This example is taken from Cornell University’s ANSYS AIM Learning Modules

Problem Specification

Separation and reattachment of turbulent flow occurs as the fluid encounters a backwards facing step and eventually reaches the floor. Consider the diagram below which illustrates the problem.

A developed, turbulent, steady flow enters through an inlet, passes over a backwards facing step and then reaches the ground. In this demonstration, an enclosure will need to be created around the step and air will be used as the flow material. Air will enter via an inlet at the top step at a speed of 0.4 m/s, with a density of 1.23 kg/m^2 and dynamic viscosity of 1.86e-5 Ns/m^2. From this data, the Reynold’s number can be calculated to be about 5000.

Pre-Analysis

The incompressible Navier-Stokes momentum and mass continuity equations are used as the governing equations for this flow. The modified Navier-Stokes and mass continuity equations are as follows, respectively.

Geometry & Mesh

Download the file here

In this video, you will learn how to import the geometry in Discovery AIM and how to generate mesh in the computational fluid domain.

Physics Setup

In this video, you will learn how to edit material properties and specify fluid flow conditions.

Results Evaluation

In this video, you will learn how to evaluate velocity vectors and velocity contour in the flow domain.

Validation

Since so many study cases have been done for this problem, it is easiest to validate our simulation against those cases. In this tutorial, “Backward-Facing Step Flows for Various Expansion Ratios at Low and Moderate Reynolds Numbers” by G. Biswas, M. Breuer, and F. Durst will be our study case. A streamline plot from the study case is shown below. It can be seen that the simulated flow behaved just as we had expected it to in our problem specification and the case study.