WHITE OAK, Md. --
The measurement of velocity in a hypersonic wind tunnel facility is no easy feat.
Along with the difficulties posed by the extremely high speeds, the measurement process must be non-intrusive to prevent disturbance of the flow field, a possibility that is only heightened due to the low densities within the tunnel.
But, thanks to a Small Business Innovation Research, or SBIR, program, accurate measurements at rates greater than 100 kHz, or 100,000 cycles per second, can now be obtained in the highest wind tunnel speed available at Arnold Engineering Development Complex Hypervelocity Wind Tunnel 9 in White Oak, Maryland.
This SBIR was funded to Spectral Energies, LLC to devise a method for measuring velocity using the Krypton Tagging Velocimetry (KTV) technique at a data rate of 100 kHz. This endeavor recently proved successful, as the method developed through the SBIR was demonstrated at Mach 18 in Tunnel 9.
The KTV technique was already shown to be successful at a lower data rate and lower tunnel speeds. The diagnostic was initially developed and demonstrated at Tunnel 9 during the summers of 2014 to 2017 by Nick Parziale, a professor at the New Jersey-based Stevens Institute of Technology who was the recipient of a Summer Faculty Fellowship Program. This system was demonstrated at both Mach 10 and Mach 14.
“The initial demonstrations, while extremely successful, resulting in a number of papers and journal articles, were performed at a data acquisition rate of only 10 Hz,” said Mike Smith, Tunnel 9 physicist.
Tunnel 9 run times generally last for only a few seconds. Because of this, a two-phase SBIR was initiated. The goal of the first phase, which began in early 2017, was to determine the feasibility of increasing the measurement data rate to 100 kHz, which would yield a more statistically-significant sample as well as provide the ability to capture transient phenomena in hypersonic environments.
The aim of Phase II of the SBIR, which got underway in late 2018, was to develop the hardware to accomplish this measurement and eventually perform a demonstration at Tunnel 9.
The hardware developed as a result of this effort, known as an Optical Parametric Oscillator, or OPO, was coupled with a pump laser (ns pulse width) initially developed under a separate AF SBIR to provide the necessary output wavelengths and intensities for the measurement. When velocity is measured in a hypersonic wind tunnel, the sample time must be very short in order to near-instantaneously determine the velocity without blurring the sample.
“Thus, a high-energy, burst-mode laser is the perfect excitation source for this measurement,” Smith said.
The high-speed OPO-based KTV technique was put to use during a short throat qualification project in Tunnel 9 with the Mach 18 nozzle installed following the recent calibration of the Mach 18 capability.
“A new diagnostic is generally demonstrated when opportunities present themselves,” Smith said. “The throat qualification effort was begun as an extension of efforts to produce a ceramic throat in order to continue the development of the Mach 18 test capability at Tunnel 9. The effort consisted of four tunnel runs whose purpose was to validate some of the ceramic throats. This was an excellent series of runs for us to demonstrate the burst-mode laser-based KTV measurements.”
Two measurement techniques were demonstrated during the throat qualification, one of which was KTV at the desired 100 kHz pulse rate. For this diagnostic, a specific amount of Krypton gas was inserted into the Mach 18 heater vessel prior to heating. The OPO was then “pumped” with the burst-mode laser operating at a repetition rate of 100 kHz. The resultant OPO output wavelength was 212.56 nanometers (nm), which is the required wavelength to produce a long-lived fluorescence at a wavelength of 769 nm.
The metastable fluorescing line that results is monitored a few tens of microseconds later using an intensified camera. An image depicting a series of lines is produced. The single “write line” in the image is the initial fluorescence created by the laser pulse, and the multiple read lines show the dissolution of the write line over time as it travels downstream. In this case, images were recorded at a rate of 100 kHz.
The flow field velocity is determined by measuring the separation distance between the initial and dispersed lines and dividing by the separation between the intensifier gate times. The shape of the line can be used to investigate the flow turbulence.
The other technique was Picosecond Laser Electronic Excitation Tagging, or PLEET. This diagnostic was used to take an additional measurement at a pulse rate of 100 kHz using the KTV pump laser. Although not part of the SBIR, the opportunity to conduct this demonstration presented itself during the throat qualification process. PLEET does not require an atomic (or molecular) seed gas such as Krypton. Instead, a multiphoton ionization of nitrogen followed by recombination produces a fluorescing, long-lived line which can be monitored in a manner similar to the KTV line.
The multiple-line measurements provided redundant velocity measurements.
The results of the KTV showed reduced velocity uncertainties when the technique is applied. Smith said the velocities compared “very well” to measurements made with the 10 Hz KTV technique and the 1 kHz Femtosecond Laser Electronic Excitation Tagging, or FLEET, technique. FLEET was developed during 2016-2019 using SBIR funding and uses a 100 femtosecond-width laser pulse at a pulse rate of 1,000 Hz. The FLEET system was previously demonstrated at Mach 10, Mach 14 and Mach 18 in Tunnel 9.
“The uncertainty decreases as you move further away from the write line,” Smith said. “The measured velocities agree very well with those previously measured with 10 Hz KTV, and1,000 Hz FLEET, and the calculated values using standard probe data.”
High-speed measurements of the Mach 18 flow velocity are needed to provide customers with the most accurate information on flow conditions.
“This state-of-the-art system will also permit a more complete characterization of the turbulent boundary layer,” Smith added.
Years of planning and research culminated with completion of the Tunnel 9 Mach 18 system calibration in July 2020. The capability allows for testing at speeds never before realized in an AEDC facility. The first customer runs at Mach 18 occurred in August 2020.
Smith expressed appreciation for those whose efforts helped provide an enhanced measurement technique for the new capability.
“The assistance of the AEDC SBIR coordinators, Nick Galyen and Robert Howard, is greatly appreciated,” Smith said. “Also, the concurrent SBIR to provide AEDC with a burst-mode laser, monitored by Joe Wehrmeyer, capable of measuring gas-phase temperature [based on Coherent Anti-Stokes Raman Spectroscopy] and velocity [based on PLEET] aided in the development of a necessary piece of hardware for the demonstration.”
Work is set to continue. Smith said the OPO will be delivered to Parziale for further development of the high-speed KTV technique.
“Additionally, Professor Parziale has acquired a burst-mode pump laser from Spectral Energies funded by a Navy DURIP [Defense University Research Instrumentation Program],” Smith said. “This will allow the development of turbulent boundary layer and multi-line velocity measurements for application to the Tunnel 9 Mach 14 and Mach 18 facilities. This capability will allow AEDC to more completely characterize the Tunnel 9 turbulent boundary layers of hypersonic flow fields.”
Smith added the successful KTV demonstration is just another example of SBIR projects that have benefited Tunnel 9.
“The SBIR program has been put to good use at Tunnel 9 in recent years, especially in the Mach 18 facility development,” he said.