Shock-related effects on aero-optical environment for hemisphere-on-cylinder turrets at transonic speeds

Abstract

The aero-optical environment around a hemisphere-on-cylinder turret with both flat and conformal windows was studied experimentally in flight using the Airborne Aero-Optical Laboratory-Transonic for a range of subsonic and transonic Mach numbers between 0.5 and 0.8. Above M=0.6, the local shock appeared near the top of the turret, causing additional aero-optical distortions at side-looking angles. Using time-resolved wavefronts, instantaneous shock locations were extracted and analyzed. The mean shock location was found to be near a viewing angle, α=80 deg for both window types at M=0.7 and 0.8. For M=0.8, the shock has a single frequency peak at St=0.15, the same as for the unsteady separation line, indicating a lock-in mechanism between the shock and the separated wake region. Analysis of aero-optical distortions in the wake indicated that the wake dynamics were beginning to be affected by the shock only at high transonic speed of M=0.8 for the conformal-window turret.

Modal Analysis of the Surface Pressure Field Around a Hemispherical Turret using Pressure Sensitive Paint

“…This suggests that these surface pressure structures are related to the vortical shear-layer structures, formed when the flow separates off the turret. It is consistent with the findings that aero-optical distortions in the wake, thought to be primarily due to shear-layer structures, are dominant at this Strouhal number [13]. Additionally, at this Strouhal number, there are no shock-related motions present.…”

“…Second, smaller turbulent structures near the turret base have been observed near St = 1 [27]. This higher frequency range was also associated with the most optically-aberrating structures in the turret wake [13]. Applying DMD technique, we also observed a local increase in the DMD modal energies at St = 0.15-0.2, which is close to the expected motion of the separation line and the shock movement peak at St = 0.15, see Figure 13.…”

“…To predict these shock properties, the flow dynamics must be better understood. Optical and pressure measurements in both flight and tunnel tests indicate that there is a coupling between the shock and wake motion [13,14,15,16,17].…”

Conditional Studies of the Wake Dynamics of Hemispherical Turret Using PSP

“…Mode # 2, in addition to having the main peak at StD = 0.2, also has additional peaks near StD = 0.3. Similar distinct peaks at the same range of the normalized frequencies were observed in wavefront spectra in-flight for side-viewing angles for M = 0.7 for both the hemisphere-on-cylinder [13] and hemisphere-only turrets [6] and were contributed to the American Institute of Aeronautics and Astronautics unsteadiness of the shock near the apex of the hemisphere. Mode # 2 at this Mach number is essentially non-zero only in the narrow region near apex of the hemisphere, see Figure 5, middle bottom plot, confirming that it is mostly related to the unsteady shock.…”

“…Al low subsonic speeds, the flow around the hemisphere is fully subsonic. At Mach numbers above 0.55, the flow over a hemisphere becomes locally supersonic, with a terminating unsteady shock forming near the apex of the hemisphere [6]; the shock oscillates within a few degrees of the apex [12][13][14]. Due to high density gradients associated with the shock it results in an increased amount of aero-optical distortions at side-looking angles.…”