Description
In this talk, the recent numerical and theoretical research activities on shock wave/boundary layer interactions (SBLI) in High-speed Thermo-fluid and MAV/UAV Laboratory (HTML), HKPolyU, will be introduced, with emphases on the hypersonic laminar separation flows over a compression ramp and a double cone. Three areas will be reviewed: (i) understanding the intrinsic instability and transition, (ii) thermochemical/vibrational nonequilibrium effects, and (iii) leading-edge bluntness effects.As demonstrated by recent numerical works and the earlier experiments, streamwise heat flux streaks form on the ramp/cone surface downstream of reattachment, and they are non-uniformly distributed in the spanwise/azimuthal direction. Due to the presence of intrinsic instability, the surface heat flux exhibits a low-frequency unsteadiness, which propagates in the streamwise direction. Additionally, the unsteadiness of the heat flux streaks downstream of reattachment is coupled with a pulsation of the reattachment position. Substantial success has been achieved in understanding the intrinsic instability and laminar-turbulent transition through complementary Direct numerical simulations (DNS) and theoretical studies by the Global Stability Analysis (GSA) and the triple-deck theory. Both DNS and GSA confirm that the supercritical ramp/double-cone flow is intrinsically three-dimensional, unsteady and exhibits strong spanwise/azimuthal variations in the peak heating. The global instability is shown to be closely linked with the occurrence of secondary separation beneath the primary separation bubble. Meanwhile, it is found that the shock-induced separated flow system becomes unstable when the deflection angle is beyond a certain value. A criterion is established based on a scaled deflection angle defined in the triple-deck theory to predict the global stability boundary, which depends on the free-stream conditions and geometries only. The critical deflection angle increases slightly with the wall temperature and, as the Reynolds number is further increased, the flow is strongly destabilized with the coexistence of multiple stationary and low-frequency oscillating unstable modes, leading to a transition process in a hypersonic compression-ramp/double-cone flow due to intrinsic instability of the flow system.
For thermochemical/vibrational nonequilibrium effects on SBLI, systematic studies were performed using different coupling models of vibrational excitation and dissociation, including a conventional two-temperature model as the baseline and an improved model established on elementary kinetics and validated against existing shock tube experimental data. For the double-cone flow with the highest total enthalpy, the improved model predicts a larger separation region and greater peak heat flux with relative differences of 20.3% and 29.2%, respectively, compared with the baseline two-temperature model. The differences are attributed to inaccurate modeling of the vibration–dissociation coupling effects by the conventional two-temperature model, which overestimates the post-shock degree of dissociation and underestimates the post-shock temperature. The size of the separation bubble is therefore altered due to the change in its density. For the condition with the low total enthalpy, the most representative flow model still underestimates the sizes of the separation regions for double cone flow and overestimates those for hollow-cylinder flare flow. It is concluded that inaccurate modeling of vibrational nonequilibrium may not be responsible for the discrepancies observed at the low total enthalpies. Suggestions for further study are also presented.
For leading-edge bluntness effects on SBLI, DNS of a compressible ramp flow shows that the separation bubble enlarges when the leading-edge radius is increased from zero up to a critical value. Beyond the critical radius, the separation bubble conversely shrinks as the radius is further increased. GSA demonstrates that the inherent instability in the flow field also exhibits a reversal trend, that is, the flow system firstly becomes more unstable and then tends to be more stable with increasing leading-edge radius. The growth rate and spanwise wavelength of the unstable modes identified by GSA are verified by DNS. The present study demonstrates that a proper blunting of the leading edge can suppress flow separation, reduce aerodynamic heating and stabilise the flow system for a hypersonic compression-ramp flow. Contrarily, the double-cone flow is insensitive to small bluntness in terms of shock structures, separation region sizes and surface pressure and heat flux distributions. A critical nose radius is observed, beyond which the separation bubble grows dramatically. The numerical data are analysed and interpreted based on a triple-deck formulation. It is shown that the sudden change in flow features is mainly caused by pressure overexpansion on the first cone due to leading-edge bluntness, such that the skin friction upstream of the separation is significantly reduced and the upstream pressure can no longer resist the large adverse pressure gradient induced by shock impingement. An estimation of the critical radius is established. Simulations at a higher enthalpy with the presence of both vibrational relaxation and air chemistry show a similar trend with increasing nose radius. The proposed criterion agrees well with the experimental observations.
Recent efforts on shock wave/turbulent boundary layer interactions will also be introduced.
Reference:
1. Cao, S.B., Hao, J.A., Klioutchnikov, I., Wen, C.Y., Olivier, H., Heufer, A., “Transition to turbulence in hypersonic flow over a compression ramp due to intrinsic instability,” Journal of Fluid Mechanics, accepted, 2022.
2. Hao, J.A., Fan, J.H., Cao, S. B., and Wen, C.Y., “Three-dimensionality of Hypersonic Laminar Flow over a Double Cone,” Journal of Fluid Mechanics, Vol. 935, A8, 2022.
3. Cao, S.B., Hao, J.A., Kiloutchnikov, I., Olivier, H., Heufer, A., Wen, C.Y., “Leading-edge Bluntness Effects on Hypersonic Three-dimensional Flows over a Compression Ramp,” Journal of Fluid Mechanics, Vol. 923, A27, July, 2021.
4. Hao, J.A., Cao, S.B., Wen, C.Y.*, and Olivier, H., “Occurrence of Global Instability in Hypersonic Compression Corner Flow,” Journal of Fluid Mechanics, Vol. 919, A4, July, 2021.
5. Cao, S.B., Hao, J.A.., Klioutchnikov, I., Olivier, H., Wen, C.Y., “Unsteady Effects in a Hypersonic Compression Ramp Flow with Laminar Separation,” Journal of Fluid Mechanics, Vol. 912, A3, 2021.
6. Hao, J. and Wen, C.Y., “Hypersonic Flow over Spherically Blunted Double Cones,” Journal of Fluid Mechanics, Vol. 896, A26, Aug. 10, 2020.
7. Hao, J.A., Wen, C.Y. and Wang, J.Y., “Numerical Investigation of Hypersonic Shock-Wave/Boundary-Layer Interactions over a Double-Wedge Configuration,” International Journal of Heat and Mass Transfer, Vol. 138, pp. 277-292, 2019.
8. Hao, J.A. and Wen, C.Y., “Numerical Investigation of Oxygen Thermochemical Nonequilibrium on High-Enthalpy Double-Cone Flows,” International Journal of Heat and Mass Transfer, Vol. 127, Part B, pp. 892-902, Dec. 2018.
9. Hao, J.A. and Wen*, C.Y., “Effects of Vibrational Nonequilibrium on Hypersonic Shock Wave/Laminar-Boundary-Layer Interactions”, International Communi-cations in Heat and Mass Transfer, Vol. 97, pp. 136-142, 2018.
| Period | 16 Jul 2023 → 21 Jul 2023 |
|---|---|
| Event title | The 34th International Symposium of Shock Waves (ISSW34) |
| Event type | Conference |
| Location | Daegu, Korea, Republic ofShow on map |
| Degree of Recognition | International |