摘要:

The effects produced by the occurrence of both ideal and resistive instabilities in magnetohydrodynamic (MHD) turbulence are discussed, from the point of view of the formation of coherent structures. It is shown that MHD equations spontaneously develop spatially flat or elongated structures, a fact that might be related with the intermittency of the magnetic field observed in some numerical simulations. It is suggested that these flat structures are perhaps unstable with respect to reconnecting (tearing mode type) perturbations. Such instabilities give rise to a nonlocal energy transfer in the wave number space, which modifies the usual inertial spectrum of MHD turbulence, and increases the rate of energy and correlation dissipation. An explicit nonlinear cascade model that qualitatively takes into account such a nonlocal energy transfer is built up, in order to study dynamically the balance between nonlinear energy cascade and linear tearing mode instability. Finally, an attempt is made to interpret some of the peculiar behavior of published MHD simulations on the basis of the results obtained using the present model.

ISSN:0899-8213    

DOI:10.1063/1.857598

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A geometric feature of the vorticity field in the near‐wall region is educed from an analysis of time‐resolved spanwise vorticity measurements. The results of this analysis indicate the existence of a region near the wall in which the instantaneouszcomponent of the vorticity vector is virtually always in the direction indicated by the mean shear. In accordance with the solenoidal condition of the vorticity field, this observed physical feature represents a limitation on the spatial structure of the vorticity field in the near‐wall region. The present results also reinforce the conclusion of Klewicki (Ph.D. dissertation, Michigan State University, 1989) that, for the momentum deficit thickness Reynolds number range 1010≤R&thgr;≤4850, the statistical properties of the spanwise vorticity component are invariant under an inner variable normalization fory+less than about 40.

ISSN:0899-8213    

DOI:10.1063/1.857599

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A simple model, parametrized by the Reynolds stress anisotropy, is proposed for the spectrum of weakly anisotropic turbulence. It contains a model constant that affects its region of realizability. This spectrum model is used to derive a one point closure to the rapid pressure‐strain term. The derived pressure‐strain model is linear in the Reynolds stress anisotropy and is of the same form as the closure model of Launder, Reece, and Rodi (LRR) [J. Fluid Mech.68, 537 (1975)]. The spectrum model becomes unrealizable in some regions of wave space for sufficiently large anisotropy of the Reynolds stress, and this is used to infer the region of validity of the linear closure model. It is found that the extent of the valid region is very small when the model constant is set to match rapid distortion theory, and largest for a model constant set close to the value suggested by LRR. However, even the largest valid domain does not extend very far from isotropy, suggesting inherent weakness in the linear pressure‐strain models.

ISSN:0899-8213    

DOI:10.1063/1.857600

出版商:AIP    

年代:1990

数据来源: AIP

ISSN:0899-8213    

DOI:10.1063/1.857827

出版商:AIP    

年代:1990

数据来源: AIP