# Assumption for time model with SIMPLE algorithm

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March 15, 2021, 00:24
Assumption for time model with SIMPLE algorithm
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Hello,

I´m trying to simulate a fluid damper: a double acting piston, which is moving with a sinusoidal function (10Hz) in a chamber and compresses/ expands the fluid which flows through a choke (s. picture)

On the example of my system:
when considering the massflow from one
chamber to the other while the piston is moving and the continous change of pressure because of motion - I´m thinking this solution is unsteady (time dependent), because i have different results between two time steps - right? , so according to the literature: my temporal discretization must
be 2nd order, because my physics have an unsteady behaviour (true so far?) and in case I use the SIMPLE algorithm: is it right, that it actually solves a steady state solution INSIDE the time step? So my transient behaviour would be a "compound of many steady state solution in each time step"?

According to this, my question is: Do i really NEED to use the 2nd order temporal discretization or is it possible to use the 1st order?
I would really appreciate if someone could explain it as simple as possible, because I´m totaly new in this theme

P.S.:
I used the following considerations in STAR-CCM:
By using the Unsteady Implicit Solver and choosing the segregated flow,
I would choose the 2nd order temporal disretization. When solving with
SIMPLE, than i choose a larger time step with a higher number of
inner iterations and hopefully the solution converges inside the timestep?
Attached Images
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 Tags simple, star ccm+, temporal discretization, time model