Thread: Solving a more difficult math equation in c programming

The program is supposed to solve for U_T (terminal velocity of a particle falling in water)
Everything can be found except Re and U_T
Re is basically 100 < Re <10000. I TRIED to create a while loop for Re. I am more used to Matlab than C programing, but it needs to be done in C.
The while loop should stop when F_D + F_B = W
After it breaks I want it to display the U_T and Re used
The difficult part is that U_T is used in another equation. I am wondering if I need to take it out of the loop.

I was wondering if you guys can tell me if I am using the while loop correctly?
And if you can tell me how to make it print the U_T and Re values when it is done.

Code:

// by Eric
// Math program to solve for terminal velocity of particle falling into water

#include<stdio.h>
#include<math.h>
 
int main(int argc,char *argv[])
	{
		int D;			//diameter
		int rho_p;		//density of sand particle
		int rho_f;		//density of fluid
		int mu_f;		//dynamic viscosity of fluid
		int g;			//gravity
		int F_D;		//Drag Force
		int C_D;		//Coefficent of Drag
		int U_T;		//Terminal Velocity solving for
		int F_B;		//Force of Bounacy
		int m			//mass
		int Re			//Reynolds Number
		int W			//Weight
		int PI			//pi
	
		D = 300*10^(-6);	// diameter of sand particle in units of meters
		rho_p = 2000;		// density of sand particle in units of kg/m^3
		rho_f = 998.2;			// density of fluid (kg/m^3) at 68 F
		mu_f = 1.002*10^(-3);				// Dynamic Viscosity of fluid(kg/m*s) at 68 F
		g = 9.81;			// gravity (m/s^2)	
		PI = 3.141592653589793;
		
		//Re=((U_T·D·rho_f) / (mu_f));
		//U_T=((Re*mu_f)/(D*rho_f));
		//F_D=C_D·((PI·D*D) / (4))*0.5·rho_f·U_T*U_T;
		//F_B=rho_f·((PI*pow (D, 3)) / (6))·g;
		W=m·g;
		m=rho_p·((PI*pow (D, 3)) / (6));
		//C_D=((24) / (Re))*pow ((1+0.27·Re), 0.43)+0.47·(1-exp (-0.04*pow (Re, 0.38)));
		
		
		Re=100;
		while (Re <= 10000)
		{
			U_T=((Re*mu_f)/(D*rho_f));
			F_D=C_D·((PI·D*D) / (4))*0.5·rho_f·U_T*U_T;
			F_B=rho_f·((PI*pow (D, 3)) / (6))·g;
			C_D=((24) / (Re))*pow ((1+0.27·Re), 0.43)+0.47·(1-exp (-0.04*pow (Re, 0.38)));
			Re++;
			if (F_D+F_B == W)
				break;
		}
		return 0;
	}

3 things that jumped out at me within 5 seconds of reading your code:

1. int is for integers only. Anything that needs to use numbers with fractional components needs to use floating point types (double is preferred).
2. There is no "power of" operator in C, so 300 * 10^(-6) doesn't do what you think. Use the pow function in math.h. ^ is the bitwise "exclusive or" operator.
3. I don't know what that "floating dot" character is in C_D·((PI·D*D) , but it's not a valid operator in C.

What those three things tell me is that you don't really know C and haven't made much of an effort to learn it. You basically tried to wrap matlab code in a main function and threw in a couple header files. C is not Matlab. They are different languages with different purposes. Check out some online tutorials (here and elsewhere). Grab a book and start reading, doing the examples along the way.