Laminar flow

Laminar flow is non-turbulent flow in smooth parallel (non-intersecting) paths in layers that have different velocities. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids.

The term laminar flow describes one of the three types (the other two are transitional and turbulent) of behavior that a boundary layer can exhibit.

When a fluid (such as air) moves (or flows) past a solid surface, such as an airplane wing, a thin layer develops adjacent to the surface where frictional forces tend to retard the motion of the fluid. This layer is defined as the boundary layer. 

In general, the most desirable state is one with a high degree of laminar flow, i.e., exhibits a low degree of friction drag. In particular, a high degree of laminar flow reduces the amount of fuel consumed by an aircraft and also increases its flight range.

Description

To explain the concept of laminar regime in a simple and intuitive way, suppose to inject, by means of a syringe, a colored fluid (inseminating), let’s call it fluid B, inside a flow of another transparent fluid, let’s call it fluid A (note that fluid is a term generally and daily used to indicate a liquid, but in thermodynamics and physics for fluid we indicate a continuous medium not having its own form, characterized by physical properties that identify the thermodynamic state, such as temperature, pressure, density; a fluid can therefore also be a gas or a steam). If the velocity of the fluid is sufficiently low (with respect to its viscosity) it will be noticed that this colored fluid thread, fluid B, does not mix with fluid A, and will remain, so to speak, confined in a “virtual cylinder” that keeps it separated from A.

As the velocity of fluid A increases, it will be seen that fluid B will remain confined in its virtual cylinder only for a short while, after which there will be a progressive breaking up of this virtual cylinder, and it will be noticed that B begins to mix with A flowing around it. This mixing has initially the appearance of small undulations of the walls of the thread of B, which proceeding will become vortices, first small and then larger. If we give to the phenomenon a sufficient time, we will see how downstream we will find ourselves in the condition of not being able to distinguish the fluid B (inseminating) from A (transparent), we will see a single indistinct fluid, perhaps no longer transparent in color.

We can say that in the first case (low speed), we are in the presence of a laminar flow, that is a flow in which all the fluid threads that make up the motion field, always remain parallel to themselves, without ever mixing, like many small “lamellas” or “laminae” all parallel, hence the definition of laminar. Observing a laminar motion it is even difficult to capture the feeling of movement because everything is always equal to itself and there are no transient and unstable phenomena such as vibrations or vortices that commonly give our perception the awareness that a fluid is moving.

In the second case (higher speeds), we can say we are in a turbulent regime, where inertial phenomena (due to speed) such as vortices, win on viscous phenomena (which tend to keep everything parallel), and play an action of mixing of fluid threads between them, breaking the original parallelism (maintained instead in a laminar flow).

In everyday experience we can observe all this (even if it is not a rigorous experiment) simply by watching the smoke rising from a cigarette resting on an ashtray. The smoke is in this case fluid B (inseminating), while the air in the room is fluid A. The heat from the cigarette heats the air and creates an upward convective motion (hot air is lighter and tends to rise, while cold air, heavier, tends to fall). It is easy to notice that at the beginning, the smoke is very concentrated and does not mix with the air (laminar regime), then you notice the first instabilities (transition regime) and finally the smoke mixes rapidly with the air dispersing in it (turbulent regime).

It contrasts with turbulent flows, which are dominated by recirculation, vortices and apparent randomness. The transition from laminar to turbulent flow depends on the value of the Reynolds number: there is a critical value for which below this the motion is laminar, above evolves gradually in turbulent. As a general rule, the fluid is considered laminar if Re < 2000, turbulent if Re > 4000, transition if it falls between these values.

Laminar-turbulent transition

The process of a laminar fluid flow becoming turbulent is known as a laminar-turbulent transition (or transitional flow). The main parameter characterizing transition is the Reynolds number.

Each of these flows behaves in different manners in terms of their frictional energy loss while flowing and have different equations that predict their behavior. Transition is often described as a process proceeding through a series of stages. “Transitional flow” can refer to transition in either direction, that is laminar-turbulent transitional or turbulent-laminar, transitional flow.

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