Flow Structure Visualization nearAircraft Models in Low-Speed WaterTunnel (Aircraft Aerodynamic Configurations)

Aerodynamics and heat-exchange processes in flying vehicles


Аuthors

Golovkin M. A.*, Golovkina E. V.*

Central Aerohydrodynamic Institute named after N.E. Zhukovsky, TsAGI, 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

*e-mail: spintest@tsagi.ru

Abstract

Results have been given of a flow structure visualization in the neighborhood of a number of models of aircraft, such as MiG-23, MiG-29, Su-27, «Buran» orbital vehicle, delta-winged and forward swept wing aircraft models, and the models with intakes mounted on the upper surface of lifting body. The investigations were intended for revealing specific features of flows and vortex structures generated during flow over aircraft elements.

Model tests have been performed in TsAGI HT-150 and HT-400 very low-speed water tunnels (flow velocity of ) wherein the velocity is formed by the gravity of water flowing out from the large container disposed above into the working section. Flow visualization was accomplished by the method of colored jets. Colored liquid in the form of aqueous solution of aniline colors was fed to the model inner cavities through the sting «by gravity» due to altitude difference from containers disposed by ~3 meters higher the model level. The liquid flowed out through the drainage pinholes on the model outer surface and drifted by approach flow tangentially to the surface of the aircraft model. When required, the visualization of external flow was carried out using streamlets flowing out from the «comb», the thin tubes positioned upstream in front of the model. HT-150 water tunnel was equipped with the mirrors located on the right and on the left of its working section, which made it possible to take views or carry out video filming of three model projections simultaneously.

As a result of the conducted investigations some specific features were revealed of flows and vortex structures generated during flow over a number of aircraft models.

Visualization of flow over the MiG-23 aircraft model at a sweep angle χ=47°of wing outer panels at the leading edge showed that in the presence of sliding at the angles of attack α above 17° the windward vortex core generated by confluence of forebody and wing extension vortex cores, approaches the vertical tail and passes above it. Due to the strong downwash induced by the united vortex core in the vertical tail region, the rolling and yawing instability of the aircraft model develops. The leeward united vortex core shedding from the forebody and wing extension propagates along the right wing outer panel, which causes the still greater development of rolling instability at these angles of attack. As a consequence of the investigations on visualization a mechanism of yawing and rolling stability loss of the MiG-23 aircraft model confirmed in wind tunnels was revealed and the original ways of improving these aerodynamic characteristics were found.

Visualization of flow structure on the MiG-23 aircraft model with a sweep angle χ=74° at the leading edge of wing outer panels showed that the above-mentioned united vortex cores (windward and leeward ones) descend in the tip wing sections and pass at a considerable distance from the vertical tail. Therefore, the noted above degradation of lateral static stability characteristics for the model with χ=74° did not occur, that is confirmed by wind tunnel tests.

Investigations of flow structure in the neighborhood of the delta-winged model showed that the vortex cores generated nearby the wing leading edges slightly rise from the tip chord of vertical tail. In the presence of sliding the vortex structure is not as much skewed as in case of the MiG-23 model with χ=74°; as a result, one might expect that the lateral characteristics of such model must be satisfactory.

Visualization of flow over the forward swept wing model has revealed the following flow features. Tip vortex cores of the wing can be plainly seen. Due to the flow in the boundary layer of such wing directed from tip sections to the plane of symmetry up to the very large angles of attack, a greatly considerable (spanwise) part of the tip sections is flowed round without flow separation. Nearby the trailing edges of wing the vortex cores are generated which move to the plane of model symmetry. The vortex cores of wing extension can also be plainly seen, which unite with the said trailing edge vortex cores. Such vortex structure and the absence of flow-separation phenomena in the tip wing sections define good lateral characteristics of such model up to the very large angles of attack, which is confirmed by the model wind tunnel tests.

For the model with air intakes on the lifting body it is shown that the vortex cores generated on its upper surface, including if there is sliding, do not ingest into the engine intake.

Detailed investigations into visualization of flow over the «Buran» orbital vehicle model conducted over a wide range of angles of attack α and sideslip β have revealed the following main features. At α=15°, β=0 the sufficiently powerful vortex cores are visualized which are generated on the forebody and wing extension and passing at some distance from the fuselage. On the backside of cockpit canopy, an extensive separated flow is developed and two more other vortex cores are generated which move along the fuselage upper surface. In this case, the flow over the wing extension is the one without separation. As the angle of attack increases, the wing extension vortex cores lose the stability and their «burst» takes place (at α=25° it occurs about in the middle of the wing root chord and at α=30° it does immediately behind the wing extension) so that practically all wing outer panels are surrounded by separated flow. If there is sliding, the vortex structure is markedly warped so that the windward vortex core generated above the cockpit canopy and passing over the fuselage approaches the vertical tail and the leeward vortex core moves away from it.

During flow structure visualization nearby the MiG-29 and Su-27 aircraft models the following features are revealed. On the wing extensions of the models sufficiently powerful vortex cores are generated which uniting with the forebody vortex cores, pass nearby (inside) the tail panels. At α=25°the vortex cores «burst» in the vicinity of a quarter of wing chord. If there is sliding at α=15° the windward vortex core passes inside the right panel of vertical tail; the leeward forebody and wing extension vortex cores do not unite so that the wing extension vortex core passes inside the left panel of vertical tail and the forebody one does nearby the trailing edge of left wing panel. At α=20° and in the presence of sliding the vortex cores «burst» before reaching the vertical tail, with the leeward vortex core «bursting» up-stream of the windward one. At α=20°, β=4° the vortex cores «burst» in the vicinity of the quarter of root chord of wing. Drooped leading edges on all said conditions are flowed round without flow separation.

The results on visualization found use in improving aerodynamics of the said aircraft, in analysis of force tests in wind tunnels, and may be useful in developing other aircraft.

In addition to wind tunnel tests, investigations into the flow visualization nearby the aircraft models in the low-speed water tunnel are the high-performance and low-cost method for revealing features of flow over aircraft models.

Keywords:

water tunnel, method of colored jets, vortex structures, wing extension

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