If the mass flux j m passes through the area at an angle θ to the area normal n ^ ^ , then j m ⋅ ⋅ n ^ ^ = j m cos ⁡ ⁡ θ θ where · is the dot product of the unit vectors. That is, the component of mass flux passing through the surface (i.e. normal to it) is j m cos θ. While the component of mass flux passing tangential to the area is given by j m sin θ, there is no mass flux actually passing through the area in the tangential direction. The only component of mass flux passing normal to the area is the cosine component.