Author : Yunxiang Chen
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (15 download)
Book Synopsis Quantifying Flow Resistance in Natural Environments by : Yunxiang Chen
Download or read book Quantifying Flow Resistance in Natural Environments written by Yunxiang Chen and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Quantifying flow resistance is important for understanding and predicting energy and momentum transfer processes in natural environments such as ice-melting in the polar ice sheet and sediment transport in mountain streams. Flow resistance represents the retarding forces experienced by flow and the associated energy loss (dissipated through heat) in these systems. Flow resistance is mainly due to skin friction and form drag. Among many factors, the geometry of flow conduit and channel plays the dominant role. Though important for predicting the ice-sheet dynamics and mountain stream bed evolution, a general methodology for accurate and rapid quantification of flow resistance in natural environments has not been established due to either the inaccessibility of harsh polar environments or the lack of accurate measurements of realistic mountain streams. Many of existing flow resistance predictors are empirical and indirect. To fill this knowledge gap, this thesis work tried to directly quantify the flow resistance in these two geophysical settings using a workflow combing structure-from-motion (SfM) photogrammetry and computational fluid dynamics (CFD) models. Based on the high-resolution topographic data of a realistic subglacial conduit and three mountain streambeds, a series of CFD simulations were performed. The simulations based on a realistic subglacial conduit surface quantified the bulk flow resistance, in the form of Darcy-Weisbach friction factor, as around 2.41. Additional CFD simulations based on three simplified conduits revealed that cross-sectional shape and size variations of, and sinuosity in, this conduit dominate (~95%) the bulk flow resistance, whereas the contribution from surface roughness due to bottom rocks and icy roof is relatively unimportant (~5%). This result suggests that most glaciological models, which use surface roughness to quantify resistance and ignore the effects of cross-sectional variation and sinuosity, may significantly underestimate the flow resistance in realistic subglacial conduits. To evaluate the implications of the CFD simulated flow resistance, an open-source one-dimensional subglacial conduit model, conduitFoam, was developed and used to model the subglacial conduit dynamics and its dependence on different forcing conditions, such as entrance water head, ice-melting/creep-closure rate, discharge, water velocity, conduit size, and effective pressure. With the CFD simulated flow resistance, the results show that realistic subglacial conduits may have a smaller water velocity and effective pressure but larger conduit size compared to those currently predicted in glaciology models due to the choosing of a much smaller friction factor in the range 0.01-0.5. Real conduits also need longer time to reach a quasi-steady state because they have much larger flow resistance than the ones used in these models. This finding suggests that a re-evaluation of the effect of subglacial flow resistance may be necessary for ice-sheet or climate models.Applying the same workflow of combining SfM and CFD to mountain streams, we quantified the microtopography of mountain streambeds and directly calculated the flow resistance. The roughness of the streambeds can mainly be represented by the standard deviation of the surface microtopography. A new resistance relationship between the standard deviation and the flow resistance was then established. This new resistance formula links the flow resistance directly to the surface features which are easily quantifiable with data acquired from SfM photogrammetry. With the workflow and the new formula, it is possible to rapidly quantify flow resistance in natural mountain streams which may be hard to access. This thesis work further tested the applicability of traditional resistance relationship/formulas. CFD simulations with rough pipes reconstructed from different detrending/smoothing methods show that most traditional resistance relationships for rough pipes are still valid when relative surface roughness is less than 20%. Under this scenario, the hydraulic roughness required by traditional rough pipe theories can be estimated by 1.1-1.4 times of the surface roughness. For surfaces with relative surface roughness larger than 20%, direct simulations are necessary to determine the flow resistance.