圆柱尾流的绝对不稳定性

ABSOLUTE INSTABILITY IN CRICULAR CYLINDER WAKES

在水槽和低湍流度水洞中进行亚临界雷诺数圆柱尾流稳定性实验来流速度由零缓慢增长到一定值后保持不变，稳定足够长时间后，在流向某站位处给流场一个有限幅值的脉冲扰动，测量扰动前后相当长时间内下游尾流速度信号的变化情况当雷诺数处于高亚临界值时，未受扰动的尾流速度脉动很小，处于定常状态，但对近尾流进行脉冲扰动后，能够激发出不衰减的旋涡脱落发现扰动位置限制在圆柱后一定范围内才能有效，再往下游则扰动随时间衰减．说明圆柱近尾流中存在一个绝对不稳定区，在该区域内的扰动将在当地放大，经过复杂的演化，最后形成不衰减的旋涡脱落.

The stability of circular cylinder wake at low Reynolds numbers has been studied experimently. The experiments were separatly carried out in a propeller-driven water channel and in a gravity-driven low turbulence water tunnel. The test section of water channel was 15 cm in width, 10cm in depth and 100 cm in length, and the test section of water tunnel was 4 cm in width.10cm in height and 100 cm in length. The cylinder model used in channel and tunnel were 15 cm long, 0.2 cm in diameter and 4cm long, 1 cm in diameter respectively. The model cylinder was placed horizentally across the test section, both ends in close contact with the vertical walls of the test section. Turbulence intensity of the oncoming flow. was 0.8% in channel and 0.25% in tunnel.The apparatus used both in water channel and water tunnel that applied disturbances to wake flow field was made up of a thin tube, a cylindric container, a round thin aluminium plate and an elastic membrane. The thin tube made of stainless steel, outer diameter 0.12cm, inner diameter 0.08cm, was in inserted into the water tunnel from above, perpendiculely penetrating through the upper wall. The end of the steel tube was less than 0.5 cm deep in water so that no disturbances were imposed to the wake flow by the tube. The end out of the tunnel was connected to the container. The container was sealed by the elastic membrane at the upper end. The round plate, with diameter slightly smaller than the inner diameter of the container, was glued to the elastic membrane, forming a plate that could move up and down in the container. Water from the tunnel filled the container and it was kept at appropriate height to keep the membrane in equilibrium. When an impulsive force was applied upon the membrane, an impulse jet from the thin tube entered the flow field, imposing a disturbance to the wake, then water was sucked back into the container, the membrane backed to the equilibrium. It was tested that one impulse force could cause just one impulse jet, the second time disturbing never appeared. In the test, velocity of the oncoming flow was slowly increased from zero to a certain value and then kept constant there to allow the flow to stabilize. An impulse disturbance was applied to the cylinder wake at some point and the downstream velocity was recorded in long periods before and after the disturbance by a hot film probe located at a point 26 diameters downstream the cylinder.When the Reynolds number is smaller than the critical one, fluctuation of the undisturbed wake velocity is very small. But disturbances applied to the near wake excite vortex shedding that does not decay. It is observed that the disturbance applied must be within certain range behind the cylinder, otherwise, when the disturbance applied to a point some distance downstream that range, it decays. The experimental results indicate that there is an absolutely unstable zone in the near wake. Disturbances within the zone locally amplify and spread, after complex transitions,develop into vortex shedding in the end. Downstream the absolute instability zone, is a broad area called convective instability zone. Impulse disturbance applied to the convective instability zone can not excite non-decay vortex shedding.