# 2.10. MMS06: Pressure outflow BC, single-phase, 3D, curl-based functions¶

## 2.10.1. Description¶

The pressure outflow boundary condition in MFIX is verified using the techniques described in [5] where the manufactured solution is selected such that it satisfies both the divergence-free constraint and the pressure outflow condition. Specifically, this boundary condition requires that the pressure and all velocity components at the outflow have zero gradients normal to the wall at the boundary. For verification of the pressure outflow condition, the manufactured solution for the velocity field is given by Eq.2.48 while that for pressure is given by Eq.2.49 [6].

(2.48)$\overrightarrow{V} = {\overrightarrow{V}}_{0} + S^{3}\left( \overrightarrow{\nabla} \times \overrightarrow{H} \right) + 3S^{2}\left( \nabla S \times \overrightarrow{H} \right)$
(2.49)$P = P_{0} + S^{2}P_{1}$

Here, $$\overrightarrow{V}$$ is the velocity field vector, $$S$$ is the mathematical equation of the boundary tested (here, $$S \equiv y = 1$$), $$P_{0}$$ is a non-zero scalar constant, $$P_{1}$$ represents the sinusoidal terms of the general manufactured solution, and $$\overrightarrow{H}$$ is a general vector field consisting of sinusoidal expressions.

## 2.10.2. Setup¶

This case is setup for single-phase flows on a domain with unit dimensions; the boundary tested is the North boundary (i.e., $$y = 1$$).

Table 2.18 MMS-06 Setup, Initial and Boundary Conditions.

Computational/Physical model

Single-phase (no solids)

No gravity

Turbulence equations are not solved (Laminar)

Non-uniform mesh

Central scheme

Geometry

Coordinate system

Cartesian

Domain length, $$L$$ (x)

1.0

(m)

Domain height, $$H$$ (y)

1.0

(m)

Domain width, $$W$$ (z)

1.0

(m)

Material

Fluid density, $$\rho_{g}$$

1.0

(kg·m-3)

Fluid viscosity, $$\mu_{g}$$

1.0

(Pa·s)

Initial Conditions

Pressure (gauge), $$P_{g}$$

0.0

(Pa)

Fluid x-velocity, $$u_{g}$$

5.0

(m·s-1)

Fluid y-velocity, $$v_{g}$$

5.0

(m·s-1)

Fluid z-velocity, $$w_{g}$$

5.0

(m·s-1)

Boundary Conditions

North boundary

Pressure outflow

All other boundaries

Mass inflow (MMS)

Material properties selected to ensure comparable contribution from convection and diffusion terms.

The manufactured solution is imposed on all boundaries (i.e., Dirichlet specification).

## 2.10.3. Results¶

Numerical solutions were obtained using the Central discretization scheme for 8x8, 16x16, 32x32, 64x64, and 128x128 grid meshes. The observed order of accuracy matches the formal order as shown in Figure 2‑15 for both the velocity variables and the pressure. Unlike the test cases verifying the no-slip and free-slip boundary conditions, no convergence issues were encountered since the problem has a physically-realistic outflow boundary.

Fig. 2.18 Observed orders of accuracy for pressure outflow verification using $$\mathbf{L}_{\mathbf{2}}$$ and $$\mathbf{L}_{\mathbf{\infty}}$$ norms of the discretization error.