Generally,  if there is an oblique body through the duct, the viscosity force cause the swirl or Recirculations near the boundary can make 'Reverse Flow' problems. As recommended, it can disappear after many iterations and then the reversed flow will disappear. The solution of this case can be listed below:

1. Increase the mesh number and quality.

2. Increase the duct length after the oblique body about 11 times the diameter of the oblique.

3. This problem appears in ANSYS FLUENT and the solution is to switch the outlet boundary condition from 'Outlet Pressure' to ' Outflow'.

4.If the length of the duct is short, I recommend using ANSYS CFX and make the outlet condition as 'opening' boundary condition.

Dear Abdullah Ishaque,

"Reversed flow" is one of many error or warning messages that one encounters whilst running simulations in FLUENT. However, unlike the "turbulent viscosity ratio" error, the reversed flow error/warning may not lead to solution divergence (like in your case). 

The reversed flow error message is displayed when velocity vectors point inwards into the domain at an outlet boundary.....where.....ideally one would expect them to be pointing outwards. Hence, rather than (entirety of) the flow exiting the domain from the outlet, a part of it might "reverse" and re-enter the domain at the outlet making it essentially an "inlet".....this would be true if a vortex exits from the outlet.

Reversed flow would most commonly occur in flow past bluff bodies if the outlet boundary is modeled too close to the object. Whilst this can be corrected by shifting the boundary away from said object, the error is virtually unavoidable in some situations such as numerical wave generation and round jet simulations.

I hope this helps.

Best regards.

Shaswat 

I don't think so, I have seen my simulations getting converged even after reverse flow in the initial iterations.

 I agree with Mr. Panda. If the results after a reasonable number of iterations show unphysical behavior, check your boundary conditions.

It is a warning rather than an error. It might be physical, in which case as said by others moving the outlet may be a good option. It can also commonly occur in initial phases of calculations, but it is not worth worrying about on its own.

The cause of the "Reverse Flow" phenomenon is improper initialization. If possible initialize with the actual expected values of the flow in the direction of the flow. If there are different domains with different flows, initialize and patch these domains.

probably, you used the pressure outlet, try using outflow ! maybe it 'll useful (I already try it), 

generally, the reversed flow can't influence the results but it'll influence the convergence.

Good luck.

Swirl / Recirculations near the boundary can cause 'Reverse Flow' issues. As suggested by peers, it can vanish after some initial iterations. In many cases you will get convergence even with 'reversed flow' but it is always better to eliminate. Some exercise you can do  are:

1.  if vortex formation/recirculation of flow near an outlet boundary, increase the downstream length.

2. Mesh Quality

3. In FLUENT, if problem arise with 'Pressure boundary condition' try switch to 'Outflow'. In CFX, try 'opening'. Interestingly, In CFX you will receive a warning message asking you to switch to 'opening' boundary condition.

Generally,  if there is an oblique body through the duct, the viscosity force cause the swirl or Recirculations near the boundary can make 'Reverse Flow' problems. As recommended, it can disappear after many iterations and then the reversed flow will disappear. The solution of this case can be listed below:

1. Increase the mesh number and quality.

2. Increase the duct length after the oblique body about 11 times the diameter of the oblique.

3. This problem appears in ANSYS FLUENT and the solution is to switch the outlet boundary condition from 'Outlet Pressure' to ' Outflow'.

4.If the length of the duct is short, I recommend using ANSYS CFX and make the outlet condition as 'opening' boundary condition.

Reversed flow can occur when pressure gradients due to geometry mean that the flow wants to re-enter the domain at the location where you have placed an outlet. Some flow reversal can be tolerated, but will have an effect on mass-conservation and thus convergence