Aero acoustics

Those Aero acoustics concerns itself with the emergence and propagation aerodynamically produced noises and their reduction. The meaning of the aero acoustics increased in the aircraft industry and in the vehicle industry in the last years strongly. In the vehicle acoustics this is justified by the increasing comfort consciousness of the customers. The amount of that Aero-acoustic wind tunnels therefore in the past constantly increased. The experimental investigations are supplemented by those numeric aero acoustics.

Surge noises by three different noise developing mechanisms are essentially caused:

• Flow rate by small openings;
• Change application of pressure of firm surfaces;
• turbulent shear stresses.

Lighthill analogy

Schematic representation of the emitter types relevant in the aero acoustics (red and black marking 180° phase shift indicates)

For the characterisation of the individual mechanisms idealized approximation models can (Lighthill analogy) to be consulted:

• Change-pressure-afflicted flow rates can through Monopoly emitter are represented. A monopoly emitter represents simply spoken a breathing ball and radiates accordingly in all directions equal. Examples of this kind of acoustic sources are leakages in seal systems or the exhaust delta of a vehicle.

• The acoustic effect of the change application of pressure of a firm surface can by one Dipole emitter are represented. This kind of noise emission is always then present, if a free or replaced current hits a surface. At vehicles there is z. B. a multiplicity of areas with replaced current.

• Turbulent shear stresses produce Quadrupole emitter. Such emitters develop for example in turbulent shear layers or in the wake of a vehicle.

As already mentions above, the intensities of these three kinds of source are quite different. For one Source of monopoly we receive v with the flow rate, that Density <math> \rho </math>, that Speed of sound C and that Mach number MA

<math>

I_m \propto \frac{\rho}{c} \cdot v^4 = \rho \cdot mA \cdot v^3 </math> ,

for one Dipolquellle

<math>

I_d \propto \frac{\rho}{c^3} \cdot v^6 = \rho \cdot Ma^3 \cdot v^3 </math>

and for one Source of quadrupole

<math>

I_q \propto \frac{\rho}{c^5} \cdot v^8 = \rho \cdot Ma^5 \cdot v^3 </math> .

Meaning of the sound formation mechanisms

The comparison of the intensities shows, that with low flow rates (Mach numbers smaller than 1) the source of monopoly is most effective, followed of the source of dipole. The smallest radiation is produced by sources of quadrupole, in the aero acoustics of road vehicles, Buildings etc.. to be in most cases neglected can. If a source of monopoly is present, this will be thus usually the loudest source. Only if all sources of monopoly are eliminated, one of the remaining sources of dipole can dominate.

Characteristics of aero-acoustic sources

As can be taken out of the above equations, is those Sound power a source of monopoly proportionally to 4. Power of the incident-flow velocity, while the sound power of a source of dipole with the 6. Power of the speed rises. Since the effective aerodynamic noise production mechanisms can be represented by road vehicles and airplanes generally by a mixture of monopoly and dipole emitters, becomes frequent in the experiment a rise of the sound power with the 4. to 6. Power of the speed observes.

When aero acoustic measurements therefore the speed must be kept very exactly. Already small deviations in the attitude can lead to clear level changes. This means, that aero-acoustic measurements outside Aero-acoustic wind tunnels with unforeseeable wind conditions only under reservation are expressive, if the relative incident-flow velocity and direction not to be included.

With motor vehicles the distribution of the flow rate over the entire finish is very uneven. Therefore the potential suggestion for noise is dependent on the suggestion place of various sizes. One presupposes dipole behavior, like that the sound 9 railways produced at a place is louder than at a neighbouring, if the local prevailing there Pressure coefficients cp -1 and/or. 0 amounts to. With pressure coefficients of 0 and -2 (one for the region around those A-column one Passenger car not uncommon value) this difference amounts to even 14 railways. This shows, that the positioning of attachments, z.B. Outside mirrors, of great importance for the aero-acoustic behavior of a vehicle to be can.

The frequency of the radiated noise depends on the characteristic dimensions of the washed construction unit and the incident-flow velocity. For the vehicle body and its one knows attachments and details the associated frequencies by the equation

<math>

f = SR \cdot \frac{v}{l} </math>

measure, whereby l a characteristic dimension (z.B. And SR the Strouhalzahl represents height or width) of the individual construction unit or detail. Generally the Strouhalzahl with approximately 1 can be accepted for attachments. For cylindrical parts it is to be set however with 0,2. As characteristic dimension we here the diameter selected. Thus z results.B. for a radio antenna on a vehicle roof with a diameter of 5 mm with an incident-flow velocity v of 40 ms a frequency f of approx.. 1,600 cycles per second. Antennas can be noticeable thus by annoying whistlers (s. also Kármán vortex trail).

Literature

• Goldstein, Marvin E.: Aeroacoustics. New York: McGraw Hill Book company, 1976
• Hucho, Wolf Heinrich: Aerodynamics of the automobile - fluid mechanics, Thermodynamics, Driving dynamics, Comfort. Wiesbaden: Vieweg, 2005, ISBN 3528039590

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