Turbulent Fluxes and Formation Region Closure in the Wake of a 2D Cylinder

Abstract

Bluff bodies, such as cylinders, cause flow separation and turbulent wake formation when fluid flows around them. The shape and size of the recirculation (or vortex formation) region in the near wake is responsible for the pressure exerted at the base of the cylinder, which leads to increased drag and complex fluid dynamics. These characteristics are critical in engineering applications like aerodynamics and hydrodynamics. This thesis focuses on the turbulent wake behind a two-dimensional blunt body with a chord-to-thickness ratio of 10, with particular attention to the recirculation region and its evolution under varying flow conditions. Reynolds numbers spanning from Re(D) = 7000 to 40000 were considered, where D is the diameter of the blunt trailing edge. A previous experimental database obtained from a Time-Resolved Particle Image Velocimetry (TR-PIV) technique provided the flow data for this study. Building on this, Computational Fluid Dynamics (CFD) simulations were performed using ANSYS Fluent with the SST k-ω turbulence model. The experimental data was used solely to calibrate and validate these simulations. The simulations were then used to reconstruct the two-dimensional mean wake and quantify the wake mass fluxes using the mass entrainment model. The main findings demonstrate that the recirculation region length is primarily governed by the balance of fluid fluxes entering and leaving this region, as proposed by Gerrard in 1966. As the Reynolds number increases, the recirculation bubble length initially decreases until it saturates. The fluxes within the wake remain balanced all the time. Additionally, it was shown that a larger recirculation region behind a bluff body corresponds to a lower exchange rate of fluid between the wake and the surrounding free-stream flow. As Reynolds number increases, the entrainment by the shear layer increases, and the recirculation bubble shrinks in size as the shear layer transitions from a laminar to a turbulent state, causing enhanced entrainment. However, the rate of increase in entrainment becomes more gradual at higher Reynolds numbers.

Date of Award	27 Aug 2024
Original language	American English
Supervisor	Vladimir Parezanovic (Supervisor)