I am trying to simulate a simple air flow in a pipe of length 70mm with boundary condition as mentioned below:
inlet set as pressure inlet = 50000Pa and outlet as wall
However after few seconds of running the simulation, there is starvation of air observed towards the outlet of the pipe. Can anyone help out with this?
as for simulation i suggest you to practice the software daily!!! but remember one thing it works on logic!!! i mean the know set of equations!!! in case you are doing a research and have derived some new equations then you need to be more practical!!! rather relying on Simulation!!!
What is the idea of simulation? How should it work?
You apply pressure on inlet and wall on oulet, so air cannont escape the pipe and will be compressed. But where air should com from? You need to define some mass flow on inlet to let air in.
Basically Ansys is working with energy balance, until it comes to a convergence. If you want to run for a flow simulation in a pipe, then the boundaries should be pressure at the inlet as you mention and pressure or velocity at the outlet. Hopefully it helps.
Pavel Lyubarskyy, Ansys fluent calculates the mass flow rate at the inlet depending on the pressure specified at the inlet. But with the pressure inlet condition, i see a very low value of mass flow rate at the inlet. I am expecting a high species (air) concentration at the outlet since the outlet is blocked.
Muhammad Ibnu Sulton Damiat I am considering the outlet to be fully blocked by setting it as wall and then trying to simulate a flow.
Geethu Varghese I suppose you have only single air phase, right?
Imagine you have a pipe with one blocked or welded edge under the ambient pressure 1atm or 101325 Pa. What would happen to the air inside it? Right, nothing. If you increase the pressure on inlet some air will flow in but for steady state solution you will get the stationary condition like the one under the ambient pressure. If your simulation is transient, you can model a time depended air compression in the pipe.
Pavel Lyubarskyy I am running a single phase transient simulation. There is a lot of reverse flow happening as there is no way for air to escape and thus leading to recirculation. I have added air concentration as a user defined scalar in Ansys and given an initial concentration of air at the inlet boundary. I am expecting air to accumulate at the outlet as time proceeds.
Geethu Varghese why should air "accumulate" at the "outlet"? Is the BC for pressure at inlet constant? Then by starting the simulation the air begins to flow in and "recirculate" until the pressure "outside" (your inlet BC) and inside in pipe will balance out. The distribution of the air or pressure distribution after defined time will be equal in whole domain. In-between the unequal pressure and velocity distributions in the pipe would lead to unequal air concentrations. The air must be compressible phase and you would most probably have turbulences till everything balance out and calms down. It’s my perception of the problem, if I understood you right :)
Pavel Lyubarskyy Thank you for your answer.
BC for pressure at inlet is considered to be constant. Initially I see a high concentration of species at the outlet but, through time this behavior is reversed and starvation of species at the outlet is observed. I agree to what you say, there is a lot of reverse flow occurring. But, imagine a gas cylinder pumped with gas at a constant pressure, after certain time, gas is accumulated towards the downstream of the cylinder right? I am expecting something similar.
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