Aerospace Science and Technology 11 (2007) 110–118 www.elsevier.com/locate/aescte Flowfield analysis of a linear clustered plug nozzle with round-to-square modules Marco Geron, Renato Paciorri, Francesco Nasuti ∗ , Filippo Sabetta University of Rome “La Sapienza”, Department of Mechanics and Aeronautics, Via Eudossiana 18, Rome, Italy Received 11 May 2006; received in revised form 19 July 2006; accepted 21 August 2006 Available online 17 October 2006 Abstract The plug nozzle is one of the advanced expansion devices proposed to improve the overall performance of launcher liquid rocket engines. The present work investigates the three-dimensional flow field generated on this kind of nozzle by partitioning the primary nozzle into modules. A linear plug nozzle has been designed together with modules having two different geometries: a rectangular cross section and round-to-square module. Numerical simulations have been carried out considering the case where all modules of the primary nozzle are active and the case where one module is turned off. The solutions are compared and specific three-dimensional flow structures taking place inside the modules and on the plug are identified. The relationship between these structures and the skin friction distribution within the module and along the plug surface is investigated. Finally, the effect on performance of these three-dimensional flow features is emphasized.  2006 Elsevier Masson SAS. All rights reserved. PACS: 47.60.+i Keywords: Flows in nozzles 1. Introduction In the design of the new generation space launchers an im- portant role is played by the performance of the engine expan- sion system working in a varying pressure environment. In this framework a great interest has been devoted to the linear plug nozzle, which has been the object of severals studies on winged reusable launch vehicles carried out in the last decade [5–9,19]. The plug nozzle is an external-expansion nozzle that yields self adaptation of the exhaust jet to varying ambient pressure ratios, in a certain range of the launcher trajectory. This self-adapting capability allows high nozzle expansion ratios while avoiding the risks of flow separation that would exist in equivalent bell nozzles. The plug nozzle is made of a primary internal expan- sion nozzle and an external expansion ramp, referred to as the plug surface. The internal expansion would be ideally limited to the subsonic part. Nevertheless, such a design requires ex- tremely large turning of the plug surface in case of large area * Corresponding author. E-mail address: francesco.nasuti@uniroma1.it (F. Nasuti). ratio nozzles. For this reason, the design optimization suggests that the first part of the expansion is made by conventional supersonic nozzles [6]. Moreover, most of the different engi- neering solutions proposed for plug nozzles have the following common feature: the primary expansion is made through a clus- ter of bell nozzles (or modules) exhausting onto a common linear plug surface [4,10,22]. The primary nozzle partitioning allows easier manufactur- ing, lower thermal loads, easier cooling and higher thrust vector capability. However, clustering causes additional performance losses due to three-dimensional flow inside the modules and to the interaction of jets exhausting from adjacent modules. For these reasons the three-dimensional features have to be stud- ied in depth to better predict the engine performance and the expected mechanical and thermal loads in nominal operating conditions, both at sea level and altitude, and for differentially throttled modules as well. In fact, the thrust vectoring could be achieved by differential throttling of modules and, when thrust requirement is reduced in the final part of the ascent, some of the modules could be intentionally shut down. 1270-9638/$ – see front matter  2006 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2006.08.004