The influence of local rectangular cavities on the evolution of the second-mode disturbances in a incoming Mach number 6.0 hypersonic boundary layer over a flat plate was investigated by using direct umerical simulation. The local scattering effect was quantified by a transmission coefficient defined as the ratio of perturbation amplitude downstream cavity to upstream cavity. It was found from the numerical results that

for a shallow cavity

thesecond-mode disturbance with a relatively low frequency was enhanced by the cavity

while the opposite was true when the frequency was high. In most of the cases

the transmission coefficient reduced with the increase of the cavity depth

implying a weaker enhancement or a stronger suppression effect. When the cavity depth exceeded a critical value

the dependence of the transmission coefficient on cavity depth was changed to a positive correlation

implying the emergence of a new regime. The critical depth relies on the disturbance frequency: it is larger for a lower frequency.