View of the reed is from the front, with the plate on top and comb below.

The flow is traveling from top to bottom. Accordingly, this reed is operating in its “closing” mode.

The red region represents high velocity, and blue represents slow flow. An increase of pressure above the reed causes it to close. In the process, flow separation is observed on both lateral edges.  Due to the high velocity, there are some unsteady vortices formed downstream.

When the reed is nearly shut, there is a very strong aerodynamic instability observed. In some cases (e.g. certain brands of harmonicas which will not be mentioned), this instability will lead to torsional flutter of the reed, known to most players as that annoying “dog whistle” sound.

-Simulation courtesy of Dr. Greg Burgreen