Figure 9 - uploaded by Bojan Vukasinovic
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The forebody tip () and the CW () and CCW () vortex centroid positions at x/D = 9 during the model pitch up motion during the stable (a, = 49˚-49˚-50˚) and unstable (c, = 51˚-51˚-52˚) body coupling. The corresponding positive (magenta) and negative (green) angular orientation of the vortex pair is shown for the stable (b) and unstable (d) coupling.
Source publication
View Video Presentation: https://doi.org/10.2514/6.2021-2610.vid The aerodynamic instability of the inclined slender cylindrical model (L/D = 11) with an ogive forebody is investigated experimentally at high angles of attack, α = 45° to 60°. The interaction between asymmetric forebody vortices at high incidence induces bi-directional net side force...
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... each vortex pair is connected with the green line when its slope is negative and with the magenta line when positive. In addition, a subset of the yaw ' and the vortex orientation angle evolution over time is shown in Figures 9b and d, during the stable and unstable body coupling, respectively. The model stable response in reflected by the nearly invariant forebody tip realizations over time, as seen in Figure 9a. ...
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... addition, a subset of the yaw ' and the vortex orientation angle evolution over time is shown in Figures 9b and d, during the stable and unstable body coupling, respectively. The model stable response in reflected by the nearly invariant forebody tip realizations over time, as seen in Figure 9a. Furthermore, it is seen in the same plot that both CW and CCW vortex realizations remain tightly clustered, having the same tilt at all times. ...
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... it is seen in the same plot that both CW and CCW vortex realizations remain tightly clustered, having the same tilt at all times. This state is further quantified in Figure 9b, where nearly invariant ' is measured over this subset in time. At the same time, the vortex pair slope also remains nearly invariant at about = -45. ...
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... the same time, there is much wider scatter of the vortex cores, where the vortex pairs orientation is not preserved but rather switches in bimodal fashion, alternating between the positive and negative slopes. This is further quantified in Figure 9d. It should be noted that some vortex detections, especially during the unstable motion, do not reliably result in the detections of both vortices and such are discarded; hence there are some dropouts in the time evolutions of the vortex pair slopes in Figure 9d. ...
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... is further quantified in Figure 9d. It should be noted that some vortex detections, especially during the unstable motion, do not reliably result in the detections of both vortices and such are discarded; hence there are some dropouts in the time evolutions of the vortex pair slopes in Figure 9d. Nonetheless, it can be seen in these time evolutions, relative to the model yaw, that the switching of the vortex pair orientation (i.e., the sign) occurs about extrema of the model yaw deflections, where the vortex pair temporarily switches its orientation on the approach to the peak excursion and then subsequently reverts back. ...
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... the vortex structure in the mean suggests that there is a bias of the leading CW vortex in the wake, implying that there is a bias in the positive net side force during this unsteady motion. Knowing that the forebody vortices naturally coupled to the stable body response are stable themselves (e.g., Figure 9a), it is expected that the flow control stabilized body response would also couple to the stable dominant vortices. ...
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... to the analysis of the switch between the stable and unstable body state during the pitch up motion (Figure 9), analysis of the body and vortex pair timedependent coupling for the uncontrolled and controlled cases shown in Figure 13 is shown in Figure 14. (Figure 14d) case. ...
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... the unstable body coupling, forebody tip's trajectory indicates predominantly yaw deflections accompanied with some pitch variations, while the vortex pair orientation switches between the two states, although its orientation is biased towards the positive tilt (Figure 14a). The difference relative to the pitch-up unstable mode (Figure 9b) is in the preferred switch of the vortex pair tilt, which occurs only on one excursion side in the model yaw, as indicated in Figure 14b. As the body becomes stabilized by the flow control, both the forebody tip (Figure 14c) and the vortex pair tilt become stabilized, having the yaw of about ' = 0 and the tilt angle of = 20. ...
Citations
... Besides the direct forebody vortex pair control over their initial domain, indirect control approach was proposed by Lee et al. (2021a), utilizing a synthetic jet to control the near wake at the forebody juncture, indirectly affecting the forebody vortex pair evolution and subsequently altering resulting net side forces. In a companion investigation (Lee et al., 2021b), this indirect control of forebody vortex evolution was extended to suppress instabilities of the axisymmetric model within a narrow range of high angles of attack that triggered the model unstable response. ...
... The present work particularly focuses on the control-induced aerodynamic states of the body at high angles of attack, regardless of what such states would be realized in the absence of control. Particularly, this is motivated by an apparent random orientation of the naturally-evolving vortex/flow asymmetry, which was documented in literature (e.g., Hunt 1982, Porter et al. 2012, and Mahadevan et al. 2018) and in the prior work by Lee et al. (2021aLee et al. ( , 2021b. Hence, the present work focuses on the tailored realization of bi-directional aerodynamic load realizations for introducing the flow asymmetries of preferred orientation, including the enforced symmetry that results in suppression of the side loads. ...
Dynamically-controlled aerodynamic bleed across the segmented surface of the forebody of a cylinder platform model at high angles of incidence (30⁰ < α < 60⁰) is explored for manipulation of the aerodynamic side forces and can either null or effect a desired side load. It is shown that localized interaction of the bleed with the flow near the forebody's front stagnation point forces flow symmetry about the cylinder and a nearly balanced resultant side force, while bleed interactions with the formation of each forebody vortex results in its premature displacement away from the cylinder while the other vortex remains in close proximity to surface and bends towards the axis of symmetry. Switching the bleed actuation about the azimuthal point of symmetry can lead to rapid variations in the side force coefficient between C s -3.5 to 3.5 within azimuthal bleed orientations of 15. It is shown that the evolution of the forebody vortex pair trajectories and their circulations are good indicators of the magnitude and direction of the resultant side force. The present results indicate that typical random side load on an axisymmetric body at high angles of incidence can be overcome by controllable azimuthal aerodynamic bleed.
... An indirect control of the forebody vortices was proposed by Lee et al. (2021a) who used a synthetic jet to control the wake at the forebody juncture and thereby alter the resulting net side forces. In a follow-on investigation, Lee et al. (2021b) extended this indirect forebody vortex control to suppress instabilities of the axisymmetric model at a narrow subrange of the high angles of attack. ...
... The axisymmetric body is comprised of three major modules: the ogive forebody, the central cylindrical body, and the aft control module, which is kept inactive in the current investigation. The prior work (Lee et al., 2021a and2021b) focused on direct control of the separating flow on the leeward side of the body, which, in turn, altered the dominant wake vortical composition through its inherent coupling. During the present investigation, the aerodynamic loads on the model are controlled by manipulation of the symmetry of the forebody vortices and, consequently, their interactions with the cylinder's wake using the aerodynamic bleed over the forebody. ...
... The present work particularly focuses on the forebody vortex pair evolution over the body, while the prior work (Lee et al., 2021a and2021b) placed emphasis on the wake interactions of these vortices, both in the presence and absence of flow control. To illustrate the base flow over the model with all the bleed ports closed, the color raster plots of the streamwise vorticity relative to the body coordinate system (x') at five cross stream planes along the model are shown in Figure 4 for the pitch angle = 50. ...
