Hello There, 　　I am using the Comsol 5.3a CFD module to study laminar flow in simple geometry. the relevant COMSOL file and geormetry discription photo have been attached. 　　there are two parallel headers, connected together using and annulus(concentric cylinders, where water flows between two cylinders). water enters from the front side of the first header and outs from the backside of the second header. 　　the mass balance is ok between inlet and outlet. However, the mass balance is not correct between both ends of the annulus.? Can somebody explain why there is a significant mass balance error for both ends of the annulus? Is there any way to improve the mass balance error? It should be mentioned that I used a normal velocity field which has explained in(https://www.comsol.com/blogs/how-to-calculate-mass-conservation-and-energy-balance/) to calculate the mass balance. 　　laminar flow (which is checked by Re#)-no gravity effect- incompressible flow. 　　P.s. I refine the mesh (using both physics and user controlled meshing)and improved the mass balance error but still, there is a considerable error. 　　looking forward balanceflow瑜伽视频 for any suggestion, idea,... Regards, Hassan