A question posed 1

[twobits]

In discussion with others a question was raised.

On a calm day there are two identical trucks pulling identical trailers at a very high rate of speed. They are on the same road and the distance between the two is far enough to not affect each other. If one truck has a far greater mass than the other, is the air turbulence different from one to the other?

Also, if equal acceleration was experienced would the answer be the same?

 

[ivymike]

yeah, that's right.. I was trying to make a "gliding truck" in my mind - where speed could be held constant too. Then it would have inadequate lift to fly, and would fall to the ground.

 

[jistre]

Actually, I'd pay to see a semi fall to the ground from a couple of thousand feet.

It would be spectacular.

 

[TheTick]

At the same aoa, a heavier glider needs to generate more lift. It does this by flying faster than the lighter glider.

And the glider flies faster by falling faster, usually by increasing aoa. No free lunch for gliders.

 

[MintJulep]

Not by increasing aoa, but yes, if you are going faster on the same downwardly inclined flight path (same aoa), than you are going down faster.

The advantage comes when you leave the idealized world of "still air", and start flying in air with vertical currents.

By flying faster, you can spend less time in sinking air, producing a considerable performance advantage in distance traveled over the ground vs altitude loss.

 

[rb1957]

nah, i don't buy it ... if a heavier glider was beneficial, then they (the glider pilots)'d be packing on the pounds.

i think given two identical gliders (or trucks?) the lighter one will perform better. i think it'd gain more height in the thermal, and have a better glide path between.

 

[GregLocock]

I guess all those ballast tanks on gliders are there to hold drinking water then?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[KENAT]

Trim ;-)

KENAT, probably the least qualified checker you'll ever meet...

 

[dvd]

It would be interesting (and off topic) to discuss the momentum change when dropping water ballast from a glider in flight. I imagine that the momentum equation would be a good place to start, but since my fluids books are all upstairs, I can imagine the glider speeding up as the water was being jettisoned. The direction of the discharge nozzle could also come into play.

Otherwise, I agree that vortex shedding would account for huge differences in velocity and pressure felt from passing trucks.

 

[TheTick]

Anyhoo...

I think we can safely say: identical shapes in identical conditions = same aero effects regardless of mass.

 

[sailoday28]

twobits and TheTick,
Velocity is a vector. A speed of 30 mph north and a speed of 30 mph south are different.

Also a constant speed, say 30 mph on a turn or curve is an acceleration.

With regards to impact of mass, I agree that if everything else is identical, mass is irrevelant.

Regards

 

[Kwan]

Nope, mass not be irrevelant. Mass will affect the dynamic condition of the trucks. The difference in mass will affect the vibration characteristics of the truck and will change the drag (think about aeroelasticity). However, this difference is minimal and would be considered irrevelant in the real world. As a question in a collegiate course this may be significant as we all know college engineering course do not teach real world situations. hahahahah just my humble opinion.

 

[sailoday28]

Correction to my last post. Mass will matter when acceleration occurs. Forces will vary because there is acceleration.
Both vehicles going around the same radius turn at the same speed will be accelerating and different mass means forces on the vehicles will be different.

 

[GregLocock]

First order: there is no difference

Second order: ride heights will be different. response to buffeting will be different. etc. Whether these produce a MEASURABLE difference is another matter entirely.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rmw]

If we are going to get collegiate and discuss all aspects, then the tire heat from the heavier vehicle would have to be considered with respect to vortex shedding etc around the tires and undercarriage, minuscule as that might be, but collegiate none the less.

The loaded radius of the tires would change also contributing to the difference in the ride height of the 'box' so the undercarriage air currents would be ever so slightly different making again an almost negligible difference.

The exhaust flow from the engine and the engine cooling flows would be different, for what ever minor effects those would have on the aerodynamics of the 'box'.

My vote is that there is no real world difference but a stimulating discussion none the less.

rmw

 

[TheTick]

different ride height = not identical

 

[JStephen]

There are occasions where a body oscillates with the fluid flow around it, and in that case, the body mass would enter into the design. An example would be the design of a tall stack, where wind-induced oscillation is a common problem.

However, I'm not aware of any such effect on a truck. For this to be happening, there would have to be a wind-induced oscillation of the truck, and I've never noticed or heard of anyone else noticing such an effect. Trucks are often rough-riding, but that is due to ground effects, not air-induced vibration. You may get occasional vibrations in a tarp or tie-down, but I think this would be imperceptible in the overall wake of the truck.

 

[IRstuff]

Near as I can tell, the air suspension ride height will only change by about an inch from empty to full load (pg. 18):

http://www.hendrickson-intl.com/pdfs/trailer_PDFs/l761.pdf

TTFN

FAQ731-376

 

[MintJulep]

Do trailers with air springs have leveling valves to maintain a constant ride height?

 

[rmw]

Yes. MCI motor coach busses don't however. (Or at least the older models didn't.)

rmw

 

[KENAT]

So I made an assumption, which maybe I didn't state clearly enough "if all other factors are equal " was meant to cover that the tires, ride height... were all compensated to have the same geometric properties on both trucks.

Agree that if they are identical in just being the 'same make & model' then one say empty and the other up at max load there would in practice be slight differences.

KENAT, probably the least qualified checker you'll ever meet...