摘要:

An experimental investigation of passive scalar mixing in turbulent pipe flow is carried out using a new non-intrusive scalar initialization technique. The measurements support a recently predicted similarity scaling of concentration spectra in flows that are unbounded in one direction. Reflecting this scaling, the scalar variance exhibits a power-law rather than exponential decay, indicating that the traditional plug-flow reactor picture of turbulent pipe-flow mixing omits key physical mechanisms.©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869211

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Predictability problem for two-dimensional decaying turbulence is addressed by means of numerical simulations. Qualitative and quantitative comparisons with previous results obtained by closure approximations are critically examined. It is found that, as for other features of two-dimensional turbulence, the role of coherent vortices is essential for a correct interpretation of the results. A Lagrangian, vortex-based, definition for the growth of incertitudes leads in general to an enhancement of the predictability time. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869227

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The alignments between the vorticity, the vortex stretching vector, the pressure Hessian eigenvectors and the strain rate eigenvectors are computed and discussed in the case of the Burgers’ vortex and the Burgers’ layer. It is shown that the main physical properties of these models can be deduced from these alignments. Following this example, the alignments between these vectors in turbulent flows are interpreted as dominated by stretched, coherent and locally quasi-bidimensional regions. This induces a new and safer classification for the strain rate and the pressure Hessian eigenvalues. ©1997 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.869228

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this work, we present from first principles a direct numerical simulation (DNS) of a fully turbulent channel flow of a dilute polymer solution. The polymer chains are modeled as finitely extensible and elastic dumbbells. The simulation algorithm is based on a semi-implicit, time-splitting technique which uses spectral approximations in the spatial coordinates. The computations are carried out on a CRAY T3D parallel computer. The simulations are carried out under fully turbulent conditions albeit, due to computational constraints, not at as high Reynolds number as that usually encountered in polymer-induced drag reduction experiments. In order to compensate for the lower Reynolds number, we simulate more elastic fluids than the ones encountered in drag reduction experiments resulting in Weissenberg numbers (a dimensionless number characterizing the flow elasticity) of similar magnitude. The simulations show that the polymer induces several changes in the turbulent flow characteristics, all of them consistent with available experimental results. In particular, we have observed, associated with drag reduction, a decrease in the streamwise vorticity fluctuations and an increase in the average spacing between the streamwise streaks of low speed fluid within the buffer layer. These findings suggest a partial inhibition of turbulence generating events within the buffer layer by the macromolecules after the onset of drag reduction. This inhibition is further shown to be associated with an enhanced effective viscosity attributed to the extensional thickening properties of polymer solutions, as proposed in the past by Metzner, Lumley and other investigators. Using the simulation results obtained for different sets of parameter values which modify the relaxational and extensional properties of the model, we propose a set of criteria for the onset of drag reduction. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869229

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The evolution of large turbulent eddies has been investigated in seven supersonic shear layers with average convective Mach numbersMcranging from 0.22 to 0.86 and with large variation in density and velocity ratios. A two-laser, single-detector planar laser-induced fluorescence technique was used to visualize the flow and its evolution. Two-dimensional pattern matching yielded the convective velocity of the eddies. ForMc>0.3, fast and slow modes of eddy propagation were detected in supersonic–subsonic and supersonic–supersonic combinations, respectively. An empirical model for the convective velocity is proposed. Plan views reveal coexistence of two- and three-dimensional disturbances. Interaction among eddies appears significantly suppressed. The findings have direct impact on supersonic jet noise and are very relevant to supersonic combustion. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869230

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In the present study, we develop a full second order model and a new flamelet closure relation for the mean chemical source term to describe turbulent reactive flows with premixed reactants. This description ensures that non-gradient and counter-gradient diffusion which can appear in such flames, are taken into account in 2-D or 3-D geometries. 2-D calculations are performed in the case of a confined turbulent premixed flame stabilized by a backward facing step, a configuration which reveals experimentally counter-gradient diffusion and combustion induced production of turbulence. Numerical results are found to be in excellent agreement with measurements of Shepherd &etal; [Nineteenth Symposium (International) on Combustion(The Combustion Institute, Pittsburgh, 1982)]. The important influence of counter-gradient diffusion on first order quantities, i.e., mean velocity and length of the recirculation zone is shown. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869231

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Transport properties of a dilute gas subjected to arbitrarily large velocity and temperature gradients (steady planar Couette flow) are determined. The results are obtained from the so-called ellipsoidal statistical (ES) kinetic model, which is an extension of the well-known BGK kinetic model to account for the correct Prandtl number. At a hydrodynamic level, the solution is characterized by constant pressure, and linear velocity and parabolic temperature profiles with respect to a scaled variable. The transport coefficients are explicitly evaluated as nonlinear functions of the shear rate. A comparison with previous results derived from a perturbative solution of the Boltzmann equation as well as from other kinetic models is carried out. Such a comparison shows that the ES predictions are in better agreement with the Boltzmann results than those of the other approximations. In addition, the velocity distribution function is also computed. Although the shear rates required for observing non-Newtonian effects are experimentally unrealizable, the conclusions obtained here may be relevant for analyzing computer results. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869232

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this work we investigate numerically turbulent flow of low electrical conductivity fluid subject to electro-magnetic (EMHD) forcing. The configuration is similar to the one considered in the experimental work of Henoch and Stace [Phys. Fluids7, 1371 (1995)] but in a channel geometry. The lower wall of the channel is covered with alternating streamwise electrodes and magnets to create a Lorentz force in the positive streamwise direction. Two cases are considered in detail corresponding to interaction parameter values of 0.4 (case 1) and 0.1 (case 2). The effect of switchingoffandonthe electrodes is also studied for the two cases. At the Reynolds number considered (Re&tgr;≈200), a drag increase was obtained for all cases, in agreement with the experiments of Henoch and Stace. A Reynolds stress analysis was performed based on a new decomposition of thegradientsnormal to the wall of the Reynolds stress−u′v′.It was found that the vortex stretching termw′w2′and the spanwise variation of the stress componentu′w′are responsible for the drag increase. More specifically, the term∂(u′w′)/∂x3is associated with secondary vortical motions in the near-wall and becomes large and positive for large shear stress in regions where fluid is moving toward the wall. In contrast, negative values are associated with regions of lower shear where fluid is being lifted away from the wall. Unlike the unperturbed flow, in the controlled flow high speed near-wall streamwise jets are present (case 1) even in the time-averaged fields. Other changes in turbulence structure are quantified using streak spacing, vortex lines, vorticity quadrant analysis, and plots of the rms value of the vorticity angle. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869210

出版商:AIP    

年代:1997

数据来源: AIP