The Influence of Inlet Flow Velocity on the Flow of Water in a Straight Plane Channel

Objective:

To model and investigate the influence of inlet flow velocity on the flow of water in a straight plane channel, using Computational Fluid Dynamics.

Apparatus:

ANSYS CFX will be used as currently available at QUT.

Problem:

For a straight channel of height H=20 mm and length L=1 m, as shown below, create a two-dimensional CFD model. You will create an initial mesh and run the simulations for an inlet velocity of 0.1 m/s. Then you will refine the mesh 2 times and re-run the simulations for Uf= 0.1 m/s. Finally, you will run the simulations for inlet velocities of 0.2 m/s and 0.5 m/s using the refined mesh to determine axial velocity profiles at different x locations along the channel.

 

The fluid flowing through the channel is water, with the following properties at 10 °C:

Density, U = 1,000 kg/m3

Dynamic viscosity, P = 1.307 x 10-3 Pa/s

The boundary conditions for all the CFD models are:

Inlet velocity, Uf= 0.1 m/s; 0.2 m/s; 0.5 m/s ::Outlet Pressure = atmospheric pressure :: Velocity at walls = 0 m/s (no slip)

Flow = Laminar (first case, Uf= 0.1 m/s) then Turbulent using default settings (second and third cases 0.2 m/s; 0.5 m/s).

Procedure and discussion points:

1. Create the geometry in DesignModeler

2. Build the grid in Ansys Mesher. First, use the Automatic mesh with no modification of the settings. Discuss the quality of the mesh obtained.

3. Set boundary conditions in Ansys-CFD and solver parameters

4. Run CFX for Uf = 0.1 m/s

5. Make a grid refinement study: Create and discuss 2 refined meshes, one finer than the other and check grid convergence. Run the simulations for Uf = 0.1 m/s for these 2 additional grids.

6. For the most refined mesh, check the iterative convergence and compare the axial velocity profiles u(y) at x=0.1 m, x=0.5 m and x=0.9 m. Discuss.

7. Using the refined grid, run CFX for Uf = 0.2 m/s and Uf = 0.5 m/s

8. Chart the axial velocity profiles u(y) at x=0.1 m, x=0.5 m and x=0.9 m for the 3 inlet velocities against the analytical velocity profile for a laminar plane Poiseuille flow, given below:

− ( ) = 6

9. Calculate the Reynolds number for each inlet velocity and determine the corresponding regime (laminar, transitional, turbulent). Explain the differences observed on the velocity profiles obtained compared to the analytical profile.
10. Discuss any observations about the boundary layer. Discuss the effect grid refinement has on the results.