Dynamics of Single and Twin Circular Jets in Cross Flow

Propulsion induced effects encountered by moderate- to high-disk loading STOVL or VSTOL aircraft out-of-ground effect during hover and transition between hover and wing-borne flight are discussed. Descriptions of the fluid flow phenomena are presented along with an indication of the trends obtained from experimental investigations. In particular, three problem areas are reviewed: (1) the performance losses sustained by a VSTOL aircraft hovering out-of-ground effect, (2) the induced aerodynamic effects encountered as a VSTOL aircraft flies on the combination of powered and aerodynamic lifts between hover and cruise out-of-ground effect, and (3) the aerodynamic characteristics caused by deflected thrust during maneuvering flight over a wide range of both angle of attack and Mach number.

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Propulsion-induced effects caused by out-of-ground effects

This Masters research is broadly divided into two sections: the first is a study of single jets in crossflow and the second is a study of a twin jet configuration in crossflow. The single jet tests are further divided into those conducted with circular nozzle injectors and those with noncircular nozzle injectors. The circular nozzle injectors were tested in confined and unconfined flow conditions. Also, the diameter and the blowing ratio for the circular nozzle injectors, in confined and unconfined flow, were manipulated to study their effects. The noncircular nozzle injectors consisted of a triangular and a bluff geometry, each aligned in two different orientations with respect to the crossflow, which were studied and compared to the baseline circular nozzle at the same conditions. In the second main study on twin jets, the interaction between the two jets was investigated and the spacing between the injectors as well as the blowing ratio were manipulated to test their effects. All experimental work is done using the Particle Image Velocimetry (PIV) flow diagnostics in subsonic windtunnels at various UC facilities. The behavior of the jet in crossflow in the single jet tests was characterized using the following parameters: penetration and trajectory, decay of mean centerline velocity, spreading rates of the windward and leeward jet boundaries, formation of the region of reverse flow, an approximation of the mass entrainment characteristics based on the trajectory of the jet, and an analysis of the turbulent kinetic energy of the flowfield as generated by the behavior of the jet. It was found that the penetration and the size of the reverse flow region grow with increasing blowing ratios. These effects are further magnified for increasing jet diameter. For the noncircular injector tests, additional analysis was performed pertaining to large scale structures as influenced by the properties of the boundary layers shed from the various walls of the injector nozzle. This incorporated an investigation of the vortices generated on the surfaces of the jet as well as in the flowfield. It was found here that jets injected from high aspect ratio nozzle geometries penetrate the least while those with low aspect ratios penetrate the highest into the crossflow. The curvature of the nozzle walls also has an effect on the outcome of penetration and other parameters. For twin jet tests, it was found that the presence of a second jet upstream of another jet generates a shielding effect that enhances the penetration of the rear jet or shielded jet. This shielding effect also results in a large region of reverse flow forming between the two jets compared to that formed leeward of a single jet at the same conditions. Also, the reverse flow region of leeward of the rear jet is considerably reduced due to the acceleration of the crossflow fluid as it maneuvers about the extra blockage of the front jet.

An experimental study of single and twin transverse jets in subsonic crossflow

The interaction between engine exhaust jets and the freestream affects the aerodynamic and stability characteristics of VTOL aircraft during the transition from hover to forward flight. This interaction is often modeled as a simple uniform jet issuing from a flat plate into a subsonic crossflow. The distribution of pressures induced by the jet on the surface of the plate can be used to predict the lift loss and pitching moment for a full-scale aircraft. The uniform jet model has limitations because an actual turbofan engine generates a rather nonuniform exit velocity profile. The purpose of this work is to study the effect of a nonuniform velocity profile on the surface pressure distribution. The mutual interaction of dual jets also is investigated in side-by-side and tandem configurations. Detailed pressure distributions are presented for two jet-to-freestream velocity ratios of 2.2 and 4.0. One important finding is that a nonuniform jet with a high velocity periphery and a low velocity core has a higher effective velocity ratio than a uniform jet with the same mass flow.

C. L. Moore

Papers

Effects of non-uniform velocity profiles on dual jets in a crossflow