國立臺灣海洋大學大型空蝕水槽大尺度流場SPIV量測

SPIV MEASUREMENTS OF LARGE-SCALE FLOW IN LARGE CAVITATION TUNNEL AT NATIONAL TAIWAN OCEAN UNIVERSITY

這是台灣第一次在大型封閉循環式水槽中進行大尺度流場的場域式細微量測。我們使用立體式質點影像測速法（stereoscopic particle image velocimetry，SPIV）於國立臺灣海洋大學的大型空蝕水槽（Large Cavitation Tunnel，LCT）測試段中量測其入流之平均流場與紊流特性。SPIV技術的核心乃是三維校正程序，可將兩台如人雙眼注視的PIV相機所拍攝的二維質點影像位移向量組合並轉換成量測面上的三維速度向量，由此我們可獲得大量的瞬時三維速度向量分佈，再經過系集平均（ensemble averaging）和雷諾分解（Reynolds decomposition）後可得三維平均流場及代表紊流特性的雷諾應力張量（Reynolds stress tensor）分佈。從量測結果發現，LCT測試段截面中心區域之入流軸向平均速度ZV的分佈均勻度佳，而名義入流速度（LCT控制儀表板上所顯示之壓差式流速儀的測量值）與SPIV測量的ZV有一不大但無法忽略的差距，顯示LCT的壓差式流速儀有必要進行檢查和校正；入流紊流強度（turbulence intensity）在名義入流速度6 m／s時約為4％，紊流特性接近等向性（isotropic）。

This was the first time for Taiwan to conduct such a detailed field measurement of large-scale flows inside a large, close-loop, circulation type of water tunnel. Stereoscopic Particle Image Velocimetry (SPIV) was used to measure the mean velocity fields and turbulence characteristics of the inflow to the test section of the Large Cavitation Tunnel (LCT) located at the National Taiwan Ocean University (NTOU). The core of the SPIV technique consists in the three-dimensional (3D) calibration procedures, which transform the two-dimensional (2D) displacements of particle images acquired by two PIV cameras into 3D velocity vectors. There-fore, a large amount of 3D instantaneous velocity-vector distributions can be obtained and used to calculate the 3D mean velocity field and Reynolds stress tensors with the statistical operations of ensemble averaging and Reynolds decomposition. The measure-ment results show that the distribution of the longitudinal mean velocity Vz over the central area of the inflow to the test section is quite uniform, while the nominal inflow velocity (gauge value on the LCT control panel that is measured with a pressure-differential flowmeter) is deviated from the SPIV-measured Vz by a small but not negligible amount, indicating that the LCT’s flowmeter needs to be inspected and calibrated; the turbulence intensity in the central area of the inflow to the test section is about 4% at a nominal inflow velocity of 6 m/s; the inflow turbulence is nearly isotropic.