Reverse flow at Outlet

ARUN K RAJ · March 5, 2015, 05:06

I am dealing with a reverse flw scenario at outlet.

I am trying to obtain condensation over vertical surface. When i run the file with zero gravity i get the condensing layers bt with gravity set as 9.81 in constant properties i notice a reverse flow at the outlet. So is there any influence of g with reverse flow at outlet. I am guessing that it has smthng to do with pressure and velocity boundary conditions. The pressure boundary is causing the flow problem mstly (i guess so). Any suitable combinations of pressure and velocity filed can make this disappear????

In my case i have a fixed value of velocity at inlet and fixed value of pressure at outlet.
Can anyone suggest me a way to properly set the boundary conditions for 0/p and 0/u with a fixed value of inlet velocity and fixed value of pressure at outlet?????

fluid23 · March 6, 2015, 18:44

What kind of geometry do you have? If you have a pipe flow you can probably just extend your pipe out a bit (i.e. move your boundary condition). If this is a pressure outlet on an external boundary condition, you can probably ignore it if it is far enogh away from what you are ineterested in.

I get reveresed flow quite often when doing external aero analysis of helicopters in hover condition.

ARUN K RAJ · March 7, 2015, 14:41

Hi Matt,

My analysis is regarding flow btw two parallel plates held at constant temperature. I indeed got favuorable results in absence of "g" using inlet velocity (0/U1) and a outlet pressure(0/p). Bt when i add "g" the problem of reverse flow happens within the entire domain i.e, reciruclates happens at mid regions, outlet and sometimes at inlet even. I have noticed that if i fix any value as outlet pressure for a fixed inlet velocity the pressure frm the inlet to outlet starts decreasing (becomes negative) with respect to a fixedvalue uniform 0 assigned at outlet condition for pressure.
The boundary fields assigned are:

0/U1
internalField uniform (0 -0.004 0);
boundaryFields

inlet -- fixed value, uniform (0 -0.004 0);
outlet -- zeroGradient;

0/p
internalField uniform 0;
boundaryFields

inlet -- zeroGradient;
outlet -- fixedValue, uniform 0;

When i put "g" as zero the above conditions works. i get positive pressure from inlet to fixed value 0 till outlet. bt when i put "g" as -9.81 the pressure decreases in the domain and that causes reverse flow. is there any suitable boundary conditions to figure this out. ( for me i need the value of velocity as -0.004 at inlet, so any suitable suggestions for the same)?

fluid23 · March 8, 2015, 13:21

That is interesting. I am not sure what to tell you. It sounds like you have very low Reynolds number, yes? It could be that boyant convection (which is driven by gravity and temperature) is dominating your flow field at low speeds. That is only a guess, though. I haven't done much work with boyant convective flows so I don't know what ot expect.

ARUN K RAJ · March 18, 2015, 10:43

Hi Matt,
Indeed, your thoughts abt buoyancy effects was true. temperature n gravity causes a pressure reduction within the internal field which causes a higher pressure at outlet( even though its fixed at atm pressure). due to pressure difference flow starts to reverse.

Till now, i wasnt able to solve the problem. Bt by keeping the inlet of pressure as fixedFluxPressure BC the solver seems to run widout reverse flow, bt still pressure contours really show negative pressures inside the domain.

March 5, 2015, 05:06	Reverse flow at Outlet	#1
ARUN K RAJ New Member ARUN Join Date: Jan 2015 Location: Kerala Posts: 18 Rep Power: 11	I am dealing with a reverse flw scenario at outlet. I am trying to obtain condensation over vertical surface. When i run the file with zero gravity i get the condensing layers bt with gravity set as 9.81 in constant properties i notice a reverse flow at the outlet. So is there any influence of g with reverse flow at outlet. I am guessing that it has smthng to do with pressure and velocity boundary conditions. The pressure boundary is causing the flow problem mstly (i guess so). Any suitable combinations of pressure and velocity filed can make this disappear???? In my case i have a fixed value of velocity at inlet and fixed value of pressure at outlet. Can anyone suggest me a way to properly set the boundary conditions for 0/p and 0/u with a fixed value of inlet velocity and fixed value of pressure at outlet?????

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March 6, 2015, 18:44		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	What kind of geometry do you have? If you have a pipe flow you can probably just extend your pipe out a bit (i.e. move your boundary condition). If this is a pressure outlet on an external boundary condition, you can probably ignore it if it is far enogh away from what you are ineterested in. I get reveresed flow quite often when doing external aero analysis of helicopters in hover condition.

March 7, 2015, 14:41		#3
ARUN K RAJ New Member ARUN Join Date: Jan 2015 Location: Kerala Posts: 18 Rep Power: 11	Hi Matt, My analysis is regarding flow btw two parallel plates held at constant temperature. I indeed got favuorable results in absence of "g" using inlet velocity (0/U1) and a outlet pressure(0/p). Bt when i add "g" the problem of reverse flow happens within the entire domain i.e, reciruclates happens at mid regions, outlet and sometimes at inlet even. I have noticed that if i fix any value as outlet pressure for a fixed inlet velocity the pressure frm the inlet to outlet starts decreasing (becomes negative) with respect to a fixedvalue uniform 0 assigned at outlet condition for pressure. The boundary fields assigned are: 0/U1 internalField uniform (0 -0.004 0); boundaryFields inlet -- fixed value, uniform (0 -0.004 0); outlet -- zeroGradient; 0/p internalField uniform 0; boundaryFields inlet -- zeroGradient; outlet -- fixedValue, uniform 0; When i put "g" as zero the above conditions works. i get positive pressure from inlet to fixed value 0 till outlet. bt when i put "g" as -9.81 the pressure decreases in the domain and that causes reverse flow. is there any suitable boundary conditions to figure this out. ( for me i need the value of velocity as -0.004 at inlet, so any suitable suggestions for the same)?

March 8, 2015, 13:21		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	That is interesting. I am not sure what to tell you. It sounds like you have very low Reynolds number, yes? It could be that boyant convection (which is driven by gravity and temperature) is dominating your flow field at low speeds. That is only a guess, though. I haven't done much work with boyant convective flows so I don't know what ot expect.

March 18, 2015, 10:43		#5
ARUN K RAJ New Member ARUN Join Date: Jan 2015 Location: Kerala Posts: 18 Rep Power: 11	Hi Matt, Indeed, your thoughts abt buoyancy effects was true. temperature n gravity causes a pressure reduction within the internal field which causes a higher pressure at outlet( even though its fixed at atm pressure). due to pressure difference flow starts to reverse. Till now, i wasnt able to solve the problem. Bt by keeping the inlet of pressure as fixedFluxPressure BC the solver seems to run widout reverse flow, bt still pressure contours really show negative pressures inside the domain.