Can a flow created by a fan show an Edge effect when hitting a wall?

[T C]

TL;DR

I want to know whether a flow created by a simple fan will show Edge Effect when hitting a flat and vertical surface.

We all know about Edge effect i.e. when an airflow hits the walls of buildings and its velocity will increase. Some market available shroud augmented wind turbine types are claiming that they used this effect to increase the output of their own designed wind turbine. But, all these are about natural wind flows. Now, let's think of a scenario that a simple table/stand fan is set before a straight and plain wall and it's switched on. I want to know that whether the flow created by the fan will show the same Edge Effect in this case just like the natural flows or not.

 

[Baluncore]

We all know about Edge effect i.e. when an airflow hits the walls of buildings and its velocity will increase.

No, we do not.
Are you suggesting that a sharp corner or edge will provide a source of eddies, because some corner is not sufficiently rounded to allow the Conda effect?

I think you need to attach a diagram, with some flow lines, showing at least a 2D view.

 

[Lnewqban]

Please, see:
https://www.sciencedirect.com/topics/engineering/boundary-layer-separation

The size of the straight and plain wall respect to the induced airstream should be an important factor.
The radial velocity of the air over the surface (in a direction perpendicular to the airstream) should get reduced as it flows away from the center of impact (as the volume remains equal).

 

[T C]

No, we do not.
Are you suggesting that a sharp corner or edge will provide a source of eddies, because some corner is not sufficiently rounded to allow the Conda effect?

I think you need to attach a diagram, with some flow lines, showing at least a 2D view.

That's the answer that I have got after asking Gemini. [AI reference link deleted by the Mentors]

Please, see:
https://www.sciencedirect.com/topics/engineering/boundary-layer-separation

The size of the straight and plain wall respect to the induced airstream should be an important factor.
The radial velocity of the air over the surface (in a direction perpendicular to the airstream) should get reduced as it flows away from the center of impact (as the volume remains equal).

Ok. Let's make it more precise. We have a 30 cm diameter fan fitted before a circular flat plate of 60 cm diameter and the centres of both are aligned. I want to know whether the flow will accelerate and will stick to the surface after the fan is switched on or not.

 

[Baluncore]

We have a 30 cm diameter fan fitted before a circular flat plate of 60 cm diameter and the centres of both are aligned.

What is the axial separation, between the fan outlet, and the circular plate?

 

[T C]

What is the axial separation, between the fan outlet, and the circular plate?

Suppose it's 10 cm.

 

[Baluncore]

Suppose it's 10 cm.

The outlet of the fan has an area of π * 15² = 707 cm².

The circumference of the fan outlet is π * 30 cm = 94.25 cm.
The area of the annular exit is 94.25 cm * 10 cm = 942.5 cm².

Since the annular exit has an area greater than the fan outlet, the air will flow at a lower velocity from the annular exit, than from the outlet of the fan. There will be a stagnation point in the centre of the plate, with air spreading radially, out across the plate. The air will not immediately wrap around the edge of the flat plate, but will continue to spread radially.

 

[T C]

Thanks for your reply. When you are saying spreading rapidly, does that mean increase in velocity after hitting the plate?

 

[Baluncore]

When you are saying spreading rapidly, does that mean increase in velocity after hitting the plate?

No.
I wrote "spreading radially", not "spreading rapidly".

 

[T C]

Will it stick to the surface while spreading radially and is there any chance of increase in velocity due to such radial spreading?

 

[russ_watters]

We all know about Edge effect...

Is "edge effect" even a real term of art in fluids? Googling it, the only top, relevant hit I get for it is this thread. Got a source for it/what you are claiming it means?

Some market available shroud augmented wind turbine types are claiming that they used this effect to increase the output of their own designed wind turbine. But, all these are about natural wind flows.

Same; source?

 

[russ_watters]

That's the answer that I have got after asking Gemini. [AI reference link deleted by the Mentors]

Ok. Let's make it more precise. We have a 30 cm diameter fan fitted before a circular flat plate of 60 cm diameter and the centres of both are aligned. I want to know whether the flow will accelerate....

Really oddly put when you add "when the fan is switched on", but no, the airflow slows down as it approaches and goes around the plate (or a building). [Edit] Clarification: it can accelerate around/over the parallel section, as if it's a very badly shaped wing.

Note, air can speed up in places when flowing around a properly shaped object, such as a wing.

 

[T C]

Kindly go through this video. It shows how the flow encountering the wall accelerates.

 

[T C]

Same; source?

Here is another video.

 

[russ_watters]

Kindly go through this video. It shows how the flow encountering the wall accelerates.

For both videos, please give me the timestamp where they say "Edge Effect". I'm not going to go through them looking for it.

 

[T C]

You can hear it in this video at 1:40. This video belongs to the same wind turbine. And, just to add, this paper shows that the output from a turbine increases sufficiently if a nozzle, shroud and diffuser is added.

 

[russ_watters]

Ok, I see it, and see that you capitalized "Edge" but not "effect" because that's how it is shown in the video subtitles. Near as I can tell, it's typically called "corner effect" and is about what happens whe a fluid flows around a sharp corner (it accelerates):
https://www.simscale.com/blog/building-aerodynamics-and-wind-effects/

What you describe in the OP and post #4 isn't that, it's flow hitting a perpendicular flat plate and spreading out. I think what you are hoping to do is make that flat plate actually be a turbine, with blades around the edge. Am I right?

 

[FactChecker]

Kindly go through this video. It shows how the flow encountering the wall accelerates.

I don't see any use of the term "edge effect" in this video.

 

[FactChecker]

Here is another video.

The term "edge effect" is used at 15:50 in this video. Here is that part of the transcript:

the VX175 has a specially engineered,
omnidirectional shroud. This allows it to be mounted on the tops of buildings and capture
energy from the turbulent airflow. The idea is to get even higher efficiencies out of higher
speeds, particularly from the constantly changing, rough winds common in cities.
Here’s how Stumpf described it:
“We focus on wind acceleration that can be seen in urban environments
due to wind “hitting” a wall and being redirected upwards and accelerated. We
call it the edge effect…The turbine is built in a way that it can harness that
upwards skewed wind and accelerate it once more inside of the turbine.”

It sounds like it is related to turbulence at the edge.

 

[T C]

What you describe in the OP and post #4 isn't that, it's flow hitting a perpendicular flat plate and spreading out. I think what you are hoping to do is make that flat plate actually be a turbine, with blades around the edge. Am I right?

Not at all. In all the videos, it has been shown that the flow accelerates. I just want to know whether such acceleration will occur in my described scenario or not.

 

[T C]

“We focus on wind acceleration that can be seen in urban environments
due to wind “hitting” a wall and being redirected upwards and accelerated. We
call it the edge effect…The turbine is built in a way that it can harness that
upwards skewed wind and accelerate it once more inside of the turbine.”

This acceleration is my point of interest.

 

[russ_watters]

Not at all. In all the videos, it has been shown that the flow accelerates. I just want to know whether such acceleration will occur in my described scenario or not.

What you described is not what is shown in the video/edge effect, so no.

 

[Lnewqban]

Will it stick to the surface while spreading radially and is there any chance of increase in velocity due to such radial spreading?

Please, see:
https://en.wikipedia.org/wiki/Coandă_effect

Note that the edge of the circular plate should not affect the flow much, since both are parallel to each other.
Once the flow goes over the edge, the surface friction disappear but interaction with air in relative repose begins.

 

[FactChecker]

The velocity at the center of impact with the wall is zero, so there is acceleration above zero around that. But it is another question, whether the velocity ever gets higher than the free air-stream velocity. Once the issue of turbulence comes up, things get complicated, and far beyond my amateur knowledge.

the VX175 has a specially engineered,
omnidirectional shroud. This allows it to be mounted on the tops of buildings and capture
energy from the turbulent airflow. The idea is to get even higher efficiencies out of higher
speeds, particularly from the constantly changing, rough winds common in cities.
Here’s how Stumpf described it:
“We focus on wind acceleration that can be seen in urban environments
due to wind “hitting” a wall and being redirected upwards and accelerated. We
call it the edge effect…The turbine is built in a way that it can harness that
upwards skewed wind and accelerate it once more inside of the turbine.”

To me, it sounds like he is talking about the advantage of positioning the turbine away from the stagnated center of impact with the wall. Any velocity above zero is "accelerated" from there. Given a broad wind hitting a building, probably the best place to mount a turbine would be at the edge of the wall.

 

[T C]

Just got this video where the force exerted by a jet (whether liquid or gaseous) is explained. After hitting the surface, the jet will move radially i.e. perpendicular to the direction of motion and the velocity will become zero in one sense that it's now going perpendicular to the direction of the previous motion. But that doesn't mean that it will become devoid of velocity at all.

 

[FactChecker]

If a stream with circular symmetry hits a wall, the center of the stream will have a zero velocity at the wall. The symmetry allows no lateral velocity there. That is at a single point on the wall. Other points will have lateral/radial velocity. IMO, a wide freestream airflow would have lateral velocities along the wall that increase toward the edge of the wall. I don't think the velocity would exceed the freestream velocity, but I don't know that authoritatively. One of the linked articles implies that the best place to mount a turbine would be on the edge of the wall (actually at the edge of the top of the building?).

 

[renormalize]

TL;DR: I want to know whether a flow created by a simple fan will show Edge Effect when hitting a flat and vertical surface.

It's not difficult to find technical papers online that analyze 2D laminar flow around a rectangular cylinder. Here's one example picked more or less at random: Effect of Corner Curvature of Square Cylinder on Flow Transition and Heat Transfer
This paper computes and compares the flow patterns around squares for various Reynolds numbers and degrees of corner-rounding:

It also displays the magnitude of the flow velocity (flow speed) for different roundings:

Note that without exception the flow speed is higher (yellow & green) on the left where the flow enters and it's lower (blue) near the cylinder surfaces. So evidently it's not true that the flow "speeds up" as it encounters the front surface and proceeds to flow "radially" (vertically) there.
Added comment: Actually it is true that the flow speed is zero at the front stagnation point and then accelerates as it moves up or down toward the front corners (i.e., the iso-speed contours get closer together there). It's just that the flow never speeds up enough to equal the incoming flow at the left. That said, the highest value of this sub-incoming speed does occur at the front corners, so one of those corners is the best place to locate a turbine if you can't otherwise raise it directly into the higher-speed free-flowing region.

 

[T C]

If this paper is correct, then the company is lying from the very beginning. The simulation shown in their videos are quite opposite to the findings of this research. And, as per this research, wind turbines can perform better a little higher point instead of the edge of the building.

 

[T C]

t also displays the magnitude of the flow velocity (flow speed) for different roundings:

I am curious to see (and know) the outcome of this research if it's a half circle.

 

[renormalize]

I am curious to see (and know) the outcome of this research if it's a half circle.

How is the "half-circle" oriented relative to the flow? Like:

or

or some other way?