The modulation of particles on coherent structure of turbulent boundary layer in dilute liquid-solid two-phase flow with PIV

Abstract

Liquid-solidtwo-phase wall turbulent flow is an important flow pattern in engineering field. Studying the modulation of particles on fluid plays a significant role in revealing the mechanism of momentum, heat and mass transfer, improving production efficiency and decreasing energy consumption. The statistical characteristics and coherent structure in the flat plate turbulent boundary layer in clean water condition and near-neutrally liquid-solid two-phase flow with 200 um polystyrene particles (dp+=2.29,dp+=duτv) measured by means of PIV technique were compared and analyzed. The variation rule of turbulent statistic characteristics such as mean velocity profile, turbulence intensity and Reynolds shear stress of fluid with polystyrene particles and clean water under the same free flow velocity was investigated. Based on the locally-average velocity structure function and condition sampling technique, the coherent structure was detected, and the two-dimensional spatial topologies such as the fluctuation velocity, Reynolds shear stress and spanwise vorticity in ejection and sweep events at different wall heights under two working conditions were extracted with phase-average method. It was found that in the buffer layer the flow velocity with particles was greater than that of clean water, while there was little difference between them in the logarithmic law region. Compared with the clean water, the streamwise turbulent intensity was decreased in particle-laden flow, whereas the normal turbulent intensity, the comprehensive turbulence intensity and the Reynolds shear stress were all increased, with the expense of a slightly increase of the friction drag in the fluid with particles by 3.57%. In the bursting of coherent structure, the addition of polystyrene particles suppressed the fluid streamwise fluctuation in a turbulent boundary layer, gave a rise to the normal fluctuation and Reynolds shear stress, while inhibited the streamwise development of the spanwise vortex and promoted its normal development. The addition of particles facilitated the fluctuation of the fluid microcluster, and as well as the transverse momentum transfer in the log law region of turbulent boundary layer. The particles could increase the probability of the coherent structure ejection events and had less influence on the sweep events. The number and intensity of the ejection event was the key to improve the flow normal turbulence intensity and even to improve the energy exchange near the wall. The presence of appropriate scale particles in wall turbulence could play an important role in strengthening the transfer of momentum, heat and mass, and promoting mixing and reaction in the engineering, only by slightly increasing the fraction resistance.