A Stress Mapping Immersed Boundary Method for Viscous Flows

This work introduces an immersed boundary method for two-dimensional simulation of incompressible Navier-Stokes equations. The method uses flow field mapping on the immersed boundary and performs a contour integration to calculate immersed boundary forces. This takes into account the relative location of the immersed boundary inside the background grid elements by using inverse distance weights, and also considers the curvature of the immersed boundary edges. The governing equations of the fluid mechanics are solved using a Galerkin-Least squares finite element formulation. The model is validated against a stationary and a vertically oscillating circular cylinder in a cross flow. The results of the model show acceptable accuracy when compared to experimental and numerical results.

An Improved Procedure for Strongly Coupled Prediction of Sailing Yacht Performance

In this paper, an improved procedure for strongly coupled prediction of sailing yacht performance is developed. The procedure uses 3D RANS CFD to compute the hydrodynamic forces. When coupled to a rigid body motion solver and a sail force model, along with a rudder control algorithm, this allows sailing yacht performance to be predicted within CFD software. The procedure provides improved convergence when compared to a previously published method. The grid motion scheme, partially using overset grid techniques, means that correct alignment between the free surface and the background grid is ensured even at large heel angles. The capabilities are demonstrated with performance predictions for the SYRF 14 m yacht, at one true wind speed, over a range of true wind angles, with up- and downwind sailsets. The results are compared to predictions from the ORC-VPP for a yacht with similar main particulars.

A $$C^1$$-continuous Trace-Finite-Cell-Method for linear thin shell analysis on implicitly defined surfaces

A Trace-Finite-Cell-Method for the numerical analysis of thin shells is presented combining concepts of the TraceFEM and the Finite-Cell-Method. As an underlying shell model we use the Koiter model, which we re-derive in strong form based on first principles of continuum mechanics by recasting well-known relations formulated in local coordinates to a formulation independent of a parametrization. The field approximation is constructed by restricting shape functions defined on a structured background grid on the shell surface. As shape functions we use on a background grid the tensor product of cubic splines. This yields $$C^1$$ C 1 -continuous approximation spaces, which are required by the governing equations of fourth order. The parametrization-free formulation allows a natural implementation of the proposed method and manufactured solutions on arbitrary geometries for code verification. Thus, the implementation is verified by a convergence analysis where the error is computed with an exact manufactured solution. Furthermore, benchmark tests are investigated.

Background-Grid Based Mapping Approach to Film Cooling Meshing: Part I — Strategies and Test Cases

Film cooling technique is commonly adopted in modern gas turbine engines to protect high-temperature components from erosion and damage caused by thermal stress. To improve film cooling effectiveness, many efficient prediction tools have been developed and have shown promising results, which are helpful for turbine aero-thermal design. For film cooling, evidence has shown that it is strongly affected by the momentum and heat transport in the boundary layer when hot gas and coolant are mixed downstream of the ejection. From the view of resolution accuracy in the boundary layer, structured grids will be the primary choice in fluid domain. However, the high-pressure gas turbine blades usually have several hundreds of cooling holes with different configurations and arrangements. Numerical simulations often face a big challenge in multi-block structured-grid generations when a large number of cooling holes are involved on curved hole-to-mainstream interfaces. Conventional block-splitting and mesh-generation for all holes are quite time-consuming and cumbersome, because the copying, translating and rotating manipulations cannot be applied on curved hole-to-mainstream interfaces directly. To solve these difficulties, this paper presents a novel mesh-generation strategy, which is a background-grid based mapping (BGBM) method, to generate multi-block structured grids for film-cooled blade efficiently without modifying the existing meshing tools and solvers, which is convenient for CFD users. It consists of three main steps: At first, the correspondence between physical space and computational space is established by two sets of background grids. Then, the sectional curves of geometry features in physical space are projected to the computational space. With these treatments, the curved hole-to-mainstream interfaces are flattened in computational space, where grids can be quickly generated with block copying, translating, rotating and merging manipulations. Thereafter, meshes in computational space are mapped back to the physical space based on the correspondence between physical and computational spaces, and high-quality structured-meshes can be obtained for numerical simulations. To demonstrate the presented meshing strategy, several typical cases with film cooling are selected for testing, including single cooling hole on curved surface, multiple rows of cooling holes on curved surface and NASA C3X vane with multiple hole arrays. In these cases, different holes, including the cylindrical holes and shaped holes with different ejection angles and arrangements, on curved interfaces are taken into consideration. The quality of generated structured grids for each test case is illustrated, which is able to meet the requirement of CFD solver. With the generated meshes, conjugate heat transfer performance in the turbine vane with different cooling arrangements is investigated and also validated with the existing experimental data.

Background-Grid Based Mapping Approach to Film Cooling Meshing: Part II — Applications in Vane With Landed Trailing-Edge Cutback

As gas turbine inlet temperature continuously increases, blade trailing edge suffers an extremely high thermal load due to the thin structure and constraint of internal convective cooling arrangements. To overcome these difficulties, pressure-side cutback, which is strengthened with multiple internal structures and land extensions, is widely used in blades to protect trailing edge from high thermal stresses. Due to the geometrical complexity and strong interactions between coolant flow and mainstream, sophisticated heat transfer and flow patterns exist in the cutback region, which presents a great challenge for the trailing edge cutback design. To understand the heat transfer and aerodynamic performance in blade with trailing edge cutback, CFD method has become an efficient tool which provides deep insights into the flow mechanisms and heat transfer characteristics in the detailed region. To accurately resolve the flow and heat transfer performance in a turbine blade with trailing edge cutback, structured grids are preferred because of higher resolution in flow/heat transfer prediction than unstructured grids, especially in boundary layers. However, for a blade with landed trailing edge cutback, few researchers tried to employ structured grids to predict aero-thermal performance due to the geometrical complexity. In this paper, the Background-Grid Based Mapping (BGBM) method proposed in Part I of this study was adopted to generate multi-block structured grids for a gas turbine vane with landed trailing edge cutback. With the coordinate transformation strategies, multi-block structured grids for the vane with landed trailing edge cutback were generated conveniently. With the generated structured grids, flow and heat transfer performance in vane were investigated using RANS (Reynolds-Averaged Navier-Stokes) equations solutions combined with transitional turbulence model. Effects of land extensions on the heat transfer and aerodynamic performance were analyzed, as well as the effects of inflow turbulence intensity, mainstream Reynolds number and ejection rate. The results show that heat transfer coefficients on vane surface, total pressure loss coefficient and energy loss coefficient in vane are all increased with the increase of inflow turbulence intensity. However, heat transfer coefficients on cutback and trailing edge surface are not sensitive to inflow turbulence intensity. At the same inflow turbulence intensity, the aerodynamic loss in vane is decreased with increasing the Reynolds number of mainstream. The increase of ejection rate significantly increases the heat transfer coefficients on cutback surface. Compared with the vane without land extensions, heat transfer coefficients and pressure coefficients on vane surface are reduced and the heat transfer coefficients on cutback surface are increased for the vane with land extensions. In the case of Re = 2.0 × 106, the area-averaged heat transfer coefficient on landed cutback is 14.46% higher than the cutback without lands. Compared with the experimental data, predictions with structured grids based on BGBM method are more agreeable than those with unstructured grids.

Performance of Overset Mesh in Modeling the Wake of Sharp-Edge Bodies

The wake dynamics of sharp-edge rigid panels is examined using Overset Grid Assembly (OGA) utilized in OpenFOAM, an open-source platform. The OGA method is an efficient solution technique based on overlap of a single or multiple moving grids on a stationary background grid. Five test cases for a stationary panel at different angle of attack are compared with available computational data, which show a good agreement in predicting global flow variables, such as mean drag. The models also provided accurate results in predicting the main flow features and structures. The flow past a pitching square panel is also investigated at two Reynolds numbers. The study of surface pressure distribution and shear forces acting on the panel suggests that a higher streamwise pressure gradient exists for the high Reynolds number case, which leads to an increase in lift, whereas the highly viscous effects at low Reynolds number lead to an increased drag production. The wake visualizations for the stationary and pitching motion cases show that the vortex shedding and wake characteristics are captured accurately using the OGA method.