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]

do you want to include all the little things such as differences in ride height, angle of the cab, and the shape of the tires, or is there a specific effect you're hoping that someone brings up? Also, why do you ask about turbulence? Is total aerodynamic drag what you're after, or turbulence itself (and if the latter, quanitified how and where?)?

 

[twobits]

Thank you ivymike for the questions.

Each truck is an exact duplicate of the other. All elevation or section views would look identical. A plan view would be identical. The same tires, mud flaps, the same everything.

Let's consider the turbulence at a distance if a motorcycle was passing the truck in the middle of the next lane. Don't take into account the motorcycle's velocity or acceleration. That would make it way too complicated. Just a distance of eight feet from the side of the truck. Let's include the total drag also, good point.

 

[TheTick]

Five minutes in the penalty box for saying "high rate of speed". Such talk is acceptable for idiot newscasters but not for engineering professionals! If speed is constant then the "rate of speed" = 0. Do you mean they are accelerating?

 

[ivymike]

okay, assuming that the mass of the trucks did not influence their shape...

The turbulence experienced by the passer-by and the drag on the truck are both determined by:
* The shape of the vehicle (assumed same)
* The state and properties of the fluid through which it travels (assumed same)
* The velocity of the vehicle (assumed same)

I'm going to have to go with "the turbulence is the same, the drag is the same" but add that I don't mean that every individual molecule of air would approach the motorcycle in the exact same way in each case, just that the measurable characteristics of the flow around the vehicles would be very very similar.

I suppose that there might be some immeasurably small difference due to the influence of gravitational attraction between the truck and its surroundings, but considering that is probably less important than counting all of the smashed bees on the bumper and mirrors.

Why, what'd you think you'd hear?

 

[jistre]

If we're talking completely identical trucks except for the mass and ideal conditions, the mass difference does not affect the onset of turbulence. Take a look at the formula for the Reynolds number, and there is no mass term for the body. The Reynolds number is the ratio of inertial effects in a flow regime to the viscous effects. Turbulence occurs when the inertial effects at a given scale overwhelm the viscous effects at that scale, so a high Reynolds number indicates that the inertial effects have become dominant and turbulence is occurring. Since the Reynolds number has no term for the body's mass and cannot be affected by it, turbulence is not affected by the body's mass and depends on the fluid density, the fluid visocity, the flow velocity, and the characteristic length of the body. So, two identical bodies save for their mass in two identical flows will generate identical Reynolds numbers and should have identical levels of turbulence.

Applying the same argument to the equation for the the force generated due to aerodynamic drag, there is again no mass term for the body. Since the mass of the body does not appear in the equation, the math says that bodies identical except for their mass in identical flows will experience identical aerodynamic drag forces.

What should change between the two are the EFFECTS of the turbulence and the drag. If the magnitude of the forces on both the bodies due to turbulence and drag are equal, then F=m*a shows that the lighter body will experience more acceleration due to the the turbulence and drag.

 

[handleman]

Drag, turbulence, etc. are related only to the external geometry. Mass has nothing to do with it, except in that if you have two identical trucks, one loaded and one empty, the load on the one truck will affect its external geometry as IvyMike mentioned. The only other possibility I can think of would be if the empty truck were so light that its own turbulence could alter its path through the air by bouncing it around.

I seriously doubt that either of these possible differences would be noticeable to a motorcycle rider, whether at 8 feet or 8 inches.

 

[twobits]

TheTick,
5 minutes in the penalty box is acceptable for idiot sportscasters but not for engineering professionals.
How can speed mean anything other than velocity? Speed is distance per time, velocity. A reference to rates of speed is merely implicating different values of velocity. Come on now theTick. A high rate of speed is implicating a reference of velocity. A velocity of moving slow to a velocity of moving fast. I believe most everyone picked up on it. The only rate of speed that would be zero would be if there was no change in distance for a given amount of time. Here is a thought you might like, rate of interest. We all know what interest is but the rate is quanitfying the interest. Let me get back to the question posed.

The question comes from observations and a genuine interest to answer questions. We approached it using different equations. I did find a reference to a body moving through air, air with no wind, air not moving. It referred to the turbulence increasing relative to the velocity squared. Unassociated, the equation of F*d=1/2*m*(velocitysquared) is one that I remember from years ago to evaluate a force to stop a body over a defined distance. This equation has nothing to do with the question posed but is interesting. If the two trucks I was talking about all of a sudden lost their source of power propelling them the heavier one would travel further before coming to a stop. It's force was greater. We all understand that and it is not a part of the question posed.

This is just something I thought interesting. Have you ever stood along the side of a road(hopefully not because your bike ran out of gas), or maybe under an overpass while you're waiting out the rain before you get back on the road and notice differences in what you see? I cannot find any equation where mass would make a difference. I just thought I might ask.

 

[ivymike]

perhaps you would also find the concept of "ballistic coefficient" interesting...

http://en.wikipedia.org/wiki/Ballistic_coefficient

 

[KENAT]

twobits, I'll assume for the first part you mean the trucks have constant velocity (while it may seam like semantics on the whole rate/speed/velocity when I read your OP I too started to wonder if you meant the trucks where rapidly accelerating).

As with the others, if all other factors are equal I'd expect no observable difference in the aerodynamic effects soley due to the mass difference.

If they aceelerate at the same rate again aerodynamic effects will be the same assuming everything else equal.


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

 

[TheTick]

Consider this:

How many wind-tunnel experiments validate shapes that are carved from solid blocks that represent objects that are basically hollow in real life?

 

[RossABQ]

The primary difference the motorcyclist will notice is that the heavier truck will be pouring sooty black smoke up the rider's helmet. It can go anywhere but always seems to end up there.

As an aside, during the '73 oil embargo, when 55 mph speed limits were first imposed, the truckers were whining that the measure saved no fuel compared to 65 but cost them a lot of time. One of my professors at Univ. of Illinois did a test and found that there was very little difference in the total drag (and fuel consumption) on a semi-truck at 55 and 65, seeming to verify the truckers' claims. (Remember this is pre-Anteater KenWorths, typical block-shaped Macks).

 

[handleman]

Speed (in this context) is defined as the rate of change in position per unit time. Speed in itself is a rate. It is just as redundant to say "rate of speed" as it is to say "ATM machine". Rate of interest is different. Interest is the actual money deposited in your account. The rate of interest is how quickly the money accrues in your account.

Shall we move this discussion to the Grammar forum?

 

[twobits]

This was fun but I have to get back to work. I'll be back later with something else to stir you up with.

 

[CorBlimeyLimey]

twobits ...

Did you not notice the at the beginning of TheTicks post? That meant he was just having a 'poke' (i.e. a friendly dig) at the use of such a non-technical phrase. The phrase "at a very high speed" would have sufficed; "rate of" was superfluous.

Sometimes the humour here can be very subtle, other times it's like being hit over the head with a 2x4 or .

 

[MintJulep]

I agree that if we are talking DRAG only, mass does not enter the equation.

However, consider this:

A truck moving through the air is likely to produce a net vertical lift (or downforce).

Since Lift and Drag are related, is it possible that there is a second-order effect?

I'm thinking not.

If I recall correctly, L/D is a function of angle of attack.
Since our case is a constant aoa the lift must be constant wrt drag, and thus must also be only a function of geometry, and not of mass.

 

[jistre]

I just thought of a way where the heavier truck could be felt more.

It swerved trying to miss the lighter truck and ran over you.

Seek medical attention.

Now, let's get a little out there and depart from the equations and think about the real world. As these trucks are barreling down the highway, bearing down on you, some of the energy generated by their engines is going into overcoming the drag and setting up the turbulence. The energy put into the air sets it swirling and some of it has to push back against the truck. Think of vortex shedding and how it can destroy structures. Now, some of that energy is going to rock the truck and make it move against its suspension, dissipating some of it. It reasons that the lighter truck will tend to be able to rock more than the heavier truck due to differences in inertia, natural frequency, etc. Could it be that the lighter truck is able to dissipate more of the energy it put into the air via its suspension than the heavier truck is able to? Then, even though they generate the same forces when they fly down the highway, the heavier truck is unable to reabsorb as much of it, and releases more net energy into its surroundings (you) than the lighter truck can.

I got thinking about crumple zones and thought of this. It's what happens as the afternoon drags on. The mind wanders.

 

[ivymike]

In any case, lift would not depend on weight for the example above.

If you were talking about a pair of gliders, and you held angle-of-attack constant, then drag would be the same, lift would be the same, and one of the two planes would hit the ground much sooner than the other (guess which).

 

[minerk]

To expand on theTick's post regarding wind tunnel experiments:

Let's imagine the trucks are stationary and the fluid (air) is flowing around them. Does the mass of the truck have an effect then? No.

As jistre has pointed out there may be some other effects that are possible. I'll add that the heavier truck would also flatten the tires a bit more, resulting in less ground clearance...again, probably not altering the turbulence enough to measure or observe.

 

[MintJulep]

Actually no.

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

Or more correctly, the thrust for a glider is the component of the lift vector that is aligned with the flight path. At the same aoa the heavier glider has more thrust than the lighter glider, and thus goes faster (albeit with higher drag) at the same L/D.

That's why gliders have the capability of adding ballast.