Experimental Investigation of Reynolds Number Effect on Wind Turbine Profiles in the Cryogenic Wind Tunnel Cologne DNW-KKK

Wind turbine size has increased continuously and correspondingly also its Reynolds numbers. The Reynolds number effect can therefore no longer be ignored in design and optimization of wind turbines. Reliable profile test data should be available. A suitable facility for testing wind turbine profiles at high Reynolds numbers is the Cryogenic Wind Tunnel Cologne DNW-KKK. By means of injecting liquid nitrogen the tunnel can be cooled down to 100 K and the Reynolds number therefore can be raised accordingly. The maximum Reynolds number for 2D profile tests can reach 27x10^6. In this paper the test uncertainty and the flow quality of DNW-KKK were analyzed. Then some test results on the Reynolds number effect of the wind turbine profiles will be presented. The Reynolds number effect is different from model to model. Especially for thick profiles and flow control devices the Reynolds number effect is not always like the description in literature.

Experimental Investigation of Flow Instabilities in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study aimed at investigating the dynamics of a laminar separation bubble, from the origin of separation up to the breakdown to turbulence of the large scale co- herent structures generated as a consequence of the Kelvin-Helmholtz instability process. Measurements have been performed along a fiat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble at low Reynolds number condition. Measurements have been carried out by means of comple- mentary techniques： hot-wire （HW） anemometry, Laser Doppler Velocirnetry （LDV） and Particle Image Veloci- metry （PIV）. The high accuracy 2-dimensional LDV results allow investigating reverse flow magnitude and both Reynolds normal and shear stress distributions along the separated flow region, while the high frequency response of the HW anemometer allows analyzing the amplification process of flow oscillations induced by instability mechanisms. PIV results complement the flow field analysis providing information on the generation and evolu- tion of the large scale coherent structures shed as a consequence of the separated shear layer roll-up, through in- stantaneous velocity vector maps. The simultaneous analysis of the data obtained by means of the different meas- uring techniques allows an in depth view of the instability mechanisms involved in the transition/reattachrnent processes of the separated shear layer.

Combined Experimental and Numerical Investigations on the Roughness Effects on the Aerodynamic Performances of LPT Blades

The aerodynamic performance of a high-load low-pressure turbine blade cascade has been analyzed for three different distributed surface roughness levels(Ra) for steady and unsteady inflows. Results from CFD simulations and experiments are presented for two different Reynolds numbers(300000 and 70000 representative of take-off and cruise conditions, respectively) in order to evaluate the roughness effects for two typical operating conditions. Computational fluid dynamics has been used to support and interpret experimental results, analyzing in detail the flow field on the blade surface and evaluating the non-dimensional local roughness parameters, further contributing to understand how and where roughness have some influence on the aerodynamic performance of the blade. The total pressure distributions in the wake region have been measured by means of a five-hole miniaturized pressure probe for the different flow conditions, allowing the evaluation of profile losses and of their dependence on the surface finish, as well as a direct comparison with the simulations. Results reported in the paper clearly highlight that only at the highest Reynolds number tested(Re=300000) surface roughness have some influence on the blade performance, both for steady and unsteady incoming flows. In this flow condition profile losses grow as the surface roughness increases, while no appreciable variations have been found at the lowest Reynolds number. The boundary layer evolution and the wake structure have shown that this trend is due to a thickening of the suction side boundary layer associated to an anticipation of transition process. On the other side, no effects have been observed on the pressure side boundary layer.

Numerical Study of Improving Aerodynamic Performance of Low Solidity LPT Cascade through Increasing Trailing Edge Thickness

This paper presents a new idea to reduce the solidity of low-pressure turbine(LPT) blade cascades,while remain the structural integrity of LPT blade.Aerodynamic performance of a low solidity LPT cascade was improved by increasing blade trailing edge thickness(TET).The solidity of the LPT cascade blade can be reduced by about12.5% through increasing the TET of the blade without a significant drop in energy efficiency.For the low solidity LPT cascade,increasing the TET can decrease energy loss by 23.30% and increase the flow turning angle by1.86% for Reynolds number(Re) of 25,000 and freestream turbulence intensities(FSTT) of 2.35%.The flow control mechanism governing behavior around the trailing edge of an LPT cascade is also presented.The results show that appropriate TET is important for the optimal design of high-lift load LPT blade cascades.