Sedimentation fall velocity 1

[Fishead]

Hi,

I must admit to being a mechanical engineer, thrust upon a water treatment project... Help! I'm looking at terminal velocities for sand particles falling out of water in a settling pond. The equations that I'm getting out of "Unit Operations and Processes in Environmental Engineering" are telling me that sand will fall out more quickly in a turbulent water flow than in a laminar flow... Am I nuts to believe it?

Thanks
GArth

 

[BigInch]

I'm no expert on this, so wait for some more opinions.

It does seem counterintuitive, however I did a little search and turned up this doc. It suggests that the drag factor of particles in turbulent flow regimes is less than the drag factor of the similar particles in laminar flow. It also points out that the data from which the Stokes equations with which we are most familiar are restricted to Nre flows, <=1. How about that? I don't seem to remember my professor mentioning that somewhat limiting factor, but then again, I was busy with other things at the time.

http://www.ntu.edu.sg/home/cnscheng/Publications/reprint/Paper for web _settling_.pdf

http://virtualpipeline.spaces.msn.com

 

[bimr]

Yes, that is what you will get from the classic laws of sedimentation formed by Newton and Stokes. But, you should note that the settling of particles in dilute suspension as defined by the classic laws has no direct application in water treatment.

However, the classic laws are the basis for the hypothetical relationship between settling velocity and overflow rate.

In practice, sedimentation basin design factors must be adjusted to allow for the effects of inlet and outlet turbulence, short-circuiting, scour, sludge storage, and velocity gradients. As BigInch notes, sedimentation basins are based on laminar flow conditions.

In practice, when a storm "turns a lake over", the suspended solids concentration is much higher from the turbulence.

 

[Fishead]

Hey, thanks alot. Yes, I did notice that the CD was quite a bit lower in turbulence, but I'm getting a 4X speed increase which sounds a bit questionable...

Thanks for you help!
Garth

 

[Fishead]

Thanks bimr,

Yeah, I know I need to keep it laminar, but I just started questioning the equations because like you said, turbulent flow is when sediment gets suspended...

I suppose one needs to believe the book...

Thanks a lot!