Chevrons are the sawtooth pattern seen on the trailing edges of some turbojet engine nozzles. As hot air from the engine core mixes with cooler air blowing through the engine fan, the shaped edges serve to smooth the mixing, which reduces turbulence that creates noise. Chevron nozzles act as great attenuators of jet noise. The chevrons reduce jet blast noise by controlling the way the air mixes after passing through and around the jet engine. Chevron nozzles help to significantly reduce pressure fluctuations and thus jet noise.

Please read the below paper:

"Numerical Simulations of Noise Reduction Devices for Aero Engines"

by Haukur Elvar Hafsteinsson