Wayne Hale’s Blog: Tripping the Boundary Layer – Part 1

As I start this series, it occurs to me that “tripping the boundary layer” could be an article on social change – maybe I’ll do that.

But for today it is an engineering subject. So buckle your seatbelt and hold your hat, we are off on an adventure in rocket science!

Aviation has been driven by the desire to fly higher and faster. Great strides have been made, especially up to the middle 1960’s. But for the last few decades aircraft have been at a plateau in terms of speed and altitude. With the exception of rocket powered X planes, the boundary of high performance jets has been just faster than Mach 3 and up to about 100,000 ft. Even though there is the perennial dream of hypersonic transports carrying passengers across the globe in a fraction of today’s aircraft, we don’t seem to be advancing on that dream.

Part of the problem is we don’t understand how to avoid tripping the boundary layer. There is precious little data at hypersonic speeds, and computer simulations are no good without data and the formulae derived from data to predict these things: garbage in; garbage out.

So, to start this discussion off, let us define the terms. (What the dickens are we talking about?)! What’s a boundary layer and what does it mean to trip one?

In aviation, the boundary layer is a thin film of air closest to the wing, body, or engine of an aircraft. At the molecular level, the air immediately adjacent to the airplane is dragged along with the plane. Infinitesimally farther away, the air is being carried along at some fraction of the speed of the airplane, and at a longer way away from the airplane, the air is not moving at all, or at least not being dragged by the airplane. That distant air is called the “free stream” and the close by air – which is affected by the passage of the aircraft – is called the boundary layer. Typically aerospace engineers consider the boundary layer to be that close in part of the air that is being dragged along by the passing of the aircraft at a speed of 5% or more of the airplane. These boundary layers are thin, inches or fractions of an inch. They are important because the boundary layer causes most of the drag and most of the heating when an airplane is in flight.

Boundary layers, like all fluid flows, is either laminar or turbulent. Laminar flow is smooth, turbulent flow is, . . . well, . . . turbulent. You can see a good youtube video of this here:

http://uk.youtube.com/watch?v=NplrDarMDF8

And there is a really good wikipedia article on turbulence here: http://en.wikipedia.org/wiki/Turbulence

So why is all of this important? Exactly at this time there is a large effort by many companies and government agencies to develop hypersonic aircraft. NASA has even sponsored a couple of test flights. The problem, as it is for all types of aircraft flight, is drag and heating. When the boundary layer over the wings or in the engine is laminar, there is low drag and low heating; and when the boundary layer is turbulent, drag and heating increase dramatically. All boundary layers can be “tripped” or transition from laminar to turbulent flow.

In some of these experimental aircraft the engines [called SCRAM jets for Supersonic Combustion Ram jet engines] have only operated for a fraction of a second or a very few seconds. Why? Because the designers do not know how to cool them; they don’t understand when or whether the boundary layer inside the engine is turbulent or laminar.

In some of these experimental aircraft, the engine begins to melt as soon as it is turned on; hence the extremely short operating times.

This is no good for a hypersonic passenger aircraft which might carry a hundred people from New York to Tokyo in a couple of hours.

Why do we not understand this phenomenon? Because it cannot be recreated in a wind tunnel or other experimental apparatus. The wind tunnels that have long enough flow durations to study this phenomenon run only up to about Mach 6. These hypersonic engines need to perform at Mach 8 or 10 or 12. There are “wind tunnels” that operate at high Mach numbers but only for fractions of a second; not long enough to understand the way in which a boundary layer works.

No aircraft fly that fast, missiles can achieve it briefly, but there is one platform that spends a serious amount of time flying through the atmosphere at speeds above Mach 6:

Its the space shuttle.

Tomorrow I’ll talk about an experiment that will be on the next shuttle flight. An experiment which will study tripping the boundary layer.

With this knowledge, the designers just might be able to make a major advancement toward hypersonic passenger aircraft.

To hold your attention until my next post, here is a true story:

Around 1900 a young graduate student in physics was trying to do research on a problem that could earn him a doctorate degree. He started out studying the transition from laminar to turbulent flow in fluids. After months of work and study, he concluded that this problem was too hard. He would concentrate on an easier subject: atomic physics. His name was Niels Bohr and he won the Nobel prize for physics in 1922 for his work in quantum mechanics. And he was right; turbulence is harder. And we don’t understand it yet.