TECHNOLOGY AREA(S): Air Platform, Sensors, Weapons

ACQUISITION PROGRAM: ONR Hypersonics D&I Program

OBJECTIVE: Develop and demonstrate non-intrusive diagnostics to: (1) quantify the spatiotemporal evolution of raindrops, ice crystals (clouds), and snow during high-speed aerobreakup; and (2) simultaneously quantify aerobreakup effects on the surrounding gas parameters such as velocity, composition, and thermodynamic state variables.

DESCRIPTION: The effect of adverse weather on hypersonic flight conditions is not well understood. Hydrometeors such as rain, hail, snow, and ice disrupt the flow field and impinge on the vehicle surface. Because the impact forces approximately scale as the velocity squared, hypersonic vehicles are at greater risk of damage [Ref 1].� The modeling and understanding of weather encounters are challenging because the vehicle flow field alters the impact boundary conditions as a function of time. Since existing computational models cannot capture these complex, multiscale finite-rate processes, weather effects are currently estimated via component testing and empirically derived correlations.

At altitudes below 4 km, rain is the most prevalent weather encounter. When raindrops are suddenly exposed to a high-speed gas flow, they deform and shatter due to shear-induced entrainment and/or interfacial flow instabilities. This phenomenon, known as aerobreakup, increases in intensity with Weber number. Aerobreakup experiments have typically been performed in shock tubes at moderate supersonic Mach numbers, M < 2. With the exception of recent laser-induced florescence (LIF) measurements [Ref 2], aerobreakup measurements strictly use path-integrated visualization techniques that have led to erroneous physical interpretations [Ref 3]. Weather effects are not limited to low-altitude flight since clouds such as cumulonimbus can reach altitudes exceeding 20 km. Prolonged exposure to high-altitude solid hydrometeors can produce surface roughness that initiate the flow instabilities responsible for laminar-turbulent boundary layer transition.

The development of improved numerical tools requires validation against high-quality experimental data that captures both the spatiotemporal evolution of the hydrometeors during aerobreakup and the effect of the aerobreakup on the surrounding flow field. Such measurements are currently unavailable in ground-test facilities due to the lack of advanced non-intrusive diagnostics. Recent advances in instrumentation and measurement techniques such as high-speed intensified camera, tunable pulsed-burst lasers, and tomographic reconstruction algorithms [Refs 4-7] can be leveraged to develop improved instrumentation for quantitative assessment of weather effects on high-speed flows. Four-Dimensional X-ray imaging [Ref 8] recently used to study an optically complex spray seems promising to provide measurements of droplet structures and liquid density in the near field.


KEY REQUIREMENTS OF THE INSTRUMENTATION SUITE AND PARAMETERS:
� The non-intrusive instrument (or suite of instruments) needs to provide three-dimensional, time-resolved measurements of the aerobreakup process including the atomization of small droplets.
� The non-intrusive instrument (or suite of instruments) needs to provide gas phase measurements (velocity, state variables and composition) for freestream Mach numbers above 3 and conditions corresponding to altitudes below 4 km for raindrops and up to 20 km for ice crystals (present in clouds).
� Primary droplet diameters of approximately 0.3 to 3 mm and secondary droplet diameters of 0.03 to 0.2 mm need to be resolved by the instruments. Ice crystals can be significantly smaller (1 to 50 microns). Therefore, the instrument suites must provide an adequate range of magnifications and spatial resolutions to resolve the wide range of relevant spatial scales.
� Ideally, the instrument must be suitable for measurements with solid particles used as surrogates for hydrometeors (with diameters between 0.03 and 3 mm).
� The non-intrusive instrument (or suite of instruments) needs to operate in various types of large-scale ground-test facilities such as shock tubes, ballistic ranges (light-gas gun), and wind tunnels.
� Ideally, the non-intrusive instrument (or suite of instruments) must be portable (on a cart or set of carts) for usage in multiple facilities.

PHASE I: Design a non-intrusive instrument or a suite of non-intrusive instruments to quantify the spatiotemporal evolution (three-dimensional, time-resolved measurements) of raindrops, ice crystals (clouds), and snow during high-speed aerobreakup, and concurrently quantify aerobreakup effects on the surrounding gas. (Note: Preferably, benchtop demonstrations of the instrument concepts shall occur in a shock tube or other facilities producing a relevant environment.) Develop a Phase II plan.

PHASE II: Refine and optimize the instrument and/or suite to produce a viable prototype. Demonstrate the performance of the prototype in a relevant ground test facility such as a ballistic range (light-gas gun), wind tunnel, or shock tube. Produce relevant data to quantify the spatiotemporal evolution of raindrops, ice crystals (clouds), and snow during high-speed aerobreakup and the effect of the aerobreakup on the surrounding gas. Assess interactions with single and multiple droplets (corresponding to relevant precipitation rates).

PHASE III DUAL USE APPLICATIONS: Private companies have demonstrated interest in commercial hypersonic flight. It will be important to assess the impact of weather effects on commercial systems. The developed instrumentation suite could be used in high-speed ground test facilities operated by the commercial sector.

REFERENCES:

1. Moylan, B., Landrum, B. and Russell, G. "Investigation of the Physical Phenomena Associated with Rain Impacts on Supersonic and Hypersonic Flight Vehicles.�,� Procedia Engineering, 2013, pp. 223-231. https://www.sciencedirect.com/science/article/pii/S1877705813009326/pdf?md5=068388223a25f6b1823be3307a1b8d06&pid=1-s2.0-S1877705813009326-main.pdf

2. Theofanous, T.G., Mitkin, V.V., Ng, C.L., Chang, C.H., Deng, X. and Sushchikh, S. "The physics of aerobreakup. II. Viscous liquids", , Physics of Fluids, 2012. https://aip.scitation.org/doi/full/10.1063/1.3680867

3. Moylan, B. �Raindrop Demise in a High-Speed Projectile Flowfield: A Dissertation.�� University of Alabama in Huntsville, 2010. https://www.worldcat.org/title/raindrop-demise-in-a-high-speed-projectile-flowfield-a-dissertation/oclc/673870343

4. Thurow, B., Jiang, N. and Lempert, W. "Review of ultra-high repetition rate laser diagnostics for fluid dynamic measurements.", Measurement Science and Technology, 2013. https://iopscience.iop.org/article/10.1088/0957-0233/24/1/012002/meta

5. Wellander, R., Richter, M. and Alden, M. "Time-resolved (kHz) 3D imaging of OH PLIF in a flame.", Experiments in Fluids, 2014. https://link.springer.com/article/10.1007/s00348-014-1764-y

6. Roy, S., Jiang, N., Hsu, P., Slipchenko, M., Felver, J., Estevadeordal, J. and J. Gord. "Development of a three-legged, high-speed, burst-mode laser system for simultaneous measurements of velocity and scalars in reacting flows."� Optics Letters, 2018. https://www.osapublishing.org/ol/viewmedia.cfm?uri=ol-43-11-2704&seq=0

7. Halls, B., Hsu, P., Jiang, N., Legge, E., Felver, J., Slipchenko, M., Roy, S., Meyer, T. and Gord. J. "kHz-rate four-dimensional fluorescence tomography using an ultraviolet-tunable narrowband burst-mode optical parametric oscillator.", Optica, 2017. https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-8-897

8. Halls, B.R., Rahman, N., Meyer, T.R., Lightfoot, M.D.A., Slipchenko, M.N, Roy, S. and Gord, J.R.� "Four-Dimensional X-ray Imaging of Multiphase Flows." Imaging and Applied Optics, 2018. https://www.osapublishing.org/abstract.cfm?uri=LACSEA-2018-LM3C.2

KEYWORDS: Weather Effects; Multiphase Flows; Laser Diagnostics; Tomography; Hypersonics; High-speed Flows; Ground Test