Take a look at this underwater flow video. Camber Cam Best Of on Vimeo
The waves were chest to head high rights and 15 to 16 second interval swell. The wave face is on the right side of the viewer’s screen and the transverse flow peels off the outboard rail on the left side of the viewer’s screen.
Pretty cool.
Really shows how dynamic the flow of water over a board is and now fast it can change.
Seems like when the board is on rail (either left or right), the flow is clean with little turbulence. But, when transitioning from one rail to the other with the whole bottom engaged, there is a lot of turbulence. I guess that makes sense, but more extreme than I would have thought. This may be HPSB specific.
Interesting that only the strings at the air-water interface show a directional flow vector.
The strings at the tail appear to remain parallel to the stringer.
Nice video idea.
I would love to see some slow motion footage of that vid.
I like the bit with the flames and the fireball.
There’s a slow motion function for YouTube but not for vimeo afaik… Other than frame by frame:
http://anilyzer.com/?page_id=7&vId=220834809&type=vimeo#playbackRate=1&seekTo=0
@ Mr. Stoneburner,
‘‘The strings at the tail appear to remain parallel to the stringer.’’ You just looked at a real world example of the ‘‘relative flow’’ related to the directional motion of the surfboard. This condition was at the heart of what I call my ‘‘Phil Edwards tutorial’’ circa 1960. It was that discussion with Phil, that was instrumental in redirecting how I thought about surfboard design, and especially fin design. That hour long discussion with Phil, some fifty seven years ago, continues to influence my work on surfboards, to this day. This one time, I’ll give you props, for your observational skill.
Major props for doing this. I think you are on to something here. Theory can be tested with With more objective view. Two points one would be to film the surfer and board from shore and synchronize with your under water footage. Another thought would be to work out a system to add dye to the water to get a better view of the waters flow around the board.
Now tell us about the Board. To get a better understanding of what we were looking at in regards to the flow. From my observation the board seemed to have a shallow concave At some points it looked like air was under the forward part of the board. I also noticed that some of the directional strips were lifting off the bottom of the board. Again thanks I find this fascinating
While the video is very cool and enlightening, you need to think about what you are seeing. You are not looking at the flow of water under the board, you are looking at a board being propelled over the water. By tying the telltales onto the board, you are looking at some kind of vector resultant. If you want to see what the water is doing as a result of the board displacing it, you need to figure out a way to measure displacement vectors without tying them to the board.
The streamers may need to ride a bit above the boards surface.
Close proximity and/or direct contact with the board’s bottom surface could affect streamer movement (e.g. [Henri] Coandă Effect).
Just a thought…
Very nice video!
How did you mount the camera and how much did it change the way the board surfed?
Artz: The board has a half inch concave. See the diagram here and explanation. http://www.cambersurfboards.com/home/ld-summary/ It is not the depth of the concave but the angle that is critical. The bottom is flat across the center except the outboard 10% relative to the flow angle. This approximates the area of a flap on the trailing edge of a wing. The shape looks extreme in cross section but when you look at the bottom shape based on transverse flow, it looks like the bottom of a high performance glider wing. Take a piece of tape and put it at 45 and 60 degree angles across your board and then look at the line created.
Good question about the 3D aspect of the flow. See the CFD (computational fluid dynamics) simulation at the bottom of the page at http://www.cambersurfboards.com/home/ld-summary/
done by Scott Graham who is the hydrodynamacist that was on the Team New Zealand design team that came up with the break through hydrofoils on the previous America’s Cup. The CFD simulation shows the depth of the flow field.
Wooddave: There is some slow motion footage here if you click on the clip “Rail to Rail. http://www.cambersurfboards.com/videos/
You can find explanations of the flow during different maneuvers here. http://www.cambersurfboards.com/home/ld-technical/
In summary, the purpose of the Camber Cam footage was intended to validate Scott’s CFD work. There are a number of assumptions such as trim angle, wave slope, speed, etc that go into setting up a CFD simulation. Therefore, real time data is critical to calibrate.
LeeV: The vector issues were a concern. We built another board that that had stand-offs to place the streamers deeper. The results were basically the same. The boundary layer is relatively thin and the flow not that turbulent. Therefore, the flow inside and outside the boundary travels in similar directions.
MrMik: The fin is mounted on a tall center fin about 8 inches deep so it can “see” over the rocker line and have a decent angle to view the streamers near the nose. It does add some drag but we found a small diameter camera with good resolution. A Go Pro would have far to much frontal drag.
All good questions. I can ping Scott if you need more detail.
If you look at the classic theory of planing by Lindsay Lord, you would expect to see the water “flow” at nearly right angles to the stringer; with the “flow” being in opposite directions to the stagnant line. The video shows, what I assume to be, the root spray going off the outside rail. It also shows the direction of “flow” on the other side of the stagnant line (where the root spray starts) to be directed to the rear of the board rather than off the inside rail as theorized by Lord. Either Lord’s theory is all wet (pun intended) or the directions are being affected by the motion of the hull going over the water. The tell-tails are being dragged by the forward motion of the board not the “flow” of the water. That is why I believe the tell-tails are showing a resultant direction. Now the resultant is probably more applicable to bottom design that not but it is not a complete picture the hydrodynamics of surfboard planing.
LeeV,
I have not read Lindsey Lord’s work.
Is the planing hull/surface he discusses a longitudinally flat bottom rather than convex, especially in the rear half (i.e. no rocker)?
Flat bottom. There are downloads of his work on the innerwebs but if you search planing hull design and look at the images, you’ll see a diagram showing all the forces and flow directions and how they are derived. Could be that the whole theory has been debunked but most recent technical work has been done on boat hulls. In most articles, the authors specifically exclude surfboards from the discussions. Lord’s research works for me and explains how surfing works and nearly every aspect of surfboard design. Elegant solution.
See PM.
Hi Lee –
Sorry for the length of following but this is really complicated. Rather than respond in bits and pieces, here is the whole explanation.
You are exactly correct. The flow under the bottom of the board is the vector sum of the water flowing up the face of the wave (and mostly across the board), the axial flow due to the forward motion of the board, and the presence of the board of course also affects the flow field. It’s easier to think of the inflow being comprised of two flow fields, one flow field aligned with the centerline of the board, and a transverse flow field perpendicular to the centerline of the board with the classic flat plate planning theory applicable to both flow fields. (One of the fundamental premises of potential flow (no viscosity) theory & computer programs is that the potential flow fields are linearly additive). Since the angle of attack of the bottom of the board is higher relative to the transverse component of the flow, this component of the flow is slowed down more than the axial flow component and much of the lift is generated by the transverse flow. For instance if the board were not moving forward, it could still support the riders weight by planning on the flow moving up the face of the wave just like the guys riding on those stationary wave rides. Ie you can take the fins off and ride your board sideways into the beach…
The flow velocity relative to the board bottom in the axial flow direction is much higher than the flow velocity in the transverse direction, and that is why the streamers are mostly aligned with the centerline of the board except near the rails and along the stagnation line just behind the spray root. Even though the axial flow velocity is higher, the trim angle of the board is lower relative to this component of the flow, and therefore the effect of the board on this axial component of the flow is typically lower and the lift generated may be less than the lift associated with the transverse component of the flow when riding across the wave face.
One of the classic planning theories is 2D + time where a planing boat is represented by a 2D wedge being quickly immersed. Ie where the keel line intersects the water is where the Vee shaped wedge is just contacting the water surface, and the transom of the boat is where the wedge has plunged down to the depth of the keel at the transom. The total pressure on the boat bottom is calculated by using the pressure associated with the transverse flow field generated by immersion of the wedge at each instant in time as it is immersed, and the axial flow is basically ignored. A surfboard bottom can also be thought of a single sided wedge being immersed into the wave face with the forward rail to wave intersection being analogous to the forward keel line intersection, and the tail of the board the transom.
On a surfboard, the flow angles, velocities, relative magnitude of the two components of lift etc. are obviously varying continuously with the direction and speed of the surfer/board and the flow field associated with the wave. However the angle of the contact line across the bottom of the board is in general an indication of the relative importance of the two components of flow in generating lift. The more parallel the contact line is in relation to the centerline of the board, the more the lift is associated with the transverse flow. Conversely the more perpendicular the contact line is relative to centerline, the more the lift is associated with the axial flow. When you are going straight down the face of the wave the contact line is perpendicular to centerline, and the all of the lift is associated with the axial flow. When taking a high line in the pocket of a steep wave, the lift is mostly due to transverse flow.
The concave concentrated along the rail of the board is designed to exploit the transverse component of the flow, and generate higher pressures on the bottom of the board and therefore increased lift. Increasing lift associated with the transverse flow, results in reduced lift and reduced trim angles associated with the axial flow for a given total amount of lift/rider weight. Since the pressure on the bottom of the board acts perpendicular to the surface of the board, lower axial trim angle results in less drag and a faster board.
Computer simulations I have done indicated that even when the flow is 100% axial, this bottom shape generated about 5% more lift, and with moderate amounts of transverse flow about 15% more lift. The increased lift with 100% axial flow allows you to take off earlier, and the increased lift/reduced drag with some transverse flow gives you more speed on the face of the wave to make sections etc.
Very good work!
This is how CFD should be done, not just the simulation but also validating the results in practice as much as possible!
Great work!
Brilliant posts
Snark and hydroguy .
Sadly I cannot like a post more than once
Best post I’ve read (watched) on this
Forum.
People can spout opinion about how water flows under a board and. It seems like a lot of voodoo shamism show me the Video and I start paying attention…
some of the witch doctors start making sense some start making non-sense.
There is so much here I need to re-read and re-watch about 4 times then I may try to add something more intelligent to this conversation
than this thank you
LIKE LIKE LIKE
@ Snark & Hydro_Guy:
It would be interesting to see streamer response to a hard lean bottom turn.
