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The influence of the length-to-depth aspect ratio and of wall asymmetry on the main vortical flow structures evolving in rectangular two-dimensional deep cavities is studied experimentally using wall-pressure and particle image velocimetry (PIV) measurements. Wall-pressure and cavity flow statistics have been analyzed and shown that the flow features are strongly affected especially by the asymmetry. An emphasis is given concerning the behavior of the shear layer oscillations that are compared to the analytical deep-cavity model prediction proposed by P.J.W. Block (NASA Tech. Note. 1976). The results show good agreement with Block's model if the value of the convection velocity is properly adjusted. Stochastic estimation of the cavity flows demonstrates that convective structures are involved downstream of the cavity along the wall and highlights the physical nature of the pressure-producing flow structures. Key words: vortex dynamics, asymmetric deep cavity, self-sustain oscillations, simultaneous PIV and pressure measurements. En utilisant des mesures de la pression du mur et de PIV, nous etudions experimentalement l'influence du rapport de forme longueur versus profondeur et de l'asymetrie du mur sur la structure d'ecoulement tourbillonnant principal evoluant dans des cavites profondes a deux dimensions rectangulaires. La pression du mur et la statistique d'ecoulement dans la cavite sont analysees et montrent que les caracteristiques d'ecoulement sont fortement affectees surtout par l'asymetrie. Nous mettons l'emphase sur le comportement des oscillations de la couche cisaillee, qui sont alors comparees aux predictions du modele analytique pour cavite profonde propose par P.J.W. Block (NASA Tech. Note. 1976). Les resultats montrent un bon accord avec le modele de Block, si la valeur de la vitesse de convexion est ajustee de facon appropriee. L'estime stochastique des ecoulements dans la cavite montre que les structures convectives existent en aval de la cavite le long du mur et souligne la nature physique des structures d'ecoulement produisant de la pression. [Traduit par la Redaction] Mots-cles: tourbillon, dynamique, cavite profonde asymetrique, oscillations auto-entretenues, mesures simultanees de PIV et de la pression., 1. Introduction Understanding the unsteady features of cavity flows presents great interest for the design of many engineering applications. Modelling these dynamics can be used to design and enhance feedback [...]

Tailings may undergo desiccation stress history under varied climatic and depositional parameters. While tailings substantially dewatered prior to deposition may experience desiccation under the greatest range of climatic variation, even conventionally deposited tailings may desiccate in arid climates at lower rates of rise. Bench-scale research has shown that the stress history imparted by desiccation substantially improves strength in gold tailings. The present study further investigates this phenomenon by simulating multi-layer deposition of high-density tailings using a modular drying box, 0.7 m by 1 min plan. The box is instrumented for directly measuring evaporation, drainage, water content, vertical volume change, and matric suction. Additional measurements included total suction at the surface as well as observations of crack development. The dewatering behaviour conforms to that predicted by previously published generic modelling, specifically that the presence of partially desiccated tailings initially accelerates, but then decelerates dewatering of fresh tailings. The shear behaviour of samples obtained using buried tubes and by driving thin-wall tubes into the multi-layer simulation are compared with shear behaviour of samples from bench-scale experiments. Shear strength of samples from the multi-layer simulation is independent of the sampling method, and shows higher strength than the bench-scale samples. The higher strength may be due to the greater number of wet-dry cycles or other age-related processes. Key words: tailings, desiccation, drying box, simple shear, cyclic. Les refus peuvent subir une etude des contraintes deja subies de dessiccation sous des parametres climatiques et sedimentaires varies. Malgre que des residus deshydrates en grande partie avant le depot peuvent eprouver la dessiccation sous la plus grande portee de variations climatiques, meme les residus conventionnellement deposes peuvent se dessecher dans les climats arides a des taux inferieurs d'elevation. Des recherches a l'echelle de banc d'essai ont montre que l'etude des contraintes deja subie conferee par la dessiccation ameliore considerablement la resistance dans les residus d'or. La presente etude examine davantage ce phenomene en simulant des depots de residus a multicouche a haute densite a l'aide d'une boite de sechage modulaire, 0,7 m par 1 m en plan. La boite est instrumentee pour mesurer directement l'evaporation, le drainage, la teneur en eau, le changement de volume vertical et la succion matricielle. Des mesures supplementaires inclus la succion totale a la surface et des observations du developpement de la fissure. Le comportement de deshydratation est conforme a celle predite par la modelisation generique deja publiee, plus precisement que la presence de residus desseches partiellement accelere initialement, mais puis ralentit la deshydratation des residus frais. Le comportement en cisaillement d'echantillons obtenus a l'aide de tubes enterres, en enfoncant des tubes a paroi mince dans la simulation a multicouche, est compare avec le comportement en cisaillement des experimentations a l'echelle de banc d'essai. La resistance au cisaillement des echantillons de la simulation a multicouches est independante de la methode d'echantillonnage, et montre une plus grande resistance que les echantillons a l'echelle de banc d'essai. La resistance la plus elevee peut etre a cause du plus grand nombre de cycles de mouillage- sechage ou d'autres processus lies a l'age. [Traduit par la Redaction] Mots-cles: residus, dessiccation, boite de sechage, cisaillement simple, cyclique., Introduction In the last few decades, more than 50 tailings impoundments have failed due to seismic activity, liquefaction, foundation failure, etc. Many of these failures have been attributed to liquefaction [...]

This paper examines the role of waves in the development of the instability of an unbounded transverse mixing layer in shallow waters. In supercritical state waves become significant. The matching of the wave-like asymptotic solution in the far field to the solutions near the inflection and the returning surfaces poses a difficult mathematical problem. We therefore conducted numerical simulations to find solutions directly from the shallow water equations. The simulations found that the growing waves in the unstable current had a consistent structure throughout the linear stage of the instability's development. The excitation, reflection, and transmission of waves produced a modulating instability that maintained a consistent structure as the amplitude of the instability increased over many orders of magnitude. This role of the waves on the supercritical instability, as delineated by our numerical simulations, is not solely describable by the classical analytical method. Key words: shear instabilities, shallow flow, waves in currents, direct numerical simulations. Cet article examine le role des vagues dans le developpement de l'instabilite d'une couche de melange transversal illimitee en eaux peu profondes. En etat supercritique, les vagues deviennent importantes. L'adequation de la solution asymptotique d'ondulation en champ lointain avec les solutions pres de l'inflexion et des surfaces de retour pose un probleme mathematique difficile. Nous avons donc effectue des simulations numeriques afin de trouver des solutions a meme les equations d'eau peu profonde. Les simulations ont revele que les vagues croissantes dans un courant instable avaient une structure coherente au long de la phase lineaire du developpement de l'instabilite. L'excitation, la reflexion et la transmission des vagues ont genere une instabilite qui a maintenu une structure coherente alors que l'amplitude de l'instabilite augmentait sur plusieurs ordres de grandeur. Ce role des vagues en matiere de l'instabilite supercritique, tel que defini dans nos simulations numeriques, n'est pas represente uniquement par la methode analytique classique. [Traduit par la Redaction] Mots-cles: instabilites causees par le cisaillement, debit en eau peu profonde, vagues dans les courants, simulations numeriques directes., Introduction The transverse mixing layer in shallow waters is a shear flow developed between free streams of different velocities. The mixing layer in deep water is unstable according to Rayleigh's [...]

The sudden appearance of localized turbulent patches in an otherwise quiescent flow was first observed by Osborne Reynolds for pipe flow (1) and has since been found to be the [...], Over a century of research into the origin of turbulence in wall-bounded shear flows has resulted in a puzzling picture in which turbulence appears in a variety of different states competing with laminar background flow (1-6). At moderate flow speeds, turbulence is confined to localized patches; it is only at higher speeds that the entire flow becomes turbulent. The origin of the different states encountered during this transition, the front dynamics of the turbulent regions and the transformation to full turbulence have yet to be explained. By combining experiments, theory and computer simulations, here we uncover a bifurcation scenario that explains the transformation to fully turbulent pipe flow and describe the front dynamics of the different states encountered in the process. Key to resolving this problem is the interpretation of the flow as a bistable system with nonlinear propagation (advection) of turbulent fronts. These findings bridge the gap between our understanding of the onset of turbulence (7) and fully turbulent flows (8,9).