Flow physics and control for improved tailless vehicle aerodynamics via leading-edge vortex manipulation

Rojas Carvajal, Tomas, Emilio
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Gandhi, Farhan
Hicken, Jason
Letchford, Christopher
Amitay, Michael
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Aeronautical engineering
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This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
Full Citation
The use of active flow control using finite-span synthetic jet actuators to affect the aerodynamicloads on a generic tailless chined forebody delta wing was examined experimentally in multiple stages performed in two different wind tunnels. First, the flowfield around the generic model having a chined forebody and a simple delta wing configuration was measured at different angles of attack, with or without a yaw angle. This was done using Oil Flow Visualizations (OFV) and Stereoscopic Particle Image Velocimetry (SPIV) at a mean chord-based Reynolds number of 2.3 x 10^5. The detailed flowfield measurements, using SPIV, were conducted for the cases where the angles of attack were 20° and 30°, and yaw angles of 0° and 5°. The flowfield over the model was seen to exhibit pairs of leading edge vortices similar to the flowfield around a double delta wing except for the influence of the fuselage, particularly in the forebody region where the chine vortex formed over the convex portion and followed its curvature. The development and interaction of the chine and wing vortices were measured and analyzed. It was found that the downstream evolution of these vortices and their interaction depended on the angle of attack and yaw angle. Increasing the angle of attack resulted in wake-like vortices while increasing the yaw angle yielded a wake-like vortex on the windward side and a jet-like vortex on the leeward side. The interaction of the windward side vortex with the physical barrier of the forebody surface was observed to greatly affect its behavior and the interaction downstream. In some combinations of angle of attack and yaw angle, the merged vortex exhibited a breakdown. The analysis of the flowfield and vortex dynamics served to provide insight into manipulating the flowfield using physics-based flow control. Based on the full-model flowfield, the effect of flow control was examined using a half-model with removable forebodies, each of which was equipped with a pair of synthetic jets located as close as possible to the leading edge. Three different jet orientations were explored, one employing surface-normal SJs, one employing horizontal synthetic jets, and one employing SJs Angled 45° away from the leading edge. Apart from a baseline for comparison taken with the jets' orifices covered, six cases were explored with actuation, one with each jet individually actuated and one with both jets actuated together with and without the pulse modulation at the helical mode frequency. In all cases, the synthetic jets were activated with ?b = 1.667 (?mu = 7.15 ∗ 10^-5 per jet). Tuft visualizations were used to qualitatively compare the flowfield with that of the full model. Then, aerodynamic load measurements were conducted to explore the effects of flow control. These measurements were followed by detailed flow field measurements using SPIV to shed light on the reasons for these effects. It was found that the surface-normal jets had a much larger effect on the aerodynamic coefficients, especially the lift, than the other two orientations. The maximum calculated increase in drag for the surface-normal jets was around 16% from ? = 28° to ? = 32° whereas modest, single-digit percentages in lift occur for the other two orientations. Therefore, the chined forebody with the surface normal synthetic jets was chosen for detailed flow measurements. Since the increased lift is a consequence of increased vortex lift, it was accompanied by an increase in vortex drag. The reason for the difference between the jet orientations was also investigated. It was observed that, despite causing a local increase in the forebody vortex circulation, the horizontal jets blew close enough to the chine that they negatively affected the feeding of the vortex by the shear layer, causing a low-velocity region over the leading edge that decreased the vortex lift. On the other hand, the upwards-oriented jets presented the opposite behavior, blowing too close to the vortex core and not strengthening the vortex to the same degree. This also caused the SJ to impinge on the core, pushing it away from the surface and also decreasing the vortex lift. The surface-normal jets yielded the largest performance enhancement by adding circulation to the vortex while simultaneously reducing its distance to the surface and the leading edge. The jets acted in a quasisteady manner, and since the chine vortex acts as an oscillator at the helical mode frequency, the interaction between the jet and the chine vortex locks to this frequency, causing the effects to superpose. The enhanced chine vortex induced a larger velocity on the wing vortex, causing an earlier merge of the two vortices, and also a more jet-like merged vortex downstream. This resulted in an increase in nose-down pitching moment, increased lift, and increased drag, all of which are desired for takeoff and landing. In addition, the synthetic jets were actuated using a pulse-modulated waveform, where the modulation frequency was near the helical mode frequency. This made the actuation much more energy-efficient as, opposite to the non-modulated actuation, the pulse modulation near the helical mode frequency caused the helical mode to lock onto the actuation frequency. This meant the addition of momentum from the jets to the vortex is approximately the same as without pulse modulation, despite the jets being off during parts of the actuation cycle. Furthermore, when using pulse modulation, actuating using either jet (or both jets) produced the same overall result. This was because the excited helical mode is a global instability mode, and the slightly different location of the jets is unimportant to the overall effect of the jets. Performing triple decomposition of the velocity vectors to estimate the vorticity transport equation using the timeresolved and phase-averaged data showed that this addition of circulation, core velocity, and vortex lift primarily affected the mean values. The spectral behavior of the modes associated with the vortical motion was seen to be unaffected beyond the appearance of a peak at the pulse modulation frequency and the actuation decreased the wandering. In parallel, the flowfield associated with the interaction of a synthetic jet with an isolated induced vortex over a flat plate was explored to aid in understanding the mechanisms at play in the chined forebody-delta wing model. It was seen that the train of vortex rings produced by the SJ at skewed and pitched angles to the crossflow broke down into smaller-scale structures that interact with each other to generate a single streamwise vortex downstream. The vortical structures generated by the synthetic jet were seen to strengthen the induced streamwise vortex (generated by a vortex generator) passing over the jet at the correct location and distance. In addition, it was seen that a streamwise vortex passing over the middle of the angled jet could be pulled closer to the wall by the induced velocities caused by the vortex rings passing under it. This was also seen over the second jet on the actuated chined forebody. The present work focused on active flow control via synthetic jets. Future work can explore the use of passive control via surface-mounted, low-aspect-ratio cantilevered circular pins. Therefore, the flowfield associated with chamfered pins when immersed in a laminar boundary layer was analyzed. These pins were originally considered for implementation into the chined forebody as an option to the finite-span synthetic jets but were not selected in the end. Two chamfered pins, where the chamfer encompassed either half of the pin's planform or its full planform, were analyzed with the chamfer at various skew angles with respect to the freestream and were compared to a pin without a chamfer. All pins exhibited a complex flowfield, including an array of streamwise vortical structures. The chamfered pins resulted in two additional counter-rotating streamwise vortices, named Chamfered Induced Vortices (CIVs). It was shown that changing the skew angle resulted in a change in the strength of these vortical structures, their direction of rotation, and as a result, net circulation produced. Comparing the two chamfered pins, the pin where the chamfer encompassed half of its planform produced stronger CIVs. These effects are discussed in detail to provide insight into a future use of these pins as flow control devices.
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