This approach offers immense flexibility in generating different grids in each block, particularly the increased mesh densities in blocks 2 and 5 capturing the developing boundary layer along the chord line between the leading and trailing edges and the expanding shock layers along the airfoil. The entire fluid region is subdivided into six contiguous blocks. One approach that can be suitably considered, especially for RANS simulations, is the block-structured or multi-block mesh (see Chapter 6) shown in Figure 7.54. Two different grid topologies that could be specifically used to solve the problem are illustrated in Figures 7.54 and 7.55. Here, the complete three-dimensional Navier–Stokes equations are solved to accommodate the spectrum of varying length scales that exists within the complicated fluid phenomena along the streamwise ( x), spanwise ( y), and vertical ( z) directions.ĬFD simulation: One challenging aspect of the CFD example of fluid flowing over an airfoil is the generation of appropriate meshes surrounding the geometry. At flows that are just below the speed of sound ( Ma = 0.2 is considered for the present problem), currently available computational hardware permits the use of DNS techniques to study the onset of flow instability and subsequent transition to turbulence subject to different inlet conditions. Numerical calculations are thus carried out in a two-dimensional fluid domain, and the calculations also include parametric investigations of the influence of different angles of attack on the expanding shock layers as the fluid travels past the airfoil. Because only time-averaged results are of primary interest, especially in RANS simulation, and since the length b is infinite, such conditions or assumptions mean that the flow is truly two-dimensional there is negligible spanwise variation of flow patterns and forces for a constant-chord airfoil. The Reynolds-averaged Navier–Stokes (RANS) equations are solved along with the shear stress transport (SST) turbulence model for supersonic flow ( Ma = 2.5) over the wing geometry. The CFD example in this section considers the subsonic and supersonic flows past an infinitely long airfoil. Īngle of attack ( incidence), α-the angle between the direction of the relative motion and the chord line.Span, b-the length of the airfoil in the direction perpendicular to the cross-section of the wing Ĭhord, c-the length of chord line between the leading and trailing edges.Ĭhord line-a straight line linking the centers of curvature of the leading and trailing edges.Trailing edge-the rear, or downstream, edge Leading edge-the front, or upstream, edge, facing the direction of flow There are a number of accepted terms related to an airfoil familiarization with them is necessary in order to understand the discussion of the flow past such geometry.įigure 7.53. This particular geometry has been specifically chosen because of the numerous aerodynamic investigative studies that have been carried out in research and design practices. Problem considered: In order to illustrate CFD application to high-speed flows, the fluid flowing past an NACA0012 airfoil is considered herein. Nevertheless, what is necessary to move the craft forward is the drag that absorbs the engine power. For an aircraft to remain in the airspace, the creation of lift on the wing surface is of paramount importance. As a measure of an airfoil's usefulness, for example, as a wing section of an aircraft, the ratio of lift to drag must be sufficiently large that it is capable of producing high lift at a small penalty of drag. The primary purpose for the construction of a streamlined airfoil is to minimize the drag imposed on the body. It will also experience the counteracting influence of drag while placed in a fluid stream. Chaoqun Liu, in Computational Fluid Dynamics (Second Edition), 2013 7.4.5.2 Subsonic and Supersonic Flows over a WingĪn airfoil can be defined as a streamlined body designed specifically to produce lift.
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