Abstract
This study investigates the flow structures behind an atmospheric entry capsule at Mach number 0.4 through an improved detached eddy simulation and a modal analysis. The simulated flowfields reveal relatively low-frequency peaks of St ≈ 0.016 and St = 0.17–0.2 in the aerodynamic coefficient variation, where St is the nondimensional frequency. Then, the dominant fluid structures that cause the frequency peaks are identified through dynamic mode decomposition and the compressive-sensing-based mode selection method. Many of the dominant fluid phenomena have a frequency of St ≈ 0.2. In this frequency range, the fluid phenomena are mainly characterized with a large-scale vortex shedding separated from the capsule’s shoulder part and with a helical fluid structure in the wake. Moreover, the variation in the lift coefficient of the capsule is mainly attributed to the large-scale vortex shedding phenomenon. Furthermore, a fluid phenomenon at a frequency of St = O(0.01) is found, which describes the pulsation, or periodic growth or shrinkage, of the recirculation bubble, accompanied by pressure fluctuation behind the capsule that exerts a large drag fluctuation of the capsule. Additionally, this phenomenon seems related to the dynamic instability phenomena of the capsule, as indicated by its time scale, which is close to that of the capsule’s attitude motion.
Highlights
Atmospheric entry capsules tend to be dynamically unstable at a wide range of subsonic and supersonic speeds.1–5 For example, the pitching angle of the capsule oscillates at a limit-cycle state, or its oscillation amplitude grows and diverges
With respect to the dynamic instability of the atmospheric entry capsule, if the Strouhal number is defined as St = fD/U∞ using capsule diameter D, uniform flow velocity U∞, and frequency f, the typical oscillation frequency of the capsule observed in the experiment is St = O(0.01)
Using a mode selection method based on compressive sensing,20 we identify dynamic mode decomposition (DMD) modes representing dominant fluid phenomena and clarify the fluctuation of aerodynamic forces these fluid phenomena give to the capsule
Summary
Atmospheric entry capsules tend to be dynamically unstable at a wide range of subsonic and supersonic speeds. For example, the pitching angle of the capsule oscillates at a limit-cycle state, or its oscillation amplitude grows and diverges. The pitching angle of the capsule oscillates at a limit-cycle state, or its oscillation amplitude grows and diverges. As such properties largely relate to the success or failure of a mission, they should be designed to accurately predict the dynamic instability of the capsule, a phenomenon assumingly attributed to the near-wake fluid phenomenon of the bluff-body-shaped capsule. Past studies on atmospheric entry capsules have not reported fluid phenomena at a frequency on the order of St = O(0.01) they have generally confirmed the vortex shedding phenomena of St = O(0.1) and separated shear layer instability of St = O(1.0)
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