what will be the most effective dihedral angle in the most open, most closed or mixed wing? I simulated 3 models in the wind tunnel. the wingspan and the shape seen from above is the same.
can someone help to interpret these graphics of the wind tunnel?
my impressions as user because I already tested the duotone and the other models:
lower dihedral, greater pressure on the sail but the wing is less drivable
greater dihedral less wind pressure and more drivable is the wing,but wing
who knows the option of leaving the dihedral in two different angles is the option to leave the wing with good pressure on the sail not excessive and not too loose
best regards .
Eric
Re: wing foils shapes
Posted: Tue Aug 11, 2020 6:50 pm
by Mossy 757
Eric if you're looking for intelligent analysis regarding wind tunnel data at Kiteforum.com, I've got some bad news for you...
Re: wing foils shapes
Posted: Tue Aug 11, 2020 10:49 pm
by bragnouff
Isn't your simulation doing the assumption that the wind hits the plane of the wing at a 90 degrees angle? (wind coming from the left on your pic)
This would be wrong because, the AOA of the wing isn't necessarily that closed, at least not on every part of the profile. And secondly, like for any sail, apparent wind plays a major role, in particular, in changing that angle between wind and sail.
Not too sure if you can isolate dihedral parameter from profile. I mean, you can, you just did, but not too sure if this leads to any kind of valid interpretation.
Re: wing foils shapes
Posted: Fri Aug 14, 2020 1:25 pm
by ericmsil
bragnouff
for the pressure on the sail I tried to simulate a constant lateral wind and the position of the sail at 60 degrees on a constant edge without changing position. Do you think it would be more valid to analyze them at 90 degrees? And for the leading edge to analyze the dihedrals who knows analyze the dihedrals in a flat position to the ground to understand which dihedral shape offers less drag, I mean in the neutral position of the wing.
tks
Eric
Re: wing foils shapes
Posted: Fri Aug 14, 2020 3:29 pm
by Kiterpep
Eric, what Bragnouff means is that the angle of attack of the wing sections is currently 90 degrees. This is not correct since you will be moving forward. Let's say you move at 20 km/h (not sure if that is a logical kind of wingsurfing speed?) that's approximately 6 m/s. So you get a headwind of 6 m/s, which needs to be added to the lateral wind. If you draw the triangle, you will see that in this case the angle of attack of the sections will be 60 degrees. That angle of attack would still be way too high to use the wing efficiently, which is why a crosswind sailing wingsurfer will probably hold his wing to reduce it, like in this video at 00:19 you can see him pushing the trailing edge away.
In your simulations, the flow is coming from the side, which is not how you want your wing to work, since it will be very inefficient. Because the angle of attack is so high, all the flow has separated. If you add a wind component into the pictures (i.e. headwind), your simulations might become much more realistic and useful.
Re: wing foils shapes
Posted: Fri Aug 14, 2020 10:59 pm
by bragnouff
OK, I didn't really see that "60 degrees" wing angle, I assumed from the arrows on the graphs that it was a straight 90. I guess it is not as bad, but I have no idea if this is still the correct AOA that the wings are meant to be operated at, factoring the apparent wind. It's probably less than that.
I guess, you could get a series of simulation with different angles, and you'll quickly see what leads to less disturbance, and would give a hint about the likely AOA to be used. But then, because various parts of the profile have different AOA, that would still be a gross approximation in terms of behaviour of the wing.
Re: wing foils shapes
Posted: Thu Aug 20, 2020 3:57 pm
by ericmsil
hi guys.
my simulation was done like this side wind
and just to see how the slope of the dihedrals would influence the lateral wind pressure.
atached file xxx with position wind and wing angle 60 degrees.
my simulation was done like this side wind
and just to see how the slope of the dihedrals would influence the lateral wind pressure.
atached file xxx with position wind and wing angle 60 degrees.
Ok, so your first pic confirms really what we initially thought (or feared), that the wind is hitting the wing laterally. And your 60 degrees is the angle between the approximate plane of the wing and the horizontal plane.
Because we're dealing with a wing (and not a flat surface) with a profile, it is critical to factor in the angle of attach(s) (and the right one(s), while at it, but surely this isn't 90). Otherwise, you'll just produce cute colorful graphs that are totally meaningless and unrelated to real world. The most powerful tools are still completely useless if you don't feed them the right params.
Re: wing foils shapes
Posted: Tue Aug 25, 2020 12:34 pm
by Kiterpep
To explain your results to you a bit, in the darker-coloured parts of your simulation (blue/green etcetera), the flow around the airfoil has separated from its surface. In this region, we call the flow stalled, and it will essentially be standing still i.e. at a very low velocity. This creates a huge amount of drag and no lift at all. What you are doing is comparable to holding a plank instead of a wing.
Just to repeat, your simulations are quite useless like this. You should probably study some things about aerodynamics before diving into CFD, some suggestions:
If you want to gain some useful design inputs, you need to reduce the angle of attack in your simulation. In effect this can be done by just adding a -y component to your inflow velocity, which is created by the winger surfing forward (similar to the headwind you experience when you ride your bike on a windless day).