| An experimental study was performed with a single-sensor hot-wire probe in the wall region of a nominally zero pressure gradient turbulent boundary layer over smooth and V-groove riblet plates of {dollar}hsp{lcub}+{rcub} sim{dollar} 14, {dollar}ssp{lcub}+{rcub} sim{dollar} 28 at {dollar}Resb{lcub}theta{rcub} simeq{dollar} 1200. The shear stress variation in the spanwise direction and the alteration of the turbulent characteristics were directly measured. The spanwise average shear stress on riblet groove was obtained and contour lines were fitted to the measurement for all the statistics. Net drag reduction of about 4% was found for this case, in substantial agreement with previous direct measurement. The synthetic analysis, at {dollar}ysp{lcub}+{rcub} sim{dollar} 30, of a modified Kovasznay-type probe was performed and an improvement algorithm was developed. The performance of a cross-shaped four-sensor probe in the turbulent boundary layer in the highly sheared region ({dollar}ysp{lcub}+{rcub} sim{dollar} 4) is also presented. To further understand the flow structure inside the riblet, four-sensor hot-wire probes of a modified version of the original Kovasznay-type and of a cross-shaped were used over the two test surfaces at {dollar}Resb{lcub}theta{rcub} simeq{dollar} 2500 and 2100, respectively. Statistical properties of the fluctuating velocity components and the Reynolds stress, including joint probability density functions and frequency spectra of data obtained over the smooth and the peak and over the valley of the riblet plate are presented. |
