Examples of Turbulent Flow

What is turbulent flow
In fluid dynamics, turbulent flow is characterized by the irregular movement of particles (one can say chaotic) of the fluid. 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 behaviour of turbulent flow regime is of importance 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 said to be the “last unsolved problem in classical mathemetical 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.

See also: Internal Flow

See also: External Flow

Examples of Turbulent Flow

Table of Reynolds Numbers

Example: Turbulent flow in a primary piping

Continuity Equation - Flow Rates through Reactor
Example of flow rates in a reactor. It is an illustrative example, data do not represent any reactor design.

The primary circuit of typical PWRs is divided into 4 independent loops (piping diameter ~ 700mm), each loop comprises a steam generator and one main coolant pump. the primary piping flow velocity is constant and equal to 17 m/s. The Reynolds number inside the primary piping is equal to:

ReD = 17 [m/s] x 0.7 [m] / 0.12×10-6 [m2/s] = 99 000 000.

This fully satisfies the turbulent conditions.

Example: Turbulent flow in a reactor core

Hydraulic Diameter
The hydraulic diameter of fuel rods bundle.

Inside the reactor pressure vessel of PWR, the coolant first flows down outside the reactor core (through the downcomer). From the bottom of the pressure vessel, the flow is reversed up through the core, where the coolant temperature increases as it passes through the fuel rods and the assemblies formed by them. The Reynolds number inside the fuel channel is equal to:

ReDH = 5 [m/s] x 0.02 [m] / 0.12×10-6 [m2/s] = 833 000.

This also fully satisfies the turbulent conditions.

See also: Hydraulic Diameter

Example: Smoke rising from a cigarette

Reynolds number - smokeFor the first few centimeters, the flow is certainly laminar. However, at some point from the leading edge the flow will naturally transition to turbulent flow as its Reynolds number increases. The Reynolds number increases as its flow velocity and characteristic length are both increasing.

Reactor Physics and Thermal Hydraulics:

See above:

Turbulent Flow