Vortex dynamics, non-normality and small-scale statistics in a separated flow around a finite wing
File(s)
Author(s)
Bilbao-Ludena, Juan Carlos
Type
Thesis
Abstract
This thesis focuses on the analysis of vorticity and small-scale dynamics in a three-dimensional separated shear flow. The effect of large scale (time-average) motion as well as the non-normality of the small-scale statistics are examined. Direct numerical simulations (DNS) were employed to investigate the transitional flow around a finite NACA 0018 wing at a $10^\circ$ angle of attack. We first analyse how each of the identified large-scale streamwise vortices forms and sustains or loses its strength in some of the most inhomogeneous regions around the tip and the near wake. This region is highly convoluted, strongly three-dimensional, multi-scale and far from being self-similar. We show how the main wingtip vortex emerges from the amalgamation and interaction of the different vortices in the near wake. The wall-normal term $-2\langle u_{2}u_{2}\rangle \frac{\partial\langle U_2\rangle}{\partial x_{2}}$ was responsible for maintaining $\langle u_{2}u_{2}\rangle$ primarily in the spiral wakes in the near field. Subsequently, we study the small-scale dynamics along the transitional region and across the recirculating bubble under the influence of the mean strain-rate field. The non-normality of the Velocity Gradient Tensor (VGT) is analysed using the Schur decomposition which separates the local, eigenvalue-based dynamics from non-local effects. We uncover new features along the transitional flow and across the recirculation zone and departures from Homogeneous Isotropic Turbulence (HIT) are quantified. We extend the Schur decomposition to mean fields for the first time. Finally, we study the Lamb vector which is the source of non-linearities in the Navier-Stokes equations (NSE). The effect of the potential component of the Lamb vector is quantified. As the flow transitions, the presence of the potential part decreases but stagnates rapidly after the flow becomes fully turbulent. This suggests that the potential part becomes less crucial as the flow is less dependent on the mean shear.
Version
Open Access
Date Issued
2023-11-06
Date Awarded
2024-03-01
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Papadakis, George
Sponsor
PRONABEC Peru (Firm)
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)