Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number
Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number
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并流浮力射流的研究：雷诺数和理查森数影响的实验
DOI：
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发表时间：
1992
影响因子：
3.7
通讯作者：
B. Cantwell
中科院分区：
文献类型：
--
作者：
E. C. Subbarao;B. Cantwell
实验已经进行了一个垂直射流的氦气进入并流的空气在一个固定的出口速度比为2.0。在所研究的所有实验条件下，流动都表现出很强的自激周期性。射流的固有频率特性、底层流动结构以及向湍流的过渡已在广泛的流动条件下进行了研究。实验是在一个可变压力的设施，这使得人们有可能改变雷诺数和理查森数独立。一个频闪纹影系统用于流动可视化和单分量激光多普勒测速仪被用来测量速度的轴向分量。该流动表现出几个有趣的特征。同向流的存在消除了在静止环境中浮力羽流典型的随机曲折。在高理查森数条件下氦喷流的周期性是惊人的。在这些条件下，向湍流的转变由射流和自由流流体的快速但高度结构化和可重复的破裂和混合组成。在Ri = 1.6时，可以看到流动的三维结构从一个循环到另一个循环重复。随着雷诺数或理查森数的增加，转捩点移近射流出口。纵向不稳定性的波长随Richardson数的增加而增加。在低Richardson数下，固有频率在惯性时间尺度上缩放，τ1 = D/Uj，其中D是射流直径，Uj是平均射流出口速度。在高Richardson数下，固有频率在浮力时间尺度上成比例，τ2 = [ρjD/g（ρ∞ -ρj）]½，其中g是重力加速度，ρj和ρ∞分别是射流和自由流密度。从一种流态到另一种流态的过渡发生在Richardson数从0.7到1的窄范围内。一个浮力Strouhal数是用来关联的高理查森数的频率行为。
Experiments have been carried out on a vertical jet of helium issuing into a co-flow of air at a fixed exit velocity ratio of 2.0. At all the experimental conditions studied, the flow exhibits a strong self-excited periodicity. The natural frequency behaviour of the jet, the underlying flow structure, and the transition to turbulence have been studied over a wide range of flow conditions. The experiments were conducted in a variable-pressure facility which made it possible to vary the Reynolds number and Richardson number independently. A stroboscopic schlieren system was used for flow visualization and single-component laser-Doppler anemometry was used to measure the axial component of velocity. The flow exhibits several interesting features. The presence of co-flow eliminates the random meandering typical of buoyant plumes in a quiescent environment. The periodicity of the helium jet under high-Richardson-number conditions is striking. Under these conditions transition to turbulence consists of a rapid but highly structured and repeatable breakdown and intermingling of jet and free-stream fluid. At Ri = 1.6 the three-dimensional structure of the flow is seen to repeat from cycle to cycle. The point of transition moves closer to the jet exit as either the Reynolds number or the Richardson number increases. The wavelength of the longitudinal instability increases with Richardson number. At low Richardson numbers, the natural frequency scales on an inertial timescale, τ1 = D/Uj where D is the jet diameter and Uj is the mean jet exit velocity. At high Richardson number, the natural frequency scales on a buoyancy timescale, τ2 = [ρjD/g(ρ∞ -ρj)]½ where g is the gravitational acceleration and ρj and ρ∞ are the jet and free-stream densities respectively. The transition from one flow regime to another occurs over a narrow range of Richardson numbers from 0.7 to 1. A buoyancy Strouhal number is used to correlate the high-Richardson-number frequency behaviour.