以 WRF-CFD 耦合方式重建 NA-706 飛航事故風場

Abstract

2018年2月5日，一架編號為NA-706的UH-60M型直升機，於台北時間23:48:36自蘭嶼機場起飛後約81秒後，墜落於蘭嶼島西南側外海3.5公里處，機上6人全數罹難。本研究企圖利用多尺度耦合的模擬方式重建NA-706事故現場的風場，透過詳細的流場資訊，判斷直升機飛行過程中是否曾遭受到極端的氣流影響而可能造成此次的意外。 多尺度耦合模擬的方法是將中尺度模擬結果作為小尺度模擬的邊界條件，再進行小尺度的模擬以得到更細微的流場資訊；本研究中尺度模擬是利用數值天氣預報模型 (Weather Research and Forecasting, WRF) 所得之模擬結果，由台大林博雄教授研究團隊所提供，小尺度則採用計算流體力學 (Computational Fluid Dynamics, CFD) 來進行模擬。本研究乃透過Monin-Obukhov相似理論 (Monin-Obukhov similarity theory, MOST) 和梯度法(gradient method) 來補足中尺度在近地表區域因解析度不足而缺失的風場資訊，並使用距離的倒數為權重 (inverse distance weighting, IDW) 的插值法將中尺度的模擬結果內插至小尺度邊界上，再透過質量校正解決小尺度計算域邊界質量不守恆的情形。最後再使用SST k-ω紊流模型進行RANS (Reynolds-Averaged Navier-Stokes equation)模擬。 觀察中尺度的模擬結果可發現，風向在高度超過3,000公尺是穩定的西風，在3,000公尺到1,500公尺左右有回流區產生，在其之下是吹東風，到地表前逐漸轉向變成北北東風，因此蘭嶼機場當時是處於背風側。為了找出合適的CFD模擬設置，研究中比較不同計算域高度(3,000、4,000公尺)及頂部邊界條件(速度入口、對稱型邊界條件)的模擬結果，發現太低的計算域高度會導致低空產生不合理的加速現象，尤其在山地的上空，應該避免使用。將事故機的飛行軌跡放入CFD模擬結果比較，發現事故機在飛行過程中確實受到大氣流經蘭嶼島產生的尾流影響。但由飛行軌跡周遭的流線圖可觀察到，除了在起飛位置附近有因地形造成的小漩渦外，在中後段的飛行過程並沒有產生明顯的紊流結構，沿著飛行軌跡的紊流動能分布也未看出有出現超過顯著紊流等級的情況。

On February 5, 2018, a UH-60M helicopter numbered NA-706 crashed 3.5 km southwest of Lantau Island about 81 seconds after taking off from Lantau Airport at 23:48:36 Taipei time; all six people on board perished in the accident. This study attempts to reconstruct the wind field at the accident site of NA-706 by using a multi-scale simulation to judge whether the helicopter was affected by extreme air currents during the flight that may have caused the accident. The multi-scale simulation involves using the results of a mesoscale simulation as the boundary conditions for a microscale simulation to obtain more detailed flow field information. In this study, the results of the mesoscale simulation were obtained by using the numerical weather research and forecasting (WRF) model, which was provided by Prof. Bo-Hsiung Lin's research team at NTU, and the microscale simulations were conducted using computational fluid dynamics (CFD). The Monin-Obukhov similarity theory (MOST) and the gradient method are used to supplement the wind field information in the near-surface region which is absent in the mesoscale simulation. The inverse distance weighting (IDW) interpolation method is used to interpolate the mesoscale simulation results to the microscale boundaries, followed by a correction of mass flow rate in order to preserve the mass conservation. Finally, a RANS (Reynolds-Averaged Navier-Stokes equation) simulation with the SST k-ω turbulent flow model is performed. As seen from the mesoscale simulation result, the wind direction is steady westward at altitudes above 3,000 meters, and there are circulation zones between 3,000 meters and 1,500 meters, below which the easterly wind blows and gradually changes to north-northeast wind before reaching the surface. It implies Lanyu Airport was on the leeward side. The microscale simulation suggests a simulation domain at least 4,000m high is necessary in order not to cause unreasonable acceleration phenomenon in the lower atmosphere, especially in the vicinity of mountains. The CFD simulation results show that the accident helicopter was affected by the wake generated by the air flow over Lantau Island. However, only the small vortices caused by the terrain are observed near the takeoff position and no significant turbulence is found in the middle and rear part of the flight trajectory. Besides, the turbulent kinetic energy along the flight trajectory did not exceed the noticeable turbulence level.