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OBJECTIVE: Develop and validate very high-speed, non-intrusive, time-resolved instrument for velocity measurements at rates up to 100 kHz and rotational/vibrational temperature measurements at rates as fast as possible using the laser and camera system supplied for the velocity measurement. The diagnostic must be capable of measurements throughout the entire run period in the new AEDC Tunnel 9 Mach 14, and the existing Mach 18 and VKF hypersonic ground test facilities. DESCRIPTION: The AEDC Hypervelocity Tunnel 9 is a key facility for providing critical high Mach number data in support of the development of the nation's hypersonic flight vehicles. Tunnel 9 provides Mach 7, 8, 10, 14, and 18 capabilities and, in the near future, new state-of-the-art Mach 8 and Mach 14 facilities. Forces, moments, and surface quantities to support the validation of CFD models are routinely measured in the hypersonic facilities. However, these measurements are not sufficient to provide a complete understanding of the hypersonic flow physics needed to produce accurate modelling. The characterization of a hypersonic wind tunnel flow requires a knowledge of its freestream turbulence, which includes vorticity, entropy, and acoustic fluctuations. Traditionally, the characterization of free stream fluctuations (tunnel noise) in large scale hypersonic wind tunnels has been limited to high frequency Pitot pressure measurements. Such measurements are performed behind a normal shock which distorts the spectral content and the magnitude of the fluctuations as a function of the probe tip geometry (1). The accuracy of this method is dependent on the accuracy of the tunnel noise measurements at high frequencies. A non-intrusive measurement of the freestream velocity and temperature (both translational and vibrational) is needed to characterize the turbulence in the hypersonic wind tunnel flow fields. The vibrational nonequilibrium processes occurring in expanding hypersonic flow fields result in the freezing of the vibrational energy at a much higher temperature than the translational temperature. This is a very important quantity to measure because this vibrational energy is not converted to kinetic energy due to the reduction of collisions during the expansion process. This results in lower free stream translational velocities as well as changes to other freestream quantities. These measurements are needed to determine the true Mach number of the flow field. The velocity and vibrational and rotational temperatures were recently measured as part of the AEDC Tunnel 9 Mach 18 nozzle development using the FLEET (Femtosecond Laser Electronic Excitation Tagging) and the Hybrid CARS (Coherent Anti-Stokes Raman Spectroscopy) techniques, respectively (2). These techniques were also demonstrated in the AEDC Tunnel 9 in the existing Mach 14 facility (3, 4). The measurements were performed at a rate of 1 kHz, which was state-of-the-art at the time. The results were very helpful in the initial design of the Mach 18 facility (5). However, to provide a much better statistical sample, a much faster data rate, up to 100 kHz is required for velocity and as fast as possible for CARS are required. A measurement technique that can discriminate between the disturbance modes (vorticity, entropy and acoustic) is highly desirable for the soon to be deployed Mach 14 facility and the existing Mach 18 facility. Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) are not acceptable due to the high stagnation temperature of the flow and the impracticality of seeding the core flow. Approaches such as Rayleigh scattering, interferometry, and schlieren methods show promise, but have yet to be successfully applied for turbulence measurements in large scale hypersonic T&E facilities. The goal of this effort is to develop and demonstrate an instrument suitable for non-intrusive turbulence measurements at rates up to 100 kHz in large-scale hypersonic wind tunnels. Previously measurements have been made at rates of 1 kHz which was state-of-the-art at the time of its implementation. However, the requested 100 kHz rate (continuous, not burst mode) is needed to achieve the desired requirements. An instrument suite would also be acceptable, considering the multimodal nature of the freestream disturbances in hypersonic wind tunnels. Typical test conditions for the USAF AEDC Hypervelocity Wind Tunnel 9 are: Mach number 8 to 18, run time 0.25 to 5 seconds, stagnation temperature up to 1850 K, static temperature 40 to 200 K, static pressure 3.5 to 10,000 Pa, static density 0.0003 to 0.56 kg/m3, and velocity 1370 to 2070 m/s. It is desired that the velocity and temperatures are measured throughout the entire run time at a rate near 100 kHz. Test section optical access is typically gained through two thick (50 mm) BK7 glass windows located 1.5 to 2 m apart, but smaller diameter (thinner) inserts with other optical materials can be implemented. Additionally, 75 mm diameter ports are available at the exit of the Mach 18 nozzle and the future new Mach 14 nozzle. The test gas is nitrogen. Hypersonic wind tunnel facilities also produce low-frequency vibrations during testing which must be tolerated by the proposed instrument. While this solicitation does not require global data, multiple point measurements are required to obtain turbulence length scale and convection velocity magnitudes and directions. Finally, it is very important that the instrument be able to reject the sidewall boundary-layer located over the test cell windows. PHASE I: Demonstrate the feasibility of a non-intrusive instrument capable of high-speed flow velocity measurements at rates near 100 kHz and CARS measurements as fast as possible using the existing velocity hardware. The demonstration needs to be done in a supersonic or hypersonic wind tunnel facility. PHASE II: Develop and demonstrate the prototype system in the AEDC Tunnel 9 (or other large scale hypersonic wind tunnel with relevant properties). Deliver the prototype system hardware and software (including data reduction software and operational manuals) to AEDC Tunnel 9. PHASE III DUAL USE APPLICATIONS: The instrument can be marketed for non-intrusive velocimetry and temperature measurements in high-speed wind tunnels. REFERENCES: 1. R. S. Chaudhry, G. V. Candler. Recovery of Freestream Acoustic Disturbances from Stagnation Pressure Spectrum in Hypersonic Flow, AIAA Paper 2016-2059. 2. Dogariu, A., et al, Velocity and temperature measurements in Mach 18 nitrogen flow at Tunnel 9 , AIAA SCITECH Forum, January 2021. 3. Dogariu, A., et al, Hypersonic Flow Velocity Measurements Using FLEET, CLEO: Applications and Technology 2018, Combustion and Hypersonic Flow Diagnostics, San Jose, CA, May 2018. 4. Dogariu, A., et al, Single shot temperature measurements using Coherent Anti-Stokes Raman Scattering in Mach 14 flow at the hypervelocity AEDC Tunnel 9, AIAA Paper2019-1089, AIAA SCITECH Forum, 07-11 January, 2019, San Diego, CA. 5. J.J. Korte et al, Tunnel No. 9 Hypervelocity Freestream Conditions for Vibrational Frozen Flow, AEDC-TR-21-H-2. KEYWORDS: turbulence; hypersonic; instrumentation; non-intrusive; seedless; wind-tunnel
