In the development of several outside mirror designs for vehicles, a high frequency noise (whistling) phenomenon was experienced. First impression was that this might be due to another source on the vehicle (such as water management channels) or a cavity noise; however, upon further investigation the source was found to be the mirror housing.
This “laminar whistle” is related to the separation of a laminar boundary layer near the trailing edges of the mirror housing. When there is a free stream impingement on the mirror housing, the boundary layer starts out as laminar, but as the boundary layer travels from the impingement point, distance, speed, and roughness combine to trigger the transition turbulent. However, when the transition is not complete, pressure fluctuations can cause rapidly changing flow patterns that sound like a whistle to the observer.
Because the laminar boundary layer has very little energy, it does not allow the flow to stay attached on curved surfaces. It also causes separations in areas that lead to higher static pressure (such as a parting line at the trailing edge of the mirror housing).
Flow visualization was performed and a region was identified on the mirror housing where the laminar separation occurred. Boundary layer trips in the form of wires or sandpaper were shown to eliminate the whistle by introducing turbulence into the boundary layer (artificially). Other methods of forced transition were also proven to be effective, such as heavy surface graining, bumps, and grooves.
Another alternative might be to utilize natural transition techniques, such as distance (longitudinal depth) or local speed (abrupt contours for local peak velocities). This can be implemented as either subtle or not-so-subtle changes to the mirror design.
The observed “laminar flow whistle” phenomenon will be discussed along with diagnosis through flow visualization. Methods to deal with the laminar boundary layer condition will be presented as well.