加裝導流板的接面流RANS與LES計算研究

A COMPUTATIONAL STUDY OF JUNCTION FLOW WITH A STRAKE BY RANS AND LES

導流板是接面流用以消除馬蹄渦的有效工具，我們設計一個導流板來進行消除馬蹄渦的研究，此導流板在中間對稱面上的曲線為四分之一橢圓，而在任一水平截面上亦為橢圓的一部份。在本論文中，入流攻角為0°；我們採用計算流體力學的方式來進行相關的研究。我們設計兩個不同幾何尺寸的導流板，兩者的高度相同、但長度不同，透過RANS（Reynolds-averaged Navier-Stokes equations）與LES（large eddy simulation）的計算，以了解流場的特性、細微流場的特徵、以及馬蹄渦被抑制的程度。相關計算研究顯示：透過兩者的計算，我們發現馬蹄渦的抑制與導流板的長度有關；RANS的計算就足以確定馬蹄渦受到抑制的程度，此一議題不須透過LES的計算來分析；不過，LES的計算顯示出許多非穩態流場的現象，包括渦流的生成、融合、衰減、不穩定的運動、不同的渦流模態等，這是RANS計算無法看到的。

Strake is an effective way to suppress horseshoe vortex in junction flows. In the present study, we designed a strake for this purpose. Its shape is a quarter ellipse on the vertical symmetric plane and a part of another ellipse on any horizontal cross section. We studied how the horseshoe vortex was mitigated by CFD when the flow incidence was at zero angle of attack. We designed two different strakes of the same height but different lengths. Then both RANS and LES computa-tions were conducted in order to understand their hydrodynamic performances and how well the horseshoe vortex was mitigated. The results show that the length of strake is an important factor controlling the mitigation of horseshoe vortex. In addition, it is sufficient from the RANS computation to judge how well the horseshoe vortes can be miti-gated. However, LES computations do reveal some unsteady flow characteristics such as vortex emergence, merging, and decay, and different vortical motion, which cannot be observed by RANS computations.