The Benard-Von Karman instability : an experimental study near the threshold

The evolution of the velocity components of the wake behind a cylinder has been investigated in the range 40 < Re < 300 using a moving fringe Laser Doppler anemometer. The order parameter (amplitude of the perpendicular velocity fluctuations) obeys Landau's law. We confirm the existence of several modes and in particular, we observe and interpret a three-dimensional motion that occurs when two modes co-exist The dependence of critical quantities on the aspect ratio is also given.