tailieunhanh - Computational Fluid Dynamics 2011 Part 8

Tham khảo tài liệu 'computational fluid dynamics 2011 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 204 Computational Fluid Dynamics Fig. 2. SOFIA telescope carried on the Boeing 747SP Schmid et al. 2009 a b Fig. 3. Snapshots of the predicted vorticity patterns across the cavity opening a URANS and b DES Schmid et al. 2009 Coming back to the ground-based telescope discussion which represents the main focus of the present chapter a campaign of scaled-model wind-tunnel measurements and CFD simulations was undertaken at the National Research Council of Canada to estimate wind loads on a very large optical telescope VLOT housed within a spherical calotte. The tests were performed for various wind speeds to examine Reynolds number effects and VLOT orientations see Cooper et al. 2005 and 2004a . The measurements revealed the existence of significant pressure fluctuations inside the enclosure owing to the formation of a shear layer across the enclosure opening. As many as four modal frequencies were detected depending on the wind speed. The number of modal frequencies decreased with increasing wind speed. The mean pressure inside the enclosure and on the primary mirror surface was roughly uniform. Later on the effect of the enclosure venting was investigated experimentally by Cooper et al. 2004b by drilling two rows of circular vents around the enclosure. The amplitude of the periodic pressure fluctuations that were measured in Cooper et al. 2005 was significantly reduced. The shear layer oscillatory modes were reduced to a single mode with smaller pressure fluctuation amplitude. In parallel with the aforementioned experimental studies Mamou et al. 2004a-b and Tahi et al. 2005a numerically investigated the wind loads on a full-scale and scaled model. Comparisons with WT measurements Cooper et al. 2005 showed good agreement for the mean pressure on the enclosure inside and outside surfaces as well on the primary mirror surface. However some discrepancies between CFD and WT data were observed for the pressure fluctuations and the oscillatory modal frequencies. It was .