A series of experimental investigations into different manifestations of the vortex ring state

Pickles, David Jonathan (2021) A series of experimental investigations into different manifestations of the vortex ring state. PhD thesis, University of Glasgow.

Abstract

The fluid dynamics of a helicopter rotor present many challenging phenomena. The flow is dominated by a system of intertwined helical vortices that trail from the rotor blades, which persist and remain in the vicinity of the rotor for many rotor revolutions. Of interest here is the vortex ring state (VRS), which is typically associated with the descent of a rotor into its own wake. In the vortex ring state the trailed vortex system collapses from its usual helical structure to form a toroidal vortex ring of the same scale as the rotor diameter. The vortex ring is highly unsteady, and sheds off and reforms, leading to large unpredictable thrust fluctuations. It is currently believed that the mutual-inductance instability of helical vortices is responsible for the breakdown of the rotor wake into the toroidal form associated with the vortex ring state. Subsequently, computational investigations have revolved around modelling the trailed vortex system for extended periods of time. Experimental investigations have also typically been configured to generate a trailed vortex wake. The aim of this work is to explore the role blade tip vortices have on the breakdown of the helical rotor wake into the unsteady toroidal form associated with the vortex ring state. In order to achieve this, a series of experimental investigations into the structure of the flow field produced by a shrouded rotor, a unshrouded rotor, a ventilated open core annular jet and a rotor with a large root cut out were performed. Each of these experimental configurations produced mean velocity profiles that were notionally similar. With a ring of high velocity flow surrounding a low velocity core. However, the structure of the wake produced by each configuration differed considerably. Unlike the wake produced by an unshrouded rotor, the wake produced by the shrouded rotor and the ventilated open core annular jet did not contain a helical vortex system. Despite this, in axial descent, each of the experimental configurations investigated entered states analogous to the vortex ring state of an isolated rotor. The results presented in this thesis show that the mechanism by which the wake produced by the shrouded rotor and the ventilated open core annular jet broke down into the toroidal form associated with the VRS of an unshrouded rotor were similar. Indicating that the breakdown of the rotor wake, into the toroidal form of the VRS is a result of the interaction between an axially induced flow, which consists of a low velocity core surrounded by a ring of higher velocity air, and a uniform counterflow.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: vortex ring state, shrouded rotor, ventilated open core annular jet.
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering > Autonomous Systems and Connectivity
• Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
• Supervisor's Name: Green, Dr. Richard and Busse, Dr. Angela
• Date of Award: 2021
• Depositing User: Mr David Pickles
• Unique ID: glathesis:2021-82158
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 05 May 2021 08:55
• Last Modified: 05 May 2021 09:01
• Thesis DOI: 10.5525/gla.thesis.82158
• URI: https://theses.gla.ac.uk/id/eprint/82158