Particle Image Velocimetry Investigation Of Coherent Structures In Wall-bounded Turbulent Flows And Their Passive Control By Riblets

The purpose of this thesis is to study multi-scale coherent structures in the wallbounded turbulent flows and their passive control by riblets. To do this, 2D TRPIV(two-dimensional time-resolved particle image velocimetry), TR-SPIV(timeresolved stereoscopic PIV) and TR-TPIV(time-resolved tomographic PIV) techniques have been at first used to obtain the velocity vector databases for the turbulent boundary layer(TBL) flows over the smooth plate and riblets, respectively, supplemented by the flow visualizations and HWA(hot-wire anemometry) techniques for verifying their reliability of the databases.Secondly, typical characteristic spatial scales, such as the dissipation scale, the shear scale, Taylor micro-scales and integral scales in the TBL over smooth plates when Reθ≈2460 have been calculated from the three-dimensional three-component(3D3C) velocity vector database. Results show that the dissipation scale slowly increases along the wall normal height( +) upwards; while the shear scale does not change significantly with + at the near-wall region; the integral scales do not change with the normal positions. The spatial lengths in the streamwise direction of multi-scale eddies have been acquired by means of the spatial auto-correlation method. In terms of the small-scale cases, the spatial characteristic lengths of multi-scale eddies are roughly equal along the streamwsie and spanwise direction at near the wall while these spatial lengths of eddies increase in general at + direction. However, at the outer region of the TBL, these spatial lengths gradually become equal. For the large-scale cases, these spatial lengths of eddies increase firstly along + direction, and then there is a slower increase trend. The streamwise lengths are always greater than spanwise lengths, with the normal lengths minimum.An asymmetric quadrupole coherent structure with a saddle point in the dynamics system has been detected by the improved quadrant splitting method(ISQM) and extracted by a new conditional sampling phase-averaged technique. This statistical model of quadrupole for coherent structures has a closer similarity at + direction, compared to the one in the other two directions. There is a relationship among such topological coherent structure and Reynolds stress bursting events, as well as the fluctuating vorticity. When other burst events are surveyed(the first-quadrant event Q1 and the third-quadrant event Q3), a fascinating bursting period circularly occurs: Q4-S-Q2-Q3-Q2-Q1-Q4-S-Q2-Q3-Q2-Q1 in the center of such topological structures along the streamwise direction. In addition, the probability of the Q2 bursting event occurrence is slightly higher than that of the Q4 event occurrence. The spatial saddle singularity point almost simultaneously appears with typical Q2 or Q4 events. It looks like a shape of X chromosome, which may be the universal structure in the TBL.Then, analysis on the database of the 2D TR-PIV measurements on the zero pressure gradient TBLs over smooth plate and riblets show that when the peak-to-peak spacing of riblets, s+ locating in the viscous linear regime of the drag-reducing curves, the space-time-averaged velocity nondimensionalized by the skin friction velocity of both plates are almost the same at the viscous near-wall region; while there is a clear shift upwards at the logarithmic layer where the riblets have the most shark-skin effect. In this case, the drag reduction is as a result of the dampened spanwise behaviors of the quasi-streamwise vortices above riblets. When s+ increases to the nearly optimum regime, the dimensionless velocities at near-wall region over riblets are larger than their counterpart, showing a thickened buffer layer of TBL and then they become equal at the log layer. Besides, turbulent intensity and Reynolds shear stress of the TBL over the drag-reducing riblets are both smaller than the ones over the smooth plate at the same +, indicating that the turbulence has been weakened. In addition, the skewness of velocity fluctuation has been decreased at the inner layer over drag-reducing riblets, although the flatness of streamwise velocity fluctuation is hardly affected.Riblets have a drag reduction of shark-skin effect on the quadrupole coherent structure and its bursting events. It significantly decreases the wall-normal fluctuating velocity during the Q2 and Q4 events. Thus, the weakened quadrupole coherent structure pairs are less able to induce the low-speed fluid at the near wall due to their reduced vorticity by the riblets. The frequency and strength of the bursting events are both decreased by riblets, which breaks the self-sustaining process of turbulence and thereby reduces the drag friction since the streamwise vortices are unable to lift up to form a hairpin vortex owing to the less sufficiently strong Q2 events. The analysis of TR-TPIV velocity database has further verified the influences on quadrupole coherent structure in the TBL over riblets. The 3D statistical model for coherent structures in the wall-bounded TBL exists at different + with a similar topological shape even in the outer layer. Riblets weaken the amplitudes of their spanwise vorticity when Q2 and Q4 events occur at the near wall, having most influence on the Q4 events which contribute to the high friction in particular. To sum, riblets has a shark-skin effect on the whole turbulent boundary layer.Lastly, flow visualizations and TR-SPIV measurements on the TBL flows over the drag-increasing riblets have shown a kind of spanwise Kelvin-Helmholtz-like rollers which may be generated as a result of Kelvin-Helmholtz instability due to the streamwise velocity inflexion just over the riblet peaks. The roller structures begin to appear below +≈26 above the riblets when the riblets spacing is up to near or beyond the optimum. The typical streamwise wavelengths of the spanwise rollers +≈40-90; their spanwise wavelengths +≥ 100 and their streamwise spacings between the Kelvin-Helmholtz-like rollers ?+≈150-200. The drag reduction breakdown or even becoming a drag increase case is duo to the onset of the Kelvin-Helmholtz instability.