Engineering School of Sustainable Infrastructure and Environment, U. of Florida, Gainesville, FL 32611, USA.
Water Res. 2017 May 15;115:347-359. doi: 10.1016/j.watres.2017.02.053. Epub 2017 Feb 23.
Stormwater, and also wastewater unit operations (UOs) to a much lower extent, are subject to unsteady hydrodynamic and particulate matter (PM) fluxes. Simulating fully transient clarification of hetero-disperse PM requires much greater computational expense compared to steady simulations. An alternative to fully unsteady methods are stepwise steady (SS) methods which use stepwise steady flow transport and fate to approximate unsteady PM clarification of a UO during transient hydraulic loadings such as rainfall-runoff. The rationale is reduced computational effort for computational fluid dynamics (CFD) compared to simulating continuous unsteadiness of such events. An implicit solution stepwise steady (IS) method is one approach which builds upon previous SS methods. The IS method computes steady flows that are representative of unsteady PM transport throughout an unsteady loading. This method departs from some previous SS methods that assume PM fate can be simulated with an instantaneous clarifier (basin) influent flowrate coupled with a PM input. In this study, various SS methods were tested for basins of varying size and residence time to examine PM fate. Differences between SS methods were a function of turnover fraction indicating the role of unsteady flowrates on PM transport for larger basins of longer residence times. The breakpoint turnover fraction was between two and three. The IS method best approximated unsteady behavior of larger basins. These methods identified limitations when utilizing standard event-based loading analysis for larger basins. For basins with a turnover fraction less than two, the majority of effluent PM did not originate from the event-based flow; originating from previous event loadings or existing storage. Inter- and multiple event processes and interactions, that are dependent on this inflow turnover fraction, are not accounted for by single event-based inflow models. Results suggest the use of long-term continuous modeling combined with the IS method for hydraulic, PM and chemical loadings to a UO when the turnover fraction is less than three.
雨水以及废水处理单元操作（UOs）在很大程度上受到不稳定水动力和颗粒物（PM）通量的影响。与稳态模拟相比，模拟多相 PM 的完全瞬态澄清需要更大的计算开销。替代完全非稳态方法的是逐步稳态（SS）方法，该方法使用逐步稳态水流输运和归宿来近似 UO 在雨水径流等瞬态水力负荷下的非稳态 PM 澄清。其基本原理是与模拟此类事件的连续非稳态相比，计算流体动力学（CFD）的计算工作量减少。一种隐式逐步稳态（IS）方法是对以前的 SS 方法的改进。IS 方法计算代表整个非稳态负荷过程中 PM 输运的稳态流。该方法与一些之前的 SS 方法不同，后者假设 PM 归宿可以用瞬时澄清器（水池）进水流量与 PM 输入耦合来模拟。在这项研究中，测试了不同大小和停留时间的水池的各种 SS 方法，以研究 PM 归宿。SS 方法之间的差异取决于周转率分数，这表明非稳态流速对较大停留时间的较大水池中 PM 输运的作用。转折点周转率分数在 2 和 3 之间。IS 方法最能近似较大水池的非稳态行为。这些方法确定了在使用更大水池的标准基于事件的加载分析时的局限性。对于周转率分数小于 2 的水池，大部分出流水体中的 PM 并非来自基于事件的水流，而是来自先前的事件负荷或现有的存储。相互作用和多重事件过程以及依赖于该流入周转率分数的相互作用，不受单个基于事件的流入模型的影响。结果表明，当周转率分数小于 3 时，对于 UO 的水力、PM 和化学负荷，使用长期连续模型并结合 IS 方法。
