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Miscellaneous Debris => Weather sensors, design, analysis, discussion, home brew => Tech Corner => Rain Gauges => Topic started by: miraculon on April 29, 2016, 09:03:58 AM

Title: Conical shaped rain gauges vs. straight sided
Post by: miraculon on April 29, 2016, 09:03:58 AM
I was thinking of putting this subject in the Davis area, but decided to make it "generic" and post it here.

I have noticed in the various discussions about the Davis VP2 rain gauge versus others is that there seems to be a common thread regarding more accurate alternative gauges. The other gauges, CoCoRaHS, RainWise, Texas, etc. all have straight sided outer cylinders. The Davis is conically shaped (a frustum).

I have spent some time searching for "aerodynamics of a frustum", but most of the papers are aeronautical and study airflow against the narrow end of the frustum. I realize that the Davis rain collector isn't a true frustum, since it has the funnel at the small end, but this shouldn't matter with respect to side airflow which applies to the rain gauge.

What I am wondering about is whether there is some upward spiral of airflow around the cone outer that creates an updraft effect that counters the rain entering the funnel. If this is the case, this could explain why RainWise, et al seem to have better results.

This is one of those "don't overlook the obvious" things. The Davis is unique for the outer cylinder shape, at least in the pro/pro-sumer market.

Any thoughts on this, or any data like wind tunnel studies, etc.?

Greg H.
Title: Re: Conical shaped rain gauges vs. straight sided
Post by: DaleReid on April 29, 2016, 10:13:46 AM
I wonder how much is functional vs. esthetic design for some of these.

Yet this must get the attention of some, since airports may have those little loin cloth like things hanging around their rain gauge installations to mitigate side winds.  Not sure what they are called but a sort of circle wire suspending panels of metal all a few feet in diameter.

In addition, if you look at the RM Young capacitive gauge, the 50202
 http://www.youngusa.com/products/3/17.html
it has a complex upper portion, although it seems mainly to give a knife edge to the collecting lip and then perhaps just functional on the way down to house the innards.

I can't imagine someone at Young didn't think of the impact on collection that wind would have.

Yet I think that unless you are concerned about the fine mist that seems to be easily disturbed by light drafts, that any impact of the air above the lip would have much impact on the diversion of drops away from the collector.

I wonder if you could get one of those things that makes smoke and see how the air flow around a few of these might play out on a still day with some fans running at a slight distance to give as good of a wind tunnel as you can get without being NASA or NOAA?  I am amazed at the ground effect (air plane term) and when the car advertisers want to show how aerodynamic their car shapes are, the smoke never touches the car, but stays a few inches away as the boundary layer deflects it.

I'll stop rattling but wanted to mention an interesting car I had, an old Toyota hatchback we got as our first ever new car right after we were married.  That thing would seldom need windshield wipers when you were going over 45 mph for most snow and a lot of light rain, and the hatchback would also stay darn clear of snow build up on the way down the road. 

It was a noticable effect and never had another car that did that type of precip clearing.

How do they make those smoke columns on those wind tunnel tests, anyway?  Some huge cigarette behind the scenes?  A nice incense punk?  Just curious.
Title: Re: Conical shaped rain gauges vs. straight sided
Post by: miraculon on April 29, 2016, 11:00:28 AM
I am thinking about buying an incense stick and seeing how that works. I have a feeling that the wind tunnel smoke generators are way more expensive than I want to get into for this experiment.

I used to own a pickup (Ford Ranger) and before I got the cap for the bed, whenever it rained the back window would stay pretty much dry. It amazed me on how hard it could rain and the window would stay clear. With the cap, however it was a totally different story. It would get quite dirty after a few minutes of driving in wet, salty conditions. I now have a Ford Flex with a rear wiper, but the aerodynamics must be similar because the rear window also gets just as bad. At least I can clean it now.

Greg H.



Title: Re: Conical shaped rain gauges vs. straight sided
Post by: miraculon on April 29, 2016, 11:20:05 AM
I found this on YouTube:


It does appear that there is some lift occurring.

There is another version on Flometric's web site that can be made full screen and seems to be more real-time.

http://www.flometrics.com/project/separated-flow-truncated-pyramid/ (http://www.flometrics.com/project/separated-flow-truncated-pyramid/)

Greg H.
Title: Re: Conical shaped rain gauges vs. straight sided
Post by: WXWIZARD on April 29, 2016, 11:29:19 AM
Just take a look at all the countries in the world that have shields for their precip gauges,  the common theme is they all direct windflow downward,  the davis rain gauge cone shape is just the opposite.  That I believe is why the davis gauge is so inconsitant in the studies that have been done.
Title: Re: Conical shaped rain gauges vs. straight sided
Post by: Old Tele man on July 01, 2018, 05:27:11 PM
Just take a look at all the countries in the world that have shields for their precip gauges,  the common theme is they all direct windflow downward,  the davis rain gauge cone shape is just the opposite.  That I believe is why the davis gauge is so inconsitant in the studies that have been done.
That's WHY Davis switched to their new "aero cone."