at low-subsonic speeds

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Deep cavity configuration at subsonic velocity could be found in many industrial processes, ranging from windows and sunroofs in automobiles and over space between two consecutive train vehicles. These cavities may induce aeroacoustic couplings between the cavity shear layer oscillations and the acoustic modes of the installation. This aero-acoustic coupling can leads to serious damages of vehicles due to resonance of high pressure fluctuation level around the cavity. The study of deep cavity at low velocity presents a great practical interest to suppress acoustic noise. In spite of numerous studies devoted to the cavity and its control, very few of them relate to the deep cavity configuration at low velocity. The focus of the present study is to apply a passive control to the case of the deep cavity flow at relatively low velocities. A Detailed Experimental study of flow over a deep cavity was conducted towards understanding the attenuation of tones using a spanwise cylinder. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary. A “no control” cavity was compared with a similar configuration using a cylinder on the leading edge of the cavity. Parametric changes of the spanwise cylinder such as the distance from the wall are studied. Maximum control across the range of studied velocities occurs for a particular position of the spanwise cylinder. Reductions in sound pressure levels (SPL) of up to 36 dB were obtained. Moreover, a shaped cylinder was also studied and shows that the attenuation of tones is not due to high frequency pulsing as suggested in literature, but to an increase of the cavity shear layer thickness due to change in the mean axial velocity profiles.vk201

Modelo de circulación de brisa de mar y tierra

Se obtiene una solución numérica para la Temperatura potencial y la Velocidad a partir de una teoría dinámica de la circulación de brisa de mar y tierra. Los resultados obtenidos a lo largo de 16 horas de integración reproducen adecuadamente las características observadas en este sist£ ma de circulación.A numerical solution for Potential temperature and Velocity is obtain ed from a dynamical theory of sea and land breeze circulation. Results after sixteen hours of integration adequately reproduce the characteristics observed in this circulation system.Asociación Argentina de Geofísicos y Geodesta

General behaviour of Bianchi VI_0 solutions with an exponential-potential scalar field

The solutions to the Einstein-Klein-Gordon equations without a cosmological constant are investigated for an exponential potential in a Bianchi VI_0 metric. There exists a two-parameter family of solutions which have a power-law inflationary behaviour when the exponent of the potential, k, satisfies k^2<2. In addition, there exists a two-parameter family of singular solutions for all k^2 values. A simple anisotropic exact solution is found to be stable when 2<k^2.Comment: 10 pages, no figures. To be published in General Relativity and Gravitatio

Latest Cretaceous climatic and environmental change in the South Atlantic region

Latest Maastrichtian climate change caused by Deccan volcanism has been invoked as a cause of mass extinction at the Cretaceous-Paleogene (K-Pg) boundary (~66.0 Ma). Yet late Maastrichtian climate and ecological changes are poorly documented, in particular on the Southern Hemisphere. Here we present upper Maastrichtian-lower Danian climate and biotic records from the Bajada del Jagüel (BJ) shelf site (Neuquén Basin, Argentina), employing the TEX86 paleothermometer, marine palynology (dinoflagellate cysts), and micropaleontology (foraminifera). These records are correlated to the astronomically tuned Ocean Drilling Program Site 1262 (Walvis Ridge). Collectively, we use these records to assess climatic and ecological effects of Deccan volcanism in the Southern Atlantic region. Both the TEX86-based sea surface temperature (SST) record at BJ and the bulk carbonate δ18O-based SST record of Site 1262 show a latest Maastrichtian warming of ~2.5-4°C, at 450 to 150 kyr before the K-Pg boundary, coinciding with the a large Deccan outpouring phase. Benthic foraminiferal and dinocyst assemblage changes indicate that this warming resulted in enhanced runoff and stratification of the water column, likely resulting from more humid climate conditions in the Neuquén Basin. These climate conditions could have been caused by an expanding and strengthening thermal low over the South American continent. Biotic changes in response to late Maastrichtian environmental changes are rather limited, when compared to the major turnovers observed at many K-Pg boundary sites worldwide. This suggests that environmental perturbations during the latest Maastrichtian warming event were less severe than those following the K-Pg boundary impact

Scaling of statistics in wall-bounded turbulent flows

High-resolution laser Doppler anemometry (Lda) and hot-wire anemometry (Hwa) measurements are utilized to study a zero-pressure-gradient turbulent boundary layer over the range of momentum thickness Reynolds number of 1170–3720. The primary objective is to investigate the near-wall behavior of this type of flow. We are particularly interested in possible Reynolds- and Kármán-number dependencies. The experimental results are in excellent agreement with most recent direct numerical simulations (Dns), which allow direct comparison of detailed results such as peak value and position of streamwise Reynolds stress, wall values of skewness and flatness factors, and turbulence dissipation rate. Systematic changes of some of these parameters with Kármán number are found when scaled with the inner parameters. A remedy seems to be the alternative mixed scaling that is based on , instead of , which admits direct influence of the outer velocity scale on the wall parameters