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Numerical simulations were conducted to investigate dynamic instability of a Japanese lift-type reentry capsule, which is named H-II transfer vehicle Recovery Vehicle (HRV), at Mach 1.2 by comparing two capsule shapes of an original HRV model and a model with a different shoulder angle of 10°. The 10° model has a thicker aft-body with a smaller shoulder angle than the original model. A previous pitch-direction One-Degree Of Freedom (1DOF) experiment revealed that the original model was dynamically stable, whereas the 10° model was dynamically unstable at Mach 1.2. So as to reproduce the same trends as the experiment, a high-order numerical scheme and zonal detached eddy simulation were employed. In fixed angle simulations, we found that the static longitudinal stability of the 10° model is higher than that of the original model in contrast with the observed dynamic stability. 1DOF free oscillation simulations successfully reproduced the same trends as the experiment. We elucidated that the main factor of the dynamic instability of the HRV-type capsule is a hysteresis of a boundary layer separation on the upper side of the capsule. The hysteresis is induced by a variation of momentum in a boundary layer due to capsule motion. We also found that for HRV-type capsules, the lower the static stability within the statically stable range, the higher the dynamic stability might be. In addition, on the lower side, a sign of the work exerted by surrounding flow is different between the original model and the 10° model. A dynamic mode decomposition analysis indicated that the vortex structures on the capsule wake might induce this difference.
Okano et al. (Mon,) studied this question.