Performance enhancement of a vertical tail using synthetic jet actuators

Abstract

An investigation into the spanwise spacing of active jets demonstrated that, for some conditions, when the individual strength of the jets was held constant, larger jet spacing was more effective. Also, similar experiments were conducted when the total area–based momentum did not change. In this case, side force enhancement was greater when the spacing and individual area–based jet momentum were larger. Additionally, the jets’ skew angle was investigated; it was found that the most favorable angle, among the ones tested, was when the initial trajectory of the jets was perpendicular to the hinge–line.

Scaling was briefly studied by comparing results obtained on the 1/19th and 1/9th scale models. It was found that side force improvement for both models was similar when the orifices were scaled dimensionally (as opposed to non–dimensionally). Although it was not clear as to why, the data, in general, indicated that for the largest performance enhancement the area–based momentum that the synthetic jets generate should be maximized.

The Stereo PIV data also suggested that the outboard sides of the vortex rings were favorable for separation control, and the inboard sides might have had a detrimental effect. This provided a partial explanation to the effect of the orifices’ skew angle, and indicated that separation could perhaps be more efficiently controlled by altering the weighted contribution of the two sides of the vortex rings.

Additional Stereo PIV experiments, which captured a region corresponding to two to three jets, were conducted at two spanwise regions (inboard and outboard). In the outboard region, the jets were less capable of reattaching the flow, simply because the separation (without flow control) was more severe. In addition, in this region, increasing the number of active jets augmented the spanwise velocity and the crossflow streamwise vorticity near the rudder surface, whereas the opposite trend was obtained in the inboard region. This (presumably) resulted in jets having beneficial and detrimental effects on one another at the inboard and outboard regions, respectively, which was related to the effect of the spanwise jet spacing.

The performance enhancement of a vertical tail using an array of finite span synthetic jet actuators was investigated in wind tunnel experiments on 1/25th, 1/19th, and 1/9th scale models. The models were sweptback, tapered, and instrumented with multiple synthetic jets placed upstream of the rudder hinge–line with the objective of controlling the separation that commenced over the rudder when it was deflected to high angles. By controlling the separation, the side force generated by the tail was augmented, which may allow for the size of the tail to be decreased. This would reduce the weight, drag, and corresponding fuel costs of an airplane.

The synthetic jets augmented the side force coefficient by a maximum of 0.17 (34%). Furthermore, the performance of the vertical tail was dependent on jet strength. It was expected that the increase in side force would continue to grow as the momentum generation capabilities of the jets improved.

A primary goal of the research was to determine the effect of multiple synthetic jet parameters on side force enhancement. Experiments were conducted with various jet orifice configurations, which indicated the following: the spacing between adjacent orifice edges and the orifices’ aspect ratio were relatively unimportant; in terms of quantifying jet strength, the area–based (3–D) momentum coefficient of the jets was a more dominant parameter than the blowing ratio or the chord–based (2–D) momentum coefficient. Moreover, the most favorable spanwise location of control was, in general, found to be near the mid–span at lower rudder deflections and toward the inboard section of the model at larger deflections. The optimal chordwise location was hypothesized to be on the stabilizer as close as possible to the hinge–line.

Detailed stereoscopic particle image velocimetry (Stereo PIV) experiments explored the formation and interaction of a single synthetic jet with the crossflow near the mid–span of the rudder (and at a moderate rudder deflection). These experiments showed that without control, the flow separated at the rudder hinge–line, and on the rudder there was a significant spanwise velocity component oriented outboard. When a single jet was activated, a train of vortex rings were ejected from the jet’s orifice. They maintained their coherence over the stabilizer, and then rotated, warped, and began to breakdown over the rudder. The outboard sides of the vortex rings were the most resilient to the crossflow. Furthermore, the jet reattached the flow along and outboard of its trajectory. Specifically, it appeared to create a virtual wall, inhibiting the strong spanwise crossflow over the rudder.