[[alternative]]Identification Flutter Derivatives Using Forced Actuation Technique(III)

計畫編號：NSC98-2221-E032-023研究期間：200908~201007研究經費：604,000[[abstract]]我國近年來經濟高度成長，加快公共建設如高速公路或快速道路等的興建步伐。其中，橋樑常扮演交通運輸上的樞紐，橋樑結構之安全與否，直接間接地對附近地區經濟造成衝擊。橫跨河川的交通運輸橋樑，因聯接距離較長，在力學以及造型美觀的雙重考量下，工程上常採用纜索支撐橋樑設計，加上現今高強度且輕質建材之相繼發明，跨度愈來愈長的橋樑逐漸出現。我國近年來亦逐漸出現跨度較長的纜索支撐橋樑，隨著橋樑跨度的增長，增加了橋樑柔軟度，將使得這些纜索支撐橋樑受風力影響的振動行為愈來愈顯著，在可能發生較大變形之情況下，可能因發散型態之顫振現象出現，在某一臨界風速下，會形成橋樑動態不穩定而崩塌，對於橋樑結構之安全構成相當威脅，1940年美國之Tacoma 吊橋在風速二十米左右即發生顫振崩塌之例證可為殷鑑，所以工程上必須對顫振現象有深入的了解。 顫振臨界風速值來自顫振導數(Flutter Derivative)之計算，當風速增加至某一特定值時，自身擾動風力與原結構結合開始出現不穩定，此時之風速即為顫振臨界風速，因此，顫振導數值決定顫振行為。土木橋樑斷面有別於機翼之流線型特性，其形狀多半為鈍體，因此顫振導數無適合之理論式可使用，必須藉由風洞試驗予以識別。傳統自由振動識別法常伴隨三個主要缺點，其一，由於自由振動之歷時短，試驗者操作之細膩度及風洞周遭天候環境常嚴重影響識別結果；其二，受制於結構本身的勁度與阻尼以及風洞最大風速之限制，常導致無法提高約化風速的範圍；其三，識別出顫振導數之對應頻率為自由振動頻率，並非對應至理論上之外來振動頻率，因此所識別結果與真實值會有差異。 本計畫之研究目標為提出一套創新的氣彈實驗設計與流程，克服傳統自由振動識別法之缺點，提高識別結果之準確度。本研究利用間接強制振動方式驅動斷面模型之振動，藉由反應量測以逆向方式有系統地進行氣彈互制力之識別，可歸類為逆向問題(Inverse Problem)之範疇。工作時程預計在三年內完成，第一年(2007/8~2008/7)之工作已完成，內容為建構強制振動實驗架構，提出識別非耦合項顫振導數之方法與流程，並以流線型平板斷面模型進行實驗驗證；現在正進行第二年(2008/8~2009/7)之工作，主要工作內容為提出識別耦合項顫振導數之方法與流程，並以流線型平板斷面模型進行實驗驗證，目前已完成部分工作；接續之第三年(2009/8~2010/7)之主要工作內容則是利用前兩年之研究成果，有系統地識別一系列典型橋面版斷面之顫振導數，並與以往文獻之結果比較，確認此強制振動識別法之可行性。 本計畫提出創新氣彈互制之識別概念，目前國內外尚無相關或類似之研究，可提升國內橋梁風工程在國際間之能見度。 Among many infrastructures, bridge structures are specifically crucial in terms of the development of a country since they in general are responsible for linking cities culturally and economically. Most of the cross-river bridges, due to their longer spans, are considered to be of cable-supported type to meet both the esthetical and mechanical needs. In addition, the trend toward using newly developed stronger and lighter materials in construction further makes the design of cable-supported bridges with longer span plausible. However, the increased flexibility induced by the longer span will aggravate the wind effect on bridge structures. In some occasions, wind blow may trigger the occurrence of flutter – the most prominent aeroelasticity that can even cause the structural instability under a critical wind speed (flutter speed). A typical example is the collapse of Tacoma Narrows suspension bridge in 1940. Basically, the flutter speed can be obtained from the so-called flutter derivatives, which are the essential parameters to represent the self-excited forces. Because of the bluff body nature of bridge decks in civil infrastructures, the flutter derivatives are best determined by directly performing wind tunnel tests on bridge section models. As we know, the methodology of the conventional free vibration approach has been well developed and widely used in many practices to date. However, the typical shortcomings out of it lie in (1) the lack of consistency because of high sensitivity of free vibration responses to test condition or environment; (2) the limitation on the reduced velocity due to the constraints from the existent stiffness in the experimental model and the capacity of the wind tunnel facility; and (3) the discrepancy inherently inherited by treating the free vibration frequency as the excitation frequency. To overcome these, this project proposes a new approach to identify flutter derivatives using white-noise forced actuation technique, which can be categorized to the scope of inverse problem. A two-axes actuating device, which is composed of two independent electric servo-motors, was used to indirectly drive the motion of the bridge section model through the serial connection of springs. This project is scheduled to be completed in three years. The tasks in the 1st year (2007/8-2008/7) have been completed, which were to develop the identification scheme and technique for determining the uncoupled flutter derivatives, and verify it by performing wind tunnel tests for a chamfered plate section model that simulates thin plate. The on-going tasks in the 2nd year (2008/8-2009/7) is to develop the identification scheme and technique for determining the coupled flutter derivatives, and also perform verification tests for the same section model. So far, these tasks have been partially completed. Based on the results in the 1st and 2nd year, the mission in the 3rd year (2009/8-2010/7) is devoted to the systematic determination of flutter derivatives for a series of section models with typical shapes by using the proposed approach, and make comparisons with the results in the existent literature. This idea of identification for aeroelasticity in this research is firstly initiated in the wind engineering field, therefore, it is expected that a successful outcome will be promisingly contributed to the international wind engineering society.[[sponsorship]]行政院國家科學委員