Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session W15: General Fluid Dynamics: Obstacles, Flow Constrictions (10:00am - 10:45am CST)Interactive On Demand
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W15.00001: The Flow Physics of Social Distancers: Uncovering Patterns in Pandemic-Era Pedestrian Flows using Particle-Based Simulations Kelby Kramer, Gerald Wang In the era of COVID-19, the practice of social distancing has become a widespread and essential strategy for mitigating disease transmission. As people gradually return to shared indoor spaces in Fall and Winter 2020, the desire to maintain social distance will create new challenges for pedestrian flows in confined environments (e.g. corridors), even when buildings are kept at relatively low density. In this work, we report the results of 2D particle-based simulations modeling confined pedestrian counter-flows, with individual pedestrians described as active particles trying to maintain a target velocity while avoiding collisions. By systematically varying two quantities -- the pedestrian volume fraction $\phi$ and the ``social distance" $d$, a characteristic lengthscale reflecting mutual agreements about desirable inter-pedestrian spacing -- we compute fundamental diagrams for confined and socially distanced pedestrian flows, which show average pedestrian velocity $\bar v$ as a function of $\phi$ and $d$. These results illustrate the sensitive dependence of $\bar v$ on $d$, including a jamming transition, even at modest values of $\phi$. These results underscore the need for careful planning as large, shared indoor spaces return to any appreciable levels of occupation. [Preview Abstract] |
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W15.00002: Sea Stars Generate Downforce to Stay Attached to Surfaces Mark Hermes Intertidal sea stars often function in environments with extreme hydrodynamic loads that can compromise their ability to remain attached to---and move on---surfaces. While behavioral responses such as burrowing into sand or sheltering in rock crevices can help minimize hydrodynamic loads, previous work shows that sea stars also alter body shape in response to flow conditions. This morphological plasticity suggests that sea star body shape and size may play an important hydrodynamic role. Here, we show through laboratory water channel experiments that pentaradial sea stars generate downforce rather than lift. This downforce is created because the sea star bodies serve as pronounced ramps that divert fluid away from the surface. The hydrodynamic forces generated by surface-mounted bodies like hemispheres, cylinders, pyramids, and cubes have been characterized in many prior studies. However, none of these studies report downforce generation. The discovery of downforce generation may explain why sea stars are shaped as they are: the pentaradial geometry can aid adhesion to the surface in the presence of high hydrodynamic loads. Further, the morphological plasticity observed in nature could be attributed to the need for increased downforce in wave-exposed locations. [Preview Abstract] |
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W15.00003: A Study of Xylem Filtration for Fecal Coliform Removal Mira M. Wiley, Frank G. Jacobitz In Uganda, there is a need for a practical, inexpensive, and effective water filtration device to prevent the contraction of water-borne diseases. A previous study found pine xylem to be capable of filtering fecal coliform bacteria through pressure-driven filtration (Boutilier, et. al. 2014). The current work shows that eucalyptus xylem has the same capability. A prototype for a filtration device using xylem has been developed and tested. Testing revealed the device to be effective using only the force of gravity. Through an application of Darcy’s Law to the data collected from long-term flow rate experiments using pine xylem, its permeability factor was determined. The permeability factor average is 0.45 kg/(s*m*MPa) with a standard deviation of 0.06. Previous literature determined the range to be 0.5-0.6 kg/(s*m*MPa). The discrepancy in these values may be due to a variation in the xylem area, as a portion of the cross-section conducts water, while the remainder is impermeable. The disparity may also be due to slight differences in the properties of the pine. The close alignment of these values and initial results show promise for the filtration device in rural Ugandan communities. [Preview Abstract] |
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