Application of an implicit dual-time stepping multi-block solver to 3D unsteady flows R. Steijl a , P. Nayyar a* , M.A. Woodgate a , K.J. Badcock a and G.N. Barakos a a CFD Laboratory, Department of Aerospace Engineering, University of Glasgow, Glasgow, G12 8QQ, United Kingdom This work discusses the application of a parallel implicit CFD method to challenging 3D unsteady flow problems in aerospace engineering: transonic cavity flows and the flow field around a helicopter rotor in forward flight. The paper discusses the computational details of simulations using the HPCx supercomputer (1600 processors) of Daresbury Lab., U.K. and a Beowulf cluster (100 processors) of the CFD Laboratory of the University of Glas- gow. The results show that accurate simulations based on Large-Eddy Simulation (LES) and Detached-Eddy Simulation (DES) at realistic Reynolds numbers require impractical run times on the Beowulf cluster. A simulation of a full helicopter geometry is similarly beyond the limits of the 100-processor Beowulf cluster. 1. INTRODUCTION Many of the present CFD applications in aerospace engineering involve unsteady three- dimensional aerodynamic problems. In contrast to steady state flows, which can typically be tackled in a matter of hours on a multi-processor machine or on a Beowulf cluster, unsteady flows require days of CPU time. This paper presents the application of a parallel, unfactored, implicit method for the solution of the three-dimensional unsteady Euler/Navier-Stokes equations on multi-block structured meshes [1]. For time-accurate simulations, dual time-stepping is used. The solver and its performance on Linux clusters was previously discussed in refs.[2] and [3]. The application examples presented here are for three-dimensional unsteady aerody- namic problems: transonic cavity flow and flow around a helicopter rotor in forward flight. The simulations were carried out on the HPCx supercomputer of the Daresbury Lab. in the UK[6] and the local Beowulf cluster (comprising 100 Pentium 4 processors). The CFD method and its parallelization are described in Sections 2 The application to 3D unsteady flow problems is described in Section 3, while conclusions are drawn in Section 4. * Present address: Aircraft Research Association Ltd., Manton Lane, Bedford, Bedfordshire, England MK41 7PF 1