tag:blogger.com,1999:blog-44859485200856782.post2223276235419771142..comments2024-02-24T05:19:32.569-10:00Comments on Zen Waterman: Some thoughts on water flowing over a Stand Up Paddle- by Robert StehlikZen Watermanhttp://www.blogger.com/profile/04991107799679502367noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-44859485200856782.post-17485569191725577752011-05-02T07:37:54.059-10:002011-05-02T07:37:54.059-10:00Here is a good illustration of what's happenin...Here is a good illustration of what's happening with a golf ball.<br />http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml<br /><br /><br />"Since the laminar boundary layer around the smooth sphere separates so rapidly, it creates a very large wake over the entire rear face. This large wake maximizes the region of low pressure and, therefore, results in the maximum difference in pressure between the front and rear faces."<br /><br />In comparison, lets look a flat plate. First, perpendicular to the flow:<br />http://en.wikipedia.org/wiki/Drag_coefficient<br /><br /><br />"fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface." Negative pressure builds at the back because of the eddies that form along the edges which adds to the effective drag. The flow around the plate will be turbulent even at very low velocities.<br /><br />But parallel to the flow:<br />http://www.roymech.co.uk/Related/Fluids/Fluids_Drag.html<br /><br /><br />You'll notice that there is no advantage to creating turbulent flow earlier on plate, like there was with the sphere. You won't get less wake or any other advantage because pressure drag is negligible to skin friction. <br /><br />So lets talk numbers. Drag Coefficient depends on the Reynold's Number which is a function of velocity, but, common estimates for the drag coefficient of sphere is about 0.47. A flat plate perpendicular to the flow is about 1.17. A flat plate parallel in laminar flow is about 0.001, and in turbulent flow about 0.005. http://www.engineeringtoolbox.com/drag-coefficient-d_627.html<br /><br />Drag force depends on the drag coefficient and a "frontal area" which we can just assume is the same for all cases: for the parallel plate, it's actually the side area, if you get what I'm saying. As such, the drag force for the parallel plate is 2-3 OOM less than the other two scenarios. So then you get into the question about laminar vs turbulent. Crunching some numbers:<br /><br />http://en.wikipedia.org/wiki/Reynolds_number<br /><br />L is the length of the blade. Assuming I can get the paddle in the water in about 0.5 sec, in sea water, your looking at a Re of about 1.1 * 10^9. Re > 10^6 is so we are way turbulent.<br /><br />Essentially, what I'm trying to argue is that I don't think there is an advantage to making the flow turbulent on catch and release. I'm guessing that a quick catch will get you turbulent even without the dimples. Even if there was and effect caused by the dimples, it would be so minuscule compared to rest of the stroke, that it wouldn't be noticeable to the paddler. Look at the mechanics of a stroke. As soon as the blade enters the water, the perpendicular flow is going to be all that the paddler feels. At the release, there's going to be so much cavitation at the back of the paddle, as soon as you stop applying forward pressure, the built up pressure on the face and suction on the back will pull the blade into the void…not really a void but a low pressure volume. Again, it will already be turbulent flow, so dimples aren't going to do anything.<br /><br />Of course, the focus of my masters was mechanics. I haven't studied fluids since my undergrad, so, Robert, how'd i do?Zen Watermanhttps://www.blogger.com/profile/04991107799679502367noreply@blogger.com