Laminar Flow
Posted: Fri Aug 19, 2011 7:52 pm
Laminar flow is the best choice in all situations other than a riot blast, right? Wrong. Turbulent flow has advantages as well. It our situation, it usually tends to hinder more then help however.
Laminat flow is the flow of a fluid devided into destinct laminae, or layers. These layers slide past each other with minimal friction. The movement between these laminae is nonexistent in true laminar flow. Now, for us, this is ideal. However, it is very difficult to acheive. We can, however, utalize the properties of laminar flow if we do obtain it. In a ball valve, for example, flow is often restricted due to the design. If the valve slopes into the hole, the small hole will experience a loss of pressure, and an increase of velocity. So even though the hole is small, it need not restrict flow at all.
Ben is always telling us to avoid bends in the piping. This, however, is especially important farther from the PC, where the pipe is usually straiter anyway. Don't add unnescesary bends, but having some may not cause turbulent flow.
The reason for this is the Reynold's number or Re. The Re is defined as the ratio of density to viscosity or the fluid, multiplied by velocity and distance. For our water guns, the density is roughly 1, as is the viscosity at room temperature. Thus, in all practical terms, the Re for our water is velocity*length, unless I got my units messed up. The bends in pipe increase velocity, which is why they contribute to the turn to turbulent flow. The Re for 10 cm of pipe carrying water at 100 cm/s then is 1000. For an average CPH, then, the Re is about 2000. This number is low enough to maintain laminar flow in a fairly smooth pipe, such as what we use. Later on in the path, the fittings may be enough to intiate the transition to turbulent flow. However, if a nozzle accelerates the water to 10 m/s, or 1000 cm/s, the transition to turbulent flow is complete at a distance of just 4 cm, due to the fact that the critical Re an less due to drag and wind, etc. This does not mean that your stream starts breakup there, just that the laminae have disapated and the stream is now triying to tear itself apart, with inertia now prevailing over viscosity.
Turbulent flows have higher drag, but also higher energy. Laminar flows contain a thin slow boundry layer, where a turbulent flow has a thick fast boundary layer. This enables turbulent flows to better avoid separation bubbles, which don't really matter for us. However, they have a higher energy, which is useful. Also, the faster moving bounary layer means less of that annoying drip from the stream in some cases, though the edges are often sheared off.
In essence, laminar flows have range, while turbulent flows have better stream speed. A turbulent flow will also induce higher output from a nozzle. This knowledge may prove useful in designing nozzles and guns for the future
Also, for those of you who read my entire rant, thank you. For those of you that did not, that's OK too.
Laminat flow is the flow of a fluid devided into destinct laminae, or layers. These layers slide past each other with minimal friction. The movement between these laminae is nonexistent in true laminar flow. Now, for us, this is ideal. However, it is very difficult to acheive. We can, however, utalize the properties of laminar flow if we do obtain it. In a ball valve, for example, flow is often restricted due to the design. If the valve slopes into the hole, the small hole will experience a loss of pressure, and an increase of velocity. So even though the hole is small, it need not restrict flow at all.
Ben is always telling us to avoid bends in the piping. This, however, is especially important farther from the PC, where the pipe is usually straiter anyway. Don't add unnescesary bends, but having some may not cause turbulent flow.
The reason for this is the Reynold's number or Re. The Re is defined as the ratio of density to viscosity or the fluid, multiplied by velocity and distance. For our water guns, the density is roughly 1, as is the viscosity at room temperature. Thus, in all practical terms, the Re for our water is velocity*length, unless I got my units messed up. The bends in pipe increase velocity, which is why they contribute to the turn to turbulent flow. The Re for 10 cm of pipe carrying water at 100 cm/s then is 1000. For an average CPH, then, the Re is about 2000. This number is low enough to maintain laminar flow in a fairly smooth pipe, such as what we use. Later on in the path, the fittings may be enough to intiate the transition to turbulent flow. However, if a nozzle accelerates the water to 10 m/s, or 1000 cm/s, the transition to turbulent flow is complete at a distance of just 4 cm, due to the fact that the critical Re an less due to drag and wind, etc. This does not mean that your stream starts breakup there, just that the laminae have disapated and the stream is now triying to tear itself apart, with inertia now prevailing over viscosity.
Turbulent flows have higher drag, but also higher energy. Laminar flows contain a thin slow boundry layer, where a turbulent flow has a thick fast boundary layer. This enables turbulent flows to better avoid separation bubbles, which don't really matter for us. However, they have a higher energy, which is useful. Also, the faster moving bounary layer means less of that annoying drip from the stream in some cases, though the edges are often sheared off.
In essence, laminar flows have range, while turbulent flows have better stream speed. A turbulent flow will also induce higher output from a nozzle. This knowledge may prove useful in designing nozzles and guns for the future
Also, for those of you who read my entire rant, thank you. For those of you that did not, that's OK too.