抽氣開口在側風下所形成的捕集區──數值計算

Capture Envelope of a Hood Opening under Crosswind Condition--A Numerical Approach

吸氣開口於側風中會在開口前方形成──捕集區，所有位於捕集區內的流線均會導入開口：所有位於捕集區外的流線則導入側風下游無限遠處。吸氣開口對氣流的捕集能力可由捕集區所涵蓋的範圍決定。在本研究中，一位於無限平面㆖的圓形開口用以模擬計算外裝形氣罩的捕集能力。流場則採用勢流理論計算，將吸氣開口所產生的流場直接疊加於橫向均勻流場。捕集區的範圍則可利用分隔流線求得。經初步探討發現捕集區範圍參數對開口直徑的比值與側風對開口面平均風速比值，fcV /V，相關。將多次計算結果加以整理後，更發現捕集區的幾何形狀可根據 Rankine’s 鈍體幾何形狀修正而得。當 V1/V2→∞上述結果與 Rankine’s 鈍體幾何形狀完全相符。當 V1/V2< 2時，則需更進醫步的修正，使捕集區經驗幾何形狀與計算結果更為吻合。

The flow induced by an exhaust opening will form a capture envelope in the presence of a crosswind. All streamlines located within the envelope will lead to the opening; those outside the envelope will lead to infinite downstream. The capture performance of the opening under crosswind then is determined by the extent of the capture envelope. In this study, the opening on an infinite wall served as a model of an exterior hood used in a local exhaust device. The flow field was computed by potential flow theory by superposing the exhaust airflow on the uniform crosswind. The boundary of the capture envelope then was determined from the limiting streamlines found by iterative computation. The geometrical parameters of the envelope normalized to the diameter of the opening were found to be a function of the ratio of exhausted face velocity at the opening to the crosswind velocity, f c V /V . Based on the results obtained by extensive computations, and a pattern modified from the classical 'Rankine's nose' problem, the empirical geometry of the envelope was obtained. The result was consistent with the case of 'Rankine's nose' at → ∞ f c V /V ; while / < 2 f c V V , the pattern was further modified to fit the computed results.