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Flow Processes in Rocket Engine Nozzles with Focus on Flow Separation and Side-Loads

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Flow Processes in Rocket Engine Nozzles with Focus on Flow Separation and Side-Loads TRITA-MEK Technical Report 2002:09 ISSN 0348-467X ISRN KTH/MEK/TR--02/09-SE FLOW PROCESSES IN ROCKET ENGINE NOZZLES WITH FOCUS ON FLOW SEPARATION AND SIDE-LOADS Jan Östlund Licentiate Thesis Stockholm, 2002 Royal Institute of Technology Department of Mechanics TRITA-MEK Technical Report 2002:09 ISSN 0348-467X ISRN KTH/MEK/TR--02/09-SE FLOW PROCESSES IN ROCKET ENGINE NOZZLES WITH FOCUS ON FLOW SEPARATION AND SIDE-LOADS by Jan Östlund May 2002 Technical reports from Royal Institute of Technology Department of Mechanics S-100 44 Stockholm, Sweden ABSTRACT The increasing demand for higher performance in rocket launchers promotes the development of nozzles with higher performance, which is basically achieved by increasing the expansion ratio. However, this may lead to flow separation and ensuing unstationary, asymmetric forces, so-called side-loads, which may present life-limiting constraints on both the nozzle itself and other engine components. Substantial gains can be made in the engine performance if this problem can be overcome, and hence different methods of separation control have been suggested, however none has so far been implemented in full scale, due to the uncertainties involved in modelling and predicting the flow phenomena involved. The present thesis presents a comprehensive, up-to-date review of supersonic flow separation and side-loads in internal nozzle flows with ensuing side-loads. In addition to results available in the literature, it also contains previously unpublished material based on this author’s work, whose main contributions are (i) discovery the role of transition between different separation patterns for side-load generation, (ii) experimental verification of side-loads due to aeroelastic effects and (iii) contributions to the analysis and scaling of side-loads. A physical description of turbulent shock wave boundary layer interactions is given, based on theoretical concepts, computational results and experimental observation. This is followed by an in-depth discussion of different approaches for predicting the phenomena. This includes methods for predicting shock-induced separation, models for predicting side-load levels and aeroelastic coupling effects. Examples are presented to illustrate the status of various methods, and their advantages and shortcomings are discussed. The third part of the thesis focuses on how to design sub-scale models that are able to capture the relevant physics of the full-scale rocket engine nozzle. Scaling laws like those presented in here are indispensable for extracting side-load correlations from sub-scale tests and applying them to full-scale nozzles. The present work was performed at VAC's Space Propulsion Division within the framework of European space cooperation. Keywords: turbulent, boundary layer, shock wave, interaction, intermittent, overexpanded, rocket nozzle, flow separation, side-load, models, criteria, prediction, review. TABLE OF CONTENTS 1 INTRODUCTION .............................................................................................................................................1 2 NOZZLE FUNDAMENTALS...........................................................................................................................5 3 NOZZLE CONTOUR DESIGN AND FLOW FIELD......................................................................................8 3.1 INTRODUCTORY REMARKS ................................................................................................................................9 3.1.1 Losses.....................................................................................................................................................9 3.1.2 Computational methods...........................................................................................................................9 3.1.3 Initial expension region.........................................................................................................................10 3.2 CONICAL NOZZLES .........................................................................................................................................10 3.3 IDEAL NOZZLE................................................................................................................................................11 3.3.1 Truncated Ideal Contoured nozzles (TIC)...............................................................................................13 3.3.2 Compressed Truncated Ideal Contoured nozzles (CTIC) ........................................................................14 3.4 THRUST OPTIMISED CONTOURED NOZZLES (TOC)............................................................................................14 3.5 PARABOLIC BELL NOZZLES (TOP) ....................................................................................................................18 3.5.1 Influence of skewed parabola design parameters on the flow field..........................................................20 3.6 DIRECTLY OPTIMISED NOZZLES.......................................................................................................................22 3.7 DESIGN CONSIDERATIONS OF CONVENTIONAL ROCKET NOZZLE ........................................................................22 4 EXHAUST PLUME PATTERN......................................................................................................................24 5 FUNDAMENTALS OF FLOW SEPARATION .............................................................................................27 5.1 FLOW SEPARATION AS A BOUNDARY LAYER PHENOMENON...............................................................................27 5.2 SHOCK-WAVE BOUNDARY LAYER INTERACTIONS .............................................................................................28 5.2.1 The basic interactions ...........................................................................................................................28 5.2.2 The free interaction concept ..................................................................................................................30 1.1.3 The separation length............................................................................................................................33 1.1.4 Unsteadiness and 3-dimensional effects.................................................................................................34 6 FLOW SEPARATION IN ROCKET NOZZLES...........................................................................................40 6.1 FREE SHOCK SEPARATION ...............................................................................................................................40 6.2 RESTRICTED SHOCK SEPARATION....................................................................................................................42 6.3 CRITERA FOR FLOW SEPARATION PREDICTION IN ROCKET NOZZLES ..................................................................43 6.3.1 Free shock separation criteria...............................................................................................................43 1.1.2 Restricted shock separation criteria.......................................................................................................51 7 MEASURMENT OF FLOW SEPARATION AND SIDE LOADS ................................................................53 7.1 STATIC WALL PRESSURE MEASUREMENTS........................................................................................................53 7.2 FLUCTUATING WALL PRESSURE MEASUREMENTS .............................................................................................54 7.3 SIDE LOAD MEASURMENTS..............................................................................................................................55 7.3.1 Determination of the system frequency response function.......................................................................58 8 SIDE-LOADS – PHYSICAL ORIGINS AND MODELS FOR PREDICTION.............................................62 8.1 SIDE-LOADS DUE TO TRANSITION IN SEPARATION PATTERN ..............................................................................62 8.1.1 Origin of side load: observations of the VOLVO S1 nozzle flow..............................................................62 8.1.2 Side-load model ....................................................................................................................................66 8.2 SIDE-LOADS DUE TO TILTED SEPARATION LINE.................................................................................................68 8.3 SIDE-LOADS DUE TO RANDOM PRESSURE PULSATION........................................................................................73 8.4 SIDE-LOADS DUE TO AEROELASTIC COUPLING..................................................................................................79 8.4.1 Aeroelastic analysis...............................................................................................................................79 8.4.2 Experimental verification of the aeroelastic analysis..............................................................................85 9 FIELD MEASUREMENT TECHNIQUES IN OVEREXPANDED NOZZLES............................................89 9.1 SHOCK VISUALISATION...................................................................................................................................89 9.2 INFRARED CAMERA IMAGING ..........................................................................................................................91
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