Surrogate Models for Transonic Aerodynamics for Multidisciplinary Design Optimization

Multidisciplinary design optimization (MDO) requires many designs to be evaluated while searching for an optimum. As a result, the calculations done to evaluate the designs must be quick and simple to have a reasonable turn-around time. This makes aerodynamic calculations in the transonic regime difficult. Running computational fluid dynamics (CFD) calculations within the MDO code would be too computationally expensive. Instead, CFD is used outside the MDO to find two-dimensional aerodynamic properties of a chosen airfoil shape, BACJ, at a number of points over a range of thickness-to-chord ratios, free-stream Mach numbers, and lift coefficients. These points are used to generate surrogate models which can be used for the two-dimensional aerodynamic calculations required by the MDO computational design environment. Strip theory is used to relate these two-dimensional results to the three-dimensional wing. Models are developed for the center of pressure location, the lift curve slope, the wave drag, and the maximum allowable lift coefficient before buffet. These models have good agreement with the original CFD results for the airfoil. The models are integrated into the aerodynamic and aeroelastic sections of the MDO code. Surrogate Models for Transonic Aerodynamics for Multidisciplinary Design Optimization Molly C. Segee GENERAL AUDIENCE ABSTRACT Multidisciplinary design optimization (MDO), when applied to aircraft design, is a method of balancing the aerodynamic, structural, and control requirements of a design, which are often conflicting, to find an optimum design. This method has the potential to produce better, more efficient airplane designs as well as to save time and expense in the design process. Finding an optimum design with MDO requires the performance of many potential designs to be evaluated. The evaluation methods used must be quick and not require too many computational resources to keep the process from taking a long time. This greatly limits the aerodynamic calculations which can be done. Most commercial airplanes have cruise Mach numbers in the transonic regime. This means that there are regions of both subsonic and supersonic flow around the airplane, as well as shocks. The differences in behavior of subsonic and supersonic flow as well as the sudden changes in the flow caused by shocks usually require the use of an evaluation method called computational fluid dynamics (CFD) to accurately evaluate the aerodynamic performance. However, CFD takes too long to be used for calculations in the MDO code. Instead, CFD is used outside the MDO code to find the aerodynamic performance of an airfoil at several points over a range of thickness-to-chord ratios, free-stream Mach numbers, and lift coefficients. Next, functions are found which closely match the CFD results. These functions are then used by the MDO to calculate aerodynamic performance. This method allows aerodynamic calculations to be done in the MDO to better match real world physics without significantly changing the time required. This improves the accuracy and usefulness of designs found by the MDO.