When the cross section of the flow tube decreases, the flow speed increases, and therefore the pressure decreases.
can someone explain to me why this is true, i would think that as the cross section decreases the pressure would also increase .
This is related to "Continuity Equation" of fluid mechanisam.(Assuming fluid as incompressible)
if we have two cross-sections of areas A1 and A2 having velocities V1 and V2 respectively .Then according to continuity equation
A1*V1=A2*V2 or we can write
V2=(A1*V1)÷A2
V2 Is inversly proportional to the A2.
so velocity increases as the area decreases.
further we have a theorem in fluid mechanics called "Bernouli's theorem".
which states that the sum of all energies at any cross-section is constant.
So if the velocity(i.e kinetic energy) increases at any section there will be decrease in pressure(i.e pressure energy)