Alan J. Bilanin

AGDISP: The Aircraft Spray Dispersion Model, Code Development and Experimental Validation

ABSTRACT OVER the last 4 years the USDA Forest Service and the DOD Atmospheric Sciences Laboratory have continued the development of the AGDISP (AGricultural DISPersal) computer code that predicts the deposition of material released from helicopters or fixed-wing aircraft. The features of the code are reviewed and the predictive capability of the code is assessed by comparison with recent test data. Code applications and limitations are also discussed.

Vortex interactions and decay in aircraft wakes

The dynamic interaction of aircraft wake vortices was investigated using both inviscid and viscous models. For the viscous model, a computer code was developed using a second-order closure model of turbulent transport. The phenomenon of vortex merging which results in the rapid aging of a vortex wake was examined in detail. It was shown that the redistribution of vorticity during merging results from both convective and diffusive mechanisms.

Turbulent Trailing Vortices in Stratified Fluids

The effects of stable atmospheric-density stratification, vortex core size, and turbulent scale on the descent of aircraft-trailing vortices are investigated using numerical solutions of a second-order closure turbulence model. A Boeing 747 vortex descent is simulated numerically and compared to reported measurements of descent distance, velocity, and circulation profiles. It is concluded that the pair was halted in its descent by a diffuse region of countersign vorticity primarily outboard and above the vortex cores. It is shown that the core size and turbulent macroscale have significant effects on vortex behavior through their influence on turbulence production, diffusion, and dissipation.

Computation of wake/exhaust mixing downstream of advanced transport aircraft

The mixing of engine exhaust with the vortical wake of high speed aircraft operating in the stratosphere can play an important role in the formation of chemical products that deplete atmospheric ozone. An accurate analysis of this type of interaction is therefore necessary as a part of the assessment of the impact of proposed High Speed Civil Transport (HSCT) designs on atmospheric chemistry. This paper describes modifications to the parabolic Navier-Stokes flow field analysis in the UNIWAKE unified aircraft wake model to accommodate the computation of wake/exhaust mixing and the simulation of reacting flow. The present implementation uses a passive chemistry model in which the reacting species are convected and diffused by the fluid dynamic solution but in which the evolution of the species does not affect the flow field. The resulting analysis, UNIWAKE/PCHEM (Passive CHEMistry) has been applied to the analysis of wake/exhaust flows downstream of representative HSCT configurations. The major elements of the flow field model are described, as are the results of sample calculations illustrating the behavior of the thermal exhaust plume and the production of species important to the modeling of condensation in the wake. Appropriate steps for further development of the UNIWAKE/PCHEM model are also outlined.

Turbulent Vortices in Stratified Fluids

In the present paper, calculations, made with the finite difference axisymmetric WAKE computer code, of the influence of turbulence and stratification on the behavior of vortex rings are compared with experimental data. Calculations, made with the two-dimensional version of the code, are used to study the behavior of vortex pairs in stably stratified atmospheres for a range of Froude numbers. Stratification is shown to have a profound effect on the radius of a vortex ring descending into a stably stratified fluid. The separation of the vortices of a vortex pair remains nearly constant or decreases monotonically with increasing penetration of a stably stratified fluid, depending on whether the stratification is discontinuous or linear. An analysis based on an energy balance is used to assess the maximum descent of a vortex pair in a stably stratified fluid.

Design, fabrication, and test planning for an SMA-actuated vortex wake control system

This paper describes ongoing design and fabrication work on a vortex wake control system for submarines that employs SMA-actuated devices. Previous work has described the theoretical basis and feasibility studies for this system, which is based on a novel wake control scheme known as vortex leveraging. The critical item in the realization of this system is a Smart Vortex Leveraging Tab (SVLT), whose design and fabrication is the principal focus of this work. This paper outlines the background of the effort and the design principles involved, but will chiefly deal with three closely interrelated topics; the hydrodynamic design requirements and control surface layout for the vortex leveraging system; the detail design and fabrication techniques being used in the construction of a prototype SVLT; and the test planning and experiment design process currently underway for test of both the overall vortex leveraging concept and SVLT device itself.

Test results for an SMA-actuated vortex wake control system

This paper describes recent test result obtained on a prototype SMA-actuated foil that serves as a key element in a vortex wake control scheme for lifting surfaces. Previous papers have described the theoretical basis and feasibility studies for this scheme - which is based on a novel wake control known as vortex leveraging - as well as prior work on device design, test planning, and fabrication. The critical item in the realization of this scheme is a Smart Vortex Leveraging Tab (SVLT), a device designed to provide perturbations in the vortex system downstream of lifting surfaces at frequencies and amplitudes carefully selected to accelerate overall wake breakup. The paper summarizes the background of the effort, but focuses on the detail design and fabrication techniques used in the construction of a prototype SVLT and the results of water tunnel tests of a near full-scale prototype device.

LONG-TIME AIRCRAFT VORTEX WAKE PREDICTIONS AND PLUME EVOLUTION CONSEQUENCES

This paper summarizes ongoing work on the prediction of wake/exhaust mixing for subsonic and supersonic aircraft, with particular emphasis on long-time wake effects. The primary objective of this work addresses the need for a continuous model prediction of the wake of a subsonic aircraft (from the trailing edge of the wing to times late downstream), its entrainment of engine products, and the consequences of the chemistry of these products on the atmosphere; in particular, the process of exhaust mixing, dispersion and chemical reactions in the upper atmosphere, and the assessment of atmospheric effects of commercial aviation. This work is part of the current Subsonic Assessment element of the NASA Atmospheric Effects of Aviation Program. The modeling described here brings together two validated computer models: UNIWAKE, a near-wake model for vortex roll-up and entrainment, and SCIPUFF, a widely used atmospheric plume dispersion model. The coupled model tracks gas-phase species, temperature and velocity fields, and passive tracers in the presence of realistic wind fields, over periods of 24 hours or more after initiation of the wake trail. It facilitates coupling with existing models of photochemical kinetics and aerosol growth dynamics contributed by other investigators, and permits fields of multiple overlapping plumes to be examined, including the simulation of active flight corridors such as those planned for the North Atlantic Flight Corridor and scheduled for Summer 1997.

Computational and Experimental Studies in Multipair Wake Vortex Instabilities

Control of trailing vortex wakes of lifting surfaces such as aircraft wings and submarine control planes is an important challenge for both military and civil applications. This paper summarizes the development and testing of a method for mitigating adverse vortex wake effects using active control of auxiliary aerodynamic surfaces mounted on the primary lifting wing. The governing concept involves a method for introducing small, secondary vortices of periodic, time-varying strength to promote the deintensification of the primary vortex wake system. Computational analyses of wake breakup using this “vortex leveraging” (VL) strategy have showed up to an order of magnitude increase in the dissipation rate of concentrated vortex pair wakes relative to natural mechanisms. The work described here was among the first efforts to explore this class of mitigation methods and the discussion below summarizes the foundational analysis and validation work on this concept. Computational modeling involved application of fast vortex methods to identify potentially promising methods for exploiting the instabilities of multipair vortex wakes. Validation of the concept was done by way of tow tank experiments that both provided flow visualization of predicted wake on wake interactions and measurements of vortex transport indicating substantial mitigation of wake effects.