How to create a turbofan exhaust mixer in CATIA V5
Enter GSD, and create a circle in sketcher.
Offset a plane from the plane you created the circle with. Enter sketcher on the new plane and create the lobed shape shown. Dimensions aren't critical, since this isn't a technically correct mixer.
Model should look like this:
Extrude each sketch a little bit. The dimensions don't matter, because they'll be used as tangency supports later.
Create a multi-section surface with the two sketches. Use the extrusions as Supports for tangency. Make sure the Closing Points are correct.
Hide the extrusions and sketches. Thicken the multisection surface a little bit.
Enter sketcher on a new plane 90 degrees from the first 2 planes. Make the sketch shown. I kind of guessed on the dimensions and made it look right.
Make a 360 degree rotational extrusion with the sketch.
Split the mixer with the rotated extrusion and cut off the back part.
You should end up with something like this: