How Can Boundary Layer Suction Reduce Flow Separation in Pipe Expansion?

In summary, the conversation discusses a scenario involving a flow separation issue in a pipe expansion where the angle of expansion is 30 degrees, resulting in a fully developed flow with a Reynolds number of 5000+. This expansion causes a head loss due to localized flow separation, resulting in eddies and vortices that eventually disperse as heat at the Kolmogorov length scale. The K factor resistant coefficient associated with this configuration is 0.46. The topic of boundary layer suction is also brought up, with the idea that it can be used to reduce drag associated with flow separation in the pipe. It is explained that suction creates a localized low pressure zone near the wall, resulting in a favorable pressure gradient that promotes a fuller
  • #1
Timtam
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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 DPMy 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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  • #2
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.
 

1. What is boundary layer suction and how does it work?

Boundary layer suction is a technique used to control the flow of a fluid over a surface. It involves the removal of a thin layer of fluid from the surface, which reduces the thickness of the boundary layer and increases the velocity of the fluid near the surface. This helps to delay flow separation, which occurs when the flow separates from the surface and creates turbulence and drag.

2. How does boundary layer suction reduce flow separation in pipe expansion?

In pipe expansion, the flow of fluid through a pipe expands rapidly, creating a region of low pressure near the walls. This low pressure can cause the flow to separate from the walls, leading to turbulence and pressure loss. By using boundary layer suction, the thickness of the boundary layer is reduced, which helps to maintain a smooth flow and prevent flow separation.

3. What are the benefits of using boundary layer suction in pipe expansion?

The main benefit of using boundary layer suction in pipe expansion is that it helps to reduce pressure loss and improve the overall efficiency of the system. It also helps to maintain a more uniform flow, which can be important in applications where precise flow control is necessary.

4. Are there any limitations or drawbacks to using boundary layer suction in pipe expansion?

One limitation of using boundary layer suction in pipe expansion is that it requires additional equipment and energy to operate. This can increase the complexity and cost of the system. Additionally, the effectiveness of boundary layer suction may be limited in certain flow conditions or at high flow rates.

5. How can boundary layer suction be implemented in pipe expansion systems?

There are several methods for implementing boundary layer suction in pipe expansion systems. One common approach is to use a suction slot or small holes along the walls of the pipe to remove a thin layer of fluid. Another method is to use a porous material lining the walls of the pipe, which allows for constant suction along the entire length of the pipe. The specific implementation will depend on the specific application and design considerations.

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