Boundary Layer Thickness is Non-Developed Internal Flow

Obad · January 31, 2017, 12:50

Hi guys,

I'm simulating the flow through a complex shaped nozzle with undeveloped flow (due to mixing and flow separation). Now I would like to determine (approximate) the boundary layer thickness to get the flow properties at the boundary layer edge.
The criterion with 99% of the free stream velocity doesn't work in that case, because there is no uniform free stream...

Do you have any ideas?

Cheers!

FMDenaro · January 31, 2017, 13:34

Sorry, but I think that your question is not well posed, for your problem it appears not correct to talk about a real BL ... Maybe you can give us the details so that the question is better addressed for this case.

Obad · February 1, 2017, 05:48

My case deals with the determination of the heat transfer over the nozzle walls. In order to proplery resolve the flow next to the wall I'm using a hybrid grid with a structured grid (prismatic layers) at the wall.

To check if the used prismatic layer is thick enough I wanted to measure the boundary layer extend. I tried to define the boundary layer thicknes by using the total temperature profile which according to compressible boundary layer theory should show an overshoot close to the boundary layer edge. However, in some parts of the flow this overshoot appears to happen much further away from the wall as you would expect it to be...

Additionally I would like to define the heat transfer coefficient by using the boundary layer edge properties for defining the recovery temperature.

FMDenaro · February 1, 2017, 06:11

Without details of your flow problem I can just have an idea...but to define a boundary layer thickness you should argue that it exists an external inviscid-like region...and from what you wrote I see a totally internal confined flow.

Obad · February 1, 2017, 08:07

As I said it's the flow through an ordinary rocket nozzle. The nozzle is overexpanded which leads to flow separation.

There definietely is a inner region with constant (almost zero) vorticity which could be considered as the potential flow. However, defining the boundary layer as the region with vorticity leads to very high vaules (approx. 40cm). Since it is an accelerated flow I suppose the boundary layer should be much thinner.

January 31, 2017, 12:50	Boundary Layer Thickness is Non-Developed Internal Flow	#1
Obad Member Obad Join Date: Sep 2013 Posts: 42 Rep Power: 12	Hi guys, I'm simulating the flow through a complex shaped nozzle with undeveloped flow (due to mixing and flow separation). Now I would like to determine (approximate) the boundary layer thickness to get the flow properties at the boundary layer edge. The criterion with 99% of the free stream velocity doesn't work in that case, because there is no uniform free stream... Do you have any ideas? Cheers!

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January 31, 2017, 13:34		#2
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	Sorry, but I think that your question is not well posed, for your problem it appears not correct to talk about a real BL ... Maybe you can give us the details so that the question is better addressed for this case.

February 1, 2017, 05:48		#3
Obad Member Obad Join Date: Sep 2013 Posts: 42 Rep Power: 12	My case deals with the determination of the heat transfer over the nozzle walls. In order to proplery resolve the flow next to the wall I'm using a hybrid grid with a structured grid (prismatic layers) at the wall. To check if the used prismatic layer is thick enough I wanted to measure the boundary layer extend. I tried to define the boundary layer thicknes by using the total temperature profile which according to compressible boundary layer theory should show an overshoot close to the boundary layer edge. However, in some parts of the flow this overshoot appears to happen much further away from the wall as you would expect it to be... Additionally I would like to define the heat transfer coefficient by using the boundary layer edge properties for defining the recovery temperature.

February 1, 2017, 06:11		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	Without details of your flow problem I can just have an idea...but to define a boundary layer thickness you should argue that it exists an external inviscid-like region...and from what you wrote I see a totally internal confined flow.

February 1, 2017, 08:07		#5
Obad Member Obad Join Date: Sep 2013 Posts: 42 Rep Power: 12	As I said it's the flow through an ordinary rocket nozzle. The nozzle is overexpanded which leads to flow separation. There definietely is a inner region with constant (almost zero) vorticity which could be considered as the potential flow. However, defining the boundary layer as the region with vorticity leads to very high vaules (approx. 40cm). Since it is an accelerated flow I suppose the boundary layer should be much thinner.