What is Turbulence?

Now anyone who has a background in the field of aeronautics or has ever traveled by air has definitely come across the term 'turbulence'. It is generally associated with the unsteady flight of an aircraft. In the following article, we're gonna clear some commonly existing myths about turbulence and make you understand this chaotic phenomenon.

Types of Flows:

Here we shall discuss the three types of air flows based on streamlines.
• Laminar Flow - All the streamlines move smoothly over one another and also parallel. The flow is smooth.
• Transient Flow - In this flow, the streamlines are time-dependent. Their direction varies over time.
• Turbulent Flow - All the streamlines collide with one another. This results in an unpredictable and unfavorable distribution of streamlines.

What is Turbulence?

With relation to Fluid Dynamics, a flow is said to be turbulent if the streamlines do not flow smoothly and parallel to one another, but rather flow in a random and unpredictable manner and cause a significant change in the pressure.

With relation to Reynolds Number, a flow is said to be turbulent if its Reynolds Number is over 4000.
In simple terms, turbulence is a random disturbance which is caused due to various phenomena that greatly affects the direction of the flow of a fluid (Air or Water) due to which random and unfavorable changes in energy occur.

Diagrammatically it is represented as

Turbulence can be segregated into further subgroups,

• Clear-Air Turbulence It is caused by variations in the jet stream. It ramps up in winter, when the jet stream — zippy air currents in the Earth's atmosphere — migrates south, and often plagues flight paths over the Pacific.
• Convective Turbulence - It is created by thunderstorms and often occurs in the summer.
• Low-Level Turbulence -  It is associated with strong winds, terrain, and buildings.
• Wake Vortex Turbulence -  It is defined as turbulence which is generated by the passage of an aircraft in flight. It generates from the tip of the wings. Because there is a pressure gradient between the two surfaces of the wings, the air from below flows up at the tip of the wings.
• Mountain Wave Turbulence - The flow of air near mountainous regions is highly turbulent because the mountains act as an obstruction to the airflow.

What Happens When a Plane Encounters Turbulence?

To understand that we must first understand how a plane's wing generates lift. They effectively cause air on the upper surface to flow significantly faster than the air on the lower surface. The flow is mostly streamlined and laminar in nature. This causes the pressure below the wing to be higher than the upper surface, thus generating a lifting force. Now when the air flow turns turbulent, the pressure over and under the wing changes in a rather sudden and abrupt manner causing unfavorable lift distribution over the wing and hence resulting in the jerky motion of the plane. Plane crash due to turbulence is very rare. A pilot can effectively receive a prior warning before entering a turbulent air zone. Here are a few ways as to how,

• Eyesight - The most obvious way is just by looking outside and observing the sky. Large billowing clouds, called cumulus clouds, indicated pockets of unstable air (the clouds are rising because the air under them is as well). If the pilots must fly through these clouds then it's a safe bet that there will be some turbulence.
• Weather Radar - Just like using your eyes, except the radar can see further through the haze and other clouds. Typically this is useful for finding embedded thunderstorms, but it can also be useful to find areas of potential turbulence.
• Communication - Pilots talk. Both to each other and to air traffic controllers. En route controllers frequently ask pilots for PIREPS (pilot reports), to build an accurate picture of the flight conditions at different altitudes. Often commercial aircraft will request to change altitudes or deviate around weather/ turbulence, so it is in the best interest of the controller to know ahead of time where the bad flight conditions are and have a game plan of how to route traffic. This makes it much easier for the controller to route traffic, rather than getting request after request from individual aircraft.

A plane can also encounter turbulence when it transitions from subsonic speeds to supersonic speeds, but we shall discuss that later in our upcoming posts on Supersonic Flows.

So, the next that you're traveling by air and you hear the captain announce "Hello ladies and gentlemen, strap on your seatbelts. We're gonna encounter some turbulence." Be calm and relaxed, there is absolutely nothing to fear.
Planes are engineered to take a remarkable amount of punishment, and they have to meet stress limits for both positive and negative G-loads. The level of turbulence required to dislodge an engine or bend a wing spar is something even the most frequent flier won't experience in a lifetime of traveling.