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Wave generation, propagation and absorption in CFD simulations of free surface flows

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  1. Tez No: 508368
  2. Yazar: BÜLENT DÜZ
  3. Danışmanlar: Dr. RENE H.M. HUIJSMANS, Dr. ARTHUR E. P. VELDMAN
  4. Tez Türü: Doktora
  5. Konular: Deniz Bilimleri, Makine Mühendisliği, Marine Science, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2015
  8. Dil: İngilizce
  9. Üniversite: Technische Universiteit Delft (Delft University of Technology)
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 177

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Özet (Çeviri)

THIS thesis presented the outcome of a four-year doctorate study on two popular topics in CFD simulation of interfacial and free surface flows: free surface modeling and absorbing boundary conditions. Numerical methods that address these topics were implemented in the CFD simulation tool ComFLOW. Spurious energy dissipation in numerical wave simulations is a widely-known problem, which manifests itself as loss of wave height accompanied by phase shift, and can have several causes such as, the discretization method used for the governing equations applied inside the flow domain, numerical modeling of the moving free surface, and the boundary conditions applied at the free surface. Among these, the free surface treatment is an important factor, and its effect on wave dissipation was studied extensively. The VOF implementation in ComFLOWbefore this work was the donor-acceptor method introduced by Hirt and Nichols [1], and a local height function (LHF) [2] to overcome the bottlenecks which originate from this VOF technique such as violation of mass conservation, and spurious flotsam and jetsam [3]. Although implementation of the LHF reduced the amount of flotsam and jetsam considerably and improved mass conservation [4], the overall VOF method produced a significant amount of wave damping. After carefully analyzing strengths and weaknesses of many VOF techniques in the literature, several methods were implemented, and their designs were explained in detail. Performance of these methods were investigated through a number of test problems including simulation of propagating waves where strong nonlinear behavior was dominant in the flow. In all the test cases, the VOF methods implemented in this work outperformed the previous VOF implementation by a clear margin without increasing computational costs in a disproportionate manner. Especially in simulations of propagating waves, a significant reduction in wave damping was observed. Numerical simulations of wave phenomena necessarily have to be carried out in a limited computational domain. This implies that incoming waves should be prescribed properly, and the outgoing waves should leave the domain without causing reflections. Sometimes both tasks need to be performed over the same boundary simultaneously. In this thesis we presented a boundary condition building upon the work in [5]. The Sommerfeld condition [6] formed the backbone of the work in [5], which is perfectly absorbing for a single wave component with one phase speed. The phase speed was substituted by an approximation of the dispersion relation from the linear theory in terms of the wave number. Subsequently the wave number was replaced by the second-order derivative of the velocity potential in the vertical direction. The innovative aspect of this design is that the phase speed featuring in the boundary condition, which is usually unknown due to dispersion, is now determined from the local flow solution in an implicit fashion. Allowing incoming and outgoing waves to travel simultaneously over the same boundary was achieved by following the idea of Carpenter [7]. In this work we preferred Higdon operators [8] as the starting point of the design The reason for this decision lies in the fact that the first-order Higdon operator is nonreflecting for a wave impinging on the boundary with an angle of incidence, while the Sommerfeld condition provides full absorption only at the normal incidence. This allowed us to generate and absorb oblique regular and long-crested irregular waves. However, looking out at the sea, we rarely observe waves traveling in a specific direction. On the contrary, the sea surface is rather chaotic with waves of different characteristics propagating in different directions. Therefore, the boundary condition needed a further modification such that generating and absorbing short-crested waves become possible. This was achieved by considering the second-order Higdon operator which has much more favorable directional properties than the first-order variant. Into the second-order operator we then incorporated the improvements concerning dispersive effects thatwere explained previously. The performance of the boundary conditionswas assessed in a number of tests. In all the tests the boundary condition that was presented in this work showed a good performance demonstrating its capability of accounting for dispersive and directional effects of free surface water waves. Finally, results from validation studies were presented. Two tests were considered: breaking dam, and wave run-up on a semi-submersible. A good agreement between the results from the numerical simulations and the experiments was observed.

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