# Changing the speed of sound

Hello to DualSPHysics team

In my case study, the fluid has a very slow velocity so I thought it is a good idea to artificially decrease the speed of sound. Therefore, a longer time step can be chosen for stability reasons (by the CFL condition).

I mean: Artificial speed of sound>10*maximum velocity of fluid

But I observed that decreasing the speed of sound had no effect on run time.

Please advise me if there is a problem with my constant definition:

<constantsdef>

<lattice bound="1" fluid="1" />

<gravity x="0" y="0" z="-9.81" comment="Gravitational acceleration" units_comment="m/s^2" />

<rhop0 value="5000" comment="Reference density of the fluid" units_comment="kg/m^3" />

<hswl value="0" auto="true" comment="Maximum still water level to calculate speedofsound using coefsound" units_comment="metres (m)" />

<gamma value="7" comment="Polytropic constant for water used in the state equation" />

<speedsystem value="0" auto="true" comment="Maximum system speed (by default the dam-break propagation is used)" />

<coefsound value="20" comment="Coefficient to multiply speedsystem" />

<speedsound value="10" auto="false" comment="Speed of sound to use in the simulation (by default speedofsound=coefsound*speedsystem)" />

<coefh value="1" comment="Coefficient to calculate the smoothing length (h=coefh*sqrt(3*dp^2) in 3D)" />

<cflnumber value="0.8" comment="Coefficient to multiply dt" />

<h value="0" auto="true" units_comment="metres (m)" />

<b value="0" auto="true" units_comment="Pascal (Pa)" />

<massbound value="0" auto="true" units_comment="kg" />

<massfluid value="0" auto="true" units_comment="kg" />

</constantsdef>

Thank you for your consideration

## Comments

• Please, check the Case.out generated by GenCase

However, this should be ok since you defined Cs=10 m/s

Note that initial time step (the very first one) will be h/Cs, and after that the time step for each step is computed as variable using information from particles (forces, Cs and visco)

Regards

• Alex I might misunderstand your comment, but it seems like you mention forces, speed of sound and viscosity. The documentation mentions:

I see forces in f_a, I see speed of sound in the viscous term and viscosity in the viscous term.

This is all what you meant right? There is no "hidden" time stepping term?

Kind regards

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