In this second entry related to Computational Fluid Dynamics (CFD) we will use a simple example to illustrate the working method and the fundamental concepts of this type of simulations. We will use the Acusolve solver together with its pre-processor, Hyperworks CFD.
The case study is a mixing elbow. We have a main cold water inlet, to which a higher temperature flow is added. We want to study the dynamic and thermal behaviour of the flow, for which we will obtain the velocity and temperature fields. The dimensions and flow conditions at the inlets are shown in the picture.
The steps for this example, as is usual in most CFD simulations, are as follows:
Pre-processing with Hyperworks CFD
- Geometry preparation: The geometry for this case can be created directly from the Hyperworks CFD interface or imported from CAD software with simple adaptation within the software.
- Definition of the physics of the problem: We selected a stationary analysis considering heat transfer, with incompressible (liquid) flow and SST turbulence model.
- Boundary conditions: These will be the indicated in the diagram for the inlet and outlet. For the walls, for this first example we will give non-slip and thermally adiabatic conditions.
- Materials: The fluid domain will be water in this example. For the walls, a thermal model of the pipe material could be defined to study its heat exchange. In this example the simulation will be simplified by neglecting this effect.
- Meshing: The Hyperworks CFD interface allows you to define controls for each area to be meshed in order to refine the regions that require it, generally those of greatest interest. In this case, the area where the lower tube meets the elbow could be refined. When defining the meshing, we must take into account:
- A fundamental concept in these simulations is the boundary layer. Due to the complexity of this topic, its definition will be left for a specific tutorial.
- In order to choose the mesh size we must consider the desired accuracy, the phenomena to be modelled and the available computational capacity. For this example, an average element size between 10 and 100 mm may be reasonable for the objectives pursued. The image shows a meshing of the geometry with 20 mm average element size.
Resolution with Acusolve
- Resolution: Acusolve adapts the resolution strategy to the physics defined for the problem, activating and solving in this case the equations of Flow, Turbulence and Temperature. These correspond to the Navier Stokes equations with the chosen turbulence model and the conservation of energy.
Post-processing with Hyperworks CFD
- Results and Reports: During problem solving, the convergence of the model and the evolution of the quantities and flows can be checked. At the end of the simulation, Hyperworks CFD offers many post-processing possibilities to obtain and visualise the desired information. In the images we can see the main results for this case: the temperature (in Kelvin) and velocity (in m/s) fields in a transverse plane.
The steps to follow using Hyperworks CFD and Acusolve are fully detailed in the tutorial available in our Downloads section. In the next post we will see how to define a simulation and visualise results in a transient analysis.