Enhanced heat transfer in a heat exchanger square-duct with discrete V-finned tape inserts

The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45° discrete V-finned tapes(DFT).The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000.The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated.Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively.Several V-finned tape characteristics were introduced such as fin- to duct-height ratio or blockage ratio(R_B=e/H = 0.075,0.1,0.15 and 0.2),fin pitch to duct height ratio(R_P= P/H=0.5,1.0,1.5 and 2.0) and fin attack angle,α = 45°.The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of R_B but the decrease of R_P.The inserted square-duct at R_B = 0.2 and R_P = 0.5 provides the highest heat transfer and friction factor while the one with R_B= 0.1 and R_P= 1.5 yields the highest thermal performance.Also,a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.

Study for reasonable value of friction coefficient between main cable and saddle

The value of friction coefficient between the main cable and saddle, relates to not only the anti-slippage stability of three-tower suspension bridge, but also the reasonable stiffness of the middle tower and the magnitude of rigidity of the whole bridge. First, the paper does some comparative studies about the relevant provisions of international norms, and then, summarizes the relevant load test results both at home and abroad. Finally, the paper draws the appropriate anti-slippage safety factor for the most unfavorable load in accordance with international load standards, and discusses the rationality and feasibility of the friction coefficient of 0.2 between main cable and saddle.

Turbulence Model Investigations on the Boundary Layer Flow with Adverse Pressure Gradients

In this paper, a numerical study of flow in the turbulence boundary layer with adverse and pressure gradients （APGs） is conducted by using Reynolds-averaged Navier-Stokes （RANS） equations. This research chooses six typical turbulence models, which are critical to the computing precision, and to evaluating the issue of APGs. Local frictional resistance coefficient is compared between numerical and experimental results. The same comparisons of dimensionless averaged velocity profiles are also performed. It is found that results generated by Wilcox （2006） k-co are most close to the experimental data. Meanwhile, turbulent quantities such as turbulent kinetic energy and Reynolds-stress are also studied.

Intermittent slipping of landslide regulated by dilatancy evolution and velocity-weakening friction law: an efficient numerical scheme

When a block of dense sandy soil moves downhill, the shear-induced soil dilatancy along the basal shear boundary produces a negative value of excess pore pressure that increases the basal frictional resistance. Dilatancy angle,Ψ, the degree to which the basal soil dilates due to the shear, normally evolves during slope failure. A study by other researchers shows that if Ψ is constant, the block of dense soil will remain stable(or unstable) sliding when the velocity-weakening rate of the basal friction coefficient of the block is small(or large) enough. Moreover, during unstable sliding processes, the block of dense soil exhibits "periodic" patterns of intermittent slipping. Here, we used a more efficient and accurate numerical scheme to revisit that study. We expanded their model by assuming Ψ evolves during slope failure. Consequently, we acquired completely different results. For instance, even though the velocity-weakening rate of the friction coefficient is fixed at the same smaller(or larger) value that those researchers use, the stable(or unstable) steady states of landslide they predict will inversely change to unstable(or stable) when Ψ decreases(or increases) with the increase of slide displacement to a value small(or large) enough. Particularly, in unstable processes, the soil block exhibits "aperiodic" styles of intermittent slipping, instead of "periodic". We found out that the stick states appearing later last longer(or shorter) in the case of decreasing(or increasing) Ψ. Moreover, because the basic states of landslides with impacts of dilatancy evolution are not steady nor periodic, traditional stability-analysis methods cannot be "directly" used to analyze the stability of such landslides. Here, we broke through this technical problem to a degree. We showed that combining a concept called "quasi-steady-state approximation" with a traditional stability-analysis technique can qualitatively predict the instability onset of the landslides. Through this study, we demonstrated that the combination of Chebyshev collocation(CC) and 4^(th)-order Runge-Kutta methods is more accurate and efficient than the numerical scheme those researchers use.

Accurate measurement of wall skin friction by single-pixel ensemble correlation

The present work deals with accurately estimating wall-skin friction from near-wall mean velocity by means of PIV measurement.The estimation accuracy relies on the spatial resolution and the precision of the resolved velocity profile inside the viscous sublayer,which is a big challenge for conventional window-based correlation method(K?hler C J,et al.Exp Fluids,2012,52:1641–1656).With the help of single-pixel ensemble correlation,the ensemble-averaged velocity vector can be resolved at significant spatial resolution,thus improving the measurement accuracy.To demonstrate the feasibility of this single-pixel ensemble correlation method,we first study the velocity estimation precision in a case of steady near-wall flow.Synthetic particle images are used to investigate the effect of different image parameters.It is found that the velocity RMS-uncertainty level of the single-pixel ensemble correlation method can be equivalent to the conventional window correlation method once the effective particle number used for the ensemble correlation is large enough.Furthermore,a canonical turbulent boundary layer is synthetically simulated based on velocity statistics resolved by previous Direct Numerical Simulation(DNS)work(Schlatter P,et al.J Fluid Mech,2010,659:116–126).The relative error of wall skin friction coefficient is shown to be one-order smaller than that of the window correlation method.And the optimization strategy to further minimize the measurement uncertainty is discussed in the last part.

Effect of slope inclination on step friction coefficient of human being

Effect of slope inclination on step friction coefficient of human being was studied by gait friction tester developed by Henan University of Science and Technology.It is beneficial to improving the safety of human beings while walking on a ramp.Based on the trajectory of mass center of body(MCOB)and the mechanical analysis of human body while walking,conclusion could be drawn that the required step frictional coefficient(RSFC)exhibited an"M"shaped curve during the foot contacting with the ground and got its maximum and minimum values when MCOB of the subject was at the highest and lowest position respectively.Moreover,the first peak of RSFC decreases firstly and then increases,while the second peak of RSFC increases with the increase of slope inclination at the experimental condition.The position of pivotal point is decided by the relative size of slope inclination and the angle between the plumb line through MCOB and the line from foot to MCOB.