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

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

Title of Document: ACTIVE JET ACOUSTIC CONTROL OF LOW FREQUENCY, IN-PLANE HELICOPTER HARMONIC NOISE D. Caleb Sargent, Doctor of Philosophy, 2012 Directed By: Professor Fredric H. Schmitz Department of Aerospace Engineering A new approach to reducing low frequency, in-plane harmonic noise of helicopter rotors is explored theoretically and experimentally in this dissertation. The active jet acoustic control methodology employs on-blade, tip located unsteady air blowing to produce an acoustic anti-noise waveform that reduces or cancels the observed noise at targeted positions in the acoustic far-field of the rotor system. This effectively reduces the distance at which the helicopter rotor can be aurally detected. An extended theoretical model of the subsonic air jet, which is modeled as both a source of mass and momentum, is presented. The model is applied to a baseline, fullscale, medium weight helicopter rotor for both steady and unsteady blowing. Significant reductions in low frequency, in-plane harmonic noise are shown to be possible for the theoretical rotor system by using physically reasonable unsteady jet velocities. A new model-scale active jet acoustic control experimental test rotor system is described in detail. Experimental measurements conducted in the University of Maryland Acoustic Chamber for the ~1/7 rotor, operated at a full-scale hover tip Mach number of MH = 0.661, indicate that active jet acoustic control is a viable option for reducing low frequency, in-plane harmonic noise. Good correlation between theoretical predictions and measured data for four valve control cases are observed in both the time and frequency domains. Model-scale limitations of the tipjet blowing experiment limited the peak noise level reductions to 30%. However, theory suggests that if the limitations of the model-scale controller are mitigated, much larger noise reductions are possible. ACTIVE JET ACOUSTIC CONTROL OF LOW FREQUENCY, IN-PLANE HELICOPTER HARMONIC NOISE

Active jet acoustic control of low frequency, in-plane helicopter harmonic noise

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

: Numerous research efforts have been conducted by different investigators to alter the characteristics of the tip vortex generated by a helicopter blade in order to alleviate the blade-vortex interaction problem as well as the noise problem associated with impulsive loading. The analytical investigation shows that it should be possible to significantly alter the characteristics of the trailing tip vortex for all flight conditions in a beneficial manner by injecting an airstream directly into the forming tip vortex. Analytical expressions were developed for the initial and final states of the vortex in order to evaluate the effects of mass flow injection on the vortex strength, swirl velocity distribution, vortex core pressure, vortex core size and the induced drag on the blade. On the basis of the results that were obtained, it was shown that the required mass flow may be obtained from centrifugal pumping action by venting the blade and therefore the desired modification can be obtained apparently without significant performance penalties which would be unacceptable. (Author)

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0704804&Location=U2&doc=GetTRDoc.pdf

Stephen A. Rinehart

Papers

Study of modification of rotor tip vortex by aerodynamic means