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sameer mohrir August 9, 2000 00:44

wake
 
I want to ask the forum a question. How wakes are formed behind two dimensional / three dimensional bodies and how it is minimize


Ushio Yuki August 10, 2000 10:49

Re: wake
 
This is all something that you can probably find in a fluids text book, and would be easier to answer with pictures. Basically, as flow moves around an object (e.g. a cylinder), it will come back together only if there is enough freestream pressure to push the flow back together or if there is enough lateral momentum (perpendicular to streamwise) to bring the flow back together. At very low flows, flow is laminar and freestream pressure is great enough to keep the flow attached to the object. At slightly higher flow rates, flow is still laminar and flow separates because the pressure is not great enough to force flow back together and the laminar boundary layer does not have any lateral momentum. At high flows, the flow trips into turbulence, where there are eddys in the boundary layer, and hence some portions have lateral momentum, which allows the flow to come back together. Tripping the flow into turbulence can minimize that kind of flow separation.

However, if you're talking about oscillating wakes at low flows, that comes from a pressure instability, and that can be minimized by streamlining the object.

sameer mohrir August 10, 2000 23:11

Re: wake
 
That means wake can be minimize by slight change in the geometry of the object.

Leonard Lorentzen August 11, 2000 01:56

Re: wake
 
A proper definition of a wake would be a region of vorticity which is not close to the surface of a body. This means that wakes are found behind any body moving through a fluid, even when the flow remains attached. (These wakes are sometimes denoted 'thin wakes'). The wakes are due to viscous effects in the flow, and represent the energy left behind in the fluid. You can determine the drag of the body by integrating the total pressure loss across the wake. This means that you "minimize the wake" by reducing the drag. Typically you do this by streamlining the body, delay transition to turbulence, avoid separating boundary layers, etc. Aeronautical engineers have been working with this problem for about a century.

Best regards

Leonard


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