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MTB you didn’t test it, you ran a calculation, which is not the same as testing it.
You’re right. However, a description of the the flow around a cylinder was a significant object of interest to fluid dynamicists–both theoretical and experimental. Hence considerable effort was devoted to understanding it by numerous persons until theory and experiment were in excellent agreement. Moreover, the accuracy of their measurements is substantially better than either you or I are likely to approach, let alone exceed–especially in a bathtub. Therefore I put much greater confidence in the results of those studies than subjective observations in the ocean or subjective?/objective? observations in a bathtub. If you wish, I will provide you with a plot summarizing the various measurements of the drag coefficient of a circular cylinder perpendicular to the flow as a function of Reynolds Number so that you can judge for your self the degree of precision and theoretical/experimental agreement that they achieved.
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…… I suggest that you try a couple of pieces of pipe of different length and diamter in the bathtub and find out for yourself how they behave. . . I won’t suggest that you spend 10 years testing them in the surf as I have done.
Your suggestion was to see how a pair of cylinders behave in a bathtub–a rather general and non-specific request. I did (in a simulated bathtub)…and apparently identified a behavior that you weren’t aware of after 10 years of testing.
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My contention was that the longer smaller diameter tunnel will stall at a lower angle of attack than the shorter larger diameter tunnel. The fact that, once stalled, the smaller diameter tunnel offers more resistance to an angle of attack of 90 degrees than does the larger diameter tunnel is interesting but does not answer the question. .
Your original contention was that small aspect ratio annular foils stall at a smaller angle of attack than large aspect ratio foils. You provided no definition of what constitutes a large aspect ratio vs a low aspect ratio foil. I provided such a semi-objective definition (“semi-objective” because there is a finite transition region from one state to the other).
You chose to suggest an experiment with a pair of pipes whose aspect ratios (according to the criteria I presented) would both fall into the category of low aspect ratio annular foils.
You also appear to define the onset of a “stalled” condition as when the flow though the interior of the pipe stops. This is not the conventional definition, so it is up to you to demonstrate that your definition is equivalent to the conventional definition. In short, you need to demonstrate that the lift force (i.e. the component of the force on the cylinder perpendicular to the direction free stream flow) achieves a maximum when this flow condition occurs.
Sharp edged foils (as would seem to be the case for a piece of pipe) can, in some conditions, show an abrupt reduction in the rate of increase of lift with increasing angle of attack beyond some reference AOA, but the lift still continues to increase (although much more slowly) with increasing AOA. Moreover, some foils (I am told), that have a first peak in the lift vs AOA curve will even generate a second maximum that exceeds the first maximum. Hence it is important to quantitatively describe what sort of a curve you generated, and if/how your working definition of “stalled” differs from the conventional definition(s).
What you also did was to claim that the flow around the smaller diameter, but longer tube, presents less resistance to the approaching water than does the larger diameter, but shorter tube. Thus you claim that the flow towards the cylinder would be more inclined to go around the tube than down it, and hence the cessation of flow through the tube. I showed that the resistance to flow around the small tube was more than three times greater than that of the bigger diameter tube at an AOA of 90 degrees (a fact of which you apparently were not aware).
I would expect that the ratio of the resistance to flow around the small diameter tube to that of the larger tube (having equal areas) would decline as the inclination (AOA) is reduced. The question becomes how rapidly does the reduction occur, and does your observed cessation of flow take place at when the ratio of drags on the two tubes reverses?
I don’t know if the latter situation is the case or not since you didn’t present any data to support when a the cessation of flow occurs, nor what the lift and drag forces are as a function of the angle of attack. However, my gut feeling is that what causes the cessation of flow through the tube is not so much associated with the resistance to flow around the tube (your hypothesis) as it is to the growth of the region of separated flow downstream from the entrance to the tube (my hypothesis).
For a “sharp-edged” entrance, the latter would seem to be the case for a section of pipe (i.e. having a small wall thickness relative to the diameter) since a small region of separated flow would be expected to occur immediately downstream of the entrance. As the AOA is increased, the length of this separated flow increases and the thickness of this region of separated flow increases. The dimensions of the separated flow can rapidly increase as the AOA approaches a critical value. Ultimately, at a sufficiently large AOA, this region of separated flow can span the diameter of the cylinder, resulting in a “blockage” of the flow of water down the tube. The smaller the diameter and the longer the length of the cylinder (i.e. the smaller the aspect ratio), the smaller the AOA at which this will occur. Moreover, this onset of blockage may occur abruptly with an increase in AOA (although this still needs to be demonstrated). In contrast, the resistance to flow around the tube can be expected to occur at a (relatively) steady (or only slowly changing) rate with increasing AOA, so it is more difficult to imagine what would cause the abrupt cessation of flow down the tube that you described.
mtb
