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# CFD Analysis of The Boundary Layer Separation in Water Flow

06/12/2017 12:43 PM

I need simple CFD simulation of transition of water flow from a converging cone to a straight pipe. What is good maximum cone angle, where no boundary layer separation will take place?

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#1

### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/12/2017 1:36 PM

If someone was to assist you,... some more information may help such as flow rate?

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#2

### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/12/2017 2:58 PM

I'm thinking the shallower the angle the better. Here is a plot of pressure drop factor as a function of angle:

As the water enters the reducer, the velocity increases, inversely proportional to the crossectional area (constant volume flow). The onset of turbulence is determined by the Reynold's number of the flow.

"For flow in a pipe of diameter D, experimental observations show that for "fully developed" flow,[n 2] laminar flow occurs when ReD < 1000 and turbulent flow occurs when ReD > 2000.[14] In the interval between 1000 and 2000, laminar and turbulent flows are possible and are called "transition" flows, depending on other factors, such as pipe roughness and flow uniformity. This result is generalized to non-circular channels using the hydraulic diameter, allowing a transition Reynolds number to be calculated for other shapes of channel.

These transition Reynolds numbers are also called critical Reynolds numbers, and were studied by Osborne Reynolds around 1895.[5]The critical Reynolds number is different for every geometry.[15] "

https://en.wikipedia.org/wiki/Reynolds_number

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#8
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### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 3:36 PM

Just opened your Piping Calculations Manual link, and happened to look at page 197 and there's a mistake. Same as in a recent discussion on this forum (link below), rated power of a motor. He calculates water power OK (I assume, haven't checked all details) and divides by pump efficiency. That would give him required motor output power, but he then divides that by motor efficiency and calls that the motor power. It is of course the electrical input power.

I know the difference is covered by the margin allowed, but a manual like this should get it right!

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#9
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### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 3:50 PM

Who he who called electric power the motor power? See M itty bitty ft? M R mice!

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### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/14/2017 3:23 AM

I couldn't agree more!

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#3

### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 12:12 AM

I would think that depends heavily upon the viscosity, and the pressure behind the liquid in question!

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### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 5:52 AM

<...pressure...> Ahem, not pressure. Er, um.....a local velocity, a dimension, and the viscosity and density of the fluid, actually.

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#4

### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 5:16 AM

A: Anything where the Reynolds Number is less than 2300 or so.

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#6
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### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 11:28 AM

Basically, PWSlack is correct. Any flow condition that remains essentially laminar will not separate the boundary layer, but turbulent flow will disrupt the boundary layer to whatever extent is possible at a given Reynolds number.

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#7

### Re: CFD Analysis of The Boundary Layer Separation in Water Flow

06/13/2017 11:59 AM

Simplistically, bottom-line, other variables being equal, the longer the cone, the smaller the angle, the less (disruptive) is the transistion...

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