# I have a fluid dynamics question, what to do?

## Background

Flows of gasses or liquids can be studied in 3 different ways: analytically, experimentally or numerically. An analytical approach is only possible for the easiest problems, such as a laminar flow in a straight pipe. Experiments are often too expensive and time-demanding. Consequently, a numerical approach is a popular choice. With modern software and advanced processing clusters, almost any type of flow can be simulated. This research area is referred to as CFD: Computational Fluid Dynamics.

Performing a correct CFD analysis is not straightforward, and much expertise and knowledge about the subject is necessary before a good analysis can be made. We usually divide a problem into 3 components:

- The customer’s question
- The geometry
- The boundary conditions

**The customer’s question**

A well-posed question is of prime importance, as we adapt our approach to problem

Therefore, it is important that the customer knows what he wants to know. Is a single pressure drop value needed? Or is extended knowledge about the flowfield necessary? Is an “order of magnitude” of the pressure drop sufficient, or are at least 4 decimal places necessary?

In the process of formulating the question, we also try to understand the flow itself. Does the temperature play a role? Is the flow composed of a single fluid, or a mixture between multiple phases? What about the presence of membranes, filters or porous media? Membrane filters are often modelled as porous media with a replacing resistance coefficient, which is often determined experimentally. The (computational) costs increase with the complexity of a flow. Therefore, we prefer to simplify a flow just to heart of the problem.

A clear question enables us to perform a correct analysis. The results we deliver are adapted to the demands of the customer.

**Geometry**

A good mesh quality is crucial for a correct CFD analysis. In a mesh, the flow domain is divided in a large number of tiny volumes. The conservation laws (mass, momentum, energy) are applied to these control volumes. By doing so, the flow field is calculated.

In the figure, a pipe tee is shown. The flow goes through the pipe, so we make a mesh of the flow domain. In the figure we show a typical mesh, in which we always locally refine the mesh close to the walls. Here the velocity gradient is large, and a fine mesh is needed for a correct simulation.

To increase the mesh quality, we often simplify the geometry. A too detailed geometry results in meshing issues, whereas many components hardly affect the flowfield. Typical examples of “troublemakers” are:

- Nuts, bolts, threads, etc
- Thin walls
- Sharp edges
- Thin flow passages

We always process the geometry, in close consultation with the customer, so that a good mesh is made without losing important geometry features.

Often, a customer seeks an ideal design for his problem. In those cases, the geometry is not known a priori, but part of the customer’s question. In the design of the optimal geometry, we of course take the manufacturability into account.

**Boundary conditions**

After a mesh is created, we pre-process the CFD simulation and we have to set our boundary conditions. This starts with defining the global properties of our flow domain. The fluid must be known, with the density and viscosity being the most crucial parameters. For more complex flows, such as multiphase flows these parameters must be known for all flow components. A multiphase flow is a mixture between multiple immiscible components, such as a liquid-gas mixture, a fluid with solid particles, or immiscible liquids, such as an oil-water mixture. For problems involving heat transfer, also the thermal material properties must be known. The same holds for compressible fluids, such as high-speed gasses.

Hereafter we set the boundary conditions of the domain.

- Inlet
- Something must be known about the amount of fluid entering the domain. This can be a (mass) flow rate, but also a pressure or a velocity.
- If applicable, a temperature must be given.

- Outlet
- Just as with the inlet, something must be known about the flow leaving the domain. Often this is the (ambient) pressure, but a (mass) flow rate is also possible.

- Wall
- The “no-slip condition” is the condition that the fluid directly at the wall has the same velocity as the wall itself. Usually the walls are non-moving, just as the fluid close to the wall. Walls with a speed also occur, for instance in turbomachinery. Sometimes wall roughness plays a role, affecting the flow field and the friction.
- If heat transfer plays a role, it must be clear if the walls have some temperature or whether the walls are adiabatic (no heat transfer).

**The customer’s question**

We now return to the original question of the costumer. With a well-posed question, a clear geometry and evident boundary conditions, it is possible to perform a concise CFD analysis. With a well-posed question, it is also possible to give a clear answer. CFD specialists are always able to help you to define a problem. With this article, we hope to have been able to assist you in defining your fluid problem.