Basiliscus

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Basiliscus last won the day on October 24 2020

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About Basiliscus

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    Port Gamble, WA, USA

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  1. Basiliscus

    Boats and foils comparison

    I think the 2D codes are pretty good, so long as the outer flow is 2D. As I understand it, the Tollmien-Schlichting waves are pretty much 2D in the laminar boundary layer as they undergo linear amplification. Once things get nonlinear and the boundary layer transitions to turbulent flow, then there are 3D structures like hairpin vortices in the boundary layer. However the averaged mixing effect of these 3D structures is accounted for by the parameters of the integral boundary layer calculation. It's when the outer flow is 3D that the integral boundary layer method starts to lose accuracy. Spanwise pressure gradients and attachment line flows are not accounted for in the 2D calculation. There is also the superposition of outer flows, such as at strut/bulb/wing junctions, that alters the local velocities from those calculated from the 2D shape. This large eddy simulation of an AC72 wingsail section done by Mario Caponetto is along the lines of what you propose. It's a good reminder that what looks to be a nice smooth edge to the boundary layer in MSES or Xfoil is anything but in real life.
  2. Basiliscus

    The new sailing twin skin setup

    I think it would be interesting to take the actual AC75 D tube shape and extend it aft to form a teardrop shape. The mast chord would be maybe 20% of the total chord. Given the low apparent wind angles for these boats, I think a large wingmast would be more appropriate. The main drawback I see to that is it would be very stiff in chordwise bending.
  3. Basiliscus

    The new sailing twin skin setup

    How about comparing to a large wingmast with single mainsail? They are about equal in speed to the rigid wingsail rigs in landyachts, which sail at similar apparent wind angles. The wingmast could have a similar sized D tube as the twin mainsail mast. That would make for a good comparison that minimized the differences in the rig and concentrated on the differences in the sails. FWIW, landyacht sails are made flat, with no broadseaming. They get enough camber just from the air loads and mast rotation. One thing I've always wondered, should the trailing edge planform of the mast be convex or concave? Most of the older C-class masts seemed to have convex planforms, but I'd think that would feed draft into the sail when the mast rotation was flattened - just the opposite of what one would want to do. Concave should pull draft out when flattened. A hooked planform for the mast would affect the twist at the head when rotated, but I don't know if that would be a good thing or a bad thing.
  4. Basiliscus

    AC boats "wind shadow"

    Well, let's see. A 10 deg apparent wind angle at 45 degrees to the true wind means the yacht is going 3.3 times the true wind speed. Think they're doing that?
  5. Basiliscus

    AC boats "wind shadow"

    The wake always trails at the apparent wind angle. This was really apparent when I was landsailing, because the landyachts would kick up a dust cloud that got entrained in the lower vortex trailing from the sail. The yacht continually laid down vortex at the speed of the yacht, which then drifted sideways with the true wind. The result was the axis of the vortex was aligned with the apparent wind vector. On these yachts, the apparent wind is on the order of 15 - 20 degrees upwind and down. So even when going downwind, the leading boat can dump on the boat behind.
  6. Basiliscus

    Boats and foils comparison

    I don't know about your specs, but there are lots of foiling sailboats available. The range from Dave Clark's UFO to beach cats (IFly15, Whisper, ETF26, Viper, many more) to the GC32, TF10, G4. If you want a monohull, there's the Quant 23. With a 10m length, you're talking a daysailer and not a cruiser or open ocean racer, and a lot of people have put a lot of time and money developing foiling daysailers. I think the GC32 comes closest to your specs. Personally, I think a multihull is the way to go, rather than a ballasted monohull. There are a lot of reasons for this, among them having maximum righting moment to accelerate to takeoff, and not having to lift ballast.
  7. Basiliscus

    Boats and foils comparison

    0.09. All the dimensions are in fraction of the chord.
  8. Basiliscus

    Boats and foils comparison

    Yes. Go to OPER->VPAR->X. Positive values of the trip location are distances from the leading edge. Negative values are distances from the stagnation point. The latter can be helpful when working with high lift sections that have the stagnation point well to the windward side of the leading edge.
  9. Basiliscus

    Boats and foils comparison

    AC75 foil wing section design, part 6 In light of the free transition results in my last post, I've made another iteration on the section design. I reduced the thickness from 12% to 11% to bump up the high-speed cavitation, and I gave the rooftop on the pressure distribution a small favorable slope to encourage laminar flow. I still think the flap is probably too thin to be practical. Here's what the new shape looks like: As before, the design pressure distribution has no net lift over the forward half of the section: I also ensured the leading edge pressure peak wasn't too high at the takeoff condition: A comparison of the profile drag with the previous section showed a savings of about 11 counts (more than 12%) for both fixed and free transition at high-speed: Here are the polars assuming Ncrit=3 for all the flap deflections: The cavitation envelopes show the cavitation speed is an honest 45 kt, now, and at a slightly higher loading that would allow increasing the loading from 49 kN/m^2 to 52 kN/m^2, saving another 6% in profile drag. The higher loading might need a bit of a tweak to the leading edge shape for the higher takeoff CL. I think I'm going to leave it here. This idiot needs his village to provide guidance as to where the design needs to be improved in order to make things feasible for the other engineering disciplines. The main points I've tried to make with these posts are: - Cavitation drives the hydrofoil section design at both high and low speeds - The local velocities due to thickness make it hard to have thick sections for structural stiffness and ballast volume, while still having cavitation onset speeds above 40 kt. - At high speed, the forward half of the section has very similar pressures on both surfaces, with all of the lift generated by the aft half of the foil, especially the flap. - As a result, the shape of the forward half of the section is nearly symmetrical. - Given that the heaviest part of the section shape is essentially dictated by the physics, the 20% rule allows for considerable tailoring of the section by swapping out flaps. - Inverse design methods, like those of Xfoil, are essential to producing sections that are tailored to the specific design requirements. No team will be using off-the-shelf NACA or Eppler sections, although those may provide reasonable starting points. - The AC75s are operating near the edges of what is physically possible in hydrofoil design. For those interested in playing with this shape, I've attached a zip file with the section coordinates and polar data. H143_Data.zip
  10. Basiliscus

    Boats and foils comparison

    Ncrit=9 is Xfoil's default value, and it is appropriate for airfoils at high altitude. It corresponds to ambient turbulence values that are less than 0.1% of the freestream. Here are some values of Ncrit and their associated turbulence levels: 9.00 ( 0.070 % turb. level ) 5.00 ( 0.371 % turb. level ) 3.00 ( 0.854 % turb. level ) 1.00 ( 1.966 % turb. level ) The main thing is not what the actual turbulence levels are, but picking Ncrit so the transition location matches experiment, as it is the main means in Xfoil of adjusting transition. I don't know what the right value is for foils in Auckland. I suspect it is somewhere between 1 and 3, but that's just a guess. I think Mark Drela uses at least 5 - 7 for model airplanes, so that may be appropriate for sails, as both are operating in the near-ground atmosphere. As a practical matter, I don't see how there would be any laminar flow past the mast/sail junction, due to the roughness of the junction. So I would artificially trip the flow there.
  11. Basiliscus

    Boats and foils comparison

    I think you're beginning to see how the uncertainties multiply in the design process. At first we only had to be concerned with shaping for low- and high-speed cavitation at potential flap angles. Now we're starting to be concerned with drag, too, and that means we need to take into account a range of ambient conditions. And so far, it's only been about one section, the average chord. There will need to be sections designed at a half dozen or more spanwise locations. The number of case that need to be run multiplies quickly.
  12. Basiliscus

    Boats and foils comparison

    AC75 foil wing section design, part 5 So far, cavitation has been like the Star Wars trash compactor, putting the squeeze on the speed range from both ends. The tentacles reaching up to pull the design down into the muck are the various sources of drag. And profile drag is all about the development of the boundary layer. Absent cavitation or flow separation, the principal source of profile drag is skin friction. When the boundary layer is laminar, the skin friction is low. Once it transitions to turbulent flow, the mixing produced by eddies brings higher speed flow closer to the surface and raises the skin friction. So the farther back laminar flow can be maintained, the lower the skin friction will be. This also delays the growth of the turbulent boundary layer and minimizes the pressure drag from the boundary layer effectively changing the shape of the section. Xfoil uses the e^n method to predict when transition will occur. This is based on the idea that the water the foil is flying through is not perfectly still, but has eddies and other small motions. These disturb the boundary layer as they get swept into it. The boundary layer itself can amplify or dampen these disturbances. If the disturbances reach a critical size, the flow will become nonlinear and generate eddies of its own, becoming turbulent. If the disturbances are initially very small, then they have to be amplified a great deal to reach the critical size. But if they are much larger already when they enter the boundary layer, then they need only be amplified a modest amount before they become critical and transition occurs. Xfoil allows one to assume what the amplification factor is (actually the logarithm of the amplification factor) with the parameter Ncrit. Ncrit is the critical value of the exponent when the critical amplification factor is represented as e^Ncrit. This is how Xfoil accounts for the effects of the external flow conditions on boundary layer transition. A favorable pressure gradient will dampen disturbances in the boundary layer and extend laminar flow. An adverse pressure gradient will amplify disturbances and lead to earlier transition. If the adverse pressure gradient leads to laminar separation, the separated flow is highly unstable and transition will occur very quickly, leading to turbulent reattachment (if the adverse pressure gradient is not too severe) forming a laminar separation bubble. So the two main routes to transition on these wings are linear amplification of disturbances or the formation of a laminar separation bubble. The proper value to use for Ncrit is hotly debated. Anecdotal evidence, much of it based on hot film experiments on IACC keel bulbs, has transition occurring at lower Reynolds numbers in water compared to air. This could be due to lots of factors, such as turbulence from wave action, and contaminants like air bubbles or sediment. It could even change from day to day, as the sea state changes or runoff washes more sediment into the water. The prudent thing to do is to run all the calculations assuming a range of values for Ncrit. Here are the polars for the H142 section with undeflected flap. The Reynolds numbers are based on the average chord of 410 mm, and are varied with speed according to the design loading of 49 kN/m^2. The red curves have the boundary layer tripped to be fully turbulent. The plot on the right side for each case shows where transition is occurring on the upper and lower surfaces. As the angle of attack increases, the changes in slope of the pressure distribution drive transition forward on the upper surface and back on the lower surface. The laminar boundary layer is not able to negotiate the hinge line on the lower surface, which is why all of the transition lines congregate there. There is potentially a saving in drag upwards of 25% at the high speed CL of 0.2, and more at somewhat lower speeds. If the laminar flow can be realized and Ncrit turns out to be high. Conjuring Dirty Harry, "You've got to ask yourself one question. Do I feel lucky?" Laminar flow requires total commitment by the whole team - it's not just a design choice. The foils have to be accurately constructed to the design shape. The surface has to be smooth, free of waviness, and polished. And it has to be maintained in pristine condition. Here are the polars for the other flap deflections. The humps and hollows in the drag curves are due to rapid changes in the transition location with angle of attack that change the proportion of laminar vs turbulent extents of the boundary layer. -5 deg flap +5 deg flap: 10 deg flap: Here are the various flap deflections assuming the boundary layer is fully turbulent: Assuming Ncrit = 1, with free transition: Assuming Ncrit = 3 Assuming Ncrit = 5 My guess, and it's only a guess, is Ncrit will be somewhere between 1 and 3. I wouldn't bet on it being higher than that, but teams may have better information than I do. Next up: Going thinner.
  13. Basiliscus

    Boats and foils comparison

    In my last post, I made a cut-and-paste error in the cavitation plot. Here's the corrected version: The state of play in that post was the design had the 12% thickness and could take off at 18 kt, but only had a 40 kt top speed. In order to get to 46 kt, the minimum Cp could be no more than -0.34. The plan for extending the high-speed cavitation onset was to use aft loading to shift the lift from the main part of the foil to the flap region. That would bring the upper and lower surface pressure distributions together. (Sorry for the red annotations. Yellow just didn't stand out on a white background.) Here's where that approach led: There's very little load carried by the main part of the foil. The pressures are pretty much all due to thickness. The flap got so thin that I added a blunt trailing edge just to give it some meat. I expect there would be a lot of anguish on the part of the structures and controls engineers because at high speed, most of the weight of the boat is sitting on the flap. The hinge moments are going to be horrendous. But it's still 12% thick. The high lift case looks OK - the min Cp is greater than -2.0: There's nothing remarkable in the drag polars: The cavitation envelopes show I've almost met the requirements: Takeoff cavitation looks good, but I'm just shy of 45 kt at the high speed end. So far, the effort has been fairly straight-forward. It could even be done using Xfoil by some guy on Sailing Anarchy, ffs. But now the team needs to make some decisions. The first thing that can be done is to sharpen the pencil, and do some analysis using RANS to see just what the behavior of the section will be past the incipient cavitation speed. I'm pretty sure there wouldn't be any visible cavitation at 45 kt, but cavitation will have an effect between there and 50 kt. Just how bad it would be is going to take some sophisticated CFD. This section might be the kind of thing a team would use for its first foil to get some experimental data on how bad the cavitation would be and what kind of tradeoffs would be needed for the next set. The sailors are going to need to weigh in on how important the 45+ speed range is, given the forecast wind conditions for the Match. If higher speeds will be needed to be competitive, then the thickness ratio needs to come down. Reducing the thickness ratio can be done in two ways. One way is to keep the physical thickness and make the foil wider, increasing the area. This does two things. It lowers the thickness ratio, which will allow the rooftops in the pressure distributions to come down. It will also reduce the foil loading. That will make the takeoff easier and may help the boat stay foilborne in marginal conditions. But the added wetted area will add drag. At the high speed end, this means trading some additional drag at speeds leading up to 45 kt, but delaying the drag rise due to cavitation to higher speeds. Extra drag can hurt the takeoff performance, too. Whether or not this is a winning move will have to be examined using the VPP and the race model program. The other way to reduce the thickness ratio is to make the foil physically thinner. This means the structural problems become more severe and there's less room for ballast. It could force the decision as to whether or not to have a bulb. And that is one reason why we're seeing different approaches to the foil design among the teams.
  14. Basiliscus

    Boats and foils comparison

    Previously, the H140 section was designed to overcome the deficiencies of the Eppler E908 section with regard to the AC75 design requirements. The H140 made a significant gain in thickness and the high-speed cavitation onset was promising, but it was subject to leading edge cavitation at takeoff speed. The next iteration, H141, was aimed at addressing takeoff speed. Here is how negative the minimum pressure coefficient can be to avoid cavitation at each speed. The minimum pressure can be no more than -2.21 in order to take off at 18 kt. I first tried to address the takeoff by just modifying the leading edge pressure distribution. However, this added thickness that ruined the high speed cavitation. And there was flow separation at the trailing edge of the flap. The 20% chord flap was just too short. So I enlarged the flap to 30% chord. That, plus the leading edge shaping I'd already done, solved the takeoff speed problem. The high speed cavitation onset occurred at too low a lift coefficient, so I added camber to the flap to get more aft loading. I couldn't stand to add more lift to the main part of the section without running afoul of high speed cavitation. Here is what the new shape looks like: The nose is a little fuller, which reduces the leading edge suction peak at takeoff. The depression at the flap hinge is moved forward to 70% chord. The flap has a bit more hooked shape to give the aft loading. Here is what the new pressure distribution looks like: And here is the effect of deflecting the flap: Two degrees of angle of attack gets the 10 degree flap case to takeoff lift: There's a leading edge suction peak, but it doesn't get to the limit for takeoff. And, for the sake of completeness, here are the drag polars (assuming fully turbulent boundary layers): The cavitation plot shows takeoff is feasible at 18 kt with 10 degrees of flap: The high speed cavitation onset is 40 kt. That's not going to be competitive. So the next goal is to extend the high speed cavitation onset without giving up on takeoff and while preserving as much thickness as possible.
  15. Basiliscus

    Boats and foils comparison

    My guess is Xfoil is used a lot. These foils are operating a decent Reynolds numbers and are subject to cavitation before they get to high enough angles of attack that they'd stall from flow separation. So the potential flow theory + integral boundary layer method that Xfoil uses is fairly accurate until cavitation begins. A RANS code will tell you what a shape will do, but it doesn't tell you what the shape should be. That's the beauty of Xfoil's inverse design capabilities, and that of the Eppler code that preceded it. If the design uses a slotted flap, then MSES is needed to design and analyze the shape. There are optimizing codes that are wrapped around Xfoil and can iterate a design automatically. The 3D effects are important, and Xfoil is strictly 2D. You can use a panel code to calculate the 3D pressures and see what areas will be cavitation free. Like Xfoil, a panel code won't tell you what happens after cavitation occurs. But since the goal is to avoid cavitation, a panel code is still very useful. You can run Xfoil and a panel code on a laptop and iterate the design very quickly. Once cavitation occurs, only a RANS code can calculate what the drag rise will be. One of Brittain's aviation pioneers, Lord Brabazon, said, "Compared to designing yachts, designing airplanes is child's play." There's a reason they call it "the art of design."