Separation Unsteadiness in Fin-Induced Shock-Wave / Boundary-Layer Interactions

Shock-wave / turbulent boundary-layer interactions are complex, unavoidable phenomena in high-speed flight vehicles which often result in localized regions of significantly elevated pressures and heating rates. Interactions between strong shocks and turbulent boundary layers can result in unsteady separated flows with associated undesirable outcomes on flight vehicle performance and survivability. The present work investigates the separation unsteadiness in two such interactions (a Mach 3 blunt-fin flow and a Mach 2 sharp-fin flow) using numerical simulations based on a detached-eddy simulation method, where a synthetic inflow turbulence technique was implemented to study the effects of flow unsteadiness in the incoming boundary layer. Because the detached-eddy simulation approach used in this work can produce a realistic flowfield in the absence of incoming disturbances, the approach is uniquely capable of demonstrating the role of incoming disturbances on separation unsteadiness.

In the Mach 3 blunt-fin induced interaction, large-scale, low-frequency motion of the separation shock was observed around St_D = 0.0268 (St_delta = 0.007), with and without synthetic inflow turbulence; the separation region in this flow behaved like a self-excited oscillator. Conditional averages of the data indicated that the low-frequency separation shock motion was induced by the dynamics of the separated flow region, supporting a downstream mechanism of separation unsteadiness. Nonetheless, significant correlation was found between the high-frequency motion of the shock and streamwise velocity perturbations of the incoming flow. A time-periodic, streamwise body force was simulated via momentum and energy source terms to inject small, controlled disturbances into the incoming boundary-layer flow; the wall-normal profile of the forcing amplitude was designed from conditional averages of the baseline flow, to emulate incoming flow disturbances that produced correlated separation motion. The frequency of forcing was found to have a significant effect on the phase-averaged response of the flow. With a forcing frequency representative of the low-frequency unsteadiness in the baseline flow, a strong response was obtained where the separation motion was phase locked to the imposed forcing. These findings showed that even in a flow that oscillates on its own, the separation motion can be modulated upstream forcing of a particular form.

As for the three-dimensional swept interaction induced by a sharp-fin in a Mach 2 flow, separation unsteadiness was found at a higher frequency range compared to nominally two-dimensional interactions. Moreover, the separation shock foot and separation position exhibited different unsteady characteristics; the fluctuations in the separation shock foot and separation position occurred around St_delta = 0.03 and St_delta = 0.1, respectively. Unlike the blunt-fin case, separation motion in the sharp-fin interaction appeared to be strongly driven by disturbances in the incoming boundary-layer flow; without these disturbances, the simulated flow was completely steady. The separated flow region demonstrated features consistent of a selective amplifier or damped resonator. Streamwise velocity fluctuations in the incoming flow significantly influenced the separation motion downstream and its influence appeared to persist to some extent with the crossflow in the interaction region. Simulations with upstream forcing demonstrated modulation of the separation unsteadiness, where different effects on the separation shock motion and separation position motion were obtained. Both the frequency and spanwise form of forcing significantly affected the phase-averaged response. For the tested parameters, the strongest response was obtained with a forcing frequency representative of the separation shock unsteadiness and a spanwise form that leveraged the swept structure of the sharp-fin flow.

As a whole, this work implies possibilities for flow control of separation unsteadiness in a range of shock-wave / turbulent boundary-layer interactions with different unsteady characteristics and varying dependence on upstream disturbances.