  This work is motivated by CFD simulations from a realistic landing gear
performed modeling only the half bottom of the aircraft fuselage [15]. Hence,
in this previous analysis, the aeroacoustic predictions had to employ
incomplete FWH surfaces. In the present study, a discussion on the closed
surface requirement is provided for the frequency domain FWH formulation,
although we believe that the approach presented here can also be applied for
the time-domain methodology. We show that all sources computed on the closed
permeable surface may play an important role if the acoustic prediction is
sought at observer positions distributed along the entire circular arc.
However, results obtained for 2D and 3D model problems show that, if wisely
designed, a permeable FWH surface composed only by a finite patch, i.e., an
incomplete surface, can be employed to accurately predict the noise at specific
observer positions. In this case, the surface must be placed between the line
of sight from the sources to the observers, for example, in flyover or sideline
locations. In the current applications, results from finite surfaces agree with
the expected values as long as the source magnitudes decay on elements distant
from the true incident source, i.e., the airframe. Hence, in order for this
approach to be valid, the sources cannot be truncated. The planar setup is
beneficial in airframe noise applications since it avoids contamination by
quadrupole sources crossing the boundaries and also may simplify the surface
design.
