以二階紊流模式計算紊流流場

Abstract

本文採用 k-ε模式及雷諾應力傳輸模式應用於數紊流場的數值模擬，這些流場包括有：(1)背階紊流場，(2)不具旋轉的突張管流場，(3)數種旋轉流場。預測得到的結果以實驗數據來作比較。 對於非旋轉流場，兩種模式都獲得不錯的預測結果，‘k-ε 模式結果顯示在迴流區域的徑向擴散傳遞性較強。在背階流場雷諾應力模式得到的再接觸長度較 k-ε 模式長，但在突張管流場，再接觸長度則較短。 對於數重旋轉流場，兩種模式在遠離中心渦流核心的區域都能得到正確的平均流，而其預測結果與實驗數據最不相符等的地方為沿著中心線區域，在些區域，兩種紊流均無法正確地預測因渦漩所引起的軸向速度的強度。但大致，雷諾應力模式在平均流的預測仍優於 k-□ 模式。 本文也將兩種紊流模式所預測的應力場作一比較，可以發現不管在無旋轉或有旋轉的流場中，雷諾應力模式均得較接近實驗數據的結果。

Numerical simulations were appied to a number of turbulent flows, including:(1) flow past a backward-facing step, (2) flow through a sudden-expansion pipe without swirl and (3) various swirling flows, using an eddy-viscosity type k-ε model and Reynolds stress transport model (RSTM). The predicted mean and turbulent results were compared with measurements. For the non-swirling cases, the flow field were well represented by the two models, and the k-ε model predictions showed a slightly higher level of radial diffusive transport across the shear layer in the recirculation zone. In the case of flow past a backward-facing step the RSTM led to a larger reattachment length, but it yielded a smaller reattachment length for the flow through an axisymmetric diffuser. For the swirling cases,both models gave accurate values of the mean flow in regions remote from the central vortex core, the biggest discrepancies between predicticons and measurements occurred along the centreline in which the two models failed to reproduce correctly the strength of the decay of swirl-induced deceleration of the axial velocity. Generally, the RSTM provided good agreement with measured meana velocity profiles. The performance of the two turbulent closure models in the prediction of stress field is also presented. The RSTM predictions were found to be in good agreement with the experimental data, regardless of the flows.