Of course, the most common type of flow that CFD deals with is incompressible fluids. There are three types of these flows:
3 Types of flows
1. Steady-state (Time-Averaged)
A steady-state flow is when the velocity field varies with time, but its average value is defined as overall space.
2. Unsteady-state (Instantaneous)
It means that the instantaneous velocity field varies with time and has infinite values at any point in space.
Transient flow varies in time, either unsteady or both transient and dynamic.
The difference between what we call transient and unsteady is not very significant since many publications will also call an unsteady flow a transient.
3 Types of steady-state, incompressible flows
1. Free Streamline (FS)
It is where neither the fluid nor anybody in it influences each other, except for gravity which acts on every part of the fluid.
A free streamline flow cannot be directly computed because there is no velocity. Still, you can find it using Dimensional Analysis since its equation only contains dimensional variables, which will simplify to zero if appropriately scaled.
2. Source / Sink (SS)
A Source or a Sink is when fluid enters from / leaves to infinity, respectively. Uncharted territory here, so nothing to explain3) Doublet (DS).
A fluid element or volume has one part that moves opposite to another part. They both move at the same speed but in opposite directions.
3. Doublet (D)
A doublet is a point in which a fluid with a velocity V enters the domain at one point and leaves it at another.
A source or a sink can connect by positioning two of these points on either side of each other with the same total velocity across them, forming an hourglass-shaped path for the fluid.
6 Types of unsteady flows
1. Source / Sink (SS)
Same as steady-state, this models fluid entering or leaving the domain.
2. Vortex (V)
It is when a doublet is moving rotationally.
It is commonly used to model blade tip vortices in turbomachinery, creating significant drag and vibration if modelled incorrectly in CFD.
3. Wake (W)
Awake is when all fluid elements behind a body experience an instantaneous change in velocity due to the body’s passing, which could be either an object or even another flow.
The most common case would be modelling the wake behind an aircraft wing where the incoming flow is decelerated abruptly.
Another one is modelling the wake behind an object. For example, a car or train where the flow ahead of it is accelerated to increase the drag on the object.
4. Vortex Street (VS)
When many vortices are situated close together with high velocities, they can create a region called a vortex street which can have drastic effects on aircraft performance, especially during take-off and landing phases.
The issue with this is that these vortices have large rotational velocities, which can cause damage to aircraft surfaces if they collide with them too often.
5. Vortex Dipole (VD)
A dipole is two equal but oppositely rotating doublets. These are common in turbomachinery applications, although the CFD engineer might not as commonly implement them.
Depending on software limitations, the geometry of parts is so closely packed together that it can become computationally costly or even close to impossible to model.
The problem with these models is that both vortices may experience each other’s wake flow which may cause inaccuracies in simulations.
6. Wake Dipole (WD)
The Awake dipole extends the previous one by adding another vortex at an angle offset from the first two.
It would allow more accurate modelling of blade tip vortices since there will be one in-between two blades and another off to the side.
One issue with this is that it may cause computational issues depending on software limitations.
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It is why there is a huge need for accurate information when predicting whether or not to invest in a specific stock.
That is why we introduced you to different computational fluid dynamics (CFD).