Validation Case: Non-Newtonian Flow Through Expansion Channel

The non-Newtonian flow through an expansion channel validation case belongs to fluid dynamics. This test case aims to validate the following parameter:

• Non-Newtonian fluid with the power law model

The simulation results of SimScale were compared to the analytical results presented in [Tanner, 1992 apud Manica]\(^1\).

Geometry

The geometry consists of a pseudo-2D channel, with a 3:1 expansion ratio, as seen in Figure 1:

The dimensions of the channel are given in Table 1:

	A	B	C	D	E	F	G	H
x \([m]\)	0	0	5	5	30	30	5	5
y \([m]\)	-0.5	0.5	0.5	1.5	1.5	-1.5	-1.5	-0.5
z \([m]\)	0.25	0.25	0.25	0.25	0.25	0.25	0.25	0.25

The corresponding nodes marked with an apostrophe (‘) are translated 0.5 m along the negative z-direction.

Analysis Type and Mesh

Tool Type: OPENFOAM®

Analysis Type: Steady-state incompressible flow

Turbulence Model: Laminar

Mesh and Element Types: Both meshes used in this validation case are hexahedral meshes created locally and imported to SimScale. Cases A through D were run with an intermediate fineness mesh (30700 cells) and with a fine mesh (77600 cells).

In Table 2, an outline of the cases is presented, including information regarding the mesh and the power-law model parameters.

Case	Mesh Type	Cells	Element Type	\(K\ [m^2/s]\)	\(N\)	\(\nu_{min}\ [m^2/s]\)	\(\nu_{max}\ [m^2/s]\)
(A)	blockMesh	30700 – 77600	3D hexahedral	0.008838835	0.5	0.000001	0.5
(B)	blockMesh	30700 – 77600	3D hexahedral	0.0125	1	0.000001	0.5
(C)	blockMesh	30700 – 77600	3D hexahedral	0.01767767	1.5	0.000001	0.5
(D)	blockMesh	30700 – 77600	3D hexahedral	0.025	2	0.000001	0.5

Find below the 30700 cells intermediate fineness hexahedral mesh. It contains a single cell in the z-direction.

Simulation Setup

Material:

• Viscosity model: Power law non-Newtonian model, with coefficients as described in Table 2;
• \((\rho)\) Density: 1 \(kg/m^3\).

Boundary Conditions:

Before defining the boundary conditions, the current nomenclature will be used for the rest of this documentation:

In the table below, the configuration for both velocity and pressure are given at each of the boundaries:

	Boundary type	Velocity \([m/s]\)	Pressure \([Pa]\)
Inlet	Velocity inlet	Fixed value: 0.5 in the x-direction	Zero gradient
Outlet	Pressure outlet	Zero gradient	Fixed value: 0
Sides	Empty 2D	Empty 2D	Empty 2D
Top and bottom	Wall	Fixed value: 0	Zero gradient

Reference Solution

A dimensionless analytical solution for the developed velocity profiles within a channel is presented by Tanner [Tanner, 1992 apud Manica]\(^1\).

Result Comparison

The numerical simulation results for the various power indexes \(N\), ranging from 0.5 to 2, are compared with the analytical solution. For all cases, the Reynolds number was kept at 40.

The generalized Reynolds number formulation for non-Newtonian flows is as follows (SI units apply for all parameters):

$$Re=\frac{\rho\ V^{2-N} H^{N} }{K} \tag{1}$$

Where:

• \(\rho\) is the density;
• \(V\) is the flow inlet velocity;
• \(H\) is the inlet height;
• \(N\) is the power index from the power law formulation;
• \(K\) is the consistency factor from the power law formulation. The consistency factor was adjusted for each case, to maintain \(Re\) = 40.

Cases A through D were run with both intermediate and fine meshes. The presented results are those from the intermediate mesh, as no further improvement was observed with the finer discretization.

The figure below shows the normalized velocity profiles in the fully developed region downstream of the expansion, at \(x\) = 28 \(m\). The \(y\) coordinates, represented in the horizontal axis of Figure 4, are normalized from 0 (center of the channel) to 1 (top of the channel).

Note that, in the figure above, ‘n’ represents the same power index \(N\) used within the workbench.

SimScale’s results show very good agreement with the analytical solution for all configurations.

In Figure 5, we can see how the velocity profile is developing along the expansion channel for case A (\(N\) = 0.5) with the intermediate fineness mesh.

Last updated: August 13th, 2020