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Numerical simulation of a flag behind a flat plate in a uniform flow

Sanchez MuÃƒÂ±oz, Jonathan

[Thesis]. Manchester, UK: The University of Manchester; 2019.

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Abstract

The dynamic response of a flexible flag in a uniform cross flow was studied using the Lattice Boltzmann Method combined with the Immersed Boundary Method. In the first instance the flag was alone, while subsequently it was attached to the aft of a rigid flat plate, positioned at various angles of incidence to the flow. Two different boundary conditions were used to describe the objects; the rigid flat plate was defined via the staircase approximation while the flexible flag was simulated using the immersed boundary method. Prior to the examination of the aforementioned cases a significant amount of code testing was undertaken towards the release of an in-houce code, LUMA (Lattice Boltzmann The University of Manchester) which has now been released via an academic journal publication in SoftwareX (2018). A mesh refinement study and a hardware performance analysis were completed in order to identify the most optimal use mode for this study. The flexible flag performance can be controlled through three main parameters Reynolds number, reduced velocity and mass ratio . The incidence angle of the flat plate is introduced as the new control parameter. The combination of these parameters modify the dynamical states of the flexible flag as well as downstream flow. By varying these parameters over the ranges (100<=Re<=400), (20 <=alpha<= 40), (5<= reduced velocity <= 30) and (0.05<= mass ratio<= 0.1); we have identified four dynamical states, of which only three were previously observed in the literature. I) A regular flapping state wherein the flexible flag displaces uniformly with very small amplitude, and does not modify its shape. II) A pseudo-steady state where the flexible flag remains trapped by vortex shedding of the flate plate. III) A regular high displacement flapping state, whereby the flexible flag displaces uniformly. IV) An irregular flapping state, where the flexible flag describes a highly unsteady displacement cycle of an aperiodic nature. The dynamical states were compared with a rigid flag to quantify drag reduction and comparisons were made to configurations with rigid body counterparts. In cases where the dynamic nature of the flexible filament appeared to lock-in to the frequency of the unsteady shedding was reduced,in some cases resulting in drag reduction.