Wind Tunnel Honeycomb Screen and Calculations

fredi · July 21, 2021, 13:04

Hi everybody and very glad to discover this very interesting forum.

I am helping someone who is not expert (like myself) in Fluid Dynamics to design a simple subsonic wind tunnel and we have read some papers and general guidelines for that and our progress is not very bad.

Our main problem is with the screen of honeycomb and the details of it and we would be very glad to understand different parameters.

As I understand the D/L for the screen should be between 6 and 8 and I do not quite get what it means and how should we design the cells.

There is also the notion of porosity and the fact that it must be bigger than 0.8 and this also confuses us a lot as to what it is and how it is going to modify/dictate the shapes and numbers of the cells.

Any help and thoughts will be greatly appretaiated.

LuckyTran · July 21, 2021, 14:43

The porosity is the open area ratio. A high porosity means the honeycomb is a smaller blockage on the flow cross sectional area and generally means a lower pressure drop. Because of hexagonal close packing, the porosity is controlled by the thickness of the honeycomb walls. So choose a honeycomb that has thin walls relative to the size of the honeycomb cells.

The other parameter is the size of the honeycomb cells (the length of the sides of the hexagon). This size, you choose based on the desired length scale.

Finally there is the thickness of the honeycomb in the flow direction. The longer this is, the more the honeycomb acts as a flow straightener that makes the flow more uniform. But a longer honeycomb also has more pressure drop. So you design for the desired performance.

fredi · July 22, 2021, 03:53

Thanks a lot LuckyTran, I was wrong on my first comprehension of the issues of porosity.

May I please ask on how to quantify the desired cells dimension? How should I choose the length and D (apparently the cell hydraulic diameter) to obtain the value of 6?

LuckyTran · July 22, 2021, 10:21

Are you asking what is hydraulic diameter?

I don't know what you're reading to find a requirement of 6-8 for D/L. There is entrance length (but that is normally L/D). Otherwise, what is not clear about choosing a and b such that the ratio comes out to be a certain number? Do you not know what D is or L?

fredi · July 23, 2021, 02:22

You are right LuckyTran and I apologies for not providing enough details and information.

I was referring to this paper:

https://www.researchgate.net/publica...nd_Calibration

In the page 4 of the pdf file of it there are some calculation and statements that the (L honey/ D h honey) must be at least 6. I do not quite get it and the reason I did not give details on my previous post was that I thought maybe this relation is very famous and every involved person will instantly realize that.

The mentioned paper has enough details about the honeycomb inlet but I have not been able to get practical calculations possibly because of my lack of knowldege on the terminology.

LuckyTran · July 23, 2021, 17:10

Well it certainly does not help that you wrote D/L (which is the reciprocal relation) of L/D.... Every involved person would know it, but you gave the opposite of what was expected just to confuse everyone.

It helps even less that the paper you are reading doesn't even have the nomenclature defined. The L/D is the streamwise length of the honeycomb in the flow direction. There are various ranges (like 5-10, and so on) that you will find in various publications. Basically, your honeycomb needs to look like a tube bundle. The purpose of this length, is to laminarize the oncoming flow and kill the turbulence as well as reduce lateral velocity component and make the flow more straight.

fredi · July 25, 2021, 12:25

Sorry again, I do not know why I made such a mistake.

Thanks for the direction and hint.

My guess is that if I try to make arrays of honeycomb to satisfy Lh/Dh honey = 6 or so then I might be in my way to get a laminar flow.

I still do not know how to set the thickness of the walls for the honeycomb(s H in the given paper). The paper and your answers give some clues but due to my lack of knowledge and experience I do not clearly see it. of course it is my bad and I hope i am not wasting your time here.

LuckyTran · July 26, 2021, 02:58

Let's say you have a given honeycomb cell size. You make the honeycomb length long enough to satisfy your L/D criteria. Then you make the thickness thin enough to satisfy the porosity criteria.

Flow conditioners don't always have to laminarize the flow. So don't get stuck up on that comment. Flow conditioners do a lot of things. Sometimes they are for producing uniform laminar oncoming flow. Sometimes they are for producing turbulence.

July 21, 2021, 13:04	Wind Tunnel Honeycomb Screen and Calculations	#1
fredi New Member Please select a state (mandatory for US) Join Date: Jul 2021 Posts: 13 Rep Power: 4	Hi everybody and very glad to discover this very interesting forum. I am helping someone who is not expert (like myself) in Fluid Dynamics to design a simple subsonic wind tunnel and we have read some papers and general guidelines for that and our progress is not very bad. Our main problem is with the screen of honeycomb and the details of it and we would be very glad to understand different parameters. As I understand the D/L for the screen should be between 6 and 8 and I do not quite get what it means and how should we design the cells. There is also the notion of porosity and the fact that it must be bigger than 0.8 and this also confuses us a lot as to what it is and how it is going to modify/dictate the shapes and numbers of the cells. Any help and thoughts will be greatly appretaiated.

July 21, 2021, 14:43		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	The porosity is the open area ratio. A high porosity means the honeycomb is a smaller blockage on the flow cross sectional area and generally means a lower pressure drop. Because of hexagonal close packing, the porosity is controlled by the thickness of the honeycomb walls. So choose a honeycomb that has thin walls relative to the size of the honeycomb cells. The other parameter is the size of the honeycomb cells (the length of the sides of the hexagon). This size, you choose based on the desired length scale. Finally there is the thickness of the honeycomb in the flow direction. The longer this is, the more the honeycomb acts as a flow straightener that makes the flow more uniform. But a longer honeycomb also has more pressure drop. So you design for the desired performance.

July 22, 2021, 03:53		#3
fredi New Member Please select a state (mandatory for US) Join Date: Jul 2021 Posts: 13 Rep Power: 4	Thanks a lot LuckyTran, I was wrong on my first comprehension of the issues of porosity. May I please ask on how to quantify the desired cells dimension? How should I choose the length and D (apparently the cell hydraulic diameter) to obtain the value of 6?

July 22, 2021, 10:21		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	Are you asking what is hydraulic diameter? I don't know what you're reading to find a requirement of 6-8 for D/L. There is entrance length (but that is normally L/D). Otherwise, what is not clear about choosing a and b such that the ratio comes out to be a certain number? Do you not know what D is or L?

July 23, 2021, 02:22		#5
fredi New Member Please select a state (mandatory for US) Join Date: Jul 2021 Posts: 13 Rep Power: 4	You are right LuckyTran and I apologies for not providing enough details and information. I was referring to this paper: https://www.researchgate.net/publica...nd_Calibration In the page 4 of the pdf file of it there are some calculation and statements that the (L honey/ D h honey) must be at least 6. I do not quite get it and the reason I did not give details on my previous post was that I thought maybe this relation is very famous and every involved person will instantly realize that. The mentioned paper has enough details about the honeycomb inlet but I have not been able to get practical calculations possibly because of my lack of knowldege on the terminology.

July 23, 2021, 17:10		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	Well it certainly does not help that you wrote D/L (which is the reciprocal relation) of L/D.... Every involved person would know it, but you gave the opposite of what was expected just to confuse everyone. It helps even less that the paper you are reading doesn't even have the nomenclature defined. The L/D is the streamwise length of the honeycomb in the flow direction. There are various ranges (like 5-10, and so on) that you will find in various publications. Basically, your honeycomb needs to look like a tube bundle. The purpose of this length, is to laminarize the oncoming flow and kill the turbulence as well as reduce lateral velocity component and make the flow more straight.

July 25, 2021, 12:25		#7
fredi New Member Please select a state (mandatory for US) Join Date: Jul 2021 Posts: 13 Rep Power: 4	Sorry again, I do not know why I made such a mistake. Thanks for the direction and hint. My guess is that if I try to make arrays of honeycomb to satisfy Lh/Dh honey = 6 or so then I might be in my way to get a laminar flow. I still do not know how to set the thickness of the walls for the honeycomb(s H in the given paper). The paper and your answers give some clues but due to my lack of knowledge and experience I do not clearly see it. of course it is my bad and I hope i am not wasting your time here.

July 26, 2021, 02:58		#8
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,673 Rep Power: 65	Let's say you have a given honeycomb cell size. You make the honeycomb length long enough to satisfy your L/D criteria. Then you make the thickness thin enough to satisfy the porosity criteria. Flow conditioners don't always have to laminarize the flow. So don't get stuck up on that comment. Flow conditioners do a lot of things. Sometimes they are for producing uniform laminar oncoming flow. Sometimes they are for producing turbulence.