# Questions about eddy

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 November 27, 2013, 10:58 Questions about eddy #1 New Member   MMS Join Date: Nov 2013 Posts: 29 Rep Power: 3 As a newcomer in the field of turbulence, I am struggling to find out answer of some basic questions regarding turbulence. when we are talking about large scale or small scale eddies in turbulent motion actually what does it means. Here I have listed some questions: 1. What is the physical meaning of large scale and small scale eddy and how these eddy lengths are measured? 2. Apart from that what does mean by the time scale of eddy? 3. Addition, what is basic difference between eddies and vortices? [Most of the cases I have found that these terms are used as same terminology] It would be better if someone help me to understand these initial things in simple way. Thanks. Last edited by cfdmms; December 1, 2013 at 09:14.

November 27, 2013, 12:17
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by cfdmms As a newcomer in the field of turbulence, I am struggling to find out answer of some basic questions regarding turbulence. when we are talking about large scale or small scale eddies in turbulent motion actually what does it means. Here I have listed some questions: 1. What is the physical meaning of large scale and small scale eddy and how these eddy lengths are measured? 2. Apart from that what does mean by the time scale of eddy? 3. Addition, what is basic difference between eddies and vortices? [Most of the cases I have found that these terms are used as same terminology] It would be better if someone help me to understand these initial things in simple way. Thanks.

Hello, what books are you reading?

Roughly speaking, an eddy is a also vortical structure, but the term is also associated to the fact that is time-life is quite long to make it "observable".
Large eddies are characterized by having a time-life longer than the characteristic turnover time, they have a characteristic lenght that is associated to a wavenumber in the inertial part of the energy spectrum.
Small eddies are rapidly dissipated by the action of the molecular dissipation, they are at the end of the inertial region, form the Taylor microscale until to the Kolmogorov one.

November 27, 2013, 13:10
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 Originally Posted by FMDenaro Hello, what books are you reading? Roughly speaking, an eddy is a also vortical structure, but the term is also associated to the fact that is time-life is quite long to make it "observable". Large eddies are characterized by having a time-life longer than the characteristic turnover time, they have a characteristic lenght that is associated to a wavenumber in the inertial part of the energy spectrum. Small eddies are rapidly dissipated by the action of the molecular dissipation, they are at the end of the inertial region, form the Taylor microscale until to the Kolmogorov one.
Presently I am reading the POPE one though I need something which will describe these basic things in more easy way.

Another question is, at the last stage of energy cascade it is said that energy is dissipating through heat due to viscous effect. My question, where the heat is going?

Addition, how different characteristic length of eddies are accounted in numerical simulation? is it possible to measure the eddy size practically or it just a theoretical concept?

 November 27, 2013, 13:27 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 1,588 Rep Power: 20 good book... mechanical energy is converted in internal energy, if your flow is compressible, the mechanism is enclosed in the governing equations. For incompressible constant-density flows, the temperature equaation is not considered. the Taylor and Kologorov scales can be theoretically estimated, the largest inertial scale is bounded by the characteristic lenght of the the geometry of your flow problem

December 10, 2013, 02:32
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 Originally Posted by FMDenaro good book... mechanical energy is converted in internal energy, if your flow is compressible, the mechanism is enclosed in the governing equations. For incompressible constant-density flows, the temperature equaation is not considered. the Taylor and Kologorov scales can be theoretically estimated, the largest inertial scale is bounded by the characteristic lenght of the the geometry of your flow problem
I have got that for LES its need to resolve inertial subrange. And for LES another important parameter is Yplus. How these can be correlate during LES?
For engineering application it is Yplus<30, what is the selection criteria of Yplus for other types flow (like pipe flow, lid driven or buoyancy driven cavity flow).
Response would be highly appreciated. Thanks.

December 10, 2013, 12:16
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Filippo Maria Denaro
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 Originally Posted by cfdmms Thanks for your reply. I have got that for LES its need to resolve inertial subrange. And for LES another important parameter is Yplus. How these can be correlate during LES? For engineering application it is Yplus<30, what is the selection criteria of Yplus for other types flow (like pipe flow, lid driven or buoyancy driven cavity flow). Response would be highly appreciated. Thanks.
Inertial range in the energy spectrum and y+ wall-coordinate are two totally different concepts...

December 23, 2013, 12:06
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 Originally Posted by FMDenaro Inertial range in the energy spectrum and y+ wall-coordinate are two totally different concepts...
I am bit confused about this what does it mean by LES need to resolve upto inertial subrange? Does it mean for every single eddy generate in the fluid flow according to energy cascade?

December 23, 2013, 12:22
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 Originally Posted by cfdmms I am bit confused about this what does it mean by LES need to resolve upto inertial subrange? Does it mean for every single eddy generate in the fluid flow according to energy cascade?
it means that your LES grid size h_LES must be as small as that k_LES=pi/h_LES lies in the inertial range of the energy spectrum.

December 24, 2013, 02:01
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 Originally Posted by FMDenaro it means that your LES grid size h_LES must be as small as that k_LES=pi/h_LES lies in the inertial range of the energy spectrum.
Hi FMDenaro! Thanks for your reply. It will be a great help if you kindly help me to understand these following points.

According to your reply does it mean we will have to calculate the energy spectrum E(k) against the wave numbers (k) and check it whether it falls in the k^-5/3 region?

I have a additional question, in LES how filter separates large and small eddies?

Last edited by cfdmms; December 30, 2013 at 09:44.

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