Modeling Of Turbulent Separated Flows For Aerodynamic Applications PDF Download

"Employing the advection-diffusion-reaction equation as a model problem, a multiscale stable finite element formulation is presented for the Spalart-Allmaras turbulence model, and for the Zonal Detached-Eddy Simulation (ZDES) with a vorticity-based subgrid length scale. The multiscale method arises from a decomposition of the scalar field into coarse (resolved) and fine (unresolved) scales. Modeling of the unresolved scales corrects the lack of stability of the standard Galerkin formulation. The proposed method possesses superior properties like those of the Streamline Upwind/Petrov-Galerkin method and the Galerkin/Least-Squares method. The stabilization terms appear naturally and are effective for turbulent computations in which reaction-dominated effects strongly influence the boundary layer prediction.Validation of the method is accomplished via a two-dimensional problem in which skewed advection goes through a unit square, and a three-dimensional problem where turbulent unsteady flow passes over tandem cylinders. The boundary layer separation, free shear layer roll-up, vortex shedding from the upstream cylinder, and interaction with the downstream cylinder, are well reproduced. Good agreement with experimental measurements gives credence to the accuracy of ZDES in modeling turbulent separated flows.Following validation, two problems related to turbulent flows in aerospace and wind engineering applications are investigated. The first study is the evaluation of the performance degradation of ice-contaminated airfoils, including a NACA 23012 airfoil with a spanwise ice ridge, and a GLC-305 airfoil with a leading edge horn-shape glaze ice. Appropriate spanwise domain size and sufficient grid density are determined to enhance the reliability of the simulations. A comparison of lift coefficient and flow field variables demonstrates the added advantage of the ZDES model for the standard Spalart-Allmaras turbulence model, as illustrated by massively separated flows at pre-stall conditions of an icing airfoil. The vorticity-based subgrid length scale in ZDES prevents the delayed development of instabilities in the shear layer and a subsequent late transition to a fully turbulent flow. Spectral analysis and instantaneous visualization of turbulent structures are also highlighted.Another application of interest is wind-induced vibrations of tall buildings. In this numerical procedure, the natural unsteady wind in the atmospheric boundary layer is modeled with artificial inflow turbulence generation. The turbulent flow is simulated by the stabilized ZDES model, and a conservative quadrature-projection scheme is adopted to transfer unsteady loads from fluid to structural nodes. The aerodynamic damping that represents the fluid-structure interaction mechanism is determined by empirical functions extracted from wind tunnel experiments. Eventually, the unsteady motion of the building is solved via a structural modal analysis. The flow solutions and the structural responses in terms of mean and root mean squared quantities are compared with experimental measurements, over a wide range of reduced velocities. The significance of turbulent inflow conditions and aeroelastic effects is highlighted. This numerical procedure provides predictions of good accuracy and can be considered a preliminary design tool to evaluate unsteady wind effects on tall buildings." --