The turbulent axisymmetric bluff body wake: analysis and control through experiment and simulation
Creators: name: Zhu, Taihang
Publisher: name: Imperial College London
Year: 2023

description: The three-dimensional turbulent bluff body wake is a complex phenomenon occurring near the back of bluff bodies, such as road vehicles. In this thesis, with a combination of numerical simulation and wind tunnel experiments, the three-dimensional turbulent wake generated by an axisymmetric bluff body with a blunt trailing edge is studied at $ Re_D=1.88 \times 10^5 $. Synchronized, time-resolved PIV and base pressure measurements and 3D compressible LES are used to study the coherent structures of the unforced wake. The POD analysis confirms the persistence of the laminar unstable eigenmodes at high-Reynolds numbers (here, $ Re_D=1.88 \times 10^5 $). These correspond to a steady spatial-symmetry breaking mode and asymmetric unsteady vortex shedding modes in the near wake. Additionally, an unsteady, axisymmetric bubble pumping mode representing the streamwise pulsation of the wake is found. Conditional POD is used to extract the asymmetric modes in the wake of the axisymmetric body. The asymmetry of the wake, quantified using the centre of pressure (CoP), is correlated with the base pressure using conditional averaging, showing that in the most probable symmetry-breaking state, a low-pressure (high drag) region is found. Away from the symmetry-breaking state, there are two high-pressure (low drag) regions, with the wake asymmetric and the other one with the wake symmetric. A conditional average based on the base pressure shows that the pulsation of the wake causes the wake transition between high/low base pressure conditions. The effect of the high-frequency pulsed jet forcing on the turbulent wake for drag reduction is studied using both wind tunnel experiments and 3D LES. In the vicinity of the separation point, a high-frequency, high-amplitude pulsed jet forcing is used to control the wake. At the optimal forcing condition ($f=700$ Hz or $ St_D = 9.17 $, $ C_\mu=0.27 $), a base pressure recovery of about $ 88\% $ is achieved. The pressure gradients calculated from the velocity field using the 2D RANS equations show the relationship between the pressure and velocity field, and are used to analyse the mechanism underlying the base pressure recovery. Specifically, the analysis shows that the pressure near the base receives contributions from two mean velocity terms ($\displaystyle - U \frac{\partial V}{\partial x} $ and $ \displaystyle- V\frac{\partial V}{\partial y} $) and two stress gradient terms ($ \displaystyle- \frac{\partial \overline{uv}}{\partial x} $ and $\displaystyle - \frac{\partial \overline{v^2}}{\partial y} $). It shows that at the optimal forcing condition, the pulsed jet increases the base pressure in two ways. Firstly, it reshapes the wake and significantly increases the pressure via the mean velocity terms. Most of the pressure recovery comes from the first mean velocity term, i.e., $ \displaystyle- U\frac{\partial V}{\partial x} $. Secondly, it reduces the gradient of the Reynolds normal stress $\displaystyle \frac{\partial \overline{v^2}}{\partial x} $ and modifies the Reynolds shear stress $ \displaystyle \overline{uv} $, thus increasing the pressure also. Both the numerical and the experimental results show that the mean flow modification caused by the high-frequency forcing is associated with a reshaping of the separation streamline and a global narrowing of the wake. The pulsed jet generates a series of convected vortices, which produces a streamwise gradient of the vertical velocity $ \displaystyle \frac{\partial V}{\partial x} $ between two adjacent vortices due to their rotation. A vertical velocity near the separation point is generated by the rotation of the vortex there and, as a result, the separation streamline becomes concave. The 3D pulsed jet structure identified in the numerical study shows that the vortex concentrates near the separation point and in the form of a vortex ring advecting downstream. At the optimal forcing frequency, up to the blowing coefficient $ C_\mu=0.11 $, the pulsed jet globally suppresses all the azimuthal modes without any mode selection, while above this threshold, the pulsed jet amplifies the amplitude of the azimuthal modes ($ m = 0, \pm 1, \pm 2 $) near the vortex shedding frequency ($St_D \sim 0.2$). The SPOD result shows that the symmetry-breaking mode ($ St_D \to 0$) and the vortex shedding mode ($ St_D \sim 0.2$) in the unforced wake are found in the optimally forced wake. An important conclusion is that the aerodynamic reshape of the wake is a promising method for reducing drag, on the condition that in the vicinity of the base, the pressure contribution from the mean velocity terms ($ \displaystyle - U\frac{\partial V}{\partial x} $ and $ \displaystyle - V\frac{\partial V}{\partial y} $) can be increased. Additionally, pressure recovery can be achieved by modifying the Reynolds stress gradients ($ \displaystyle - \frac{\partial \overline{uv}}{\partial x}$ or $ \displaystyle - \frac{\partial \overline{v^2}}{\partial y} $).