Cfd Assignment 2r4a4p

Computational Fluid Dynamics (CFD) Assignment A Background and Aim A backward-facing step is commonly used as a benchmark for validation of numerous flow characteristics, including physical models, multiphase flows and fundamental numerical methods. This model also has a broad range of applications in industry, such as HVAC and combustion. The aims of simulating this model are: 1. To learn the process of creating and exporting a mesh for quality CFD modelling. 2. To learn how to set boundary conditions and process a numerical model. 3. To explore the post-processing abilities of the CFD code to analyse numerical results. 4. To practice writing concise and well developed professional reports. Problem Description The student is required to run laminar flows through a backward-facing step as detailed below: (10, 1)

(-5, 1)

Inlet

(0, 0)

Outlet

(-5, 0)

(0, -1)

(10, -1)

Figure 1 Illustration of the 2D geometry of a backward-facing step

The dimensions given for the geometry are dimensionless as they are normalised for the characteristic length. Bear in mind that for the case of backward-facing steps, the characteristic length is the step height (in this case 1 unit high). The fluid enters at the Inlet and exits the channel from the Outlet. The normalised inlet velocity and fluid properties that are also normalised for the characteristic length are provided as: Inlet velocity: Fluid properties:

ux = 1 and uy = 0 density, ρ = 1 dynamic viscosity, μ = 1/Re, where Re is the Reynolds number

The outlet should be defined as an outflow and the other boundaries should be left as no-slip walls. As turbulent flow is not considered, turbulent settings are neglected. There will be no heat transfer within the system. Dimensions may have to be altered to ensure developed flow at the outlet.

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Instructions 1. Model creation: (a) Use ANSYS DesignModeler to create the two dimensional backward-facing step as shown in Figure 1. (b) Use ANSYS Mesh to generate a uniformed mesh over the domain. Check the mesh quality to ensure that a proper mesh quality is achieved. (c) Given Re = 100, use ANSYS Pre to define the model settings. This includes (i) specify the material properties of the fluid; (ii) choose the appropriate fluid flow model; (iii) assign the appropriate boundary conditions for the model; (v) assign the initial values of the model. 2. Use ANSYS Solver to run the simulation and then use ANSYS Post to get the velocity and pressure contours and velocity vectors for the case of Re = 100. Ensure that the developed flow is present at the outlet and at least a close-to-developed flow is present prior to the step. Discuss any observed flow characteristics using the physical parameters. 3. Mesh independence test: increase your current mesh density by increasing its fineness (and use the same flow settings for Re = 100) and determine the sensitivity of the mesh fineness to the reattachment point of the recirculation zone. Discuss this change in reattachment location with the mesh refinement, and choose the appropriate mesh for the next stage of the investigation. Provide the explanations for areas that may require local mesh refinement (note: the local mesh refinement is not required for this assignment). 4. Using the mesh chosen from Step (3), perform simulations for Reynolds numbers of 50, 150 and 200. Compile your results and create a graph highlighting the relationship between Reynolds numbers and reattachment points, using physical parameters to explain the phenomenon. Provide your own conclusion on this observation. 5. Change the ‘Top’ boundary condition from “No-slip” wall to “Specified-shear” (maintaining Shear stresses of X & Y components at ZERO Pascal) for the highest Reynolds number case and discuss the results obtained there in relation to using two different boundary conditions.

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