Longitudinal vortices have tremendous practical utility for flow control (control by vortices) and in some cases have exhibited tremendous potential for causing harm if uncontrolled (i.e. control of vortices is required). Vortex control has thus far been carried out via multitudinous approaches in an empirical fashion, aided by the essentially inviscid nature of much of longitudinal vortex behaviour. Further refinement and several applications of vortex flow control require knowledge regarding the detailed flow physics of longitudinal vortices such as transition, transitional flow regimes, turbulence structure and modelling, and interaction with shock waves, other vortices and surfaces. This paper summarises vortex control applications and extant techniques for the control of longitudinal vortices produced by bodies, leading edges, tips, and intersections.

Longitudinal vortex control: techniques and applications

The effectiveness of various candidate aircraft-wing devices for attenuation of trailing vortices generated by large aircraft is evaluated on basis of results of experiments conducted with a 0.03-scale model of a Boeing 747 transport aircraft using a technique developed at the HYDRONAUTICS Ship Model Basin. Emphasis is on the effects produced by these devices in the far-field (up to 8 kilometers downstream of full-scale generating aircraft) where the unaltered vortex-wakes could still be hazardous to small following aircraft. The evaluation is based primarily on quantitative measurements of the respective vortex velocity distributions made by means of hot-film probe traverses in a transverse plane at selected stations downstream. The effects of these altered wakes on rolling moment induced on a small following aircraft are also studied using a modified lifting-surface theory with a synthesized Gates Learjet as a typical example. Lift and drag measurements concurrently obtained in the model tests are used to appraise the effects of each device investigated on the performance characteristics of the generating aircraft.

Evaluation of effectiveness of various devices for attenuation of trailing vortices based on model tests in a large towing basin

It is demonstrated that axial air injection into a core of a vortex can beneficially spread out the vorticity concentrated in it and prematurely age it. It is also shown that the phenomenon is more nearly governed by the momentum flux of injection than by mass flow.

Response of a trailing vortex to axial injection into the core.

Vortex flows produced by submersibles typically unfavorably influence key figures of merit such as acoustic and nonacoustic stealth, control effectiveness/maneuverability, and propulsor efficiency/body drag. Sources of such organized, primarily longitudinal, vorticity include the basic body (nose and sides) and appendages (both base/intersection and tip regions) such as the fairwater, dive planes, rear control surfaces, and propulsor stators/tips. Two fundamentally different vortex control approaches are available: (1) deintensification of the amplitude and/or organization of the vortex during its initiation process; and (2) downstream vortex disablement. Vortex control techniques applicable to the initiation region (deintensification approach) include transverse pressure gradient minimization via altered body cross section, appendage dillets, fillets, and sweep, and various appendage tip and spanload treatment along with the use of active controls to minimize control surface size and motions. Vortex disablement can be accomplished either via use of control vortices (which can also be used to steer the vortices off-board), direct unwinding, inducement of vortex bursting, or segmentation/tailoring for enhanced dissipation. Submersible-applicable vortex control technology is also included derived from various aeronautical applications such as mitigation of the wing wake vortex hazard and flight aircraft maneuverability at high angle of attack as well as the status of vortex effects upon, and mitigation of, nonlinear control forces on submersibles. Specific suggestions for submersible-applicable vortex control techniques are presented.

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Control of submersible vortex flows

The results of a recent wind tunnel test program to evaluate the effectiveness of the Tip Air Mass Injection (TAMI) system in modifying the blade tip vortex occurring during helicopter flight is described with attention to the effect of this modification on the impulsive noise. The measurement program is explained, and the correlation between experimental and predicted results is discussed. Topics considered include the effect of descent rate on noise pressure time histories, the effect of air mass injection on noise, and the analysis based on a dB(A) weighted approach. Impulsive noise generated by the interaction of a helicopter rotor blade and the concentrated tip vortex during forward flight descent is a primary contributor to acoustic annoyance as it draws early attention to the presence of the helicopter.

Some measured and calculated effects of a tip vortex modification device on impulsive noise

: An experimental program was conducted to investigate the modifications of a tip vortex which could be obtained by injecting the core of a tip vortex with a stream of air. Wind tunnel tests of an airfoil model were conducted. The results obtained from flow-visualization studies, balance data, and vortex-meter measurements show how the strength of the tip vortex can be reduced significantly by the injection of a linear mass flow of air into the core of the tip vortex. The reduction in drag obtained with injection is important since it allows the recovery of power required to inject the tip vortex core with mass flow.

Stephen A Rinehart

Papers

An Experimental Study of Tip Vortex Modification by Mass Flow Injection