A parameterization of flow separation over subaqueous dunes

Flow separation plays a key role in the development of dunes, and modeling the complicated flow behavior inside the flow separation zone requires much computational effort. To make a first step toward modeling dune development at reasonable temporal and spatial scales, a parameterization of the shape of the flow separation zone over two-dimensional dunes is proposed herein, in order to avoid modeling the complex flow inside the flow separation zone. Flow separation behind dunes, with an angle-of-repose slip face, is characterized by a large circulating leeside eddy, where a separation streamline forms the upper boundary of the recirculating eddy. Experimental data of turbulent flow over two-dimensional subaqueous bed forms are used to parameterize this separation streamline. The bed forms have various heights and height to length ratios, and a wide range of flow conditions is analyzed. This paper shows that the shape of the flow separation zone can be approximated by a third-order polynomial as a function of the distance away from the flow separation point. The coefficients of the polynomial can be estimated, independent of flow conditions, on the basis of bed form shape at the flow separation point and a constant angle of the separation streamline at the flow reattachment point. \ud \u

Practice patterns and clinical outcomes in acute appendicitis differ in the elderly patient

Background: Appendicitis is the most frequent global abdominal surgical emergency. An ageing population, who often exhibit atypical symptoms and delayed presentations, challenge conventional diagnostic and treatment paradigms. Objectives: This study aims to delineate disparities in presentation, management, and outcomes between elderly patients and younger adults suffering from acute appendicitis. Methods: This subgroup analysis forms part of ESTES SnapAppy, a time-bound multi-center prospective, observational cohort study. It includes patients aged 15 years and above who underwent laparoscopic appendectomy during a defined 90-day observational period across multiple centers. Statistical comparisons were performed using appropriate tests with significance set at p < 0.05. Results: The study cohort comprised 521 elderly patients (≥65 years) and 4,092 younger adults (18–64 years). Elderly patients presented later (mean duration of symptoms: 7.88 vs. 3.56 days; p < 0.001) and frequently required computed tomography (CT) scans for diagnosis (86.1% vs. 54.0%; p < 0.001). The incidence of complicated appendicitis was higher in the elderly (46.7% vs. 20.7%; p < 0.001). Delays in surgical intervention were notable in the elderly (85.0% operated within 24 h vs. 88.7%; p = 0.018), with longer operative times (71.1 vs. 60.3 min; p < 0.001). Postoperative complications were significantly higher in the elderly (27.9% vs. 12.9%; p < 0.001), including severe complications (6.9% vs. 2.4%; p < 0.001) and prolonged hospital stays (7.9 vs. 3.6 days; p < 0.001). Conclusions: Our findings highlight significant differences in the clinical course and outcomes of acute appendicitis in the elderly compared to younger patients, suggesting a need for age-adapted diagnostic pathways and treatment strategies to improve outcomes in this vulnerable population

Modeling river dune evolution using a parameterization of flow separation

This paper presents an idealized morphodynamic model to predict river dune evolution. The flow field is solved in a vertical plane assuming hydrostatic pressure conditions. The sediment transport is computed using a Meyer-Peter–Müller type of equation, including gravitational bed slope effects and a critical bed shear stress. To avoid the necessity of modeling the complex flow inside the flow separation zone, we follow an approach similar to one used earlier to simulate the dynamics of wind-blown desert dunes. In case of flow separation, the separation streamline acts as an artificial bed and sediment avalanches down the leeside distributing evenly on the leeside at the angle of repose. Model results show that bed slope effects play a crucial role in the determination of the fastest-growing wavelength from a linear analysis. Flow separation is shown to be crucial to take into account if the dune lee exceeds a certain threshold slope. If flow separation is not included, dune shapes are incorrectly predicted and the dune height saturates at an early stage of bed form evolution, yielding an underprediction of dune height and time to equilibrium. The local bed slope at the dune crest plays a critical role for obtaining an equilibrium dune height. The simulation model is able to predict the main characteristics of dune evolution, such as dune asymmetry, dune growth, and saturation at a certain dune height. Dune dimensions, migration rates, and times to equilibrium compare reasonably well to various data sets

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