What is the mode of a square waveguide \$TE_{11}\$ that is deformed into a circle?
Dominant mode in rectangular waveguide is TE10 and in circular
  waveguide is TE11.
  
  Rectangular to circular waveguide transition convert dominant TE10
  mode of rectangular waveguide to TE11 dominant mode of circular
  waveguide and vice versa.


Source: http://www.rfwireless-world.com/Terminology/Rectangular-waveguide-vs-Circular-waveguide.html