Rocket Exhaust: 2D Velocity Measurements

Another application of laser diagnostic measurements is within the complex and harsh flow-field of rocket exhaust. The cap shock pattern is observed in the plumes of over-expanded thrust optimized rocket nozzles at the low pressure ratios that occur during start-up and shut-down transients. These nozzles exhibit both free and restricted shock separation, and the related flow transitions can result in damaging instabilities. Instantaneous 2D planar velocity measurements obtained here with Hydroxyl Tagging Velocimetry (HTV), in which molecular tags are written into the flow by laser photo-dissociation and tracked by laser-induced fluorescence, provide a detailed reference data set in the cap shock region. Measured velocity fields are compared to axisymmetric numerical simulation and are in good agreement.

Shown to the right is a schematic of the HTV experimental setup. Lines of OH are written into the flow-field using an ArF Excimer laser (193 nm), and the displaced lines are imaged using planar laser induced fluorescence with 283 nm laser light.

 

 

 

 

The data is reduced using the newly developed Template Matching Method, and the raw data is shown to the left. The dotted lines show the initial OH grid position, and the solid lines show the displaced grid—highly deformed by the large variations in velocity.

The resulting velocity field measurement is shown below.

 

Adapted from:
M. C. Ramsey, R. W. Pitz, T P. Jenkins, Y. Matsutomi, C. Yoon and W. E. Anderson, “Planar 2D Velocity Measurements in the Cap Shock Pattern of a Thrust Optimized Rocket Nozzle,” Shock Waves, 22, pp. 39-46, 2012.