While we typically think about boundary layers as a small region near the surface of an object–be it airplane, golf ball, or engine wall–boundary layers can be enormous, like the planetary boundary layer, the part of the atmosphere directly affected by the earth’s surface. Shown above is a flow visualization of the boundary layer in an urban area; note the models of buildings. In these atmospheric boundary layers, buildings, trees, and even mountains act like a random rough surface over which the air moves. This roughness drives the fluid to turbulent motion, clear here from the unsteadiness and intermittency of the boundary layer as well as the large variation in scale between the largest and smallest eddies and whorls. In the atmosphere, the difference in scale between the largest and smallest eddies can vary more than five orders of magnitude.
Tag: boundary layer
Flow Over Swept Wings
Flow over a swept wing behaves very differently than a straight fixed wing or an airfoil. Instead of flowing straight along the chord of the wing in a two-dimensional fashion, air is also directed along the wing, parallel to the leading edge. The above oil flow visualization on a swept wing airplane model shows this curvature of streamlines. As a result of this three-dimensional flow behavior, boundary layers on swept wings are subject to the crossflow instability, which manifests as co-rotating vortices aligned to within a few degrees of the streamlines. Triggering this boundary layer instability can lead to turbulence and higher drag for the aircraft.
Transition to Turbulence
Smoke introduced into the boundary layer of a cone rotating in a stream highlights the transition from laminar to turbulent flow. On the left side of the picture, the boundary layer is uniform and steady, i.e. laminar, until environmental disturbances cause the formation of spiral vortices. These vortices remain stable until further growing disturbances cause them to develop a lacy structure, which soon breaks down into fully turbulent flow. Understanding the underlying physics of these disturbances and their growth is part of the field of stability and transition in fluid mechanics. (Photo credit: R. Kobayashi, Y. Kohama, and M. Kurosawa; taken from Van Dyke’s An Album of Fluid Motion)
Vortex Street Sim
This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.
Airfoil Boundary Layer
This video shows the turbulent boundary layer on a NACA 0010 airfoil at high angle of attack (15 degrees). Notice how substantial the variations are in the boundary layer over time. At one instant the boundary layer is thick and smoke-filled and in another we see freestream fluid (non-smoke) reaching nearly to the surface. This variability, known as intermittency, is characteristic of turbulent flows, and is part of what makes them difficult to model.
Meandering Mississippi
This satellite photo of the Mississippi River south of Memphis, TN shows how the river’s course has changed over time. When a river bends, the water near the inner bank flows faster than the water by the outer bank. This difference in speeds actually creates a vortical secondary flow in the boundary layer of the river that erodes sediment from the outer bank and deposits it on the inner bank. This increases the meander of the river bend. If this continues long enough, the river bend can get pinched off into an oxbow lake, like the ones scattered to either side of the current river path.
Aerodynamics with Bill Nye and Samuel L. Jackson
Bill Nye, Samuel Jackson, golf balls, Reynolds number, dimples, and boundary layers. It doesn’t get much better than this. – Khristopher O (submitter)
It definitely beats Jackson’s other foray into aerodynamics! The dimples on a golf ball cause turbulent boundary layers, which actually decrease drag on the ball and make it fly farther. Why bluff bodies experience a reduction in drag as speed (and thus Reynolds number) increases was a matter of great confusion for fluid mechanicians early in the twentieth century, but it’s not too hard to see why it happens with some flow visualization.
On the top sphere, the laminar boundary layer separates from the sphere just past its shoulder. This results in a pressure loss on the backside of the sphere and, thus, an increase in drag. On the bottom sphere, a trip-wire placed just before the shoulder causes a turbulent boundary layer, which separates from the sphere farther along the backside. This late separation results in a thinner wake and a smaller pressure loss behind the sphere, thereby reducing the overall drag when compared to the laminar case. (Photo credit: An Album of Fluid Motion)
Hotwire Anemometry
Hotwire anemometry is used in experimental fluid dynamics to measure velocities with high temporal resolution. The boundary layer crosswire probe shown here was used for turbulence research. Between the prongs, which are about the thickness of a sewing needle, are tiny wires about 3 microns in diameter. A human hair is about 80 microns in diameter. Hotwires actually measure voltage; when part of an electrical circuit, the hotwire’s temperature rises above ambient. As air flows over the wire, it cools, which causes the wire’s resistance to drop. By tracking this change in resistance, it is possible to determine the speed of the air moving over the wire.
Godspeed, Discovery!
The space shuttle, despite three decades of service, remains a triumph of engineering. Although it is nominally a space vehicle, fluid dynamics are vital throughout its operation. From the combustion in the engine to the overexpansion of the exhaust gases; from the turbulent plume of the shuttle’s wake to the life support and waste management systems on orbit, fluid mechanics cannot be escaped. Countless simulations and experiments have helped determine the forces, temperatures, and flight profiles for the vehicle during ascent and re-entry. Experiments have flown as payloads and hundreds of astronauts have “performed experiments in fluid mechanics” in microgravity. Since STS-114, flow transition experiments have even been mounted on the orbiter wing. The effort and love put into making these machines fly is staggering, but all things end. Godspeed to Discovery and her crew on this, her final mission!
Smokestack Plumes
On a cold and windy day, the plume from a smokestack sometimes sinks downstream of the stack instead of immediately rising (Figure 1). This isn’t an effect of temperature–after all, the exhaust should be warm compared to the ambient, which would make it rise. It’s actually caused by vorticity.
Figure 2: Simple geometry (side view) In Figure 2, we see a simplified geometry. The wind is blowing from right to left, and its velocity varies with height due to the atmospheric boundary layer. Mathematically, vorticity is the curl of the velocity vector, and because we have a velocity gradient, there is positive (counterclockwise) vorticity generated.
Figure 3: Vortex lines (top view) According to Helmholtz, we can imagine this vorticity as a bunch of infinite vortex lines convecting toward the smokestack, shown in Figure 3. Those vortex lines pile up against the windward side of the smokestack–Helmholtz says that vortex lines can’t end in a fluid–and get stretched out in the wake of the stack. If we could stand upstream of the smokestack and look at the caught vortex line, we would see a downward velocity immediately behind the smokestack and an upward velocity to either side of the stack. It’s this downward velocity that pulls the smokestack’s plume downward.
Figure 4: Vortex wrapped around stack Now Helmholtz’s theories actually apply to inviscid flows and the real world has viscosity in it–slight though its effects might be–and that’s why this effect will fade. The vortex lines can’t sit against the smokestack forever; viscosity dissipates them.
