The 1D flow tube only refers fluid materials and allows modeling of internal flow using the mass flow rate times specific heat method. Once the 1D fluid tube property applies to curves or 1D elements, they are treated as 1D fluid flow, and will therefore be represented in the SINDA model as one-way conductors. The upstream will be the starting point of the curve or the first node of the 1D element. If you need to reverse the fluid flow direction, you can either create the curve again by changing the start and end points, or input a negative mass flow rate in the fluid material referred by 1D flow tube property.