Davide, Gatti (2015)
Turbulent Skin-Friction Drag Reduction via Spanwise Wall Oscillations.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
Abstract
The present work aims at contributing to the development of active flow control via spanwise wall oscillations for turbulent skin-friction drag reduction with numerical and experimental tools, both improving our understanding of their mechanisms and to easing laboratory implementation with a novel actuator principle: the dielectric elastomer actuators (DEA). Dielectric elastomer actuators, capable of large in-plane wall motions, are developed and thoroughly characterized for suitability in flow-control applications. The influence of a major design parameter, the amount of pre-stretch, on static and dynamic actuation is extensively discussed and an optimal combination is found, which yields the best trade-off between actuation amplitude and frequency. DEA-based spanwise oscillating surfaces are tested in a laboratory, low-Reynolds number (Re), fully developed turbulent channel flow, where they yield an actively-induced relative decrease in skin-friction of about 5%. Measurements of the real power required to impose the spanwise oscillations are performed and enable to evaluate the control performance in terms of energetic efficiency. A negative yet promising net power saving, budget between the energy saving and expenditure, is achieved which is several order of magnitude larger than previous experiments with conventional actuator technologies. Direct numerical simulations (DNS) of fully developed turbulent channel flows are performed to study the effects of inhomogeneous actuation on control performance. Spatial onset transients are investigated as source of the commonly-observed discrepancies between laboratory and numerical experiments, by directly comparing the experimental investigations to their numerical idealized representation. The rate at which the drag reduction deteriorates at increasing Re is investigated by exploring a new DNS dataset of turbulent channel flows controlled through an alternative spanwise wall forcing method: the streamwise-traveling waves of spanwise wall velocity. A mathematical relationship between the Reynolds number and the drag reduction is found to predict the dominant part of this decay, which is not specific to the particular control technique but rather inherent in the definition of drag reduction rate.
Item Type: | Ph.D. Thesis | ||||
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Erschienen: | 2015 | ||||
Creators: | Davide, Gatti | ||||
Type of entry: | Primary publication | ||||
Title: | Turbulent Skin-Friction Drag Reduction via Spanwise Wall Oscillations | ||||
Language: | English | ||||
Referees: | Tropea, Prof. Cameon ; Frohnapfel, Prof. Bettina ; Schlaak, Prof. Helmut | ||||
Date: | 8 September 2015 | ||||
Place of Publication: | Darmstadt | ||||
Refereed: | 17 December 2014 | ||||
URL / URN: | http://tuprints.ulb.tu-darmstadt.de/4963 | ||||
Abstract: | The present work aims at contributing to the development of active flow control via spanwise wall oscillations for turbulent skin-friction drag reduction with numerical and experimental tools, both improving our understanding of their mechanisms and to easing laboratory implementation with a novel actuator principle: the dielectric elastomer actuators (DEA). Dielectric elastomer actuators, capable of large in-plane wall motions, are developed and thoroughly characterized for suitability in flow-control applications. The influence of a major design parameter, the amount of pre-stretch, on static and dynamic actuation is extensively discussed and an optimal combination is found, which yields the best trade-off between actuation amplitude and frequency. DEA-based spanwise oscillating surfaces are tested in a laboratory, low-Reynolds number (Re), fully developed turbulent channel flow, where they yield an actively-induced relative decrease in skin-friction of about 5%. Measurements of the real power required to impose the spanwise oscillations are performed and enable to evaluate the control performance in terms of energetic efficiency. A negative yet promising net power saving, budget between the energy saving and expenditure, is achieved which is several order of magnitude larger than previous experiments with conventional actuator technologies. Direct numerical simulations (DNS) of fully developed turbulent channel flows are performed to study the effects of inhomogeneous actuation on control performance. Spatial onset transients are investigated as source of the commonly-observed discrepancies between laboratory and numerical experiments, by directly comparing the experimental investigations to their numerical idealized representation. The rate at which the drag reduction deteriorates at increasing Re is investigated by exploring a new DNS dataset of turbulent channel flows controlled through an alternative spanwise wall forcing method: the streamwise-traveling waves of spanwise wall velocity. A mathematical relationship between the Reynolds number and the drag reduction is found to predict the dominant part of this decay, which is not specific to the particular control technique but rather inherent in the definition of drag reduction rate. |
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URN: | urn:nbn:de:tuda-tuprints-49635 | ||||
Classification DDC: | 600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften und Maschinenbau | ||||
Divisions: | 16 Department of Mechanical Engineering > Fluid Mechanics and Aerodynamics (SLA) 18 Department of Electrical Engineering and Information Technology > Microtechnology and Electromechanical Systems 18 Department of Electrical Engineering and Information Technology > Institute for Electromechanical Design (dissolved 18.12.2018) 16 Department of Mechanical Engineering 18 Department of Electrical Engineering and Information Technology |
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Date Deposited: | 27 Sep 2015 19:55 | ||||
Last Modified: | 27 Sep 2015 19:55 | ||||
PPN: | |||||
Referees: | Tropea, Prof. Cameon ; Frohnapfel, Prof. Bettina ; Schlaak, Prof. Helmut | ||||
Refereed / Verteidigung / mdl. Prüfung: | 17 December 2014 | ||||
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