The good vortex

 

Vortex Generators on a wing.
Vortex Generators on a wing.

Highlights:

  • What is a vortex generator?
  • Why are they used?
  • What is boundary layer separation?
  • How does wake increase the draft?
  • This week in Invisible History: The 3 stroke gas engine. Domain: Automobile.
  • Discuss the article here.

If you’re an automobile enthusiast like me, I’m sure that you’ve wondered what the small little bumps on the 8th generation Mitsubishi Evo are. Am I right? I was a kid when I first noticed this on aeroplane wings. The question of why is it put there, fascinated me. These are called vortex generators. They help in limiting the flow separation from the surface. But how do they work and what exactly is their purpose? Let’s find out!

Wing stall

While I was in the Formula Student team for my university, we used to stick threads on the wings to see the airflow over it. Have you ever heard of wing stalls? It is when the flow over the wing is separated. Let's get into the details.

A wing has an airfoil profile which tries to cut the air as it whips through it. The pressure difference between the top and the bottom side dictates whether the body will have a lift or will have downforce. Air has an adhesive behaviour which makes a layer of air stick to the surface. This layer is called the boundary layer. The thickness of the velocity boundary layer usually is defined as the distance from the solid body to the point at which the air velocity is 99% of the freestream velocity (the surface velocity of the unobstructed flow).

Air flow over a wing
A good diagram of the flow characteristics over a wing, courtesy of http://aerospaceengineeringblog.com


At the end of the wing, the momentum of the airflow sometimes separates itself from the surface. This phenomenon is called boundary layer separation. This separation creates a low-pressure area on the body behind the separation point. This phenomenon is also caused by something called "adverse pressure gradient." In simple words, the air has a low-pressure peak near the front of the wing (where it accelerates to maximum velocity). The pressure slowly increases as you move further back (as it returns more towards freestream velocity). In a car, the low-pressure peak is near the roofline and the high-pressure area more towards the boot.

Why is boundary layer separation bad?

As mentioned before, the area of the body behind the separation point has a lower pressure area. This area is called a wake. For any vehicle which has to travel at high speed, the pressure difference is significant. Having a low-pressure area behind increases the drag on the body. And drag is the enemy of any racecar or aeroplane. This drag force increases further as the car goes faster. It is directly proportional to the square of velocity. Therefore, the boundary layer separation has to be delayed as much as possible. The vortex generators are what make this possible.

Wing without vortex generators
The separation seen in wings, courtesy of https://www.boldmethod.com


Vortex Generator

There is a cool video by Real Engineering on the importance of endplates on the wings of a plane. The endplates are used to separate the high-pressure region (below) and low-pressure region (up). If the endplates are not present, the high-pressure air will try to get to the low-pressure area and form a vortex at the edge of the wing. This is precisely what the vortex generator takes advantage of.

The separation being eliminated
The separation being eliminated by the VG, courtesy of https://www.boldmethod.com


A vortex generator (VG) is a small wing element which is placed at a strategic position to induce vortices in airflow and make the boundary layer stick to the surface. The VG gives energy to the air and makes the air to be forced down to the body. The most significant advantage of this is the less wake area left behind the body. The resultant of this is the reduction in drag force.

Vortex Generator in action

VG in old jet planes
VG in old jet planes, courtesy of https://drivetribe.com


F1 cars are covered in vortex generators, perhaps the most obvious ones are those mounted to the sidepods, courtesy of https://drivetribe.com
VGs on cars to decrease the drag force, courtesy of https://drivetribe.com

Your turn now

Vortex generators are one of the elements which sound fancy and look cool but are very simple and work effectively. But people tend to go for the shiny stuff and add vortex generators on their coupes to generate more downforce or decrease drag. But that is not how it works. The location of the vortex generators are very strategic and needs rigorous testing before it is mounted on the car.

So next time if you see some bumps or small elements on the body of a car or plane, think about the testing and the simulations that the design engineer must have done to keep the boundary layer stuck to the body.

This week in invisible history

The clue in last week’s article was “The universal meridian.”

On 13th October 1884, Greenwich was adopted as the universal meridian. Greenwich lies on the River Thames, a few miles from central London. At the direction of the U.S. President, 41 representatives from 25 countries met in Washington, DC, for the International Meridian Conference. Several important principles were established at the conference. They were: a single world meridian passing through the principal Transit Instrument at the Observatory at Greenwich; that all longitude would be calculated both east and west from this meridien up to 180°; a universal day; and studies of the decimal system to the division of time and space. Resolution 2, fixing the Meridian at Greenwich was passed 22-1 (San Domingo voted against, France & Brazil abstained).

Laser projected from the Royal Observatory in Greenwich, marking the Prime meridian.

This time Rider8797 from Discord answered the clue correctly.

The clue for this week is: The 3 stroke gas engine. The domain is Automobile.

Inspired? Do write to us with your answers to get featured in the next article. Or send in your suggestions to make your weekend read absolutely amazing!

See you next weekend. Have a good one!

Cheers!

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Reference links

  1. Micro Vortex Generators - Micro AeroDynamics Inc
  2. Land Shorter! Benefits".

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