Transition Prescription and Prediction

The modeling of laminar-turbulent transition in RANS flow solvers is an inevitable requirement for the correct numerical simulation of physical phenomena in industrially relevant flow problems, especially those concerned with aerodynamics in the aerospace industry. In many cases it is not possible to obtain quantitatively sufficient results, e.g. the drag coefficient of aircraft, or even qualitatively correct results such as suction peaks at the nose of a highly inclined airfoil (take-off/landing condition) or the trailing edge separation induced by airfoil oscillations, when laminar-turbulent transition is not taken into account. For industrial applications a RANS solver must provide the possibility of a general transition prescription method, which is moreover a necessary condition for the coupling of the flow solver with a method for transition prediction. The transition prescription method must be able to be applied to 2-dimensional and 3-dimensional geometric configurations consisting of several components, each having an individual set of transition points, e.g. one or more transition lines on each part of a 3-d body. Moreover it must be able to treat general transition lines in space on general surfaces such as highly curved transition lines on highly curved surfaces. The transition prescription method should be independent from the topology of the computational grid and the basic algorithms should work on structured as well as on unstructured grids. As by a "pure" transition prescription method the transition locations are imposed and thus a RANS flow solver would always be dependent on experimental knowledge about the transition on locations, transition prediction is the next step. Numerous transition prediction methods are available, ranging in cost and complexity from "simple" empirical methods to "complex" non-linear stability approaches. For the sake of developing a basic algorithmic infrastructure, the former can be used to quickly and efficiently complete the first step, the coupling of a RANS flow solver and an empirical transition criterion, which build a program system that is able to autonomously and automatically handle transitional flows. Further, the empirical transition methods can be used to provide qualitative insights into the benefits of considering natural transition for aircraft design and optimization. This report has been made during the European project AVTAC - Advanced Viscous Flow Simulation Tools for Complete Aircraft Design. Within the project, the report was published as deliverable AVTAC/DEL/DLR/D3.2C5.