CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Trapezoidal microchannel and slip velocity. Help needed.

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 28, 2014, 14:36
Question Trapezoidal microchannel and slip velocity. Help needed.
  #1
New Member
 
Join Date: Jul 2013
Posts: 1
Rep Power: 0
thapostol is on a distinguished road
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.

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!

Last edited by thapostol; March 28, 2014 at 14:41. Reason: typos
thapostol is offline   Reply With Quote

Reply

Tags
laminar incompressible, microchannel, slip velocity, udf

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to set slip velocity and temperature jump awuunity CFX 10 July 2, 2018 08:40
Slip flow condition in a microchannel itzal CFX 2 August 6, 2009 03:43
slip flow in a microchannel Siddharth FLUENT 1 July 23, 2007 11:23
velocity slip at wall SA FLUENT 0 March 30, 2003 20:32
Compressible slip flow in microchannel by FEMLAB? Sander Nagoo COMSOL 2 September 10, 2002 04:39


All times are GMT -4. The time now is 14:28.