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  1. The fluid-structure interaction (FSI) of a compliant panel under a compression-ramp-induced shock/boundary-layer interaction (SBLI) has been studied in Mach 2 flow. Simultaneous high-speed measurements of the velocity field and the panel displacement were conducted using 50 kHz particle image velocimetry (PIV) and 5 kHz stereoscopic digital image correlation (DIC). The mean effect of the panel displacement has been evaluated by monitoring the change in velocity profiles along the streamwise direction (x), upstream of the separated flow region. Streamwise (u) velocity near the panel surface has been shown to change its magnitude in response to the wall shape. Furthermore, the strong cross-correlation between fluctuations of the wall-normal panel displacement and the transverse (v) velocity can be explained by the flow remaining tangent to the wall surface as the panel deforms. This latter result is consistent with the panel motion being sufficiently low frequency compared to flow convective time scales that the flow is quasi-steady In addition, assessment of the correlation between the separation shock position and panel displacement seems to suggest that when the panel is bulged down (concave up) at the downstream end of the panel, a larger separated flow is generated and the shock moves upstream. This observation remainsmore »speculative, but is consistent with the flow undergoing greater compression for the bulged down case.« less
    Free, publicly-accessible full text available July 1, 2023
  2. This work investigates surface pressure unsteadiness on a compliant panel under a shockwave/boundary-layer interaction (SBLI) induced by a 2D compression ramp with an angle of 20o in a Mach 2 wind tunnel. High-speed digital image correlation (DIC) and fast-response pressure-sensitive paint (PSP) measurements are used to measure the panel displacement and panel and ramp-face surface pressure fluctuations at 5kHz and 20kHz, respectively. The data reduction technique of POD (proper orthogonal decomposition) was employed both for pressure and displacement fields. POD mode distribution for the pressure fields reveals that the first six modes have 60% of the total energy and exhibit low-frequency content for both rigid and compliant panels. The vibration of the compliant panel was seen to alter the energy distribution of the high energy modes as compared to the rigid panel case. The cross-correlations between the displacement and pressure modes were made using the time coefficients. This analysis shows significant correlations were present among the lower modes. The highest correlation was between displacement mode 1 and the pressure mode 4, which stemmed from the upstream of the intermittent region. The analysis was also made for the surrogate shock foot and reattachment lines. The correlation shows that panel vibration lowersmore »the correlation between the shock foot and reattachment line when compared with the rigid panel case.« less
  3. A thin compliant panel was tested in a Mach 2 wind tunnel. The panel was mounted flush with the tunnel floor and was of dimensions L=121.9 mm (chord), W=63.5 mm (span) and h=0.254 mm (thickness). A 20 degree compression ramp was placed 5 mm downstream of the model, which induced a shock/boundary layer interaction with fully separated flow over parts of the panel. Full-field deformation was measured using Digital Image Correlation and the surface pressure field was obtained from fast-response pressure-sensitive paint. Analysis of the shock foot motion was performed using a curve-fitting method. Comparison of the shock motion between a rigid and compliant panel case showed no difference in the size of the intermittent region but found that the shock motion over the compliant panel is affected by the panel vibration. Proper Orthogonal Decomposition revealed that the surface pressure is dominated by low-frequency unsteady shock motion, in both cases (rigid and compliant panel). The sixth POD mode clearly shows the streamwise shock foot motion oscillates at the first panel vibration frequency. The surface pressure field upstream of the shock foot is dominated by piston-theory aerodynamics and thus correlated to the slope of the compliant panel. The Sparse Identification ofmore »Nonlinear Dynamic Systems algorithm was employed to find low-order representations of the system dynamics. Linear stiffness matrices could be consistently recovered. The measurement noise however prevented extraction of additional relations, such as linear damping matrices or forcing terms from the surface pressure.« less
  4. This experimental study focuses on fluid-structure interaction (FSI) for a thin compliant panel under a shock/boundary layer interaction (SBLI) generated by a 2D compression ramp in a Mach 2 wind tunnel. In previous work, we have studied the FSI for this configuration using simultaneous fast-response pressure-sensitive paint (PSP) and digital image correlation (DIC). Simultaneous PSP/DIC allows for examination of the relationship between the dynamic panel displacement and surface pressure loading, respectively. Spectral analysis showed that pressure fluctuations within the interaction region and shock-foot unsteadiness tend to lock to the first mode resonant frequency of the compliant panel. The current study aims to utilize synchronous high-speed stereoscopic PIV (25 kHz) and DIC (5 kHz) techniques to better understand the coupling between the flow field and the panel displacement field. The PIV is obtained in a streamwise-spanwise plane located at 15% of the boundary layer height. Thin compliant polycarbonate panel with thicknesses of 1 mm is utilized, which has a first-mode vibrational frequency of 407 Hz. The 1 mm panel out-of-plane displacement amplitude was up to 15% of the boundary layer thickness. The analysis includes low-pass and band-pass filtering of the velocity data, including the surrogate separation line, and cross-correlation analysis betweenmore »panel displacement and velocity. The results indicate a clear coupling of the panel motion and velocity field, but the spectral analysis suffers from limited time records associated with the pulse-burst laser used for PIV. Future work will focus on collecting more data to improve the statistical convergence of the results.« less
  5. The vibration of a compliant panel under a shock / boundary layer interaction (SBLI) induced by a compression ramp in a Mach 2 flow, is investigated experimentally. The panel is made from brass shim stock of length (streamwise), width (spanwise) and thickness of 122 mm by 63.5 mm by 0.25 mm, respectively. The 20° compression ramp is placed near the downstream edge of the compliant panel, and it creates a shock-induced turbulent separated flow that extends over the downstream 20% of the panel. Large pressure fluctuations occur in the region of the separation shock foot unsteadiness. The pressure fluctuations increase vibration amplitudes of the higher panel modes, especially the second mode, which has an antinode near the shock foot region. In this work, the authors use structural modifications of the baseline compliant panel to mitigate vibrations induced by the large pressure fluctuations of the shock foot unsteadiness. A thin rib is attached in the spanwise direction to the lee side of the panel at the location of SBLI. In one configuration, the rib is attached to the panel using epoxy adhesive, which creates a stiff connection. In another configuration, the rib is attached to the panel via double-sided viscoelastic tape,more »which adds significant damping to the system. The panel vibration and surface pressure field are measured using stereoscopic digital image correlation and pressure sensitive paint. Results show that especially the second vibration mode of the panel is reduced through the addition of the rib. This effect is more pronounced in the case where the viscoelastic tape was used, where a 72% reduction in vibration is observed.« less
  6. Flight vehicles that operate in the supersonic regime can be subject to adverse fluid-structure interactions due to their lightweight design. The presence of geometric obstructions such as control surfaces or fins can induce compression shocks that can interact with the boundary layer, leading to flow separation. The interaction of flow, compression shock and structural dynamics is very difficult to model and currently only poorly understood. This work investigates experimentally the interaction between a compliant panel in a Mach 2 flow under a ramp-induced shock-wave/boundary layer interaction (SWBLI). Brass panels of length 4.8" and width 2.5" and different thicknesses (h=0.020", 0.016", 0.012" and 0.010") are investigated. Tests are performed both with and without a compression ramp installed. This direct comparison allows characterization of the effect of the SWBLI on the system dynamics. High-speed stereoscopic digital image correlation (DIC) and fast-response pressure sensitive paint (PSP) are used to obtain simultaneous full field deformation and surface pressure of the panels. The results show that the shock induced by the 20compression ramp leads to separation of the turbulent boundary layer close to the ramp starting at about 80% of the panel length. This results in a region of large pressure fluctuations which primarily increasemore »the vibration amplitude of the second panel mode. Analysis of the fundamental mode, which contains most of the vibration energy of the panel, shows that the SWBLI does not lead to changes of this mode, neither in frequency, amplitude or mode shape. On the other hand, analysis of the shock foot motion shows that the shock primarily oscillates at the fundamental frequency of the panel. This means that while the shock and panel oscillate at the same frequency, it is not two-way coupling. The panel vibration dictates the motion of the shock, but the shock (or rather the SWBLI) does not modify the fundamental panel vibration beyond the forcing provided by the turbulent boundary layer. Full field surface pressure predictions are made using linearized potential flow theory, which relates the local slope of the panel to the surface pressure. Results are found to be in good agreement in the region of attached flow.« less
  7. This experimental study aims to investigate and compare unsteady surface pressure fluctuations on rigid and compliant panels under a shock-wave/boundary-layer interaction (SBLI) generated by a 20o compression ramp in a Mach 2 wind tunnel. The compliant panel was made of 1mm thick polycarbonate and had a first mode resonant frequency of 407 Hz. High-speed simultaneous pressure-sensitive paint (PSP) and digital image correlation (DIC) techniques allow for examination of the panel surface pressure and panel displacement, respectively, with acquisition frequencies of 20 kHz and 5 kHz respectively. The PSP measurements were also made on the face of the rigid compression ramp and so the effect of fluid-structure interaction on the reattachment dynamics could be explored. The rigid panel pressure measurements demonstrated the spectral content inherent to the SBLI. Spectral analysis of the surface pressure fields revealed that the SBLI shock foot and separation shear layer behave as low- and high-frequency filters, respectively, and the compliant panel showed the similar behavior with the peak frequency at the first mode frequency of the panel (~407Hz). The spectral comparison of pressure fields also depicted that the other modes of the structural panel affected the flow ( at frequency 624 Hz) near the compliant panel’smore »mid-region. The results indicate the panel dynamics have a strong effect on separation- and reattachment-flow dynamics. Modes obtained by spectral proper orthogonal decomposition (SPOD) show good agreement with modes obtained from bandpass filtered displacement and pressure fields.« less