We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns i...We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(onthe order of viscous wall units)spanwise meandering motions.The probability density functions(PDFs)ofwall shear stresses show a clear discrepancy in high stress range with those from direct numerical simu-lation(DNS),which is attributed to the unresolved streamwise and spanwise motions by DNS.Moreover,the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenousisotropic turbulence,indicating strong intermittency in the sublayer.We find a significant fraction of highaccelerations is associated with the small-scale meandering motions.Our study helps explain the effectof sublayer-scale roughness on reducing drag and flow separation reported in the literature.展开更多
Several methods for evaluating the sublayer suspension beneath old pavement with falling weight deflectormeter(FWD), were summarized and the respective advantages and disadvantages were analyzed. Based on these method...Several methods for evaluating the sublayer suspension beneath old pavement with falling weight deflectormeter(FWD), were summarized and the respective advantages and disadvantages were analyzed. Based on these methods, the evaluation principles were improved and a new type of the neural network, functional-link neural network was proposed to evaluate the sublayer suspension with FWD test results. The concept of function link, learning method of functional-link neural network and the establishment process of neural network model were studied in detail. Based on the old pavement over-repairing engineering of Kaiping section, Guangdong Province in G325 National Highway, the application of functional-link neural network in evaluation of sublayer suspension beneath old pavement based on FWD test data on the spot was investigated. When learning rate is 0.1 and training cycles are 405, the functional-link network error is less than 0.000 1, while the optimum chosen 4-8-1 BP needs over 10 000 training cycles to reach the same accuracy with less precise evaluation results. Therefore, in contrast to common BP neural network,the functional-link neural network adopts single layer structure to learn and calculate, which simplifies the network, accelerates the convergence speed and improves the accuracy. Moreover the trained functional-link neural network can be (adopted) to directly evaluate the sublayer suspension based on FWD test data on the site. Engineering practice indicates that the functional-link neural model gains very excellent results and effectively guides the pavement over-repairing construction.展开更多
To explore the influence of sublayer numbers on the structure evolution and thermal stress level,an alternate coating consisting of ZrC-10 vol.%SiC and ZrC-70 vol.%SiC sublayers was designed in this work.With a basica...To explore the influence of sublayer numbers on the structure evolution and thermal stress level,an alternate coating consisting of ZrC-10 vol.%SiC and ZrC-70 vol.%SiC sublayers was designed in this work.With a basically consistent general thickness,three coatings constituted by 2,4 and 6 sublayers were prepared by plasma spraying,which then were assessed using an oxyacetylene torch under cyclic and long-term exposure.The coating with 6 sublayers was supposed to be the one with the best ablation property,finite element analysis also evidenced its least thermal stress among all these samples.After being ablated for 240 s,the linear and mass ablation rates of the coating with 6 sublayers decreased by 51.27%and 14.41%as compared to that with 4 sublayers.Post-test analysis proved the existence of Si-based products,which help the outmost surface to develop a dense profile.Additionally,the yielded alternate dense/porous scale had a preferable toughness,allowing it to maintain a good integrity.展开更多
The spike phenomena,namely strong normal velocity fluctuations,in the viscous sublayer have been discovered through detailed investigation of the databank of the direct numerical simulation of a turbulent channel flow...The spike phenomena,namely strong normal velocity fluctuations,in the viscous sublayer have been discovered through detailed investigation of the databank of the direct numerical simulation of a turbulent channel flow.Their characteristic features are disclosed and their effects on the generation of turbulence,and correlation with the vortex structures are studied.展开更多
文摘We employ novel digital Fresnel reflection holography to capture the 3D flows within the viscous sublayerof a smooth-wall turbulent channel flow at Reτ=400.The measurements reveal unsteady and diverse flow patterns in the sublayer including nearly uniform high and low speed flows and strong small-scale(onthe order of viscous wall units)spanwise meandering motions.The probability density functions(PDFs)ofwall shear stresses show a clear discrepancy in high stress range with those from direct numerical simu-lation(DNS),which is attributed to the unresolved streamwise and spanwise motions by DNS.Moreover,the PDF of Lagrangian particle accelerations yields a stretched exponential shape like that in homogenousisotropic turbulence,indicating strong intermittency in the sublayer.We find a significant fraction of highaccelerations is associated with the small-scale meandering motions.Our study helps explain the effectof sublayer-scale roughness on reducing drag and flow separation reported in the literature.
文摘Several methods for evaluating the sublayer suspension beneath old pavement with falling weight deflectormeter(FWD), were summarized and the respective advantages and disadvantages were analyzed. Based on these methods, the evaluation principles were improved and a new type of the neural network, functional-link neural network was proposed to evaluate the sublayer suspension with FWD test results. The concept of function link, learning method of functional-link neural network and the establishment process of neural network model were studied in detail. Based on the old pavement over-repairing engineering of Kaiping section, Guangdong Province in G325 National Highway, the application of functional-link neural network in evaluation of sublayer suspension beneath old pavement based on FWD test data on the spot was investigated. When learning rate is 0.1 and training cycles are 405, the functional-link network error is less than 0.000 1, while the optimum chosen 4-8-1 BP needs over 10 000 training cycles to reach the same accuracy with less precise evaluation results. Therefore, in contrast to common BP neural network,the functional-link neural network adopts single layer structure to learn and calculate, which simplifies the network, accelerates the convergence speed and improves the accuracy. Moreover the trained functional-link neural network can be (adopted) to directly evaluate the sublayer suspension based on FWD test data on the site. Engineering practice indicates that the functional-link neural model gains very excellent results and effectively guides the pavement over-repairing construction.
基金supported by the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical Univer-sity(No.CX2021006)the National Natural Science Foundation of China(Nos.91860203,51727804 and 52130205)+1 种基金the Fundamental Research Funds for the Central Universities(No.3102019TS0409)the Creative Research Foundation of Science and Technology on Thermo-structural Composite Materials Laboratory。
文摘To explore the influence of sublayer numbers on the structure evolution and thermal stress level,an alternate coating consisting of ZrC-10 vol.%SiC and ZrC-70 vol.%SiC sublayers was designed in this work.With a basically consistent general thickness,three coatings constituted by 2,4 and 6 sublayers were prepared by plasma spraying,which then were assessed using an oxyacetylene torch under cyclic and long-term exposure.The coating with 6 sublayers was supposed to be the one with the best ablation property,finite element analysis also evidenced its least thermal stress among all these samples.After being ablated for 240 s,the linear and mass ablation rates of the coating with 6 sublayers decreased by 51.27%and 14.41%as compared to that with 4 sublayers.Post-test analysis proved the existence of Si-based products,which help the outmost surface to develop a dense profile.Additionally,the yielded alternate dense/porous scale had a preferable toughness,allowing it to maintain a good integrity.
基金Project supported by the National Natural Science Foundation of China.
文摘The spike phenomena,namely strong normal velocity fluctuations,in the viscous sublayer have been discovered through detailed investigation of the databank of the direct numerical simulation of a turbulent channel flow.Their characteristic features are disclosed and their effects on the generation of turbulence,and correlation with the vortex structures are studied.
