Ok, let's take a break here. When you pour sugar into tea and stir it with your spoon, the leftover, undissolved sugar gathers in the middle of the cup. This is the work of the secondary flow. When the spoon creates a vortex in the cup, pressure around the edge of the cup becomes higher than that of the middle, due to centrifugal force. However, near the base, the current speed is reduced by viscosity, so centrifugal force is not as strong. In the end, near the base water acted on by the pressure gap is forced to the middle of the cup. The layer at which the work of viscosity is clearly evident is called the viscous boundary layer. Tea that has passed through the viscous boundary layer and lost momentum spreads from the middle of the cup and stops the overall motion of the tea.