Flow can be divided in two main types: 1) laminar and 2) turbulent.
Laminar flow is characterized by fluid particles following smooth paths
in parallel layers with no or little disruption between the layers (Figure 1). Laminar flows have a parabolic velocity profile. The velocity of the flow is at its lowest at the walls and highest in the center of the stream.
F I G U RE 1.
Laminar flow in a closed pipe. The length of the arrows shows the approximate velocity of the fluid flow.
Turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity (Figure 2).
F I G U RE 2.
Turbulent flow in a closed pipe.
One of the main focus points of wind noise testing in the aerospace industry is measurement in boundary layers, where there is considerable interest in separating the acoustic signal from flow-induced turbulent or hydrodynamic noise. And special attention is paid to determine the exact location where the
flow goes from laminate (relatively quiet) to turbulent and noisy. That location is called the transition region (Figure 3).
F IG U R E 3.
Boundary layers in flow.
The transition region is also very important when dealing with stall in an airfoil (such as the wings of a plane). In aeronautics, stall is the condition
where maximum lift is achieved and where, if the angle of attack increases
or decreases, lift begins to decrease. When the angle of attack increases, the separated flow region moves forward and affects lift and increases drag. This means that the optimal angle (maximum lift and minimal drag) correlates to
the location of the separation region (Figure 4). This optimal angle also ensures minimum fuel consumption and noise production.
F I G U RE 4.
Flow around an airfoil.
Microphones or pressure sensors can be used to monitor the separation
region because turbulence has a characteristic acoustic signature and can be differentiated from laminar flow acoustic signatures. The level of the turbulence increases from the separation region to the separated flow, and results in
vortical systems forming behind obstacles. These vortices and turbulence travel with a velocity comparable to the flow velocity and are the main sources of
hydrodynamic noise. When the speed is high enough, the acoustic component is typically very small in amplitude compared to the hydrodynamic noise.
One of the particularities of hydrodynamic noise is that turbulence propagates with the flow and not with the speed of sound. This results in much shorter wavelengths. In a wind tunnel, turbulence travels in a motion parallel to the
microphone diaphragm, so special care must be taken to avoid “microphone size effect” at higher frequencies.
