## Laminar Flow

In fluid dynamics,** laminar flow** is characterized by **smooth or regular paths** of fluid particles, in contrast to **turbulent flow**, which is characterized by the **irregular movement** of particles of the fluid. The fluid flows in **parallel layers** (with **minimal lateral mixing**), with no disruption between the layers. Therefore the laminar flow is also referred to as **streamline or viscous flow**.

The term streamline flow is descriptive of the flow because, in laminar flow, layers of water flow over one another at **different speeds** with virtually no mixing between layers. Fluid particles move indefinite and observable paths or streamlines.

When a fluid is flowing through a **closed channel** such as a pipe or between two flat plates, either of two types of flow (laminar flow or turbulent flow) may occur depending on the **velocity**, **viscosity** of the fluid, and the **size of the pipe (or on the Reynolds number)**. Laminar flow tends to occur at lower velocities and high viscosity.

## Turbulent Flow

In fluid dynamics, **turbulent flow** is characterized by the fluid’s **irregular movement** of particles (one can say **chaotic**). In contrast to laminar flow, the fluid **does not flow in parallel layers**, the **lateral mixing is very high**, and there is a disruption between the layers. Turbulence is also characterized by **recirculation, eddies**, and **apparent randomness**. In turbulent flow, the speed of the fluid at a point is continuously undergoing changes in both **magnitude and direction**.

Detailed knowledge of the behavior of turbulent flow regimes is important in engineering because **most industrial flows**, especially those in nuclear engineering, **are turbulent**. Unfortunately, the highly intermittent and irregular character of turbulence **complicates all analyses**. In fact, turbulence is often the “**last unsolved problem in classical mathematical physics**.”

The main tool available for their analysis is **CFD analysis**. CFD is a branch of fluid mechanics that uses numerical analysis and algorithms to solve and analyze problems that involve **turbulent fluid flows**. It is widely accepted that **the Navier–Stokes equations** (or simplified **Reynolds-averaged Navier–Stokes equations**) are capable of exhibiting turbulent solutions, and these equations are the basis for essentially all CFD codes.