Reply To: Polyflow pressure contour issue

[Prashanth]

Hello, please find the answers to your questions..

-- First of all, the pressure variable (p) does not necessarily correspond to the pressure that could be experimentally measured with an appropriate device. The experimentally measured quantity corresponds actually to a given component of the force density applied to a given surface (with its normal vector).There is probably a historical reason for the confusion: for an incompressible Newtonian fluid in a simple Poiseuille flow, the calculated pressure corresponds to the one that could be measured. Notwithstanding, for a general complex flow, the pressure variable should at first be understood as the unknown (Lagrange multiplier) associated with the incompressibility constraint

To illustrate this, let us consider a simple extensional flow along the z-direction for a Newtonian fluid filament. We assume that a zero force is applied on the border. The total stress tensor S is diagonal, and given by:

Szz = -p + 2 eta . e-dot , Sxx = Syy = -p - eta . e-dot ,

where eta is the viscosity, while e-dot is the extension rate (positive). Since we assumed that a zero force is applied on the border of the filament, we have:

p = -eta . e-dot

Hence, the pressure can be as negative as one wants.

 

-- Yes, it looks blocky since you have defined only 11 color bands (by default). You can increase the # of contours to make it smooth..