Consider, you have a closed loop arbitrary shape. If you make a circle with perimeter equal to the perimeter of that arbitrary shape, that is called as equivalent circle. In equivalent circle the surface area of the circle may not be the same with that of arbitrary shape. If you want to make a circle for that arbitrary shape, in which the area of the circle is same as that arbitrary shape area, then you have to go for Hydraulic diameter. If you are dealing with the physical quantities that depends on the area like Mass flow rate etc, then you have to use Hydraulic diameter where the area is constant. In some places you have to deal with the quantities which depends on the wall length etc that time you can use equivalent circle.
Consider, you have a closed loop arbitrary shape. If you make a circle with perimeter equal to the perimeter of that arbitrary shape, that is called as equivalent circle. In equivalent circle the surface area of the circle may not be the same with that of arbitrary shape. If you want to make a circle for that arbitrary shape, in which the area of the circle is same as that arbitrary shape area, then you have to go for Hydraulic diameter. If you are dealing with the physical quantities that depends on the area like Mass flow rate etc, then you have to use Hydraulic diameter where the area is constant. In some places you have to deal with the quantities which depends on the wall length etc that time you can use equivalent circle.
Hello Kalyana Raman,
Thanks for the information. I learnt that the concept of hydraulic diameter can be used when we use irregular geometry. However, for flow in pipes or parallel plates, researchers have used hydraulic diameter in place of equivalent diameter. Is there any specific reason for this? Please let me know.
In a internal flows, mass flow rate should be maintained constant when you change the shape. If you use equivalent diameter there is a possibility of change in cross sectional area which in turn can vary the mass flow rate if you give velocity as input. I think that may be the reason for using hydraulic diameter which maintains the same area.
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