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To study the influence of hydrogen on the fatigue strength of AISI type 304 metastable austenitic stainless steel, specimens were cathodically charged with hydrogen. Using tensioncompression fatigue tests, the behavior of fatigue crack growth from a small drill hole in the hydrogen-charged specimen was compared with that of noncharged specimen. Hydrogen charging led to a marked increase in the crack growth rate. Typical characteristics of hydrogen effect were observed in the slip band morphology and fatigue striation. To elucidate the behavior of hydrogen diffusion microscopically in the fatigue process, the hydrogen emission from the specimens was visualized using the hydrogen microprint technique (HMT). In the hydrogen-charged specimen, hydrogen emissions were mainly observed in the vicinity of the fatigue crack. Comparison between the HMT image and the etched microstructure image revealed that the slip bands worked as a pathway for hydrogen to move preferentially. Hydrogencharging resulted in a significant change in the phase transformation behavior in the fatigue process. In the noncharged specimen, a massive type a¢ martensite was observed in the vicinity of the fatigue crack. On the other hand, in the hydrogen-charged specimen, large amounts of e martensite and a smaller amount of a¢ martensite were observed along the slip bands. The results indicated that solute hydrogen facilitated the e martensitic transformation in the fatigue process. Comparison between the results of HMT and EBSD inferred that martensitic transformations as well as plastic deformation itself can enhance the mobility of hydrogen.

Mean wind force coefficients of nacelles are investigated by a wind tunnel test and are compared with those in current codes, such as the Germanischer Lloyd Guideline 2010 (GL2010) and Eurocode, in order to clarify the effects of the ground, presence of a hub, turbulence in the incident flow and nacelle length on these coefficients. Formulas for the mean wind force coefficients are proposed as a function of yaw angles. It is found that mean wind force coefficients of wind turbine nacelles specified in GL2010 are underestimated in comparison with those obtained by wind tunnel tests.Pressure measurements of a nacelle are also conducted. Notably, the mean pressure coefficients for design load case 6.2 (DLC6.2) are significantly larger than those for design load case 6.1 (DLC6.1) in IEC61400-1. Maximum and minimum mean pressure coefficients are proposed for the DLC6.1 and DLC6.2 by the wind tunnel test, which are similar to those in Eurocode and are larger than those proposed in GL2010.

Instantaneous velocity distributions in the viscous and roughness sublayers and in the lower parts of the logarithmiclaw region of flat-plate boundary layers over smooth and rough surfaces are measured using a hot-wire rake consisting of two X-wire and nine single-sensor probes in order to investigate the turbulence structure and similarity of the instantaneous and filtered velocity profiles near the wall. The results can provide useful information as to the treatment of flow in this region in a large-eddy type simulation of high Reynolds-number flows over smooth and rough walls. The instantaneous velocity profiles feature large-scale structures but random fluctuations of considerable amplitude are superimposed and no similarity that is applicable to instantaneous unconditioned profiles can be derived. Conditionally averaged profiles based on the magnitudes of the wall shear and normal velocity in the buffer layer, however, show trends related to the sweep-ejection type events. The filtered profiles normalized by the filtered instantaneous friction velocity show similarity that progressively extends its region from the immediate neighborhood of the wall to the log layer as the filter size is increased. The minimum streamwise length of the filter size required to show logarithmic similarity is found to be as large as 1800 wall units. Similar results are obtained for rough-wall boundary layers if the instantaneous friction velocity is replaced by the equivalent velocity scale implied by the velocity just outside the roughness-influenced sublayer at about 10 viscous units from the wall, or about 1.5 times the height of the roughness bars from the wall. The sub-grid stress components have are obtained from the data and implication for modeling is discussed.

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