Research Notes #12: Hypersonic Boundary Layer Transition & TPG Optimization
1. Objective & Scope
This document consolidates findings from Wind Tunnel Facility B-42 (Run Series VT-2025-03) and associated Reynolds-Averaged Navier-Stokes (RANS) simulations. The primary objective is to characterize boundary layer transition onset over the forward fuselage and upper wing surface at Mach 5.8–6.2, and to evaluate Transpiration Cooling Plate (TCP) performance under simulated flight heat flux conditions.
Secondary goals include validating the SST-kω turbulence model against schlieren imaging data and establishing updated Thermal Protection System (TPS) margin thresholds for Phase III flight testing.
2. Theoretical Framework
Boundary layer transition in hypersonic flows is governed by the amplification of second-mode Mack instabilities. The critical Reynolds number \(Re_{crit}\) is approximated using the empirical correlation:
\( Re_{\theta,crit} \approx 180 \cdot (M_e)^{-0.4} \cdot (\theta/L)^{0.6} \)
Where \(M_e\) is the edge Mach number, \(\theta\) is the momentum thickness, and \(L\) is the characteristic length. Transpiration cooling effectiveness \(\eta_{tcp}\) is modeled as:
\( \eta_{tcp} = \frac{q_{adiabatic} - q_{cooled}}{q_{adiabatic}} \times 100\% \)
Derivation Notes & Assumptions
Assumes perfect gas behavior, calorically constant specific heats, and fully developed turbulent flow downstream of transition. Real gas effects become significant above 1,800K but are neglected for baseline correlation. Surface roughness assumed Ra < 3.2μm per MIL-STD-1246C.
3. Test Matrix & Setup
| Run ID | Mach No. | Reynolds (1/m) | Static Pressure (atm) | Wall Temp (K) | Injection Rate (g/s) |
|---|---|---|---|---|---|
| VT-03-101 | 5.8 | 8.2×10⁶ | 0.12 | 300 | 0.0 |
| VT-03-102 | 5.9 | 8.5×10⁶ | 0.14 | 450 | 0.0 |
| VT-03-103 | 6.0 | 9.1×10⁶ | 0.15 | 600 | 0.0 |
| VT-03-104 | 6.1 | 9.4×10⁶ | 0.16 | 600 | 0.8 |
| VT-03-105 | 6.2 | 9.8×10⁶ | 0.18 | 600 | 1.2 |
Instrumentation: IR thermography (FLIR A95sc, 12μm resolution), piezoresistive pressure taps (100kHz sampling), hot-film anemometry for transition detection, and high-speed schlieren (Phantom v2640, 50kfps).
4. Experimental Data
Average Heat Flux vs. Mach Number (Forward Fuselage Station X=0.42m)
Raw Sensor Calibration Logs
TC-Cal-2025-03-12: IR emissivity calibrated to 0.85±0.02 using blackbody furnace.
Pressure tap drift: +0.004 atm post-run, within tolerance.
Injection mass flow verified via Coriolis meter (±0.5% accuracy).
Transition onset detected at X=0.38m (M=5.9), X=0.35m (M=6.0).
CFD mesh: 28.4M cells, y+ < 1.0 across TPG zones.
5. Analysis & Observations
- Transition onset shifted upstream by ~8% compared to baseline predictions (Note #09), consistent with increased surface roughness from previous thermal cycling.
- TCP injection at 0.8–1.2 g/s reduced peak heat flux by 14–18%, but introduced local flow separation bubbles observed in schlieren (see Fig. B-42.3).
- RANS SST-kω overpredicted turbulent skin friction by ~11% in the reattachment zone. LES validation recommended for Phase III.
- Material response: Silicon Carbide (SiC) TPS tiles maintained structural integrity at 1,450K. Microcracking observed at thermal gradient interfaces >800K/mm.
⚠️ Anomaly: Run VT-03-105 exhibited unsteady pressure oscillations at 2.4kHz, correlating with acoustic resonance in the injection manifold. Requires flow restrictor redesign.
6. Recommendations
- Implement porous Ti-6Al-4V injection plates to mitigate flow separation (CFD parametric study queued for Cluster-7).
- Update transition prediction model using eN-method with modified roughness function \(H(k_s^+)\).
- Proceed to cold-flow acoustic testing before next hot-fire campaign to resolve 2.4kHz resonance.
- Recommend TPS tile interface redesign with compliant ceramic fiber interlayer to reduce thermal stress concentration.
7. References
- Anderson, J.D. (2006). Hypersonic and High-Temperature Gas Dynamics. AIAA Education Series.
- Rostova, E. et al. (2024). "TPG Optimization for Mach 6 Cruise Vehicles." AeroVance Technical Report TR-2024-08.
- ASTM E2661-21. Standard Practice for Thermal Protection System Materials Testing.
- Menter, F.R. (1994). "Two-Equation Eddy-Viscosity Turbulence Models for Engineering Applications." AIAA Journal, 32(8), 1598-1605.
- Internal CFD Validation Guidelines v3.2 (AeroVance Comp. Fluids Division).