Supersonic PIV measurements in an open jet flow

Abstract

Some preliminary supersonic PIV measurements were done in a simple open free jet to provide CFD validation. Although difficulties were initially encountered with light reflection or insufficient seeding, satisfactory results were eventually obtained at Ma 2, 2.5 and 3. The qualitative agreement was very good when comparing streamline patterns for both PIV and CFD results. For quantitative evaluation, velocity profiles were completed at Ma 2 and the results were very close at the four selected vertical positions. At Ma 2.5 and 3, the differences were more significant due mostly to insufficient seeding for the PIV or inadequate grid refinement for the CFD calculations. These initial tests therefore meet the objectives of getting the knowhow to properly make PIV measurements and obtain data to validate CFD calculations. * Graduate Student, Mechanical Engineering Professor, Mechanical Engineering, Member AIAA * Student, Mechanical Engineering § NSERC Post-Doctoral Fellow, Propulsion Group ' Scientist, Propulsion Group, Member AIAA * Scientist, Propulsion Group, Senior Member AIAA Copyright © 2001 by the Department of National Defence and Universite Laval, Canada. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Introduction Although it remains a challenge for measuring supersonic flow, Particle Image Velocimetry (PIV) is a maturing technology.''' A validation study was undertaken to compare PIV measurements with those of Computational Fluid Dynamics (CFD) calculations. The main motivation for this study is to provide suitable validation for a more elaborate Thrust Vector Control (TVC) system with multiple vanes installed in the exhaust plume of a rocket motor near the exit plane of the nozzle. With an eventual measurement of lift and drag forces, it will then be possible to complete a more detailed validation. The Open Jet Facility The open jet consists of a high-pressure tank held at 420 kPa discharging vertically to atmosphere in a blow-down mode through a converging/diverging nozzle. The exit plane has a 25 mm diameter. The first series of results present PIV measurements done in the exhaust jet of a supersonic nozzle at three different velocities: Ma 2, 2.5 and 3. Once supersonic conditions were confirmed, a small vane was installed in the supersonic flow to measure flow at different vane angles with the same three Mach numbers 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. using the PIV system. Experiments were also conducted successfully at Mach numbers below one and above three with and without the vane insert. PIV Velocity Measurements The PIV measurements were done with the FlowMaster III system provided by the German company La Vision. A double pulse Continuum Nd-YAG laser served to produce a light sheet to illuminate the flow seeded with very fine ethyl glycol droplets produced with a TSI atomizer. Two successive illuminations of the flow field allowed recording of the droplet displacement by one double-frame highresolution 1280 x 1024 CCD camera. The 2D-PIV software provided by La Vision was used for processing the recorded images and the correlation window was set at 32x32. Different seeding materials were used but ethyl glycol was found to provide the right amount of light scattering for the measurements and minimized erosion effects on the vane insert. Many problems were encountered with light reflection from shining surfaces and inadequate seeding. Covering the exposed metal surfaces with absorbing black paint eliminated most of the light reflections. Also rust particles from the air reservoir significantly eroded the painted surface of the small vane, which consequently increased the reflected surface and saturated PIV measurements. It was decided that a graphite vane would be used to prevent both erosion and reflection. Inadequate seeding was the result of having to limit the reservoir pressure to the operating limit of 420 kPa gauge for the droplet generator. With the air reservoir operating at about the same pressure, the droplet feed rate was limited and most of the time insufficient. Consequently the droplet generator feed rate was further increased with a sandblast gun discharging directly into the air reservoir. These two combined to properly seed the flow and allowed adequate image recordings; otherwise very poor image quality resulted.