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Research Overview
Air-breathing hypersonic vehicles live or die by their intake. It must compress the incoming flow efficiently, stay started across the flight envelope, and do so within a geometry the airframe can actually accommodate — requirements that pull against one another. Our work attacks this from two sides: active flow control to keep the inlet started under off-design and transient conditions, and inverse design methods that widen the geometric freedom available to the designer in the first place. The variable Busemann streamline stacking (VBSS) method, developed in this lab, is our main vehicle for the latter.
Inlet Unstart Control via Active Boundary-layer Suction
Inlet Unstart Control via Active Boundary-layer Suction
Inlet Unstart
Inlet Unstart
Inlet unstart is a catastrophic failure mode of super- and hypersonic air-breathing vehicles. The intakes of such systems are designed to operate with supersonic internal flows. However, when the captured mass flow exceeds the maximum the isolator/combustor can pass, an unstart shock forms to turn the downstream flow subsonic, eventually propagating to the inlet. When this shock system is disgorged from the inlet, the whole internal flow turns subsonic, causing several technical issues, including reduced mass capture, increased drag, and combustor malfunction.
Therefore, mitigating unstart is a central operational requirement.
Test Schematic
Test Schematic
Inlet unstart was experimentally investigated in a hypersonic wind tunnel ACT-1 at the University of Notre Dame.
The model has a single ramp of 12 degrees and a perforated plate in the isolator region for active boundary-layer suction.
Test Apparatus
Test Apparatus
The model was installed in a free-jet configuration with freestream Mach number of 6.
An active boundary-layer suction was performed via a pre-evacuated chamber that drove suction flow from the isolator.
unstart_weak.mp4High-speed Schlieren Results
High-speed Schlieren Results
Case 1-1: non-suction case
- The inlet was unstarted in 305 ms.
Case 1-2: suction activated 10 ms after the jet injection
- The inlet remained started throughout the run.
Case 1-3: suction activated 20 ms after the jet injection
- The inlet was unstarted in 325 ms (20 ms delay)
In summary, properly timed suction can delay or even prevent unstart.
Similar experiments under high-enthalpy conditions, where choking is driven by combustion, are reported in K. Kang et al., AIAA Journal 59.8 (2021).
Inlet Buzz Mitigation via Active Boundary-layer Suction
Inlet Buzz Mitigation via Active Boundary-layer Suction
Inlet Buzz
Inlet Buzz
Inlet buzz is a self-sustained flow oscillation that can follow inlet unstart.
Due to the temporal evolution of the choking flow and spillage, the internal flow alternates between choked and unchoked states, periodically expelling and re-ingesting the unstart shock.
The consequences of inlet buzz include acoustic noise (hence the name), unsteady aerodynamic loads and performance fluctuations.
Test schematic
Test schematic
An identical model above was tested for inlet buzz mitigation.
Two jet plenums with different pressures were used for creating a rapid drop of injection pressure, simulating fuel cut-off after unstart.
Suction was concurrently activated with the cut-off.
Visual Analysis
Visual Analysis
Left figures present Schlieren intensity traces over time at the leading edge of the cowl.
In the non-suction case (left), the inlet still exhibited an oscillatory flow, represented by the several peak intensities despite the injection cut-off.
On the other hand, the suction case showed almost immediate restart after the cut-off.
buzz.mp4High-speed Schlieren Result
High-speed Schlieren Result
High-speed Schlieren shows the entire sequence of inlet buzz to restart.
In the non-suction case (top), there are still flow oscillations after the jet cut-off (at 250 ms).
In the suction case (bottom), the flow stabilizes quickly after the cut-off and the suction activation at 250 ms.
High-speed Intake Designs
High-speed Intake Designs
Multi-ramp Intakes
Constructed via multiple oblique/conical shockwaves
Easier to predict performance
Substantial shock-induced losses
Streamline-traced Intakes
(Hexafly-int)
Constructed via the streamline-tracing technique, which requires a base flow
Performance determined by the base flow (usually Busemann flow) and the capture profile.
Shape Transition Intakes
(REST intake from NASA)
Constructed via multiple stream-traced intake flow fields
Enables shape transition (rectangular to elliptical cross-section)
Requires a transition (shape morphing) function
Busemann Streamline
Busemann Streamline
Busemann streamline is a special set of solutions from Taylor-Maccoll equation. By solving the equation backwards (downstream to upstream) with a specific termination shock condition, the solution produces an axisymmetric irrotational flow field that consists of an isentropic compression and conical shock. This gradual compression allows the resulting flow field (along the streamline) to be highly efficient, achieving superior total pressure recovery.
However, this distributed compression makes the compression field inherently long, inducing greater skin-friction losses in practice.
Classical Streamline-tracing Technique
Classical Streamline-tracing Technique
To overcome the excessive length limitation of Busemann streamlines, the streamline-tracing technique is often employed.
This technique traces a set of streamlines along a given capture profile, reducing the wetted area of the compression field.
Variable Busemann Streamline Stacking Method
Variable Busemann Streamline Stacking Method
Although the streamline-tracing technique allowed better performances under viscous conditions, its design methodology was rigid: with a given capture profile and the base flow (constructed from an axisymmetric Busemann flow), the intake geometry was essentially determined.
The variable Busemann streamline stacking (VBSS) method was proposed to deliver greater degrees of design freedom. Under this framework, the compression field is constructed from a bottom-up approach: each constituent streamline is computed at every azimuthal station to satisfy a prescribed design requirement.
As a demonstration, shape transition intakes are generated via the VBSS method by calculating the required contraction ratio of each streamline.
Profiled Truncation Method
Profiled Truncation Method
Another application of the VBSS technique is to vary the truncation angle of Busemann streamlines. Since Busemann streamlines are long, they are often truncated: the leading portion of the streamline is removed. By varying this truncation angle, one can decouple the capture area from the leading-edge design, enabling greater design freedom for optimized performances.
Future Work
Future Work
The intake sits between two components that want different things. Upstream, a waverider airframe delivers high lift-to-drag but imposes a flat, sharp-edged capture; downstream, the combustor prefers a rounded cross-section for structural and flow-uniformity reasons. The intake must reconcile the two.
This makes shape transition not a convenience but a requirement — and the VBSS method, which builds the compression field streamline by streamline, is well suited to prescribing it. We are extending the framework toward a unified treatment of waverider-intake integration, so that the airframe, the intake, and the combustor can be designed as one flow path rather than three.
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