water analogy- a simple calculator

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I guess I've never tried it on complex systems. Mostly I'm just trying to guess what the time constant of the system will be.

There must be some software company that would sell you such a thing. Ever read the book "Hot air rises and Heat sinks" Not all that technical, but an enjoyable read.

George H.

George Herold:

If the car has ABS and you brake at high speed, it seems to barely slow down. As the speed decreases the deceleration increases.

This is due to the fact that the brakes must dissipate the kinetic energy "stored" in the running car without exceeding the tyres grip, energy that is proportional to the square of speed.

Oh it's about doing thermal things with an electrical analogy.

There's John's list. If current -> watts of heat flow then the charge is the amount of heat in Joules. And Q=3DCV so C =3D Q/V and heat capacity is Joules/degree. If you haven't used this analogy I guess it's confusing. But once you get it, you can trun thermal circuits into electrical circuits. Which are just easier for me to grasp.

George H.

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Everything was pretty much frozen here right around Xmass. We snow- shoed and X-contry skied down the creek to Angle Falls.

And then it all melted.

George H.

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$$$$$$$$$$$$$$$!!!

I need to play with Sonnet Lite, the free EM simulator. Maybe it can be hijacked for 2D thermal analysis.

I have it. Not impressed. Steinberg's "Cooling Techniques for Electronic Equipment" is pretty good, but I wish he's stick to SI units. He freely mixes inches, feet, BTUs, deg F, degC, calories, joules, cm, meters, whatever.

John

I haven't stuck an accelerometer in the car, no. To be really probative, it would need a measure of the pressure in the master cylinder anyway, which is more difficult.

Try it on your way home from work--you'll come right off the back of the seat if you don't ease off on the pedal.

I'm not really a mechanical guy myself. (I've known some mechanical guys who were pretty life-like, though.)

I'm assuming that the stiction grabs briefly at each stopping point, then lets go after stretching out whatever little asperities have become interlocked on the surface. That will slow down the acceleration in the neighbourhood of the peaks, but they'll still appear smooth because the stiction loss is all happening while the rotor is nearly stopped. It would cause a slight change in the second derivative of the angle, i.e. a minor flattening and asymmetry of the peak, but no jaggies. Deforming the asperities should take roughly a constant force, independent of amplitude, because (in my simple model) it's limited by the yield stress of the materials. Similarly, the deformation should occur over a distance determined by the size and shape of the asperities.

Both force and distance are constant in this model, so to leading order, unsticking should take a constant amount of energy per cycle. That leads to a square-root time dependence of amplitude, at least in cases where stiction dominates.

As you point out, the energy is nearly all potential near the peaks, so the peak shape has to change a bit to account for the reduced acceleration during the sticking and unsticking. If it were instantaneous, it wouldn't require any work, and so couldn't change the shape of the curve.

Of course! Surely nobody in this august company would be so ill-bred as to confuse an argument with a quarrel. ;) ;)

Cheers

Phil Hobbs

I had a hard time giving up "condenser" and "cycles/second" and "uuF" and "mhos". Which really should have been "smho".

It's a real analogy, one that can deliver useful numerical results, not just fuzzy feelings.

The nasty part about thermal time constants is that they tend to be diffusive, like a super-lossy transmission line, essentially a string of infinitesimal RC networks. Sometimes it gets hard to close a temperature control loop because the phase shifts get crazy.

Which makes me wonder if the Spice lossy transmission line element might be useful in thermal modeling.

John

resistance.

Analogy is like popcorn.

John

By the time you know enough about the detailed thermal behaviour to specify the RC transmission line parameters, you already know the answer to the problem!

Cheers

Phil Hobbs

Cars definitely jerk as they come to a stop. I suspect it's the elasticity of the suspension, all those struts and bushings, that essentially put a spring between the brake housing and the frame. That spring winds up during braking and, when the brakes finally go from sliding to grabbing, there's a spring back.

John

Ignoring brake heating issues, wouldn't the minimal-distance braking strategy be almost constant deceleration, corresponding to constant tire-road force, just short of sliding? Power dissipated would be proportional to speed, but force would be constant.

John

It has nothing to do with tire adhesion, nor to suspension deflection. The jerk happens when you're coming to a stop, regardless of how fast you're doing it. And if the deceleration were constant, the suspension deflection would be too, at least until after the stop.

Also, car suspensions are extremely stiff in the fore-and-aft direction, because otherwise you couldn't control the car. The suspension deflection is essentially all vertical, whereas the jerk on stopping is definitely horizontal. You can watch the front suspension deflect due to the resulting torque.

Cheers

Phil Hobbs

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That part of the country is really beautiful. The Brat went to Cornell, and we got to explore a little. It's different from the mud flats of Louisiana!

We skiied at Thanksgiving and Christmas, and I hear it's still fabulous. It's been cold here, very cold (below 0F at night) in the Sierras. So far, we're having a wet winter. California survives on snowmelt.

John

The electronic div-2 capacitor thing, a charge pump converter, can approach 100% efficiency. I don't know if there's a hydraulic analog.

John

Sure, just a pulley and two buckets. If the weights are the same, the energy supplied by lowering one bucket halfway is enough to raise the other bucket to the same point.

Cheers

Phil Hobbs

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Oh thanks I'll check out the reference. One thing I hate about thermal stuff is all the different units. I can spend more time making sure I've converted the units correctly than in doing the calculation.

george H.

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Cheers

Phil Hobbs

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Yup, but the math still works the same way. Unlike AlwaysWrong, computers are good at it.

Maybe a fields simulator? Lumped analysis works fine, though.

Sure, right at the end of braking it sticks, you ease off and the 'sticking' torque decreases. I think my first encounter must have been 'alllllmost' going through the handle bars of my new three speed, with a front hand brake. I can still see the car next to me and pavement in front of me.

But static friction is no more than double(?) dynamic friction, except for velcro and thistles.

You can get a measure of the static friction by looking at the stopping point.

I don't have any good data here and the demo unit is on a truck coming back from a show.

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OK the distance is fairly short though. A wee bit of streching in the string and flexing of the box.

Ahhh I was going to say I can try fitting some data. But... The loss in clearly nearly linear in amplitude not energy. I can send some better data, but you'll have to page through the manual.

I bet we can fit all that. The data is a lot better than a 'scope can show. The insturment is not mine, and I only poked my fingers into a few places. So it's OK for me to brag a bit.

I'm seeing your 'sticking loss' being larger (proportionally) at lower tensions. Is that right? (a bit of thread gets picked up and carried along, on the return stroke the same bit gets carried back. Same energy loss per half cycle.)

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My pleasure, as always

George H.

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