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961.
为了得到壁面温度在不同来流速度、不同湍流强度条件下对边界层转捩与减阻的影响规律,本文采用Transitionk-kl-ω模型对低来流速度下无压力梯度的光滑平板进行了数值模拟。结果表明,随着来流速度的升高,壁温升高所起到的减阻效果更好,即高来流速度对壁面温度更为敏感。当来流处于中高湍流强度下时,壁温升高能起到推迟转捩的作用,且随着湍流强度的升高,转捩推迟的效果越好,但减阻效果正好相反;当来流处于低湍流强度下时,壁温升高会使得转捩提前发生。壁温升高抑制了边界层内流体的脉动程度,使得层流的稳态不易被破坏,流动更加稳定;同时,壁温升高使得边界层内流体的速度梯度减小,从而降低了壁面摩擦系数,故壁温升高能起到推迟边界层转捩与减阻的作用。 相似文献
962.
Belkacem Manser Idir Belaidi Abderrachid Hamrani Sofiane Khelladi Farid Bakir 《Comptes Rendus Mecanique》2019,347(2):141-165
A wisely chosen geometry of micro textures with the favorable relative motion of lubricated surfaces in contacts can enhance tribological characteristics. In this paper, a computational investigation related to the combined influence of bearing surface texturing and journal misalignment on the performances of hydrodynamic journal bearings is reported. To this end, a numerical analysis is performed to test three texture shapes: square “SQ”, cylindrical “CY”, and triangular “TR”, and shaft misalignment variation in angle and degree. The Reynolds equation of a thin viscous film is solved using a finite differences scheme and a mass conservation algorithm (JFO boundary conditions), taking into account the presence of textures on both full film and cavitation regions. Preliminary results are compared with benchmark data and are consistent with a positive enhancement in misaligned bearing performances (load carrying capacity and friction). The results suggest that the micro-step bearing mechanism is a key parameter, where the micro-pressure recovery action present in dimples located at the second angular part of the bearing (from 180° to 360°) can compensate for the loss on performances caused by shaft misalignment, while the micro-pressure drop effect at the full film region causes poor performances. Considering the right arrangement of textures on the contact surface, their contours geometries can have a significant impact on the performance of misaligned journal bearings, particularly at high eccentricity ratios, high misalignment degrees and when the misalignment angle α approaches to or . 相似文献
963.
A computational study has been carried out to examine the effects of driver gas contamination in the NASA HYPULSE facility at GASL when operating with a detonation driver in reflected-shock tunnel mode. Unlike high-enthalpy shock tunnels which use helium as a driver gas, the driver gas in a detonation driver consists of a mixture of water vapour and argon, which has very different chemical and thermodynamic properties than those of helium. The purpose of the present work is to quantitatively evaluate the effects of driver gas contamination on the flow properties in the test section. Two computational analyses have been performed. The first analysis examined the nozzle flow under the influence of a prescribed level of driver gas contamination. In the second analysis, the transient development of the driver gas leakage in the reflected-shock region in the shock tube is studied. The unique flow features brought about by the detonation-driver gas and the method for detecting the contamination are discussed.Received: 6 August 2002, Revised: 22 September 2003, Published online: 10 February 2004Correspondence to: R.S.M. ChueAn abridged version of this paper was presented at the 23rd International Symposium on Shock Waves at Fort Worth, Texas, July 23 to 27, 2001 相似文献
964.
We consider the inverse problem of identifying the density and elastic moduli for three-dimensional anisotropic elastic bodies, given displacement and traction measurements made at their surface. These surface measurements are modelled by the dynamic Dirichlet-to-Neumann map on a finite time interval. For linear or nonlinear anisotropic hyperelastic bodies we show that the displacement-to-traction surface measurements do not change when the density and elasticity tensor in the interior are transformed tensorially by a change of coordinates fixing the surface of the body to first order. Our main tool, a new approach in inverse problems for elastic media, is the representation of the equations of motion in a covariant form (following Marsden and Hughes, 1983) that preserves the underlying physics.In the case of classical linear elastodynamics we then investigate how the type of anisotropy changes under coordinate transformations. That is, we analyze the orbits of general linear, anisotropic elasticity tensors under the action by pull-back of diffeomorphisms that fix the surface of the elastic body to first order, and derive a pointwise characterization of parts of the orbits under this action. For example, we show that the orbit of isotropic elastic media, at any point in the body, consists of some transversely isotropic and some orthotropic elastic media. We then derive the first uniqueness result in the inverse problem for anisotropic media using surface displacement-traction data: uniqueness of three elastic moduli for tensors in the orbit of isotropic elasticity tensors.
†Partially supported by an MSRI Postdoctoral Fellowship. Research at MSRI is supported in part by NSF grant DMS-9850361. This work was conducted while the first author was a Gibbs Instructor at Yale University.
‡Partially supported by an MSRI Postdoctoral Fellowship, and by NSF grant DMS-9801664 (9996350). 相似文献
965.
966.
Tang Guiming 《Acta Mechanica Sinica》1998,14(2):139-146
An experimental study was conducted on shock wave turbulent boundary layer interactions caused by a blunt swept fin-plate
configuration at Mach numbers of 5.0, 7.8, 9.9 for a Reynolds number range of (1.0∼4.7)×107/m. Detailed heat transfer and pressure distributions were measured at fin deflection angles of up to 30° for a sweepback
angle of 67.6°. Surface oil flow patterns and liquid crystal thermograms as well as schlieren pictures of fin shock shape
were taken. The study shows that the flow was separated at deflection of 10° and secondary separation were detected at deflection
of ϑ≥20°. The heat transfer and pressure distributions on flat plate showed an extensive plateau region followed by a distinct
dip and local peak close to the fin foot. Measurements of the plateau pressure and heat transfer were in good agreement with
existing prediction methods, but pressure and heating peak measurements atM≥6 were significantly lower than predicted by the simple prediction techniques at lower Mach numbers.
The project supported by China Academy of Launch Vehicle Technology 相似文献
967.
Thin structures are generally solved by the Finite Element Method (FEM), using plate or shell finite elements which have many
limitations in applications, such as numerical locking, edge effects, length scaling and the envergence problem. Recently,
by proposing a new approach to treating the nearly-singular integrals, Liu et al. developed a BEM to successfully solve thin
structures with the thickness-to-length ratios in the micro- or nano-scales. On the other hand, the meshless Regular Hybrid
Boundary Node Method (RHBNM), which is proposed by the current authors and based on a modified functional and the Moving Least-Square
(MLS) approximation, has very promising applications for engineering problems owing to its meshless nature and dimension-reduction
advantage, and not involving any singular or nearly-singular integrals. Test examples show that the RHBNM can also be applied
readily to thin structures with high accuracy without any modification. 相似文献
968.
In this paper an approximate equation is derived to describe smooth parts of the stability boundary for linear Hamiltonian
systems, depending on arbitrary number of parameters. With this equation, we can obtain parameters corresponding to the stability
boundary, as well as to the stability and instability domains, provided that one point on the stability boundary is known.
Then differential equations describing the evolution of eigenvalues and eigenvectors along a curve on the stability boundary
surface are derived. These equations also allow us to obtain curves belonging to the stability boundary. Applications to linear
gyroscopic systems are considered and studied with examples.
The project supported by the National Science Foundations of Russia and China (10072012) 相似文献
969.
本文利用边界元方法,通过krichhoff变换将描述叶栅流动的控制方程转换成线性方程。并将广义k-J条件与边界积分方程联立求解,避免了非线性项和叶片出气角的迭代计算。完成了一种快速求解任意迥转面叶栅流场的计算程序,实用表明与其它数值方法及实验结果符合较好,具有快带、简明、实用的特点。 相似文献
970.
A combined theoretical and experimental study is presented for the interaction between crossing shock waves generated by (10°, 10°) sharp fins and a flat plate turbulent boundary layer at Mach 8.3. The theoretical model is the full 3-D mean compressible Reynolds-averaged Navier-Stokes RANS) equations incorporating the algebraic turbulent eddy viscosity model of Baldwin and Lomax. A grid refinement study indicated that adequate resolution of the flowfield has been achieved. Computed results agree well with experiment for surface pressure and surface flow patterns and for pitot pressure and yaw angle profiles in the flowfield. The computations, however, significantly overpredict surface heat transfer. Analysis of the computed flowfield results indicates the formation of complex streamline and wave structures within the interaction region.This article was processed using Springer-Verlag TEX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society. 相似文献