Application Challenges for Transition Prediction Techniques in an Unstructured CFD Code

The prediction and control of the boundary layer laminar to turbulent transition is a process of major interest for mechanical engineering applications. For example, an effective boundary layer transition control during the flight of an aircraft can be of significant importance for improving its flight endurance and maneuverability. In turbomachinery also, the accurate transition control over compressor and turbine blades has the potential to provide major increase in the overall engine performance. For the boundary layer transition prediction, numerous studies are available, focusing on the accurate numerical modeling with the use of advanced and sophisticated turbulence models. For instance, Chen et al. [1], modeled the by-pass transition in boundary layers formed on turbomachinery blades with cubic non-linear eddy-viscosity models. Walters and Leyleck [2] derived three-equation turbulence models based on the laminar kinetic energy concept and predicted with a remarkable precision the by-pass transition on simplified turbine stator and compressor blade flows. Vlahostergios et al. [3] used a cubic non-linear turbulence model coupled with the laminar kinetic energy concept for the prediction of the boundary layer separation induced transition. In the literature, there are numerous techniques that investigate accurate and sophisticated ways of transition control, (Balakumar and Hall [4] among others). A new and promising technique is the use of plasma actuators for boundary layer and flow control. The function of plasma actuators is based on the induced ionic wind that acts as a jet within the boundary layer, adding momentum and thus modifying its characteristics. In the current work, the control of by-pass transition over a flat plate with sharp leading edge, under zero freestream pressure gradient, with a single Dielectric Barrier Discharge (DBD) plasma actuator is numerically investigated. Previous works have shown that the use of DBD plasma actuators is able to control efficiently the laminar to turbulent transition. For example Ustinov et.al [5] showed experimentally the boundary layer transition delay and the drag reduction due to this delay that can be achieved by the use of a DBD plasma actuator. The DBD actuator is modeled with the approach of Suzen et al. [6]. This model uses two additional transport equations that describe the electric field and the net charge density. These equations are coupled with the Reynolds Averaged Navier-Stokes (RANS) equations in the ANSYS FLUENT commercial computational fluid dynamics (CFD) software (ANSYS@ Scientific Research, Release 15) with the use of user defined functions (UDF). The three-equation eddy-viscosity model of Walters and Cokljat [7] is adopted. This model uses an additional Figure 1: The sharp leading edge ZPG flat plate experimental setup (from ERCOFTAC site) transport equation to model the laminar kinetic energy transport. For the assessment of the modeling approach, the skin friction coefficient and the shape factor of the ERCOFTAC T3A case is used. Additionally, the laminar kinetic energy distribution and the distribution of one of the production terms affecting the laminar and turbulent kinetic energies interaction are provided. The numerical results show that the DBD plasma actuator is capable to control and delay the boundary layer transition from the laminar to turbulent regime