Porous Screen in a Non-uniform Stream

Let us now validate the Flow Simulation capability to calculate fluid flows through porous media.

Here, following Ref. 14, we consider a plane cold air flow between two parallel plates, through a porous screen installed between them, see Figure 1. At the channel inlet the air stream velocity profile is step-shaped (specified). The porous screen (gauze) levels this profile to a more uniform profile. This effect depends on the screen drag, see Ref. 14.

Figure 1. Leveling effect of a porous screen (gauze) on a non-uniform stream: (1) air stream, (2) porous screen.

The geometry model used for calculating the 2D (in XY-plane) flow is shown in Figure 2. The channel has height of 0.15 m, the inlet (upstream of the porous screen) part of the 0.3 m length, the porous screen of the 0.01 m thickness, and the outlet (downstream of the porous screen) part of the 0.35 m length. All the walls have thickness of 0.01 m.

Figure 2. The geometry model used for calculating the 2D flow between two parallel plates and through the porous screen with Flow Simulation.

Following Ref. 14, we consider porous screens (gauzes) of different drag, ζ:

z = 0.95, 1.2, 2.8, and 4.1, defined as:



where ΔP is the pressure difference between the screen sides, q = ρV2/2 is the dynamic pressure (head) of the incoming stream.

Since in Flow Simulation a porous medium’s resistance to flow is characterized by parameter k = -gradP/ρV, then for the porous screens k = V∙ζ/(2L), where V is the fluid velocity, L is the porous screen thickness. In Flow Simulation, this form of a porous medium’s resistance to flow is specified as k = (A∙V+B)/ρ, so A = ρ∙ζ/(2L), B = 0 for the porous screens under consideration. Therefore, taking L = 0.01 m and ρ = 1.2 kg/m3 into account, we specify A = 57, 72, 168, and 246 kg/m4 for the porous screens under consideration. In accordance with the screens’ nature, their permeability is specified as isotropic.

According to the experiments presented in Ref. 14, the step-shaped velocity profiles V(Y) presented in Figure 3 have been specified at the model inlet. The static pressure of 1 atm has been specified at the model outlet.

Figure 3. Inlet velocity profiles.

The air flow dynamic pressure profiles at the 0.3 m distance downstream from the porous screens, both predicted by Flow Simulation at result resolution level 5 and measured in the Ref. 14 experiments, are presented in Figure 4 for the ζ = 0 case (i.e., without screen) and Figure 5-Figure 8 for the porous screens of different ζ.

It is seen that the Flow Simulation predictions agree well, both qualitatively and quantitatively, with the experimental data both in absence of a screen and for all the porous screens (gauzes) under consideration, demonstrating the leveling effect of the gauze screens on the step-shaped incoming streams. The prediction error in the dynamic pressure maximum does not exceed 30%.

Figure 4. The dynamic pressure profiles at ζ = 0, predicted by Flow Simulation and compared to the experiments.

Figure 5. The dynamic pressure profiles at ζ = 0.95, predicted by Flow Simulation and compared to the experiments.

Figure 6. The dynamic pressure profiles at ζ = 1.2, predicted by Flow Simulation and compared to the experiments.

Figure 7. The dynamic pressure profiles at ζ = 2.8, predicted by Flow Simulation and compared to the experiments.

Figure 8. The dynamic pressure profiles at ζ = 4.1, predicted by Flow Simulation and compared to the experiments.