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Force Measurements On Rigid And Flexible Oscillating Foils

Electronic Theses of Indian Institute of Science

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Title Force Measurements On Rigid And Flexible Oscillating Foils
 
Creator Jimreeves, M
 
Subject Vibration
Foils - Oscillation
Rigid Foils
Flexible Foils
Oscillating Foils
Foils - Force Measurements
Mechanical Engineering
 
Description In the present work, we experimentally study thrust generation from sinusoidally pitched rigid and flexible foils immersed in a uniform flow. The flexible foils are made by attaching a flexible flap of known flexural rigidity and flap length to the trailing edge of a rigid foil. For such thrust generating systems, a propulsive efficiency (η) may be defined as the ratio of the useful work done to the input energy requirement. In the present experiments, the propulsive efficiency (η) of the flapping foil can be determined from direct measurement of the unsteady forces and torque on the foil. The effects of systematic variation of the flexural rigidity of the foil, from highly flexible to rigid, on the thrust and efficiency characteristics of the foil are investigated. Studying such oscillating foils helps one to understand and mimic the efficient thrust generating mechanism in fishes and other creatures that use flapping to locomote themselves.
A strain guage based loadcell is used to measure the forces normal to the foil (N) and forces along the chord of the foil (A). With a potentiometer, the instantaneous angular position (θ) is also measured, so that instantaneous lift (L) and thrust (T ) can be calculated. The measured moment (M) is used to calculate the instantaneous power input (P = Mθ˙). The foil is immersed in a uniform flow (u) set in a water tunnel, and the sinusoidal pitching (θ = θmaxsinωt) is provided by a servo motor. The Reynolds number (Re = uc/ν) in the present study is in the range of 103 to 104 .
For the case of the rigid foil, the thrust and efficiency characteristics are presented for variation of the non-dimensional flapping frequency called the ‘reduced frequency’ (k = πfc/u), which is varied in the range of 1 to 10. At small reduced frequency (k < 3), the foil experiences a mean drag, while at k > 3, the foil experiences a mean thrust that grows rapidly as the reduced frequency (k) is increased. The thrust characteristics of the rigid foil are decided mainly by the normal force’s phase with respect to θ (φCN ) and its magnitude ([CN ]), as the chord-wise force is very small compared to the normal force (A
 
Contributor Govardhan, R N
Arakeri, J H
 
Date 2013-10-04T09:59:47Z
2013-10-04T09:59:47Z
2013-10-04
2010-10
 
Type Thesis
 
Identifier http://etd.iisc.ernet.in/handle/2005/2259
http://etd.ncsi.iisc.ernet.in/abstracts/2880/G24682-Abs.pdf
 
Language en_US
 
Relation G24682