Jessica Shang

Artificial insect wings of diverse morphology for flapping-wing micro air vehicles.

The development of flapping-wing micro air vehicles (MAVs) demands a systematic exploration of the available design space to identify ways in which the unsteady mechanisms governing flapping-wing flight can best be utilized for producing optimal thrust or maneuverability. Mimicking the wing kinematics of biological flight requires examining the potential effects of wing morphology on flight performance, as wings may be specially adapted for flapping flight. For example, insect wings passively deform during flight, leading to instantaneous and potentially unpredictable changes in aerodynamic behavior. Previous studies have postulated various explanations for insect wing complexity, but there lacks a systematic approach for experimentally examining the functional significance of components of wing morphology, and for determining whether or not natural design principles can or should be used for MAVs. In this work, a novel fabrication process to create centimeter-scale wings of great complexity is introduced; via this process, a wing can be fabricated with a large range of desired mechanical and geometric characteristics. We demonstrate the versatility of the process through the creation of planar, insect-like wings with biomimetic venation patterns that approximate the mechanical properties of their natural counterparts under static loads. This process will provide a platform for studies investigating the effects of wing morphology on flight dynamics, which may lead to the design of highly maneuverable and efficient MAVs and insight into the functional morphology of natural wings.

Body torque modulation for a microrobotic fly

The Harvard Microrobotics Lab has previously demonstrated the worlds first at-scale robotic insect capable of vertical takeoff with external power. Both of the robots wings were driven by a single power actuator and 1-DOF mechanical transmission - making independent control of both wings, and therefore asymmetric flapping and the generation of a net body torque, impossible. This paper presents a method to modulate body torques by altering the kinematics of each wing transmission independently, via the introduction of two additional control actuators. Theoretical kinematic and dynamic predictions based on a pseudo-rigid body model are compared to the observed wing trajectories. Controllable body torques are necessary for the development of control algorithms for eventual stable hovering and free flight.

Spectral analysis of fluid flows using sub-Nyquist-rate PIV data

Abstract Spectral methods are ubiquitous in the analysis of dynamically evolving fluid flows. However, tools like Fourier transformation and dynamic mode decomposition (DMD) require data that satisfy the Nyquist–Shannon sampling criterion. In many fluid flow experiments, such data are impossible to acquire. We propose a new approach that combines ideas from DMD and compressed sensing to accommodate sub-Nyquist-rate sampling. Given a vector-valued signal, we take measurements randomly in time (at a sub-Nyquist rate) and project the data onto a low-dimensional subspace. We then use compressed sensing to identify the dominant frequencies in the signal and their corresponding modes. We demonstrate this method using two examples, analyzing both an artificially constructed dataset and particle image velocimetry data from the flow past a cylinder. In each case, our method correctly identifies the characteristic frequencies and oscillatory modes dominating the signal, proving it to be a capable tool for spectral analysis using sub-Nyquist-rate sampling.

Stratified thin-film flow in a rheometer

When two immiscible layered fluids are present in a rheometer, interfacial distortions driven by the centripetal pressure gradient can modify torque measurements and induce dewetting. In particular, we examine the steady-state interface shape of a thin film coating a stationary substrate beneath a second immiscible fluid that is driven by a rotating parallel-plate or cone. An asymptotic analysis of the interfacial distortion for the parallel-plate flow is compared with numerical solutions for both the parallel-plate and cone and plate configurations. We develop asymptotic criteria for dewetting of the thin film as a function of fluid and flow properties, and show that significant interfacial distortion and dewetting can occur due to secondary flow effects even at low Reynolds numbers. The distortion of the interface can result in increased or decreased torque measurements depending on the viscosity and density ratios between the two fluid layers. We relate these effects to recent experimental studies on l...

Optimization of the Assisted Bidirectional Glenn Procedure for First Stage Single Ventricle Repair

Background: First-stage single-ventricle palliation is challenging to manage, and significant interstage morbidity and mortality remain. Prior computational and in vitro studies of the assisted bidirectional Glenn (ABG), a novel first-stage procedure that has shown potential for early conversion to a more stable augmented Glenn physiology, demonstrated increased pulmonary flow and oxygen delivery while decreasing cardiac work, as compared to conventional stage-1 alternatives. This study aims to identify optimal shunt designs for the ABG to improve pulmonary flow while maintaining or decreasing superior vena caval (SVC) pressure. Methods: A representative three-dimensional model of a neonatal bidirectional Glenn (BDG) was created, with a shunt connecting the innominate artery to the SVC. The shunt design was studied as a six-parameter constrained shape optimization problem. We simulated hemodynamics for each candidate designs using a multiscale finite element flow solver and compared performance against designs with taper-less shunts, the standalone BDG, and a simplified control volume model. Three values of pulmonary vascular resistance (PVR) of 2.3, 4.3, and 7.1 WUm2 were studied. Results: Increases in pulmonary flow were generally accompanied by increases in SVC pressure, except at low PVR (2.3 WUm2), where the optimal shunt geometry achieved a 13% increase in pulmonary flow without incurring any increase in SVC pressure. Shunt outlet area was the most influential design parameter, while others had minimal effect. Conclusion: Assisted bidirectional Glenn performance is sensitive to PVR and shunt outlet diameter. An increase in pulmonary flow without a corresponding increase in SVC pressure is possible only when PVR is low.

Dynamics of a flexible superhydrophobic surface during a drop impact

In this study, coupled dynamic responses of flexible superhydrophobic surfaces during a drop impact were investigated with position sensing and high-speed imaging. A smooth polydimethylsiloxane surface was spray coated with commercially available superhydrophobic paint particles. The influence of initial and subsequent impacts of a water droplet on the surface dynamics was studied at various natural frequencies of the surface (50 < fs < 230 Hz) and Weber numbers (2 < We < 90). We discovered that the flexible superhydrophobic surface was deflected twice during contact of the droplet by an impact force of the droplet as well as its reaction force during recoil. The magnitude of the droplet reaction force was estimated to be comparable to the droplet impact force. As the Weber number increased, however, the influence of the droplet reaction force on the surface displacement was attenuated because of the instability of the droplet rim. The contact time of the droplet and surface dynamics were found to be dependent on the phase of the surface. The contact time was reduced as much as 7% when a completion of the droplet spreading matched to the upward motion of the surface. One of the two local minima of the surface position observed during the contact of the droplet was diminished by matching the instance of the droplet reaction force to the downward motion of the surface. This study provides new insight into the effect of the droplet reaction force on dynamics of flexible superhydrophobic surfaces.In this study, coupled dynamic responses of flexible superhydrophobic surfaces during a drop impact were investigated with position sensing and high-speed imaging. A smooth polydimethylsiloxane surface was spray coated with commercially available superhydrophobic paint particles. The influence of initial and subsequent impacts of a water droplet on the surface dynamics was studied at various natural frequencies of the surface (50 < fs < 230 Hz) and Weber numbers (2 < We < 90). We discovered that the flexible superhydrophobic surface was deflected twice during contact of the droplet by an impact force of the droplet as well as its reaction force during recoil. The magnitude of the droplet reaction force was estimated to be comparable to the droplet impact force. As the Weber number increased, however, the influence of the droplet reaction force on the surface displacement was attenuated because of the instability of the droplet rim. The contact time of the droplet and surface dynamics were found to be depe...

Vortex and structural dynamics of a flexible cylinder in cross-flow

A low-density, flexible cantilevered cylinder was permitted to vibrate freely under the influence of vortex shedding in the laminar flow regime. We find that the vortex-induced vibrations (VIV) of a flexible cantilever depart from those of a flexible cylinder that is fixed at both ends. In particular, we find discontinuous regions of VIV behavior – here called states – as a function of the reduced velocity U*. These states are demarcated by discrete changes in the dominant eigenmodes of the structural response as the cylinder vibrates in progressively higher structural modes with increasing U*. The contribution of structural modes can be identified readily by a modal projection of the cylinder oscillation onto known cantilever beam modes. Oscillation frequencies do not monotonically increase with U*. The wake response between different states is also found to have distinct characteristics; of particular note is the occurrence of a P+S wake over one of these regions, which is associated with a high-amplitu...