# Pyro solver - difference between inputs

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Hi!

What is the difference between solver inputs (pre-solve, velupdate, advection, post-solve)?

as for instance i may plug my gasfieldvop1 microsolver in any pyro solver inputs.

the simulation looks the same in any case.

may be for this particular microsolver it does not matter , but for another microsolver it will do ?

solver.hipnc

Edited by 90okm
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correct me if i am wrong , from what i understand ,

a quick explanation to this is ,

the solver is composed of many parts which is solving the navier stokes equation for fluids ,

according the the equation the velocity is solved and updated at every point ,

so post solve and pre solve are computed before and after solving the field , which then updates the position /density/pressure etc of the point

update velocity modifies the solved velocity field and advection uses this velocity field to advect the input points around using this filed  ,

of course there is more to solving the navier stokes equation , just to explain what these inputs are on this solver

more in depth explanation can be found here Wikipedia

https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations

Flow velocity

The solution of the Navier–Stokes equations is a flow velocity. It is a field, since it is defined at every point in a region of space and an interval of time. Once the velocity field is calculated, other quantities of interest, such as pressure or temperature, may be found. This is different from what one normally sees in classical mechanics, where solutions are typically trajectories of position of a particle or deflection of a continuum. Studying velocity instead of position makes more sense for a fluid; however for visualization purposes one can compute various trajectories.

General continuum equations

The Navier–Stokes momentum equation can be derived as a particular form of the Cauchy momentum equation. In an inertial frame of reference, the conservation form of the equations ofcontinuum motion is:[2]

Cauchy momentum equation (conservation form)

where

is the density,  is the flow velocity,  is the del operator.  is the pressure  is the identity matrix  is the deviatoric stress tensor, which has order two,  represents body accelerations (per unit mass) acting on the continuum, for example gravityinertial accelerationselectric field acceleration, and so on.

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this vimeo video explains it better

it is in russian , , but have english subtitles

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