橫向流流經兩個大小不同圓柱之流場模擬分析

Abstract

本研究主要利用非結構性寬頻元素法來進行數值模擬，當橫流流經圓柱流場時，會在圓柱後方產生週期性的渦流脫離，本研究試著影響圓柱下游的尾流，尋找改變渦流的結構且致力於降低渦流引致振動的程度，希望可以進一步的瞭解各種控制流體的方法，在降低阻力、提升升力以及抑制渦流脫離等方面來達到工程上的應用。而改善流體分離的方法有被動式控制法與主動式控制法兩種，本研究對此兩種方法分別去做研究；在被動式控制法方面，主要模擬低雷諾數(Re=100)時，探討控制圓柱的擺放角度以及兩圓柱間隔大小不同對主圓柱的影響；而在主動式控制法方面，主要是利用控制圓柱振動的方式，來探討對主圓柱所造成的影響。本研究發現當控制圓柱擺放在上方附近時，會造成主圓柱的阻力增加升力降低，其最大的程度是控制圓柱擺放在正上方90度且間隔為0時，其餘升阻力增加的程度就要看間隔比的影響。當控制圓柱擺放在抑制區時，會讓主圓柱有較佳的減阻效果，可減少阻力最多達到15%；但會讓阻力減少最多的位置是控制圓柱擺放在正前方時，此時阻力會比單一圓柱時低了35%。在模態方面，本研究依據兩圓柱的渦流是否有擺盪來做流場模態的分類，其結果發現會有三種模態，分別是流動模態一:兩圓柱的渦流都不擺盪，是屬於完全抑制的情況。而流動模態二:主圓柱的渦流有擺盪，而控制圓柱的渦流沒有擺盪，此模態有部分抑制以及互不影響兩種情況產生。而流動模態三:兩圓柱的渦流都有擺盪，是屬於完全不抑制的情況。而當控制圓柱在抑制區振動時，所引致的流場現象界於次臨界條件(Subcritical)以及過臨界條件(Supercritical)之間，因此，鎖頻區的現象跟以往文獻完全不同。

Numerical simulations with unstructured spectral element method were applied to the study of the phenomenon of cross flow around two cylinders. According to the research of Strykowski(1985), vortex shedding could be drastically inhibited with a control cylinder which in size is much smaller compared to the main cylinder. The purpose of this research is further focus on different parameters of control cylinder for drag reducing, lift increasing, and vortex inhibition. The Reynolds number of all simulations is 100. Both passive and active controls were studied. For passive controls, effects of position and gap between two cylinders were systematically investigated and three modes of flow patterns were classified. It was also found that 15% of drag reducing could be achieved at fully inhibition mode and maximum 35% of drag reducing could be achieved at partial inhibition mode. Based on different types of flow patterns, vibration was applied to the control cylinder for active controls. Effects of vibration amplitude and frequency were studied. Due to the transition between subcritical and supercritical flow, the lock-in area at fully inhibition mode is much different than previous literatures.