VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

6.2. REDUCING NOISE AT THE LEVEL OF THE SOUND SOURCE103Figure 6.4: Different sources of noise and transmission paths of the hydraulicgroup.Turbulence and vorticesVortices can occur because of bodies in a flow. They generate pure tonalcomponents (e.g. the flow over a cylinder like a chimney pipe). Tonal noiseis also generated by a flow over a cavity (for example, the slicer Figure 6.5).In channels, noise can be generated by sharp corners or valves (see Figure6.6). Appart from the coherent vortical structures flow over objects can alsoresult in turbulent noise. Turbulent noise can also be produces due to shearstresses that exist when there is a gradient in the air velocity (e.g. in the caseof the the jet produced by an air gun). Turbulence gives rises to a broadbandnoise. The following design rules should be taken into account to reduce thenoise generated by turbulence and/or vortices :1. Reduce the workload2. Reduce the pressure drops3. Reduce the outlet flow rate (for example, use a larger opening ).