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Investigation of controls on secondary circulation in a simple confluence geometry using a three‐dimensional numerical model
Author(s) -
Bradbrook K. F.,
Biron P. M.,
Lane S. N.,
Richards K. S.,
Roy A. G.
Publication year - 1998
Publication title -
hydrological processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.222
H-Index - 161
eISSN - 1099-1085
pISSN - 0885-6087
DOI - 10.1002/(sici)1099-1085(19980630)12:8<1371::aid-hyp620>3.0.co;2-c
Subject(s) - confluence , geometry , simple (philosophy) , circulation (fluid dynamics) , geology , calculus (dental) , mathematics , mechanics , physics , computer science , medicine , programming language , philosophy , dentistry , epistemology
Recent research into river channel confluences has identified confluence geometry, and particularly bed discordance, as a control on confluence flow structures and mixing processes, and this has been illustrated using both field measurements in natural confluences and laboratory measurements of simplified confluences. Generalization of the results obtained from these experiments is limited by the number of confluence geometries that can be examined in a reasonable amount of time. This limitation may be overcome by numerical models, in which confluence geometry is more readily varied, and data acquired more rapidly. This paper aims to: (i) validate the application of a three‐dimensional numerical model to a simple confluence geometry; (ii) simulate the effects of different boundary condition values upon flow structures; and (iii) interpret the implications of these simulations for river channel confluence dynamics. The model used in this research solves the three‐dimensional form of the Navier–Stokes equations and is used to simulate the flow in a parallel confluence of unequal depth channels and to investigate the effect of different combinations of velocity and depth ratio between the two tributaries. The results generally agree with empirical evidence that secondary circulation is generated in the absence of streamline curvature, but only for specific combinations of depth and velocity ratio. This research shows how understanding of the interaction of these controls is enhanced if pressure gradients are considered. The velocity ratio is the prime determinant of the cross‐stream pressure gradient that initiates cross‐stream velocities. However, for significant secondary circulation to form, cross‐stream velocities must lead to significant transfer of fluid in the cross‐stream direction. This depends on the vertical extent of the cross‐stream pressure gradient which is controlled by the depth ratio. In this study, strong secondary circulation occurred for a depth differential of 25% or more, as long as the velocity in the shallower tributary was at least as great as that in the deeper channel. This provides an important context for interpretation of previous work and for the design of new experiments in both the field and the laboratory. © 1998 John Wiley & Sons, Ltd.