196,284 research outputs found
Solutions to wave damping over time in CFD RANS simulations due to exponential generation of numerical turbulence
No full textIn ocean engineering, high-fidelity analyses like computational fluid dynamics Unsteady Reynolds Averaged Navier Stokes simulations are fundamental for the evaluation of realistic hydrodynamic loads during extreme events, which are usually overestimated by boundary element method based solvers, thus leading to the definition of an oversized wave energy converter structure. To describe multiphase flows, the most used model in RANS CFD simulations is Volume of Fluid model, which consists in creating an interface between the different fluids and assigning to each cell of the domain a percentage of each phase. Unfortunately, the model has a well-known problem in the ocean engineering community which increases with simulation time and affects the wave propagation. For the correct spatial propagation of the wave motion, it is necessary to use a very low Courant-Friedrich-Lewy factor, hence causing a noticeable increase in the computational time and speeding up the generation of artificial turbulence in the domain. The problem is intrinsic in the Volume of Fluid model and cannot be solved by refining the mesh or increasing the order of accuracy of the solvers. The numerical turbulence spreads in the whole domain and dampens the waves overtime after several wave periods (10-50). With this work, we analyse the effect of a threshold on the turbulent viscosity to reduce the production of numerical turbulent kinetic energy; theaim is to produce a general methodology to simulate long time frames in a RANS simulation with the Volume of Fluid model. The investigation is first performed in a semi-2D tank by monitoring the wave probes over time. It is then repeated in a 3D domainwith and without the limiter in the case of a floating body, to verify that the kinematics of the floater is not altered by the limiter
Optimal heuristic design of a high efficient SWATH-type vessel
No full textThe paper illustrates the shape optimization of the immersed part of the hulls of a SWATH (Small Waterplane Area Twin Hull) vessel, a particular type of catamaran that has already been used as passenger ship and fast ferry. The optimization tool aims to find the best combination of parameters, such as the shape of the immersed hulls, their distance, and draft that induces the lowest hydrodynamic resistance. This application refers to the same design vessel that has fixed displacement and cruise speed defined for specific mission requirements. Size limits are also imposed by operability requirements.The optimization makes use of a genetic evolutionary algorithm, a stochastic global search method based on the Darwinian concept of natural selection. The optimization environment has been created in Matlab and is based on a parametric geometry definition module, able to define a series of hull forms through the CST technique (Class Function/Shape Function Transformation). For the evaluation of the hydrodynamic resistance, CFD simulations are implemented in STAR-CCM+.The main reasons that motivate this research are the integration of computer-assisted parametric definition of vessels hull shapes with the CFD techniques for the prediction of hydrodynamic performances, and to assemble an optimization tool able to facilitate the design procedure through the creation of complete performance databases of customized hulls.As the amount of the total resistance reduction with an improved hull shape is impressive, the usefulness of applying automated CFD optimization techniques in the design is remarkable and should persuade the reader to consider this alternative in the design stages of a boat
Correlation between a field glucose tolerance test (FGTT) in late gestation and early lactation energy metabolism and disease incidence in dairy cows
No full textCows response to glucose tolerance test (GTT) and periparturient diseases: preliminary study.
No full textReproductive management. A field approach
No full textAnalysis of data on 763 primiparous and 1214 multiparous cows in Italy, obtained between 1 April and 31 December 2004, revealed that the average incidence of twinning was 0.39 and 6.58% respectively and that of stillbirths 4.58 and 5.10%. The incidence of retained placenta, metritis, ketosis and mastitis was 3.93, 30.53, 14.2 and 0.52% respectively for heifers vs. 13.59, 29.07, 22.56 and 2.80% for cows. The average body condition score at calving was >4.25 for 1.04% of heifers and 2.37% of cows, and it was <3 for 32.11% of heifers and 43.42% of cows. An account is given of the correlations between incidence of the various diseases and body condition and of the costs of veterinary treatment. The importance of herd management is emphasised, details are given of methods of evaluating body condition scores at different times of the reproductive cycle, and suggestions are made for various hands-on methods of avoiding and detecting diseases
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