The Added Uncertainty in Densitometry and CT Due to Fluctuating Material Distribution

X-ray based techniques, including densitometry and Computed Tomography (CT), are becoming more widely utilized for multiphase flows. These measurements are often implemented as time-averaged, or nominally time-resolved. In all but the fastest synchrotron or electron beam CT experiments, composition fluctuations at time scales beyond resolvable may be present. While it is commonly assumed that uncertainty arising from fluctuations in time can be mitigated by employing long averaging windows, analysis demonstrates that this assumption does not always hold. For examples considered, we show that the bias error in volume fraction of gas (i.e., void fraction) caused by rapidly fluctuating domain composition in time-averaged 2D densitometry or 3D CT can be 10%. Fortunately, through understanding the nature of this error, one can bound the uncertainty in the measurement attributable to time-varying composition. With synthetic (simulated) data we illustrate the behavior of this uncertainty for both monochromatic and polychromatic X-ray sources, and subsequently show how it indeed manifests in experimental data in both 2D and 3D densitometry. Other scenarios of motion artifact and noise, which are sometimes perceived as analogous, are also considered. Unlike for the time-varying gas volume fraction case, we see that simple post-processing can alleviate much of the error in these cases.