Localization, Characterization, and Modeling of Freestream Disturbances in Hypersonic Wind Tunnels | STTR

OUSD (R&E) critical technology area(s): Scaled Hypersonics

Objective: Develop and demonstrate a robust methodology to localize, characterize and model upstream noise and flow disturbances in hypersonic wind tunnels, specifically targeting high-risk, hard-to-access regions upstream of the test section (e.g., driver, reservoir, nozzle throat and walls) where traditional optical access is restricted or unavailable. Performers must implement novel non-intrusive or minimally intrusive diagnostics and coupled with multi-fidelity simulation strategies to accurately trace the evolution of unwanted thermo-fluid-dynamic disturbances from their point of origin to the test section.

Description: Hypersonic ground-test facilities are critical for evaluating aerodynamic forces, aerothermodynamic heating, and boundary-layer transition (BLT) behavior on high-speed flight vehicles. However, conventional hypersonic wind tunnels are plagued by high-intensity freestream noise present in the test section. Because these disturbances build, focus, and propagate downstream, the fundamental understanding of their evolution may require a potentially deep analysis within the upstream components of the facility.

Historically, Laufer [1] established that nozzle-wall turbulent boundary layers radiate downstream-directed Mach waves, making them a principal source of freestream noise. However, especially in high-enthalpy and hypervelocity impulse facilities, the noise field may be further enhanced by complex upstream dynamics that are not optically accessible. Examples of focus areas include, but are not limited to:

• Nozzle Throats and Walls (all tunnels): The extreme thermal and velocity gradients in the nozzle throat pose an opportunity for disturbance generation and initiation of turbulence over the nozzle walls. However, severe restricted throat geometry and high heat flux make optical diagnostic access virtually impossible. Diagnostics on tunnel walls are also very challenging although there are examples of previous successful attempts [12].
• Reflected-Shock Tunnels: Reservoir entropy, pressure fluctuations, and driver-gas contamination originate from the complex interaction between the reflected shock wave and the shock tube wall boundary layer [5, 6]. Furthermore, upstream diaphragm particulate-laden flow [7] acts as a continuous source of premature model transition downstream.
• Expansion Tubes and Tunnels: Subject to driver-gas acoustic focusing [9, 10], where upstream driver unsteadiness is focused directly into the test gas when sound speed ratios fall into unfavorable ranges. This is exacerbated by secondary diaphragm rupture wave systems [3, 11] propagating from upstream.

Characterizing these regions is exceptionally difficult because standard optical diagnostic techniques (such as Femtosecond Laser Electronic Excitation Tagging (FLEET), Focused Laser Differential Interferometry (FLDI), or Rayleigh scattering) require optical-grade line-of-sight window access, which may not withstand the extreme pressures, temperatures, or geometric constraints of regions such as the driver, reservoir, or nozzle throat.

To bridge this gap, this STTR topic focuses on the development of specialized diagnostics, informed by multi-fidelity computational flow models ranging in scope from system-scale to component-specific. Collaborative proposals from Small Business Concerns (SBCs) must address the following technical pillars:

• Upstream Noise Characterization & Modeling: Modeling facility-specific noise generation phenomena such as transient or statistically steady boundary-layer effects, driver-gas acoustic focusing, diaphragm bursting, starting from the most upstream location ultimately responsible for noise contamination of the test section.
• Diagnostics for Non-Optical Zones: Developing and demonstrating novel non-intrusive or minimally intrusive diagnostic systems for flow and acoustic characterization, capable of operating without traditional optical windows. Promising approaches include but are not limited to flush-mounted high-frequency pressure/thermal sensor arrays, micro-bore fiber-optic probes, laser-based acoustic sensing, and/or hybrid data-assimilation techniques that computationally reconstruct upstream flow states from sparse wall measurements.

Phase I

This topic is soliciting Direct to Phase II (DP2) proposals only. Proposers must submit a 10-page feasibility volume providing written evidence of the following:

1. Upstream Flow & Acoustic Modeling: Successful modeling of upstream disturbance generation and propagation (e.g., driver-gas acoustic focusing, shock-tube boundary layer interactions, or nozzle throat shear layer acoustics).
2. Confined-Space Diagnostic Feasibility: Proof-of-concept or benchtop demonstration of a diagnostic method (e.g., fiber-optic-coupled probes, ultra-high-frequency flush-mounted sensors, acoustic emissions arrays, or shear sensors) demonstrating a frequency response and the physical ruggedness required to operate in high-pressure/temperature and/or non-optically accessible environments.

Phase II

The Phase II effort will be divided into the following key tasks:

• Task 1: Upstream Noise Modeling & Source Characterization - Refine computational models of the targeted wind tunnel's upstream environment, focusing on detailing how disturbances generate in the driver/reservoir and focus/propagate through the throat.
• Task 2: Upstream Diagnostic Suite Development - Build and calibrate a rugged, high-frequency flow and acoustic diagnostic system tailored for non-optical or highly confined spaces (e.g., micro-fiberoptic probes, flush-mounted acoustic arrays). Compare the novel diagnostic system developed for this task against low-order system-scale or component-specific high-fidelity predictive models.
• Task 3: Facility Integration & Baseline Upstream Characterization - Deploy the diagnostic suite developed in Task 2 on the targeted wind tunnel to map baseline fluctuations directly within the upstream driver, reservoir, or throat. The measurements will correlate and support the refinement of Task 1 efforts.

Phase II Base Period (12 Months)

The Phase II Base effort will focus on designing and validating the laboratory-scale prototype of the upstream diagnostic system and completing the engineering/integration plans for the upstream noise-mitigation hardware.

• Month 4: Diagnostic System Architecture & Preliminary Design
  • i. Deliverable: Preliminary Design Review (PDR) Document and Upstream Diagnostic Specification Report.
  • ii. 0Criteria: Selection of target wind tunnel facility completed; integration layout for upstream diagnostics (e.g., sensor ports, fiber-optic bypasses) finalized; preliminary numerical models of upstream mitigation hardware and observed environment completed.
  • iii. Associated Tasks: 1, 2
• Month 8: Diagnostic Calibration & Component Assembly
  • i. Deliverable: Confined-Space Diagnostic Calibration and Benchtop Testing Report.
  • ii. Criteria: Successful benchtop demonstration of the diagnostic tool under simulated high-pressure/temperature conditions, showing a frequency response of at least 100 kHz and the ability to capture fluctuations through restricted-access ports
  • iii. Associated Tasks: 2
• Month 12: Baseline Diagnostic System Critical Design Review (CDR)
  • i. Deliverable: CDR Document, Upstream Structural/Thermal Safety Analysis, and Final Base Report.
  • ii. Criteria: Final engineering drawings for upstream noise-mitigation hardware approved; structural/thermal safety and tunnel compatibility certified by target facility operators for high-pressure zones; integration interface finalized.
  • iii. Associated Tasks: 1, 2

Phase II Option Period (12 Months)

The Phase II Option will focus on facility integration, mapping the baseline noise directly from the upstream components, and demonstrating a quantifiable reduction in freestream noise downstream.

• Month 18: Facility Installation & Baseline Mapping
  • i. Deliverable: Baseline Upstream Characterization and Integration Report.
  • ii. Criteria: Successful integration of the diagnostic suite into the target facility's upstream components (reservoir, throat, and/or driver); baseline fluctuation measurements completed under nominal run conditions (minimum Mach 5) with and without existing noise mitigation strategies (if present in selected tunnel).
  • iii. Associated Tasks: 3
• Month 24: Upstream-to-Downstream Noise Source Correlation
  • i. Deliverable: Upstream Source Localization and Propagation Analysis Report.
  • ii. Criteria: Detailed spatial and temporal mapping of the flow path completed, successfully correlating fluctuations measured upstream (e.g., driver-gas acoustic focusing or throat shear-layer noise) with the resulting freestream acoustic noise in the downstream test section. Notional mitigation plan developed for observed flow characteristics.
  • iii. Associated Tasks: 1, 3

Phase III dual use applications

DoD/Military Applications: High-fidelity aerodynamic and aerothermodynamic characterization of hypersonic weapons systems, glide vehicles, and interceptors across Major Range and Test Facility Bases (MRTFB), such as the Arnold Engineering Development Complex (AEDC). Understanding the mechanisms for tunnel noise generation directly translates to higher-fidelity ground-test data, more accurate boundary-layer transition prediction, and accelerated flight-test qualification.

Commercial Applications: Commercial space launch vehicle design, thermal protection system (TPS) testing, and academic/commercial aerospace wind tunnel facility diagnostic upgrades. The rugged, non-optical high-frequency diagnostic systems developed under this topic have immediate commercial application in monitoring turbulent combustion chambers, gas turbine engines, and high-pressure chemical reactors where optical access is similarly restricted.

References

[1] Laufer, J., "Some Statistical Properties of the Fluctuations in the Boundary Layer of a Supersonic Nozzle," Physics of Fluids, Vol. 7, No. 3, 1964.

[2] Schneider, S. P., "The Development of Hypersonic Quiet Tunnels" Journal of Spacecraft and Rockets, V. 45, No. 4, July/August 2008, pp. 641-664.

[3] Hornung, H. G., "Performance of and Noise in High-Enthalpy Wind Tunnels," AIAA Paper 93-0185, 1993.

[4] Parziale, A. N., Shepherd, J. E., and Hornung, H. G., "Observations of Hypervelocity Boundary Layer Transition on a Cone in a Shock Tunnel," Journal of Fluid Mechanics, Vol. 748, 2014.

[5] Davies, L., "The Interaction of the Reflected Shock with the Boundary Layer in a Shock Tube and its Influence on the Duration of Hot Flow in the Reservoir," Aeronautical Research Council, CP-881, 1966.

[6] K. Hannemann, M. Schnieder, B. Reimann, and J. Martine Schramm, "The influence and the delay of driver gas contamination in HEG", AIAA 2000-2593. 21st Aerodynamic Measurement Technology and Ground Testing Conference. June 2000.

[7] Joseph S. Jewell, Nicholaus J. Parziale, Ivett A. Leyva, and Joseph E. Shepherd, "Effects of Shock-Tube Cleanliness on Hypersonic Boundary Layer Transition at High Enthalpy", AIAA Journal 2017 55:1, 332-338

[8] Trimpi, R.L., "A Preliminary Theoretical Study of the Expansion Tube, a New Device for Producing High-Enthalpy Short-Duration Hypersonic Gas Flows", NASA Technical Report 1962

[9] Paull, A. and Stalker, R.J., "Acoustic waves in shock tunnels and expansion tubes", Shock waves; Proceedings of the 18th International Symposium, Sendai, Japan, July 21-26, 1991. Vol. 1

[10] A. Dufrene, M. Sharma and J.M. Austin, "Design and Characterization of a Hypervelocity Expansion Tube Facility" Journal of Propulsion and Power, Vol 23, No. 6, 2007.

[11] Furukawa, S., et al., "Visualizing the Secondary Diaphragm Rupture in an Expansion Tube," Shock Waves, Vol. 17, No. 3, 2007.

[12] Kasper, K. M., et al.,” Pressure fluctuations beneath instability wavepackets and turbulent spots in a hypersonic boundary layer”, J. Fluid Mech., 756, pp. 1058-1091

Keywords

Hypersonics, Wind Tunnel Noise, Upstream Disturbances, Nozzle Throat, Driver-Gas Acoustics, Non-Optical Diagnostics, High-Frequency Sensors, Acoustic Mitigation, Fiber-Optic Probes, Reflected-Shock Tunnels, Expansion Tubes

TPOC-1-PoC

DARPA BAA Help Desk

Email

SBIR_BAA@darpa.mil

Ready to apply?

For additional information and to submit your full proposal package, visit the DSIP Portal.