Development of a Stability-Based Transition Transport Modeling Framework

Abstract

The physical process of a laminar boundary layer becoming turbulent is called “laminar­turbulent transition”. Since the flow physics and in particular the skin friction responsible for the drag is significantly influenced by the state of the flow, that is laminar or turbulent, transition plays an important role in external aerodynamics. For example, for rotorcraft computations it was shown that the skin friction was significantly overpredicted (depending on the geometry and the flow state) applying a fully turbulent simulation compared to a simulation that considered laminar­turbulent transition [1]. An overestimation of skin friction leads to an overestimation of power demand which needs to be avoided. However, on complex industrially relevant configurations, like a helicopter rotor, incorporating transition prediction capabilities into the simulation is not straightforward. State­of­the­art streamline­based methods require a great implementation effort, massive user input and a detailed expert knowledge. Additionally, computing helicopter rotors is inherently computationally expensive, requiring unsteady computations on a fine grid (Fig. 1), with a long computation time to establish the flow. The addition of boundary layer transition to the computation requires even finer time­steps and additional effort to compute the transition position. While streamline­based methods are relatively effective, they are not easily parallelizable to a large number of cores. Therefore, in the last decade transition transport models have been developed that aim at simplifying the incorporation and parallelization of the laminar­turbulent transition significantly. These models are only based on information available at each grid point, using transport type partial differential equations.