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Steady and unsteady flows

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 3
أستاذ المادة عدي عدنان جهاد الخيكاني       05/10/2012 11:40:57
Steady flow is defined as that in which the various parameters at any point do not change with time. Flow in which changes with time do occur is termed unsteady or non-steady. In practice, absolutely steady flow is the exception rather than the rule, but many problems may be studied effectively by assuming that the flow is steady. A particular flow may appear steady to one observer but unsteady to another. This is because all movement is relative; any motion of one body can be described only by reference to another body, often a set of coordinate axes. For example, the movement of water past the sides of a motor boat travelling at constant velocity would (apart from small fluctuations) appear steady to an observer in the boat. Such an observer would compare the water flow with an imaginary set of reference axes fixed to the boat. To someone
on a bridge, however, the same flow would appear to change with time as
the boat passed beneath the bridge. This observer would be comparing the
flow with respect to reference axes fixed relative to the bridge. Since steady flow is usually much easier to analyses than unsteady flow,
reference axes are chosen, wherever possible, so that flow with respect to
them is steady. It should be remembered, however, that Newton’s Laws of Motion are valid only if any movement of the coordinate axes takes place at constant velocity in a straight line.
The great majority of flows may be analyzed assuming the fluid motion is
steady. There are, however, three cases where unsteady effects are important. In no particular order, they are as follows. First, waves formed on free surfaces display oscillatory effects, and therefore aspects
of their motion are unsteady. A second important topic is that of liquid
flows rapidly brought to rest. Such unsteady flows can generate very large pressure surges. There is a third class of unsteady flows. In this type of flow, the boundary conditions of the flow may be steady, but the flow itself is inherently unstable. The classical example of such a case involves the flow of fluid past a circular cylinder. For a certain range of Reynolds numbers, although the velocity of the flow approaching the cylinder is steady and uniform, large eddies are shed alternately and continuously from the two sides of the cylinder to form what is known as a Karman vortex street It is useful to refer to another class of flows which are described as quasi steady. This term is applied to flows when the variables are changing slowly with time. In these situations the fundamental fluid dynamics are essentially the same as for steady flow, but account has to be taken of overall changes taking place over a period of time. An example of a quasi-steady flow is the flow that results when a large tank is drained through a small outlet pipe. Over time the lowering of the head in the tank results in a reduced flow rate from the tank.
Finally, a comment about turbulent flow is relevant. It has already been


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .