[Gmsh] Ruled surfaces and horns

[Bjørn Kolbrek]

Hi,

I'm using gmsh to create mesh for horns (for loudspeakers, but it could 
be for instruments as well), and for horns of rectangular cross section, 
it works well. But for circular/axisymmetric horns, I get some problems.

Basically, I divide the horn circumference into arcs, that together make 
complete circles. There are several circles along the horn (mostly to 
check the geometry). Then I have spline lines running along the length 
of the horn, making up the contour. Two spline lines and one arc at each 
end of the horn are then combined into a ruled surface.

This is where the problem comes. The ruled surface follows the arcs at 
the ends, but not in between. It bulges out from the circles along the 
horn. If I use only two arcs around the horn, it becomes oval. If I use 
more arcs, it becomes "flower shaped", instead of perfectly round. I 
guess the problem is that the ruled surface is concave in one direction 
an convex in the other, although the problem is still there to a small 
extent when the horn is a paraboloid. If it is a cone, there is no 
problem, but most of the horns I want to mesh are trumpet shaped.

Is there a solution or workaround to make the horn perfectly circular 
all the way?

Regards,
Bjørn