# Laminar Boundary Layer

### Key Takeaway

• The difference between a turbulent and laminar boundary layer.

• What are the characteristics of laminar fluid flow?

• How to analyze boundaries with laminar flow.

The laminar boundary layer compared with the turbulent boundary flow layer

As all matter is composed of atomic particles, the principles of quantum mechanics govern all motion. An important foundation of quantum mechanics is the Heisenberg uncertainty principle, which asserts that the level of accurate data about any physical system is limited. In other words, there is always some unknown aspect of a system’s state which cannot be known and therefore directly controlled at the atomic level.

Fortunately, the impact of this important principle is typically negligible when dealing with objects at the classical physics level. Yet, the concept of controllability often applies when dealing with physical phenomena such as aerodynamic fluid flow. The best system design is based on a thorough understanding of the difference between the naturally occurring desirable laminar boundary layer and the chaotic turbulent boundary layer.

## The Difference Between the Turbulent and Laminar Boundary Layer

In terms of their impact on aircraft flight, turbulent and laminar boundary layers can be thought of as opposites. Laminar flow is preferable, as it contributes to steady and smooth travel, while turbulence causes a bumpy ride and can threaten an aircraft’s ability to maintain its course and altitude due to increased drag, one of the principal aerodynamic forces. This difference is quite obvious from a graphical perspective, as shown below.

Laminar vs. turbulent flow profiles. Image from Bronkhorst.

As illustrated above, there is a significant difference between fluid activity for a turbulent vs. laminar boundary layer. One indication of this variance is the Reynolds number, which is above 3000 when the layer is exhibiting turbulence and typically lower than 2300 for laminar flow. There are other significant characteristics that indicate flow is laminar, as discussed below.

## The Characteristics of Laminar Fluid Flow

The dimensionless Reynolds number is an important indicator of the type of fluid flow that exists in the boundary layer and is one of the notable attributes of laminar boundary layer flow, as listed below.

### Laminar Fluid Flow Boundary Layer Attributes

⇶ Flow in layers

Laminar flow is characterized by distinctive and separate layers which slide by but do

⇶ Parallel streamlines

When modeling surfaces (for CFD analysis, as an example), fluid flow is often described

in terms of streamlines, which are imaginary lines that indicate the direction of flow. For laminar flow, these lines are parallel to each other.

⇶ Momentum convection is low

For horizontal laminar airflow, there is very little momentum convection or vertical

movement of momentum flux, which prevents the mixing of layers.

⇶ High diffusion of momentum

The distribution or diffusion of momentum in the direction of flow is high for laminar fluid

flow. This diffusion may be caused by shear stresses or pressure.

⇶ Low velocities

A low Reynolds number corresponds to lower velocity, which is a characteristic of

laminar flow. Higher Reynolds numbers indicate increased velocity and tendency for

turbulence.

Although laminar fluid flow is optimal for smooth flight, laminar boundary layers are unstable and break down as the airflow moves away from the leading edge of the aircraft surface. Subsequently, the flow transitions to a turbulent regime, which necessitates turbulent and laminar boundary layer analyses for system design optimization.

## Analyzing Boundaries With Laminar and Turbulent Flow

Aerodynamics boundary layer analysis requires the following:

• Accurate modeling of the surface shape
• Incorporation of all pertinent fluid parameters
• Knowledge of flight environmental conditions
• Understanding of the forces that impact flight
• Understanding of laminar and turbulent flow characteristics
• Ability to simulate various flight orientations (such as different angles of attack)

For optimal system design that includes accurate laminar boundary layer evaluation, the above must be included in the solution technique you employ. This is best accomplished by utilizing advanced CFD solver tools that include multiple mesh generation and fast computation capabilities, as included in the Cadence suite of CFD tools.