CFD Online Discussion Forums - Trapezoidal microchannel and slip velocity. Help needed.

Hello guys. First of all thumbs up for a great forum. Superb job!

I am trying to simulate a laminar, steady state, incompressible liquid (water) flow through a micro-channel (trapezoid cross section), with a uniform inlet-velocity (couple of mm/sec).

The simulation's velocity results are then compared to actual microPIV measurements on a super-hydrophilic and a super-hydrophobic microchannel.

The fluent solution, with no-slip wall boundary conditions, provides velocity profiles in accordance with the measurements on the super-hydrophilic microchannel.

For the actual super-hydrophobic channel, experiments provide the slip velocity (and slip length) of the flow for the small base and the sidewall of the microchannel.

Now I need a way to fix these channel-height-related slip values on the fluid domain and calculate a solution. :confused:

I tried using DEFINE_PROFILE UDFs to impose wall motion velocities (but the results are nowhere near the measurements). Also I have been trying to use fixed velocities in the fluid domain with no luck so far.

Some questions:
1) Is FLUENT able to handle non-uniform wall motion for wall boundaries (in reality no wall can move that way...)?
e.g.

Code:

Vel(x)=a*y+b // a=a1, b=b1 for 0 <= y <Y1

                            a=a2, b=b2 for Y1 <= y <y2   ...



                            a=an, b=bn  for Yn-1 <=y <=Yn.

2) Having a universal XYZ coordinate system (X along the length of microchannel, Y height between parallel bases and Z along small base), how can I input the Y coordinates of the sidewall (54 degrees angle against the parallel bases) in a UDF? Will the use of universal x[1] coordinates for the sidewall ID thread be enough for the above mentioned velocity functions??

3) Fixed velocities values in the fluid domain.
I am trying to find out whether can I fix velocities in the cells adjacent to the thread zones of wall boundaries, or not.

I think maybe something like this udf not used yet (in part used for the aforementioned velocity profiles).

Code:

#include "udf.h"

DEFINE_PROFILE(fixed_velocities,t,nv)

{

        real x[ND_ND];

        double a;

        double b;

        double y;

        cell_t c;

        Thread* t;

        int idsidewall = 6;

        int idbottom = 0;

        

        if (THREAD_ID(t)==idsidewall)

        begin_c_loop(c,t)

        {

                C_CENTROID(x,c,t);

                y=x[1];

                if ((y >= 0.) && (y < 0.000041027))

                        {

                        a = 0.731231;

                        b = 0.00125;

                        }

                else if (y < 0.000049232)

                        {

                        a = 12.181716;

                        b = 0.00078;

                        }

                else if (y < 0.000065643)

                        {

                        a = 100.544204;

                        b = -0.00357;

                        }

                else if (y < 0.000082053)

                        {

                        a = 12.796535;

                        b = 0.00219;

                        }

                else if (y < 0.000098464)

                        {

                        a = -38.389605;

                        b = 0.00639;

                        }

                else if (y < 0.000114875)

                        {

                        a = -91.403822;

                        b = 0.01161;

                        }

                else if (y < 0.000147696)

                        {

                        a = -22.850956;

                        b = 0.003735;

                        }

                else if (y < 0.00015196)

                        {

                        a = -30.435494;

                        b = 0.004855;

                        }

                else

                        {

                        a=0.;

                        b=0.;

                        }

                F_PROFILE(c,t,nv) = a*y + b;

        end_c_loop(c,t)

        }        

        

        

        if (THREAD_ID(t)==idbottom)

        begin_c_loop(c,t)

        {

                C_CENTROID(x,c,t);

                F_PROFILE(c,t,nv) = 0.00125;

        

        end_c_loop(c,t)

        }

}

If I haven't explained appropriately my case please ask for clarifications.
Any ideas, pointers and/or corrections will be much appreciated!