"Flanging" in frequency domain?
Frequency domain does not really work here. A flanger is NOT a linear time invariant system. You could try to implement this is "piece-wise" LTI and change the delay only at a frame boundary but this will mostly likely result in unacceptable artifacts. 

A flanger will require a time variant fractional delay. Rounding the delay the nearest integer delay results in unacceptable "zipper" noise.