摘要:

We present criteria for monitoring the validity of the Navier–Stokes approximation during the simulation of a rarefied gas. Our approach is based on an underlying kinetic formulation through which one can construct nondimensional non-negative definite matrices from moments of the molecular distribution. We then identify one such3×3matrix that can be evaluated intrinsically in the Navier–Stokes approximation. Our criteria are based on deviations of the eigenvalues of this matrix from their equilibrium value of unity. Not being tied to a particular benchmark problem, the resulting criteria are portable and may be applied to any Navier–Stokes simulation. We study its utility here by comparing stationary planar shock profiles computed using the Navier–Stokes equations with those computed using Monte Carlo simulations. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869849

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We study the initial value problem posed by the small amplitude free oscillations of a bubble in a viscous fluid. The solution consists of a linear combination of discrete normal modes and an integral over a continuous spectrum. The continuous spectrum dominates the solution for large times. As a result, the surface deformation ultimately decays algebraically and not as a modulated damped wave, as has sometimes been suggested. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869850

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Recent experimental results [J. Fluid Mech.345, 45 (1997)] for long-wavelength surface-tension-driven rupture of thin liquid layers (∼0.01 cm) found the onset for significantly smaller imposed temperature gradients than predicted by linear stability analyses that assume an initially flat interface with periodic boundary conditions. The presence of sidewalls and other aspects of the experiment, however, led to deformed interfaces even with no imposed temperature gradient. These sidewall effects were not due to a small system size since experiments with aspect ratios as large as 450 were significantly affected. The stability analysis presented here takes into account the effects of the deformed interface profile and shows that these effects account for some of the disagreement between experiment and theory. In addition, deviations from standard linear stability theory caused by these effects have the same qualitative behavior as the deviations seen in the experiments. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869857

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this Brief Communication, we report a new finding on a Taylor–Couette flow in which the outer cylinder is stationary and the inner cylinder is accelerated linearly from rest to a desired speed. The results show that when the acceleration(dRe/dt)is higher than a critical value of about2.2 s−1,there exists a new flow regime in which the flow pattern shows remarkable resemblance to regular Taylor vortex flow but is of shorter wavelength. However, when the acceleration is lower than2.2 s−1,a wavy flow is found to occur for the same Reynolds number range. To our knowledge, this is probably the first time that such a phenomenon has been observed. For completeness, the case of a decelerating cylinder is also investigated, and the results are found to be almost the same. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869851

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Dispersion of heavy particles from the core of a turbulent vortex with nonzero axial flow is investigated using an axisymmetric computational model, in which the coherent turbulent structures are modeled as periodic vortex rings wrapped around a columnar vortex. Two regimes of particle dispersion are distinguished, which are mapped in the space of two dimensionless parameters; the ratioL/Rof vortex ring spacing to ring radius and the ratio&Ggr;R/&Ggr;Cof ring strength to columnar vortex strength. In one regime, the outward particle drift is arrested by the ring vortices and the particles become trapped on a single wavy surface enclosing the columnar vortex core. In the other regime, the particles continue to move radially past the vortex rings and the particle dispersion is enhanced by the ring-induced velocity. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869852

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The departure from Kolmogorov (1941) scaling of high-order moments of longitudinal velocity increments in a fully developed turbulent channel flow is investigated using data obtained with hot wire anemometry (HWA), laser Doppler velocimetry (LDV) and direct numerical simulation (DNS). The magnitude of the departure increases towards the wall, reaching a local maximum at a distance of about 20 wall units. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869853

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Some aspects of a recent paper on rotating turbulence by Canuto and Dubovikov [Phys. Fluids9, 2132 (1997)] are examined from historical and scientific perspectives. Their claim to have discovered a new energy spectrum scaling law for rotating turbulence is examined in light of previous publications on this subject. We answer an objection raised to the consistency of this spectral scaling law with the Bardina-type dissipation rate transport equation derived from this law by two of the authors [Phys. Fluids8, 3172 (1996)]. Finally, some difficulties with the alternative model for the dissipation rate transport equation proposed by Canuto and Dubovikov are described in both the weak and strong rotation limits. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869854

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:1070-6631    

DOI:10.1063/1.869855

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In the publication by Hu and Mei [Phys. Fluids10, 1028 (1998)] a correction to the collision formula is described for like particles within a given size category. Here we show that the formula derived by Sundaram and Collins (1997) is already consistent with Hu and Mei (1998) and so their correction does not apply. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869856

出版商:AIP    

年代:1998

数据来源: AIP