Ashok K. Kancharala

Study of flexible fin and compliant joint stiffness on propulsive performance: theory and experiments

The caudal fin is a major source of thrust generation in fish locomotion. Along with the fin stiffness, the stiffness of the joint connecting the fish body to the tail plays a major role in the generation of thrust. This paper investigates the combined effect of fin and joint flexibility on propulsive performance using theoretical and experimental studies. For this study, fluid-structure interaction of the fin has been modeled using the 2D unsteady panel method coupled with nonlinear Euler-Bernoulli beam theory. The compliant joint has been modeled as a torsional spring at the leading edge of the fin. A comparison of self-propelled speed and efficiency with parameters such as heaving and pitching amplitude, oscillation frequency, flexibility of the fin and the compliant joint is reported. The model also predicts the optimized stiffnesses of the compliant joint and the fin for maximum efficiency. Experiments have been carried out to determine the effect of fin and joint stiffness on propulsive performance. Digital image correlation has been used to measure the deformation of the fins and the measured deformation is coupled with the hydrodynamic model to predict the performance. The predicted theoretical performance behavior closely matches the experimental values.

Enhanced hydrodynamic performance of flexible fins using macro fiber composite actuators

Recent studies on the role of body flexibility in propulsion suggest that fish have the ability to control the shape or modulate the stiffness of the fins for optimized performance. Inspired by natures ability to modulate stiffness and shape for different operating conditions, this paper investigates active control of flapping foils for thrust tailoring using Macro Fiber Composites (MFCs). A coupled piezohydroelastic model has been developed to predict the propulsive performance of an actively deforming fin. The effect of important parameters such as oscillation frequency, flexibility of the fin, applied voltage and the phase difference between applied voltage and heaving on propulsive performance are studied and reported. It is observed that distributed actuation along fin produces maximum performance through proper selection of the phase difference between heaving and voltage. The optimal phase for lower values of fin stiffness is approximately 90° and it approaches 0° for higher stiffness values. Experiments performed to determine the effect of active control using MFCs validate the theoretical results.

Influence of bending mode shape and trailing edge deflection on propulsive performance of flexible heaving fins using digital imagecorrelation

The propulsive performance of flexible flapping fins greatly depends on the stiffness of the fins along with the oscillating parameters. The bending mode shape and trailing edge deflection of the oscillating fins play a major role in the generation of thrust and efficiency. This paper examines the deformation pattern of heaving flexible foils and its dependency on propulsive performance. Experimental investigation has been carried out on fins of various lengths oscillating at their leading edge. A LaVision 2D/3D StrainMaster Digital Image Correlation (DIC) system was used to measure the deformation of the fins. It is observed that the propulsive performance can be maximized by operating at frequencies close to resonance. Trailing edge amplitude and deformation pattern together play an important role in achieving high propulsive performance even when the oscillation frequency is not close to resonant frequency.

Energy Harvesting From Droplet Interface Bilayers

Biologically inspired droplet interface bilayers have found applications in the development of hair cell sensors and other mechanotransduction applications. In this research, the flexoelectric capability of the droplet bilayers under external excitation is explored for energy harvesting. Traditionally, membrane capacitance models are being used for inferring the magnitude of the membrane deflection which do not account for the relation between the applied force or deflection and the deflection of the interfacial membrane and time dependent variations. In this work, the dynamic behavior of the droplets under external excitation has been modeled using nonlinear finite element analysis. A flexoelectric model including mechanical, electrical, and chemical sensitivities has been developed and coupled with the calculated bilayer deformations to predict the mechanotransductive response of the droplets under excitation. Using the developed framework, the possibilities of energy harvesting for different droplet configurations have been investigated and reported.Copyright

High-Fidelity Fluid Structure Coupled Simulations for Underwater Propulsion Using Flexible Biomimetic Fins

The ability of fish to maneuver in tight places, perform stable high acceleration maneuvers, and hover efficiently has inspired the development of underwater robots propelled by flexible fins mimicking those of fish. In general, fin propulsion is a challenging fluid-structure interaction (FSI) problem characterized by large structural deformation and strong added-mass effect. It was recently reported that a simplified computational model using the vortex panel method for the fluid flow is not able to accurately predict thrust generation. In this work, a high-fidelity, fluid-structure coupled computational framework is applied to predict the propulsive performance of a series of biomimetic fins of various dimensions, shapes, and stiffness. This computational framework couples a three-dimensional finite-volume Navier-Stokes computational fluid dynamics (CFD) solver and a nonlinear, finite-element computational structural dynamics (CSD) solver in a partitioned procedure. The large motion and deformation of the fluid-structure interface is handled using a validated, state-of-the-art embedded boundary method. The notorious numerical added-mass effect, that is, a numerical instability issue commonly encountered in FSI simulations involving incompressible fluid flows and light (compared to fluid) structures, is suppressed by accounting for water compressibility in the CFD model and applying a low-Mach preconditioner in the CFD solver. Both one-way and two-way coupled simulations are performed for a series of flexible fins with different thickness. Satisfactory agreement between the simulation prediction and the corresponding experimental data is achieved.Copyright

The Role of Compliant Joint and Flexibility on the Propulsive Performance of a Self Propelled Caudal Fin

Caudal fin plays an important role in the thrust generation of fish locomotion. Recent studies on the role of body flexibility in propulsion show that fish have a remarkable ability to control or modulate the stiffness of the fin for optimized propulsive performance. Along with the fin stiffness, the stiffness of the joint connecting the caudal peduncle and the fin also plays a major role in the generation of thrust. Since thrust and efficiency are dependent on various parameters, a detailed investigation would be required to understand the combined effect of fin and joint flexibility over a wide range of parameters for optimized performance. The present study provides a parametric study on the effect of flexibility of fin and the compliant joint on propulsive performance. For this investigation, fluid structure interaction of the fin has been modeled considering unsteady slender wing theory coupled with the nonlinear Euler-Bernoulli beam theory. The compliant joint has been modeled as a torsional spring at the leading edge of the fin. A comparison of Self–propelled speed (SPS) and efficiency with parameters such as heaving and pitching amplitude, oscillation frequency, flexibility of the fin and the compliant joint is reported. The model predicts the optimized stiffnesses of the compliant joint and the fin for maximum efficiency. These optimal stiffnesses vary with the motion parameters suggesting the benefits of active stiffness modulation in bio-inspired underwater robotics.Copyright

A comprehensive flexoelectric model for droplet interface bilayers acting as sensors and energy harvesters

Droplet interface bilayers have found applications in the development of biologically-inspired mechanosensors. In this research, a comprehensive flexoelectric framework has been developed to predict the mechanoelectric capabilities of the biological membrane under mechanical excitation for sensing and energy harvesting applications. The dynamic behavior of the droplets has been modeled using nonlinear finite element analysis, coupled with a flexoelectric model for predicting the resulting material polarization. This coupled model allows for the prediction of the mechanoelectrical response of the droplets under excitation. Using the developed framework, the potential for sensing and energy harvesting through lipid membranes is investigated.

Investigation on the reduction of center of mass oscillations of flexible flapping fins

Nature has been a source of inspiration for developing advanced autonomous aerial and underwater vehicles. The bodies with flapping appendages produce Center of Mass (COM) oscillations as the flapping fins generate forces oscillatory in nature. The vehicles with larger COM oscillations pose the problems of control and maneuverability and this paper discusses the effect of flexibility and other operating parameters such as heaving, pitching amplitudes and operating frequency on COM oscillations. A detailed theoretical investigation has been carried out to predict the optimal operating parameters along with the fin stiffness to reduce the COM oscillations for a given Self-Propelled Speed (SPS). Experiments have been performed to validate the theoretical results. It has been observed that the flexible fins operating at larger frequencies produce lower COM oscillations compared to stiffer fins operating at lower frequencies for a given mean thrust/SPS, and that the trailing edge amplitude along with the deformation pattern play a role in the generation of COM oscillations.

The Role of Flexibility on Center of Mass Oscillations of Self Propelled Flapping Fins

Flexibility is known to improve the propulsive performance of flapping fins. Flapping fins generate forces oscillatory in nature and this paper reports an investigation on the effect of flexibility and other parameters such as heaving, pitching amplitudes and operating frequency in reducing the center of mass oscillations of bodies attached to flapping fins. A detailed theoretical investigation has been carried out to predict the optimal operating parameters along with the fin stiffness to reduce the COM oscillations for a given self-propelled speed (SPS). Some design guidelines have been proposed which reduce COM oscillations that aid in the development of aerial and underwater robotic vehicles.Copyright

Active Stiffness Modulation of Fins Using Macro Fiber Composites

Studies on the role of body flexibility in propulsion suggest that fish have the ability to control or modulate the stiffness of the fin for optimized propulsive performance. Fins with certain stiffness might be efficient for a particular set of operating parameters but may be inefficient for other parameters. Therefore active stiffness modulation of a fin can improve the propulsive performance for a range of operating conditions. This paper discusses the preliminary experimental work on the open loop active deformation control of heaving flexible fins using Macro Fiber Composites (MFCs). The effect of important parameters such as oscillation frequency, flexibility of the fin, applied voltage and the phase difference between applied voltage and heaving on propulsive performance are studied and reported. The results indicate that propulsive performance can be improved by active control of the fins. The mean thrust improved by 30- 38% for the fins used in the experiments. The phase difference of ~90° is found to be optimal for maximized propulsive performance for the parameters considered in the study. Furthermore, there exists an optimal voltage magnitude at which the propulsive performance is a maximum for the range of operating conditions.