CFD analysis of complete helicopter configurations – lessons learnt from the GOAHEAD project

The GOAHEAD project, funded under the 6th European Funding framework, provided valuable measurements of flow parameters for a realistic helicopter configuration. The wind tunnel investigations included an extensive set of conditions from cruise at high speed, to very high speed flight as well as high disk loading cases. Several GOAHEAD partners, including the University of Liverpool, contributed CFD simulations for the full helicopter in the blind test phase (prior to the wind tunnel test), as well as, the post-wind tunnel test phase. The Helicopter Multi Block solver (HMB) of Liverpool University was the only in-house code from the UK to be used in this project. To account for the relative motion of rotor(s) and fuselage, the sliding-plane approach was used. As a first step of the project, a family of multi-block CFD meshes was developed at Liverpool designed to work with the sliding-plane method. For the blind test phase, the tail rotor was omitted. Using the lessons learnt from this first phase, a more advanced multi-block topology was developed for the post-wind tunnel phase of the project, which allowed main and tail rotors to be included. The pre-test computations for the economic cruise condition were found to be in good agreement with the experiments when comparing surface pressure at various places on the fuselage considering the relative coarseness of the employed grids. Also, the CFD results of the various partners agreed reasonably well. As expected, the main discrepancies were in the separated-flow regions at the back of the helicopter. The improved meshes used in the post-test phase resulted in better spatial resolution of the flow in addition to having the added complexity of the tail rotor. These new sets of results were in better agreement with measurements and were also performed on finer meshes. Clearly, the quality of the CFD mesh is key for accurate predictions and an educated guess of the flow regions where severe interactions of flow structures will occur is of importance for such complex CFD computations. Remarkably, the efficiency of the CFD solver was high, and CFD analyses on meshes of up to 30 million cells were performed during this project