Sound generation in the flow behind a rectangular cylinder of various aspect ratios at low Mach numbers

• The sound generation in the wake of rectangular cylinder is studied by DNS.
• Lift dipole becomes the strongest when the aspect ratio of the cylinder is 0.5.
• Sound by secondary vortex street is extremely weaker than that by primary one.
• Sound with the frequency of drag fluctuation 2St1 becomes significant with decreasing the aspect ratio.
• Monopole sound source is formed at 2St1 and becomes significant with increasing Mach number.

The development of wake vortices and the associated sound generation in the two-dimensional flow past a rectangular cylinder with the aspect ratio from 0.1 to 5.0 at a Reynolds number of 150 and at Mach numbers below 0.3 were studied by direct Navier–Stokes simulations using a compact scheme. The location of vortex formation moved toward the cylinder with decreasing aspect ratio AR, and the lift dipole due to periodic vortex shedding became the strongest at AR = 0.5. For AR ⩽ 0.5, a secondary vortex street was developed in the downstream region, but the associated sound was extremely weak compared to the sound associated with the primary vortex shedding. The sound with the frequency of drag fluctuation was pronounced for AR < 1 and became stronger with decreasing AR to approach that of the lift-dipole at AR = 0.1. We also found that a monopole sound source, in addition to a dipole sound source, was formed at twice the shedding frequency, and the former became pronounced with increasing Mach number.