Marc Ramsey

Assessment of the Application of Hydroxyl Tagging Velocimetry to Rocket Engine Exhausts

Hydroxyl tagging velocimetry (HTV) is a non-intrusive technique for velocity measurement in gas flows. A molecular tag is written within a flow in the form of hydroxyl radicals (OH) created by photo-dissociation of water vapor in air or combustion products with 193 nm ArF excimer laser light. This tag is imaged at discrete points in time by laser induced fluorescence (LIF) stimulated by OH(A-X)(1←0) excitation with an Nd:YAG pumped dye laser tuned near 283 nm. Flow velocity data can be determined by analysis of tag displacement over time. HTV has been proposed as a velocity diagnostic for hydrogen fueled rocket exhaust. The anticipated environment is simulated in the exhaust of a laboratory H2/O2/N2 Hencken burner flame, and key performance parameters for the method are evaluated. The demonstrated 200μs useful tag lifetime is two orders of magnitude longer than required, and agreement is found between analytical prediction and experiment. Displacement determination with accuracy less than 0.75 detector pixels is demonstrated within the signal-to-noise ratio range of 85-1.5 observed at delay times varying from 50ns-200μs after the tag is written. Signal sensitivity to temperature and mixture ratio variations is reported. Tag emission spectra are presented and the appropriate optical filters for signal detection are discussed. Results indicate that HTV can be successfully applied as a velocity diagnostic in rocket exhaust.

Studies on thin films as short pulse laser debris shields

Optical properties of various thin films such as Nitrocellulose, Mylar, and Polyimide were investigated with respect to their application as laser debris shields. Studies on optical and spectral transmission quality, absorption, stress induced birefringence, and damage threshold have been performed. Scalability to large apertures was also considered. Studies were performed of how focusing geometry, target alignment, and mechanical components can help mitigate target debris traveling back to the focusing optic.

Experimental Velocity Profiles in the Cap Shock Pattern of a Thrust Optimized Rocket Nozzle

The cap shock pattern is observed in the plumes of over-expanded thrust optimized rocket nozzles at the low pressure ratios that typically occur during start-up and shut-down transients. This shock pattern is related to restricted shock separation, and associated flow transitions can result in damaging side loads. Instantaneous 2D planar velocity measurements are obtained here with hydroxyl tagging velocimetry, in which molecular tags are written into the flow by laser photo-dissociation and tracked by laser-induced fluorescence. Measured velocity profiles are compared with numerical simulation and show promise as a detailed validation tool.

Investigation of a Bow Shock in a Shock Tube Flow Facility Using Hydroxyl Tagging Velocimetry (HTV)

Hydroxyl tagging velocimetry (HTV) is applied to a shock tube located at the Arnold Engineering Development Center, Arnold AFB, TN. In HTV, an 11x11 grid of OH molecules is formed by dissociating H2O in the stream with an ArF laser at 193 nm. The 11 x 11 grid is tracked over a fixed time interval to determine the velocity in the plane of the grid at the crossing points. The undisplaced grid is imaged by 193-nm induced fluorescence before the experiment has begun at atmospheric pressure and the displaced image is imaged by 282-nm induced fluorescence of OH. Both images are recorded by an intensified interline CCD camera mounted above the field of view. The images are used to determine the velocity downstream of the shock wave for two test articles, a 10-deg half-angle blunt cone and a 30-deg half-angle cone. The HTV images are analyzed via a spatial correlation technique that locates the displacement between the image pairs. HTV is successfully demonstrated to obtain multipoint, spatially resolved 2D data in a highly accelerated flow.

Dual-Pulse Hydroxyl Tagging Velocimetry (HTV) in Jet Engine Exhausts

Hydroxyl tagging velocimetry (HTV) is applied to measure the centerline velocities of a turbojet engine exhaust from a J85-GE-5 turbojet engine located in Tullahoma, Tennessee at the Arnold Engineering Development Center. In HTV, an 11x11 grid of OH molecules is formed by dissociating H2O in the exhaust stream with an ArF laser at 193 nm. The 11x11 grid is tracked over a fixed time interval to determine the velocity in the plane of the grid at the crossing points. In order to eliminate the deleterious effects of the engine vibrations on the optical system, the HTV measurement is made using a dual-pulse method to capture both the initial and final grid images in a short time interval (3-9 μs). The undisplaced grid is imaged by 193 nm induced fluorescence and the displaced image is imaged by 282 nm induced fluorescence of OH. Both images are recorded by an intensified interline CCD camera operating in dual-pulse mode to capture the two images, undisplaced and displaced, in quick succession. In spite of the low signal-to-noise ratio of the undisplaced images (about 1), the image pairs are analyzed to determine the velocity via computer software that locates the displacement between the image pairs using a spatial correlation technique. By eliminating engine vibration effects on the grid displacement, the dual-pulse HTV method records reduced rms velocity variations in the engine exhaust when compared to a single-image HTV method.

Megabar cavitation collapse

Summary form only given. A megabar plasma has been produced in a precisely controlled cavitation event with a repetition rate that can range from single shot to several per minute.