In actual life flow is rotational and has viscosity. The assumption of inviscid flow, in physical meaning, means there's no friction between surface and respective layers in the boundary which means there's no resistance against the flow due to firction In mathematical meaning the navier stokes equations are greatly simplified to euler equations by dropping viscosity terms, You can already guess that by assuming there's no friction between the flow and the surfacs that the skin friction drag is neglected and assumed to be equal to zero Now Irrotational flow assumption, this results in further simplification to euler equations to have the potential flow equations, that's another assumption which means that fluid particles do not spin around its center, "but" that doesn't mean that stream lines and do not rotate and vortices are not generated. That's a bit complicated and need time and thinking But you can imagine it like the following: Earth spins around itself and orpits around sun If i applied irrotational assumption on this example Then i will say that earth do not spin around its cente axis but still rotates around the sun. In simple words, particles are not rotating but streamlines are Now the question that comes to your "mind so what if particles are not spining"? This means that on the boundary layer level the adverse pressure gradient do not cause separation because particles will not decelerate or spin and will continue in its direction untill trailing edge and then satisfy kutta condition. So, inviscid flow ( no friction between flow and surface) And irrotational (adverse pressure gradient has no effect) You can concloude that there's no separation from the airfoil surface and hence both form drag and skin friction drag are neglected! This takes us to d'Alembert paradox, which states that for inviscid and irrotational flow, drag is zero! These 2 assumptions makes it not possible to calculate drag, but get we still can calculate lift. Prandtl took these assumptions and made "lifting line theory" which took advantage of the simplified equations of these assumptions to calculate 3d lift

@Mohamed Sayed Hi Dr Mohamed, actually Simscale can generate tetrahedrals and hexahedrals and can combine both using Hexcore option and that's the one i am using actually And about Spaceclaim it works for me very well for alot of cases actually, but sometimes it doesn't make sense why this straightforward, simple geometry is making problems in fluent mesh or SpaceClaim، Things that repair and prepare taps and even my eyes can't capture xD, i think it somehow affected how this cad model was drawn in the first place or what tools we used during creating this cad in the cad software. But with some manipulation and modifications it gets working. Sometimes this same cad gets done easily in simscale. But i guess that's how it is to get cfd results and good mesh.