Turbulence and Boundary Layers

The amount of turbulence in a flow dramatically affects sediment transport. In this video, I review the Reynolds number and the characteristics of the flow near the boundary layer. For a beautiful example of a video showing the change in Reynolds number with depth in the flow, watch: http://www.youtube.com/watch?v=cUTkqZeiMow

Please note that this is a very simplified description of the boundary layer in a smooth channel with a low turbulence flow. Real rivers have irregular sides and rough beds that disrupt the boundary layer, complicating the picture substantially.

You can also see my lecture notes on this topic in the second half of this blog posting: http://dawnssedstrat.blogspot.com/2011/01/lecture-2-walthers-law-and-fluid-fl...

Category:

Education

Tags:

• sedimentology
• sediment
• stratigraphy
• geology
• science

[champboy1990]

i thought the boundary between laminar flow and transitional flow gave a Re of around 2000? that's according to the moody diagram..

[sumnerd]

@champboy1990 Re is defined slightly differently as the geometry of flow changes. I am using one for flow in an infinitely wide channel. A Re eqn for flow in a pipe (the Moody chart) uses diameter rather than depth, and pipes have more friction per volume of flow. The Re eqn for pipes defines the transition from laminar to transitional at a different Re rather than including the geometry differences in the equation. The key universal point is that the flow is laminar close to solid surfaces.

[kiben7]

if the reynolds number is <1, there's a reversible flow, but what characteristiques has that kind of flow?

[sumnerd]

If Re < 1, the flow is laminar and reversible. There's a great video of reversible flow in corn syrup, posted by catwebster. Paste this text in after youtube... watch?v=p08_KlTKP50

[limewaveZ]

More questions!!! Is the laminar sublayer the same as the boundary layer (sorry my questions are more related to aerodynamics) If so then how is it possible to have turbulent boundary layers (at higher Re numbers). Thanks

[sumnerd]

The boundary layer is the zone where the boundary influences the flow speed and includes, but is larger than the laminar sublayer. It consists of the zone from the base of the flow to the depth where the average flow speed is reached in my drawing. The boundary layer is transitional to turbulent for most of it's thickness. The laminar sublayer becomes very thin to non-existent as Re gets very high (especially in air which has low viscosity).

[emreturkoz]

thank you so much for such a compact and all-in-one video.

[kghrib]

thank you

i think the origin of reynolds number is very mysterious

[giogio147]

mamma mia come cazzo stai messa male.....

[MRburitz]

I just got a 47/100 on my sed rocks test. I hope this thing can help me

[limewaveZ]

Makes sense! Thanks for clarifying.