密排粗糙床面明渠水流的紊动特性

Turbulence characteristics of open channel flow with dense roughness bed

为揭示密排粗糙床面对明渠水流紊动参数的定量影响规律,采用不同直径的圆形玻璃珠制作了3种密排加糙床面,基于光滑床面和加糙床面明渠紊流PIV流场测试资料,探讨了加糙后紊动强度、雷诺应力和紊动能等参数的变化特征。结果表明:床面粗糙对紊动参数的影响主要体现在内区,而外区紊动参数的变化相对较小。加糙后纵向和垂向紊动强度相对值u’^+、v’^+在内区呈现不同的变化规律,流量相同时,粗糙雷诺数Re_*越大,v’^+越大,而u’^+越小;床面粗糙对内区水流有抑制纵向紊动和加强垂向紊动的双重作用,而对外区水流有加强纵、垂双向紊动的作用;床面加糙对垂向紊动强度的影响总体大于纵向。加糙后内、外区的雷诺应力绝对值均增大,内区增幅尤为明显,雷诺应力增大的原因在于水流质点混掺强度和频次均增大。不同工况紊动能相对值E^+在内区差异明显,随着相对水深η的增大,E^+差异逐渐减小,在水面附近趋于一致;加糙后内区紊动能绝对值总体减小,而外区紊动能绝对值有所增大,内、外区紊动能变化与床面粗糙抑制水流纵向紊动强度和加强质点垂向混掺作用等因素有关。

To study the quantitative influence of bed dense roughness on the turbulence parameters of open channel flow,3 kinds of dense roughness beds were made by round glass beads with different sizes. Based on the instantaneous velocity data investigated by Particle Image Velocimetry( PIV) for open-channel flows over smooth and rough beds,variation characteristics of turbulence parameters such as turbulence intensity,Reynolds stress and turbulent kinetic energy after roughening were discussed. Results show that the influence of bed roughness on turbulence parameters mainly occurs in the inner region,and the change of turbulence parameters in the outer region is relatively small. With the bed roughness increased,the variation trend of the relative longitudinal turbulence intensity u’^+ and the relative vertical turbulence intensity v’^+ in the inner region is different. As fixed flow,the larger the roughness Reynolds number Re_*,the larger the v’^+,and the smaller the u’.To inner region flow,the bed roughness has the dual effects of inhibiting the longitudinal turbulence and enhancing the vertical turbulence. However,to the outer region flow,it has the influence of strengthening the longitudinal and vertical turbulence. The influence of bed roughness on the vertical turbulence intensity is overall greater than that of the longitudinal. After increasing the roughness,the absolute values of the Reynolds stress increase both in inner region and outer region. The mechanism of Reynolds stress increase is the enhancing of mixing intensity and frequency of particles. The difference of the relative turbulent kinetic energy E^+ in the inner region for different conditions is obvious,and with the increase of relative water depth η,the difference of E^+ is gradually smaller and tends to be consistent near the water surface. Absolute value of turbulent kinetic energy in the inner region decreases overall,but the absolute value is increase in the outer region. The variation of turbulent kinetic energy is related to the bed roughness effects of inhibiting the longitudinal turbulence and strengthening the vertical mixing of the particles.