4,712 research outputs found
Passive scalar mixing of a turbulent jet emitted into homogeneous, isotropic turbulence
No full textAlthough most jets, whether they be natural or industrial in origin, are emitted into a turbulent environment, almost all previous research on turbulent jets has dealt with jets emitted into quiescent or laminar background flows. The present work extends the work of Khorsandi, Gaskin and Mydlarski, J. Fluid Mech., 2013 – who studied the effect of background turbulence on the velocity field of a turbulent jet emitted into turbulent surroundings – to the study of passive scalar mixing of a jet released into a turbulent flow. To this end, the experiments described herein use planar laser-induced fluorescence to study the mixing of a (high-Schmidt-number) passive scalar within a turbulent jet that is emitted into a quasi-homogeneous, isotropic, zero-mean-flow turbulent background. We examine herein statistics of the jet’s scalar field, and compare them to those of a jet emitted into a quiescent background
INVESTIGATION OF MECHANICAL BEHAVIOR OF INTERFACES IN NANOSTRUCTURED METALS
No full text
将常规多晶材料的粗晶粒尺寸缩小到纳米尺度时,这些纳米晶体材料会呈现出与其对应的粗晶材料迥异的物理现象.与材料力学行为最相关的是强度及塑形变形机理这两个方面.考虑到晶界的变形与破坏可能是纳米晶体材料低塑性的根源,克服纳米晶体材料中强度与韧性之间存在的“熊掌和鱼不可兼得”的问题,也通常称为晶界工程.在众多的晶界中,孪晶界面被发现可同时保持材料的强度和韧性.本文主要就纳米金属材料中界面的力学行为做一个简要述,包含晶界的强化力学机理以及新型孪晶界面的力学行为与揭示内在尺度效应的模型研究。</p
Direct Numerical Simulations of Turbulent Mixing Layers Between Two Fluids of Large Density Difference
No full textIn numerous practical applications, shear layers exist between fluids of strongly differing densities. At high Atwood numbers, the large variations in density introduce important effects that have recently been observed in other flows (e.g., Livescu and Ristorcelli, J. Fluid Mech., 605:145–180, 2008). To investigate the inertial variable density effects on the instability growth and structure of mixing layers, we perform very large Direct Numerical Simulations of planar mixing layers between two miscible fluids, each with different density. The DNS domain sizes accommodate large extents of mode pairings, based on the most unstable modes obtained from linear stability analysis. The results display the overall statistical effects on the turbulence and mixing, as well as the structural differences that occur as Atwood number is varied. In particular, significant asymmetries are introduced by the differences in the densities of the mixing layer streams
THE LARGE DEFLECTION OF A RIGID PERFECTLY PLASTIC PORTAL FRAME SUBJECTED TO IMPULSIVE LOADING
No full textA portal frame subjected to a distributed impulse is studied on the basis of a large deflection formulation. By assuming that the material is rigid-perfectly plastic, a complete solution is constructed and then compared with the modal solution and the experimental results reported by Hashmi and Al-Hassani (1975, Int. J. Mech. Sci. 17, 513-523) and Bodner and Symonds (1979, Int. J. Solids Structures 15, 1-13). The solution agrees well with the experimental results and indicates that a great part of the input energy is absorbed by travelling plastic hinges. The possible failure regions are discussed according to the distribution of plastic work.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:A1990DL91900002&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701MechanicsSCI(E)EI1ARTICLE111225-12422
Contact velocities of small ellipsoids settling in turbulence
No full textCollisions of small and heavy non-spherical particles settling in turbulence are very important for systems such as ice clouds and proto-planetary disks where the particle spectra evolution is strongly dependent on the collision induced growth rate. Still, the influence of the particle shape on the collision probability is virtually unknown. Building on our recent investigation on the collision rate of monodisperse suspensions of ellipsoidal particles (Siewert et al., J. Fluid Mech. 758, 686-701, 2014), we show theoretically and by direct numerical simulations that the behavior of ellipsoids subject to turbulence and gravity is different from the behavior of spheres. Due to the dependence of the particle settling velocity on the particle orientation, the relative velocity at contact is influenced by turbulence. When ellipsoids differ either by mass or shape, their contact velocity is randomized by the randomized particle orientation. For particles much heavier than the fluid these orientation dependent settling velocity differences are larger than the relative velocities directly induced by the turbulent fluctuations
Settling of finite-size particles in isotropically forced, homogeneous turbulence: interface-resolved simulations
No full textWe have simulated the gravity-induced settling of finite-size particles in a turbulent background flow which is forced in a statistically-stationary fashion. The simulations are accurately resolving the solid-fluid interface with the aid of an immersed boundary technique [1]. The parameters of the simulation are (apart from background turbulence) identical to those of reference [2], where particle clustering was observed at a Galileo number of 178 and a solid volume fraction of 0.005. In the present case, it is found that a relative turbulence intensity of 0.24 leads to the disappearance of the clusters; as a consequence, the increase in average particle settling velocity found in [2] also vanishes. [1] M. Uhlmann. An immersed boundary method with direct forcing for the simulation of particulate flows. J. Comput. Phys., 209(2):448–476, 2005. [2] M. Uhlmann and T. Doychev. Sedimentation of a dilute suspension of rigid spheres at intermediate Galileo numbers: the effect of clustering upon the particle motion. J. Fluid Mech., 752:310–348, 2014
Statistics of the subgrid scales after the shock-turbulence interaction
No full textThe interaction of a normal shock with isotropic turbulence (IT) represents a basic problem for studying some of the phenomena associated with high speed flows, such as hypersonic flight, supersonic combustion and Inertial Confinement Fusion (ICF). In general, in practical applications, the shock width is much smaller than the turbulence scales and the upstream turbulent Mach number is modest. In this case, recent high resolution shock-resolved Direct Numerical Simulations (DNS) (Ryu and Livescu, J. Fluid Mech., 756, R1, 2014) show that the interaction can be described by the Linear Interaction Approximation (LIA). By using LIA to alleviate the need to solve the shock, DNS post-shock data can be generated at much higher Reynolds numbers than previously possible. Here, such results with Taylor Reynolds number around are used to investigate the properties of the subgrid scales (SGS). In particular, it is shown that the shock interaction decreases the asymmetry of the SGS dissipation PDF as the shock Mach number increases, with a significant enhancement in size of the regions and magnitude of backscatter
Comments on the articles "Hyperbolic thermoelasticity: A review of recent literature" (Chandrasekharaiah DS, 1998, appl mech rev 51(12), 705-729) and "Thermoelasticity with second sound: a review" (Chandrasekharaiah DS, 1986, appl mech rev 39(3), 355-376)
No full textThis review article is a continuation of a previous article by the author, Thermoelasticity with second sound: A review, which appeared in this journal in March, 1986 (Appl Mech Rev39 (3) 355-376). Here, attention is focused on papers published during the past 10-12 years. Contributions to the theory of thermoelasticity with thermal relaxation and the temperature-rate dependent thermoelasticity theory are reviewed. The recently developed theory of thermoelasticity without energy dissipation is described, and its characteristic features highlighted. A glance is made at the new thermoelasticity theory which includes the so-called dual-phase-lag effects. There are 338 references
Acoustic radiation due to scattering of T-S wave by the mean-flow distortion induced by steady local suction
No full textSubstantial sound waves can be generated by boundary-layer instability modes when the latter are scattered by a rapid mean-flow distortion. This is a rather generic mechanism and operates when an oncoming T-S wave is scattered by a steady local suction slot. This paper focuses on this problem by extending a recently developed Local Scattering Theory (Wu & Dong, J. Fluid Mech. submitted), where a so-called transmission coefficient, defined as the ratio of the T-S wave amplitude downstream of the scatter to that upstream, is introduced to characterize the effect of a local scatter on boundary-layer instability and transition. As in the earlier work, the mathematical formulation is based on triple-deck formulism, but in order to accommodate the acoustic far field, which was not considered in the paper mentioned, the unsteady terms in the upper deck, which play a leading-order role in radiation, are retained, and the influence of the radiated sound on the near-wall perturbation is included. The upper deck equation for the pressure is the Helmholtz equation rather than the Laplace equation. This leads to a modified pressure-displacement relation, which is coupled with the linearized boundary-layer equations in the lower deck. Discretization of the whole system formulates a generalized eigenvalue problem, which is solved numerically. It is found that suction suppresses oncoming T-S waves, and this effect increases with the suction velocity and the slot width. The directivity is ndependent of the flow parameters only when the Mach number is low. The intensity of the radiated sound in general increases with the frequency, the suction velocity and the width of the suction slot. Interestingly, for O(1) suction velocities, the radiated sound is very weak, indicating that the gain of stabilizing effect does not cause aeroacoustic penalty
The mean velocity profile of a smooth-flat-plate turbulent boundary layer at high Reynolds number
No full textSmooth flat-plate turbulent boundary layers (TBLs) have been studied for nearly a century. However, there is a relative dearth of measurements at Reynolds numbers typical of full-scale marine and aerospace transportation systems (Reθ = Ueθ-vandgt; 105, where Ue = free-stream speed, θ= TBL momentum thickness and v= kinematic viscosity). This paper presents new experimental results for the TBL that forms on a smooth flat plate at nominal Reθ values of 0.5×105, 1.0×105 and 1.5×105. Nominal boundary layer thicknesses (δ) were 80-90mm, and Karman numbers (δ+) were 17000, 32000 and 47000, respectively. The experiments were conducted in the William B. Morgan Large Cavitation Channel on a polished (k+ andlt; 0.2) flat-plate test model 12.9m long and 3.05m wide at water flow speeds up to 20ms1. Direct measurements of static pressure and mean wall shear stress were obtained with pressure taps and floating-plate skin friction force balances. The TBL developed a mild favourable pressure gradient that led to a streamwise flow speed increase of ~2.5percent over the 11m long test surface, and was consistent with test section sidewall and model surface boundary-layer growth. At each Reθ, mean streamwise velocity profile pairs, separated by 24cm, were measured more than 10m from the model's leading edge using conventional laser Doppler velocimetry. Between these profile pairs, a unique near-wall implementation of particle tracking velocimetry was used to measure the near-wall velocity profile. The composite profile measurements span the wall-normal coordinate range from y+ andlt; 1 to y andgt; 2δ. To within experimental uncertainty, the measured mean velocity profiles can be fit using traditional zero-pressure-gradient (ZPG) TBL asymptotics with some modifications for the mild favourable pressure gradient. The fitted profile pairs satisfy the von-Kármán momentum integral equation to within 1percent. However, the profiles reported here show distinct differences from equivalent ZPG profiles. The near-wall indicator function has more prominent extrema, the log-law constants differ slightly, and the profiles' wake component is less pronounced. © 2010 Cambridge University Press.Afzal N, 2001, ACTA MECH, V151, P195, DOI 10.1007-BF01246918; Barenblatt GI, 2000, PHYS FLUIDS, V12, P2159, DOI 10.1063-1.1287613; BENEDICT RP, 1984, FUNDAMENTALS TEMPERA, P340; Bourassa C, 2009, J FLUID MECH, V634, P359, DOI 10.1017-S0022112009007289; Buschmann MH, 2003, AIAA J, V41, P565, DOI 10.2514-2.1994; Compton DA, 1996, EXP FLUIDS, V22, P111, DOI 10.1007-s003480050028; Compton DA, 1997, J FLUID MECH, V350, P189, DOI 10.1017-S0022112097007106; DeGraaff DB, 2000, J FLUID MECH, V422, P319, DOI 10.1017-S0022112000001713; Elbing BR, 2008, J FLUID MECH, V612, P201, DOI 10.1017-S0022112008003029; Etter RJ, 2005, MEAS SCI TECHNOL, V16, P1701, DOI 10.1088-0957-0233-16-9-001; Fernholz HH, 1996, PROG AEROSP SCI, V32, P245, DOI 10.1016-0376-0421(95)00007-0; FERNHOLZ HH, 1995, PHYS FLUIDS, V7, P1275, DOI 10.1063-1.868516; Fife P, 2005, J FLUID MECH, V532, P165, DOI 10.1017-S0022112005003988; Gad-el-Hak M., 1994, APPL MECH REV, V47, P307, DOI DOI 10.1115-1.3111083; George W. K., 1997, APPL MECH REV, V50, P689, DOI [10.1115-1.3101858, DOI 10.1115-1.3101858]; Knobloch K, 2002, IUTAM S REYN NUMB SC, P11; Kunkel GJ, 2006, J FLUID MECH, V548, P375, DOI 10.1017-S0022112005007780; Launder B. E., 1972, MATH MODELS TURBULEN; LINDGREN B, 2004, A H M M H J A V H J, V502, P127; Marusic I, 1997, PHYS FLUIDS, V9, P3718, DOI 10.1063-1.869509; Marusic I, 2010, PHYS FLUIDS, V22, DOI 10.1063-1.3453711; Marusic I, 2003, PHYS FLUIDS, V15, P2461, DOI 10.1063-1.1589014; McKeon BJ, 2007, PHILOS T R SOC A, V365, P635, DOI 10.1098-rsta.2006.1952; Metzger MM, 2001, PHYS FLUIDS, V13, P692, DOI 10.1063-1.1344894; Metzger MM, 2001, PHYS FLUIDS, V13, P1819, DOI 10.1063-1.1368852; Monkewitz PA, 2008, PHYS FLUIDS, V20, DOI 10.1063-1.2972935; Monkewitz PA, 2007, PHYS FLUIDS, V19, DOI 10.1063-1.2780196; Nagib HM, 2008, PHYS FLUIDS, V20, DOI 10.1063-1.3006423; Nagib HM, 2004, IUTAM S 100 YEARS BO, P383; Osterlund JM, 2000, PHYS FLUIDS, V12, P2360, DOI 10.1063-1.1287660; OSTERLUND JM, 2000, A H M H, V12, P1; OSTERLUND JM, 1999, 30 AIAA FLUID DYN C; PANTON RC, 2002, PHYS FLUIDS, V14, P180; Park J. T., 2003, P 4 ASME JSME JOINT; Pope S. B., 2000, TURBULENT FLOWS; SADDOUGHI SG, 1994, J FLUID MECH, V268, P333, DOI 10.1017-S0022112094001370; Sanders WC, 2006, J FLUID MECH, V552, P353, DOI 10.1017-S0022112006008688; SCHULTZGRUNOW F, 1941, 1718 NACA, P1; Sreenivasan K. R., 1989, EXP FLUID, V46, P159; Wei T, 2005, J FLUID MECH, V522, P303, DOI 10.1017-S0022112004001958; White F. M., 2006, VISCOUS FLUID FLOW; Winkel ES, 2009, J FLUID MECH, V621, P259, DOI 10.1017-S002211200800487410101
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