Evaluation of surface shear stress 



75. The main source of the surface shear stress is the wind. The shear 

 stress at the water surface produced by the wind is usually computed using the 

 turbulent shear stress formula given as Equation 45, where Cd is the drag 

 coefficient, p^ is the density of air, and U„ is the wind speed. 



T = C P -^ (^5) 



^ s ^D y a 2 



A typical value of the drag coefficient is 2 x 10"'' (Hsu 1988). 

 Evaluation of transition losses 



76. Transition loss is the energy loss accompanying sharp expansions or 

 contractions of the flow area and may be expressed as Equation 46, where S^ is 

 the rate of loss with longitudinal distance y, and K^ is an empirical form- 

 drag coefficient. The partial derivatives of the transition loss term with 

 respect to the independent variables are needed in Equations 33 and 34. These 

 derivatives are computed from Equation 46. 



dy \ 2g) 



gAS=gAK, — i — \ (^^) 



A value of K^ = Q .5 is recommended as an initial estimate, and K^ should never 

 exceed 1 . 



77. In DYNLETl , values of the transition loss term Sg between nodes i 

 and i+1 and its derivatives with respect to the independent variables are 

 given in Equations 47 to 49 , 



S. = K^ 



Oil _ Qi_ 



)i 2 !i2 



-^ (47) 



2gr 



^ = iC -^ (48) 



9^i ^' gA\ 



32 



