仿生葉片風力機之暫態流場模擬與噪音分析

本篇論文分析風力機的噪聲頻率，得到特定噪音強度與頻率，以提供工程減噪處理的依據。內容分為兩個部分。第一個部分為驗證模擬，以現有的風力機實驗量測資料( NREL Phase VI )來進行模擬參數設置，並以穩定旋轉速度設置旋轉流體的模擬，以求得風力機於穩態中的流場資訊。　　第二個部分求解仿生風力機由靜止開始轉動至穩定時的流場及聲場資訊。仿生葉片以青楓（Green Maple）的雙翅果種子葉片為基礎，設計了以青楓種子葉片幾何外型的五葉片風力發電機，並以此風力發電機的模型進行模擬。全文使用商用軟體ANSYS Fluent中的 turbulence models 計算風力機於風速為3、5、7、9及11 m/s時的流場狀態，並以Acoustics-Ffowcs-Williams & Hawkings 演算模型計算以求得穩定旋轉速度時，頻率為2000 Hz內之噪音的頻譜分析。　　流場分析之結果顯示風力機葉片尖端產生較強的紊流強度之分布，葉片尖端同時也是壓力場、速度場、渦度場變化劇烈的地方，這些變化劇烈的地方更是與聲功率水平有著高度的關聯性。聲學計算結果發現當風速落在3、5 m/s，風力機產生的噪音源的主要頻段落在4~8 Hz的低頻率區域；風速落在7、9、11 m/s，風力機產生的噪音源的主要頻率為8 Hz的低頻率區域。頻譜分析的位置隨著越遠離風力機，整體噪聲頻率由葉片尖端後方的以低頻為主的噪音轉變為遠場區域的高頻噪音，葉尖所產生的大顆渦流破碎成小顆渦流，進而渦流之能量會逐漸被流體的黏度所消耗，結果整體的噪聲能量也會隨著離風力機下游距離的增加而下降。

關鍵字

仿生葉片,氣動聲學,噪聲分析,FW-H model,ANSYS Fluent

並列摘要

The present study analyzes the noise frequency of the wind turbine in order to obtain the specific noise intensity and frequency, and enhances noise reduction setup. This study has two main parts: The first part is the verification of CFD simulation. The experimental measurement data of existing wind turbine (NREL Phase VI) is used to setup the simulation parameters. A steady rotating speed of the turbine is superimposed on the wind flow, a rotating flow simulation condition, to obtain the simulated steady flow field information of the wind turbine. The second part is focused on solving the steady flow field and sound field information of the wind turbine with bionic blades. A transient simulation of the wind turbine, which is started from stationary to steady rotation, is performed. Based on the seed leaf geometric contour of the Green Maple, the present study designed a five-blade bionic wind turbine and used it to conduct the simulation. The wind speed conditions are 3, 5, 7, 9 and 11 m/s, with the turbulence model and the Acoustics FW-H model; the commercial software ANSYS Fluent is used to compute the flow and sound fields. Finally, the simulation obtains the frequency spectrum, which is below 2000 Hz, of noise generated by the steady rotating wind turbine. The results of the flow field analysis show that the blade tip of wind turbine produces stronger turbulence intensity distribution. The region near the tip of the blade is also a place with drastic changes of the pressure, velocity, and vorticity fields. These intensive changes are highly correlated with sound power levels. The acoustic calculation results show that when the wind speeds are at 3 and 5 m/s, the main frequency band of the noise source generated by the wind turbine is within the low frequency region from 4 to 8 Hz. When the wind speeds are at 7, 9, and 11 m/s, the main frequency of the noise source generated by the wind turbine is at 8 Hz. When the position is further downstream away from the wind turbine, the spectrum analysis shows that the overall noise frequency is converted from low frequency-based noise, right behind the blade tip, to high frequency dominated noise in the far-field region. The large eddies generated by the tip of the blade break into smaller eddies, and the energy of eddies is gradually dissipated by the viscosity of the fluid. Consequently, the overall noise energy also decreases with increasing downstream distance from the wind turbine.