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run the code there comes "-1.#IND",what is the matter??

qwerwan5 (7)
here is code ,when n=500,"-1.#IND" appears. what is the problem???

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#include<iostream>
#include<fstream>
#include<sstream>
#include<string>
#include<iomanip>
#include<cstdlib>
#include<cmath>

using namespace std;

const int Q=9;
const int NX=400;
const int NY=40;
const double U0=0.1;

int i,j,ip,jp,inext,jnext,k,n;
int Re=200;
double niu=U0*NY/Re,tau=3.0*niu+0.5;

int e[Q][2]={{0,0},{1,0},{0,1},{-1,0},{0,-1},{1,1},{-1,1},{-1,-1},{1,-1}};
double w[Q]={4.0/9,1.0/9,1.0/9,1.0/9,1.0/9,1.0/36,1.0/36,1.0/36,1.0/36};

double rho0=1.0,rho[NX+1][NY+1];
double ux0[NX+1][NY+1],uy0[NX+1][NY+1],ux[NX+1][NY+1],uy[NX+1][NY+1];
double f[NX+1][NY+1][Q],F[NX+1][NY+1][Q];

void init();
void evolution();
void output(int);
double feq(int i,int j,int k);

int main()
{
	using namespace std;

	init();
	for(n=0;n<=20000;n++)
	{
		evolution();
		if(n>0)
		{
			if(n%100==0)  cout<<"The ux and uy of [NX/2][NY/2] is:"<<setiosflags(ios::scientific)<<ux[NX/2][NY/2]<<" , "<<uy[NX/2][NY/2]<<endl;
			if(n%100==0)  output(n);
		}
	}
	system("pause");
	return 0;
}

void init()
{
	std::cout<<"tau="<<tau<<endl;

	for(i=0;i<=400;i++)
		for(j=0;j<=40;j++)	
			{
				rho[i][j]=rho0;
				ux[i][j]=0;
				uy[i][j]=0;
				
                for(k=0;k<Q;k++)
				{
					f[i][j][k]=feq(i,j,k);
				}
			}

	for(j=20;j<=40;j++)
	{
		ux[0][j]=U0;
	}

	if(i=40 && j<=20)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}
	if(j=20 && i<=40)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}
}

double feq(int i,int j, int k)
{
	double eu,uu,feq;
	eu=e[k][0]*ux[i][j]+e[k][1]*uy[i][j];
	uu=ux[i][j]*ux[i][j]+uy[i][j]*uy[i][j];
	feq=w[k]*rho[i][j]*(1+3.0*eu+4.5*eu*eu-1.5*uu);
	return feq;
}

void evolution()
{
	for(i=0;i<=400;i++)
		for(j=0;j<=40;j++)
			for(k=0;k<Q;k++)
			{
				ip=i-e[k][0];
				jp=j-e[k][1];
				inext=i+e[k][0];
				jnext=j+e[k][1];
				F[i][j][k]=f[ip][jp][k]+(feq(ip,jp,k)-f[ip][jp][k])/tau;
			}

	for(i=0;i<=400;i++)
		for(j=0;j<=40;j++)
		{
			ux0[i][j]=ux[i][j];
			uy0[i][j]=uy[i][j];
			rho[i][j]=0;
			ux[i][j]=0;
			uy[i][j]=0;

			for(k=0;k<Q;k++)
			{
				f[i][j][k]=F[i][j][k];
				rho[i][j]+=f[i][j][k];
				ux[i][j]+=e[k][0]*f[i][j][k];
				uy[i][j]+=e[k][1]*f[i][j][k];
			}
			ux[i][j]/=rho[i][j];
			uy[i][j]/=rho[i][j];
		}

		
	for(i=1;i<=400;i++)
		for(k=0;k<Q;k++)
		{
			f[i][NY][4]=f[i][NY-1][2];
			f[i][NY][7]=f[ip][NY-1][5];
			f[i][NY][8]=f[inext][NY-1][6];
		}
	for(i=41;i<=400;i++)
		for(k=0;k<Q;k++)
		{
			f[i][0][2]=f[i][1][4];
			f[i][0][6]=f[ip][1][8];
			f[i][0][5]=f[inext][1][7];
		}
	for(i=1;i<=40;i++)
		for(k=0;k<Q;k++)
		{
			f[i][20][2]=f[i][21][4];
			f[i][20][6]=f[ip][21][8];
			f[i][20][5]=f[inext][21][7];
		}
	for(j=1;j<=20;j++)
		for(k=0;k<Q;k++)
		{
			f[40][j][1]=f[41][j][3];
			f[40][j][5]=f[41][jnext][7];
			f[40][j][8]=f[41][jnext][6];
		}
	for(j=21;j<40;j++)
		for(k=0;k<Q;k++)
		{
			rho[0][j]=rho[1][j];
			f[0][j][k]=feq(0,j,k)+f[1][j][k]-feq(1,j,k);
			ux[0][j]=U0;
		}
	for(j=0;j<=NY;j++)
		for(k=0;k<Q;k++)
		{
			rho[NX][j]=rho[NX-1][j];
			f[NX][j][k]=feq(NX,j,k)+f[NX-1][j][k]-feq(NX-1,j,k);
			ux[NX][j]=ux[NX-1][j];
		}
}

void output(int)
{
	ostringstream name;
	name<<"cavity_"<<n<<".dat";
	ofstream out(name.str().c_str());
	out<<"Title=\"LBM 2D cavity Flow\"\n"<<
		"VARIABLES= \"X\",\"Y\",\"U\",\"V\"\n"<<"ZONE T=\"BOX\",I="
		<<NX+1<<",J="<<NY+1<<",F=POINT"<<endl;
	for(j = 0;j <= NY;j++)
		for(i = 0;i <=NX;i++)
		{
			out<<double(i)<<" "<<double(j)<<" "<<
				ux[i][j]<<" "<<uy[i][j]<<endl;
		}

}
helios (17530)
* Lines 122-123 contain divisions by variables. Check for zeroness of the divisor.
* Ensure everything is properly initialized.
* Ensure all array accesses happen within bounds (have fun!).
* Ensure all floating point operations are given values that make the operation mathematically well-defined.
Last edited on
cppnoob25 (165)
I haven't found an answer to your problem but I did spot an error.

Lines 72 and 77 should look like this I'm guessing.

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if(i==40 && j<=20)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}
if(j==20 && i<=40)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}


Yours look like this
Check the if statements logic.

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if(i=40 && j<=20)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}
if(j=20 && i<=40)
	{
		ux[i][j]=0;
		uy[i][j]=0;
	}


See helios for correction below on this.
Last edited on
helios (17530)
Actually, if you make that change that code becomes dead, since those conditions are never met at that point.
I think this is what OP meant to do:
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for (j = 0; j <= 20; j++){
    ux[40][j] = 0;
    uy[40][j] = 0;
}
for (i = 0; i <= 40; i++){
    ux[i][20] = 0;
    uy[i][20] = 0;
}
Now the code executes properly, but it does nothing because the previous loops had already completely initialized ux and uy.
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