# Velocity Verlet Integration for SPH Fluids

I am trying to implement a SPH (Smoothed Particle Hydrodynamics) based fluid solver and would like to use a second order integration method, for updating velocities and positions of the fluid particles. Some resources suggest using the velocity Verlet method, which to my knowledge, is as follows:

1. $v^{n+1/2}=v^n+\frac{\Delta t}{2}a^n$
2. $x^{n+1}=x^{n}+\Delta t v^{n+1/2}$
3. $a_{n+1} = f(...)$
4. $v^{n+1}=v^{n+1/2}+\frac{\Delta t}{2}a^{n+1}$

Where $a^0 = f(x^0, v^0)$ is computed beforehand. I would like to know how to proceed in step 3. To compute the acceleration both, the particle position and velocity, are needed. I guess position and velocity at time $n+1$ are required, but how am I am supposed to obtain $v^{n+1}$? I am sorry if this a silly question, but I found the resources I have found so far a little irritating...

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Although it might not serve you since it is an older question, computing the acceleration at step 3. can be done using a predicted $v^{n+1}$ velocity, i.e. either via an Euler step, $v^{n+1}=v^n + \Delta t a^n$ or by simply using $v^{n+1/2}$ as in a half-step approach. Then, step 4. can be interpreted as correcting the velocity.

Otherwise, if you want to, you can treat 4. as being an implicit update and simply use a non-linear solver to find $v^{n+1}$ (probably not the correct terminology, but this results in an implicit-explicit update, which may cost you a lot of energy on the long run - i.e., the integrator is no longer symplectic).

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I am struggling with this kind of problems and what I think the answer about step 4 being implicit is a good one. But in SPH you'll need also to iterate for convergence, both for compressible and incompressible.

In my case I am dealing with incompressible SPH for a Newtonian fluid, so I end up having something like

$$v^{n+1}=v^{n+1/2}+\frac{\Delta t}{2} (\mu \nabla^2 v^{n+1} - \nabla p^{n+1})$$

I then do the usual trick of defining an intermediate velocity, writing $$v^{n+1}=v^* - \frac{\Delta t}{2} \nabla p^{n+1},$$ with $$v^*:=v^{n+1/2}+\frac{\Delta t}{2} \mu \nabla^2 v^{n+1} ,$$ and the Poisson pressure equation must be solved in order $v^{n+1}$ be divergence-free: $$\frac{\Delta t}{2} \nabla^2 p^{n+1} = \nabla\cdot v^{n+1}$$

The resulting pressure is plugged back into $v^{n+1}$ until convergence is achieved.

Good luck!

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. – jameselmore Jul 13 '15 at 22:15
I am not sure I follow. The original question is unclear about what kind of SPH is considered, above all incompressible vs compressible. I have written a rather detailed answer for the former, which is the one I currently know best. I would request clarification to the original question, but, as you say, I am unable to do so yet. – Daniel Duque Jul 15 '15 at 3:58
To be honest, I didn't write that comment. I think that this post must have come through the review queues and I may have accidentally marked this post as an incomplete answer. Looking at it now, it's clear I was confused. Please excuse me! – jameselmore Jul 15 '15 at 11:51