Aeroacoustics

The aeroacoustics or flow acoustics deals with the emergence and spread of aerodynamically generated noise and its reduction. The importance of aeroacoustics has greatly increased in recent years in the aviation industry and in the automotive industry. In the vehicle acoustics this is due to the growing awareness of customer comfort. The number of aero -acoustic wind tunnels has therefore been increasing steadily in the past. Are complemented by experimental investigations, the numerical aeroacoustics.

Air flow and is caused by three different noise generation mechanisms essentially:

• Volume flow through small openings;
• Wechseldruckbeaufschlagung solid surfaces;
• Turbulent shear stresses.

Acoustic analogy

To characterize the individual mechanisms idealized approximation models ( Acoustic analogy ) can be used:

• Alternating pressure Lossy volume flows can be represented by monopole radiator. A monopole radiator is simply speaking is a breathing sphere, and can radiate accordingly equal in all directions. Examples of this type of sound sources are sirens, leaks in the seal system or the exhaust outlet of a vehicle.
• The acoustic effect of Wechseldruckbeaufschlagung a solid surface can be represented by a dipole antenna. This kind of noise emission is always present when a free or separated flow impinging on a surface. On motor vehicles, this applies for example to the caster behind the mirrors, if this again collides with the vehicle structure.
• Turbulent shear stresses produce Quadrupolstrahler. Such emitters are formed for example in turbulent shear layers or in the wake of a vehicle.

As already mentioned above, the intensities of these three types of sources are quite different. For a monopole source is obtained with the flow velocity v, density, speed of sound c and the Mach number Ma

For a dipole source

And for a Quadrupolquelle

Importance of sound generation mechanisms

The comparison of the intensities shows that at low flow speeds ( Mach numbers less than 1), the monopole source is most effective, followed by the dipole source. The lowest radiation generated by Quadrupolquellen that can be so neglected in the aeroacoustics of road vehicles, buildings in most cases. If a monopole source is present, it will therefore be the loudest source in general. Only when all sources are eliminated monopoly, one of the remaining dipole sources can dominate.

Properties of aeroacoustic sources

As can be seen from the above equations, the sound power of a monopole source is proportional to the 4th power of the flow velocity, while the sound power of a dipole source with the sixth power of the velocity increases. Since the effective aerodynamic noise generation mechanisms of road vehicles and aircraft in general can be represented by a mixture of monopole and dipole, an increase in sound power is often observed with the 4th to the 6th power of the velocity in the experiment.

In aeroacoustic measurements, therefore, the speed must be maintained very accurately. Even small deviations in the setting can lead to significant changes in level. This means that aeroacoustic measurements outside Aero -acoustic wind tunnels in unpredictable wind conditions are meaningful only with reservations when the relative flow velocity and direction are not also detected.

For motor vehicles, the distribution of flow velocity over the entire vehicle surface is very uneven. Therefore, the potential noise excitation is dependent on the excitation location of different sizes. Substituting dipole behavior advance so the sound produced in a location is 9 dB louder than on an adjacent, if the prevailing local pressure coefficient cp be -1 or 0. When the pressure coefficient of 0 and -2 (one for the region around the A -pillar of a car not unusual value ), this difference is even 14 dB. This shows that the positioning of attachments, such as outside mirrors, can be of great importance for the aero-acoustic behavior of a vehicle.

The frequency of the radiated noise is a function of the characteristic dimensions of the flow around the component, and the flow velocity. For the vehicle body and its attachments and details you can see the associated frequencies by the equation

Estimate, where l is a characteristic dimension (eg height or width) of each component or details and Sr represents the Strouhal number. It can generally be assumed to be about 1 for attachments, the Strouhal number. However, for cylindrical parts, they should be assumed to 0.2. As a characteristic dimension here, the diameter is chosen. Thus arises eg for a radio antenna on a vehicle roof with a diameter of 5 mm at a flow velocity v of 40 m / s, a frequency f of about 1600 Hz antennas can therefore by annoying whistling notice (see also Kármán vortex street ).