外文翻譯--使用cfd模型分析規(guī)模和粗糙度對(duì)反弧泄洪洞的影響-wenkub
2023-05-19 08:01:08 本頁(yè)面
【正文】 increase in severity as the ratio of prototype to model size increases or the number of physical processes to be replicated simultaneously increases. Laboratory effects arise because of limitations in space, model constructability, instrumentation, or measurement. Generally, steady nonuniform flow characteristics in open channel flow with hydraulic structures can be explained as a following relationship (ASCE, 2020). 9 where Sw is water surface slope, So is channel bottom slope, h is water depth, k is roughness height of solid boundary, V is flow velocity, g is gravitational acceleration, and vare dynamic iscosity, density, surface tension of water, respectively. Eq. (1) states that water surface profile is expressed as bottom slope, relative roughness height, Froude number, Reynolds number and Weber number. Similarity of variables in Eq. (1) between scaled model and prototype is maintained for the hydraulic model to properly replicate features of a plicated prototype flow situation. Generally, geometric similarity (So) is achieved and experiments are carried out by using Froude number similarity in the hydraulic Table 1. Approximate Values of Roughness Height, k model on the open channel flow and hydraulic structures. Water is used to analyze the flow characteristics of scaled model, thus modeling accuracy is promised because the properties of water are not scaled. So, a small scale model may causes a failure to simulate the forces attendant to fluid properties such as viscosity and surface tension, to exhibit different flow behavior than that of a prototype. Moreover, relative roughness height of the scaled model cannot be exactly reproduced because materials of experiment are limited. Previous study on the scale limits of hydraulic models leads to some guidelines. The Bureau of Reclamation (1980) used length scale ratios of Lr = 30~100 for models of spillways on large dams. And model flow depths over a spillway crest should be at least 75 mm for the spillway’s design operating range. The average roughness height for a given surface can be determined by experiments. Table 1 gives values of roughness height for several kinds of material which are used for construction of hydraulics structures and scaled models (Hager, 1999). To determine quantitatively how scale and roughness effects influence the model results, 10 it is possible to use a series of scale models with different surface roughness including prototype. But the hydraulic model experiments are expensive, timeconsuming, and there are many difficulties in measuring the data in detail. Today, with the advance in puter technology and more efficient CFD codes, the flow behavior over ogeespillways can be investigated numerically in a reasonable amount of time and cost. Equations and Computational Scheme The mercially available CFD package, FLOW3D, uses the finitevolume approach to solve the RANS equations by the implementation of the Fractional Area / Volume Obstacle Representation (FAVOR) method to define an obstacle (Flow Science, 2020). The general governing RANS and continuity equations for inpressible flow, including the FAVOR variables, are given by where ui represent the velocities in the xi directions which are x, y, zdirections。 fi represents the Reynolds stresses for which a turbulence model is required for closure. To numerically solve the rapidly varying flow over an ogee crest, it is important that the free surface is accurately tracked. In FLOW3D, free surface is defined in terms of the volume of fluid (VOF) function which represents the volume of fraction occupied by the fluid. A twoequation renormalized group theory models
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