Lyle N. Long

Object-Oriented Unsteady Vortex Lattice Method for Flapping Flight

The unsteady vortex lattice method is used to model the oscillating plunging, pitching, twisting, and flapping motions of a finite-aspect-ratio wing. Its potential applications include design and analysis of small unmanned air vehicles and in the study of the high-frequency flapping flight of birds and other small flyers. The results are verified by theory and, in the plunging and pitching cases, by experimental data. The model includes free-wake relaxation, vortex stretching, and vortex dissipation effects and is implemented using object-oriented computing techniques

Computation of sound radiating from engine inlets

A hybrid method has been developed to calculate sound radiation from turbofan engine inlets. Given the acoustic source at an interface near the fan, the method solves the full three-dimensional, time-dependent Euler equations in the near field and passes the solution to a moving-surface Kirchhoff method for far-field predictions. The Kirchhoff method enables one to limit the size of the computational domain where the essential acoustic signals are captured accurately by a high-order accurate flow solver and then to extrapolate these signals to the far field exactly only within the discretization error on the Kirchhoff surface. The steady flowfield is required by the hybrid approach and a high-order accurate multigrid method has been implemented to enhance convergence of steady-state calculations. The computations are all carried out on parallel processors using essentially high-performance Fortran language. Results indicate very good agreement with available numerical and analytical solutions.

3-D Time-Accurate CFD Simulations of Wind Turbine Rotor Flow Fields

This paper presents the results of three-dimensional and time-accurate Computational Fluid Dynamics (CFD) simulations of the flow field around the National Renewable Energy Laboratory (NREL) Phase VI horizontal axis wind turbine rotor. The 3-D, unsteady, parallel, finite volume flow solver, PUMA2, is used for the simulations. The solutions are obtained using unstructured moving grids rotating with the turbine blades. Three different flow cases with different wind speeds and wind yaw angles are investigated: 7 m/s with 0◦ yaw (pre-stall case I), 7 m/s with 30◦ yaw (prestall, yawed case II), and 15 m/s with 0◦ yaw (post-stall case III). Results from the inviscid simulations for these three cases and comparisons with the experimental data are presented. Some information on the current work in progress towards Large Eddy Simulations (LES), including details about the viscous grid and the implementation of wall-functions, are also discussed. The inviscid results show that the flow is attached for cases I and II, with the latter having an asymmetrical wake structure, whereas there is massive separation over the entire blade span in case III. There are considerable spanwise pressure variations in addition to the chordwise variations, in all three cases. Comparisons of sectional pressure coefficient distributions with experimental data show good agreement. These threedimensional and time-accurate CFD results can be used for the far-field noise predictions based on the Ffowcs Williams Hawkings method (FWH), which can provide a first-principles prediction of both the noise and the underlying turbulent flow that generates the noise, in the context of the wind turbine application.

LANDING GEAR AERODYNAMIC NOISE PREDICTION USING UNSTRUCTURED GRIDS

Aerodynamic noise from a landing gear in a uniform flow is computed using the Ffowcs Williams-Hawkings (FW-H) equation. The time accurate flow data on the integration surface is obtained using a finite volume low-order flow solver on an unstructured grid. The Ffowcs Williams-Hawkings equation is solved using surface integrals over the landing gear surface and over a permeable surface away from the landing gear. Two geometric configurations are tested in order to assess the impact of two lateral struts on the sound level and directivity in the far-field. Predictions from the Ffowcs Williams-Hawkings code are compared with direct calculations by the flow solver at several observer locations inside the computational domain. The permeable Ffowcs Williams-Hawkings surface predictions match those of the flow solver in the near-field. Far-field noise calculations coincide for both integration surfaces. The increase in drag observed between the two landing gear configurations is reflected in the sound pressure level and directivity mainly in the streamwise direction.

Character Recognition using Spiking Neural Networks

A spiking neural network model is used to identify characters in a character set. The network is a two layered structure consisting of integrate-and-fire and active dendrite neurons. There are both excitatory and inhibitory connections in the network. Spike time dependent plasticity (STDP) is used for training. The winner take all mechanism is enforced by the lateral inhibitory connections. It is found that most of the characters are recognized in a character set consisting of 48 characters. The network is trained successfully with increased resolution of the characters. Also, addition of uniform random noise does not decrease its recognition capability.

Optimal Path Planning of UAVs Using Direct Collocation with Nonlinear Programming

A trajectory generation algorithm using direct collocation with nonlinear programming is successfully demonstrated in simulation. Direct collocation, which approximates the states and controls with piecewise polynomials, has been widely used in space and manned aircraft applications, but has only seen limited use in UAV applications. The algorithm is successfully applied to the generation of a UAV trajectory that provides maximum viewing time for a camera mounted on the UAV. The target can be stationary or moving. Multiple UAVs are considered. In this case, the objective is to provide maximum sensor coverage time using a combination of the UAVs. No specific initial guesses are required to ensure the algorithm is successful.

A Small Semi-Autonomous Rotary-Wing Unmanned Air Vehicle (UAV)

Small radio controlled (R/C) rotary-wing UAVs have many potential military and civilian applications, but can be very difficult to fly. Small and lightweight sensors and computers can be used to implement a control system to make these vehicles easier to fly. To develop a control system for a small UAV, an 8-bit microcontroller has been interfaced with MEMS (Micro-Electro-Mechanical Systems) gyroscopes, an R/C transmitter and receiver, and motor drivers. A single angular degree of freedom test bed has been developed to test these electronics and successful pilot-in-the-loop PI control has been achieved for this test system. A quadrotor with a stability augmentation system that uses these electronics to control the vehicle has also been developed. The future goals of this research are to incorporate more sensors to increase the level of autonomy for UAV operation.

A Time-Domain Implementation of Surface Acoustic Impedance Condition with and Without Flow

The impedance condition in computational aeroacoustic applications is required in order to model acoustically treated walls. The application of this condition in time-domain methods, however, is extremely difficult because of the convolutions involved. In this paper, a time-domain method is developed which overcomes the computational difficulties associated with these convolutions. This method builds on the z-transform from control and signal processing theory and the z-domain model of the impedance. The idea of using the z-domain operations originates from the computational electromagnetics community. When the impedance is expressed in the z-domain with a rational function, the inverse z-transform of the impedance condition results in only infinite impulse response type, digital, recursive filter operations. These operations, unlike convolutions, require only limited past-time knowledge of the acoustic pressures and velocities on the surface. Examples of one- and two-dimensional problems with and without flow indicate that the method promises success in aeroacoustic applications.

Simulation of Helicopter Shipboard Launch and Recovery with Time-Accurate Airwakes

A simulation of the helicopter/ship dynamic interface has been developed and applied to simulate a UH-60A operating from an LHA class ship. Time accurate CFD solutions of the LHA airwake are interfaced with a flight dynamics simulation based on the GENHEL model. The flight dynamics model was updated to include improved inflow modeling and gust penetration effects of the ship airwake. A maneuver controller was used to simulate pilot control inputs for specified approach and departure trajectories. The CFD solutions show significant time varying flow effects in the airwake. Time histories of the aircraft angular rate and pilot control activity indicate that the time varying nature of the airwake has significant effect on aircraft response and pilot workload.

Molecular dynamics simulations of droplet evaporation

The complete evaporation of a three-dimensional submicron droplet under subcritical conditions has been modeled using molecular dynamics. The two-phase system consisted of 2048 argon atoms modeled using a Lennard-Jones 12-6 potential distributed between a single droplet and its surrounding vapor. The system was first allowed to relax to equilibrium, then the droplet was evaporated by increasing the temperature of the vapor phase atoms at the boundaries of the system until only the vapor phase remained. The computed evaporation rate agrees with that predicted by the Knudsen aerosol theory.