Effects of turbulence boundary conditions on Spalart-Allmaras RANS simulations for active flow control applications

We assess the suitability of Reynolds-Averaged Navier-Stokes (RANS) simulation using the Spalart-Allmaras (SA) turbulence model as a closure in analysing the performance of fluidic Active Flow Control (AFC) applications. In particular, we focus on the optimal set of actuation parameters found by Tousi et al. (Appl Math Model, 2021) and Tousi et al. (Aerospace Sci Technol 127:107679, 2022) for a SD7003 airfoil at a Reynolds number Re=6x10(4 )and post-stall angle of attack alpha=14(degrees) fitted with a Synthetic Jet Actuator (SJA). The Large Eddy Simulation (LES) presented in that work is taken as the reference to identify the best choice of boundary conditions for the turbulence field nu at both domain inlet and jet orifice in two-dimensional SA-RANS computations. Although SA-RANS is far less accurate than LES, our findings show that it can still predict macroscopic aggregates such as lift and drag coefficients quite satisfactorily and at a much lower computational cost, provided that turbulence levels of the actuator jet are set to a realistic value. An adequate value of nu is instrumental in capturing the correct flow behaviour of the reattached boundary layers for close-to-optimal actuated cases. This validates the use of RANS-SA as a reliable and cost-effective simulation method for the preliminary optimisation of SJA parameters in AFC applications, provided that thorough sensitivity analysis on turbulence-related boundary conditions is performed. Given the strong sensitivity of flow detachment on the laminar or turbulent nature of boundary layers, our results suggest that such analyses are particularly indispensable for vastly separated flow scenarios in general, notably for bluff bodies at moderate transcritical Reynolds numbers.