Circulating, vortex or rotational flows are important in nature and industry flows. These flows are called vortexes in nature, seen in air and sea storms. In industry, they are also called Rotating flow, such as the flow around a spinning rocket (or the flow inside a rotating tube), or rotational flows, such as the fluid flow inside turbomachines (pumps, compressors, turbines, blowers) are known. In this section, we introduce these flows in nature and industry.
Vortex is the circular rotation of a part of the fluid around a fixed axis. This rotation causes the components around the vortex to be sucked toward its center. The rotational speed of the fluid has an inverse relationship with its distance from the center of rotation, and it is higher near the center of the vortex than around the vortex. As we move away from the center, this speed decreases. The flow of water poured into the sink, or the big storms that happen in the oceans and seas, are examples of vortices, and in all these phenomena, we see that the flow speed in the center is higher than on the sides.
Vortices are formed under certain conditions and have different characteristics than flow areas.
– The fluid pressure in the center of the vortex is lower than the other points, as it was said that the speed in the center is maximum. As we move away from the center, the pressure increases, and the speed decreases. The relationship between increasing and decreasing velocity and pressure can be described using Bernoulli’s equation.
– Because there is a low-pressure area in the center of the vortex, if our fluid is water, it may evaporate due to the low pressure of the water in the center, and the shape of the vortex in the center will change.
– The center of each vortex can be identified using the vortex line. Each particle of fluid in the vortex is circulating this line. The vortex line can be continuous from the beginning to the end of the fluid or form a closed loop. For example, when the plane moves on the ground, one side of the vortex is connected to the engine, and the other is connected to the ground. The created vortices can throw small objects into the aircraft engine and cause damage.
The strength of a vortex is measured using a number called “Circulation.” If two or more vortices with different rotations rotate in the same direction, then they tend to merge and form a giant vortex. In this case, the power of the formed vortex is equal to the sum of the powers of two or more vortices that make up it. For example, small vortices are formed at the trailing edge of an airfoil or an airplane blade. These vortices are formed due to the pressure difference between the upper and lower areas of the airfoil.
If we look at an airplane from the back, there is a clockwise vortex on the left wing and a counter-clockwise vortex on the right wing. This vortex is shown in the image below. Note that these two vortices will not merge due to the opposite direction of rotation.
Rotating and Swirling flows
So far, we have introduced the eddy flows in nature. In industry, there are also important flows, such as eddy flows, called swirling or rotating flows. Rotational flows in the external flow are called flows passing through axisymmetric objects that rotate around their axis (such as the flow on rockets). The same definition applies to internal flows, such as the flow inside a rotating tube. One of the most famous rotating flows is the flow in turbomachines such as turbines, compressors, pumps, fans, and any axially symmetric object that rotates around its axis.
As mentioned above, the rotating flow is specific to axially symmetric objects and geometries. These geometries can be tubes, cones, and shapes like these. Rotating flows have a rotational movement around their axis, although they also have an axial flow in some cases. Swirling flows can be placed under a set of rotating flows. However, due to the symmetry of the geometry of the objects that flow outside or inside them, the simulation of these types of flows can be simplified using relations 2D-Axisymmetric modeling. This causes the computational space to change from three to two dimensions, reducing the computational cost.
Rotating flows generally occur inside turbomachines. Also, the airflow around the helicopter propeller, airplane propeller, wind turbines, and other similar things are considered rotational flows. These flows cannot be simulated with two-dimensional assumptions and in two-dimensional space.
The important issue in periodic currents is whether it is periodic or non-periodic. Axial turbomachines (fans, compressors, and turbines) have periodic flow, and radial turbomachines have non-periodic flow. Solving only one part of the entire computational domain, whose flow is repeated in other parts, is possible in periodic flows. For example, in a turbine where the flow between all blades is similar and repeats, it is possible to model the flow around only one blade and generalize the answers obtained to other blades.
Simulation of rotating flows using CFD
Considering the application and importance of rotational flows in analyzing turbomachines and other geometries where the flow outside or inside them is rotational. Axisymmetric solvers have been developed to analyze these flows in CFD software such as Fluent. These solvers are provided to make simulating these types of flows easy and inexpensive. Below we can see the picture related to the setting of this type of solver in Fluent.
We can choose one of the Axisymmetric or Axisymmetric Swirl solvers according to the problem we want to simulate. In another section, we will introduce these solvers and their required settings.