of different studies extremely difficult. For example, some used relatively 

 crude instruments such as a strain gage to determine the bottom shear stress. 

 In our studies, the bottom stress is derived from detailed flow measurements 

 within the boundary layer. 



Table 6.4 



Composition of Fresh-Water Sediments 

 Used in Previous Entrainment Experiments 









ALKALI 







SEDIMENT TYPE 



CLAY 



QUARTZ 



FELDSPAR 



DOLOMITE 



CALCITE 



Shale based 



67.7% 



21.8% 



4.2% 







Western basin 



37.9% 



30.4% 



14.9% 



12.7% 



4.1% 



of Lake Erie 













Central basin 



33.7% 



43.0% 



17.5% 



6.8% 





of Lake Erie 













Pond 



66.1% 



27.4% 



7.5% 







Despite its many advantages, rotating annulus flume usually contains 

 secondary flow in the radial plane. The nature and strength of secondary flow 

 depend strongly on flume configuration and flow Reynolds number. For flumes 

 with relatively low aspect ratio, such as the one shown in Fig. 6.7(a), the 

 secondary flow usually consists of a single circulation cell. At very high 

 flume aspect ratio, Taylor-Goertler instability may lead to the occurrence of 

 multiple circulation cells. A detailed quantitative study of secondary flow 

 and its contribution to the entrainment and deposition of sediments is lacking 

 and is much needed. Based on tangential flow measurements and boundary layer 

 formulae, we found that the radial flow was generally less than 10% of the 

 tangential flow in our flume with moderate rotational rates (<1 rad/sec). A 

 more detailed analysis can be performed by using a turbulent transport model 

 in conjuction with skin friction and/or Reynolds stress measurement. 



147 



