|M.Sc Student||Mendelson Jonathan|
|Subject||Hydro-Elastic Model for a flexible Fin|
|Department||Department of Mechanical Engineering||Supervisor||Professor Nitai Drimer|
|Full Thesis text|
Batoid fishes such as the Manta Ray have inspired many Bio-Mimetic concepts of underwater propulsion. A new concept for realizing this motion is presented, consisting of a flexible fin in which a few hydraulic muscles are imbedded. The bending of these muscles due to internal pressure actuates the fin to create flapping, twisting and wave-like propulsive motions.
A new concept of hydraulic muscle is developed for this purpose, dubbed HELM- Hydraulic Equal strain Linear Muscle- which indicates its features. The HELM is composed of a relatively stiff tube with triangular cutouts in which membranes of low tension rigidity are implanted. Variation of the internal pressure causes uniformly distributed strain variation in the membranes, which bends the actuator. The HELM has many design parameters which allow it to be tailored for specific applications.
According to beam theory a linear relation is obtained between the internal pressure and bending angle at a given external moment load. The theory is validated on a few HELM prototypes which are constructed using different membrane materials. Quasi- static actuation efficiency testing on a HELM composed of nitrile rubber reveals energy losses which stem from hysteresis in the membrane material. Following testing of the properties of the materials used we suggest using natural rubber for future prototypes, as it presents minimal energy loss by hysteresis.
A hydro- elastic
simulation is devised to be used for design optimization. It computes the
thrust and propulsion efficiency of a given fin configuration and motion
profile (i.e. pressure supplied to the HELMs) by coupling hydrodynamic loads with
Hydrodynamic loads are computed by a panel method which solves the unsteady potential flow problem at each time step, assuming a thin lifting surface representing the contour of the fin. Hydrodynamic loads are supplied to the equations of motion which are integrated in discrete time steps using the implicit Newmark-β method.
Numerical instabilities were encountered related to the added mass of fluid accelerated by the fin. The instabilities occur when the added mass, which is a result of the hydrodynamic solution, is in the order of the actual mass of the fin. A novel method of overcoming this issue is presented by adding an estimated (by existing databases) added mass to the known mass of the fin, which stabilizes the iterative time stepping process.
Experiments of a full scale propulsion fin prototype have been conducted in a towing tank. Good agreement was found between recorded motion and that simulated using the pressure input. Total efficiency was found to be limited by high energy dissipation in the fin and HELMs which were at first made of an elastomer with high hysteresis. Characterization of materials shows that by selecting better HELM materials (natural rubber) losses by hysteresis can be minimized.
A few locomotion cases were investigated using the simulation to find optimal motion and design parameters. Hydrodynamic efficiencies peaking at 84.2% were computed.