Bernoulli effect on new vent design

[NatureFriend]

TL;DR Summary

Bernoulli ; Thermal dynamics ; Global Warming ; Nature

First off don't quit reading if you understand basic Bernoulli effect. I am nowhere close to the caliber of minds on this forum but I have a problem and lack understanding that science may help answer.

Preface: Temperature extremes are stressing wildlife populations. I happen to be an advocate of one species, the Purple Martin (PUMA). The PUMA nests almost exclusively in man made housing. With rising temperatures nestlings are stressed. The PUMA is in perile. I am attempting to come up with a "better mouse trap" for venting this housing to increase brood survival.

I would like to apply the Bernoulli effect but am uncertain how. The vent is essentially a small funnel wind vane that faces into the wind. Wind would come into the funnel and cross over a hole that comes from inside the housing. My hope is that this will create a vacuum in the house pulling hot air from the nest cavity out the funnel. Is my theory sound? I also don't even know whether to face the large or the small diameter of the funnel into the wind ? Any help or suggestions greatly appreciated.

 

[Rive]

pulling hot air from the nest cavity

I'm a bit doubtful about that. Nest cavities are often about a specific micro-climate (needed for the incubation and raising of chicks), and venting would seriously disrupt that. Even might dry the chicks - but even more likely that the birds would just not occupy a windy cavity.

Needs an expert on their nesting habits, I'm afraid.

Maybe some shades (shade nets), instead? Or just changing the placement of nests?

 

[Baluncore]

The length of the access tube, the thermal mass and insulation properties of the nest will be important. Given a shorter access tube, the birds may be willing to ventilate the nests themselves. We have no way of knowing if more or less ventilation is required.

I would start by data logging the temperature of nests to determine the optimum temperature range and the tolerated daily variation. The changing behaviour of the birds at different temperatures would indicate stress. Has that study been done?

 

[NatureFriend]

Rive, I believe there is a current study being done (using data loggers) about the effectiveness of certain types of venting currently being used by the active PUMA community. I will update you on specifics. It is recognized by that community that high heat and humidity lead to mortality, stress, change in behavior of nestlings. I am trying to come up with a new "mouse trap" using some basic thermodynamics beyond the current state of the art which is essentially drilling a hole in the side of the housing (aka plastic gourd) to let warm air escape. Unfortunately, I have only rudimentary (high school physics 50 years ago) knowledge ot thermo.

Baluncore, suggestions with the proper length of tube and funnel would be helpful. Attached are some pics of a protype I've been using (successfully). I am also attempting to combine Bernoulli with what I call "smoke stack" effect. I'm sure there's a better name for it.

 

[NatureFriend]

This is where I got the idea.

 

[Lnewqban]

This is where I got the idea.

Welcome, @NatureFriend !

Please, see:
https://en.wikipedia.org/wiki/Vacuum_ejector#Air_ejector

 

[NatureFriend]

Ah, so it is a Venturi air pump design I seek that can operate at normal air speeds? Can they? Based on the sketches online the windward and leeward side of the cone can be equal in diameter? Not constricted at one end as I have it in my prototype. Is the length of the cone critical or just the size and location of the constriction? I would think there is some sort of meter/gauge available to measure the vacuum (albeit small)? Lots of question but I am enjoying asking them and learning as I go. Thanks for the welcome.

 

[Baluncore]

I would think there is some sort of meter/gauge available to measure the vacuum (albeit small)?

The internal pressure "depression" will be very small because the opening is large, also pressure is not what you need to know. I would employ a "mass air flow sensor", MAF, to measure the air flow directly.

When it is sufficiently windy to drive a directional vane, then I expect there would be sufficient ventilation through the access hole.
Moist air is less dense than dry air, so I expect a tapered chimney would draw hot and/or moist air from the nest chamber in a self regulated way. A chimney would continue to function in still weather. A thin-walled black chimney will draw better when heated by sunlight.

 

[Lnewqban]

This is where I got the idea.

Regarding the device shown in your picture in post #5, it seems to be two L-shaped ducts (inlet and outlet) mounted on a gyratory common platform rather that a venturi (Bernoulli) effect.

Copied from
https://sustainabilityworkshop.venturewell.org/node/1029.html

"For example, the specially-designed wind cowls in the BedZED development use the faster winds above rooftops for passive ventilation. They have both intake and outlet, so that fast rooftop winds get scooped into the buildings, and the larger outlets create lower pressures to naturally suck air out. The stack effect also helps pull air out through the same exhaust vent."

From your pictures of the actual nest in post #4, it seems that the venting device can't orientate itself toward the wind, as it seems to be a simple PVC tee.
It may be manually oriented into the prevailing wind direction.
Are my observations correct?

Also, is the entrance of the nest the only way for fresh air to come inside?
Any means to drain the nest in case wind-driven rain penetrates?

This is an interesting breading process:
https://en.wikipedia.org/wiki/Purple_martin#Breeding

 

[NatureFriend]

Lnewqban, The "T" coupling or "vane" sits loosely on the verticle pvc so it is able to orient into the wind. It works well.

There are 4 small weep holes at the bottom of the nest cavity for drainage. Nesting material stacked on top make them ineffective for ventilation. The manufacturer of this style of house (aka gourd) has two approx. 1/8" holes at the top to allow hot air to escape. Not sufficient IMO. Mortality in my bird colony is much higher during spells of hot weather. Young birds still unable to fly actually fall or jump from the gourds. In the PUMA community they are called "jumpers".

I had thought about wind driven rain in my design. That is the reason for the pvc horizontals extending from the "T". Not shown in the pics is that I cut weep holes on the underside of these extensions to allow for drainage prior to reaching the verticle down pipe. After two seasons of monitoring, I've had no moisture infiltration into the nest cavity.

Any thoughts on how I could teak this design to make it more effective? I appreciate all the feed back.

 

[NatureFriend]

The internal pressure "depression" will be very small because the opening is large, also pressure is not what you need to know. I would employ a "mass air flow sensor", MAF, to measure the air flow directly.

When it is sufficiently windy to drive a directional vane, then I expect there would be sufficient ventilation through the access hole.
Moist air is less dense than dry air, so I expect a tapered chimney would draw hot and/or moist air from the nest chamber in a self regulated way. A chimney would continue to function in still weather. A thin-walled black chimney will draw better when heated by sunlight.

I had researched manometers but also doubted the cheapies would register much, if at all. Will look into MAF. Good suggestion on the thin walled verticle. I'll look around for a source. Are you suggesting the taper be larger at the top or the bottom? My guess is top. I was going to try to explain it in physics terms but I didn't wanted to be laughed off this forum. Besides I'm probably wrong. Thanks again

 

[Rive]

I am trying to come up with a new "mouse trap"

I do understand that. What I'm wondering about is, that maybe it would be more effective to modify the placement of the "mouse trap" instead.

We have some kind of similar issues with decreasing population of (barn & other) swallows, and while we too came up with various types of artificial nests, a common trait is that none (!) of those gets direct sunlight, ever.

I know that purple martins are not barn swallows, but these 'gourd-trees' still looks (very!) weird for me, especially in the context of heat management.

What we have (for open areas) are like this (mind the placement, not the size!):

Ps.: if you are decidedly only after ventilation improvements then sorry, and I just won't bother you again with these

 

[Baluncore]

Are you suggesting the taper be larger at the top or the bottom? My guess is top.

Larger at the bottom, so air from the nest starts to rise slowly, then accelerates, so it needs less sectional area. The small hole at the top can have a flat hat above, to keep out heavy rain. The stem of the gourd has about the right shape to start the chimney.

 

[NatureFriend]

Rive, are you suggesting altering the placement of the housing to provide adequate shade? Unfortunately, the PUMA will not tolerate being too close to trees and are attracted to open areas. The going theory here is that predators hang out in "shady" places. Agreed, there are other things that can be done to mitigate heat. Others have tried house shading, misting, etc. I just happen to working on venting. The current most common way of adding venting to these gourds is to attach a 90 degree pvc elbow at the top of the gourd.The elbow points down to limit moisture infiltration. Simple and cheap. My concern (not backed by data) is that the elbow pointing down resists warm air from leaving. These set ups are still plagued with heat issues.

Your first picture happens to be the same as one of my units. If you look closely to the pic you can see these after market pvc elbow vents.

 

[Lnewqban]

Lnewqban, The "T" coupling or "vane" sits loosely on the verticle pvc so it is able to orient into the wind. It works well.

In that case, the inlet diameter should be bigger than the diameter of the vertical opening.
Research the Ventury effect of carburators.

There are 4 small weep holes at the bottom of the nest cavity for drainage. Nesting material stacked on top make them ineffective for ventilation. The manufacturer of this style of house (aka gourd) has two approx. 1/8" holes at the top to allow hot air to escape. Not sufficient IMO.

In that case, your ventilation system may be by-passing air from those holes, not helping in ventilating and cooling the space.
I would make new holes around the perimeter of the recipient, just above the top layer of the nest material.
That way, fresh air would sweep the hole interior volume, rather than moving from entrance to exhaust pipe.

Mortality in my bird colony is much higher during spells of hot weather. Young birds still unable to fly actually fall or jump from the gourds. In the PUMA community they are called "jumpers".

Is the picture showing sand or snow?
The recipient seems to be receiving a good deal of radiation from below in either case.
You can have sunny hot days with no breeze.
For those conditions, the birds will suffer, but that could be reduced with external insulation and some weather resistant shield.

As for the jumpers, could nets be installed under the entrance of each recipient?

 

[NatureFriend]

Lnewqban, I wasn't clear. The weep holes at the bottom of the gourd are intended for drainage only. Not for air induction or circulation. Drilling additional holes along the gourd body would be problematic. The picture is in fact snow but nesting season is late spring into summer. Yes, the "venturi wind vane" would be ineffective without a breeze. The black stove pipe vent is the back up for those days.

 

[Baluncore]

The weep holes at the bottom of the gourd are intended for drainage only. Not for air induction or circulation.

I would agree with that, but only in the first analysis.
There are often hidden or forgotten benefits. Parasites, that remain in the nest after fledging, may fall through the holes before the new tenants arrive next season. Ants (room service) can clean the nest while it is vacant.

The floor of a bee hive may have a mesh that prevents some of the parasites that fall off a returning bee, from transferring to another bee inside the hive. The mesh also makes it easier to examine the parasite burden present in the hive.

 

[NatureFriend]

Baluncore, you are right about the parasites. I was only attempting to stay focused on the heat venting issue. It is easy to get off topic by all the alternative things one can do to improve brood success.

I have also thought about your suggestion about tapering the stack. I could do this by installing a larger diameter pipe into the gourd and then adding a reducer say halfway up. I would like to understand the theory behind why this should increase the stack effect.

 

[Baluncore]

It is easy to get off topic by all the alternative things one can do to improve brood success.

The integration of many features will produce the optimum design. Fixating on, or rejecting one feature, will surprise you later when you find you have taken two steps backwards, where you thought you were going forwards.

I would like to understand the theory behind why this should increase the stack effect.

As the air accelerates upwards in the stack, there should be no sudden step change in section. A step change in section forces a sudden change of air velocity, wasting what little pressure is available, while causing turbulence that throttles the flow.

The advantage of a tapered stack that is wide at the bottom, is that the airspeed in the nest area is low, so stagnant pockets of air will be less likely to form, leading to more even conditions.

A compromise would be to have the bottom part tapered, with the top part parallel. I would probably mould the stack into the gourd profile with the stem up, forming the initial taper at the base of the stack.

While the top half of the stack should be black and thin walled, the nest chamber should be well insulated and white to reflect the sunlight.

 

[NatureFriend]

I am not fixated on one solution. I do many things to assure maximum brood success e.g. nest checks, changing nest material, controlling parasites, controlling predators, controlling competition for housing, bird banding, rescuing jumpers, supplemental feeding, etc., etc. One issue that has vexed me and the PUMA community has been the increasing high temperature within housing. It is as good a place for me to think and tinker with my limited resources of time, money and energy. I value this discussion but don't want it to wander too far. I need help understanding the physics of venting. No offense. I just felt I needed a little defense.

I think I understand the stagnant pocket and abrupt transition. It's not clear to me what you are describing here: "A compromise would be to have the bottom part tapered, with the top part parallel. I would probably mould the stack into the gourd profile with the stem up, forming the initial taper at the base of the stack."

I really don't have the resources to mould something. I am hoping to construct this with readily available off the shelf items so that others can build their own at low cost. That is why I suggested the reducer but now know...bad idea. I don't believe I could find a UV resistant tube that tapers with the correct diameters needed.

Thanks for hanging in there with me on this.

 

[Baluncore]

What are the fake gourd nest boxes made from?

 

[NatureFriend]

p.s. When I've made some adjustments, I intend to devise an experiment to test the new against a standard. That will be a later discussion but my guess is it will involve data loggers which I know little about. That will be a later topic...I don't want to wander off this topic

 

[NatureFriend]

I'll get back to you on the plastic.

 

[NatureFriend]

HDPE w/ UV inhibitor.

 

[hutchphd]

My two cents: Do not underestimate the effect of th black and white paint (i.e. the albedo) of the sections for temperature control. In my experience a thin layer of black paint blocks radiation damage to plastic (pvc) pretty well. Reflective white paint might be cost effective also. Two cents please.

 

[Baluncore]

HDPE would make a really hot nesting box during the day, and cold at night.
Is it injected, rotomolded or 3D printed?
Do you know a local geek who can 3D print a prototype with a chimney?

Thick paper-mache would have better insulation properties than HDPE.
It could be made by children in school from newspapers and paste, on blow-up molds made of silicon rubber. Once dry, deflate the mold to extract it. Give the structure a coat of white exterior paint, but paint the chimney black.
What better way to introduce the next generation kids, through symbiotic construction, to teamwork, recycling, nature, annual migration and protected species.

 

[Lnewqban]

Lnewqban, I wasn't clear. The weep holes at the bottom of the gourd are intended for drainage only. Not for air induction or circulation. Drilling additional holes along the gourd body would be problematic. The picture is in fact snow but nesting season is late spring into summer. Yes, the "venturi wind vane" would be ineffective without a breeze. The black stove pipe vent is the back up for those days.

You were clear.
The by-pass or short cut for air I was referring to was between the chimney and the "1/8" holes at the top to allow hot air to escape."
You want your precarious ventilation [edited misspelling] to sweep over the nest, avoiding a stagnant bubble of hot air there.

I believe that external insulation of the nest would eliminate a good deal of necessary ventilation.
Once radiation energy goes inside, it heats everything, which also radiates out into the nest.
Ventilation can only do so much for that.

Research construction insulation material that is suitable for outdoor use.
In form of spray expanding foam, it will be easy to apply over the exterior surface, and also reapply when needed.

Think of how nature solve that temperature problem.
Perhaps the natural nests are built in cavities which walls have some natural insulation or thermal inertia or shade.

If everything fails, you could consider commercially available small exhaust fan powered by a builtin solar panel and controlled by a builtin thermostat.
I know a friend that installed that device for his pigeon's house, and it has been working flawlessly.

 

[NatureFriend]

So many good ideas here.

Lnewqban, Google failed me on "precary". Would having the chimney exit the gourd more towards its' peak resolve your concern?

Baluncore, my guess is it's injection molded. The gourds have a seam where the two halves have been joined. The inventor told me several years ago it would cost 10K for any changes to the mold. I also thought about 3D printing but geeks are far and few between in my world...but I'll look.

All, your concerns about insulation, materials,etc needs exploring as well. I just want to focus on the vent for now so its not overwhelming. I am learning a lot. Thanks

 

[Lnewqban]

Lnewqban, Google failed me on "precary". Would having the chimney exit the gourd more towards its' peak resolve your concern?

Apologies.
I should have written "precarious ventilation" instead.
Let's blame it to my poor English.

No, closing the mentioned holes located at the top of the sphere would.
The idea is to eliminate any draft between those and the chimney.

Again, very sorry.

 

[Tom.G]

For a tapered vent, a 'crevice tool' for a vacuum cleaner is one idea; probably too expensive though. The one I just measured has a circular area (for hose attachment) of 1.2 sq.in. with the flattened end area of 0.33 sq.in. The taper, though, is only 1 in. long, so a bit sudden.

Another possibility is PVC 'pump discharge hose'. Check the Big Box hardware stores for some. It runs about USD 1.70 to 4.00+ per foot. The corrugated style is moderately stiff, available in 6-foot lengths, and could probably be shaped after heating in boiling water.

Have Fun!
Tom

 

[Baluncore]

Have Fun!

Maybe a shorter form of the vuvuzela. $2.50 each.
https://www.amazon.com/dp/B0834N3JXY/?tag=pfamazon01-20

 

[NatureFriend]

Thank you all.

 

[NatureFriend]

Baluncore, I have one (final) question. You suggested thinning the wall of the upper chimney. Was the purpose to encourage more heat absorption thus increasing the stack effect? If so, could increasing the surface area for heat absorption by adding short, black ridges along this section accomplish the same thing?

 

[Baluncore]

If so, could increasing the surface area for heat absorption by adding short, black ridges along this section accomplish the same thing?

It might, but there are disadvantages. I recommend a thin black metal tube, so the sunlight heated surface is in closer contact with the rising internal air. There will also be an external air current moving up the outside of the stack, that will cool the surface. If ridges were added, they would need to be vertical inside the stack, and horizontal outside. That would increase the internal exchange area, but slow it by increasing the surface area drag, while also providing accommodation for spiders, that will block the airflow.

Unfortunately, the use of PE for construction will supply most of the heat to the nest space, and so drive the stack. The fine detail of the upper stack will be relatively unimportant, too little and too late to have a significant effect. The only way to change that would be to insulate the nest box, and paint it externally, reflective white.
The more I think about it, the more I like paper-mache, laid up by child labour, on a hanging party balloon, partly filled with water to give it the gourd shape with a tapered stem-stack.

 

[NatureFriend]

There are thousands currently being used of the PE design. Reportedly they can last 20 years. I am trying to come up with a vent for these existing and surely thousands more to be sold over time.

I have a good idea now what needs to be done to make the vent more effective. Thank you