Local flow resistance with step cross-section constriction in which the ratio between the length l and the diameter d is relatively small (Figure B 18):
The smallest flow cross-section is outside the geometric cross-section constriction. As in these types of flow resistances the viscous fluid friction is very low, the flow rate is dependent on the pressure difference and not the viscosity.
The approximate flow rate through an orifice can be determined with the aid of Bernoulli's equation:
The simplest orifice is washer with hole of diameter d drilled into it (fixed orifice). However, orifices with a changing cross-section are used in the majority of cases. The most common forms are the needle throttle (Figure B 19 a) (risk of clogging with small opening), the gap throttle (b) (not for small oil flows, resolution poor) and the longitudinal notch (c) (good for small oil flows, good resolution).
Another form of local flow resistance is the nozzle. The orifice is the mating piece for the throttle. In practical usage often a differentiation is not made between an orifice and a nozzle, as the hydraulic resistances in practice often have mixed forms.
Figure B 18: Flow through an orifice
Figure B 19: Adjustable orifice forms