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):
![9af9f04757ec9ab03e26a47f332d8cfec6ff73b2 blende_01.gif](/fileadmin/smc/files/9af9f04757ec9ab03e26a47f332d8cfec6ff73b2.gif)
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:
![d33393f94a4bf296695698cae49445343160dba0 blende_02](/fileadmin/smc/files/d33393f94a4bf296695698cae49445343160dba0.gif)
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.
![4607d33f51290664c2a4f81bd1ee7ff08aeb2930 0001_blende](/fileadmin/smc/files/4607d33f51290664c2a4f81bd1ee7ff08aeb2930.gif)
Figure B 18: Flow through an orifice
![46c90ba50ef2268e614d61247dd85e807c17abc3 0002_blende](/fileadmin/smc/files/46c90ba50ef2268e614d61247dd85e807c17abc3.gif)
Figure B 19: Adjustable orifice forms