An algorithm of super-convergence for the Garlerkin FEM (finite element method) for the second order non-self-adjoint boundary-value problem is proposed based on the improved displacement mode. The new displacement mode is constructed by combining the displacement mode of the conventional finite element and the high-order displacement mode based on the Galerkin method, and the element equilibrium equation is derived using the integral form. A representative example is presented for the Hermite element in this paper, the accuracies of the displacements and the derivatives accuracy of nodes and elements have reached the order of h4.
The thermocapillary convection and its instability is an important problem in the microgravity fluid science. Its study will not only improve our understanding of the fluid behavior in microgravity conditions but also benefit the space and terrestrial applications such as the crystal growth and the film preparation. This paper studies the thermocapillary convection in thin liquid layers contained in an open rectangular cavity with differently heated sidewalls. In our experiments, a rectangular cavity of l = 52mm and w = 36mm is used, and the silicone oil with the kinetic viscosity of 1cSt is chosen as the working fluid whose Prandtl number is 16.2 at 25℃. The particle image velocimetry (PIV) is employed to observe and measure the flow structure in the thin liquid layers. Multiple flow states are observed within the parameter range examined. It is found that the transition routes depend on the thickness of the liquid layer. For thinner layers, as ΔT is increased, the flow structure in the vertical section transits first from the unicellular flow to the bicellular flow, and then to multicellular flow with several corotating rolls embedded in the main flow. And the number of the rolls decreases as ΔT is increased. The flow will eventually become time dependent and three dimensional if ΔT is even larger. While for thicker layers, the transition route is different. As ΔT is increased, the topology of the flow structure in the vertical section changes little, but different flow states can be differentiated in terms of the flow structure in the horizontal section. When ΔT is small, the flow near y = 0 is two dimensional. As ΔT is increased, shuttle structures will occur, which are symmetric with respect to y = 0. But a larger ΔT will destroy the symmetry and turn the flow in to a 3D unsteady flow.
The flow around a finite-length square prism is numerically investigated using the LES (large eddy simulation). The aspect ratio of the prism is 5. The prism is mounted on a flat wall, with one end free. The thickness of the boundary layer on the flat wall is very small to be negligible. The Reynolds number based on the free-stream oncoming velocity and the prism width is 3 900. Based on the simulation results, it is found that the near wake is highly three dimensional under the effects of the free-end downwash flow. Two typical flow states were observed in the near wake: first, the spanwise vortices are staggered arranged similar to the Karman vortex street; second, the spanwise vortices are quasi-symmetrically arranged without obvious periodicity. These two typical states occur alternately, which affects the aerodynamic forces on the prism.
As a major digestive and absorptive organ of humans, the intestinal tract is narrow and winding with a complicated surface structure, so complications such as tissue injuries and capsule stagnations frequently occur in the diagnosis process, which thus inspires considerable researches concerning the frictional characteristics of the intestinal tract and the capsule endoscope, which play an important part in treating the above mentioned diseases and developing harm-free endoscopes. This paper briefly reviews the categories of capsule endoscopes and outlines the frictional characteristics between the capsule endoscope and the intestinal tract, the microrobot and intestinal tract, concentrating on two aspects: (1) the researches concerning the friction reduction of the capsule endoscope, including research methods of the frictional characteristics, the relationship between the capsule structure and the frictional resistance, the researches of the frictional mechanism, the prediction model of the frictional resistance and the optimization of the capsule structure; (2) the researches of the friction increase for the realization of autonomous walking of in vivo micro-robot, including the methods of increasing frictional force and the related mechanisms. Finally, some scientific questions for further studies are raised.
This paper first briefly reviews the types of the dynamic instability of helicopter rotor/airframe, including the isolated blade dynamic instabilities such as the rotor flap-pitch coupling, pitch-lag instability, the coupled flap-lag aeroelastic instability, the flap-lag-pitch coupled instability, and the coupled rotor/airframe instabilities, such as the ground resonance and the air resonance. The related studies are reviewed from 3 aspects, the aerodynamic and structural numerical models with high precisions, the numerical methods of dynamic stability, and the model testing. The major fields of the analytical technology for dynamic stability of helicopter rotor/airframe are discussed, including the rotor aeroelastic stability analysis using the coupled computational fluid dynamics/computational structural dynamics, the dynamic stability analysis of composite rotor with consideration of material uncertainty, the dynamic stability analysis of coupled rotor/airframe with lag damper, and helicopters with advanced configurations. In the end, the future development of dynamic stability of helicopter rotor/airframe is commented.
Based on a seven-species and six-reaction chemical kinetical mode, the real gas effects on the flow field and the aerodynamic characteristics of the Apollo return command module in a chemical nonequilibrium flow at high temperature are investigated numerically. The code presented here for the chemical non-equilibrium flow is validated through the flight data and the experimental data. Results show that the real gas effects are prominent in some regions of quite thin shock wave layers close to the wall surface, and the outstanding distance of the shock wave is reduced, and the drag coefficient and the lift coefficient are increased with the largest amplitude at a small attack angle. The real gas effect results in some additional downward force moment, moving the pressure centre backward. The larger the freestream Mach number, the greater the real gas effect on the aerodynamics of the return command module.
The rudder is always placed behind the propeller to absorb the energy in the wake flow, thus to achieve a more optimized maneuverability. According to the characteristics of the propeller flow, six kinds of rudder arrangement were adopted by varying the lateral and longitudinal distances away from the propeller. In each case, the free-running model test was carried out in the lake, including the turning circle test, the zigzag test and the pull-out test. The speed drop and the static roll angle agree well with the general rules of maneuverability. By testing the parameters and analyzing the test data, different effects on the turning ability, the course-keeping ability, and the response to intermediate and small helm angles were achieved with different rudder arrangements.
The interaction between the temperature field the pressure field and the moisture field in the multicoil pipe freezing process is a problem of non-linearity and multi-fields, involving many factors. The temperature field, the pressure field and the moisture field in the freezing process are studied by the model experiment based on the Gubei Mine in Huainan, the variations of the temperature filed, the pressure field and the moisture field againt the freezing time are obtained. The results can be used to establish the coupling model of the moister fields, the temperature fields and the pressure fields to be used in mining engineering practice.
Combining the experiments and the damage mechanical theory analysis, the freeze-thaw cycling experiment was conducted for the red sandstone in the water-saturated state, the mechanical property tests with different number of freeze-thaw cycles were carried out, the rock deterioration process of the freeze-thaw damage was analyzed, then the deformation and failure laws and the damage propagation mechanical characteristics were studied systematically for rock. It is shown that the deterioration modes of the freeze-thaw damage for the red sandstone include the spalling mode and the fracture mode. The greater the number of the freeze-thaw cycles the more intensified the reduction of the strength and the elastic modulus will result, with the increased compaction property, the weakened elasticity and the enhanced plasticity. Through the nonlinear evolution of the rock damage, the initial defect of meso-structure within the rock will lead to the deterioration of the macro mechanical behavior finally.
A global oriented method of measuring of the threshold pressure gradient (TPG) in micro-channels was studied in this paper, in order to more accurately measure the TPG in the low permeability porous media. The method includes the static measurement and the steady flow experiments, and can be defined as the global-oriented method to measure the threshold pressure gradient, which can be used to analyze the pressure responses when the deionized water changes from the static state to the steady flow state in micro-channels. The experiments indicate that the threshold pressure gradient in the steady flow experiments is 14.5 times of that obtained by the static method, which shows that the previously published experimental values are too large, and are difficult to be used in the oil development. The results show that the global-oriented method of measuring the TPG can be applied for low permeability cores to obtain more precise parameters, and the TPG is verified when the liquid flows in micro-channels.
For an unbounded domain, a local high-order transmitting boundary is implemented based on the scaled boundary finite element method and the continued-fraction expansion. An improved continued-fraction expansion is employed to solve the dynamic stiffness matrix equation of an unbounded domain. Compared to the previous approach, it is numerically more robust for large-scale systems and arbitrarily high orders of expansion. The equation of this boundary is a system of first-order ordinary differential equations in the time domain. The stability of the high-order boundary depends on the general eigenproblem of the coefficient matrices. Possible spurious modes can be eliminated using the spectral shifting technique. The accuracy and the robustness of the high-order transmitting boundary are verified by two numerical examples.