摘要:

The description of turbulent mixing and chemical reactions by Lagrangian probability density function methods offers some significant advantages over other methods, mainly due to the simulation of mixing processes and the exact treatment of chemical transformations. A key problem of such methods is the information on the time scales of processes, because they determine the dynamics and intensity of mixing. This question is considered for stratified flow. Different models are presented for the development of these time scales in time and their stationary spatial patterns in dependence on shear and stratification. The model predictions are shown to be in agreement with large-eddy simulations of stratified homogeneous shear flow. Two further applications of these models are considered: the description of transitions between flow regimes (characterized by different scaling quantities) in the stationary atmospheric surface layer and, second, the simulation of buoyant plume rise. It is shown that the predictions of the stationary frequency model agree with measured data. The consideration of limit cases of this model leads to connections between second-order closure parameters and (critical) flow numbers that characterize these transitions. These relationships are shown to be very advantageous for the application of closure models. A new flow number that characterizes the transition to free convective flow under unstable stratification is introduced here in analogy to the critical gradient Richardson number, which characterizes the onset of turbulence in stably stratified flow. The second application provides a new theory for buoyant plume rise. Two parameters that describe the turbulent mixing in the entrainment and extrainment stages of plume rise are explained as ratios of the relevant time scales. The two-thirds power law of buoyant plume rise, which is observed for nonturbulent and neutrally stratified flow, is obtained without having to make ad hoc assumptions. For turbulent flow, the plume’s leveling-off is calculated in accord with measurements. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869618

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

As an incident shock wave hits a material interface between two fluids of different densities, the interface becomes unstable. Small disturbances at the interface start to grow. This interfacial instability is known as a Richtmyer–Meshkov (RM) instability. It plays an important role in the studies of inertial confinement fusion and supernova. The majority of studies of the RM instability were in plane geometry—namely, plane shocks in Cartesian coordinates. We present a systematic numerical study of the RM instability driven by cylindrical shocks for both the imploding and exploding cases. The imploding (exploding) case refers to a cylindrical shock colliding with the material interface from the outside in (inside out). The phenomenon of reshock caused by the waves reflected from the origin is also studied. A qualitative understanding of this system has been achieved. Detailed studies of the growth rate of the fingers at the unstable interface are presented. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869624

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Previous investigations have demonstrated that a compressible reacting mixing layer can develop two instability modes in addition to the more common central mode that exists unaccompanied in incompressible nonreacting flows. These two additional modes are termed “outer” because of their association with the fast and slow free streams. Numerical simulations have shown that mixing layers dominated by outer modes have a lower global reaction rate in comparison to a flow structure governed by the central mode. Therefore, the presence of these modes has important consequences for applications in supersonic combustion. Results are presented from a parametric study of the compressible reacting mixing layer’s regime space using linear stability analysis. The focus of our work is to develop a better understanding for the combined effects of compressibility, heat release and the ratios of density, equivalence, and velocity on the instability characteristics of each mode and on the structure predicted to result in a turbulent reacting mixing layer. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869619

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Mixing enhancement results are presented for compressible (convective Mach number 0.63) planar shear layers perturbed by 2D and 3D disturbances located within the supersonic-side splitter tip boundary layer. The disturbances were parametrically varied in shape, spacing, and thickness, and for each geometry time-resolved end-, side-, and plan-view visualizations of mixed fluid were obtained. The mixing layer thickness and growth rate are measured directly from the averaged images. As an indicator of the pressure loss induced by each disturbance geometry, the streamwise static pressure distribution is also recorded. The visualizations reveal that discrete 3D disturbances induce appreciable spanwise convolution, streamwise structure, and thickening of the mixing layer with disturbances as thin as 5&percent; of the boundary layer displacement thickness. The optimal disturbance appears to have an angle of 30° to the streamwise direction and be located at the splitter tip, rather than upstream. Panoramic side-views show that the far-field growth rate increases (45&percent; in one case) for certain discrete 3D disturbances but not 2D disturbances, despite equivalent area blockage. For the most promising geometry, quantitative measurements of the mixing layer thickness, probability of mixed fluid, and mixing efficiency were made using cold chemistry planar laser-induced fluorescence. The perturbed layer shows a slight improvement (7&percent;) in mixing efficiency and a large increase (48&percent;) in layer thickness, indicating that gains in the total amount of mixed fluid occur primarily by layer thickening. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869620

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The nonlinear Boltzmann equation for the steady planar Poiseuille flow generated by an external fieldgis exactly solved through orderg2.It is shown that the pressure and temperature profiles, as well as the momentum and heat fluxes, are in qualitative disagreement with the Navier–Stokes predictions. For instance, the temperature has a local minimum at the middle layer instead of a maximum. Also, a longitudinal component of the heat flux exists in the absence of gradients along that direction and normal stress differences appear although the flow is incompressible. To account for theseg2-order effects, which are relevant when the hydrodynamic quantities change over a characteristic length of the order of the mean free path, it is shown that the Chapman–Enskog expansion should be carried out three steps beyond the Navier–Stokes level. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869621

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The classical result of Smoluchowski [Z. Phys. Chem.92, 129 (1917)] for the collision rate of monodisperse particles in a laminar shear flow is shown to be inaccurate due to the inclusion of the self-collision. In the present work we extend Smoluchowski’s result by excluding the self-collision in the counting of collision pairs. A numerical simulation for particle collisions in a laminar shear flow at very low concentration is carried out to validate the extended result of Smoluchowski. Good agreement for the collision rate between the numerical simulation and the prediction based on the extended expression is obtained. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869622

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We demonstrate here the use of a non-invasive technique based on Positron Emission Tomography (PET) in visualizing and in making quantitative measurements of scalar transport through natural opaque permeable sediments. Along with various other possibilities, this technique has the potential to help improve the understanding of processes that take place at the seabeds between the porewater and the overlying water, which result in exchange of nutrients, toxins and solute. Unlike many other methods, PET is able to produce full three-dimensional pictures of the percolation of fluid through not only “constructed” but also natural porous media. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869627

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A power series mathematical solution is advanced for the problem of instability of an inviscid liquid sheet of parabolic velocity profile emanated from a nozzle into an inviscid gas. A comparison of the instability of a sheet of parabolic velocity profile with one of a uniform velocity profile is performed in order to deduce the effects of velocity profile relaxation on instability. The results show that for both antisymmetrical and symmetrical disturbances departure from uniformity of the velocity profile causes the instability to be reduced. The physical interpretation and practical implications of the present findings are discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869628

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Barenblatt [J. Fluid Mech.248, 513 (1993)] proposed a Reynolds number(R)-dependent power law for the mean velocity near the wall in fully developed turbulent shear flows. In an appropriate region, asR→∞, the envelope of the velocity profiles reduces to a logarithmic law with an additive constant that is significantly larger than that of the universal logarithmic law. This discrepancy is removed upon including a higher-order term in the power-law exponent. Using the friction (instead of average) velocity in the definition ofRdestroys the invariance of the nonvanishing higher-order terms of the power law asR→∞. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869629

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In rimming flow, a thin film of viscous liquid is entrained on the inside of a horizontally rotating cylinder. We give an explicit criterion for determining whether or not shock solutions occur and show that the location and height of these shocks can be determined using a simple lubrication theory. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869630

出版商:AIP    

年代:1998

数据来源: AIP