Showing papers on "Secondary circulation published in 1999"
TL;DR: In this paper, the laminar flow structure inside an unbaffled stirred tank generated by a 6-blade radial flow impeller was characterized using flow visualization experiments, particle imaging velocimetry (PIV) experiments, and computational fluid dynamics simulations.
161 citations
TL;DR: In this article, the authors quantified mean and turbulent flow structures over self-formed sediment stripes in a weakly bimodal sand and gravel mixture and showed that longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic, spanwise variations in bed texture and elevation.
Abstract: Longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic, spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction. This paper quantifies mean and turbulent flow structures over self-formed sediment stripes in a weakly bimodal sand and gravel mixture. Turbulence anisotropy generates two secondary circulation cells across the channel half-width, which produce a cross-stream perturbation in boundary shear stress. The interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline. Finer sediments are entrained from regions of high shear stress, transported laterally by the secondary flow, and deposited in regions of lower shear stress. Lateral changes in bed texture further enhance the near-bed secondary flow, which provides a positive feedback mechanism for stripe growth. In bimodal sediments, at shear stresses just above the entrainment threshold, stripes may replace lower-stage plane beds. At higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms.Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(1999)125:5(463)
65 citations
TL;DR: In this paper, the cross-stream velocity field is decomposed into components associated with primary and secondary velocities, as defined by Rozovskii, and the contribution of secondary circulation and helical motion to the pattern of cross stream flow at confluences is discussed.
Abstract: In this response to Lane et al. (1999) ‘Time averaged flow structure in the central region of a stream confluence: a discussion’, Earth Surface Processes and Landforms, 24, 361–367 we show that our method for decomposing the cross-stream velocity field into components associated with primary and secondary velocities, as defined by Rozovskii, is not flawed or misleading. Instead, it yields valuable information that illustrates the contribution of secondary circulation and, by inference, helical motion to the pattern of cross-stream flow at confluences. Lane et al.'s concern about our methodology can be attributed to their failure to distinguish clearly and consistently between secondary circulation and cross-stream discharge, a distinction that is central to our method of analysis, and to their inappropriate comparisons of velocity fields for the different frames of reference associated with secondary circulation and cross-stream discharge. Copyright © 1999 John Wiley & Sons, Ltd.
63 citations
TL;DR: In this article, the authors argue that the Rozovskii method that Rhoads and Kenworthy use to rotate field data to allow identification of secondary circulation cells is flawed, and can result in misleading conclusions about the nature of flow processes in confluences.
Abstract: This paper is a discussion of Rhoads and Kenworthy (1998) ‘Time-averaged flow structure in the central region of a stream confluence’ Earth Surface Processes and Landforms, 23, 171–191, that focuses upon the methods used to identify secondary circulation in river channel confluences. It argues that the Rozovskii method that Rhoads and Kenworthy use to rotate their field data to allow identification of secondary circulation cells is flawed, and can result in misleading conclusions about the nature of flow processes in confluences. It recommends that there is a re-emphasis upon helical as opposed to secondary circulation, and that recent developments in both field monitoring and numerical modelling may help significantly in this respect. Copyright © 1999 John Wiley & Sons, Ltd.
27 citations
01 Sep 1999
TL;DR: In this article, the role of double-diffusive mixing in the North Atlantic thermocline is considered, and a model of the dissipation rate is derived and used to specify the turbulent mixing rate and constrain the diapycnal advection.
Abstract: : The role of double-diffusive mixing in the North Atlantic thermocline is considered. Mixing observations are analyzed in terms of the stability parameters for shear and double-diffusive convection, and a nondimensional ratio of dissipation rates. While the model for turbulence describes most dissipation occurring in high shear, dissipation in low shear is better described by the salt-finger model, and a method for estimating the salt-finger enhancement of the diapycnal haline diffusivity over the thermal diffusivity is proposed. These calculations suggest that salt-finger driven haline flux drives diapycnal downwelling in the upper thermocline. The role of turbulence occurring above rough bathymetry in the abyssal Brazil Basin is also considered. The mixing levels along sloping bathymetry exceed the levels observed on ridge crests and canyon floors. Additionally, mixing levels modulated in phase with the spring-neap tidal cycle. A model of the dissipation rate is derived and used to specify the turbulent mixing rate and constrain the diapycnal advection in an inverse model for the steady circulation. The inverse model solution reveals the presence of a secondary circulation with zonal character. These results suggest that mixing in abyssal canyons plays an important role in the mass budget of Antarctic Bottom Water.
9 citations
01 Jan 1999
TL;DR: In this article, the cross-stream velocity field is decomposed into components associated with primary and secondary velocities, as defined by Rozovskii, and the authors show that their method for decomposing the cross stream velocity field into components is not flawed.
Abstract: In this response to Lane et al. (1999) ‘Time averaged flow structure in the central region of a stream confluence: a discussion’, Earth Surface Processes and Landforms, 24, 361‐367 we show that our method for decomposing the crossstream velocity field into components associated with primary and secondary velocities, as defined by Rozovskii, is not flawedormisleading.Instead,ityieldsvaluableinformationthatillustratesthecontributionofsecondarycirculationand,by inference,helicalmotiontothepatternofcross-streamflowatconfluences.Laneetal.’sconcernaboutourmethodologycan be attributed to their failure to distinguish clearly and consistently between secondary circulation and cross-stream discharge,adistinctionthatiscentraltoourmethodofanalysis,andtotheirinappropriatecomparisonsofvelocityfieldsfor the different frames of reference associated with secondary circulation and cross-stream discharge. Copyright 1999 John Wiley & Sons, Ltd.
7 citations
TL;DR: In this article, a computational code has been developed to analyze the non-steady flow field of the thermal diffusion column in the higher pressure region, where a secondary circulation observed near the bottom of the column moves periodically at the pressure of 0.112 MPa or more.
Abstract: A computational code has been developed to analyze the non-steady flow field of the thermal diffusion column in the higher pressure region. Numerical analyses for H2-HT are performed for the thermal diffusion column with an effective height of 1,500 mm, a hot wire of 0.15 mm and a cold wall of 15 mm in radius. In the condition that temperatures of cold wall and hot wire are 77.35 and 1,277.35 K at the pressure less than 0.112 MPa, the flow fields reach the steady state. However, a secondary circulation observed near the bottom of the column moves periodically at the pressure of 0.112 MPa or more. The frequency of the flow fields becomes larger as the gas pressure increases. The marginal pressure to make flow field apart from the steady state to the periodic state is predicted with the present analyses.
2 citations
01 Oct 1999
TL;DR: In this article, the authors show that the surface air temperature in a wide range of transition zone migration and its surrounding decreases as the transition zone migrates northward, and the net upward fluxes of the surface long wave radiation and the sensible heat decrease, and evaporation to the atmosphere increases.
Abstract: As the position of the transition zone changes obviously, that is, as the transition zone migrates to the north or the south from present position, it affects water or heat balance between the land and the atmosphere in a considerable degree and has a profound influence on climate in North China. The experiment results in this paper indicate whether in the dry case or in the wet case of the large-scale climatological background field, the surface air temperature in a wide range of the transition zone migration and its surrounding decreases as the transition zone migrates northward. Moreover, the net upward fluxes of the surface long wave radiation and the sensible heat decrease, and the evaporation to the atmosphere increases. As the transition zone migrates southward, the results are opposite. This kind of significant thermal forcing between the land and the atmosphere can excite secondary circulation or circulation cells, which interact with the large-scale circulation systems, changing the atmospheric motion, affecting the water vapor transportation and consequently having an effect on the precipitation.