Distribution of turbulent eddies behind a monopile for vortex lock-on condition due to wave current combined flow

The paper presents the results of an experimental study on wave current combined flow around a monopile (partially submerged circular cylinder) for a large Reynolds number. Measured instantaneous time series velocity signals (i.e., the sequence of discrete-time dependent velocity data) were analyzed to evaluate the statistical moments. It was found that vortex lock-on takes place when the forcing frequency of the superimposed surface wave is double that of the natural shedding (NS) frequency. Also, the recirculation region almost disappeared for vortex lock-on condition. The vortices generated due to superimposed surface waves increased the turbulence level near the shear layer at the recirculation region. Moreover, the Reynolds shear stress increased near the shear layer at the recirculation region due to vortex lock-on compared to that of NS condition. Further, the Gram-Charlier type joint probability distributions for two variables were estimated to describe the statistical properties of turbulent velocity fluctuations in different co-ordinate directions. Results from wavelet analysis of phased averaged velocity fluctuation suggested that the wave motion could modulate not only the large-scale eddies in the energy containing region, but the moderate scale eddies for the vortex lock-on condition. At the wake region of the monopile for vortex lock-on condition, it was pertinent that the wave motion induced fluctuating eddies of the background flow gained in strength and showed periodic distribution at the NS frequency (half of the induced wave frequency) in the time domain. The comparative study between NS and vortex lock-on condition showed that the induced wave motion can modulate the instantaneous small amplitude values of stream - wise and transverse velocity fluctuations (u'v') that occur at the recirculation region.