Investigations of shock–boundary layer interaction dynamics using high-bandwidth pressure field imaging

Abstract

The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.

Spanwise unsteadiness in the sidewall-confined shock-wave/boundary-layer interaction

“…Moreover, a notable region with negative coefficient is found in the separation bubble just downstream of the reference location, which is similar to the findings reported by Jenquin et al. (2023) and implies the possible relationship between the intermittent region and the bubble. The space–time correlation coefficient distributions along the spanwise line of mm are shown figure 5( b ).…”

“…Interestingly, spanwise propagating pressure perturbations are also identified in the intermittent region in an inward-turning axisymmetric compression ramp (Jenquin et al. 2023), which implies the broadness of confinement effect (caused by concave surface in their work) on the separation shock unsteadiness.…”

“…In contrast, fast PSP measurements are a promising technique owing to their spatial and temporal advantages for fully resolving the large-scale unsteadiness in 3-D flows (Peng & Liu 2020), for example, the unsteadiness in various SBLIs (Running & Juliano 2021; Liu et al. 2022 b ; Jenquin, Johnson & Narayanaswamy 2023).…”