4 results
The structure and control of a turbulent reattaching flow
- L. W. Sigurdson
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- Journal:
- Journal of Fluid Mechanics / Volume 298 / 10 September 1995
- Published online by Cambridge University Press:
- 26 April 2006, pp. 139-165
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An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied using flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved: the Kelvin-Helmholtz instability of the free shear layer and the ‘shedding’-type instability of the entire bubble. The former consists of an interaction of the shear layer vorticity with itself, the latter with its images that result because of the presence of a wall. In order to determine the optimum forcing frequency, a method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Kármán vortex shedding.
CMB Analysis of Boomerang & Maxima & the Cosmic Parameters {Ωtot,Ωbh2, Ωcdmh2, ΩΛ, ns}
- J. Richard Bond, P. A. R. Ade, A. Balbi, J. J. Bock, J. Borrill, A. Boscaleri, K. Coble, B. P. Crill, P. de Bernardis, P. Farese, P. G. Ferreira, K. Ganga, M. Giacometti, S. Hanany, E. Hivon, V. V. Hristov, A. Iacoangeli, A. H. Jaffe, A. E. Lange, A. T. Lee, L. Martinis, S. Masi, P. D. Mauskopf, A. Melchiorri, T. Montroy, C. B. Netterfield, S. Oh, E. Pascale, F. Piacentini, D. Pogosyan, S. Prunet, B. Rabii, S. Rao, P. L. Richards, G. Romeo, J. E. Ruhl, F. Scaramuzzi, D. Sforna, K. Sigurdson, G. F. Smoot, R. Stompor, C. D. Winant, J. H. P. Wu
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- Journal:
- Symposium - International Astronomical Union / Volume 201 / 2005
- Published online by Cambridge University Press:
- 26 May 2016, pp. 347-357
- Print publication:
- 2005
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We show how estimates of parameters characterizing inflation-based theories of structure formation localized over the past year when large scale structure (LSS) information from galaxy and cluster surveys was combined with the rapidly developing cosmic microwave background (CMB) data, especially from the recent Boomerang and Maxima balloon experiments. All current CMB data plus a relatively weak prior probability on the Hubble constant, age and LSS points to little mean curvature (Ωtot = 1.08±0.06) and nearly scale invariant initial fluctuations (ns = 1.03±0.08), both predictions of (non-baroque) inflation theory. We emphasize the role that degeneracy among parameters in the Lpk = 212 ± 7 position of the (first acoustic) peak plays in defining the Ωtot range upon marginalization over other variables. Though the CDM density is in the expected range (Ωcdmh2 = 0.17 ± 0.02), the baryon density Ωbh2 = 0.030 ± 0.005 is somewhat above the independent 0.019 ± 0.002 nucleosynthesis estimates. CMB+LSS gives independent evidence for dark energy (ΩΛ = 0.66 ± 0.06) at the same level as from supernova (SN1) observations, with a phenomenological quintessence equation of state limited by SN1+CMB+LSS to wQ < −0.7 cf. the wQ=−1 cosmological constant case.
7 - Free surface interaction
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 72-80
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Summary
Laminar jets can splash!
It has been observed that a liquid jet impinging on a solid surface can produce splashing. High-speed photography has revealed that, with a turbulent jet, splashing is related to the jet surface roughness. To investigate the importance of the jet shape on splashing, perturbations of known frequency or amplitude are imposed on the surface of a smooth laminar jet.
The top picture shows the unperturbed smooth jet as it spreads radially on the solid surface. The varicose deformations imposed on the jet surface alter the flow quite dramatically (center picture). As we further increase the amplitude of the oscillations, splashing starts suddenly. The bottom picture shows the beauty and complexity of splashing.
Impacting water drops
The four photographs shown here are representative of a series which recorded the structure and evolution of the vorticity generated by a water drop impacting a free surface of water in a container. The 2.8 mm diam water drop was dyed with fluorescein and released from the tip of a hypodermic needle under specific parameters, We=26, Fr=25. The Weber number (ρU2d /γ) and Froude number (U2/gd) are based on drop diameter d, impact velocity U, and surface tension γ.
Figure 1 is photographed from the side and slightly below the free surface while Fig. 2 is shot looking directly up at the free surface via a mirror. These alternative viewing angles provide a valuable tool in visualizing the threedimensional flow structure. A “primary” vortex ring can be seen convecting away from the free surface. A convoluted secondary structure can be seen wrapped around the primary core.
4 - Drops and bubbles
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- By S. Chandra, C. T. Avedisian, M. P. Brenner, X. D. Shi, J. Eggers, S. R. Nagel, M. Tjahjadi, J. M. Ottino, PH. Marmottant, E. Villermaux, B. Vukasinovic, A. Glezer, M. K. Smith, A. Lozano, C. J. Call, C. Dopazo, D. E. Nikitopoulos, A. J. Kelly, D. Frost, B. Sturtevant, M. M. Weislogel, S. Lichter, M. Manga, H. A. Stone, J. Buchholz, L. Sigurdson, B. Peck
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 42-53
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Summary
The collision of a droplet with a solid surface
The photographs displayed above show the impact, spreading, and boiling history of n-heptane droplets on a stainless steel surface. The impact velocity, Weber number, and initial droplet diameter are constant (values of 1 m/s, 43 and 1.5 mm respectively), and the view is looking down on the surface at an angle of about 30°. The photographs were taken using a spark flash method and the flash duration was 0.5 μs. The dynamic behavior illustrated in the photographs is a consequence of varying the initial surface temperature.
The effect of surface temperature on droplet shape may be seen by reading across any row; the evolution of droplet shape at various temperatures may be seen by reading down any column. An entrapped air bubble can be seen in the drop when the surface temperature is 24°C. At higher temperatures vigorous bubbling, rather like that of a droplet sizzling on a frying pan, is seen (the boiling point of n-heptane is 98°C) but the bubbles disappear as the Leidenfrost temperature of n-heptane (about 200°C) is exceeded because the droplet become levitated above a cushion of its own vapor and does not make direct contact with the surface. The droplet shape is unaffected by surface temperature in the early stage of the impact process (t≤0.8 ms) but is affected by temperature at later time (cf. t≥ 1.6 ms) because of the progressive influence of intermittent solid-liquid contact as temperature is increased.