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Large-scale secondary motions are known to occur in turbulent flows over surfaces with spanwise roughness heterogeneity. Numerical studies often use adjacent high- and low-roughness longitudinal strips to investigate these secondary rolls in boundary layers without any thermal stratification. In the present study, the effect of unstable thermal stratification on secondary rolls in a very high-Reynolds-number turbulent flow with spanwise-heterogeneous roughness is investigated by means of large-eddy simulation. The strength of the unstable stratification is systematically changed from L/h=-20 to L/h=-1, where L and h are Monin-Obukhov length and boundary-layer height, respectively. This range covers the transition from neutral stratification to unstable stratification. The results show that the positive buoyancy associated with the unstable thermal stratification acts against the roughness-induced secondary rolls. In the case of unstable stratification, secondary rolls are completely canceled out by buoyancy and replaced by new stronger convection-induced rolls rotating in opposite directions.
In this paper, in order to improve the performance of a linear parabolic collector, the thermal effects of using Al2O3-syltherm oil nanofluid with different concentrations and new flangeshaped turbulators are investigated. The simulation was performed by ANSYS-FLUENT-18.2 commercial software using Realizable k-ε two-equation turbulence model. In accordance with the results, it was realized that increasing the volume fraction of nanoparticles (to 5%) and number of turbulators causes the heat transfer coefficient (h) of the fluid to elevate and ultimately the uniform temperature is created in the absorber. For instance, at a flow rate of 4.5 kg/s and an inlet temperature of 350K, the value of h increases by about 8.5% by changing the number of turbulators from 10 to 15 sets. On the other hand, the results indicate that by changing the arrangement of the turbulators, the heat transfer efficiency of the collector can be increased by 5% for 350K, 3.5% for 450 K and 1% for 550 K inlet temperature.
The circumferential vibration of a gear pair is a parametric excitation caused by nonlinear tooth stiffness, which fluctuates with meshing. In addition, the vibration characteristics of the gear pair become complicated owing to the tooth profile error and backlash. It is considered that the circumferential vibration of the gear pair is affected by the torsional vibration of the shafts. It is important to understand quantitatively the vibration characteristics of the gear system considering the shafts. Therefore, the purpose of this research was to clarify the nonlinear vibration characteristics of a gear pair considering the influence of the shafts using theoretical methods. To achieve this objective, calculations were performed using equations of motion in which the circumferential vibration of the gear pair and the torsional vibration of the shafts were coupled. The nonlinear tooth stiffness was represented by a sine wave. The influence of tooth separation was considered by defining a nonlinear function using backlash and the tooth profile error. For the numerical calculations, both stable and unstable periodic solutions were obtained by using the shooting method. The effect of the shafts on the gear system vibration were clarified by comparing the results in the cases in which the shaft was not considered, one shaft was considered, and both shafts were considered.
The development of a general discrete element method for irregularly shaped particles is the core issue of the simulation of the dynamic behavior of granular materials. The general energy-conserving contact theory is used to establish a universal discrete element method suitable for particle contact of arbitrary shape. In this study, 3D modeling and scanning techniques are used to obtain a triangular mesh representation of the true particles containing typical concave particles. The contact volume-based energy-conserving model is used to realize the contact detection between irregularly shaped particles, and the contact force model is refined and modified to describe the contact under real conditions. The inelastic collision process between the particles and boundaries are simulated to verify the robustness of the modified contact force model and its applicability to the multi-point contact mode. In addition, the packing process and the flow process of a large number of irregular particles are simulated with the modified DEM to illustrate the applicability of the method of complex problems.
With the fast evolution of wireless and networking communication technology, applications of surface acoustic wave (SAW), or Rayleigh wave, resonators are proliferating with fast shrinking sizes and increasing frequencies. It is inevitable that the smaller resonators will be under a strong electric field with induced large deformation, which has to be described in wave propagation equations with the consideration of nonlinearity. In this study, the formal nonlinear equations of motion are constructed by introducing the nonlinear constitutive relation and strain components in a standard procedure, and the equations are simplified by the extended Galerkin method through the elimination of harmonics. The wave velocity of the nonlinear SAW is obtained from approximated nonlinear equations with boundary conditions through a rigorous solution procedure. It is shown that if the amplitude is small enough, the nonlinear results are consistent with the linear results, demonstrating an alternative procedure for nonlinear analysis of SAW devices working in nonlinear state.
We report the results of the direct numerical simulations of two-dimensional Rayleigh-Bénard convection(RBC) in order to study the influence of the periodic(PD) and confined(CF) samples on the heat transport Nu. The numerical study is conducted with the Rayleigh number(Ra) varied in the range 106 ≤ Ra ≤ 109 at a fixed Prandtl number Pr=4.3 and aspect ratio Γ=2 with the no-slip(NS) and freeslip(FS) plates. There exists a zonal flow for Ra ≥ 3×106 with the free-slip plates in the periodic sample. In all the other cases, the flow is the closed large-scale circulation(closed LSC). The striking features are that the heat transport Nu is influenced and the temperature profiles do not be influenced when the flow pattern is zonal flow.
The three-dimensional (3D) model of the middle ear is of great significance to the research of middle ear related diseases. The particular focus of this work is to simulate the impact of aircraft altitude and speed changes on the tympanic membrane (TM) during the descent phase, so as to analyze the pathogenesis of aero-otitis media and the mechanical response characteristics of TM under static pressure. The simulations showed that the stress and strain of TM increase as the altitude difference and speed of the aircraft increase, and the maximum stress and strain areas are consistent with the clinical observation of TM hyperemia. Therefore, among many prevention and treatment measures of aero-otitis media, it is a therapeutic method to directly balance the pressure difference between the inner and outer TM.
It is believed that it is going to be a sizeable mismatch between supply and demand when it comes to renewable resources. Lately, researchers are on course to compensate for the unpredictabilityof such resources by the employment of phase change materials (PCMs). Having multiple advantages, PCMs generally suffer from inadequate thermal conductivity which causes prolonged transition procedures. To tackle this issue, this study is fixated on two parameterswhich are linked to fins addition and porous media incorporation in a melting process within a triple concentric tube heat exchanger (TCTHX). The results provided by multiple cases underlined the significance of natural convection in the bare system, although finned and coppermetal-foam cases outshine buoyancy forces by roughly 45% and 97%, respectively. Material is a major determent when it comes to the selection of porous media as Al2O3 registered the weakest performance among SiC, Ni and Cu, however, it managed to speed up the process by 75% which still is much higher than the finned system, implying that porous media is of higher priority over fins. The best scenario transpiredwhile fins and copper metal foam were integrated as 26% and 97% soars in efficacy have been obtained compared to individual incorporation of porous media and fins, respectively.
Data-driven partial differential equation identification is a potential breakthrough to solve the lack of physical equations in complex dynamic systems. However, existing equation identification methods still cannot effectively identify equations from multivariable complex systems. In this work, we combine physical constraints such as dimension and direction of equation with data-driven method, and successfully identify the Navier-Stocks equations from the flow field data of Karman vortex street. This method provides an effective approach to identify partial differential equations of multivariable complex systems.
Fluctuating wall shear stress in turbulent channel flows is decomposed into small-scale and large-scale components. The large-scale fluctuating wall shear stress is computed as the footprints of the outer turbulent motions, and the small-scale one is obtained by subtracting the large-scale one from the total, which fully remove the outer influences. We show that the statistics of the small-scale fluctuating wall shear stress is Reynolds number independent at the friction Reynolds number larger than 1000, while which is Reynolds number dependent or the low-Reynolds-number effect exists at the friction Reynolds number smaller than 1000. Therefore, a critical Reynolds number that defines the emergence of universal small-scale fluctuating wall shear stress is proposed to be 1000. The total and large-scale fluctuating wall shear stress intensities approximately follow logarithmic-linear relationships with Reynolds number, and empirical fitting expressions are given in this work.