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Equipment Discussions >> Reflectors

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Can a rear fan remove the boundary layer?
      #5427894 - 09/18/12 05:32 PM

Hi everyone,

I realized some time ago that I had some serious termal issues in my scope, so I started using a 80mm rear fan with a baffle in my 8" f6 sonotube dob. It worked very well and I could notice the difference in the image with the fan on/off.

From the front of the tube, I can hear and actually feel a very light air that comes from the back of the scope passing through the cell, and I wonder if that air is helping to remove the boundary layer of the primary, since I noticed inmediate changes in the image. I ask these because I thought that only a side fan would inmediately remove the BL and that rear fans would only help to accelerate the cooling of the mirror.

Clear Skies,

Javier.


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EJN
Carpal Tunnel
*****

Reged: 11/01/05

Loc: 53 miles west of Venus
Re: Can a rear fan remove the boundary layer? new [Re: Javier1978]
      #5427908 - 09/18/12 05:40 PM

If it is strong enough. Most computer fans are not.

I bought an "outrunner" motor used for radio-control airplanes and
mounted it behind my mirror cell with a 9" APC pusher prop. I run it at
10000 RPM. Gets rid of every trace of tube currents and boundary layer.
The only problem is you have to be careful getting your hand close to it,
because the prop can take a nice chunk out of your finger. Collimate
*before* turning on the fan.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Can a rear fan remove the boundary layer? new [Re: EJN]
      #5427916 - 09/18/12 05:46 PM

WOW, thats turns observation into a quite risky hobby!

My fan is quiet and weak running with a 9 v battery.

Thanks for your asnwer.


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sixela
Postmaster
*****

Reged: 12/23/04

Loc: Boechout, Belgium
Re: Can a rear fan remove the boundary layer? new [Re: Javier1978]
      #5427991 - 09/18/12 06:22 PM

To get the fan to scrub the boundary layer you have to install a baffle in front of the mirror with a circular hole slightly wider than the mirror, so that the air gets pulled from over the mirror.

Mauro da Lio pioneered it and it works quite well.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Can a rear fan remove the boundary layer? new [Re: sixela]
      #5428017 - 09/18/12 06:32 PM

Great, thanks for the tip.

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Can a rear fan remove the boundary layer? new [Re: Javier1978]
      #5428076 - 09/18/12 07:13 PM

Hi Sixela,

I just read your 2010 post. Do you recommend to use the fan the other way round to pull air?


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TopherTheME
sage
*****

Reged: 02/11/11

Loc: Rochester, MI
Re: Can a rear fan remove the boundary layer? new [Re: Javier1978]
      #5428479 - 09/18/12 11:42 PM

Don't mean to knit-pick here but there is no removing a fluid boundary layer. If there is fluid moving over an object, a fluid boundary layer will form. You're creating a boundary layer by having the fan on, not removing it. There will always be a fluid boundary layer present in/on a scope, its not something you can get rid of, ever (unless its in a vacuum).

A thermal boundary layer will form if there is a temperature difference between the scope and the surrounding air, which there always will be. There's no getting rid of that either.

The real question should be, "can I reduce the natural convective currents in my scope enough so they don't affect seeing". To which the answer is yes. The purpose of the fan isn't to "remove boundary layers" its to create forced convection in the scope so it acclimates faster (compared to natural convection). You should only use a fan to help the scope quickly acclimate to the point where natural convection isn't noticeable or minimized. Once its near the temperature of the surrounding environment you should turn the fan off. Having air blow through the scope will only worsen its performance and the vibrations from the fan aren't going to help either.


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Can a rear fan remove the boundary layer? new [Re: TopherTheME]
      #5428517 - 09/19/12 12:27 AM

Quote:

Don't mean to knit-pick here...




Glad you did! I've been wondering about this issue myself. For a glass mirror that's typically *warmer* than the ambient air temp until it's acclimated, it's commonly portrayed as having this "boundary layer" sitting on its face, like a puddle. Yet in point of fact, the air absorbing heat from the glass will itself become warmer than ambient, and so will rise- across the face of the mirror, and thence up the tube- and thereby produce the well known & documented "chimney effect" common in tubed Newtonians. Check these out!

Yes, unless your scope's name is "Hubble", there will *always* be air directly in front of its mirror. At stake is what is that air doing? The plan is to keep it still, which a fan helps by eventually reducing the temp delta, and so the unwanted thermal effects. It will also, if blowing *UP* the tube (assuming we're aimed upwardly & viewing the sky ), scoot the temperature-laden "bad air" out the top of the tube, and much more quickly than if left to natural convection alone. Blowing up the tube works WITH convection, not against it. I believe it also smooths this air out, so it's not bubbling & billowing as it makes its way up the tube walls toward the top... which helps keep these effects farther out of the light's path, both down & back up to the secondary.

So as i see it, there are three possible states for this boundary *and* tube air, affecting light on both it's way down to the primary, as well as back up:
1) loaded with thermal energy- turbulent, churning, billowing all over the place, or
2) loaded with thermal energy- but gradually decreasing, and being whisked into a smoother, directional flow outta there, or
3) ambient- and relatively quiescent.

It's pretty easy to see the diff between 1 & 2, by switching a Newt's rear-blower fan on-and-off while viewing;
*ON* definitely & obviously improves the views!

Once you're at 3, can the fan, man, and run up the mags until the sky yells "uncle!"


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john gabriel
sage
*****

Reged: 12/13/11

Loc: Southern California
Re: Can a rear fan remove the boundary layer? new [Re: Mike B]
      #5428598 - 09/19/12 01:19 AM

Hi everyone. Thought I'd chime in here with a related question. I saw a similar discussion earlier this year (on this CN forum I think) in which fans were mentioned that have a very good score for minimum vibration, and I wonder if anyone knows what make those are or can recall that post. I was pretty new to CN then and forgot to bookmark it.

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simpleisbetter
Carpal Tunnel
*****

Reged: 04/18/11

Re: Can a rear fan remove the boundary layer? new [Re: sixela]
      #5428601 - 09/19/12 01:20 AM

I wouldn't ever consider adding to Alexis' posts, but in this case I'd follow up to say that in addition to the baffle ring just ahead of the primary, you need to install a full baffle at the rear, closing in the rear of the tube around the fan(s) so they're the only thing seen. Otherwise, air hits the mirror and never enters the tube, it just reflects back out into open air. You need the rear baffle to make the air flow forward into the tube.

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sixela
Postmaster
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Reged: 12/23/04

Loc: Boechout, Belgium
Re: Can a rear fan remove the boundary layer? new [Re: TopherTheME]
      #5428698 - 09/19/12 03:56 AM

Quote:

Don't mean to knit-pick here but there is no removing a fluid boundary layer.



We're not talking about that kind of "boundary layer", but a layer of air warmer than the ambient air, but of course we both know that (it's a good nit-pick since "boundary layer" is not such a good name for what we're talking about, but it's become established usage and I can't really find anything better that's as short).

And it's definitely possible to blow off the air and replace it frequently, which makes the temperature delta of this layer smaller and reduces the optical effects quite instantly (I can see the effects of the fan setup improve the image well before the mirror is cooled).

The aim is not to "keep it still", at least not if the mirror is still warm. Warm air that is still over a mirror will warm up a lot more and act as a lens, and unfortunately it's not going to be very well figured.

The flow is always going to be somewhat turbulent, but if the temperature deltas between the pockets of air being mixed is low, it doesn't hurt that much. Even if there is a temperature difference that you can't get rid of, making the pockets of warmer air very small and mixing them with ambient air at a small scale has different optical effects than if you leave the pockets coherent on a larger scale. There is actually peer-reviewed literature about the effects of scrubbing the air over a mirror and it was referenced on CN, but I can't find it instantly on this laptop.



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sixela
Postmaster
*****

Reged: 12/23/04

Loc: Boechout, Belgium
Re: Can a rear fan remove the boundary layer? new [Re: Javier1978]
      #5428700 - 09/19/12 04:09 AM

Quote:

Hi Sixela,

I just read your 2010 post. Do you recommend to use the fan the other way round to pull air?




Unfortantely, it really depends. The drawback to pulling air (aside from the fact you seem to need a larger fan and to put the baffle somewhat closer to the mirror) is that on some nights it can pick up hot air plumes from your body through the front of the scope, and in windy conditions fight with the natural chimney effect in strange ways, so a lot depends on the temperature difference between the mirror and the ambient temperature, the wind force and the wind direction. So I have my fan set up to be reversible.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Can a rear fan remove the boundary layer? new [Re: sixela]
      #5429603 - 09/19/12 05:26 PM

Thank to everyone for the inputs. It seems to be a complex topic. I will continue trying my actual configuration with the 80 mm fan and the baffle that seems to be working just fine with my 1" thick mirror.

Making the diaphragm doesn´t seems an easy task with so little space between the mirror and the tube.


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rockethead26
Post Laureate
*****

Reged: 10/21/09

Loc: Arizona, USA
Re: Can a rear fan remove the boundary layer? new [Re: john gabriel]
      #5429616 - 09/19/12 05:39 PM

Quote:

Hi everyone. Thought I'd chime in here with a related question. I saw a similar discussion earlier this year (on this CN forum I think) in which fans were mentioned that have a very good score for minimum vibration, and I wonder if anyone knows what make those are or can recall that post. I was pretty new to CN then and forgot to bookmark it.




I don't know if this was what you were thinking about, but it might help. This is the thread where I discussed changing out the fans in my Teeter with extra low vibration, variable speed fans.


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dvb
different Syndrome.
*****

Reged: 06/18/05

Loc: Vancouver, Canada
Re: Can a rear fan remove the boundary layer? new [Re: EJN]
      #5429650 - 09/19/12 06:11 PM

Quote:

If it is strong enough. Most computer fans are not.

I bought an "outrunner" motor used for radio-control airplanes and
mounted it behind my mirror cell with a 9" APC pusher prop. I run it at
10000 RPM. Gets rid of every trace of tube currents and boundary layer.
The only problem is you have to be careful getting your hand close to it,
because the prop can take a nice chunk out of your finger. Collimate
*before* turning on the fan.





I don't think I'll try the airplaine prop, but you do remind me that the most effective fan I used was a household electric fan that I set up behind the scope for a while before I started observing. Seems to have helped me get to ambient temperature in the tube very quickly.

I also have a computer-type fan to mount on the back, but I don't think it is as effective.


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AlBoning
professor emeritus


Reged: 03/06/11

Re: Can a rear fan remove the boundary layer? new [Re: dvb]
      #5429675 - 09/19/12 06:28 PM

Alan Adler’s article “Thermal Management in Newtonian Refractors” from S&T Jan 2002 can be found here …

http://media.skyandtelescope.com/documents/Adler-NewtThermal.pdf

and here …

http://www.crossflowfansystem.com/docs/SkyArticle_01-2002.pdf

Bryan Greer’s article “Understanding Thermal Behavior in Newtonian Reflectors” from S&T Sept 2000 can be found here …

http://www.crossflowfansystem.com/docs/SkyArticle_09-2000.pdf

Greer’s two part article that appeared in S&T May and June 2004 “Improving the Thermal Properties of Newtonian Reflectors” can be obtained by purchasing the back issues in which they appear, here …

http://www.shopatsky.com/category/magazine-back-issues/?m=leftnav_magazine-back-issues

Greer has additional information at his web site here …

http://www.fpi-protostar.com/bgreer/


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davidpitre
Post Laureate
*****

Reged: 05/10/05

Loc: Central Texas
Re: Can a rear fan remove the boundary layer? new [Re: AlBoning]
      #5436113 - 09/23/12 01:23 PM

So what advantage does the rear fan baffled to blow across the front of the mirror have over a rear fan for cooling and another fan blowing across the top of the mirror? Is the Mauro da Lio set-up supposed to be more effective than a fan blowing horizontally over the front of the mirror?

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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: Can a rear fan remove the boundary layer? new [Re: john gabriel]
      #5436403 - 09/23/12 03:43 PM

Quote:

... I saw a similar discussion earlier this year (on this CN forum I think) in which fans were mentioned that have a very good score for minimum vibration, and I wonder if anyone knows what make those are.....




Search 'maglev fan'.
They're naturals for reducing vibration.

dan k.


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cuemark8
member


Reged: 07/01/12

Loc: Imler, PA
Re: Can a rear fan remove the boundary layer? new [Re: HunterofPhotons]
      #5436470 - 09/23/12 04:24 PM

I found that I can push my primary mirror fan beyond 12 volts. I use a 14V DeWalt cordless drill battery to run my fan. It can easily handle it. It runs stronger and faster. I made a set of alligator clips to fit the terminals on the battery.
A good car battery is never 12V. More like 12.6V to 13V. By design that are rated to provide 12V while under load (i.e. cranking).
Another note. Those fans are cheap computer fans and easy to buy. If it burns up it's really not that big of a deal. At least for me.
If your looking to get more from your cooling fan this may work.


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SteveG
Post Laureate
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Reged: 09/27/06

Loc: Seattle, WA
Re: Can a rear fan remove the boundary layer? new [Re: cuemark8]
      #5438007 - 09/24/12 12:27 PM

If you want you can watch these Youtube video's I posted 2 years ago with my 10" Lightbridge. Smoke tests reveal air movement around the primary with and without baffles. There are 7 short videos:

http://www.youtube.com/watch?v=vslnABcyi8k&feature=plcp


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