As a hypersonic flow transits from continuous to more and more rarefied, both the bow shock wave and the boundary layer over a leading edge will gradually become thickening, touching, and interlacing with each other, characterizing the evolution of the flow field structure. Correspondingly, an increasing nonlinear non-Fourier feature emerges from the aeroheating instead of the linear Fourier’s law. An intuitive theoretical model is proposed, and it is found that the ratio of the non-Fourier’s part to Fourier’s part of the heat flux term is a universal flow parameter to predict and describe both the physical phenomena above-mentioned. This model leads to a physically meaningful criterion to classify hypersonic rarefied flow regimes. New conclusions obtained are validated by present direct simulation Monte Carlo results.
Flexible insect wings deform passively under the periodic loading during flapping flight. The wing flexibility is considered as one of the specific mechanisms on improving insect flight performance. The constitutive relation of the insect wing material plays a key role on the wing deformation, but has not been clearly understood yet. A viscoelastic constitutive relation model was established based on the stress relaxation experiment of a dragonfly wing (in vitro). This model was examined by the finite element analysis of the dynamic deformation response for a model insect wing under the action of the periodical inertial force in flapping. It is revealed that the viscoelastic constitutive relation is rational to characterize the biomaterial property of insect wings in contrast to the elastic one. The amplitude and form of the passive viscoelastic deformation of the wing is evidently dependent on the viscous parameters in the constitutive relation.
Because of the relatively narrow space and high density distribution of the passengers, the bus interior environment deteriorates in summer. Natural ventilating introduces a fresh natural freeze, provides the bus interior with appropriate distribution of air supply temperature and velocity field. Making good use of natural ventilation is an operating strategy ideal for improving passengers’ satisfaction, which is considered as an environmental friendly and cost effective approach. Based on CFD numerical simulation, with a whole-domain approach, this paper predicts air flow and thermal comfort in naturally ventilated bus. The outside and inside airflow is modelled simultaneously and within the same computational domain. The thermal environment in different parts of the bus interior is compared. Different vehicle velocities and conditions of windows are taken into account, analysis are made regarding to the effect of both of them on the interior thermal comfort. The result shows, air disturbance at the bus rear parts are intenser than the other parts with better thermal satisfaction; the quality of air flow in different parts makes the temperature difference in bus achieve as high as 3°C; the use of roof openings distributes the temperature more evenly.
The characteristics and mechanism of unsteady aerodynamic heating of a transient hypersonic boundary layer caused by a sudden change in surface temperature are studied. The complete time history of wall heat flux is presented with both analytical and numerical approaches. With the analytical method, the unsteady compressible boundary layer equation is solved. In the neighborhood of the initial and final steady states, the transient responses can be expressed with a steady-state solution plus a perturbation series. By combining these two solutions, a complete solution in the entire time domain is achieved. In the region in which the analytical approach is applicable, numerical results are in good agreement with the analytical results, showing reliability of the methods. The result shows two distinct features of the unsteady response. In a short period just after a sudden increase in the wall temperature, the direction of the wall heat flux is reverted, and a new inflexion near the wall occurs in the profile of the thermal boundary layer. This is a typical unsteady characteristic. However, these unsteady responses only exist in a very short period in hypersonic flows, meaning that, in a long-term aerodynamic heating process considering only unsteady surface temperature, the unsteady characteristics of the flow can be ignored, and the traditional quasi-steady aerodynamic heating prediction methods are still valid.
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