L11. Diffraction behind a gap in a breakwater


The purpose of this test is to validate the diffraction approximation in Swan. The approximation is described by Holthuijsen, Herman and Booij (2003).
The results are compared with observations reported by Yu et al.(2000). Two tests have been used, both with a gap width equal to 4 times the wave length (2 m); one test was carried out with regular incoming waves, the other with unidirectional irregular waves.

A domain with horizontal bottom is considered.
In order to simulate the unidirectional incoming waves at the Southern boundary an incident wave field is assumed with narrow directional distribution ([ms]=500) and a Jonswap spectrum over frequencies with peak period of 1.2 s. The wave field propagates in N direction.

7 cases are considered:

L11a
Swan without diffraction
L11b
Swan with standard diffration
L11c
diffraction with enhanced smoothing
L11d
diffraction with strong smoothing
L11e
standard diffraction on a grid, rotated over i90 degrees, in order to test the correct implementation
L11f
standard diffraction on a grid, with singularity approximation
L11g
as L11f, but with simplified representation of the breakwater

Graphs are shown for cases a, b and c, along lines parallel to the breakwater at various distances behind the breakwater: 0 m (i.e. in the gap), 3 m, 6 m and 9 m.

L.H. Holthuijsen, A. Herman, N. Booij (2003)
Phase-decoupled refraction-diffraction in a spectral wave model, Coastal Engineering, vol.49, PP.291-305
Yu, Y.-X., S.-X., Li, Y.S., Onyx, Wai, W.H. (2000)
Refraction and diffraction of random waves through breakwater, Ocean Engng, vol. 27, pp.489-509

© 2012: Nico Booij