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題名 | 阻塞研究--第二部份:瞬變渦流與魔子穩定度之數值模擬=Blocking Study--Modon Stability Part Ⅱ: Numerical Experiments |
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作 者 | 洪志誠; 林和; | 書刊名 | 大氣科學 |
卷期 | 25:1 1997.03[民86.03] |
頁次 | 頁99-121 |
分類號 | 328.2 |
關鍵詞 | 積分守恆量; 非均向性; 魔子; 瞬變渦流; 漸近穩定; Integral conseved quantity; Anisotropy; Modon; Transient eddy; Asymptotically stable; |
語文 | 中文(Chinese) |
中文摘要 | 本文是阻塞研究之第二部份,在無外力及摩擦的假設下,我們以系統積分保守量 探討初始動之能譜 (即,非均向性 ) 與魔子穩定度的關係。 並且以數值實驗模擬瞬變渦流 (transient eddy) 在魔子初期演變。 若控制初始擾動之平均尺度,線性實驗得到:當初始 擾動之非均向性 (在此, 非均向性定義為:波譜平面上,初始擾動能譜在 1 方向的能譜重 心與 k 方向的能譜重心之比值 ) 大於 1 為不穩定, 小於 1 為漸近穩定,等於 1 為中性 穩定。 根據資料計算得到瞬變渦流 (週期 2-10 天 ) 的非均向性約 0.4,按上面理論應屬 於穩定類型的擾動。而實驗結果與理論預期完全一致,瞬變渦流在魔子的初期演化屬於漸近 穩定。在穩定的條件下,魔子分流場提供一個有效的變形環境,可以快速吸收瞬變渦流的能 量,維持阻塞。非線性實驗得到,擾動的振幅會影響擾動與基本場間的能量交換速率。當瞬 變渦流之振幅與魔子之振幅相當時,擾動能量輸往魔子的速率最大。此外,擾動能量收支顯 示:積分初期,擾動能量趨近線性遞減率減弱,當擾動進入分流後,擾動與基本場間的能量 交換速率有加速的現象,約以時間更高次方 (大於二次方 ) 減弱。 |
英文摘要 | This paper is the second part of blocking studies. Asummed the flow is without external force and frictionless, we desigened numerical experiments and chose some integral conserved quantity to investigate the effect of anisotropy of transient disturbances on stability (the anisotropy, measured by a parameter ε defined by the ratio of the weighting spetrum of disturbances in the 1 direction to k direction in the spectrum space, see Eq. 10). We also examine the evolution of eddies embedded in a modon-like base state. The amplitude of perturbation are assumed to be infintismal, the linearized model gave results that if the disturbances are highly anisotropy in the way the ε >1, they become unstable, if the disturbances tilting to the other direction that ε <1, they are asymptotically stable, when the disturbances are isotropic that ε =1, the flow stays neutral. From the defintiion of the anisotropy (ie, ε ) of transient eddies is about 0.4, and the linearized run gives results identical to those predicated by stability analysis. The numerical simulations prove that the transient eddies is asymptotically stable. The diffluent flow of modon supports a environment when transient eddies past the diffluent section, the perturbed wave packets are brocken into south and north blanches, and loss energy to the base flow. When the amplitude of perturbation is about equal to the amplitude of modon, the nonlinear runs show that the the interaction between perturbations and modon reachs a peak. In the beginning the perturbed energy decreases linearly. When the eddies enter the diffluent area, the loss of perturbed energy accelerates, the decrease of energy fits a t-squared curve. |
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