In hydrocyclone separation, fluctuating solids contents and size distributions in the feed cause instationarities. Under these conditions, the stationary concentration equilibrium in the apparatus and the separation are affected. Control mechanisms are demanded for stabilization of the separation effect. Designing the control system, knowledge on hydrocyclone dynamics and the transitional behaviour is necessary. To simulate hydrocyclone dynamics, the transport balance considering slip velocity and turbulent particle diffusivity was used. The transitional behaviour of a 40 mm hydrocyclone at special operating conditions has been computed numerically. The following scenario was simulated: a water hydrocyclone at t = 0 is suddenly fed with a 200 g/l-suspension. The transitional period until the concentration equilibrium of all fractions is reached, takes about 2.5 s. Partition curves simulated inside the transitional period indicate a maximum. The major objective of hydrocyclone modelling is estimation of the separation in form of the partition curve. The physics of separation is highly complex because of the turbulent multiphase flow in the apparatus. Basis of modelling is the description of cyclone flow with the Navier-Stokes equations for a suspension and the calculation of the separation with the transport equation. For modelling simplifications are necessary without neglecting the essential physics. Experience show that the separation result tolerates more simplification of the flow model as of the particle transport model. (c) 2006 Elsevier Ltd. All rights reserved.