### 7.8.2. Lift Force

The lift force refers to the shear-induced lift force acting on a dispersed phase in the presence of a rotational continuous phase. Hence, it is applicable to the `Particle Model` only. Additional information on the theory is available.

You may set a non-dimensional lift coefficient either as a constant, or an expression. It should be set to 0.5 for inviscid flow around a sphere. For viscous flow, the coefficient varies from 0.01 to 0.5 in a way that is only partially understood.

Several models for the lift coefficient have been proposed in the literature. See, for example, [86] and [87], and this is still a matter for current research. ANSYS CFX has the following built-in lift models:

- The Saffman Mei Lift Force Model in the CFX-Solver Theory Guide

- The Legendre and Magnaudet Lift Force Model in the CFX-Solver Theory Guide

- The Tomiyama Lift Force Model in the CFX-Solver Theory Guide

The lift force is proportional to the continuous phase density. Hence, it is mainly significant when the dispersed phase density is either less than, or of the same order of magnitude as the continuous phase density. Also, it is proportional to the continuous phase shear rate. Hence, it is most significant in shear layers whose width is comparable to the dispersed phase mean diameter.

For example, the lift force is important for bubbly flow in a vertical pipe, when the pipe diameter is comparable to the bubble diameter. In this case, the lift force induced by the continuous phase boundary layer is responsible for pushing the bubbles towards the wall. On the other hand, for bubbly downflow, the lift force tends to push bubbles towards the pipe center, leading to the phenomenon of void coring.

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