[05:30] Macrobius: (think of it like preventing air cooling in an engine when you are driving fast)
[05:30] Macrobius: Wind can't convect if it can't get at it. Jes sayin'
[05:29] Macrobius: btw if you are worried about the vehicles I'll bet putting a tarp over them helps a lot
[05:28] Macrobius: (living in a cave at ground temp is probably starting to look attractive...)
[05:27] Macrobius: with those time gradients I'd start worrying about thermal stresses. You'll be lucky if you don't get sink holes
[05:26] Macrobius: When it is predicted -5 -- ca. Wed?
[05:25] OVERWATCH: it was 45 degrees yesterday. its 18 degrees now, this weekend it will be almost 60
[05:23] OVERWATCH: neg five
[05:23] Macrobius: (wind chill is about convection, and vehicle temp is all about conduction and radiation)
[05:23] Macrobius: They might not be a vulnerable to 'wind chill' as you are -- what's the air temp going to?
[05:22] Macrobius: yeah - they use those little light bulb heaters under the hood in MPLS for a reason
[05:21] OVERWATCH: I reckon I better double check the coolant strength in the vehicles before the polar vortex goes into full effect
[05:21] Macrobius: btw, if you don't have one, buy a 'space blanket'
[05:21] Macrobius: You like those uncovered, and 'wind burn' sucks
[05:20] Macrobius: The main problem I recall is about your eyes
[05:20] Macrobius: It's really kinda subjective after about -30 as far as wind chill is concerned. Cover skin or enjoy your new fren, frostbite.
[05:20] OVERWATCH: Chicongo is supposed to have -60 wind chill
[05:19] Macrobius: You're going to have winos stiff as a board in wherever 'downtown' is for you.
[05:19] Macrobius: I remember the Blizzards of the mid 70s got that low in the Milwaukee/Chicago area -- I'll bet it's been that long for them too.
[05:18] Macrobius: Fun fact. -40 F = -40 C. It's the one temp that's true. Also, it's the freezing point of alcohol.
[05:18] OVERWATCH: I dont ever remember it being that low at any time
[05:17] OVERWATCH: the wind chill here is gonna go down to around -40 F
That was me speaking off the cuff as a Physicist -- I'd love to hear followup discussion, anecdotes, etc.
This stuff is actually pretty important. There are three heat transfer mechanisms: Conduction, Convection, Radiation.
Convection comes in two flavours -- natural and forced. Natural convection happens when the ground (and your vehicle in thermal contact with the ground) is warmer than the air temp, so it heats the air which rises, taking the heat way with a buoyancy.
Wind Chill means that *forced convection* (from the fluid flow of the wind) is dominating natural convection. Thus, the cooling power of the convective path is moar dangerous than if the wind weren't blowing.
Now, I bet those vehicles have seen -5 F before (and the coolant in them won't matter too much unless are driving). -40 F (wind chill effective temp) is, it would seem, a 'once in a lifetime event' for them. I think the problem isn't when they are driving and generating heat internally, but when they are sitting outside, with the wind blowing over them.
There are a couple ways to model this... Imagine the wind flowing *over* the vehicle, as over an airfoil. This is a standard problem (you have a hot plate at temp X, and air flowing over it at velocity U -- what is the heat transfer by convection?)
Assuming the wind flow field isn't enough to cause lift off, you can pretend they are just a hot plate (heated by conduction from the ground through insulating tyres, which is inefficient but their main heat source, if the vehicles are idle).
The other 'pathway' is for air to circulate through the engine block, and cool the internals -- my suggestion (use a tarp) was to block this pathway. It's the same reasoning as 'drafts in my house are a bad thing'. There is mixing of colder air -- probably negligible here, since the vehicle is at air temp -- but also forced convection. That could *thwart* the heat flow from the ground, if allowed, and might freeze the engine block, colder than it's ever been. I think a cheap way to stop that pathway is worthwhile -- but I'm willing to hear otherwise.
Under the tarp, I don't think it would hurt to put a radiative barrier either. Colder things radiate less though, by a power of 4 in Temp, so that may not really be necessary (I'd have to do a calculation).
Related topic: Why Use a Refractometer? |
https://www.reichertai.com/clientupl...ractometeR.pdf
