Helmholtz resonator vs. bluff body wake region

[freebeard]

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.

[citation needed] a Google search string would suffice. Commercial product?

shovel -- I've taken to thinking of turbulence as noise, and the vehicle as an inside out musical instrument; with the optimum in making it as quiet as possible. I don't know where to take that, except maybe putting an array of microphones in areas of interest.

[big shafe]

While I understand helmholtz resonators and the idea of wave canceling, every example that I can think of (specifically noise cancellation) is done in a controlled/defined volume. How does this apply or what would be the effectiveness of this in an unbounded/uncontrolled volume? I apologize if this is somewhat archaic but my knowledge of fluid dynamics is rusty.

I also wonder about the energy needed to not only perpetuate the required speed but also the volume needed to have any affect on low pressure void.

[freebeard]

Yeah, it's that 'inside out' part that's the trick.

[freebeard]

Old Tele man -- Are you thinking of a half-round wake filler, or a big port/small volume resonant cavity? Either could work.

[Josh8loop]

very interesting

[aerohead]

As I understand it,acoustics as a branch of physics which deals with a sound pressure wave and it's interactions with it's environment.
Aerodynamics,as concerns us,is the reduction of the pressure differential between the forward stagnation point of a vehicle,and the base pressure of the vehicles turbulent wake.
The drag can only be mitigated by reducing or eliminating the flow separation responsible for the creation of the wake of which a symptom is the resonant frequency we detect.
The only prima facie evidence we possess for effective wake reduction is the empirical data from aerodynamic boat tail research.
Once the wake is eliminated,so goes the 'noise' of the wake.

[ECONORAM]

Originally Posted by christofoo

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.

Very intriguing thread. I was wondering... Klipsch loudspeakers, the BIG ones, all use horns and not cones. The bass horn is folded upon itself inside the speaker cabinet. Maybe a similar concept could be used here?
Also, I don't think 30 cubic feet is that big a device on the back of a van or a truck like mine with a topper. The end of my truck is about 20 sq ft, so a 1.5 ft long device would be 30 cu ft. My Kammback plan is for an 18" length... Hadn't considered a mod like this.

[shovel]

A horn is an excellent way of mating high pressure on the surface of a cone with the atmosphere - and is about the same idea as a 'complete' torpedo shape, but we don't have the luxury of bringing our functional, parkable road cars to a point at the tail with a slope that works at highway speeds. You can fold a horn without losing much of its effectiveness - clearly a luxury much less available on cars unless you want to model your tail after a collapsible cup and use a ram of some sort to suck it in when you slow down.

A horn is kinda the point though, rather than pushing a moving wave past stationary surfaces in air, we're pushing surfaces through air. The need to manage pressure differentials isn't terribly different in either case. So since we can't really make a full horn (20 foot long tail) , the whole point of this thread is what can we do in a shorter space to get the same effect as a horn?

Good to see we're still tossing ideas about

[Tesla]

Re read the thread and some other related stuff to try to understand the concept presented here.

As for sound and pressure waves it is to produce a matching wave phased out by 1/2 a wavelength so that the two cancel each other out.

In order to do this the resonator needs to be reactive to the wave generator to reproduce this wave length, in the simplest terms the inside of the resonator needs to take on the low pressure fluctuations of the turbulent wake, does this not mean that the low pressure inside the resonator will be acting through the neck to the rear of the vehicle resulting in the same low pressure drag on the rear of the vehicle.
It may well smooth out some of the buffetting on the rear of the vehicle by transferring it into the resonator but this does not change the net low pressure at the rear of the vehicle in comparison to the high pressure at the front of the vehicle and it is this pressure differential that is the source of form drag isn't it?

There is also a possibility of increasing drag as with a diffuser that is too steep and just results in a greater surface area exposed to the low pressure wake, the resonator internal surface area may just add to the total surface area exposed to the low pressure wake.

[Xist]

I do not know how well I would have understood this before this semester, but we study sound waves in Speech and Hearing Sciences.

To me, the easiest way would be to create a shield generator.