MLSを導入したPTVによる渦度推定法の提案と風波流速場への適用
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Copyright (c) 2009 社団法人 土木学会, 著者別名: Eto, Takeharu〈エトウ, タケハル〉[Abstract]Particle Tracking Velocimetry (PTV)により得られた多数のランダムな点における流速ベクトルから渦度を求めるために，数値計算分野のメッシュレス法として開発されたMoving Least Square (MLS)法を用いた渦度推定法を提案する．MLS法ではランダム点上の流速分布から格子点上の流速分布に変換することなく，直接渦度分布を求めることができる．提案したMLSによる渦度推定法の最適適合領域の推定式をモンテカルロシミュレーションにより求めた．一般的に用いられる格子変換および循環法との精度比較の結果，提案した渦度推定法の方が高精度で計測できることを示した．また，風波流れ場に対して高速ビデオカメラを用いてPTV計測を行い，求められた流速分布に提案した渦度推定法を適用した結果，空間的な渦度分布を連続的に追跡することが可能となった．[Abstract] A method to measure two-dimensional vorticity fields was developed by combining MLS, the Moving Least Square method, with PTV. A velocity vector at a grid point is approximated with a pair of quadratic functions fitted to velocity vectors at particle positions estimated by PTV which randomly distributed around the grid point. The vorticity is calculated by differentiating the quadratic functions. A two-dimensional Gaussian weight function is employed for fitting. The size of the interrogation area is represented with the standard deviation of the weight function. The expression of the optimum size was analytically derived with a coeffient finally determined by Monte Carlo simulation with synthesized velocity fields with random measurement errors. The accuracy of the proposed method was compared with that of a conventional grid based method, which verified superiority of the proposed one. The method was applied to measurement of vorticity distribution generated by wind waves. Three synchronized high-speed video cameras were set beside a wind-wave tunnel: two were used to capture particle images in a wider field of view and one to detect the water surface. Temporal and spatial behaviors of vorticies generated at and near a water surface can be clearly traced by the proposed method.