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jay kumar January 12, 2001 02:47

vortex shedding
 
Hello,

I got results for flow over a square cylinder through numerical simulation. Due to flow unsteadiness,behind the body, vortex shedding moves from top to bottom and vice-versa. It has also been observed as Cp was increasing at vortex shedding portion. The increment of Cp moves from top to bottom and vice-versa as vortex shedding behaves. What is the physical reason for variation of Cp?. I will be thankful for your kind advice.


John C. Chien January 14, 2001 05:03

Re: vortex shedding
 
(1). If you are using transient equations in the flow simulation, then it is possible to capture the oscillating behavior in the wake. But that does not automatically mean that the captured flow behavior is always real and physically correct. (2). You could also try to use the steady-state equations to do the simulation. The converged solution would be steady state, and you are not going to see any flow oscillations at all. (3). Both approaches can be done and are acceptable. In the end, it is really up to you to decide which approach is more appropriate for your study and goal. (4). To get a quick idea about the pressure distribution on the surface, the steady-state approach is better, from my point of view. (5). But if you are studying the transient nature of the wake, then, the transient equation approach is the right approach. (5). As for the physics, I think, it is rather simple. The wake is created by the flow separation. And the flow separation is due to the boundary layer separation which generates a new shear layer. The shear layer (or the mixing layer between the oncoming flow and the reversed flow) is always unstable. Since you have two unstable mixing layers, one on the top and the other on the bottom sides of the square cylinder, the overall fluid motion is coupled. The coupled motion is organized and is displayed as wake oscillation. (6). If the flow is oscillating, the pressure must be changing in time also. (7). I guess, it is very difficult to come up with a simple physical explanation based on the results obtained from millions of numerical operations. The reason is, the numerical result may not be realistic or accurate. (that's why the word "simulation" is used )

John January 17, 2001 22:17

Re: vortex shedding -coonected
 
Another interesting phenomenon occurs. Can you please explain.

The swirl (ratio of tangential velocity to axial velocity) is a function of vortex shedding. Swirl is reduced from inlet to outlet of a duct by guiding the flow through 2 tapered blade (chord reduces from hub to tip )rather than two straight blades. I am not able to figure out the physics behind this phenomenon

John C. Chien January 18, 2001 01:17

Re: vortex shedding -coonected
 
(1). It's hard to picture the flow field from my side. (2). Vortex shedding is normally related to the flow separation from both side of the body. On the other hand, the wake generated from a blade normally does not have flow separation. It is just formed by two merging boundary layers. The wake could become oscillating, but in most cases, it will not create vortex shedding. (3). There is a possibility, that is, when you have a very blunt trailing edge, the vortex shedding can occur. Any, each time when you have 3-D flow through a blade passage, the flow field will be highly complex because of the existence of leading edge separation vortex and the subsequent secondary flows.

John January 18, 2001 19:49

Re: vortex shedding -coonected
 
Thanks for your immediate response. Lets forget the vortex shedding. Why does swirl reduce when you taper the blades instead of using straight blades

John C. Chien January 18, 2001 21:02

Re: vortex shedding -coonected
 
(1). The blade tapering is normally used on the rotating blade to reduce the stress caused by the centrifugal force at high RPM. (2). Modern blade construction does not use straight blade because it can not respond to the radial variation of the tangential velocity. Therefore the blade staggering angle and also the shape must be changed. (3). There are other factors affecting the 3-D flow field such as the number of blades per blade row or the solidity. (4). In your case, I can only guess that, if you keep the blade geometry the same and shrink the blade from the hub to tip (tapering), then the blade loading will be smaller at the tip and it will be less effective to turn the flow in the blade trailing edge direction. And if you reduce the blade chord to zero at the tip, it will not produce any flow turning at all. At the tip, you will see only empty space. So, it is 3-D problem, the reduction in blade chord at the tip will change the solidity property there. I hope this is the answer you are looking for. Obviously an oversimplified one. But then I don't run into your kind of question anyway.

V. G. Ferreira January 27, 2001 10:25

Re: vortex shedding -coonected
 
Hi, In this context, I would like to know what boundary conditions and especifical domain that you are using, Jay. Valdemir.


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