Study On Flow Field Characteristics In Turbulent Boundary Layer On The Micro-Ribbed Surface

Turbulence is one of the most widespread fluid flow phenomena in nature and industrial society.Many problems in engineering applications are closely related to turbulence.It is possible to control turbulence due to the discovery of the coherent structure and its research has become a hot issue in the energy industry.In order to reveal the intrinsic mechanism of the coherent structure and to understand the effect of the micro-ribbed surface on the coherent structure,the technique of hydrogen bubble tracer is used to visualize the turbulent boundary layer on the smooth surface and the micro-ribbed surface with water in the vertical rectangular channel in this paper.By means of hydrogen bubble tracing and visual observation,the distribution of the low-speed and high-speed streaks,the formation of long strip,the whole process of burst and various vertical structures are introduced in detail.The flow field distribution in the boundary layer of the smooth surface and the micro-ribbed surface is measured by DPIV and the effect of micro-ribbed surface on the flow field is analyzed,especially the turbulent intensity distribution in the near wall.The influence of the micro-ribbed surface is reduced on the turbulence intensity with the increase of y+.The effect of the micro-rib structure on the turbulence intensity is mainly concentrated in the near wall of the boundary layer,while the impact on the outer region is small.By means of mathematical statistics,we get the eigenvalue distribution of coherent structure in the boundary layer of smooth surface and micro-rib surface,including the dimensionless spacing of the long strip,burst frequency and burst period.In the region of y+<30,the distribution of the long strip spacing on the smooth surface and micro-ribbed surface is plotted.It is found that there is an inflection point in the normal distribution of the dimensionless spacing of the long stripe,and the presence of micro-ribs causes the position of the inflection point to lag.The burst frequency of smooth surface and micro-ribbed surface increases with the Reynolds number,and the burst period decreases with the increase of Reynolds number.As the height of the ribs increases,the burst period increases.Relative to the height of the ribs,the effect of tilt angle on the burst frequency is less affected by the Reynolds number and the effect of micro-ribbed surfaces with different tilt angles of 30°is the same on the burst frequency and burst period.From both the burst frequency and burst period,the longitudinal micro-ribbed surface has the best flow drag reduction effect.A physical model is established for the smooth surface to describe the whole burst process and the relation between the burst period of smooth surface and the Reynolds number is deduced.On this basis,the relationship between the burst period and the Reynolds number of the longitudinal micro-ribbed surface and the inclined micro-rib surface is derived considering the influence of two factors of rib height and tilt angle,and the applicable range of the model is expanded.