# The problem of solving pressure(p) in 3D

**URL:** https://community.freefem.org/t/the-problem-of-solving-pressure-p-in-3d/65
**Category:** General Discussion
**Created:** [June 3, 2019, 3:17am UTC](https://community.freefem.org/t/the-problem-of-solving-pressure-p-in-3d/65 "2019-06-03T03:17:24Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![xlzheng](https://avatars.discourse-cdn.com/v4/letter/x/2acd7d/32.png) [@xlzheng](https://community.freefem.org/u/xlzheng)
#### Post date: [June 3, 2019, 3:17am UTC](https://community.freefem.org/t/the-problem-of-solving-pressure-p-in-3d/65/1 "2019-06-03T03:17:25Z")

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Hi,  
I want to solve pressure with the initial velocity in 3d, but it doesn’t work, the variational equation comes from the following N-S equation:  
 ![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/f3b32bb4a2c4137d82fa6845234bbf9f95fa3e64.png)  
the program as follows:

```auto
//
load "msh3"
load "tetgen"
load "medit"
//verbosity=2;

real hs4=0.02;
real voltet4=(hs4^3)/6.;
real[int] domain = [0.,0.,0.5,53,voltet4];

real x0=0.,x1=1;
real y0=0.,y1=1;
real z0=0.,z1=1;
int [int,int] L=[[1,2],[3,4],[5,6]];
int[int] refx=[0,L(0,0)],refX=[0,L(0,1)];
int[int] refy=[0,L(1,0)],refY=[0,L(1,1)];
int[int] refz=[0,L(2,0)],refZ=[0,L(2,1)];

mesh Thx=square(50,50,[y0+(y1-y0)*x,z0+(z1-z0)*y]);
mesh Thy=square(50,50,[x0+(x1-x0)*x,z0+(z1-z0)*y]);
mesh Thz=square(50,50,[x0+(x1-x0)*x,y0+(y1-y0)*y]);

mesh3 Thx0=movemesh23(Thx,transfo=[x0,x,y],label=refx);
mesh3 Thx1=movemesh23(Thx,transfo=[x1,x,y],label=refX);
mesh3 Thy0=movemesh23(Thy,transfo=[x,y0,y],label=refy);
mesh3 Thy1=movemesh23(Thy,transfo=[x,y1,y],label=refY);
mesh3 Thz0=movemesh23(Thz,transfo=[x,y,z0],label=refz);
mesh3 Thz1=movemesh23(Thz,transfo=[x,y,z1],label=refZ);

mesh3 ThH=Thx0+Thx1+Thy0+Thy1+Thz0+Thz1;
plot(ThH,wait=1);

mesh3 Th=tetg(ThH,switch="paAAYYQ",nbofregions=1,regionlist=domain);

savemesh(Th,"mesh.mesh");
medit("mesh",wait=1,Th);

// FE Space 
fespace Xh(Th,P23d); //for velocity
fespace Mh(Th,P13d); //for pressure and stress
Xh u1old,u2old,u3old;
Xh p,q;

// Physical parameter 
real re=1000,;
int k;

for(re=1000;re<=1000;re=re+1) // Loop on vicosity 
{ 
	real nu= 1./re; 
	// stop test for Newton 
	real eps=1e-6;
	// intial guess with B.C. 
	u1old=(z==1);
	u2old=0;
	u3old=0;
	verbosity=0;
	solve Pold(p,q) = 
		int3d(Th) (dx(p)*dx(q)+dy(p)*dy(q)+dz(p)*dz(q))
		-int3d(Th) (q*(dx(u1old)*dx(u1old)+dy(u2old)*dy(u2old)+dz(u3old)*dz(u3old)+2*dy(u1old)*dx(u2old)+2*dz(u1old)*dx(u3old)+2*dz(u2old)*dy(u3old)));	
	
	ofstream ppp(re+"+p.txt");
	for (int m=0;m<Th.nv;m++)
	{
		ppp<<Th(m).x<<" "<<Th(m).y<< " "<<Th(m).z<< " "<<p(Th(m).x,Th(m).y,Th(m).z)<<endl;
	}  
}

```

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<div class="post-metadata">

### Author: ![xlzheng](https://avatars.discourse-cdn.com/v4/letter/x/2acd7d/32.png) [@xlzheng](https://community.freefem.org/u/xlzheng)
#### Post date: [June 3, 2019, 10:04am UTC](https://community.freefem.org/t/the-problem-of-solving-pressure-p-in-3d/65/2 "2019-06-03T10:04:26Z")

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I think I have make it

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<div class="post-metadata">

### Author: ![sumantkr](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/sumantkr/32/2681_2.png) [@sumantkr](https://community.freefem.org/u/sumantkr)
#### Post date: [September 14, 2021, 7:13am UTC](https://community.freefem.org/t/the-problem-of-solving-pressure-p-in-3d/65/3 "2021-09-14T07:13:52Z")

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hello @xlzheng , can you please upload a complete code for such 3d problem, I am also stucked in such issues.

Thanks in advance!!!