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Which quantity can be used to visualize vortex

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Old   February 6, 2001, 11:42
Default Which quantity can be used to visualize vortex
  #1
Mike
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Could someone tell me which quantity (func(rho,u,v,w,p) ) is suitable to visualize the vortex shedding and which one for shock? By the way, I want to visualize them in Tecplot? Thank you in advance.
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Old   February 6, 2001, 15:00
Default Re: Which quantity can be used to visualize vortex
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John C. Chien
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(1). For vortex, do an array of particle tracing. (2). For shocks, use either density or pressure.
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Old   February 6, 2001, 16:37
Default vorticity -
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Patrick Godon
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Hi Mike,

I am not sure if your flow is 2D or 3D, but if your vortices are mainly 2D in nature, then you can use the vorticity, defined as the curl of the velocity. So if the vortices appear mainly in the (say) (x,y) plane (at a given z0), then compute the curl of the velocity (=vorticity) and draw the amplitude of the component of the vorticity in the -z direction (for z=z0).

If U=(u,v,w)=(Ux,Uy,Uz)

then draw the amplitude of (curl U)z to find about the vortices in the (x,y) plane.

Here is a color scale of the vorticity in a differentially rotating (quarter of a) disk with a counter rotating vortex in it: Click here .

If your flow is 3D and you have elongated vortices and/or streamwise vortices, then the tracers would probably give satisfying results.

Cheers, Patrick Godon
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Old   February 6, 2001, 17:31
Default Re: vorticity -
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Mike
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Thank you very much.
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Old   February 7, 2001, 04:01
Default Re: Which quantity can be used to visualize vortex
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Bart Prast
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For compressible flow we use mainly total pressure. A strong vortex creates a lot of dissipation so the vortex core is clearly visible in P_0. For us this is also a measure whether we have a good grid density. A coarse grid diffuses the vortex core and hence the total pressure loss very rapidly. Same holds for a shock. Density tends to vary the least of all variables. You can also calculate vorticity (rot V).
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Old   February 7, 2001, 11:29
Default Re: Which quantity can be used to visualize vortex
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Mike
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Hi,Bart, But how can you calculate Total Pressure Loss for compressible flow? What is the formula do you use? I think the calculation of total pressure loss have to assume an isentropic process.
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Old   February 7, 2001, 12:08
Default Re: vorticity -
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Patrick Godon
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Hi Mike,

Just a detail, the results I showed you with the vortex in a disk is for a compressible flow. There is in this particular case an increase of density and pressure inside anticyclonic vortices and a decrease of density and pressure inside cyclonic vortices (similar to what actually happen on earth - the low pressure systems are cyclonic - the hurricane, typhoon, etc..). However, the vortices are best seen using vorticity, since the difference in pressure and/or density is not a pronounced as in the vorticity.

Patrick Godon
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Old   February 8, 2001, 04:05
Default Re: Which quantity can be used to visualize vortex
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Bart Prast
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No, knowing the local static pressure and Machnumber you can calculate the local total pressure and hence the pressure loss uptil that point (=pressure when flow brought to rest isentropically). The only thing you assume then is a constant specific heat ratio (gamma): p0=pstat*(1+((gamma-1)/2)*Mach^2)^(gamma/(gamma-1)) For isentropic flow it is indeed: p0=pstat*(T0/Tstat)^(gamma/(gamma-1)). But T0 will not change over a shock for instance. Hope this helps.
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