KAHIBARO
Table of Contents
Smooth Flow in Layers
Laminar flow is a type of fluid motion in which the fluid moves smoothly in parallel layers. The layers slide past one another without large mixing or chaotic swirls. This is the simplest kind of real fluid flow to analyze, and it is especially important when fluid speed is low, passages are narrow, or the fluid is quite viscous.
In laminar flow, the motion at one point is orderly and predictable. If a tiny bit of dye is injected into the fluid, it tends to form a smooth streak rather than breaking up into irregular patterns. This visual behavior gives a strong clue that neighboring layers are moving in an organized way.
What “Layers” Means
Imagine a fluid moving through a horizontal pipe. The fluid near the center usually moves faster than the fluid near the wall. The fluid right at the wall is effectively at rest because of the no slip condition, which means the fluid sticks to the surface. Between the wall and the center, the speed changes gradually from zero to its maximum value.
So laminar flow does not mean every part of the fluid has the same speed. It means the flow pattern is smooth, with one layer moving past the next in an orderly manner.
Velocity Profile
A very important feature of laminar flow is the velocity profile, which tells how speed changes across the flow region. In a cylindrical pipe, steady laminar flow has a parabolic profile. The speed is largest at the center and decreases smoothly toward the walls.
For flow in a pipe of radius $R$, the speed at distance $r$ from the center is
$$
v(r) = v_{\max}\left(1 - \frac{r^2}{R^2}\right)
$$
where $v_{\max}$ is the maximum speed at the center.
The average speed is smaller than the maximum speed. For this case,
$$
v_{\text{avg}} = \frac{v_{\max}}{2}
$$
This is a special result for fully developed laminar flow in a circular pipe.
For steady laminar flow in a circular pipe,
$$
v(r) = v_{\max}\left(1 - \frac{r^2}{R^2}\right), \qquad
v_{\text{avg}} = \frac{v_{\max}}{2}
$$
The fluid speed is zero at the wall and maximum at the center.
Why Laminar Flow Happens
Laminar flow appears when viscous effects are strong enough to suppress irregular motion. Viscosity resists relative motion between neighboring layers of fluid. When this resistance dominates over the tendency of the flow to become unstable, the fluid remains smooth and layered.
This is common for honey flowing slowly, oil moving through narrow tubes, or blood moving through small vessels under normal conditions.
Everyday Examples
Laminar flow is often seen in situations where the speed is not too high and the geometry is simple.
| Situation | Why flow is laminar |
|---|---|
| Oil in a thin tube | High viscosity, narrow passage |
| Water moving slowly through a syringe needle | Small diameter, moderate speed |
| Air over a very smooth surface at low speed | Disturbances remain small |
| Blood in tiny capillaries | Very small vessel size |
A stream of smoke rising gently from an incense stick often begins as laminar flow near the source, then farther up it becomes irregular as disturbances grow.
Energy Loss and Resistance
Even though laminar flow is smooth, it still experiences resistance. The fluid loses mechanical energy because of internal friction between layers and interaction with the boundaries. To maintain steady motion, a pressure difference is usually needed.
In a pipe, the pressure must be higher at one end than the other to keep the fluid moving. The larger the viscosity, the greater the pressure difference required for the same flow rate.
This behavior leads directly to Poiseuille’s law, which is treated separately, but laminar flow is the condition under which that law applies.
Laminar flow is smooth, but it is not frictionless.
A pressure difference is needed to maintain steady laminar flow in a real viscous fluid.
Comparison with Turbulent Flow
Laminar flow is the opposite of turbulent flow in an important practical sense. In turbulent flow, the motion is irregular and involves eddies and strong mixing. In laminar flow, fluid particles follow smooth paths and mixing across the flow is much smaller.
| Feature | Laminar flow | Turbulent flow |
|---|---|---|
| Motion | Smooth and orderly | Chaotic and irregular |
| Mixing | Weak | Strong |
| Velocity changes | Predictable | Fluctuating |
| Energy loss | Smaller | Usually larger |
This chapter focuses only on the laminar case. The conditions for the onset of turbulence are discussed under Reynolds number and turbulent flow.
Streamlines in Laminar Motion
A helpful way to picture laminar flow is with streamlines. In steady laminar flow, streamlines are smooth curves that do not cross. Fluid particles move along these paths. If the flow is through a straight pipe, the streamlines are nearly parallel to the pipe axis.
A Simple Force Picture
You can think of the fluid as made of many thin sheets. If one sheet moves faster than the next, viscosity creates a shear force that resists their relative motion. This is why adjacent layers do not freely slide without resistance.
For two nearby layers, the shear stress is proportional to how rapidly the velocity changes from one layer to the next,
$$
\tau = \eta \frac{dv}{dy}
$$
where $\tau$ is shear stress, $\eta$ is viscosity, and $\frac{dv}{dy}$ is the velocity gradient perpendicular to the flow.
This relation helps explain why laminar flow can remain smooth while still dissipating energy.
In laminar flow, viscous shear is described by
$$
\tau = \eta \frac{dv}{dy}
$$
A larger velocity gradient means a larger internal frictional effect.
Fully Developed Laminar Flow
In pipe flow, the flow may enter with a more complicated speed distribution. After traveling some distance, the shape of the velocity profile becomes fixed. When that happens, the flow is called fully developed laminar flow.
In this regime, the profile no longer changes along the pipe, although the pressure continues to drop in the direction of flow. This is the standard situation used in most simple formulas for viscous flow in pipes.
Key Idea to Remember
Laminar flow is orderly fluid motion in which neighboring layers slide smoothly past one another. It is characterized by smooth streamlines, little mixing, and a well defined velocity profile. In pipes, the center moves fastest, the walls hold the fluid back, and viscosity controls the motion throughout the flow.