Flow over a backward facing step using Converge CFD.

Objective: Simulate the flow over a backward-facing step.
1. Compare Mesh (i.e. surface with edges) for all 3 base mesh sizes.
2. Show velocity and pressure contours
3. Show the plots of velocity, pressure, mass flow rate and total cell count for all the 3 base mesh sizes
4. Explain where and why separation is taking place in detail.

Given:

Once you have the base setup ready, run the case for three different base mesh sizes

• dx = dy = dz =2e-3m
• dx = dy = dz =1.5e-3m
• dx = dy = dz =1.0e-3m 

Theory:

The discovery of boundary layer theory by Ludwig Prandtl in the early twentieth century was the beginning of the extensive research on separated flows. Separated flows are common in several engineering applications such as aircraft wings, turbine, and compressor blades, diffusers, buildings, suddenly expanding pipes, combustors, etc. The characteristics of a separated flow have been studied for decades by experimentalists to understand the physics of the separated shear layers and their instability mechanisms. The instabilities in the free shear layers are the source to distinctly visible large coherent structures

The physics of separated flows, due to their instabilities, are very complex. In an attempt to simplify these flow characteristics, researchers conducted experiments on various geometries, which include rib, fence, bluff body with a splitter plate, suddenly expanding pipes, forward and backward-facing steps, cavities, and bluff bodies with blunt leading edges.

The backward-facing step is considered by most as the ideal canonical separated flow geometry because of its single fixed separation point and the wake dynamics unperturbed by the downstream disturbances.

Case  Setup:

Geometry is created using vertices as

Above vertices use to create basic geometry after that we will have to use sweep edges or loft edges to have a 3D geometry.

The case setup is done similarly shown in lectures. The only changes we need to do is in the base grid accordingly our mesh size requirement and simulation time parameters as we know finner the mesh higher the computational time, so end time choose for appropriate cycles.

3D post-processing Data ParaView:

1. Mesh Comparision

• dx = dy = dz =2e-3m

• dx = dy = dz =1.5e-3m

• dx = dy = dz =1e-3m

Closer Look at mesh size:

2. Velocity and Pressure contours

• dx = dy = dz =2e-3m

• dx = dy = dz =1.5e-3m

• dx = dy = dz =1e-3m

All the above plots are of pressure and velocity contours.

3. velocity, pressure, mass flow rate and total cell count:

• dx = dy = dz =2e-3m

• dx = dy = dz =1.5e-3m

• dx = dy = dz =1e-3

All plots of density, mass flow rate, total cell count, total pressure, static pressure, velocity compared with all mesh sizes. As we can solutions are completely converged.

4. Explain where and why separation is taking place in detail:

The physics of separated flows, due to their instabilities.
The wake characteristics behind a backward-facing step are shown in fig:

The wake of a backward-facing step has unique features mainly in two regions: the free shear layer and the low-velocity re-circulating bubble. The flow behind the backward-facing step (BFS) is complex and involves various instability mechanisms. Some of the most common features behind the step recognized are illustrated in fig:

We can also compare this with our newly obtained simulation results from converge CFD.
the flow wake can be distinguished into three main regions namely, the shear layer region, separation bubble or recirculation zone and the reattachment zone.
This very much resembles each other as we can see below:

As we can see wake created just after a backward step where recirculation bubble form.
Boundary separation begins at the step edge due to an adverse pressure gradient that develops into a thin shear layer. The recirculation zone is mainly comprised of a primary vortex in the center and a secondary vortex adjacent to the corner of the step as shown in Figure. Due to the favorable pressure gradient created by the fluid entrained, the shear layer eventually curves down towards the wall and impinges at a location known as the reattachment point.
The shear layer region is created due to fast-moving fluid or free stream fluid on top and low momentum fluid in the wake of the step. The rolling and pairing of vortices in the shear layer result in the formation of large coherent structures. Coherent structures are significant features observed in flow over a step.

Adverse pressure gradient and free stream velocity is responsible for flow separation at the backward-facing step region

Simulation is shown below:

Mesh size: 2e-3

Mesh size: 1.5e-3

Mesh size: 1e-3