## 1 Introduction  
Laser powder bed fusion (L-PBF) is one of the fastest-growing metal additive manufacturing technologies because of its capability to fabricate complex geometries with intricate substructures, such as internal channels [1; 2; 3]. In particular, the fabrication of channels and holes with small diameters has gained continuous interest in advanced manufacturing industries for producing automotive and gas turbine parts. However, the components produced by L-PBF are subjected to high surface roughness. In contrast, the surface irregularities inside of channels strongly depend on the inclination angle, leading to cross formation and stair-case defects on the upward- and downward-facing surfaces [4; 5]. These surface irregularities, in the form of microscale features on the internal surface of the channel, can be related to the hydrodynamic sand grain roughness \"\(k_{s}\)\" or mean roughness height \"\(\epsilon\)\" and were recently introduced as the mean surface topography height (_nsth_) [5]. The hydrodynamic sand grain roughness \"\(k_{s}\)\" is defined as the mean height of tightly packed protruding sand-grain-like features that are uniformly distributed inside the channel, directly impacting the fluid flow through the channel in terms of the pressure drop due to friction and an alteration from the designed mean channel diameter [6; 7; 8]. Therefore, the mean channel diameter, affected by the microscale surface irregularitiesexhibiting _msth_, needs to be carefully assessed in a non-destructive manner to understand the functionality of the channel [5]. For this purpose, experimental flow tests are generally conducted to determine the hydraulic diameter (\(d_{h}\)), the effective area (\(A_{\textit{eff}}\)) and, subsequently, the hydrodynamic sand grain roughness (\(k_{s}\)) or mean roughness height (\(\epsilon\)) [6; 9; 10]. In fluid mechanics, the pressure drop due to friction is generally known as the major loss in a pipe, or channel flow, and pressure drops due to geometrical fittings such as elbows, joints, valves, etc., are collectively known as minor losses [11; 12]. However, there are constraints related to the experimental setup regarding the channel's dimensions and geometry under investigation. Hence, different geometries inflict additional complexity in the experimental setup, which can also strongly impact the measurement accuracy. As a result, re-calibration is required whenever modifications are made to the experimental setup, resulting in longer evaluation times and higher costs. Industrial X-ray computed tomography (CT) is a powerful non-destructive technique that enables the assessment and measurement of the component and internal features without altering its actual form and allows a 3D reconstruction of the internal features of a component, such as internal channels [13; 14; 15]. Several approaches can be applied to the 3D modelling of the CT-scanned part to perform dimensional and geometrical measurements. One approach is the nominal-to-actual comparison of the CT-scanned part (actual) with the computer-aided design (CAD) geometry (nominal). However, for components with high surface variation, achieving the proper alignment between the actual and nominal geometry becomes increasingly difficult, which in turn can strongly affect the accuracy of the dimensional evaluation [16]. A perfect example of such components is parts manufactured by L-PBF, where high surface variation can originate from several causes. For example, the parts undergo high thermal gradients during production. They are fully immersed in powder at the end of the fabrication process due to the layer-by-layer building process. As a result, the surrounding powder can become attached to the borders of the part due to partial sintering or adhesion forces and can cause deviations from the nominal CAD design file [17].