<p>This study examines the effect of crystallization fouling induced surface roughness on the near‐wall turbulence and, by extension, gives first insights in the convective heat transfer performance in turbulent channel flow. The presented work extends an experimental approach to investigate the local microscale effects on fluid dynamics under fouling conditions applying Stereoscopic Micro Particle Image Velocimetry (Stereo μPIV) in a flow channel. Following on from the previous investigations, in which fouling layers were generated ex-situ in a batch reactor and inserted into a rectangular flow channel equipped with a Stereo µPIV, CaSO₄ fouling layers are now being generated within the flow channel and investigated in-situ. Additionally, additively manufactured (AM) structures are introduced into the flow channel to simulate flow conditions comparable to crystal fouling layers, to eliminate interfering effects from depositions outside the areas of interest. The presented results show a significant influence of the surface roughness, inducing turbulences into the bulk flow while altering the velocity and Turbulent Kinetic Energy (TKE) profile inside the flow channel. Furthermore, a detailed investigation of local flow velocities and turbulences during different phases of crystal fouling is presented.</p>