Orifice 

Orifice calculates size, flowrate or pressure drops for gas and liquid orifice meters based on ANSI/API-2530-1991, Part 3 (AGA-3) or the International Standard ISO-5167-1:1991. Customary or SI units.
orifice orifice size pressure drop engineering design oil & gas metering

ViziFlow 

A program to allow the very fast modeling of fluid flow, and the visualization of the streamlines and pressure distribution. Rectangular and circular sections can be entered as well as aerofoils. A flowrate and pressure gauge is provided.
fluid flow streamlines visualizing fluid flow measurement pressure aerofoil aerodynamics fluid flow modeling

Uconeer 

Freeware. 397 units in 46 categories. Includes engineering specific categories like enthalpy, heat capacity, heat transfer coefficient, torque and viscosity. Work with feet and inches. Formatting and clipboard features. Excludes archaic units.
unit conversion engineering units convert freeware measurement si system metric