Helmholtz resonator vs. bluff body wake region

[shovel]

I was doing some acoustics work (horn design) a few days ago and, being aware that acoustics and aerodynamics are effectively the same as each other, it occurred to me that there could be some benefit to applying capacitance to the wake area of a vehicle moving through air.

Being more specific, a blunt-tailed vehicle traveling through air at a given speed will leave behind a low pressure wake, and that wake will inherently resonate in the same way that a bottle resonates when you blow across it or a vented speaker enclosure resonates. This is why, for example, a tarp would flap about if you dragged it behind your vehicle.

If you could isolate the fundamental frequency of that resonant wake at a given target (air) speed (65mph? ) it would be possible to build a cabinet, chamber, whatever you want to call it that resonates at the same frequency and may significantly clean up the vehicle's wake as a result.

As an example, if you had a SUV whose wake naturally resonates at 7hz and you could build a lightweight "backpack" to fit inside its tailgate (you choose if you want the ugly part inside or outside the vehicle) occupying 30 cubic feet, with a vent around 16 square inches cross-section and 54 inches deep - that should go a long way toward cleaning up the shape of the vehicle's wake. However, that's a pretty huge enclosure with a difficult to manage vent! By cutting the enclosure size down to 10 cubic feet, with 16 square inches of vent 18 inches deep... you lose some effectiveness but the size becomes easier to manage - thankfully reality is fractal so there is a lot of room for optimization.

Since the wake's resonant frequency is completely dependent upon airspeed, air direction and the shape of the vehicle it would be most useful if some mechanical means were employed for tuning the helmholtz resonator in realtime, potentially using the same type of feedback system employed in noise cancelling headphones or simply an aircraft-type pitot speedometer.

So - any other thoughts on this?

I have no plans to build such a creature in the near future, it just seemed a novel approach I haven't seen anyone propose before so I wanted to submit it for discussion.

[raylit20]

Very curious! I love the concept and I will keep this in mind for future experiments at school (I'm a Mech. E. student).

[some_other_dave]

Probably varies with density as well, so temperature and altitude and humidity would also play some role in the frequency. (Could be too small to measure, though.) A neat idea! I wonder if the resonator could be dynamically altered for different air speeds/conditions?

-soD

[shovel]

Originally Posted by some_other_dave

I wonder if the resonator could be dynamically altered for different air speeds/conditions?

A range of tuning frequencies could be selected by telescoping the port, in a manner not drastically different from how a trombone is tuned. This is why I suggested using either a feedback system or a speed-based system for tuning such a device to its conditions, or at least to the vehicle's road speed.

Even a fixed tuning frequency would be functional over a range of speeds - and it wouldn't really be of much use at city speeds where acceleration/braking make up a greater share of a vehicle's fuel consumption - nor would it be of much use at speeds far greater than a combined highway cruise speed plus whatever headwind speed we might expect to encounter. A specific unit tuned for its host vehicle at 65mph road speed with 0 airspeed would still be within its effective tuning range against a 10mph head wind, or following a 10mph tailwind... and so forth.

I also have no idea how much benefit, overall, could be expected from this. I know that a correctly tuned helmholtz resonator attached to the bell of an acoustic horn can partially mimic the effect of a longer horn, with a larger bell - and intuition tells me that the application of this to aerodynamics would mean mimicing the effect of a longer boat-tail with a smaller wake area - but this would be extremely application and condition specific. There would be very little likelihood of a one-size-fits-all product. If anything, this idea would be best integrated into a vehicle from the factory - using structural cavities inherent to the vehicle's construction as the resonant chambers, and locating the vents where they will operate to greatest effect. The whole idea may also be completely idiotic and not offer any benefit whatsoever

I will spend a little more time refining this idea in my mind, and of course absorbing any discussion that takes place here.

I think that a practical test could be made by anyone with a van/SUV, a hitch-mounted cargo rack, and enough spare lumber and junk to build a crude box with a means of port tuning (making the port in the form of an external trombone slide, from PVC pipe, might be a good start) - then measuring the resonant wavelength of their wake with the big bulky box in place. Once you had an idea of the frequenc(ies) at which your wake resonates, tuning to meet that is academic. A large challenge here would be in keeping vent sizes and lengths manageable - we're likely dealing with some very long waves and to attack them head-on means a lot of air moving a long distance very quickly through a port. The math might tell you to make a port with 4 square inches of cross section, 45 feet long - but in addition to the obvious problem of packaging that much port, you're also asking the air in that port to travel back and forth at mach 0.5 and come out cleanly at either end - a very quick way of giving your project asthma and keeping it from working at all. Instead, a fractal approach may be called-for with either multiple chambers/vents or with taller harmonics of the offending frequency.

I haven't the time or money to fool with this at the moment, but it'd be a fun experiment to toy with and if it fails as an aero concept I'd have a cargo trunk instead So, maybe later in the year.

[ConnClark]

They have experimented using Helmholtz resonators as vortex generators. The thing to keep in mind is that the creating/maintaining the resonance is going to consume some energy.

[RiderofBikes]

Required energy aside, the input frequency would be relatively consistant(used within tuning properties), but what about the output, surely there will be some phase shift.

[christofoo]

Originally Posted by shovel

...
So - any other thoughts on this?
...

I think your hypothesis is a good one, and the numbers you picked make sense to me as well, as a starting point. I believe you could coil the duct to make it more manageable?

I wonder if vehicles would usually benefit most from dual Helmholtz resonators, top and bottom. Really you need to consider the 3d shape of the turbulence, and which zones are out of phase with one another. Someone with CFD should take a swag at this.

The thing that really strikes me is that whereas boattails may offer increased interior volume, a helmholz resonator would reduce interior volume, so for many applications (my applications ) this would seem to be the less optimal solution.

I do think this aspect makes it unlikely to get used by auto manufacturers. This is for the after-market.

I see a lot of sense in one of your suggestions, a hitch-mounted resonator for large bluff-tailed vehicles - vans and SUVs, maybe hatchbacks and wagons. 30 cuft would fit there nicely, and yet not be nearly so large and obstructive (and odd-looking?) as a boattail, nor have stiff structural requirements and be easier to fabricate. Another interesting benefit, if we ever see motion-sensor-guided road-trains, is that the wake would passively return to existence if a following vehicle entered the tight draft zone.

Good thinking.

[christofoo]

Originally Posted by shovel

I was doing some acoustics work (horn design) a few days ago and, being aware that acoustics and aerodynamics are effectively the same as each other, it occurred to me that there could be some benefit to applying capacitance to the wake area of a vehicle moving through air.
...

Wait, wait wait...

Does a resonator create capacitance?

Wouldn't a resonator actually be in phase with, and simply reinforce the amplitude of the vortex in the wake?

I accepted this without really thinking about it, sorry.

[TOOSTUBBORN2FAIL]

Depends how you tune it. My car has several helmholtz resonators inside, tuned to be out of phase from the main resonant frequencies of the car, to improve acoustics.

[christofoo]

Originally Posted by TOOSTUBBORN2FAIL

Depends how you tune it. My car has several helmholtz resonators inside, tuned to be out of phase from the main resonant frequencies of the car, to improve acoustics.

You're right.

I think the explanation I needed was something like; a Helmholtz resonator can be designed as a quarter-wave resonator. Quarter Wave Tube - DiracDelta Science & Engineering Encyclopedia