Effect of chordwise flexibility and depth of submergence on an oscillating plate underwater propulsion system
Date
2010-11-15T17:54:13Z
Authors
Barannyk, Oleksandr
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Abstract
The first part of this work was dedicated to the experimental study of basic principles of oscillating plate propulsors undergoing a combination of heave translation and pitch rotation. The oscillation kinematics are inspired by swimming mechanisms
employed by fish and some other marine animals. The primary attention was the
propulsive characteristics of such oscillating plates, which was studied by means of direct force measurements in the thrust-producing regime.
Experiments were performed at constant Reynolds number and heave amplitude.
By varying the Strouhal number, experimental depth and chordwise exibility of the
plate it was possible to investigate corresponding changes in thrust and hydromechanical efficiency. After numerous measurements it was possible to establish an optimal set of parameters, including the system's driving frequency, the ratio of rigid to flexible segment length of the plate and the range of Strouhal number, that led to a peak efficiency near 80%.
The experiments for different values of chordwise flexibility showed that greater
flexibility increases the propulsive effciency and thrust compared with similar motion of the purely rigid foil. By submerging the plate at different depths, it was observed that the proximity of the propulsor to the channel floor led to overall increase in the thrust coefficient. However the increase in thrust coefficient was pronounced in the range from middepth to the floor of the water tunnel. The special case when the upper plate's edge is tangential to the undisturbed free surface is discussed separately.
The second part of this work introduces a semianalytic approach for calculating
the influence of piezoelectric (PZT) actuators on the free vibration characteristics of an Euler-Bernoulli clamped free beam. The beam represents a simplifled version of the fish tail whose stiffness is proposed to be controlled by placing a pair of PZT actuators
in strategic regions along the caudal area of the tail. This approach, according to
earlier studies, improves efficiency if tail natural frequency matches tailbeat frequency.
The approach used an existing form of a transfer matrix technique developed for
the analysis of non-proportionally damped slender beams. The PZT dynamics were
incorporated into this recursive procedure through a modification that accounted for
the tendency of the PZT patches to couple the dynamics of the node points of the
segmented Euler-Bernoulli beam. To ensure stability of the system, an angular ve-
locity feedback law, originally motivated by vibration suppression applications, was
chosen for the PZT actuators. The sensitivities of the tail modes of vibration to the location of the PZT elements and the control gain were determined. Mode shapes for
the revised modes were determined and it was shown that the first, second and the
third modes maintained similar norms as tail shapes observed in anguilliform, sub-
carangiform, and thunniform regimes of swimming. Using a semianalytic approach,
it was shown that PZT location heavily in
uences the frequency distribution of the
modes of vibration. The control gain, when chosen within the limit of saturation
voltage, is shown to be an effective control lever for vibration suppression and at
rising the tail stiffness during rapid acceleration when the fish accelerates. However, the single PZT patch does not provide significant frequency adjustments such that different swimming modes could be employed efficiently with a single mechanical tail system primary actuator. To pursue such versatility for the sh tail, the tail structure must be very flexible to accommodate the significant frequency increase caused by the addition of the PZT material. Also, the use of additional PZT patches and negative control gains must be considered in order to use the PZT's to drop the higher modes (second and third) down into the frequency range of the primary actuation system, presuming the tail and primary actuator are designed for a thunniform regime of swimming.
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Keywords
Propulsion systems, Oscillations