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Turbulent Flow

Fundamental CFD Course Ep. 1 on Turbulence and flow Modeling 1: 1.1 Introduction, 1.2 what's a turbulent flow?


What pop ups to your mind when some one says the term “turbulent flow”? Well engineers working in fluid simulation are very much familiar with the term.

Even so, many of these engineers only care about setting the turbulent flow boundary conditions required by the simulation software they're using and do not even have a good physical understanding about what the turbulent flow actually is.

Well absolutely, anyone who does not work or familiar with flow simulation most likely does not even recognize the term itself at all.
Turbulence flow thinking
Turbulent flows are all around us, and that they affect our lifestyle . The airflow around a moving vehicle, whether it's an automobile, train, or airplane, is turbulent. Even when people walk, the airflow around them is turbulent. The airflow from an air conditioning is turbulent as is that the be due the rotating blades of an electrical fan.

Additionally, water running from a faucet is turbulent. we will say we are surrounded by turbulent flows.

This series of articles will discuss the positive and negative aspects of turbulent flows and their mechanisms. they're going to also discuss why flow calculations are so difficult..
Turbulence flow thinking

What is a turbulent flow?

As mentioned prior to this paragraph, turbulent flows are all around us. It can be identified everywhere. But, what can be the defining quality of a turbulent flow? The term “turbulent flow” literally means a flow during a turbulent state. Since it isn't helpful really to define a term by an equivalent term, what does turbulence  really mean? Since most flows are invisible, this makes it super difficult to identify a true turbulent flow. However, some turbulent flows are actually visible.

Imagine standing by a river. If there are leaves floating along on the river, they can be identified to see the flow behavior of water. The water within the river flows from upstream to downstream. However, when looking closely at the leaves that floats along on the river, there'll likely be atleast one leaf that's rotating in circular path as a part of the flow that's swirling  from upstream to downstream. In other words, there are separate movements besides the most flow, and movements like that are a turbulence flow.

Next Imagine the smoke plume rising from a candle in a room where there is no airflow. Smoke rises from the candlelight thanks to buoyancy. The smoke rises upward as a straight line, but its pattern changes at a particular moment when the flow sets off turbulent. If the movement of the plume at this transition point is closely observed, some eddies (regions where the plume recirculates upon itself) are going to be observed starting where the flow becomes turbulent. Just like the river, movements that are breaking away the most flow occur within the plume. The movements are called turbulent. These movements occur because the momentum of a flow (inertia force) has become larger than the force acting against the flow (viscosity).

Takao Itami

Software Cradle's Consulting Engineer.
The author had engaged in researches on numbercial aspects of turbulence flow in University. He graduates with a Doctoral degree from Tokyo Institute of Technology (Graduate School of Science and Engineering) through researching on Large-Eddy Compressible Turbulent Flow Simulation.