Experimental Methods for Studying and Characterizing Freely Expanding Supersonic Gaseous Jets
Authors: Holguin Ryan , Bernardin John , Morgan Robert 
Published: November 2019

This manuscript describes an experimental system that was constructed to observe and scrutinize the transient fluid mechanics of a supersonic gaseous jet freely expanding into ambient air conditions within a steel containment vessel. Measurement parameters included jet expansion angle, peak jet velocity and local velocity profile, shock propagation, vessel gas entrainment, and wall stagnation pressure. Test gases included air and helium at pressure ratios ranging from roughly 2 to 300. The measurement techniques used to characterize the gas jets included hot wire anemometry, high-frequency pressure transducer measurements, and schlieren/shadowgraph imagery. The development of a system capable of capturing the desired data presented many engineering challenges including optical alignment for schlieren imaging, synchronizing the system for consistent and repeatable data collection, development of an experimental vessel capable of incorporating measurement equipment, and accommodation for future measurement capabilities.

A vented PVC cylindrical test vessel was utilized in the preliminary stages of experimentation and set up of the gas delivery and diagnostic systems. Upon completion of the preliminary testing, a stainless steel experimental vacuum and pressure vessel, capable of accommodating a variety of diagnostics, was designed and fabricated. The gas jet delivery system consisted of a restrictive flow orifice, high pressure two stage regulator, two isolation valves, and a high pressure relief valve set to 4500 psig. Downstream from the safety manifold was a high pressure AC solenoid. This configuration was able to generate a maximum supply pressure of 4000 psig, corresponding to a maximum gas pressure ratio of 400 for a vessel at atmospheric pressure and 4,000 for vessel under low vacuum. The schlieren/shadowgraph configuration utilized for the imaging is a Z-Type configuration and possesses the advantages of both reducing aliasing effects and decreasing the overall area needed for the schlieren arrangement. Schlieren images taken were captured with a PCO Pixelfly CCD camera. A Photron high speed camera eventually replaced the Pixelfly within the schlieren arrangement expanding the imaging capabilities. A large polycarbonate enclosure was developed to enclose the entire system, shielding both the worker and the optics. Pressure and velocity sensors with high frequency response capability were selected to adequately monitor rapidly changing jet characteristics. PCB Piezotronic pressure sensors were mounted flush to the wall of the vessel opposite the gas jet inlet. A TSI one dimensional hot wire probe was inserted radially along the horizontal axis of the vessel, perpendicular to the jet flow. A NI Compact RIO data acquisition system, run with LabVIEW, was used to record the pressure measurements. For the hot wire anemometry velocity measurements, a standalone TSI IFA 300 was used to capture and process data. A dimensional analysis was performed to define the jet velocity in terms of other jet parameters, characteristic lengths, and fluid properties. The dimensional analysis results did not elucidate the substantiality of the dimensionless groupings; however, some of the missing exponents can theoretically be parameterized through additional future testing.

Initial measurements with the experimental system will be presented and discussed. Schlieren and shadowgraph images and velocity measurements of air and helium jets were captured in both the PVC and steel vessel configurations. Pressure ratios of 10 to 300 were examined for helium, while pressure ratios up to 20 were achieved for air. The data shows how the leading edge velocity, average spread angle, and Mach disk height data are all influenced by pressure ratio and gas type. Velocity frequency content, basic jet turbulence structure, and gas entrainment are also evident in the experimental data. Based on these initial measurements, an outline for ongoing experimental studies will be presented.