Aerodynamic Design and Optimization for Turbine Engine Exhaust Systems

The aerodynamic design of turbofan and turboshaft engine exhaust systems can be a very iterative process. There are several key parameters to consider for the design of a high performance exhaust system for aerospace applications. The design of these systems requires significant experience and expertise. This paper concerns the use of design of experiments and optimization with response surfaces as tools to assist the designer in determining the design that best meets the performance and integration requirements for the exhaust system. These tools are not intended to replace the designer whose expertise is needed to determine the best feasible design since the tools may present a variety of designs that meet the specified requirements. However, once a generic design/optimization system is set up, a less experienced engineer can produce reasonable results with a minimum of guidance from a senior aerodynamic design engineer. This is demonstrated by comparing the results obtained without design/optimization methods by an experienced aerodynamic designer to those obtained using design of experiments and optimization. The classical optimization problem statements are discussed, along with the Kuhn-Tucker conditions for global optimum. Methods of solution including direct methods, penalty methods and global optimizations are briefly reviewed. The motivation for using design of experiments (DOE) with response surfaces for solving engineering design/optimization problems is presented along with a brief review of the various DOE options available. The basic optimization problem statement is discussed along with methods to solve the specific problem. The application of the commercial software package, iSIGHT, to the design/optimization of a mixer/ejector for a turboshaft engine application will be presented. In this application, a one-dimensional analysis code is used to determine the overall geometric parameters for this exhaust system.