Why do passing trucks suck instead of blow?

[JD_and_Beastlet]

Nope, it's not the first line of a joke or a rhetorical question.

I would expect a passing truck or bus to be pushing a wave of high pressure air in front and beside, but I've noticed time and again that passing trucks and buses tend to pull our trailer in towards them rather than push it out. It's a point of curiosity for me and I'm interested in the explanation.

[Slowmover]

Tension on the hitch rigging determines that. Slack is a problem. One big push is all it takes.

On a grade descent it's especially critical to control for the crosswinds, natural or man made. A close passing big truck in this stretch is dangerous. Literally.

With a VPP hitch I don't feel the big truck bow wave on the trailer. But I did as it reached the truck bed with cap. Added a rear antiroll bar where none existed and upsized the front to compensate. A Panhard Rod is next. Tire pressure on TV is according to load (inside vehicle manufacturer range) to maximize footprint. TT tire pressure is to sidewall max to resist sway and run coolest.

This trailer came from factory with shocks and suspension crossmembers. Low ride height. I'll likely find a way to increase wheel travel, preferably with independent suspension upgrade. Short of that, with spring equalizers that help the wheel travel problem. As a reference, an Airstream will slide sideways where a plastic box will roll over.

Today's trailers with high clearance for slides should control for winds underneath trailer body, IMO. Flexible side skirting would be my choice. Conveyor belting is one suggestion.

A truly aero trailer has crosswinds pulling at it as they cross over. A box allows wind pressure to pile up along the length, increasing in force. The greater amount of wind also passing below the TT exacerbates the problem. The TT is lifted as it is also pushed over.

[Vulcan_Rider]

JD and Beastlet wrote:

I think we're splitting hairs here.

Probably.

The term "sway" for me means a long period oscillation and the effect of a big vehicle passing really doesn't cause but one cycle......unless the driver over-compensates.

[JD_and_Beastlet]

Vulcan Rider wrote:

JD and Beastlet wrote:

In my case I'm afraid you're both wrong. My ProPride 3P hitch eliminates sway, so the entire rig gets pulled in towards passing trucks.

And I'm afraid NOT.
What we are talking about here is NOT "sway".
It is a tiny but perceptible TURN in response to external forces.

I think we're splitting hairs here. If you want to call it a "TURN", go ahead, but I perceive no turn - just the sensation of being drawn in towards the passing truck. It certainly doesn't alarm or worry me, though it did, a little, for the few trips we took in 2008 before we installed the ProPride 3P.

[4X4Dodger]

Vulcan Rider wrote:

4X4Dodger wrote:

Sorry but the effect is the same for cars, trucks and other vehicles.

And the others are correct this is what is happening when a truck passes you.

If you are really attuned to your vehicle you will feel the initial push of the bow wave, then comes the low pressure area or feeling of being pulled in.

Once again, contradictory statements. The last one is mostly true and explains the question originally asked here. If you are being overtaken, that initial bow wave hits you in the BACK and makes you feel that you are being pushed/sucked in the opposite direction.

You really do not feel the low pressure area, hopefully, because it remains within a few inches to maybe a foot of the big vehicle in question. OUTSIDE of that area, the pressure is higher.......UNITL you get to the back where the pressure wave collapses and a vacuum is formed.

And none of that really has anything to do with the Bernolli effect.

You are right this is NOT LIFT. Using a wing and the effect of lift does not apply here.

When a truck does pass you though you will feel the "pull" of the low pressure after the turbulence of the bow wave has passed you. Keep in mind that you may feel another bow wave at the front of the trailer as it passes you, but smaller and more turbulent. But the low pressure will pull you towards the truck after that. At the end of the trailer there is yet another low pressure area right behind and caused by the trailer. This will also tend to pull you in that direction.

Again the wing analogy is completely off base here.

[Ron_Gratz]

RECOMMENDED READING

International Vehicle Aerodynamics Conference 2014

"Experimental Investigation of Aerodynamic Effects During Overtaking and Passing Maneuvers" beginning on page 31 can shed some light on the subject question of this thread.

The pressure variations generated by a passing 18-wheeler will be different from the effects of the "van" studied, and the effects on a TV/TT combination will be more complicated than the effects on a car, but you should be able to infer how the spatial and temporal pressure variations might affect a TV/TT.

The answer to the OP's question, as indicated by mowermech in the fourth post, is "Passing trucks both suck and blow".

Ron

[Vulcan_Rider]

4X4Dodger wrote:

Sorry but the effect is the same for cars, trucks and other vehicles.

And the others are correct this is what is happening when a truck passes you.

If you are really attuned to your vehicle you will feel the initial push of the bow wave, then comes the low pressure area or feeling of being pulled in.

Once again, contradictory statements. The last one is mostly true and explains the question originally asked here. If you are being overtaken, that initial bow wave hits you in the BACK and makes you feel that you are being pushed/sucked in the opposite direction.

You really do not feel the low pressure area, hopefully, because it remains within a few inches to maybe a foot of the big vehicle in question. OUTSIDE of that area, the pressure is higher.......UNITL you get to the back where the pressure wave collapses and a vacuum is formed.

And none of that really has anything to do with the Bernolli effect.

[wnjj]

BarneyS wrote:
If you are going to continue the discussion on this subject, please - when you quote somebody - edit out the previous quotes so your quote only contains the last statement you are responding to.

Got it. I edited a few old ones to clean them up. I normally try to clip the original back some. I didn't on a few of these for a few reasons:

1) Ipad's are huge pain to selectively edit on.
2) Many of the responses to my posts referred to mine in such a way that deleting mine left nothing for context. Things like "wrote that" and "(A)". Maybe in those cases no quote at all is better.
3) In my most recent post, I responded to all of the statements so left them all in.

[BarneyS]

If you are going to continue the discussion on this subject, please - when you quote somebody - edit out the previous quotes so your quote only contains the last statement you are responding to. The way you are doing it now just make the page require scrolling down to read and adds nothing to the discussion. We don't need or want quotes within quotes within quotes etc.
Thank you.
Barney

[wnjj]

Vulcan Rider wrote:
Some person, supposedly on the NASA payroll, wrote that. I certainly hope it doesn't represent an "official" position from them because......it is absolutely wrong.

The Bernoulli principle only applies to a specific set of circumstances: an enclosed tube and fluids. It is what makes carbs work....and some atomizers and some paint "guns".

The space between vehicles is not enclosed AND the things being "moved" are not fluids; they are big chunks of metal.

How can you say Bernoulli only applies to an enclosed tube and fluids and follow up with a carburetor as an example?? A carburetor venturi is an hourglass-shaped (i.e. necked down) airflow device. When the intake air passes through that confined space, the pressure drops. Fuel flows because the float bowl vents are at atmospheric pressure while the pressure in the airflow within the venturi is lower. It's actually a great comparison to two vehicles travelling beside each other.

When you say the vehicles are moving and the air isn't I agree but aerodynamics is all about the relative difference in speeds between the air and the objects. That's how wind tunnels are used to test out real world performance. The only difference between the wind tunnel and heading down the road is the air speed relative to the road surface, which is negligible compared to what's going on around the vehicles themselves.

If, as you claimed, more air in a smaller place always creates higher pressure, how do you explain the concept of a diffuser on a race car? Diffusers cram all kinds of air into a small space under the car to help "suck" it down to the track.

If you write NASA to tell them they're wrong, you'd better add a bunch more to your list.

http://www.caranddriver.com/features/the-physics-of-diffusers-how-to-make-a-car-really-suck-feature

http://www.racecar-engineering.com/technology-explained/diffusers-engineering-basics-aerodynamics/

https://en.wikipedia.org/wiki/Diffuser_(automotive)

[4X4Dodger]

Dick_B wrote:
Nope, you are all wrong.
The real explanation is that the truck IS pushing air against the sides of the trailer and tow vehicle. BUT the least stable part of that assembly is between the trailer wheels and the hitch. When that part of the assembly is pushed away from the truck it tends to also move the back of the tow vehicle away from the truck which tends to turn the front of the tow vehicle TOWARDS the truck thus giving the impression of the truck sucking in the tow vehicle.
You're welcome.

Sorry but the effect is the same for cars, trucks and other vehicles. This is why if you look at the nose cone of any launch vehicle (a rocket to you) it is not a sharp point but a dull rounded shape. In fact that shape is one of the most design intensive parts of the vehicle.

What it does is create a bow wave of air which creates a low pressure area behind it. This reduces drag all along the length of the Launch vehicle. Resulting in less fuel required to exit the atmosphere.

And the others are correct this is what is happening when a truck passes you.

If you are really attuned to your vehicle you will feel the initial push of the bow wave, then comes the low pressure area or feeling of being pulled in.

This effect is easily and readily accommodated when towing when you know what to expect. And why it is happening.

[Vulcan_Rider]

wnjj wrote:

NASA wrote:
An example of Bernoulli's principle in the real world is a car passing by a truck. The wind rushing between the two vehicles acts like a venturi tube. Because the pressure is less between the two, the car is pushed toward the truck by air pressure on the other side of the car.

Some person, supposedly on the NASA payroll, wrote that. I certainly hope it doesn't represent an "official" position from them because......it is absolutely wrong.

The Bernoulli principle only applies to a specific set of circumstances: an enclosed tube and fluids. It is what makes carbs work....and some atomizers and some paint "guns".

The space between vehicles is not enclosed AND the things being "moved" are not fluids; they are big chunks of metal.

The tiny vacuum that is created in a real Bernoulli effect is actually caused by the air rushing past the little hole in the side; no hole in the side, no vacuum.

[BenK]

Forgetting all about the many variables for this academic airfoil discussion...

The 'tube' is in this case is not symmetrical, nor is it smoothed surfaces,
nor it with both/all sides at the same speed relative to each other *AND*
the surrounding surfaces (pavement), nor is this a designed airfoil, nor
are the two surfaces attached to the same mass, nor does all of the examples
while in school...or current examples have pivot(s) on the surfaces in question,
and a big etc

*IF* one was to take the airfoil example, then go there to then ask which
one has more surface and mass

Which one has the better shape to create lift?...like these examples:


airfoil airfoils airfoilexample

[wnjj]

Vulcan Rider wrote:

wnjj wrote:


(A) This in turn accelerates the air to a higher speed within the "tube" which reduces the static pressure throughout the space between the vehicles.

(b) This is why there's a bow wake pushing you away followed by the suction pulling you in when another vehicle passes you.

(A) No it doesn't.
Your two statements above are in direct conflict with each other.

(B) UNTIL the overtaking big vehicle actually gets it's back end up to or slightly beyond your back end, the pressure wakes between the two are additive and you can't go fast enough to negate that.

This discussion is ridiculous. I quit.

Maybe you'll believe NASA. Figure 2

NASA wrote:
An example of Bernoulli's principle in the real world is a car passing by a truck. The wind rushing between the two vehicles acts like a venturi tube. Because the pressure is less between the two, the car is pushed toward the truck by air pressure on the other side of the car.

[Vulcan_Rider]

wnjj wrote:


(A) This in turn accelerates the air to a higher speed within the "tube" which reduces the static pressure throughout the space between the vehicles.

(b) This is why there's a bow wake pushing you away followed by the suction pulling you in when another vehicle passes you.

(A) No it doesn't.
Your two statements above are in direct conflict with each other.

(B) UNTIL the overtaking big vehicle actually gets it's back end up to or slightly beyond your back end, the pressure wakes between the two are additive and you can't go fast enough to negate that.

This discussion is ridiculous. I quit.