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Calibri 83ffff̙̙3f3fff3f3f33333f33333.RTU Delft Repositoryg Buuidrepository linktitleauthorcontributorpublication yearabstract
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departmentresearch group programmeprojectcoordinates)uuid:cef210d6b8764f659641ef2ef4d02209Dhttp://resolver.tudelft.nl/uuid:cef210d6b8764f659641ef2ef4d02209:Wave slamming forces on truss structures for wind turbinesChristy Ushanth, N.'ivind, A.A. (mentor); Alf, T. (mentor)Generally the foundations of offshore wind turbines are steel truss structures which are exposed to wave slamming forces due to breaking waves, typically plunging breaking waves in shallow water. Calculations show that the forces from the plunging breaking waves are governing the design responses of the structure and the foundations. However, there are considerable uncertainties on the calculated plunging breaking wave forces. This research study is to investigate the wave slamming forces acting on different sections of the truss structure for wind turbines. A physical model of 1:50 scale was built at the hydrodynamic laboratory, NTNU. A large number of experiments were carried out on various sections of the truss structure such as front section and side section. Besides, two different size individual piles places at the position of the vertical legs of the truss structure were tested in order to check the size effects. All the tests carried out for regular waves with different wave height and wave periods.The recorded total responses have been decomposed into quasi static and dynamic components. Then dynamic component of the total response is analysed using frequency response function (FRF) method or the transfer function method. The transfer function relates the impact force and the responses and an impulse hammer was used to obtain the transfer function. Duhamel integral method was used only for two individual cylinders in addition to the frequency response function method. The analysed results show that the measured slamming forces are much lesser than the calculated slamming forces in all the cases. This discrepancies could be due to the size effects, scale effects and unfavourable wave form when it hits the structure. The entrained air during breaking process also influences in the results as it is different in the small scale test and in reality. It is recommended to perform the large scale tests to overcome such discrepancies.Fbreaking waves; slamming forces; response; frequency response functionen
master thesis!Civil Engineering and GeosciencesHydraulic Engineering3CoMEM Coastal and Marine Engineering and Management)uuid:e8800b4d57434bf88766bc83bd107f28Dhttp://resolver.tudelft.nl/uuid:e8800b4d57434bf88766bc83bd107f28$Stability of stones in the surf zoneOttenheim, E.S.A.P.KFontijn, H.L. (mentor); Schiereck, G.J. (mentor); D' Angremond, K. (mentor)]
Much research has been done on the stability of stones in breaking waves, but up to now, most of these studies were based on experiments with slopes varying from 1: 1 to 1:7. The stability of stones on mild slopes, slopes not steeper than 1: 10, has not yet been researched very extensively. Applications of mild slopes in practice are for example landfalls of oil pipelines and outfalls of sewage systems. The objective of this study is to improve the theoretical knowledge of the stability of stones on mild slopes in the surf zone by researching the flow in breaking waves. The stability relations for stones on mild slopes established so far, followed the trend of experimental results quite well in a qualitative way, but the difference in stability for spilling breakers and plunging breakers was predicted too small by the relations. Probably the main reason of this imperfection is the influence of the plunging jet in a plunging breaker. Therefore, the processes which take place in plunging breakers are studied. From a study by Basco (1985) it was concluded that processes in spilling and plunging breakers are similar, albeit that the vortex systems in plunging break< ers are of a much larger scale. Experiments were carried out in the large wave flume of the Laboratory of Fluid Mechanics for a better understanding of the stability of stones on a slope subjected to wave attack. The model structure consisted of a 1: 10 impermeable slope, on which a layer of stones (Dnso =1.21 cm) was laid. Only regular waves were used, because these wave are more suitable for researching the flow in a particular wave. For three waves with different wave steepnesses, incipient motion of the stones was determined. SUbsequently, in the breaking regions of these waves, velocity measurements were carried out by means of LDV and video recordings. From the damage experiments it was concluded that maximum damage was located at about h/Ho =0.6 and that the direction of displacement of the stones depends on the breaker parameter. Furthermore, the stone displacement in upslope direction seemed to be caused by the plunging jets of the breaking waves. The velocities in the plunging jet were equivalent for the three different waves, which is in line with the fact that these waves cause incipient motion. The plunging jets of the breaking waves cause incipient motion of the stones. Up to now no theories were available for the stone stability in plunging jets. Therefore, an attempt was made to model the stone stability in a plunging jet. Two different models were considered, which both schematize the plunging jet on a stone as static forces on a single cubical stone. From the modelling it was concluded that the results deviate from experimental results. The missing of the turbulent fluctuations of the jet and to a less extent the dynamic characteristics of the stone stability were probably the main reasons of this deviation. Nevertheless, the modelling can improve the theoretical understanding of the stone stability in plunging jets. The numerical results of the stability relation by Izbash for uniform flow are close to the experimental results. Therefore, it seems that stone stability in a plunging jet is not as unfavourable as expected, compared to stone stability in uniform flow. The resulting stability equations for the stone stability in plunging breakers is in conformance with existing relations.9stone stability; Izbash; incipient motion; breaking waves)uuid:27b9df2f9e444e3690f81e8634453ac5Dhttp://resolver.tudelft.nl/uuid:27b9df2f9e444e3690f81e8634453ac5VThe influence of breaking waves on the vertical velocity distribution in the surf zone
Walstra, D.J.Stelling, G.S. (mentor)In this report we describe the development and calibration of a model which predicts the vertical distribution of the horizontal velocities induced by waves, the socalled undertow. Location of interest is the nearshore region where the flow is induced by both breaking and nonbreaking waves. The model consists out of three modules. The first module computes the properties of the incoming (breaking) waves, a second module converts the resulting dat from the first module so that it can be used as input for the third module which is a 2DY model. The 2DY model is a 2DY version of DELFT HYDRAULICS' TRISULA which is a 3D hydrostatic tidal model. The waves influence the flow in three ways. A first effect is a shear stress at the water surface which originates from the breaking of waves. Also is shown in this thesis that the waves induce a mass flux which has as a result that there is a nett flow directed offshore. The third effect is the influence of the orbital motion on the viscosity distribution. All these effects are included in the model described in this thesis. The model has been tested against measured data in the Delta flume to see how accurate the model predicts the undertow. Also published as Delft Hydraulics report H 1684;surf zone; breaking waves; velocitiy distribution; undertowEnvironmental Fluid Dynamics)uuid:5902e438de57474c8373eef33d0c1068Dhttp://resolver.tudelft.nl/uuid:5902e438de57474c8373eef33d0c1068qThe breaking of windwaves in deep water: Part I. Probalistic aspects : Part II. Observed breaking wave statisticsHerbe<drs, T.H.C.2Battjes, J.A. (mentor); Holthuijsen, L.H. (mentor)Part I Probabilistic aspects. An overview is given of literature on the statistics of breaking waves in open ocean. New approximations of the fraction of breaking waves and the distribution of breaking wave heights are presented and compared with results found in literature. Part II Observed breaking wave statistics. This part is dedicated to the results of the field observations. In this experiment visual observations of breaking waves passing a waverider buoy are carried out. The statistics of breaking waves are investigated from wave records of this buoy in which visually observed breaking waves are labelled. Part III ontbreekt.fraction; breaking waves)uuid:439d0912a38c4d4b9302c86154f011c3Dhttp://resolver.tudelft.nl/uuid:439d0912a38c4d4b9302c86154f011c3$Golfvervorming bij een hoog voorlandVan der Most, H.Battjes, J.A. (mentor)CHet rapport "Literatuuronderzoek bij een hoog voorland" bevat een systematische weergave van de bestaande kennis van het gedrag van golven in een bepaalde ondieptesituatie. Die situatie betreft een tamelijk vlak, relatief hoog liggend voorland voor een kust met daar boven een geringe waterdiepte, en wel het gebied vanaf de lijn waar de golven het voorland bereiken tot aan de lijn waar de golven zich, voornamelijk t.g.v. brekingsverschijnselen, hebben aangepast aan de waterdiepte op het voorland. Het rapport geeft een beschrijving van de verschijnselen van het breken en van de overige oorzaken van energieverliezen in de golven. De in de literatuur gegeven kwantitatieve benaderingen voor het verloop van de golfhoogte en periode in en na de brekerzone worden weergegeven. Dit leidt tot enkele praktisch hanteerbare resultaten.Rwave runup; shallow foreshore; shoaling; breaking waves; shallow water conditionsnl
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