  In this work a simple method to enforce the positivity-preserving property
for general high-order conservative schemes is proposed. The method keeps the
original scheme unchanged and detects critical numerical fluxes which may lead
to negative density and pressure, and then imposes a cut-off flux limiter to
satisfy a sufficient condition for preserving positivity. Though an extra
time-step size condition is required to maintain the formal order of accuracy,
it is less restrictive than those in previous works. A number of numerical
examples suggest that this method, when applied on an essentially
non-oscillatory base scheme, can be used to prevent positivity failure when the
flow involves vacuum or near vacuum and very strong discontinuities.
