An experimental study of the unsteady characteristics of the turbulent near wake of a turbine blade

摘要

The paper reports the results of an experimental investigation of the time-varying characteristics of the flow in the turbulent near wake behind the central blade of a large-scale turbine cascade. The blade wake has been surveyed by means of a two-component laser Doppler velocimeter (LDV) over a close experimental grid extending from the blade trailing edge to six trailing edge diameters downstream. A phase-locked ensemble averaging technique has been applied to the LDV instantaneous data in order to separate coherent contributions due to large-scale organised structures from random contributions due to turbulence. A reference signal generated by a hot-wire probe, located at a fixed position within the flow, is utilised to associate each LDV velocity realisation with the phase of the vortex shedding. The hot-wire signal provides also an univocal time reference suitable for correlating all single-point measurements, allowing to construct sequences of images which show the time evolution of the organised structures embedded in the flow. In the discussion, the mean flow properties, the periodic structure of the unsteady flow and the vortex shedding mechanism are analysed. Similarities and differences between the present results for a turbine blade wake and those for wakes of circular cylinders are highlighted. Normal stresses due to the periodic motion and those due to turbulence fluctuations are resolved separately. Both contributions have the same order of magnitude in the region close to the trailing edge, where the vortices are forming, but the normal stresses associated with the periodic fluctuations decay more rapidly in streamwise direction. (C) 2000 Elsevier Science Inc. All rights reserved. [References: 20]