Random Temperature Spike with Transient Supersonic Nozzle?


I'm running a density-based, transient, detached-Eddy-simulation of a supersonic rocket nozzle with an ideal-gas input. The inlet pressure is 1 MPa gauge pressure at 2250 K, with an outlet pressure and pressure far-field of 54 KPa. I set my operating conditions to 0 Pa (gauge pressure). I have a semi-fine mesh with 120,000 cells. 

My solution type is implicit, AUSM, least square cell based gradient, and second order upwind. Courant number is set to 2. Hybrid initialization, with a time step size of 2e-06 seconds, 10 iterations per timestep, and 1000 timesteps. I'm running with double precision and an 8 core processor.

Problem: For some reason, at around timestep 375, there is a random temperature/energy spike in my farfield that I didn't prompt (literally out of nothing), and I've attached an image of the temperature contour at that point and the boundary labels. Does anyone know why this happens? I've tried re-running this with a smaller timestep but a floating point exception persists. Thank you so much in advance, and I look forward to your helpful responses.

Boundary Labels.Explosion? Frame 1

Explosion? Frame 2


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