Steady analysis of transcritical flows in collapsible tubes with discontinuous mechanical properties: implications for arteries and veins

We study the conditions under which discontinuous mechanical properties of a collapsible tube can induce transcritical flows, i.e. the transition through the critical state where the speed index (analogous to the Mach or the Froude numbers for compressible and free surface flows, respectively) is one. Such a critical transition may strongly modify the flow properties, cause a significant reduction in the cross-sectional area of the tube, and limit the flow. General relationships are obtained for a short segment using a one-dimensional model under steady flow conditions. Marginal curves delimiting the transcritical regions are identified in terms of the speed index and the cross-sectional area ratio. Since there are many examples of such flows in physiology and medicine, we also analyse the specific case of prosthesis (graft or stent) implantation in blood vessels. We then compute transcritical conditions for the case of stiffness and reference area variations, considering a collapsible tube characterized by physiological parameters representative of both arteries and veins. The results suggest that variations in mechanical properties may induce transcritical flow in veins but is unrealistic in arterie

An eco-geomorphic model of tidal channel initiation and elaboration in progressive marsh accretional contexts

The formation and evolution of tidal networks have been described through various theories which mostly assume that tidal network development results from erosional processes, therefore emphasizing the chief role of external forcing triggering channel net erosion such as tidal currents. In contrast, in the present contribution we explore the influence of sediment supply in governing tidal channel initiation and further elaboration using an ecogeomorphic modeling framework. This deliberate choice of environmental conditions allows for the investigation of tidal network growth and development in different sedimentary contexts and provides evidences for the occurrence of both erosional and depositional channel-forming processes. Results show that these two mechanisms in reality coexist but act at different time scales: channel initiation stems from erosional processes, while channel elaboration mostly results from depositional processes. Furthermore, analyses suggest that tidal network ontogeny is accelerated as the marsh accretional activity increases, revealing the high magnitude and prevalence of the depositional processes in governing the morphodynamic evolution of the tidal network. On a second stage, we analyze the role of different initial topographic configurations in driving the development of tidal networks. Results point out an increase in network complexity over highly perturbed initial topographic surfaces, highlighting the legacy of initial conditions on channel morphological properties. Lastly, the consideration that landscape evolution depends significantly on the parameterization of the vegetation biomass distribution suggests that the claim to use uncalibrated models for vegetation dynamics is still questionable when studying real cases

Multiple states for flow through a collapsible tube with discontinuities

We study the occurrence of the multiple steady states that flows in a collapsible tube can develop under the effect of: (i)geometrical alterations (e.g.stenosis), (ii)variations of the mechanical properties of the tube wall, or (iii)variations of the external pressure acting on the conduit. Specifically, if the approaching flow is supercritical, two steady flow states are possible in a restricted region of the parameter space: one of these flow states is wholly supercritical while the other produces an elastic jump that is located upstream of the variation. In the latter case the flow undergoes a transition through critical conditions in the modified segment of the conduit. Both states being possible, the actual state is determined by the past history of the system, and the parameter values show a hysteretic behaviour when shifting from one state to the other. First we set up the problem in a theoretical framework assuming stationary conditions, and then we analyse the dynamics numerically in a one-dimensional framework. Theoretical considerations suggest that the existence of multiple states is associated with non-uniqueness of the steady-state solution, which is confirmed by numerical simulations of the fully unsteady proble

Preliminary results on the evolution of proglacial ponds in the deglaciating Alps

Deglaciation is one of the most evident effects of the ongoing climatic changes on the Alpine environment. One of its common consequences is the formation of new water bodies in the proglacial area, where proglacial lakes and ponds are increasingly relevant ecosystems for the mountain landscape. The EVERLAKE project focuses on a recent system of proglacial ponds that originated from the retreat of the Zufall/Cevedale Glacier (Plima catchment, Central/Eastern Italian Alps). The aims of the project are to: (i) provide a first hydroecological characterisation of these pond ecosystems from a physical, chemical and biological point of view; (ii) understand their evolutionary trend during the process of deglaciation, with a space-for-time substitution approach. Here, we present data collected during the ice-free season 2022, showing the seasonal development occurring in three ponds located along a gradient of distance from the Cevedale Glacier terminus (i.e., at 2700-2900 m a.s.l.). We monitored water level, temperature and electrical conductivity and assessed the origin of water through analyses of stable isotopes (δ2H, δ18O). Bathymetric measurements were performed to estimate the residence time of each waterbody. We also analysed basic water chemistry, concentrations of trace elements, benthic and planktonic chlorophyll-a and organic content. The biological communities of these poorly known aquatic ecosystems were characterised by adopting an integrated approach combining morphological observations of microalgae and 16S and 18S rRNA metabarcoding of eDNA from both benthic and planktonic samples. The ponds showed different ecological conditions related to their distance from the glacier margins

Flow Reorganization in an Anthropogenically Modified Tidal Channel Network: An Example From The Southwestern Ganges-Brahmaputra-Meghna Delta

We examine variations in discharge exchange between two parallel, 1‐ to 2‐km‐wide tidal channels (the Shibsa and the Pussur) in southwestern Bangladesh over spring‐neap, and historical timescales. Our objective is to evaluate how large‐scale, interconnected tidal channel networks respond to anthropogenic perturbation. The study area spans the boundary between the pristine Sundarbans Reserved Forest, where regular inundation of the intertidal platform maintains the fluvially abandoned delta plain, and the anthropogenically modified region to the north, where earthen embankments sequester large areas of formerly intertidal landscape. Estimates of tidal response to the embankment‐driven reduction in basin volume, and hence tidal prism, predict a corresponding decrease in size of the mainstem Shibsa channel, yet the Shibsa is widening and locally scouring even as the interconnected Pussur channel faces rapid shoaling. Rather, the Shibsa has maintained or even increased its pre‐polder tidal prism by capturing a large portion of the Pussur\u27s basin via several transverse channels that are themselves widening and deepening. We propose that an enhanced tidal setup in the Pussur and the elimination of an effective Shibsa‐Pussur flow barrier are driving this basin capture event. These results illustrate previously unrecognized channel interactions and emphasize the importance of flow reorganization in response to perturbations of interconnected, multichannel tidal networks that characterize several large tidal delta plains worldwide