1/2 



time scale of fluctuations of the dissipation can be defined as Tn=(n/e) 



2 ° 



and a micro-acceleration within the eddy as ^q'^o^^o' ^"^ ^^" ^^^° define a 



micro-shear to characterize the velocity shear within the eddy as 60=^" . 



Magnitudes of these quantities for the range of turbulence dissipation rates 



encountered in a shallow coastal environment are shown in Table 6.1. 



Table 6.1 



Characteristic Quantities for Dissipation Eddies 

 in Coastal Waters 



^ ^0 ^0 ^0/9 % 



,23 1 



(m /sec ) (ym) (sec) (---) (sec" ) 





0.001 



178 



0.03 



0.02 



33.3 



Normal 



0.01 



100 



0.01 



0.1 



100 



Condition 















0.1 



56 



0.003 



0.6 



333 





1 



32 



0.001 



3.26 



1000 



Storm 













Condition 



10 



18 



0.0003 



20.4 



3333 



Since the dissipation rate is preserved through the cascade process, the 



2/3 

 fluctuation time of an eddy of scale ^ is "^ x='^ q{^ /^ q) and the acceleration 



is ax=aQ(^/^Q) . As the eddy size (^) increases, ^x increases while ax and 



Gx decrease. 



Correlation Between Sediment Particles and Turbulent Eddies 



The extent to which sediment particles of a given size and density are 

 influenced by the turbulence depends on the relative magnitude of the particle 

 relaxation time t^, defined as w /g where w^ is the settling speed, versus the 

 fluctuation time of various turbulent eddies, t^. If t^>>t the particle 

 motion is completely decoupled from the turbulence of the flow field and hence 

 there is no influence of turbulence on the particle collision. At the other 

 extreme, if Tp<<Tq, the particles are completely correlated with all the 

 turbulent eddies. The sediment particles experience an ensemble mean shear "& 

 comprising of contributions from all eddies. In between the two extremes. 



129 



