Why do boundary layers occur

While these elaborate schemes have not quite found their way into mass production probably due to their cost, maintenance problems and risk , laminar flow wings are a very viable future technology in terms of reducing greenhouse gases as stipulated by environmental legislation.

An important driver in reducing greenhouse gases is maximising the lift-to-drag ratio of the wings, and therefore I would expect research to continue in this field for some time to come. Hello, Thank you very much for this article. I would like to know more about the approximated law given the boundary layer thickness in case of turbulent flow. Hi Kevin, thanks for your message. Only a thought but if viscosity is defined by Sutherlands law I. I guess the question is — is it worth it? Carrying that extra kit — but leading edges of modern aircraft already have heating elements for deicing — but how long would it take to get the viscosity down.

Sure, it would definitely be a matter of a cost-benefit analysis. Intuitively, my first thoughts are that the faster you are going the harder it will be to get heat into the boundary layer to reduce viscosity and skin friction. By analogy I would presume that conduction, which is also based on molecules boundcing into each other, would be low. This looks involved- it will take me a while to absorb it all. I was thinking of just making a simple spreadsheet with the derivation of viscosity from Sutherlands law over a range of temperatures probably air.

Plug the resultant viscosities into the derivation of gamma. Rainer — do you or have you worked in Switzerland- I only ask because I used to work with someone called Rainer over there. Hi David, yes, absolutely. That would be the simplest and quickest way to get a feel for how the viscosity changes the Mach number for a given pressure ratio and fun of course. But I guess the question remains could you actually get sufficient heat flux into the air with something like a de-icing device to cause an appreciable change in temperature?

I guess the fluid right at the surface of the wing will take the temperature of the heating device, but then it might drop off quite dramatically moving away from the surface. Ha, no I have never worked in Switzerland. Heating elements across the body? Given a certain wing skin temperature, how much can you expect to heat up the surrounding fluid? I just have a quick question do you calculate a drag coefficent based on combined boundary layer flow, so laminar going to turbulent flow?

I am confused on how you can calculate a drag coefficient based for this. How do you calculate a drag coefficent based on combined boundary layer flow, so laminar going to turbulent flow? Hi Robert, thanks for your comment. Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed. Laminar and turbulent boundary layers One simple example that nicely demonstrates the physics of boundary layers is the problem of flow over a flat plate. Laminar and turbulent flow in smoke. Like this: Like Loading Related Posts. December 12, at pm.

Rainer says:. December 13, at am. Aubrey Jaffer says:. January 21, at pm. David Richardson says:. November 5, at pm. November 6, at am. November 6, at pm. David says:. Robert says:. May 12, at am. May 14, at am. Leave a Reply Cancel reply Your email address will not be published. Sign-up to the monthly Aerospaced newsletter. Recent Posts Podcast Ep. By continuing to use this website, you agree to their use.

To find out more, including how to control cookies, see here: Cookie Policy. Want to learn more about aerospace engineering? Hi, no maximum lift does not necessarily correspond to the separation point being at the leading edge. As the AoA is increased the lift coefficient increases too; initially, in a linear relationship. But the increase in AoA also leads to some separation close to the trailing edge, which moves closer to the leading edge as the AoA increases.

Therefore, the closer we get to stall the more the relationship between lift coefficient and AoA becomes non-linear. An incremental change of AoA does not lead to the same degree of incremental lift coefficient anymore slope of the Cl — AoA curve decreases.

This means the maximum lift coefficient typically occurs before separation reaches the leading edge. But you can find many other versions online. For a wing tilted upward against the airflow, the convergence of air flowing into the region must result in an increase in pressure, as well as velocity shears, and the subsequent turbulent layer thereby generated will slow the flow and therefore increase the air density and subsequent pressure. Control of the underside Prandtl layer assuming it exists is, I suspect, critical to a least the flight of gliding birds e.

Such a model also explains why aircraft can fly upside-down, if the angle of attack is correct. Am I incorrect? Hi Wayne, thanks for your comment. You are absolutely right, the bottom surface of an airfoil plays an important role in creating lift.

The topic of this article was not particularly about the creation of lift per se, but more about the idea of pressure drag and how it is created. Of course lift and pressure drag are both aerodynamic phenomena, and thus, closely related. But the main focus here is pressure drag, and to illustrate its source, talking about the top surface is sufficient.

Thanks — I looked at the link — it is quite good. These 2 articles will be good for my first year engineering students. This basically says that in an inviscid fluid, no non-rotating body can generate a force, so nothing can fly. This, perhaps parodoxically, tells us the you must have viscosity in order to fly.

Aircraft essentially float on a cushion of controlled turbulence Prandtl layer. As you say, there is a lot of misunderstanding out there, and your articles do a good job of demystifying the process.

So I have a doubt. If the AoA increases boundary layer separation occurs earlier So, does the pressure of the wake increase or decrease? I have this confusion. When the AoA Angle of attack is increased for a wing, due to the formation of boundary layer separation, so there will be formation of wakes eddies , so the static pressure will increase because the fluid will lose energy and come to rest and this will reduce lift.

So my question is if static pressure in the right side of the wing increases then drag should reduce right? Because the pressure difference at the front and back will decrease? Hi, maybe the example of flow around a cylinder will help. Imagine the flow stays entirely attached to the cylinder all around its circumference, such that no wake occurs at all.

Now the pressure at the front and back of the cylinder is balanced, and in this theoretical scenario, we only have drag from skin friction. However, if the boundary layer does separate from the cylinder, then the pressure on the downstream side in the wake is essentially constant and equal to the lower pressure on the top and bottom of the cylinder, or essentially at the point where the flow separates. This pressure is lower than the large pressure at the stagnation point at the front of the cylinder.

This pressure imbalance between front and back then leads to pressure drag. Your email address will not be published. This site uses Akismet to reduce spam. Learn how your comment data is processed. Leading edge slats and trailing edge flaps on an aircraft wing. Like this: Like Loading Related Posts. November 11, at am. Rainer says:. November 11, at pm. April 8, at am. Jeongsoo says:. December 25, at am. Aaron says:. April 30, at pm.

Wayne Hocking says:. April 11, at am. April 14, at pm. Srni says:. July 25, at am. July 28, at am. Leave a Reply Cancel reply Your email address will not be published. The displacement thickness depends on the Reynolds number which is the ratio of inertial resistant to change or motion forces to viscous heavy and gluey forces and is given by the equation : Reynolds number Re equals velocity V times density r times a characteristic length l divided by the viscosity coefficient mu.

Boundary layers may be either laminar layered , or turbulent disordered depending on the value of the Reynolds number. For lower Reynolds numbers, the boundary layer is laminar and the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of the figure.

For higher Reynolds numbers, the boundary layer is turbulent and the streamwise velocity is characterized by unsteady changing with time swirling flows inside the boundary layer. The external flow reacts to the edge of the boundary layer just as it would to the physical surface of an object.

So the boundary layer gives any object an "effective" shape which is usually slightly different from the physical shape. To make things more confusing, the boundary layer may lift off or "separate" from the body and create an effective shape much different from the physical shape.

This happens because the flow in the boundary has very low energy relative to the free stream and is more easily driven by changes in pressure. Flow separation is the reason for wing stall at high angle of attack. The effects of the boundary layer on lift are contained in the lift coefficient and the effects on drag are contained in the drag coefficient.

The general fluids equations had been known for many years, but solutions to the equations did not properly describe observed flow effects like wing stalls.