Boundary Layer Suction

  • #1
Timtam
42
0
I have a question about the following scenario involving a flow separation issue in a pipe expansion

upload_2016-4-12_11-28-27.png


The angle of the expansion is 30* - doubling the diameter from 1D to 2D

We can consider this flow fully developed with a Reynolds of 5000+

Associated with this expansion is a head loss caused by localised flow separation converting flow to eddies and vortices which I understand to then cascade down to the Kolmogorov length scale where they are dispersed as heat.

If I was to zoom in the localisation might look like this

upload_2016-4-12_11-28-27.png


The K factor resistant coefficient associated with this configuration is 0.46

Current tables
upload_2016-4-12_11-28-27.png




In aerodynamics they use boundary layer control suction (removing Boundary Layer Static Pressure) or re-energizing the flow adding Dynamic Pressure) to reduce drag associated with flow separation




upload_2016-4-12_11-28-27.png



I am similarly interested in using boundary layer suction in my pipe (by reducing pressure at the start of the expansion to reduce the head loss**) but I am unsure I understand the actual mechanism that achieves this

My attempt at an explanation


Suction creates a localized low pressure zone ( ahead of the localized high pressure zone created by the stagnated flow within the boundary). This creates a new gradient with which this high pressure can disperse . As this gradient is in the streamwise direction this allows the energy to rejoin the flow as DP


My Question

Is this correct ? Would my resistance coefficient be lower than the 0.46 the pressure loss the expansion experiences without suction ?

**I am aware that adding suction will cost energy which will be more than the flow recovered: therefore will be energy deficit. I am only concerned with lowering the Resistance coefficient
 

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Answers and Replies

  • #2
boneh3ad
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Suction does create a localized low pressure zone near the wall, but the important effect here is that, while there was no wall- normal pressure gradient before (it's a boundary layer, after all), there is now a favorable pressure gradient, however slight, pointing toward the wall. This tends to draw more high-momentum fluid from the free stream down toward the wall, and promotes having a "fuller" boundary layer than would otherwise occur. The result is a boundary layer that, among other things, is more resistant to separation.

I do believe it makes sense that you would experience less flow resistance if you were to add suction as you have described.
 

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