Local Shape Optimisation of Turbine Disc Firtrees Using NURBS

This paper describe s a general shape optimisation method for a structural optimisation problem using a combined feature -based and B -spline based free form geometry modelling approach. The geometry is defined by a number of features for the global shape and by Non -Uniform Rat ional B -splines (NURBS), via the use of control points for the local shape. The coordinates of the control points along with other dimensions are chosen as design variables. A knot insertion/removal process is introduced to increase flexibility and to obta in a compact definition of the final optimised shape. Here the geometry is modelled using an intelligent CAD system (ICAD), which enables complete automation of the design -to - analysis process. Stresses are obtained from finite element analysis. A two -stage hybrid strategy is used to solve the formulated optimisation problem. Its application to the design of a turbine disc firtree root is demonstrated. This new approach is compared to a more traditional optimisation analysis using a geometry model based on straight-lines and circular - arcs. Improvements in the objective function and flexibility in geometry definition are achieved compared to designs obtained using the traditional approach.