報告書番号

N08014

タイトル（和文）

風洞実験による大気接地層の強風変動特性の把握

タイトル（英文）

Wind tunnel experiment on velocity fluctuations characteristics of a strong wind in the atmospheric surface layer

概要 （図表や脚注は「報告書全文」に掲載しております）

大気境界層内の大規模擾乱の影響を受ける接地層内の乱流構造を風洞実験により調べた．乱流境界層の長さスケールを越える大規模擾乱を，測定部風上に挿入したアクティブ乱流格子により生成した．測定部内の滑面平板上に発達する境界層下層の対数領域を対象に熱線およびPIV計測を行った．乱流統計量の鉛直分布や変動風速のパワースペクトルは観測値と良い一致を示した．大規模擾乱の影響を把握するために，擾乱を付加した実験と付加しない実験を比較した．そして，大規模擾乱が主に主流方向の変動風速と組織構造に影響すること，また，擾乱影響下では，上空にで生成された渦が地表面近傍へと侵入する流体運動が顕著となることを明らかにした．このような運動の再現には数値気象モデルに基づくラージエディーシミュレーションが適していることがわかった．

概要 （英文）

A wind-tunnel experiment was carried out to investigate the turbulence characteristics in the neutral surface layer under the influence of larger-scale disturbances, which often exist in the atmosphere. The disturbance, the scale of which is much larger than that of boundary layer shear turbulence, was generated by using an active turbulence grid installed at the upwind of the test section. The logarithmic layer of the boundary layer fully developed along a smooth flat surface was examined. The vertical profiles of turbulence statistics and spectra of velocity fluctuations agreed with the observations. Then, the effects of the larger-scale disturbance on turbulence characteristics were clarified by comparison with the experiment without a larger-scale disturbance; the larger-scale disturbance mainly affected the streamwise-velocity fluctuation and also coherence motions. These also revealed that the logarithmic layer with the larger-scale disturbance has a multi-layer structure, which corresponds to recent observations, i.e., the lower and upper sub-layer are controlled by sweep- and ejection-like events, respectively, also suggesting that the turbulence structure in the surface layer is mainly controlled by the top-down fluid motions. The mean velocity gradient and the Reynolds shear stress are, however, independent of the disturbance, and the eddy viscosity used in numerical weather models does not change so much with the larger-scale disturbance. Consequently, we found that a numerical weather model with large eddy simulation used with very fine resolution is more suitable rather than computational fluid dynamic models with boundary conditions including larger-scale disturbance to predict turbulence statistics in the near-neutral surface layer.

報告者

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