MVGs & Superchargers

General Discussion
41
Quote:
Hey you guys!!!! This is the best. How about a brief simplified "glossary/definition" of some of these terms for those who may not know what your referring to - then we can all share in the love and knowledge, e.g., Chord length, Reynolds #, etc.

On Reynolds #'s:

Remembering back to my Biological Oceanography classes, Reynolds number has to do with viscosity of fluids. A very simple analogy/example is how water feels very different to a human when compared to how water feels to a bacterium. To us humans, water is very fluid and easy to move in. To bacteria, swimming in water is more akin to a human trying to swim in molasses.

Mark,

Have there been studies on this topic? If there are any references, I’d like to read about it.

I would also think that very sophisticated measuring equipment to measure flow rates/directions/velocities would be needed. We would need numbers not pictures.

42

Josh,

Here’s some Reynold’s Number notes from report 586 Variations of the Reynolds Number that I put together to describe a thruster fin.

We first we need to determine a Reynolds number for our typical template. Where v=speed of vehicle (20MPH= 352 in/sec)L= cord length of foil (4.7")p=density of water (0.577 ounces/cubic inch)u=dynamic viscosity of water (.00002325466 ounces/ square inch) g=acceleration of gravity (386.088 inches/second squared)

RN=vLp/ug or (3524.70.577)/(0.00002325466*386.088) or RN=106,352

Reynolds Numbers of 100,000 or less fall into the definite laminar flow range. Reynolds Numbers greater than 1,000,000 fall into the turbulence range. Everything in between is “in transition”. So, thruster foils fall in the bottom of the transitional range as they approach max speed.

Now, according to the NACA Report No. 586 the accuracy of their airfoil tests:

“Hence airfoil characteristics dependent on the shape of such curves, e.g., the optimum lift coefficient and the aerodynamic center position, are considered unreliable and in most cases are not presented below an effective Reynolds Number of 800,000” (pg. 230)

43

Im glad you guys are starting to appreciate testing (hey even Einsteins theories had to be proven empirically). Mark, you are on a similar track as I have been for a while now. There are many ways to skin this cat…

  1. Clear see-thru board (lexan) with brightly colored streamers. Either take to surf or pull behind a ski boat. You can do this with a spare surfboard - attach a thin sheet of lexan about the size of a salad plate on the bottom of a board in the desired location (ahead of the fin tested). Then remove the glass/foam from the top all the way thru to the sheet. Place a streamer in the “window”. Draw a reference line parallel to the stringer. When you ride the board you may/will be able to see the streamer break away from the reference line on turns. You can get real fancy with this test but the challenge will be to see it while surfing…thats why a ski boat is likely is more controlled test.

  2. Make a mini board (say 2 feet long) model template out of lexan, bend it to put a little rocker in it and lock that in place. Place easy to see streamers along the bottom. Then place along the side of a moving ski boat (no need to move very fast) and try to determine the angles of attack as you manipulate the model to mimick surfing.

All your trying to do is estimate the angles of attack during surfing particularly the most common angles during the most common turns and maximum angles during the most severe turns. My guess is that 35 degrees is fairly extreme but even a 10 degree angle would blow all the concave scheme practices out…and dont even get me started on bonzers…:slight_smile:

One good test result if worth a thousand opinions.

“If we are all thinking the same thing, we are not thinking enough”

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“Hence airfoil characteristics dependent on the shape of such curves, e.g., the optimum lift coefficient and the aerodynamic center position, are considered unreliable and in most cases are not presented below an effective Reynolds Number of 800,000” (pg. 230)

Tomatdaum,

I’m a microbiologist, so my physics knowledge is shaky at best and my hydrodynamics knowledge is nonexistent.

What does this mean in terms of current fin design?

45

Lift? Want more lift? Cant your front fins out a couple of degrees more.

I took Fluids 1 and 2 in engineering school (BSME) and honestly, it is the most

theoritical of courses…lots a fancy calculus but very little you can wrap your arms around.

Lift? Are we trying to fly?

WHAT IS THE #1 GOAL OF THIS POSTING?

46

Josh,

   It's been a while since I pulled this information. So, I went back to the NACA site to pull the report back up to keep it in context and I'm getting an internal server problem on their search engine. But, basically it means that for the speed and size of the foils we are talking about the data is unreliable.
47

Chord length - length of surface interacting with water.

Reynolds number. Ratio of inertia mass to viscosity. Laminar flow breaks down at high Reynolds numbers, but this breakdown is not thought relevant in most surfboard fins.

There are actually people who have created flow tanks and tested water flow around low aspect ratio foils, and this information is available on the web. The flow tank is not so much, maybe 2 feet by 2 feet. With a couple holding tanks and a pump you could do experiments quite cheaply. Just let water go from one tank, down an incline, and into the other. Each run could last a minute or two. Place one fin on a board in the middle, and go from there.

But everything I saw suggested the leading edge vortex theory would be perfectly well supported in the range of Reynolds numbers, aspect ratios, and rake, in surfboards.

IF true, it would suggest that leading edge rake is the most important variable in fin design. Depth is the second. The others fall much further behind (but can have an impact, especially at lower angles of attack where linear (laminar) flow theory dominates ).

Max hold of a fin - simple - as a FIRST APPROXIMATION, it is fin depth. Chord length will have a smaller but not negligible impact.

Drive of a fin, or the increase in lift per degree of angle of attack change. To a first approximation, the leading edge rake. Foiling will have a small/negligible impact at higher angles of attack, and a substantial impact at lower angles of attack.

Stall angle. Dependent principally on leading edge rake - more upright fins stall earlier.

This is all applied assuming infinite stiffness, and a bunch of other stuff. But it meshes well with my own experiences with different fins. Fin rake is highly controlled - 30-35 degrees at the base for almost all thruster fins. For longboards there is a range of 25-30 degrees (interestingly the new Channel Islands Machado singlefin is 35 degree base rake - as though it were made for a quicker board).

And back to the original point. The MVGs, and possibly also the superchargers, may work by more reliably initiating the leading edge vortex, which would increase lift over a limited range of angles of attack (predominantly help initiate it).

I’m thinking about how to go about optimizing a fin for my own system, probably I’ll start with a rake series. And maybe throw the MVGs on the board. :slight_smile:

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Quote:

Lift? Want more lift? Cant your front fins out a couple of degrees more.

Exactly, if you want the benefits of lift you can put a deeper concave in the bottom or actually increase the cant beyond the standard/boring 4-7 degrees of cant. Bonzer side fins have what 17 degrees? That plus the concaves and you get greater lift. Greater lift “creates” more speed. Play with cant. Not Kant!

49

With all this discussion of fins…Can any of you tell us anything about the Futures Vector fins?

My brother just gave me a set that he got from work. He loves them and say they enable you to generate lots of speed.

I haven’t tried them out too much yet.

50

Josh,

  Read earlier in the thread. MarkSpindler does just that.
51

Hey Dave,

If you put MVGs in front of a rotating fin as the fin rotates the votices of them may have a very negative or possitive affect on performance because the fin will no longer have the same angle of attack as they do. I’ll be laying up you panels this weekend.

Mahalo, Rich

52

Okay, now I got past my search engine problem. The link for the 586 Variation of Reynolds Number Report is:

http://naca.larc.nasa.gov/reports/1937/naca-report-586/index.cgi?page0004.gif

From glancing back over the report I recall that much of the original lift drag data that was developed for the various NACA foils was done in a relatively small closed tunnel with a continous shaped foil spanning the entire tunnel and a constant reynold’s number. The jist of the report is that as you vary the Reynold’s number, lower R numbers yield higher degrees of variance of errors. There is a a divergence of the data called “scale affect”. Basically, the limited range of variables tested with models do not reflect accurately the results tested in full scale flight tests until the Reynolds Number is above 800,000. So, according to this report the Reynold’s numbers that surfboard fins opperate in yield results that are not reliable when comparing them to the wind tunnel models.

53

that makes perfect sense to me. If you’re testing a model of an airplane wing that is (for the sake of argument) roughly the size of a surfboard fin (say between 4 and 10 inches in length) you have to adjust for the fact (reflected in Reynolds number) that the number and density of air molecules that hit that mini-wing are NOT comparable to the number and density of air molecules that hit a full size airplane wing (say, 40 feet long or more). Even if the foil, chord etc proportions are the same, the wing will perform differently due to the qualities of the medium in which it’s acting.

Now, think about applying any such analysis of that foil to WATER molecules instead of air molecules. Not even close. Why would anyone expect that wind tunnel analysis should apply to water flow and yield reliable data?

just one non-engineer’s take on this stuff…

54

This discussion is increasingly illustrating the need for a serious academic study of the questions being posed.

Surely there has to be someone at an American University with the means to fund this kind of study.

55

Well, here is a study done IN WATER IN A FLOW TANK at Reynolds numbers comparable to surfing (actually, somewhat lower), studying leading edge vortex formation, using swept delta wings. The delta wings have 3-4 inch chord lengths, well in the range of surfboard fins.

http://www.bath.ac.uk/~ensgst/files/aiaa-2003-4021.pdf

56

OK. The NACA foils were studied in wind tunnels, though, was the point, wasn’t it?

57

NACA foils are the results of wind tunnels, 2-D modelling, and computer simulation. With high aspect ratio foils, they have reasonable predictive power. I think that is the real problem using them - they are mainly intended for limited sweep high aspect ratio wings. They’d be great if you were designing a glider, where maximal lift:drag ratios are a high design requirement. But other effects start to matter when aspect ratios lower than 5 come in.

There are plenty of studies on low aspect ratio swept wings in the range of Reynolds numbers used in surfing. Most of them use much more sweep than surfboard fins have rake, but some are more limited in sweep. The leading edge vortex effects are the reason foiling has minimal effects at larger angles of attach. Look at the foiling on the Concorde.

Unlike the Concorde’s wings, though, surfboard fins need to work at low AND high angles of attack. It is an interesting problem, be cool to have a budget and a flow tank to get down to brass tacks. But I wouldn’t expect anything revolutionary from such work.

58

Yeah,

I’m having a hard enough time trying to figure the relavence of a flat 50 degree delta plate with a 45 degree beveled leading edge in .45 m/sec. (1 MPH) flow has with surfboard fins.

I know that one way to analyse a complex problem is to break it down to it’s most fundamental components. But, in doing so, we must realize that the results only reflect one or two variables, of a much larger equation, integrated across a range that might have nothing to do with real life experience. At this point there does not appear to be any better way to analyze surf board fin perfomance than to just get out there and ride them.

59

I was just trying to identify the relevant phenomena, so that I would have some idea what variables to play with, first, in trying to optimize fins for the rotating fin system. Presumably you do the same thing for your fin system - you need someplace to start.

In that vein, if I were to study fins in a flow tank, or on a surfboard, I’d start with a rake series. Then a depth series. Then a chord length series, and then a series with different leading edge contours, and then maybe a thickness series. Or maybe I’d just stop after the first two and figure it was most of the way there, and use middle of the road chord lengths, leading edge contours, and thicknesses.

To point, I don’t think it is useful, at all, to say it is all empirical, nothing close enough to surfboard fins has published data, therefore we need to start from scratch, and ride them to figure it all out. People have ridden different fins for years, hundreds of thousands of people. Fins have a VERY narrow range of rakes, depths, thicknesses, and chord lengths. I find it entirely reasonable to start with the major effects, and go down the line from there.

A whole lot of noise is made about foiling these days. Is it the presumption that it is the major player among fin variables not yet optimized?

60

I don’t follow some of your post. For example,

"NACA foils are the results of wind tunnels, 2-D modelling, and computer simulation. "

NACA foils were developed in the early-mid 1930s. They weren’t the product of any computer simulations.

I also don’t understand your argument that NACA foils are useful only for high (>5) aspect ratios. Small boat rudders, for example, with aspect ratios of between 1 and 2 are often designed using NACA symmetrical foils.

and as for “the Concorde wing not needing to work at high AOA”, the Concorde was in fact designed to generate lift at low speeds precisely by flying at very high AOA (and using tip washout) to create vortex lift (and ground effect lift); while at the same time using a very thin foiled wing to allow supersonic flight at low AOA…