Tautliners and Bernoulli's Theorem

Make a sketch about the problem with the shape of the veichles for a better understanding.

O,K, Folks, Let's integrate the actual question!!

From the point-of-view of pure Flight Dynamics, ALL: of the Lift and Drag equations apply. (PERIOD). The particular shape of the vehichle and/or its relation to Ground Speed, are not material. Hard to do, but think about an Air Tunnel of about 3m greater than the dimensions of the Bus. Then, start to think about laminar flow in an environment approx. 6m wider in ALL Dimensions. OK, so far??

Now, rem, that the sides of the vehicle are a dynamic environment (this factoid thrown in for those that don't know it DOES NOT make a diff).

At this point, I'm going to only ask the question: Do "Bernouilli's Statements (3)", actually state that there will be a decrease in pressure.

At GSL there is 0.075 lbs/ ft. (Cubic) , By the FAR Tables (45, E) there only remains approx. 0.0045 lbs/ ft. (Cubic) at 35,000 ft.

I throw this in, only to ask the Density, sub, Air question.

So, Is your air flow laminar or turbulent??

Thanks for allowing me to let my mind go...

GLB

REM : "Any sufficiently advanced technology is indistinguishable from magic." RAH

I am intrigued by the Stop Abusing Bernoulli page. To think, I had never considered Newton's third law when contemplating lift and flight. Shame on me. I have misguided my students.

I appreciate this last answer concerning the laminar flow versus turbulent flow. I have observed these trucks and have seen different results in the fabric that would correspond to different cab designs. I have seen on occasion a very sleek aerodynamic truck with the fabric on the trailer concave at the front of the trailer and convex toward the rear of the trailer. How would we consider this? It would seem that the flow is turbulent toward the front and laminar toward the back. However, we could use the information provided by Stop Abusing Bernoulli and consider that it is merely Newton's laws in action!

Fantastic discussion!

It was about 15 years ago when I had the epiphany that most of what we're taught in the USA government schools about our history and laws, let alone aerodynamics, is false. It's amazing what happens when you start questioning stuff.

Which is why homeschooling kids in a more Socratic, classical method (the trivium of grammar, logic, rhetoric) has had such amazingly good results among demographics that should (by government reasoning) do poorly.

As a kid I never even thought of insect wings when people tried to force that hump theory on me...

I agree, great discussion. I'm newly registered here and find this forum addictive. I need to get some work done though lol!!

Don't get sucked in by these whack-jobs. Bernoulli's theorem is well documented and backed up by mountains of experimental data. The apparent high pressure on the outside of the truck is because the air flow around the truck body is most definately not laminar. Just pass one on the highway and you will instantly be subjected to massive turbulence. Don't want to do that? Look at the exhaust smoke and see just how laminar the flow appears to be. The answer to the inward bowing walls is simple the truck leaves a enormous low pressure region behind itself at speed. This tends to pull the air from inside the trailer back to fill the low pressure region resulting in a decrease in the pressure inside the truck body. It only takes a fraction of a psi to result in tremendous force... Use caution when using the internet to do "scientific" research. Owning a web site does not make you qualified to do anything except pay an occasional bill from an ISP.

How would you take into consideration Newton's third? There would be an apparent violation of Newton's third with the Bernoulli's demo of blowing air (with a straw so that it is laminar) under a paper supported by a couple of books. I guess the question is not if Bernoulli's applies here, but rather, "where is the reaction force?"... all right... OK... I'm going to draw a diagram and do some math and figure this one out....

I would agree, however, that there is a variety of factors that come into play with a wing. Perhaps we discredit some and over emphasize others.

Ok, I've been thinking about this for a whole five minutes and have come to the realization that I have completely forgotten about the airmass above the low pressure area above the wing. The reaction force would be pulling down on this air mass as well. Also consider Magdeburg hemispheres; with a small vacuum, it takes an incredible amount of force to separate them. So... We establish a laminar flow creating a decent low pressure and then we have lift and a considerable amount of due to the pull on the air mass above and below the wing...simple physics!

I'm sure I'll waffle on this for a while until I have analyzed it completely!

Don't look at it as air blowing by the truck (as in a wind tunnel), but rather consider the truck "plowing" through the air. As the blunt nose of the tractor pushes the air aside (and upward), this creates a local high pressure that the truck must pass through. The pressure acts (albeit with turbulence) against the sides of the canvas, pushing it inward. This localized high pressure is finally relieved at the back of the trailer, when it turbulently returns to "fill the void". If the truck were aerodynamic in the vertical plane (like a wing standing on its root chord), the canvas sides would indeed be "sucked" outward, as Bernoulli predicts. Many truck cabs are designed with some streamlining in the vertical plane, but for obvious reasons, the horizontal plane remains somewhat rectangular. The non-aerodynamic features of a box flying down the highway induce so much turbulence, separation and buffeting that Bernoulli's principle is insufficient to predict its characteristics. Hold a thin cardboard box (e.g., a cereal box) out of your car window at 50 mph and you'll see the same thing: the sides are "pushed" inward.

Awesome post. Straight forward, no nonsense and correct answer. Your observation is right on, in my opinion. This is the way I see it happening.

I think guest (post #11) had it right, and turbulence should not significantly alter the overall picture. There appears to be an assumption in the above 'explanation' that the pressure inside the container is at some generalised "atmospheric" pressure; but the internal pressure will correspond to the pressure at the dominant vent, where-ever that may be.

Properly executed, the cardboard box experiment will bear this out: initially make a sealed cardboard box, and puncture (then cover) holes in different places (one at a time) while you do the tests*. Hole at the back = sucked in. Hole at the front = blown out. Holes elsewhere - I await reports (I don't have anywhere safe-and-legal to perform the experiment).

Fyz

__________________________________________________________
"Everything should be made as simple as possible, but not one bit simpler." Einstein

"(I don't have anywhere safe-and-legal to perform the experiment)."

LOL! Don't forget the safety goggles, you don't want OSHA down there.

Actually, the air is dragged along with the vehicle in the boundry layer pressure field which surrounds all moving objects in fluids or gases. What you are seeing is the result of high pressure air being redistributed from the front of the vehicle to the sides. Ride a motorcycle on the highway and you'll develop a sincere appreciation of just how much air pressure a large truck can develop.

The air pressure inside the trailer is at atmospheric, no air movement at 14.7psi. The high pressure outside wall of the trailer is greater than 14.7psi. The higher pressure side will push in towards the lower pressure side. How much is based on the wall's structural rigidity and the amount of affected wall surface area. The trailer manufacturer has to road test this to comply with DOT regulations and standards. Some trailers are ribbed, or have rivets and sheetmetal overlap. This drag on the trailers wall surface pushes on the trailer wall also.

Concave or convex? - it depends where the cavity is vented to the outside world. Typically, the vents are at low pressure points (hard to avoid the way the sheeting is rigged, and a good thing too). So the pressure inside the body of the truck is below the average pressure outside it, and the sheeting pulls inwards. (That's also part of the reason some of the vents in a car are usually in the rear pillars).

It's still all down to the "Bernoulli effect", I'm afraid

Fyz