## Tags and keywords

SysPhS-1.1: 'Equations define mathematical relationships between the values of numeric variables. Equations in SysML, are constraints in constraint blocks that use properties of the blocks (parameters) as variables. In this example, constraint blocks PipeConstraint and TankConstraint define parameters and equations for pipes and tanks, respectively, as shown in Figure 61.'

SysPhS-1.1: 'The pipe constraints specify that the pressure pressureDiff across it is equal to the difference of fluid pressures opening1Pressure and opening2Pressure at each end of the pipe. The fluid flow rate through the pipe, fluidFlow, is proportional to the pressure difference by the constant resistance, which depends on the geometric properties of the pipe as well as fluidic properties. The magnitude of fluid flow rate through the pipe fluidFlow is the same as the magnitude of flow rates opening1FluidFlow and opening2FluidFlow going through the pipe’s openings, though the values differ in sign. The sum of the fluid flow rates going through the two pipe openings is zero (the fluid is assumed to be incompressible).'

SysPhS-1.1: 'The tank constraints specify that the pressure in the tank, pressure depends on the height of the fluid level in the tank, fluidHeight, as well as properties of the fluid, fluidDensity. Also, the fluid flow in the tank, fluidFlow, is related to the change in the fluid height level fluidHeight over time and the cross-sectional surface area of the tank, surfaceArea.'

The trail version of the diagram is much like the spec version except:

- The correct ValueTypes are used throughout instead of just `Real`

.

- Each ConstraintBlock definition has been set as owned by the Block that has a usage of it (just as a modelling choice).

The`resistance`

constraint appears to be obtained from the Hagen–Poiseuille equation:
```
{resistance=(8*viscosity*length)/(3.1416*(radius^4))}
{fluidFlow=pressureDiff/resistance}
```

where:

Δp is the pressure difference between the two ends,

L is the length of pipe,

μ is the dynamic viscosity,

Q is the volumetric flow rate,

R is the pipe radius,

A is the cross section of pipe.