Lily D. Chambers

Hydrodynamic pressure sensing with an artificial lateral line in steady and unsteady flows

With the overall goal being a better understanding of the sensing environment from the local perspective of a situated agent, we studied uniform flows and Kármán vortex streets in a frame of reference relevant to a fish or swimming robot. We visualized each flow regime with digital particle image velocimetry and then took local measurements using a rigid body with laterally distributed parallel pressure sensor arrays. Time and frequency domain methods were used to characterize hydrodynamically relevant scenarios in steady and unsteady flows for control applications. Here we report that a distributed pressure sensing mechanism has the capability to discriminate Kármán vortex streets from uniform flows, and determine the orientation and position of the platform with respect to the incoming flow and the centre axis of the Kármán vortex street. It also enables the computation of hydrodynamic features which may be relevant for a robot while interacting with the flow, such as vortex shedding frequency, vortex travelling speed and downstream distance between vortices. A Kármán vortex street was distinguished in this study from uniform flows by analysing the magnitude of fluctuations present in the sensor measurements and the number of sensors detecting the same dominant frequency. In the Kármán vortex street the turbulence intensity was 30% higher than that in the uniform flow and the sensors collectively sensed the vortex shedding frequency as the dominant frequency. The position and orientation of the sensor platform were determined via a comparative analysis between laterally distributed sensor arrays; the vortex travelling speed was estimated via a cross-correlation analysis among the sensors.

A fish perspective: detecting flow features while moving using an artificial lateral line in steady and unsteady flow

For underwater vehicles to successfully detect and navigate turbulent flows, sensing the fluid interactions that occur is required. Fish possess a unique sensory organ called the lateral line. Sensory units called neuromasts are distributed over their body, and provide fish with flow-related information. In this study, a three-dimensional fish-shaped head, instrumented with pressure sensors, was used to investigate the pressure signals for relevant hydrodynamic stimuli to an artificial lateral line system. Unsteady wakes were sensed with the objective to detect the edges of the hydrodynamic trail and then explore and characterize the periodicity of the vorticity. The investigated wakes (Kármán vortex streets) were formed behind a range of cylinder diameter sizes (2.5, 4.5 and 10 cm) and flow velocities (9.9, 19.6 and 26.1 cm s−1). Results highlight that moving in the flow is advantageous to characterize the flow environment when compared with static analysis. The pressure difference from foremost to side sensors in the frontal plane provides us a useful measure of transition from steady to unsteady flow. The vortex shedding frequency (VSF) and its magnitude can be used to differentiate the source size and flow speed. Moreover, the distribution of the sensing array vertically as well as the laterally allows the Kármán vortex paired vortices to be detected in the pressure signal as twice the VSF.

FILOSE for Svenning: A Flow Sensing Bioinspired Robot

The trend of biomimetic underwater robots has emerged as a search for an alternative to traditional propeller-driven underwater vehicles. The drive of this trend, as in any other areas of bioinspired and biomimetic robotics, is the belief that exploiting solutions that evolution has already optimized leads to more advanced technologies and devices. In underwater robotics, bioinspired design is expected to offer more energy-efficient, highly maneuverable, agile, robust, and stable underwater robots. The 30,000 fish species have inspired roboticists to mimic tuna [1], rays [2], boxfish [3], eels [4], and others. The development of the first commercialized fish robot Ghostswimmer by Boston Engineering and the development of fish robots for field trials with specific applications in mind (http://www.roboshoal. com) mark a new degree of maturity of this engineering discipline after decades of laboratory trials.

Modelling of a biologically inspired robotic fish driven by compliant parts

Inspired by biological swimmers such as fish, a robot composed of a rigid head, a compliant body and a rigid caudal fin was built. It has the geometrical properties of a subcarangiform swimmer of the same size. The head houses a servo-motor which actuates the compliant body and the caudal fin. It achieves this by applying a concentrated moment on a point near the compliant body base. In this paper, the dynamics of the compliant body driving the robotic fish is modelled and experimentally validated. Lighthills elongated body theory is used to define the hydrodynamic forces on the compliant part and Rayleigh proportional damping is used to model damping. Based on the assumed modes method, an energetic approach is used to write the equations of motion of the compliant body and to compute the relationship between the applied moment and the resulting lateral deflections. Experiments on the compliant body were carried out to validate the model predictions. The results showed that a good match was achieved between the measured and predicted deformations. A discussion of the swimming motions between the real fish and the robot is presented.

What information do Karman streets offer to flow sensing

In this work, we focus on biomimetic lateral line sensing in Kármán vortex streets. After generating a Kármán street in a controlled environment, we examine the hydrodynamic images obtained with digital particle image velocimetry (DPIV). On the grounds that positioning in the flow and interaction with the vortices govern bio-inspired underwater locomotion, we inspect the fluid in the swimming robot frame of reference. We spatially subsample the flow field obtained using DPIV to emulate the local flow around the body. In particular, we look at various sensor configurations in order to reliably identify the vortex shedding frequency, wake wavelength and downstream flow speed. Moreover, we propose methods that differentiate between being in and out of the Kármán street with >70% accuracy, distinguish right from left with respect to Kármán vortex street centreline (>80%) and highlight when the sensor system enters the vortex formation zone (>75%). Finally, we present a method that estimates the relative position of a sensor array with respect to the vortex formation point within 15% error margin.

Parylene conformal coating encapsulation as a method for advanced tuning of mechanical properties of an artificial hair cell

A soft Parylene conformal coating encapsulation is demonstrated to be an efficient method to control the mechanical and sensory properties of a bioinspired artificial hair cell, tuning the mechanoreceptive responsivity from a sub-linear to a super-linear behaviour such as hair cells adapt to a natural environment.

Sensing oscillations in unsteady flow for better robotic swimming efficiency

Turbulent flows are often treated as a noisy environment by control algorithms of underwater robots. However, aquatic animals such as fish have learned to take advantage of certain unsteady flow. Periodic complex flow, such as that found in the wake of cylinders has been shown to offer energy saving opportunities to fish. We built a fish-like robot with an integrated pressure sensor array housed in the head. The robot can control its tail beat synchronization with respect to the periodic oscillations in the flow behind a cylinder. We show that vortices, represented here by pressure maxima, can be detected and exploited to increase the swimming efficiency of the robot fish while it remains rigidly mounted to a force plate. Force measurements show an efficiency gain of 23% when the tail beat of the robotic fish is synchronized at a particular phase lag.

Stress-Driven Artificial Hair Cell for Flow Sensing

Bio-inspiration from natural structures and systems can be used to design innovative engineered solutions. Here natural sensor architectures inspire the design of micro-electronic-mechanical systems (MEMS) for flow sensing. In this chapter, we introduce an innovative approach to artificial flow sensing based on mimicking stereocilia and their mechanical properties. This method exploits the intrinsic differences in material properties of multilayered thin films such as thermal expansion properties, crystalline lattice order and interatomic distances. If a cantilever beam is multilayered, these properties create a stress gradient along the cantilever cross section, allowing an upwards bending, defined as ‘stress-driven geometry’. When inserted in a superficial fluid stream, the cantilever beam is deformed by the flow and acts as a fluid flow velocity sensor. It is shown that a Parylene post-processing conformal coating not only waterproofs the device, but also sets the flexural stiffness of the beam, thus tuning the dynamic range for flow measurements optimisation.

Variable stroke timing of rubber fins' duty cycle improves force

Swimming animals can tune their kinematics to achieve increased propulsive performance. To engineer effective propulsive mechanisms, a better correlation between kinematics and dynamics is required in artificial designs. Two rubber fins: one with a NACA aerofoil shape, the other with a biomimetic shape, were used in two asymmetric oscillations in a static water tank. The force generation patterns within the parameter space and the response to the change in stroke timing, were dependant on the fin. The biomimetic fin produced peak force at a similar frequency and amplitude regardless of its kinematics and duty cycle. The response of the NACA fin, however, was dependent on the duty cycle. For the NACA fin, the fast-to-centreline kinematics caused larger resultant force over a narrow range of frequencies. For the fast-to-maximum-amplitude stroke, a lower resultant force was achieved, but over a larger range of frequencies. Digital Particle Image Velocimetry (DPIV) analysis showed the wake pattern of shed vortices. We present experiments and qualitative flow analysis that relate kinematic parameters, particularly the trailing-edge angle to resultant forces.

The interaction between vortices and a biomimetic flexible fin

The fluid-structure interaction of flexible bodies in steady and unsteady flow is a key area of interest for the development of underwater vehicles. In the design of marine vehicles the flow can often be seen as an obstacle to overcome, whilst in nature a fish interacts with the flow and is capable of achieving a high level of efficiency. Therefore by understanding how fish – or flexible bodies – interact with the flow we may be able to achieve a better level of co-operation between our vehicles and their environment, potentially attaining a better efficiency in design.