## 4.18Recommended boundary conditions

This chapter covers a range of boundary conditions and their implementations. It ﬁrst describes a speciﬁcation of the basic conditions at inlet, outlet and wall boundaries for subsonic ﬂow with ﬁxed value and zero gradient.

The conditions, based on the propagation of disturbances, are described in Sec. 4.3 :

• zero gradient on at an inlet, ﬁxed value on other variables;
• ﬁxed value on at an outlet, zero gradient on other variables.

The conditions at a wall are similar to an inlet for and , but generally are represented more directly by physical models, e.g. the condition for heat ﬂux for .

### Supersonic conditions

The basic conditions for supersonic ﬂow are discussed in Sec. 4.5 . If the ﬂow speed is supersonic at an inlet, the basic condition is ﬁxed value for ; it is zero gradient for if the ﬂow is supersonic at an outlet.

### Robust, practical conditions

Section 4.6 introduced a free boundary that cannot be deﬁned as an inlet or outlet, but instead often uses the following conditions:

• total pressure for , see Sec. 4.7 ;
• inlet-outlet-velocity for , see Sec. 4.15 ;
• inlet-outlet for , see Sec. 4.10.

These conditions also respond well at an outlet, in the event that some inﬂow occurs at startup, a rotating structure passes through the boundary etc., see Sec. 4.10 .

The freestream conditions, Sec. 4.16, are eﬀective for cases with known and at a free, far-ﬁeld boundary.

The symmetry and wedge conditions enable suitable cases to be simpliﬁed as symmetric and axisymmetric, respectively.

In the presence of a body force , the zero gradient condition for at inlets and walls should be replaced by a ﬁxed gradient condition , see Sec. 4.4 .

Notes on CFD: General Principles - 4.18 Recommended boundary conditions