Numerical and Experimental Study of Bluff Body Aerodynamics

This thesis investigates the dynamics of detached flows around bluff bodies and the resulting aerodynamic loads, which are of primary importance in wind engineering. Two canonical geometries are considered: a 4:1 rectangular cylinder and a flat plate that is either fixed at large incidence or undergoing a large pitching motion. The different studies are conducted through both experimental measurements and numerical simulations. Their results are compared by using Proper Orthogonal Decomposition and Dynamic Mode Decomposition. The flow around the 4:1 rectangular cylinder is studied through dynamic pressure measurements along a cross-section combined with Unsteady Reynolds-Averaged Navier-Stokes (urans) and Delayed-Detached Eddy Simulation (ddes) results. The main objectives are two-fold: i) to improve the general knowledge about the spatio-temporal flow features, the resulting aerodynamic loads, and their variations with the incidence angle and the freestream velocity, and ii) to assess the capabilities of urans and ddes approaches to provide a sufficiently accurate estimation of the flow and aerodynamic loads at several incidences. It is shown that the rectangular cylinder involves complex separation-reattachment phenomena that are highly sensitive to the Reynolds number (Re). Consequently, the mean lift slope clα increases rapidly in the investigated range 3. 1ˆ10 ď Re ď 7. 6ˆ10. Additionally, it is shown that both urans and ddes simulations fail to accurately approximate the flow at the different incidence angles considered. Only the urans approach is able to qualitatively estimate the spatio-temporal variations of vortices for incidences below the stall angle α “ 4 ̋. Nonetheless, this stall angle is only captured by ddes. The flows around a flat plate with different configurations are selected as test cases to assess an indirect load measurement technique that uses Particle Image Velocimetry (piv) velocity fields. This technique is well suited to analyze phenomena involving moving bodies where direct load measurements could be contaminated by the structural response. The capabilities of two formulations based on the momentum balance are tested. The first method uses the integral form of the Navier-Stokes equation (inse), and the second is based on the flux equation derived by Noca et al. (1999) (noca). This second method is extended to the indirect estimation of aerodynamic moments that is not provided by the original formulation. The user-defined parameters required by the inse and nocamethods are studied to evaluate their effects on the indirect estimation. Additionally, numerical results are used to better understand the limitations of the indirect approaches. It is shown that the inse and the nocamethods perform similarly for the detached flows considered. They are able to accurately estimate the mean loads, and the phase-averaged time responses are also estimated with a reasonable accuracy as long as the corresponding piv phase-averaged fields are sufficiently converged. The unsteady results are generally negatively impacted by threedimensional effects and the lack of a clear single shedding frequency. The user-defined parameters that have the most significant effects on the accuracy of the indirect load estimations are identified and guidelines for their setting are proposed. Overall, this thesis demonstrates the added-value of integrating numerical and experimental studies, especially in the context of wind engineering where the aerodynamics of bluff civil engineering structures is very challenging to study numerically. The two approaches are complementary and enable a deeper understanding of the physics.