A review of synchrotron radiation diagnostics for fluid mechanics

X-rays are capable of making measurements in flowfields that are not amenable to optical diagnostics. Synchrotron x-rays are particularly useful as a diagnostic tool. Their tunable energy, high flux and highly collimated nature enable several diagnostics which will be discussed. Time-resolved x-ray radiography allows us to measure the line of sight density of a flowfield at microsecond time scales and micron resolution, independently of the pressure, temperature and velocity. This is particularly useful in dense multiphase flows such as fuel injection sprays. X-ray fluorescence spectroscopy allows us to measure local species concentration independent of molecular and valence state, which can be used to study turbulent mixing and reacting flows. X-ray phase contrast imaging exploits the weak diffraction of x-rays at phase interfaces to recover the morphology of complex, optically dense flows. This allows us to study multiphase flows in optically inaccessible environments, and the velocity of multiphase mixing flows at very high temporal resolution. Ultra- small angle x-ray scattering allows us to use the scattering properties of x-rays at atomic wavelengths to measure the surface area of complex highly refractive flowfields such as dense sprays. All these techniques provide unique and useful measurements which can be used to complement traditional visible-light diagnostics.