Weird velocity vector shape after step height (BFS)
 

Below is backward-facing step 1 micron microchannel with 75% step height, 500 Reynolds number. Investigation is performed on velocity vector after step height. It appears that something interesting happen.

View 1, 5 pictures (further away from step height)

http://i911.photobucket.com/albums/a...arlocklw/1.jpg

http://i911.photobucket.com/albums/a...locklw/2-1.jpg

http://i911.photobucket.com/albums/a...arlocklw/3.jpg

http://i911.photobucket.com/albums/a...arlocklw/4.jpg

http://i911.photobucket.com/albums/a...arlocklw/5.jpg

View 2, 5 pictures, same with View 1 (Further away from step height)

http://i911.photobucket.com/albums/a...arlocklw/a.jpg

http://i911.photobucket.com/albums/a...arlocklw/b.jpg

http://i911.photobucket.com/albums/a...arlocklw/c.jpg

http://i911.photobucket.com/albums/a...arlocklw/d.jpg

http://i911.photobucket.com/albums/a...arlocklw/e.jpg

There appears a egg-alike shape after step height, when the recirculation zone is over. Why is this happen? Please help for anyone know. Thank you.

this seems to be the projection method problem. Select the proper projection method for plotting vectors.

Or this may be something new. I am not sure.

Since this is about fluid velocity, I am quite sure it should already been discovered. The projection is view towards outlet, or view towards -z axis. Above views are set so that clear view of what happened can be observed. The flow type is set as laminar flow.

My original assumption is there are 2 forces acting on it. One is fluid kinetic flow at the center, another one is fluid viscosity towards call that drag the fluid. 2 forces acting on different direction, causing the egg shape things.

However, this egg shape not happen before step height. That's why I suspect step height has something to do with the formation of it. Please help, any reply is welcomed and appreciated. I will post any view required.

I changed arrow shape and discovered that fluid at side flow upwards to fill the void created when the current flow down. The egg shape are vector between vector flow down at center and vector go up at side. This thread can be closed. Thanks for your help.

so that was the visualization problem?

Can be say so, since the fluid near boundary flow at slow speed, the arrow is too small. When I normalized it, I can see the arrow direction clearly. And I notice that the horizontal and vertical line are actually arrowhead. Horizontal means fluid flow up and down, vertical means fluid flow left and right.