Book review : Chanson, H. (2004). "The Hydraulics of Open Channel Flows : An Introduction." Butterworth-Heinemann, Oxford, UK, 2nd edition, 630 pages (ISBN 0 7506 5978 5).

This is the second and expanded edition of the first edition of "The Hydraulics of Open Channel Flows : An Introduction" first published in 1999 and reprinted in 2001 and 2002

Detection of microorganisms at high altitudes

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Bubbly two-phase flow in hydraulic jumps at large Froude numbers

A hydraulic jump is a sudden, rapid transition from a supercritical flow to a subcritical flow. At large inflow Froude numbers, the jump is characterized by a significant amount of entrained air. For this paper, the bubbly two-phase flow properties of steady and strong hydraulic jumps were investigated experimentally. The results demonstrate that the strong air entrainment rate and the depth-averaged void-fraction data highlight a rapid deaeration of the jump roller. The results suggest that the hydraulic jumps are effective aerators and that the rate of detrainment is comparatively smaller at the largest Froude numbers. © 2011 American Society of Civil Engineers

Wind and boundary layers in Rayleigh-Benard convection. Part 2: boundary layer character and scaling

The effect of the wind of Rayleigh-Benard convection on the boundary layers is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is shown that the kinetic boundary layers on the top- and bottom plate have some features of both laminar and turbulent boundary layers. A continuous spectrum, as well as significant forcing due to Reynolds stresses indicates undoubtedly a turbulent character, whereas the classical integral boundary layer parameters -- the shape factor and friction factor (the latter is shown to be dominated by the pressure gradient) -- scale with Reynolds number more akin to laminar boundary layers. This apparent dual behavior is caused by the large influence of plumes impinging onto and detaching from the boundary layer. The plume-generated Reynolds stresses have a negligible effect on the friction factor at the Rayleigh numbers we consider, which indicates that they are passive with respect to momentum transfer in the wall-parallel direction. However, the effect of Reynolds stresses cannot be neglected for the thickness of the kinetic boundary layer. Using a conceptual wind model, we find that the friction factor C_f should scale proportional to the thermal boundary layer thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness lambda_u scales inversely proportional to the thermal boundary layer thickness and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted trends for C_f and \lambda_u are in agreement with DNS results

Hydraulic jumps: Turbulence and air bubble entrainment

A free-surface flow can change from a supercritical to subcritical flow with a strong dissipative phenomenon called a hydraulic jump. Herein the progress and development in turbulent hydraulic jumps are reviewed with a focus on hydraulic jumps operating at large Reynolds numbers typically encountered in natural streams and hydraulic structures. The key features of the turbulent hydraulic jumps are the highly turbulent flow motion associated with some intense air bubble entrainment at the jump toe. The state-of-the-art on the topic is discussed based upon recent theoretical analyses and physical data

Progress towards the vindication of panspermia

Theories of panspermia are rapidly coming into vogue, with the possibility of the transfer of viable bacterial cells from one planetary abode to another being generally accepted as inevitable. The panspermia models of Hoyle and Wickramasinghe require the transfer of viable bacterial cells from interstellar dust to comets and back into interplanetary and interstellar space. In such a cycle a viable fraction of as little as 10−18 at the inception of a newly formed comet/planet system suffices for cometary panspermia to dominate over competing processes for the origin and transfer of life. The well-attested survival attributes of microbes under extreme conditions, which have recently been discovered, gives credence to the panspermia hypothesis. The prediction of the theory that comets bring microbes onto the Earth at the present time is testable if aseptic collections of stratospheric air above the tropopause can be obtained. We describe a recent collection of this kind and report microbiological analysis that shows the existence of viable cells at 41 km, falling to Earth at the rate of a few tonnes per day over the entire globe. Some of these cells have been cultured in the laboratory and found to include microorganisms that are not too different from related species on the Earth. This is in fact what the Hoyle-Wickramasinghe theory predicts. The weight of evidence goes against the more conservative explanation that organisms are being lofted to the high atmosphere from the ground

Jet-induced cratering of a granular surface with application to lunar spaceports

The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but previously unrecognized mechanism for erosion dynamics. It has also shown that slow regime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cratering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories cannot be entirely correct.Comment: 13 pages, link to published version: http://cedb.asce.org/cgi/WWWdisplay.cgi?090000

Turbulent length-time scales distributions in hydraulic jumps

Air–water flow measurements were performed in hydraulic jump flows for a range of inflow Froude numbers. The experiments were conducted in a large-sized facility using phase-detection intrusive probes. The void fraction measurements showed the presence of an advective diffusion shear layer where the air concentration vertical distributions were successfully compared with an analytical solution of the advective diffusion equation for air bubbles. In the air–water shear layer, a new empirical relationship between the maximum air concentration decay as a function of both the distance from the jump toe and the inflow Froude number was derived. Air–water turbulent time and length scales were deduced from auto- and cross-correlation analyses based on the method of Chanson (2007). The result provided some characteristic transverse time and length scales of the eddy structures advecting the air bubbles in the developing shear layer. The turbulence time scale data showed an increase with the relative elevation above the bed, as well as some decrease with increasing distance from the toe. The dimensionless integral turbulent length scale Lxz/d1 was closely related to the inflow depth

Influencia de la rama de ascenso en las características de descarga de los aliviaderos de perfil estricto

[ES] Este trabajo presenta un estudio de la influencia de la rama ascendente de la cresta del aliviadero de perfil estricto, aquél cuya geometría adopta la forma de la cara inferior de una lámina vertiente sobre vertedero en pared delgada, en el proceso de desagüe. Las alteraciones geométricas de la rama ascendente con respecto al criterio convencional de diseño propuesto por el U.S. Bureau of Reclamation, conducen a variaciones en la eficiencia de la descarga y la carga sobre la cresta, esto es, en los niveles de embalse agas arriba. Partiendo de la formulación analítica del problema del vertedero, se identifican tres regímenes característicos de funcionamiento, vertedero somero, intermedio y profundo, el último de los cuales gobierna el funcionamiento de la mayoría de aliviaderos de presas. Bajo este enfoque, se ha desarrollado un programa experimental que ha permitido cuantificar la influencia de la geometría del perfil en el régimen, de descarga, encontrándose una nueva geometría de diseño simple que garantiza igual o mejor respuesta hidráulica que la del perfil clásico. Se ha verificado experimentalmente, no obstante, que la mejora en la descarga no supone un riesgo de daños por cavitación.Moñino, A.; Losada, MÁ.; Riera, J. (2006). Influencia de la rama de ascenso en las características de descarga de los aliviaderos de perfil estricto. Ingeniería del agua. 13(1):25-33. https://doi.org/10.4995/ia.2006.2883OJS2533131Abecasis F.M. (1961). Soleiras descargadoras. Alguns problemas especiais. Memoria nº 175, LNEC. ISBN 972-49-0357-5.Bradley J.N. (1952). Discharge coefficients for irregular overfall spillways. Engineering monograph nº 9, U.S. Dept. of Interior.Chanson H. (1996). Prediction of the transition nappe/skimming flow on a stepped channel. Journal of Hydraulic Research, vol. 34, nº 3.Christodoulou G.C. (1993). Energy dissipation on stepped spillways. Journal of Hydraulic Engineering, vol. 119, nº 5, pp. 644-650.Diez-Gascón J., Blanco J.L., Revilla J. & García R. (1991). Studies o the hydraulic behaviour of stepped spillways. Water Power and Dam Construction, September, pp 22-226.Falvey H. (1990). Cavitation in Chutes and spillways. Engineering monograph nº 42. U.S. Dept. of Interior.Falvey H. (2003). Hydraulic design of labyrinth weirs. A.S.C.E. Press.Hager W.H. (1992). Spillways, shock waves and air entrainment. International Commission on Large Dams, Bulletin 81.Lemos F.O. (1981). Criterios para o dimensionamento hidráulico de barragens descarregadoras. Memoria nº 556, LNEC. ISBN 972-49-0662-5.Melshemer E.S. & Murphy T.E. (1970). Investigation on various shapes of the upstream quadrant of the crest of a high spillway. RR H-70-1. U.S Army Engineer Waterways Experimental Station.Moñino A. (1999). La medida de grandes caudales. Revisión de criterios aplicados a aliviaderos de presas. Tesina de Magíster, Universidad de Granada, 150 pp.Moñino A. (2004). Sobre soluciones analíticas y experimentales de flujo en régimen libre. Tesis Doctoral, Universidad de Granada, 255 pp. ISBN 84-688-9180-0.Moñino A. & Riera J. (2002). On the incipient self aerated flow in chutes and spillways. Journal of Hydraulic Research, vol. 40, nº 1, pp. 95-97.Olsen N. & Kjellesvig H. (1998). Three dimensional numerical modelling for estimation of spillway capacity. Journal of Hydraulic Research, vol. 36, nº 5, pp. 775-784.Peterka A.J. (1963). Hydraulic design of stilling basins and energy dissipators. Engineering monograph nº 25. U.S. Dept. of Interior.Rajaratnam N. (1990). Skimming flow in stepped spillways. Journal of Hydraulic Engineering, vol. 116, nº 4, pp. 587-591.Rice C.E. & Kadavy K.C. (1996). Model study of a roller compacted concrete stepped spillway. Journal of Hydraulic Engineering, vol. 122, nº 6.Riera J. & Moñino A. (1998). Accurate measurement of large flows. Revision on criteria applied to spillways of dams. Dam Safety, Balkema, Rotterdam.U.S. Army Corps of Engineers (1995). Hydraulic design of spillways. Technical Engineering and Design Gudes, nº 12, A.S.C.E. Press.U.S. Bureau of Reclamation (1948). Studies on crests for overfall dams. Bulletin 3, Part VI, Boulder Canyon Project Final Reports.U.S. Bureau of Reclamation (1970). Design of small dams. Dossat.Wen X., Guo Y. & Fang D. (1998). Numerical modelling of spillway flow with free drop and initially unknown discharge. Journal of Hydraulic Research, vol. 36, nº 5, pp. 785-801