Unsteady Flow and Force Development in the case of a Circular Cylinder
type: CreativeWork
description: The time varying nature of many real flows has a strong effect on the resulting force experienced by aerodynamic bodies, where the transient force response can readily exceed the steady-state equivalent. This therefore poses a significant threat to small drones as well as larger aircraft that can be subjected to highly unsteady flow fields. Sensing the flow and using predictive modelling to mitigate the unsteady forces shows potential, yet requires a detailed knowledge of the aerodynamic principles at play. This work is a fundamental study into the underlying mechanisms involved in low Reynolds number unsteady aerodynamics to help facilitate future low order models (LOMs). Specifically, the focus is on the development of the unsteady force, by exploring the origin and evolution of boundary layer vorticity as well as the impact of free vorticity located in the flow. Four sets of experiments are conducted in the towing tank facilities at the University of Cambridge using a rotating and translating circular cylinder as well as a flat plate. To capture the fluid dynamic response, force balance and planar particle image velocimetry (PIV) measurements are acquired in combination, at Reynolds numbers between 4000 and 20000. It is found that whilst the potential flow `added mass' vortex sheet distribution around a stationary object immersed in an accelerating freestream is correct in shape, it ascribes the vortex sheet to the wrong origin. Instead, the vortex sheet is found to develop as a result of external vorticity that is created at the interface between the moving freestream and the quiescent surrounding. Moreover, the evolution of the boundary layer vortex sheet is investigated around a translating and rotating cylinder. The vortex sheet contributions due to kinematics and free vorticity are experimentally recovered. It is further proposed that the vortex sheet contribution due to free vorticity can be decomposed into a local and far-field component. Examining the vor…
abstract: The time varying nature of many real flows has a strong effect on the resulting force experienced by aerodynamic bodies, where the transient force response can readily exceed the steady-state equivalent. This therefore poses a significant threat to small drones as well as larger aircraft that can be subjected to highly unsteady flow fields. Sensing the flow and using predictive modelling to mitigate the unsteady forces shows potential, yet requires a detailed knowledge of the aerodynamic principles at play. This work is a fundamental study into the underlying mechanisms involved in low Reynolds number unsteady aerodynamics to help facilitate future low order models (LOMs). Specifically, the focus is on the development of the unsteady force, by exploring the origin and evolution of boundary layer vorticity as well as the impact of free vorticity located in the flow. Four sets of experiments are conducted in the towing tank facilities at the University of Cambridge using a rotating and translating circular cylinder as well as a flat plate. To capture the fluid dynamic response, force balance and planar particle image velocimetry (PIV) measurements are acquired in combination, at Reynolds numbers between 4000 and 20000. It is found that whilst the potential flow `added mass' vortex sheet distribution around a stationary object immersed in an accelerating freestream is correct in shape, it ascribes the vortex sheet to the wrong origin. Instead, the vortex sheet is found to develop as a result of external vorticity that is created at the interface between the moving freestream and the quiescent surrounding. Moreover, the evolution of the boundary layer vortex sheet is investigated around a translating and rotating cylinder. The vortex sheet contributions due to kinematics and free vorticity are experimentally recovered. It is further proposed that the vortex sheet contribution due to free vorticity can be decomposed into a local and far-field component. Examining the vor…
author: Gehlert, Pascal
language: eng
keywords: Fluid Dynamics, Vortex Sheets, Unsteady Forces, Unsteady Flow, Added Mass, Unsteady Separation
license: https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e9e81a5c-86c0-47ca-8ef5-4963205e6ba0/download
license: https://www.rioxx.net/licenses/all-rights-reserved/
date published: 2021-07-15
date modified: 2026-07-22T22:24:27Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.77949