The influences of the modification of turbulent coherent structures on temperature field and heat transfer in turbulent channel flow are studied using large eddy simulation(LES) of compressible turbulent channel flows with spanwise wall oscillation(SWO).The reliability of the LES on such problems is proved by the comparisons of the drag reduction data with those of other researches.The high consistency of coherent velocity structures and temperature structures is found based on the analyses of the turbulent flow field.When the coherent velocity structures are suppressed,the transportations of momentum and heat are reduced simultaneously,demonstrating the same trend.This shows that the turbulent coherent structures have the same effects on the transportations of momentum and heat.The averaged wall heat flux can be reduced with appropriate oscillating parameters.
The average-passage equation system （APES） provides a rigorous mathematical framework for account- ing for the unsteady blade row interaction through multi- stage compressors in steady state environment by introduc- ing deterministic correlations （DC） that need to be modeled to close the equation system. The primary purpose of this study is to provide insight into the DC characteristics and the influence of DC on the time-averaged flow field of the APES. In Part 1 of this two-part paper, firstly a 3D viscous unsteady and time-averaging flow CFD solver is developed to investi- gate the APES technique. Then steady and unsteady simu- lations are conducted in a transonic compressor stage. The results from both simulations are compared to highlight the significance of the unsteady interactions. Furthermore, the distribution characteristics of DC are studied and the DC at the rotor/stator interface are compared with their spatial cor- relations （SC）. Lastly, steady and time-averaging （employing APES with DC） simulations for the downstream stator alone are conducted employing DC derived from the unsteady re- suits. The results from steady and time-averaging simula- tions are compared with the time-averaged unsteady results. The comparisons demonstrate that the simulation employing APES with DC can reproduce the time-averaged field and the 3D viscous time-averaging flow solver is validated.
The average-passage equation system （APES） provides a rigorous mathematical framework for account- ing for the unsteady blade row interaction through multistage compressors in steady state environment by introducing de- terministic correlations （DC） that need to be modeled to close the equation system. The primary purpose of this study was to provide insight into the DC characteristics and the in- fluence of DC on the time-averaged flow field of the APES. In Part 2 of this two-part paper, the influence of DC on the time-averaged flow field was systematically studied; Several time-averaging computations boundary conditions and DC were conducted with various for the downstream stator in a transonic compressor stage, by employing the CFD solver developed in Part 1 of this two-part paper. These results were compared with the time-averaged unsteady flow field and the steady one. The study indicat;d that the circumferential- averaged DC can take into account major part of the unsteady effects on spanwise redistribution of flow fields in compres- sors. Furthermore, it demonstrated that both deterministic stresses and deterministic enthalpy fluxes are necessary to reproduce the time-averaged flow field.
Large eddy simulation （LES） of low Mach num- ber compressible turbulent channel flow with spanwise wall oscillation （SWO） is carried out. The flow field is analyzed with emphases laid on the heat transport as well as its rela- tion with momentum transport. When turbulent coherent structures are suppressed by SWO, the turbulent transports are significantly changed, however the momentum and heat transports change in the same manner, which gives the evi- dence of inherently consistent transport mechanisms between momentum and heat in turbulent boundary layers. The Reynolds analogies of all the flow cases are quite good, which confirms again the fact that the transport mechanisms of momentum and heat are consistent, which gives theoreti- cal support for controlling the wall heat flux control by using the drag reducing techniques.
This paper presents the moving mechanism of a high-speed insect-scale microrobot via electromagnetically induced vibration of two simply supported beams.The microrobot,which has a body length of 12.3 mm and a total mass of 137 mg,can achieve reciprocating lift motion of forelegs without any intermediate linkage mechanisms due to the design of an obliquely upward body tilt angle.The gait study shows that the body tilt angle prevents the forelegs from swinging backward when the feet contact the ground,which results in a forward friction force applied on the feet.During forward movement,the microrobot utilizes the elastic deformation of the simply supported beams as driving force to slide forward and its forelegs and rear legs work as pivots alternatively in a way similar to the movement of soft worms.The gait analysis also indicates that the moving direction of the microrobot is determined by whether its body tilt angle is obliquely upward or downward,and its moving speed is also related to the body tilt angle and as well as the body height.Under an applied AC voltage of 4 V,the microrobot can achieve a moving speed at 23.2 cm s1(18.9 body lengths per second),which is comparable to the fastest speed(20 cm s-1 or 20 body lengths per second)among the published insect-scale microrobots.The high-speed locomotion performance of the microrobot validates the feasibility of the presented actuation scheme and moving mechanism.
Xinyi LiuZhiwei LiuYangsheng ZhuJiaming LengMingjing QiJianmei HuangXiaojun Yan
