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DISCUSSION

E.Palm

University of Oslo, Norway

It is an interesting and important topic which is discussed in this paper. In spite of this, the available, pertinent literature on the topic is very limited. The paper is of special value since it reports experimental as well as numerical results.

The main outcome of the numerical computations is displayed in Fig.6, which shows the mean second-order forces acting on the submerged body as functions of ω2b/g1, for three different values of the pycnocline thickness, δ. As expected, it is found that for sufficiently long waves the pycnocline layer is of no importance if the body is situated from this layer. The figures suggest that the waves do not feel the pycnocline layer if kδ< about 0.5 (k incoming wave number), roughly speaking. It would be of interest to see also the data for other forms of the elliptic body, for example, a circular form or a flat plate.

The experimental and numerical results are compared in Figs. 7 and 8. It is surprising that in Figs. 7a and 7b a fair agreement is obtained for the horizontal force whereas the values for the vertical forces are more than 100% different. The authors believe it is due to nonlinear effects which obviously may be important. It is seen that for Figs. 7a and 7b the values of the second-order forces computed by the two-layer models are in reasonable agreement with those obtained from the three-layer model. This suggests that it would be of interest to apply a two-layer time-dependent nonlinear program to this problem to study the forces. Such programs exist.

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