TRANSITION IN A PIPE .1. ORIGIN OF PUFFS AND SLUGS AND FLOW IN A TURBULENT SLUG
TRANSITION IN A PIPE .1. ORIGIN OF PUFFS AND SLUGS AND FLOW IN A TURBULENT SLUG
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DOI：
10.1017/s0022112073001576
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发表时间：
1973-01-01
影响因子：
3.7
通讯作者：
CHAMPAGNE, FH
中科院分区：
文献类型：
--
作者：
WYGNANSKI, IJ;CHAMPAGNE, FH
有条件采样的热线测量是在管道中以对应于湍流开始的雷诺数进行的。管道是光滑的，仔细地对齐，所以在Re>；5×104处，湍急的弹状物自然出现。通过在入口引入扰动，可以在较低的Re处启动转变。对于光滑或只有轻微扰动的进气口，在流动在管道中完全发展之前很久，边界层中的不稳定就会发生转变。这种类型的过渡会产生占据管道整个横截面的湍动弹状物，并随着向下游流动而变长。湍流弹状物的前锋和后锋都有明确的定义。界面的速度和流体的速度之间似乎存在着一种独特的关系，通过这种关系可以防止湍流的再分层。虽然在相同的流动条件下，各个弹状物的长度不同，但弹状物的长度与管道的长度具有相同的数量级。弹状流内部的流动结构与充分发展的湍流管流中的流动结构相同。在界面附近，平均运动从层流状态变为湍流状态，速度分布出现拐点。界面附近的总湍流强度很高，可以达到管道中心速度的15%。对界面附近的流动进行了湍流能量平衡。如果界面的某一部分不夹带非湍流流体，则所有对能量平衡有贡献的项必须在界面上的某处完全消失。扩散也包括压力传递，这似乎是湍流能量转移到非湍流流体的最有可能的机制。界面处的耗散项可以忽略不计，并且随着向弹状物内部湍流能量的增加而增大。当大扰动引入入口时，在远下游观察到层流和湍流的混合流动。然而，入口附近的流动在Re低得多的情况下是湍流的。以略小于管内平均速度的速度向下游对流的湍流区，从今以后我们称之为喷雾。喷雾的前锋没有明确定义的界面，而后锋仅在中心线附近明确定义。喷雾的长度和结构与造成它的障碍物的性质无关，前提是后者足够大，足以在入口处产生湍流。喷雾将在后面更详细地讨论。
Conditionally sampled hot-wire measurements were taken in a pipe at Reynolds numbers corresponding to the onset of turbulence. The pipe was smooth and carefully aligned so that turbulent slugs appeared naturally at Re > 5 × 104. Transition could be initiated at lower Re by introducing disturbances into the inlet. For smooth or only slightly disturbed inlets, transition occurs as a result of instabilities in the boundary layer long before the flow becomes fully developed in the pipe. This type of transition gives rise to turbulent slugs which occupy the entire cross-section of the pipe, and they grow in length as they proceed downstream. The leading and trailing ‘fronts’ of a turbulent slug are clearly defined. A unique relation seems to exist between the velocity of the interface and the velocity of the fluid by which relaminarization of turbulent fluid is prevented. The length of slugs is of the same order of magnitude as the length of the pipe, although the lengths of individual slugs differ at the same flow conditions. The structure of the flow in the interior of a slug is identical to that in a fully developed turbulent pipe flow. Near the interfaces, where the mean motion changes from a laminar to a turbulent state, the velocity profiles develop inflexions. The total turbulent intensity near the interfaces is very high and it may reach 15% of the velocity at the centre of the pipe. A turbulent energy balance was made for the flow near the interfaces. All of the terms contributing to the energy balance must vanish identically somewhere on the interface if that portion of the interface does not entrain non-turbulent fluid. It appears that diffusion which also includes pressure transport is the most likely mechanism by which turbulent energy can be transferred to non-turbulent fluid. The dissipation term at the interface is negligible and increases with increasing turbulent energy towards the interior of the slug.Mixed laminar and turbulent flows were observed far downstream forwhen a large disturbance was introduced into the inlet. The flow in the vicinity of the inlet, however, was turbulent at much lower Re. The turbulent regions which are convected downstream at a velocity which is slightly smaller than the average velocity in the pipe we shall henceforth call puffs. The leading front of a puff does not have a clearly defined interface and the trailing front is clearly defined only in the vicinity of the centre-line. The length and structure of the puff is independent of the character of the obstruction which created it, provided that the latter is big enough to produce turbulent flow at the inlet. The puff will be discussed in more detail later.