Kamran Mohseni

SensorFlock: an airborne wireless sensor network of micro-air vehicles

An airborne wireless sensor network (WSN) composed of bird-sized micro aerial vehicles (MAVs) enables low cost high granularity atmospheric sensing of toxic plume behavior and storm dynamics, and provides a unique three-dimensional vantage for monitoring wildlife and ecological systems. This paper describes a complete implementation of our SensorFlock airborne WSN, spanning the development of our MAV airplane, its avionics, semi-autonomous flight control software, launch system, flock control algorithm, and wireless communication networking between MAVs. We present experimental results from flight tests of flocks of MAVs, and a characterization of wireless RF behavior in air-to-air communication as well as air-to-ground communication.

Numerical experiments on vortex ring formation

Numerical simulations are used to study the formation of vortex rings that are generated by applying a non-conservative force of long duration, simulating experimental vortex ring generation with large stroke ratio. For sufficiently long-duration forces, we investigate the extent to which properties of the leading vortex ring are invariant to the force distribution. The results confirm the existence of a universal ‘formation number’ defined by Gharib, Rambod & Shariff (1998), beyond which the leading vortex ring is separated from a trailing jet. We find that the formation process is governed by two non-dimensional parameters that are formed with three integrals of the motion (energy, circulation, and impulse) and the translation velocity of the leading vortex ring. Limiting values of the normalized energy and circulation of the leading vortex ring are found to be around 0.3 and 2.0, respectively, in agreement with the predictions of Mohseni & Gharib (1998). It is shown that under this normalization smaller variations in the circulation of the leading vortex ring are obtained than by scaling the circulation with parameters associated with the forcing. We show that by varying the spatial extent of the forcing or the forcing amplitude during the formation process, thicker rings with larger normalized circulation can be generated. Finally, the normalized energy and circulation of the leading vortex rings compare well with the same properties for vortices in the Norbury family with the same mean core radius.

Numerical simulations of the Lagrangian averaged Navier-Stokes equations for homogeneous isotropic turbulence

Capabilities for turbulence calculations of the Lagrangian averaged Navier-Stokes (LANS-alpha) equations are investigated in decaying and statistically stationary three-dimensional homogeneous and isotropic turbulence. Results of the LANS-alpha computations are analyzed by comparison with direct numerical simulation (DNS) data and large eddy simulations. Two different decaying turbulence cases at moderate and high Reynolds numbers are studied. In statistically stationary turbulence two different forcing techniques are implemented to model the energetics of the energy-containing scales. The resolved flows are examined by comparison of the energy spectra of the LANS-alpha with the DNS computations. The energy transfer and the capability of the LANS-alpha equations in representing the backscatter of energy is analyzed by comparison with the DNS data. Furthermore, the correlation between the vorticity and the eigenvectors of the rate of the resolved strain tensor is studied. We find that the LANS-alpha equations capture the gross features of the flow, while the wave activity below the scale alpha is filtered by a nonlinear redistribution of energy.

Thrust Characterization of a Bioinspired Vortex Ring Thruster for Locomotion of Underwater Robots

A new type of thrusting technology, loosely inspired by the locomotion of cephalopods, offers promising low-speed maneuvering capabilities for a new generation of underwater vehicles and robots. The actuators consist of a small cavity with a moving wall on one side and an orifice on the other side. The net effect of periodic movement of the moving wall is the ingestion of low-momentum fluid inside the cavity and then the expulsion of the fluid as a pulsatile jet from the orifice, with no net mass flux. Continuous operation of the actuator results in a synthetic jet. The actuators provide a net positive momentum flux with zero net mass flux. They are compact with no extruding components to negatively impact the vehicles drag at cruising speed. Parameters controlling the pulsatile jet and its thrust are identified. The thruster was empirically tested for a large range of frequencies and stroke ratios. The thrust characteristics of the device with respect to frequency was seen to converge to a single thrust coefficient. A model was developed to predict the thrust coefficient. The effect of the stroke ratios on the thrust coefficient is investigated. The model accurately predicts the observed thrust coefficient for stroke ratios up to five where the vortex ring pinchoff occurs. The accuracy of the model degrades for stroke ratios above the formation number where part of the expelled jet is pulled back into the cavity. Additionally, these devices have thrust tracking times faster than those reported for typical propellor-type thrusters, and deliver a fully quantized level of thrust.

A ridge tracking algorithm and error estimate for efficient computation of Lagrangian coherent structures

A ridge tracking algorithm for the computation and extraction of Lagrangian coherent structures (LCS) is developed. This algorithm takes advantage of the spatial coherence of LCS by tracking the ridges which form LCS to avoid unnecessary computations away from the ridges. We also make use of the temporal coherence of LCS by approximating the time dependent motion of the LCS with passive tracer particles. To justify this approximation, we provide an estimate of the difference between the motion of the LCS and that of tracer particles which begin on the LCS. In addition to the speedup in computational time, the ridge tracking algorithm uses less memory and results in smaller output files than the standard LCS algorithm. Finally, we apply our ridge tracking algorithm to two test cases, an analytically defined double gyre as well as the more complicated example of the numerical simulation of a swimming jellyfish. In our test cases, we find up to a 35 times speedup when compared with the standard LCS algorithm.

Digitized Heat Transfer: A New Paradigm for Thermal Management of Compact Micro Systems

This paper presents theoretical and numerical results describing digitized heat transfer (DHT), a newly developing active thermal management technique for high-power density electronics and integrated micro systems. In describing DHT, we numerically investigate the mass, momentum, and energy equations governing the flow within a translating microdroplet. Our analysis shows the existence of a pair of recirculation zones inside the droplet. This internal circulation within discrete fluid slugs results in significantly increased overall heat transfer coefficients when compared to continuous Graetz-type flows. The internal circulation drives the cold fluid in the middle of the droplet to the vicinity of the walls and creates a higher local temperature difference between the wall and the fluid in contact with the wall, resulting in higher heat transfer rates. Nusselt numbers characterizing DHT flow are also shown to exhibit periodic fluctuations with a period equal to the characteristic time scale for droplet circulation. The overall effect of discretizing a flow on heat transfer capability is described and characterized in terms of a nondimensional circulation number defined by the ratio of characteristic thermal diffusion and fluid circulation time scales. DHT coolants, including liquid metals and alloys, are proposed, and their physical properties are shown to enable handling of significantly higher heat transfer rates than classical air- or water-cooled methods. The actuation method for DHT coolant transport is also outlined, and shown to provide the capability for active, on-demand suppression of transient hot spots. This overall analysis defines the key parameters for optimization of the DHT method and forms the basis of ongoing experimental work.

The Numerical Comparison of Flow Patterns and Propulsive Performances for the Hydromedusae Sarsia Tubulosa and Aequorea Victoria

SUMMARY The thrust-generating mechanism of a prolate hydromedusa Sarsia tubulosa and an oblate hydromedusa Aequorea victoria was investigated by solving the incompressible Navier–Stokes equations in the swirl-free cylindrical coordinates. The calculations clearly show the vortex dynamics related to the thrust-generating mechanism, which is very important for understanding the underlying propulsion mechanism. The calculations for the prolate jetting hydromedusa S. tubulosa indicate the formation of a single starting vortex ring for each pulse cycle with a relatively high vortex formation number. However, the calculations for the oblate jet-paddling hydromedusa A. victoria indicate shedding of the opposite-signed vortex rings very close to each other and the formation of large induced velocities along the line of interaction as the vortices move away from the hydromedusa in the wake. In addition to this jet propulsion mechanism, the hydromedusas bell margin acts like a paddle and the highly flexible bell margin deforms in such a way that the low pressure leeward side of the bell margin has a projected area in the direction of motion. This thrust is particularly important during refilling of the subumbrella cavity where the stopping vortex causes significant pressure drag. The swimming performances based on our numerical simulations, such as swimming velocity, thrust, power requirement and efficiency, were computed and support the idea that jet propulsion is very effective for rapid body movement but is energetically costly and less efficient compared with the jet-paddling propulsion mechanism.

Simulation of flow patterns generated by the hydromedusa Aequorea victoria using an arbitrary Lagrangian–Eulerian formulation

A new geometrically conservative arbitrary Lagrangian-Eulerian (ALE) formulation is presented for the moving boundary problems in the swirl-free cylindrical coordinates. The governing equations are multiplied with the radial distance and integrated over arbitrary moving Lagrangian-Eulerian quadrilateral elements. Therefore, the continuity and the geometric conservation equations take very simple form similar to those of the Cartesian coordinates. The continuity equation is satisfied exactly within each element and a special attention is given to satisfy the geometric conservation law (GCL) at the discrete level. The equation of motion of a deforming body is solved in addition to the Navier-Stokes equations in a fully-coupled form. The mesh deformation is achieved by solving the linear elasticity equation at each time level while avoiding remeshing in order to enhance numerical robustness. The resulting algebraic linear systems are solved using an ILU(k) preconditioned GMRES method provided by the PETSc library. The present ALE method is validated for the steady and oscillatory flow around a sphere in a cylindrical tube and applied to the investigation of the flow patterns around a free-swimming hydromedusa Aequorea victoria (crystal jellyfish). The calculations for the hydromedusa indicate the shed of the opposite signed vortex rings very close to each other and the formation of large induced velocities along the line of interaction while the ring vortices moving away from the hydromedusa. In addition, the propulsion efficiency of the free-swimming hydromedusa is computed and its value is compared with values from the literature for several other species.

Dynamic Modeling and Control of Biologically Inspired Vortex Ring Thrusters for Underwater Robot Locomotion

A new type of underwater thruster was designed to provide high-accuracy, low-speed maneuvering to underwater robots. Located internal to the vehicle surface, these thrusters have a minimal effect on the forward-drag profile of the vehicle. These thrusters, whose inspiration comes from the natural propulsion of cephalopods and jellyfish, generate control forces by successive ingestion and expulsion of jets of water from a cavity mounted in the hull of the vehicle. The jetting process has no net mass flux but results in a positive momentum flux. A time-dependent thrust model was developed, which predicted the thruster dynamics as a function of time, actuation frequency, and thruster-driving parameters. A linear transfer-function model was developed to approximate both the thruster and vehicle dynamics, which led to maneuver categorization into three regimes: Cruising, Docking, and Transition. The predicted frequency response was verified through hybrid simulation to be accurate for predicting general trends and cutoff frequency.

An arbitrary Lagrangian-Eulerian formulation for the numerical simulation of flow patterns generated by the hydromedusa Aequorea victoria

A new geometrically conservative arbitrary Lagrangian-Eulerian (ALE) formulation is presented for the moving boundary problems in the swirl-free cylindrical coordinates. The governing equations are multiplied with the radial distance and integrated over arbitrary moving Lagrangian-Eulerian quadrilateral elements. Therefore, the continuity and the geometric conservation equations take very simple form similar to those of the Cartesian coordinates. The continuity equation is satisfied exactly within each element and a special attention is given to satisfy the geometric conservation law (GCL) at the discrete level. The equation of motion of a deforming body is solved in addition to the Navier-Stokes equations in a fully-coupled form. The mesh deformation is achieved by solving the linear elasticity equation at each time level while avoiding remeshing in order to enhance numerical robustness. The resulting algebraic linear systems are solved using an ILU(k) preconditioned GMRES method provided by the PETSc library. The present ALE method is validated for the steady and oscillatory flow around a sphere in a cylindrical tube and applied to the investigation of the flow patterns around a free-swimming hydromedusa Aequorea victoria (crystal jellyfish). The calculations for the hydromedusa indicate the shed of the opposite signed vortex rings very close to each other and the formation of large induced velocities along the line of interaction while the ring vortices moving away from the hydromedusa. In addition, the propulsion efficiency of the free-swimming hydromedusa is computed and its value is compared with values from the literature for several other species.