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dc.contributor.advisorGovardhan, Raghuraman N
dc.contributor.authorDavid, Jimreeves M
dc.date.accessioned2021-09-17T11:34:56Z
dc.date.available2021-09-17T11:34:56Z
dc.date.submitted2018
dc.identifier.urihttps://etd.iisc.ac.in/handle/2005/5307
dc.description.abstractThe swimming motion of fishes have long fascinated scientists and engineers. Over the past few decades, researchers have studied physical models mimicking the tail beat of fishes in an effort to extract key physical principles behind the swimming of fishes with a view to develop more efficient underwater vehicles. In this thesis, we study two idealized models related to the propulsive force generated by the oscillating tail and the turning manoeuvre of a fish. We use experimental studies involving force measurements, velocity fi eld measurements and the recently developed tool called Lagrangian Coherent Structures (LCS) to understand the developing vortical flow and the forces acting on the unsteady physical models. In the first experimental study, we investigate thrust production from a pitching flexible foil in a uniform flow which can be seen as idealized model of a swimming fish. The flexible foils studied comprise of a rigid foil in the front (chord length cR) that is pitched sinusoidally at a frequency f, with a flexible flap of length cF and flexural rigidity EI attached to its trailing edge. We investigate thrust generation for a range of flexural rigidities (EI) and flap length to total chord ratio (cF =c), with; the former being varied over 4 orders in magnitude and the later varied from 0 to about 0.7. In each case, the mean thrust (CT ) and the efficiency of thrust generation ( ) are directly measured. We nd that using a non-dimensional exural rigidity parameter (R ) de fined as R = EI=(0:5 U2c3 F ) appears to combine the independent effects of variations in EI and cF =c at a given value of the reduced frequency (k = fc=U) for the range of cF =c values studied here. At k 6, the peak mean thrust coefficient is found to be about 100% higher than the rigid foil thrust, and occurs at R value of about 8, while the peak efficiency is found to be about 300% higher than the rigid foil efficiency, and occurs at a distinctly different R value of close to 0.01. Corresponding to these two optimal flexural rigidity parameter values, we nd two distinct flap mode shapes; the peak thrust corresponding to a mode 1 type simple bending of the flap, while the peak efficiency corresponds to a mode 2 type bending of the ap. The peak thrust condition is found to be close to the `resonance' condition for the first mode natural frequency of the flexible flap in still water. PIV measurements for the flexible cases show signi ficant differences in the strength and arrangement of the wake vortices in these two cases. In the second study, we present a combined experimental and numerical study of an idealized model of the propulsive stroke of the turning manouevre known as C-start in sh. Speci fically, we use the framework of Lagrangian Coherent Structures (LCS) to describe the kinematics of the flow that results from a thin-plate performing a large angle rotation about its tip in still fluid. Temporally and spatially well-resolved velocity fields are obtained using a two-dimensional, incompressible finite-volume solver. We then implement the recently proposed variational theory of LCS to extract the hyperbolic and elliptic LCSs in the numerically generated velocity fields. Detailed LCS analysis is performed for a plate motion pro le described by _(t) = max sin2(!t) during 0 t to and zero otherwise. The stopping time to is given by to = =! = 10s, the value of max chosen to give a stopping angle of max = 90 , resulting in a Reynolds number Re = c2 max= = 785:4, where c is the plate chord length and = 10􀀀6m2=s the kinematic viscosity of water. The flow comprises a starting and a stopping vortex, resulting in a pair of oppositely signed vortices of unequal strengths that move away from the plate in a direction closely aligned with the nal plate orientation at t=to 2. The hyperbolic LCSs are shown to encompass the fluid material that gets advected away from the plate for t > to, henceforth referred to as the advected bulk. The starting and stopping vortices, identi ed using elliptic LCSs and hence more objective than Eulerian vortex detection methods, constitute only around (2=3)rd of the advected bulk area. The advected bulk is traced back to t = 0 to identify ve distinct lobes of fluid that eventually form the advected bulk, and hence map the long-term fate of various regions in the fluid 1 at t = 0. The ve different lobes of uid are then shown to be delineated by repelling LCS boundaries at t = 0. We provide the first ever direct experimental evidence for the relevance of hyperbolic and elliptic LCSs using novel dye visualization experiments, and also show that attracting hyperbolic LCS provide objective characterization of the spiral structures often observed in vortical flows. We also show that qualitatively similar LCSs persist for several other plate motion pro les and stopping angles as well. Lastly, as a continuation of the second study, we present the flow and forces created by partially and fully exible (cF =c = 0:5 ; 1) rotating plates using experiments. But to begin with, we present a numerical study to bring out the connection between the flow and the forces acting on the rigid rotating plate and an experimental study that brings out the weak dependence of the problem on the Reynolds number in the range of 5000 < Re < 16000. The partially and fully flexible plates performs a 90o rotation about the hinge using a angular velocity pro le _(t) = max sin2(!t) during 0 t to. A non-dimensional stiffness parameter, EI = EI=0:5 c2 2 maxc3 F is de fined to characterize the flexible plate, where c is the total chord, cF is the flap length, EI the flap stiffness and max the maximum angular velocity of the imposed rotation at the hinge. It is found that both the partially and fully flexible aps generate a vortex pair as a result of the imposed rotation at the hinge. A momentum parameter, Svp = j(d􀀀V 1j+jd􀀀V 2j)d=c is defi ned to qualitatively relate to the momentum associated with the bulk flow generated along the stopping angle of the plate. It was found that Svp has a broad peak around EI 0:5 with cases in the range of 0:1 < EI < 6 having 60% more Svp than the rigid case. The parameter EI was found to collapse data from force measurements for the partially and fully flexible rotating plates with low, medium and high stiffness aps. Mean forces in the direction of the stopping angle of the plate, CX, was found to take higher values than the rigid case in the range of 0:3 < EI < 1. 2en_US
dc.language.isoen_USen_US
dc.relation.ispartofseries;G29359
dc.rightsI grant Indian Institute of Science the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertationen_US
dc.subjectLagrangian Coherent Structuresen_US
dc.subjectforce measurementsen_US
dc.subjectvelocity field measurementsen_US
dc.subjectflexible foilen_US
dc.subjectfishen_US
dc.subjectswimmingen_US
dc.subject.classificationResearch Subject Categories::TECHNOLOGY::Engineering mechanics::Mechanical and thermal engineeringen_US
dc.titleFlow and Forces on Rigid and Flexible Plates in Unsteady Rotational Motionen_US
dc.typeThesisen_US
dc.degree.namePhDen_US
dc.degree.levelDoctoralen_US
dc.degree.grantorIndian Institute of Scienceen_US
dc.degree.disciplineEngineeringen_US


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