Specifying the wind data in the Unsteady Flow Data Editor’s Meteorological Data tab (Figure 4-21) automatically turns on wind forcing in the computational engine.  However, there are some additional computational options available to the user from the HEC-RAS Unsteady Computation Options and Tolerances window.  This is done by opening the Unsteady Flow Analysis window and select the menu option Options | Calculation Options and Tolerances (Figure 5-12).



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Within the HEC-RAS Unsteady Computation Options and Tolerances window (Figure 5-13), the wind Reference Frame may be specified as Eulerian or Lagrangian.  An Eulerian reference frame only takes into account the speed of the wind when computing the wind drag forces.  The Lagrangian reference frame also takes into account the speed and direction of the water. 


The wind Drag Formulation may be specified using the drop-down menu.  Users can choose from four different equations to compute the drag coefficient (i.e., Hsu (1988), Garratt (1977), Large and Pond (1981), Andreas et al. (2012)), or enter the drag coefficient as a constant value.  Please see Chapter 2 of the HEC-RAS Version 6.0 Hydraulic Reference Manual for the details of the different wind drag equations and their applicability.


Figure 5-13. HEC-RAS Unsteady Computation Options and Tolerances dialog shown the Wind Forcing options.