No flow in vertical pipe.
 

As a beginner, I have attempted to model a vertical pipe, filled with water and open at top and bottom.

problem is I am not getting any flow out of the bottom outlet. The vertical velocity refuses to budge from its initial zero value.

I have used the twoLiquidMixingFoam but in this case I use only one fluid.

I have set the 0/U file as:
FoamFile
{
version 2.0;
format ascii;
class volVectorField;
location "0";
object U;

dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{
topOutlet
{
type inletOutlet;
inletValue uniform (0 0 0);
value uniform (0 0 0);
}
bottomOutlet
{
type inletOutlet;
inletValue uniform (0 0 0);
value uniform (0 0 0);
}
pipe
{
type fixedValue;
value uniform (0 0 0);
}
}
------------------------------------------------------------------------
and the 0/p_rgh file as:
FoamFile
{
version 2.0;
format ascii;

root "";
case "";
instance "";
local "";

class volScalarField;
object p_rgh;

dimensions [1 -1 -2 0 0 0 0];

internalField uniform 0;

boundaryField
{
bottomOutlet
{
type fixedValue;
value uniform 0;
}
topOutlet
{
type fixedValue;
value uniform 0;
}
pipe
{
type zeroGradient;
}

-------------------------------------------------------
The pipe is 50 cm vertical column and having a square 4 cm by 4 cm cross-section

I attach the distribution of p_rgh, p, Ux (the vertical velocity) about 10 s after the simulation starts.

SO, what could be the possible problem?
IS there a problem with BC at bottom outlet?