報告書番号

T95032

タイトル（和文）

気流中の熱移動現象に及ぼす安定成層と乱流運動の影響

タイトル（英文）

EFFECTS OF THERMAL STRATIFICATION AND TURBULENT MOTION ON HEAT TRANSFER IN GRID-GENERATED FLOW WITH STABLE STRATIFICATION

概要 （図表や脚注は「報告書全文」に掲載しております）

夜間の都市境界層内の熱移動現象を解明するために，当所の長２０ｍ温調拡散風洞内に，温度成層の強さと乱流運動の強さの異なる５種類の安定成層乱流場を形成し，瞬間速度と瞬間温度の同時測定を行った。測定データから，気流中の乱流運動による熱移動量を表すパラメータである乱流熱フラックスなどの速度と温度に関する乱流統計量を評価した。その結果，温度成層が強く乱流運動が強い安定成層乱流場ほど浮力の効果が作用し，熱の逆勾配拡散現象が顕著に現れることを明らかにした。さらに，速度変動値と温度変動値のスペクトル解析結果から，安定成層が存在する気相格子乱流場では，熱の逆勾配拡散現象は特性周波数が１Ｈｚ～１０Ｈｚ程度の大スケールの流体運動によって起こることを明らかにした。

概要 （英文）

THE EVOLUTION OF GRID-GENERATED TURBULENCE WITH STABLE STRATIFICATION WAS STUDIED EXPERIMENTALLY IN A LOW-SPEED WIND TUNNEL TO INVESTIGATE HEAT TRANSFER IN THE NOCTURNAL URBAN BOUNDARY LAYER. STABLE STRATIFICATION WAS ACHIEVED WITH A TEMPERATURE STEP PROFILE BY HEATING THE UPPER HALFLAYER,WHILE TURBULENCE WAS PRODUCED BY A SQUARE MESH PLACED BEHIND THE HEATER. INSTANTANEOUS STREMWISE AND VERTICALVELOCITY FLUCTUATIONS AND TEMPERATURE FLUCTUATIONS WERE SIMULTANEOUSLYMEASURED BY THE COMBINED USE OF HOT-WIRE ANEMOMETERS AND A COLD-WIRE THERMOMETER. TO INVESTIGATE THE EFFECTS OF STRATIFICATION AND TURBULENT MOTION ON THE COUNTER-GRADIENT HEAT TRANSFER,MEASUREMENTS WERE CONDUCTED USING FIVE STRATIFIED FLOWS AT THE BRUNT-VAISALA FREQUENCY,NI,IN THE RANGE OF 0.979 TO 2.03 AND WITH DIFFERENT VALUES OF THE TURBULENCE REYNOLDS NUMBER,RE LAMBDA,IN THE RANGE OF 41 TO 96. A COMPARISON OF THE VERTICALHEAT FLUX CORRELATION COEFFICIENT AND TURBULENCE STATISTICE SHOWS THAT THE EFFECTS OF STABLE STRATIFICATION AND TURBULENT MOTION PROMOTE THE COUNTER GRADIENT HEAT TRANSFER. THE BEHAVIOR OF THE COSPECTRUM BETWEEN VERTICAL VELOCITY AND TEMPERATURE SUGGESTS THAT THE COUNTER GRADIENT HEAT TRANSFER APPEARS IN THE LOW-FREQUENCY REGION FOR THE THREE FLOWSWITH DIFFERENT VALUES OF THE TURBULENCE REYNOLDS NUMBER.THE RESULTS SHOW THAT COUNTER-GRADIENT HEAT TRANSFER OCCURS ONLY IN LARGE-SCALE AIR FLOWS.

報告者

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