CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > FLUENT

Transient inlet velocity - Reversed Flow

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

LinkBack Thread Tools Display Modes
Old   July 26, 2007, 10:52
Default Transient inlet velocity - Reversed Flow
Posts: n/a
Hello, I am trying to run an unsteady simulation with pulsatile velocity, approximately a sine function, contained in a boundary profile file. Even if I try a simple geometry like a cylinder there is always reversed flow expressed by negative vectors in the descending portion of the pulsatile function near the walls. Is this physically valid? How can I avoid this? Thanks in advance.


  Reply With Quote

Old   July 26, 2007, 15:22
Default Re: Transient inlet velocity - Reversed Flow
Posts: n/a

Here is what "I think" is happening

In a real internal flow with an alternating positive velocity, the minimum velocity near the walls is zero (no-slip condition). (Unless you have a small nozzle (inlet) ejecting fluid in a large tube. In this case two vortices will appear on the sides (2D model).

In the CFD modeling of a pulsating inlet velocity, as the velocity increases, most of the mass gets accelerated in the core of the flow. In order to satisfy conservation of mass across a cross section at a specific moment in time, some mass gets reversed near the walls (again, mass is consereved at this cross section at this moment in time, i.e, what goes in must come out). {Don't quote me on that!}

Please do the following and teach me:

1)See if the mass flow rate is constant in a single moment of time at different cross sections (i.e. at a point where you have a reveresed flow and at a point downstream where you don't have any).

2)See if changing the walls to Symmetry boundary condition eleminate the negative sign (Of course this is not what you want, but see its effect).

3)Try having an extremely fine grid near the walls. This might supress the problem.


  Reply With Quote


Thread Tools
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 On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Problem with assigned inlet velocity profile as a boundary condition Ozgur_ FLUENT 5 August 25, 2015 04:58
udf for transient mass flow rate inlet Fluent UDF and Scheme Programming 0 February 22, 2011 03:51
Channel flow: Inlet pressure AND uniform velocity? frmap1 Main CFD Forum 0 April 15, 2010 10:32
VOF Volume fraction in Velocity inlet aruelle FLUENT 1 March 18, 2010 12:53
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 09:11

All times are GMT -4. The time now is 07:52.