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COMPRESS provides options to account for the action
of vortex shedding. The designer must initiate the
vortex shedding analysis when desired. To do so:
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Select Codes > Wind
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Click the button labeled
"Vortex Shedding / Damping Coefficients"
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Within the Vortex Shedding Calculations / Damping Coefficients dialog
"check" the conditions for
which the vortex shedding analysis should be
performed (operating, empty and/or vacuum).
When the wind vortex shedding option is applied
COMPRESS first determines the critical wind speed
of the vessel. If this is greater than the design
wind speed then no further analysis is performed
because the necessary forcing condition for vortex
shedding would not be encountered. If the critical
wind speed is less than the design wind speed,
COMPRESS uses the method described in the National
Building Code (NBC) of Canada - 1995 to perform an
analysis for the additional loads imposed on the
vessel by the vortex shedding condition.
The vortex shedding analysis defined in the NBC
applies a static load to the top 1/3 height of the
vessel in a direction perpendicular to the wind.
The static load is determined based on the vessel
geometry, dynamic characteristics (natural
frequency) and the critical wind speed.
When the critical wind speed is less than the
design wind speed COMPRESS performs additional
calculations to determine the bending moments in
the vessel due to the combined effect of the
additional lateral force due to vortex shedding
and the basic wind load corresponding to the
critical wind speed.
Additional summary tables are provided to report
the wind shear, bending moment and deflection due
to this load condition. In "Design Mode,"
COMPRESS will increase shell thicknesses, if
necessary, to maintain the calculated stresses at
or below the allowable stress.
Note that the vortex shedding calculations
reported are based on forces resulting from a
wind speed corresponding to the critical wind
speed. For example, if the design wind speed is
90 miles per hour (mph) but the critical wind
speed is only 20 mph, the results (wind
shear, bending moment, deflection) are calculated
based on the 20 mph critical wind speed in
combination with an additional vortex shedding
force applied normal to the wind flow. Thus there
is no "mixing" of design wind speed of
90 mph with vortex shedding forces applied at
20 mph.
If the deflection due to wind (with or without
vortex shedding action) exceeds the maximum
specified in the Wind Code dialog and "Design Mode"
is used, COMPRESS will automatically increase the
shell and skirt thicknesses so as to increase the
moment of inertia to a value so that the deflection
is within the specified limits.
See these related technical papers on the
Codeware website:
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