Anna Färm

The Effect of Boundary Layers on Bulk Reacting Liners at Low Mach Number Flows

Acoustic absorbing liners are efficient and commonly used measures to reduce sound levels in many fields of application. The sound reducingperformance of the liners is dependent on the acoustic state, defined by e.g. the flow and sound field interacting with the liner. To enable liner optimization the impact of these factors on the liner performance must be predictable. Studies of the impact of these factors were performed with existing experimental, analytical and numerical methods at low Mach number flows and material used in truck engine compartments. The study showed significant impact of both flow and sound field onthe liner performance. The size of the impact of the flow depends on which of the existing methods and models that was used, implying theneed of complementary methods. A new numerical method to model the boundary layer effect was for this reason developed in this work. The method was shown to predict the impact of flow correctly compared to the Pridmore-Brown solution and the method was computationally efficient. The sound reducing performance of a liner exposed to complex sound field and grazing flow can be predicted using existing methods together with the new proposed method. Extra care has to be taken when bulk reacting liners are considered since additional complications compared to locally reacting surfaces occur in presence the of grazing flow.

On internal mean flow in porous absorbers and its effect on attenuation properties

In vehicle applications, absorbing materials are often used to attenuate sound. In, for example, exhaust systems and on noise encapsulations, the absorber is exposed to flow. This creates a boundary layer above the absorber, which affects the impedance of the surface, and hence alters the absorption properties. In addition to this effect, the flow itself may enter the absorbent material due to high pressure and forced flow paths. An investigation of the effects that internal flow in the absorber imposes on the acoustic properties is presented. One way to describe the effect is by a change in flow resistivity. The effect is investigated for typical absorbers used in noise encapsulations for trucks. The Transfer Matrix Method is applied to calculate the resulting absorption coefficient for an absorber with changed flow resistivity due to internal flow. The possibility to model the changed properties of the absorber with internal mean flow by means of Biot theory is also explored, together with a discussion on suitable experimental methods to verify and further investigate the effects.

On Sound Absorbing Characteristics and Suitable Measurement Methods

Acoustic absorbing liners are efficient and commonly used measures to reduce sound levels in many fields of application. The sound reducingperformance of the liners is dependent on the acoustic state, defined by e.g. the flow and sound field interacting with the liner. To enable liner optimization the impact of these factors on the liner performance must be predictable. Studies of the impact of these factors were performed with existing experimental, analytical and numerical methods at low Mach number flows and material used in truck engine compartments. The study showed significant impact of both flow and sound field onthe liner performance. The size of the impact of the flow depends on which of the existing methods and models that was used, implying theneed of complementary methods. A new numerical method to model the boundary layer effect was for this reason developed in this work. The method was shown to predict the impact of flow correctly compared to the Pridmore-Brown solution and the method was computationally efficient. The sound reducing performance of a liner exposed to complex sound field and grazing flow can be predicted using existing methods together with the new proposed method. Extra care has to be taken when bulk reacting liners are considered since additional complications compared to locally reacting surfaces occur in presence the of grazing flow.