When the speed of a fluid (be it air or water for example) flowing pass an obstacle is just right, a unique wave pattern forms behind the obstacle known as Vortex shedding or a vortex street. This causes the obstacle and anything behind it to experience oscillating forces perpendicular to the flow. In application, vortex shedding can explain why a string vibrates when you blow on it; how electricity cables vibrate or ‘sing’ in the wind.