Reynolds Stress Anisotropy in a Two-Stage Low Pressure Axial Turbine at Low Reynolds Numbers

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Computational fluid dynamics have become important in turbine design, because experimental tests can easily become very expensive and time consuming. The industrially used two-equation turbulence models have weaknesses in predicting the Reynolds stress anisotropy in complex flows. The free stream Reynolds stresses influence transition and separation on turbine airfoils and vice versa. Higher-order models are supposed to improve numerical prediction quality. For development and validation of these models, a good understanding of the Reynolds stress distribution is required. Therefore the full Reynolds stress tensor and its anisotropy are experimentally investigated in a two-stage low pressure axial turbine. The Reynolds stresses are resolved from 3D hot-film probe area traverses downstream of the first vane at three Reynolds numbers from 40,000 to 180,000, related to vane 1. Surface thin film gauge measurements on the suction side of the vane are used to determine transition and separation. The size of the separation bubble on the late suction side and the progress of transition vary with Reynolds number. This influences the Reynolds stress elements to different extents and thus the Reynolds stress anisotropy downstream of the vane.