We consider the instability of two vortex pairs in a counter-rotating configuration. Such configurations model vortex wakes observed behind aircraft with inboard vortices produced at the inboard flap edges and at the tips of the horizontal tail. The instability potential is characterized by an optimal perturbation analysis. This extends the analysis of Fabre & Jacquin (2000) which was restricted to particular stationary configurations, and that of Crouch (1997) which considered co-rotating configurations. A complete mapping of the optimal perturbations is presented. The optimal perturbations grow faster than the Crow (1970) instability. However, they correspond to short-wavelength perturbations mainly affecting the weaker inboard vortices. A possible strategy which consists of forcing a long wavelength is then investigated. Application of both the optimal and the long-wave optimal perturbations to reduction of vortex wake danger is discussed.

Optimal perturbations in a four-vortex aircraft wake in counter-rotating configuration

A rapidly growing instability is observed to develop between unequal-strength counter-
 rotating vortex pairs. The vortex pairs are generated in a towing tank in the wakes of wings with outboard triangular flaps. The vortices from the wing tip and the inboard tip of the flap form the counter-rotating vortex pair on each side of the wing. The flow fields are studied using flow visualization and particle image velocimetry. Both chord-
 based and circulation-based Reynolds numbers are of O(105). The circulation strength ratios of the flap- to tip-vortex pairs range from −0.4 to −0.7. The initial sinuous stage of the instability of the weaker flap vortex has a wavelength of order one wing span and becomes observable in about 15 wing spans downstream of the wing. The nearly straight vortex filaments first form loops around the stronger wing-tip vortices. The loops soon detach and form rings and move in the wake under self-induction. These vortex rings can move to the other side of the wake. The subsequent development of the instability makes the nearly quasi-steady and two-dimensional wakes unsteady and three-dimensional over a distance of 50 to 100 wing spans. A rectangular wing is also used to generate the classical wake vortex pair with the circulation ratio of −1.0, which serves as a reference flow. This counter-rotating vortex pair, under similar experimental conditions, takes over 200 spans to develop visible deformations. Velocity, vorticity and enstrophy measurements in a fixed plane, in conjuction with the flow observations, are used to quantify the behaviour of the vortex pairs. The vortices in a pair initially orbit around their vorticity centroid, which takes the pair out of the path of the wing. Once the three-dimensional interactions develop, two-dimensional kinetic energy and enstrophy drop, and enstrophy dispersion radius increases sharply. This rapid transformation of the wake into a highly three-dimensional one offers a possible way of alleviating the hazard posed by the vortex wake of transport aircraft.

Experimental study of the instability of unequal-strength counter-rotating vortex pairs

We present a combined analytical and numerical study of the instabilities of a pair of parallel unequal-strength vortices. We extend the analyses of a vortex in an external strain field (Crow, AIAA J. vol. 8, 1970, p. 2172; Widnall et al., J. Fluid Mech. vol. 66, 1974, p. 35) to include the orbital motion of the vortex pair. For counter-rotating pairs, the classic Crow-type periodic displacement perturbations are unstable for all vortex strength ratios, with fastest-growing wavelengths several times the vortex spacing. For co-rotating pairs, the orbital motion acts to suppress instability due to displacement perturbations. Instabilities in this case arise for elliptic perturbations at wavelengths that scale with the vortex core size. We also examine the influence of a second vortex pair by extending Crouch's (J. Fluid Mech. vol. 350, 1997, p. 311) analysis. Numerical results from a spectral initial-value code with subgrid-scale modelling agree with the growth rates from the theoretical models. Computations reveal the nonlinear evolution at late times, including wrapping and ring-rejection behaviour observed in experiments. A pair of co-rotating Gaussian vortices perturbed by noise develops elliptic instabilities, leading to the formation of vorticity bridges between the two vortices. The bridging is a prelude to vortex merger. Analytic, computational and experimental results agree well at circulation Reynolds numbers of order .

On cooperative instabilities of parallel vortex pairs

Since the 1989 publication of the original modeling approaches assembled into AGDISP, significant advances
have been made in the development and validation of spray drift models. The latest version of AGDISP (version 8.08) now
embodies the latest refinements to the computational engine first developed for NASA, improved by the USDA Forest Service,
and implemented by the Spray Drift Task Force and the U.S. Environmental Protection Agency into a regulatory version. This
article updates the previous summary of AGDISP, builds on a recent publication summarizing the regulatory version of the
model, and includes the most recent modeling additions of atmospheric stability effects, vortical decay, simple terrain
features, plant canopy, riparian barriers, and the aerial release of dry materials.

Technical advances in modeling aerially applied sprays

This paper focuses on full six degree-of-freedom (6-DOF) aerodynamic modeling of small UAVs at high angles of attack and high sideslip in maneuvers performed using large control surfaces at large deflections for aircraft with high thrust-to-weight ratios. Configurations such as this include many of the currently available propellerdriven RC-model airplanes that have control surfaces as large as 50% chord, deflections as high as 50 deg, and thrust-to-weight ratios near 2:1. Airplanes with these capabilities are extremely maneuverable and aerobatic, and modeling their aerodynamic behavior requires new thinking because using traditional stability derivative methods is not practical with highly nonlinear aerodynamic behavior and coupling in the presence of high propwash effects. The method described in this paper outlines a component-based approach capable of modeling these extremely maneuverable small UAVs in a full 6-DOF realtime environment over the full envelope that is defined in this paper to be the full ±180 deg range in angle of attack and sideslip. This method is the foundation of the aerodynamics model used in the RC flight simulator FS One. Piloted flight simulation results for four small RC/UAV configurations having wingspans in the range 826 mm (32.5 in) to 2540 mm (100 in) are presented to highlight results of the high-angle aerodynamics modeling approach. Maneuvers simulated include tailslides, knife edge flight, high-angle upright and inverted flight (“harriers”), rolling maneuvers at high angle (“rolling harriers”) and an inverted spin of a biplane (“blender”). For each case, the flight trajectory is presented together with time histories of aircraft state data during the maneuvers, which are discussed.

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Modeling full-envelope aerodynamics of small UAVs in realtime

The development of flight control systems to enhance aircraft safety during periods of vehicle impairment or degraded operations has been the focus of extensive work in recent years. Conditions adversely affecting aircraft flight operations and safety may result from a number of causes, including environmental disturbances, degraded flight operations, and aerodynamic upsets. To enhance the effectiveness of adaptive and envelope limiting controls systems, it is desirable to examine methods for identifying the occurrence of anomalous conditions and for assessing the impact of these conditions on the aircraft operational limits. This paper describes initial work performed toward this end, examining the use of fault detection methods applied to the aircraft for aerodynamic performance degradation identification and model-based methods for envelope prediction. Results are presented in which a model-based fault detection filter is applied to the identification of aircraft control surface and stall departure failures/upsets. This application is supported by a distributed loading aerodynamics formulation for the flight dynamics system reference model. Extensions for estimating the flight envelope due to generalized aerodynamic performance degradation are also described.

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Aircraft Flight Envelope Determination using Upset Detection and Physical Modeling Methods

Mitigation of the undesirable effects of trailing vortex wakes has been a long-standing priority for both reduction of submarine wake signature and alleviation of aircraft vortex wake hazard. A recent study established the feasibility of using relatively weak, secondary vortices with carefully selected unsteady amplitude and phasing to accelerate the breakup of the primary vortex system of a lifting surface, a technique denoted `vortex leveraging'. This paper will summarize progress on the development of SMA-actuated devices for implementing vortex leveraging for hydrodynamic applications. The methods being applied to the hydrodynamic design of these deformable Smart Vortex Leveraging Tabs (SVLTs) will be described, and the results of a preliminary assessment of SVLT performance in achieving wake breakup will be presented. Also, previous work on the design and testing of deformable control surfaces actuated via embedded SMA agonist wires will be reviewed and the design process being employed in the present applications will be discussed. Finally, the plans for near-term computational and experimental work to validate the use of SMA-driven devices for the wake mitigation task will be briefly outlined.

Implementation of vortex wake control using SMA-actuated devices

A flow control device generates counter-rotating vortices in the boundary layer of the flow in a supersonic inlet diffuser for an aircraft turbine engine. The flow control device comprises a flap attached to the duct wall for selective deployment, wherein it extends into the boundary layer, and retraction, wherein it lies substantially flush with the duct wall. In one embodiment an actuating mechanism comprising one or more shape-memory alloy wires moves the flap between two stable positions. In another embodiment the deployment height of the flap can be controlled as desired, preferably using a shape-memory alloy actuating mechanism. Typically, an array of plural flow control devices is disposed in the inlet duct for selective actuation according to a predetermined schedule.

Supersonic engine inlet diffuser with deployable vortex generators

Control of trailing vortex wakes is an important challenges for both military and civilian applications. This paper summarizes an assessment of the feasibility of mitigating adverse vortex wake effects using control surfaces actuated via Shape Memory Alloy (SMA) technology. The assessment involved a combined computational/design analysis that identified methods for introducing small secondary vortices to promote the deintensification of vortex wakes of submarines and aircraft. Computational analyses of wake breakup using this `vortex leveraging' strategy were undertaken, and showed dramatic increases in the dissipation rate of concentrated vortex wakes. This paper briefly summarizes these results and describes the preliminary design of actuation mechanisms for the deflectable surfaces that effect the required time-varying wake perturbations. These surfaces, which build on the high-force, high- deflection capabilities of SMA materials, are shown to be well suited for the very low frequency actuation requirements of the wake deintensification mission. The paper outlines the assessment of device performance capabilities and describes the sizing studies undertaken for full-scale Vortex Leveraging Tabs (VLTs) designed for use in hydrodynamic and aerodynamic applications. Results obtained to date indicate that the proposed VLTs can accelerate wake breakup by over a factor of three and can be implemented using deflectable surfaces actuated using SMAs.

Vortex wake control via smart structures technology

Fixed-vane vortex generators (VGs) have been in existence for over 50 years and are still among the most effective available flow control devices. However once such fixed VGs have been configured to improve performance in one regime, they often penalize the performance in other conditions. This paper will summarize results of an effort making VGs

Robert McKillip

Papers

Aircraft Flight Envelope Determination using Upset Detection and Physical Modeling Methods

Implementation of vortex wake control using SMA-actuated devices

Supersonic engine inlet diffuser with deployable vortex generators

Vortex wake control via smart structures technology

Development and Testing of Deployable Vortex Generators Using SMA Actuation