Visualization of the formation of separation bubbles on a bell-shaped nozzle surface in relation to serious side-load

Abstract

Elimination of serious side-loads are essential in the design of first stage-rocket nozzles. Recent studies have indicated that the major origin of side-loads is a change of separation pattern. In the present paper, shear sensitive liquid crystal (SSLC) was applied to the nozzle wall to visualize changes of separation pattern and to determine the mechanism that produces side-loads. The results showed that SSLC applied to the nozzle wall successfully visualized separation, reattachment and re-separation of the boundary layer and a rapid movement of separation points. The CFD code based on the axisymmetric Navier-Stokes equation was firmly anchored by visualized shear stress distributions, wall pressure distributions and shadow graphs. Introduction Elimination of excessive side-loads during start-up and shut-down transients is one of the most difficult issues in the design of first-stage rocket nozzles. This has long been the subject of investigation but detailed studies on such things as proper contours of the nozzle which would minimize side-loads by optimizing nozzle efficiency have not been conducted. Recently, intensive studies have been conducted to determine the origin of side-loads in the Vulcain nozzle during start-up and shut-down transients [l]. M. Frev et al. |2-4l investigated truncated perfect nozzles and thrust optimized nozzles and found * Researcher, Kakuda Space Propulsion Laboratory, NAL, Member AIAA t Senior researcher, Kakuda Space Propulsion Laboratory, NAL, Member AIAA $ Group leader, Kakuda Space Propulsion Laboratory, NAL, Member AIAA 11 Associate Senior engineer, NASDA Copyright ©2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. that the highest side-loads occur in the thrust-optimized nozzle, when the type of separation changes from free shock separation (FSS) to restricted shock separation (RSS), or vice versa. The side-loads of the truncated perfect nozzle, in which only free shock separation occurs, are significantly lower, namely, only about one-third as high [4]. The authors are investigating methodology for the design of the nozzle contour of compressed truncated perfect (CTP) nozzles, proposed by J. D. Hoffman [5], which would optimize nozzle efficiency without severe side-loads. A CTP nozzle contour is obtained by linearly compressing the truncated perfect nozzle (TP nozzle) contour in the axial direction to obtain the desired nozzle length. A discontinuity in the nozzle slope produced in the above compression procedure is e l iminated by introducing a cubic equation which smoothly connects the linearly compressed curve to the initial circular curve. If strong compression is applied to a TP nozzle with a nozzle length longer than that of the original TP nozzle, the nozzle exit angle will be smaller compared with that of the original TP nozzle, resulting in reduction of divergence loss with a probable consequent Fig. 1 Cold flow test facility