  Providing an accurate, representative sample of mass flux across large open
areas for atmospheric studies or the extreme conditions of a hypersonic engine
is challenging for traditional intrusive or point-based sensors. Here, we
demonstrate that laser absorption spectroscopy with mode-locked frequency combs
can simultaneously measure all of the components of mass flux (velocity,
temperature, pressure, and species mole fraction) with low uncertainty, spatial
resolution corresponding to the laser line of sight, and no supplemental sensor
readings. The low uncertainty is provided by the broad spectral bandwidth, high
resolution, and extremely well-known and controlled frequency axis of
stabilized, mode-locked frequency combs. We demonstrate these capabilities
using dual frequency comb spectroscopy (DCS) in the isolator of a ground-test
supersonic propulsion engine at Wright-Patterson Air Force Base. The mass flux
measurements are consistent within 3.6% of the facility-level engine air supply
values. A vertical scan of the laser beams in the isolator measures the
spatially resolved mass flux, which is compared with computational fluid
dynamics simulations. A rigorous uncertainty analysis demonstrates a instrument
uncertainty of ~0.4%, and total uncertainty (including non-instrument sources)
of ~7% for mass flux measurements. These measurements demonstrate DCS with
mode-locked frequency combs as a low-uncertainty mass flux sensor for a variety
of applications.
