Unsteady RANS method for ship motions with application to roll for a surface combatant

An unsteady Reynolds-averaged Navier-Stokes method is developed to compute motions and the resulting flow and wave fields around surface ships. Although the formulation and RANS code are generalized for six-degree-of-freedom motions, the method is demonstrated here for the viscous phenomenon of roll decay motion for a surface combatant. The method is based on an extension of CFDSHIP-IOWA (a general-purpose code for computational ship hydrodynamics) to predict ship motions with larger amplitude and non-slender geometry, in comparison to traditional linearized methods. The flow solver uses higher-order upwind discretization, PISO method for pressure-velocity coupling, a blended k-ω/k-ε two-equation turbulence model, free surface tracking approach, and structured multi-block grid systems. As an initial step, unsteady simulations of a modern surface combatant with predicted roll decay and prescribed sinusoidal roll motion are performed. Roll decay motion is simulated by releasing the model from an initial roll angular displacement and by computing the resulting roll motion. Verification of the time history of the roll motion is performed using iteration, grid, and time step studies and numerical uncertainties are shown to be less than 1%. Validation is performed by comparison with available experimental data with the predictions validated at 1.7% and 1.5% for the uncorrected and corrected solutions, respectively. For simulations with prescribed motions, the periodic response of the boundary layer to the rolling motion is described and quantified using a Fourier analysis. A spring-mass-damper system is used to compare the current non-linear predictions to traditional linear strip theory results. The method is shown to accurately predict the natural rolling frequency and roll decay rate at multiple ship speeds both without and with bilge keels, which demonstrates the ability to assess seakeeping characteristics for practical geometries.     

Evaluation of a bounded high order upwind scheme for 3D incompressible free surface flow computations

In the context of normalized variable formulation (NVF) of Leonard and total variation diminishing (TVD) constraints of Harten, this paper presents an extension of a previous work by the authors for solving unsteady incompressible flow problems. The main contributions of the paper are threefold. First, it presents the results of the development and implementation of a bounded high order upwind adaptative QUICKEST scheme in the 3D robust code (Freeflow), for the numerical solution of the full incompressible Navier–Stokes equations. Second, it reports numerical simulation results for 1D shock tube problem, 2D impinging jet and 2D/3D broken dam flows. Furthermore, these results are compared with existing analytical and experimental data. And third, it presents the application of the numerical method for solving 3D free surface flow problems.     

Comparison of surface roughness quality obtained by high speed CNC trimming and high speed robotic trimming for CFRP laminate

This paper proposes an experimental approach for evaluating the surface roughness of the CFRP parts produced by high speed CNC trimming and high speed robotic trimming under various cutting conditions. A comparison is made between the surface roughnesses obtained by the two processes. The results obtained show that, the measured profiles obtained from high speed robotic trimming are dominated by a large trajectory deviation, as compared to machine tool trimming results. After the trajectory deviation effect is discounted, the results show that for the +45° ply orientation, the surface quality obtained through high speed robotic trimming is similar to what is obtained with the CNC machine. Furthermore, a significant relationship was observed between the surface quality and the ply orientation, whatever the machining process and the cutting conditions employed. The −45° ply orientation represents the worst case in terms of surface roughness, whatever the machining process. It is 4 times higher compared with that of +45° ply orientations,The results also show that the effect of cutting conditions on surface quality is significant for both machining processes tested.     

Numerical simulation of steady planar die swell for a Newtonian fluid using the spectral element method

This paper is concerned with two dimensional numerical simulations of plane extrusion of a Newtonian fluid. The problem is discretized using the spectral element method and the free surface is evolved according to an ALE treatment. Numerical simulations are performed over a wide range of Reynolds and Weber numbers to highlight the effects of inertia and surface tension, respectively. Convergence of the numerical approximations with respect to polynomial order is demonstrated for the sensitive measures of free surface location and downstream relaxation distance. The higher the inertia the further downstream the relaxation occurs. Numerical results show good qualitative and quantitative agreement with predictions of other numerical schemes and experiments.     

Numerical simulation of the coupling problems of a solid sphere impacting on a liquid free surface

This paper presents a model, using a phase-field method, that is able to simulate the motion of a solid sphere impacting on a liquid surface, including the effects of capillary and hydrodynamic forces. The basic phenomena that were the subject of our research effort are the small scale mechanism such as the wetting property of the solid surface which control the large scale phenomena of the interaction. The coupled problem during the impact will be formulated by the inclusion of the surface energies of the solid surface in the formulation, which gives a reliable prediction of the motion of solid objects in/on/out of a liquid surface and the hydrodynamic behaviours at small scales when the inertia of fluid is less important than its surface tension. Numerical results at different surface wettabilities and impact conditions will be presented and compared with the experiments of Duez el al. [C. Duez, C. Ybert, C. Clanet, L. Bocquet, Nat. Phys. 3 (2007) 180–183] and Lee and Kim [D. Lee, H. Kim, Langmuir 24 (1) (2008) 142].     

Characterising groundwater use by vegetation using a surface energy balance model and satellite observations of land surface temperature

This study presents a novel ‘model-data’ approach to detect groundwater-dependent vegetation (GDV), through differences in modelled and observed land surface temperatures (LST) in space and time. Vegetation groundwater use is inferred where modelled LST exceeds observed LST by more than a threshold determined from consideration of systematic and random errors in model and observations. Modelled LST was derived from a surface energy balance model and LST observations were obtained from Terra-MODIS thermal imagery. The model-data approach, applied in the Condamine River Catchment, Queensland, Australia, identified GDV coincident to existing mapping. GDV were found to use groundwater up to 48% of the time and for as many as 56 consecutive days. Under driest of conditions, groundwater was estimated to contribute up to 0.2 mm h⁻¹ to total ET for GDV. The ability to both detect the location and water-use dynamics of GDV is a significant advancement on previous remote-sensing GDV methods.     

A versatile multicomponent database for the surface tension of liquid metals

Experimental surface tension data for many binary, ternary and higher order metallic systems is unfortunately currently unavailable in the literature. This could be detrimental in several practical and industrial applications. Consequently, having a theoretical model with a good predictive capability is highly desirable. In general, the surface tension of metallic alloys is predicted via the well-established Butler model. This model assumes a linear relationship between the excess Gibbs energy in the Bulk and on the surface. For many systems, this assumption is not valid, especially for systems based on elements with different bulk electronic structures, for which the Butler model fails to predict the composition dependence of the surface tension (As demonstrated in this work, the Butler model is accurate only for about 65% of metallic system for which experimental data is available). The aim of this paper is to propose an alternative to the Butler model to represent accurately the surface tension multi component liquid metals. The proposed model is an extension of the Guggenheim model of ideal solutions to take into account the difference of electronic structures between elements of a solution. The model is compared with the 36 binary and 7 ternary metallic alloys for which experimental data is available. It is shown that the accuracy of the model is higher than 95%. On that basis, the composition dependence of the surface tension of many other binary alloys for which experimental data is not available is predicted. The extension of the model to ternary and higher order systems is proposed without introducing any new parameters, i.e. by considering only the binary parameters. It is shown that the extended model provides also an accurate prediction of the surface tension for ternary metal.     

Analysis of a Discontinuous Finite Element Method for the Coupled Stokes and Darcy Problems

The coupled Stokes and Darcy flows problem is solved by the locally conservative discontinuous Galerkin method. Optimal error estimates for the fluid velocity and pressure are derived. This revised version was published online in July 2005 with corrected volume and issue numbers.     

Analysis and optimization of surface roughness in the ball burnishing process using response surface methodology and desirabilty function

In the present study, an optimization strategy based on desirability function approach (DFA) together with response surface methodology (RSM) has been used to optimize ball burnishing process of 7178 aluminium alloy. A quadratic regression model was developed to predict surface roughness using RSM with rotatable central composite design (CCD). In the development of predictive models, burnishing force, number of passes, feed rate and burnishing speed were considered as model variables. The results indicated that burnishing force and number of passes were the significant factors on the surface roughness. The predicted surface roughness values and the subsequent verification experiments under the optimal conditions were confirmed the validity of the predicted model. The absolute average error between the experimental and predicted values at the optimal combination of parameter settings for surface roughness was calculated as 2.82%.     

可逆冷带轧机速度张力系统的模糊自适应动态面反步控制

研究了基于动态面反步控制和模糊自适应逼近的可逆冷带轧机速度张力系统直接反馈线性化(direct feedback linearization,DFL)动态解耦控制问题.首先,通过构造非线性干扰观测器(nonlinear disturbance observer,NDO)削弱了模型中非匹配不确定项对系统性能的影响,进而应用DFL理论实现了速度张力非线性耦合系统的动态解耦和线性化;其次,将反步控制与动态面控制相结合完成了解耦后速度张力各线性子系统控制器的设计,且有效避免了反步控制中的"微分爆炸"现象;再次,采用模糊自适应方法对所设计控制器中的匹配不确定项进行了逼近估计,有效地提高了速度张力系统的跟踪控制精度;稳定性分析结果表明,可逆冷带轧机速度张力系统是一致最终有界的.最后,基于工业现场的实际数据进行仿真对比研究,仿真结果验证了本文所提方法的有效性.