Flow separation in diffuser section of wind tunnel

[bdcrown007]

For my research purpose, I made a wind tunnel which has 0.6m rectangular section [0.2X0.2sq. meter] and 1.4m long diffuser section having porous top [top wall angle is 20deg] as follows:

Wind tunnel speed is 15m/s. Even using 20deg inclination of porous top wall, there is no flow separation over bottom plate. How can I produce flow separation on bottom plate? I also tested wind tunnel without any top wall at diffuser section on that case still no flow separation have been produced on bottom plate. Any idea?

One point should be noted that due to space limitation in our lab total length of tunnel [2m] could not be extended.

any idea please?

 

[boneh3ad]

Is the wall just porous or is there any sort of suction applied. Suction would help you out. The goal should be to get an adverse pressure gradient along the bottom.

 

[bdcrown007]

At first I tried with punching metal sheet which have holes of 2mm dia, as no separation have been detected using tufts, then I use metallic wire screen, but no such achievement. Then I used two cross flow fan that are placed almost middle of diffuser top wall with same angle.
http://www.blower.com.tw/images/pdc/crvd-crsd-crqd-crfd/crvd-025-1-cross-flow-fan.jpg [Broken]
But may be fan was not sufficient ...

 

[boneh3ad]

I don't think you get what I mean. If you want separated flow, you need an adverse pressure gradient that either "pushes against" the boundary layer flow or "pulls up on" the boundary layer.

In other words, you need to have mass diffusing through your ceiling or some floor curvature.

 

[Travis_King]

Wait? Does your tunnel terminate at the end of the porous wall? If so, you are turning the flow at the wall. You'll probably have some turbulence at the corner, but you won't see any separation on the bottom wall if all you are doing is turning the flow.

You are probably just creating a nice pocket of relatively laminar flow on the bottom surface.

 

[bdcrown007]

@boneh3ad, Yes, I understand. I am thinking some efficient meaning of suction through upper wall to make adverse pressure gradient.

 

[bdcrown007]

@Travis_King, I want to produce flow separation over bottom plate by applying suction on porous top wall. I used two cross flow fan at the middle position of top porous wall with same angle [20deg] but no such separation over bottom plate. Even I remove the top porous wall but there is no separated flow over bottom wall.

You told about turbulence in corner, which corner bottom wall or upper wall? I tested without porous top wall also.

 

[Travis_King]

What I was asking was, is your wind tunnel really only 2m? Does it end at that left wall there after the diffuser? Is the air you are pumping in channeled out of the tunnel?

 

[bdcrown007]

@Travis_King, sorry I missed the point. This one is open type wind tunnel. The tunnel is only 2m long. The exit is open and 1m away from brick wall of Lab. Room air is sucked by tunnel fan and air is escaped to room again at exit.

 

[Travis_King]

Ah ok. I think a problem you have is that with the air escaping into the lower pressure room out of the end of the tunnel, you aren't getting much suction at the top, where the air will have to both turn and speed up (through those holes).

Air is like anything else, path of least resistance.

 

[bdcrown007]

Ah ok. I think a problem you have is that with the air escaping into the lower pressure room out of the end of the tunnel, you aren't getting much suction at the top, where the air will have to both turn and speed up (through those holes).

Air is like anything else, path of least resistance.

This is a good point. I also think about room pressure, possibly door should be open during operation. If you have any more suggestion, please let me know.

 

[boneh3ad]

The door being open or closed isn't going to make a lick of difference if your tunnel is drawing air from the room and exhausting into the same room. The only way your pressure in the room would change is over time as the air heats up as a result of the tunnel, which will not be a large effect.

If you want air to go through the holes in your diffuser, you need to artificially lower the pressure behind those holes to create suction because as Travis mentioned, not much will go through them otherwise. You can do that a number of ways, but a vacuum pump may be a good place to start your search.

At any rate, don't worry about such minor things such as the door in your room being open or closed. Instead, try and solve your fundamental problems. You need a pressure gradient acting on the boundary layer to separate it, and it is your job to implement that. Opening the door isn't going to create it. If you look at your situation, your tunnel design is what needs work, and if you insist on doing it with diffuser roof suction, then you really need to create a chamber behind those holes where you can hold the pressure in the chamber at a lower value than the room around it. You need something to encourage the flow to go through those holes and I suspect you will need a fairly substantial mass flow through those holes if you want to get the kind of effect you want on your boundary layer. It will be made even more difficult by the fact that your boundary layer is almost certainly turbulent.

 

[bdcrown007]

The door being open or closed isn't going to make a lick of difference if your tunnel is drawing air from the room and exhausting into the same room. The only way your pressure in the room would change is over time as the air heats up as a result of the tunnel, which will not be a large effect.

If you want air to go through the holes in your diffuser, you need to artificially lower the pressure behind those holes to create suction because as Travis mentioned, not much will go through them otherwise. You can do that a number of ways, but a vacuum pump may be a good place to start your search.

At any rate, don't worry about such minor things such as the door in your room being open or closed. Instead, try and solve your fundamental problems. You need a pressure gradient acting on the boundary layer to separate it, and it is your job to implement that. Opening the door isn't going to create it. If you look at your situation, your tunnel design is what needs work, and if you insist on doing it with diffuser roof suction, then you really need to create a chamber behind those holes where you can hold the pressure in the chamber at a lower value than the room around it. You need something to encourage the flow to go through those holes and I suspect you will need a fairly substantial mass flow through those holes if you want to get the kind of effect you want on your boundary layer. It will be made even more difficult by the fact that your boundary layer is almost certainly turbulent.

thanks for you suggestion. I want to use vacuum pump coupling with a suction hood like in kitchen we use above the oven.