Large-scale anisotropy in stably stratified rotating flows.

We present results from direct numerical simulations of the Boussinesq equations in the presence of rotation and/or stratification, both in the vertical direction. The runs are forced isotropically and randomly at small scales and have spatial resolutions of up to 1024(3) grid points and Reynolds numbers of ≈1000. We first show that solutions with negative energy flux and inverse cascades develop in rotating turbulence, whether or not stratification is present. However, the purely stratified case is characterized instead by an early-time, highly anisotropic transfer to large scales with almost zero net isotropic energy flux. This is consistent with previous studies that observed the development of vertically sheared horizontal winds, although only at substantially later times. However, and unlike previous works, when sufficient scale separation is allowed between the forcing scale and the domain size, the kinetic energy displays a perpendicular (horizontal) spectrum with power-law behavior compatible with ∼k(⊥)(-5/3), including in the absence of rotation. In this latter purely stratified case, such a spectrum is the result of a direct cascade of the energy contained in the large-scale horizontal wind, as is evidenced by a strong positive flux of energy in the parallel direction at all scales including the largest resolved scales.

The relationship between exposure during pregnancy to cigarette smoking and cocaine use and placenta previa.

OBJECTIVE: This study examined the relationship between two maternal exposures, cigarette smoking and cocaine use, and placenta previa. STUDY DESIGN: A hospital-based case-control study was conducted. Three hundred four cases of placenta previa were compared with 2732 controls with respect to demographic characteristics, substance use, and perinatal characteristics. Logistic regression was used to examine the individual effects of cigarette smoking and cocaine use on placenta previa, independent of other known risk factors. RESULTS: A dose-response relationship between smoking cigarettes and placenta previa was observed independent of other known risk factors (ptrend < 0.01). Pregnant women who smoked > or = 20 cigarettes per day were over two times more likely to experience a placenta previa relative to nonsmokers (odds ratio 2.3, 95% confidence interval 1.5 to 3.5). Pregnant women who used cocaine were 1.4 times (95% confidence interval 0.8 to 2.4) as likely to experience a placenta previa as nonusers. CONCLUSIONS: The previously observed association between smoking and placenta previa is supported by the dose-response relationship observed in this study. The potential association of cocaine with placenta previa needs more exploration.