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Master Thesis Report Verification of 2D Fitting Results interactively as in CFX, the quality of the mesh was inspected visually based on its aspect ratio, skew- ness and smoothness. The aspect ratio is a ratio of the longest length to the shortest mesh length. The skewness is a measure of the angle between the associated sides of the mesh. For the case of a triangular mesh, a 60◦ angle between the sides is optimal. Finally, the smoothness is a measure of the transition in cell size of the mesh domain. A large change in cell size across different mesh represents a bad smoothness. The mesh topology is dependent on parameters such as the y-plus (y+), total thickness of the boundary layer and the number of layers, local edge length as well as the fineness. A description of these meshing parameter and their associated values will be described below. y-plus (y+) The y-plus is known as the non dimensional wall distance. It is defined as the physical distance from the wall to the first layer of the boundary layer. A smaller value of the y+ implies a closer node distance between the first boundary layer and the wall. This leads to a finer mesh and hence a better ability to capture the flow characteristic accurately. Mathematically, the equation can be expressed as follows: y+ = y√ρτw (5.19) μ Where μ is the dynamic viscosity of the air, τw is the wall shear stress and ρ is the density of the air. For the various blade profiles tested, it is considered good practice to keep the y+ value less than 5 for all cases, especially in regions with important boundary layer phenomena such as the separation zone. In UMG 2, this is basically controlled by a parameter which defines the thickness of the first boundary layer (BL). Overall thickness of BL and number of layers The overall thickness of the boundary layer influences how accurately viscous effect due to shear stress is being modelled. It has an influence on the overall prediction of momentum deficit. The number of layer affects the discretization within the boundary layer and its ability to accurately predict the flow characteristics within the wall region. Through a series of extensive testing, it was found that this parameter has a large influence on the blade loading. An optimum number of 28 layers along with a thickness of 0.007 was used for the various blade profile. This was decided based on the skewness as well as the aspect ratio of the mesh within the boundary layer. Fineness and Local edge control The fineness parameter describes the growth rate of the mesh. Within UMG 2, the mesh within the boundary layer is structured while the mesh outside of the boundary layer is unstructured. The fineness of both set of meshes are controlled within the spacing control file of UMG2. Figure 5.4: Illustration of Mesh Zone 44PDF Image | Aerodynamic Design of the NASA Rotor 67 for Non Uniform Inflow
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