Showing papers on "Secondary circulation published in 1979"

Secondary Flow and Shear Stress at River Bends

Abstract: Field measurements are presented to illustrate the pattern of secondary circulation at river bends, the effect of the circulation on boundary shear stress disbribution, and the way in which that effect changes with discharge. The field results show that, in addition to the classical main cell, there can be a small cell of opposite circulation close to the outer bank provided the bank is steep. The circulation distorts the primary isovel pattern and thereby affects the positions and relative magnitudes of the boundary shear stress peaks associated with regions of downwelling and high velocity. Secondary circulation appears to be weakest at low and high discharges and to be strongest, with its greatest effect on the shear stress distribution, at medium discharges. Generally the distribution of shear stress at a section becomes more uniform as discharge increases and at high discharges is determined mainly by primary flow effects.

Forced secondary circulations in hurricanes

Abstract: In this study, application of Eliassen's (1951) forced secondary circulation model to the hurricane vortex is reexamined. It is shown by scale analysis that the symmetric tangential flow is in hydrostatic and gradient balance if the asymmetric part of the tangential flow and the radial and vertical symmetric motions are appreciably smaller than the symmetric tangential flow. When surface friction and cumulus transports of heat and momentum are included, the forced symmetric radial and vertical motions may be diagnosed. The approximations on which this system of balanced equations is based are valid everywhere in the hurricane vortex except in the upper tropospheric outflow layer. In the absence of Fickian diffusion of momentum and heat, both deep inflow in the outer vortex and subsidence within the eye are forced by radial gradients of convective heating. If corresponding gradients of cooling arise in the upper troposphere by mechanisms such as detrainment from overshooting cumuli, descent may be induced in the lower stratosphere above and around the eye. Convective momentum transports act to weaken the thermally induced secondary flow somewhat. If the vertical resolution of the finite difference mesh is too coarse to resolve the detrainment layer properly, the model may diagnose an unrealistic secondary flow with ascent throughout the eye. In the absence of all cumulus processes, the frictionally converged air no longer rises to the tropopause but rather flows outward in a shallow layer in the lower troposphere.

Shear-Stress Distribution in Stable Channel Bends

Abstract: A theoretical and experimental investigation is made concerning the relationships between secondary flow phenomena in alluvial channel bends and boundary shear-stress distribution and related stable bed topography generated in the vicinity of same. The physical model, a wide rectangular channel system in which two-dimensional flow conditions were reproduced, incorporated an adjustable bend test section and bends having central angles of 45—, 60—, and 70? were adopted in the study. Particular emphasis was placed on an examination of the secondary circulation decay process along that portion of the straight channel reach immediately following the bend section. Observed stable bed topography within the bend and along the downstream decay zone related favorably to the steady-state boundary shear-stress distributions obtained from a numerical model of the flow field therein.