Flow 2D simulation fails when using air instead of water 回答済み

I used the Flow 2D simulation as a starting point. I changed the flow velocity (30 m/s) and the medium from water to air:

symb vel = 340

matr
  wvsp on 
  type elas
  prop air 1.225 340 0
  vdmp air $freqdamp db 0.002 0 1e+06 2 0.01 /* needs changing for air
  hrgl air visc 0.02

However, the simulation appears to crash. I've attached two screenshots from the output mp4 showing the crash. 

Last good frame:

First bad frame:

This cloud of energy floats away in the wind field!

What could be the problem? Should I decrease the step size? Here are the relevant parameters:

c Flow
symbx vflow = 30				/* flow velocity
symb nsc = 20					/* boxes at edge to scale flow

c Frequency and mesh
symb freqint = 0.5e6			/* Center frequency
symb freqdamp = $freqint		/* Damping frequency
symb vel = 340					/* Minimum velocity to mesh for
symb wavelen = $vel / $freqint	
symb nelem = 15				/* Number of elements per wavelength for mesh (minimum 100, increase for accuracy but at expense of longer computation time)
symb box = $wavelen / $nelem

c Simulation time
symb ncycles = 200				/* Number of cycles
symb nloop = 20					/* Number of times to plot the simulation
symb tfact = 0.6				/* Time stability factor

c Calculations including Meshing
symb simtime = $ncycles / $freqint
symb pipeLen2 = $pipeLen / 2
symb txDist = $pipeRad / cos ( $txAng * 3.14 / 180 ) + 10e-3
symb txTop = $txDist + $txThk
symb ygap = $txTop + 3e-3
symb ybeg = 0 - $pipeRad - $ygap
symb ang = $txAng + 90