Choice of Pressure Boundary Condition in FloWorks
At SOLIDWORKS World 2008, Seth and I had the privilege of getting into a discussion with Bill Dziedzic, FloWorks Technical Specialist at SOLIDWORKS Corp. and Arvind Krishnan, Product manager of FloWorks, about the correct usage of the pressure boundary condition in COSMOS FloWorks. It is to be kept in mind that FloWorks gives a user three choices when assigning a pressure boundary condition – static pressure, total pressure or environmental pressure.
The formal definitions of Static and Total Pressure are as follows:
Static pressure is the pressure indicated by a measuring device moving with the flow or by a device that introduces no velocity change to the flow. The usual method for measuring static pressure in a flow along a wall is to drill a small hole normal to the surface of the wall and connect the opening to a manometer or pressure gage. In the region of the flow away from the wall, static pressure can be measured by introducing a probe, which in effect creates a wall.
Total pressure is the pressure measured by bringing the flow to rest isentropically (without loss). A device for measuring total pressure is the Pitot tube, an open-ended tube facing directly into the flow, where the kinetic energy of the fluid is converted into potential energy.
Bernoulli’s equation states that Total Pressure = Static Pressure + Dynamic Pressure.
CHOOSING THE RIGHT BOUNDARY CONDITION
The decision to select Static or Total Pressure should be based off real testing procedures (whether the pressure at the outlet is measured by a manometer, or a pitot tube arrangement). In the event that one is unsure, it is always a good idea to pick the Environmental Pressure condition. This ensures that any potential reversals in fluid flow (like vortices across a pressure opening) are accounted for.
NOTE 1: If, during the calculation, a vortex crosses an opening with the Environment pressure condition specified, the pressure will be considered as the total pressure at the part of opening through which the flow enters the model, and as the static pressure at the part of the opening through which the flow leaves the model.
NOTE 2: It is important to remember that FloWorks uses Absolute Pressure values. Hence, 2psig should be specified as 16.69 psi in absolute terms (2 + 14.69psi atmospheric).