TRAPPED MODES EMBEDDED IN THE CONTINUOUS-SPECTRUM

The existence of trapped modes due to rigid obstacles placed symmetric ally in between parallel walls having either Neumann or Dirichlet cond itions imposed upon them are well known to occur for frequencies below the continuous spectrum or channel cut-off and for a range of geometr ic configurations. Zn this paper, we provide convincing numerical evid ence for an additional isolated trapped mode of both Neumann and Diric hlet type embedded in the continuous spectrum (or above the channel cu t-off) in the case of a rigid circular cylinder placed on the centre-p lane of the channel. Thus, for each type of mode we give results showi ng that there is just one cylinder size and wave frequency at which th e trapped mode occurs.