I cannot solve the vpasolve using a symbolic function. Symbolic equal to Inf

Hey there I really appreciate if someone can help. I have no ideia why I my symbolic equantion is Inf. I used the same scrip to solve another equation and it worked well.

Thanks in advance.

ds= 3;

rho_w= 3;

cp_w = 5;

hl_s = [2 , 2, 2, 2];

ls= 3;

mcv=9;

cp_al=7;

vs = [1, 2, 3, 4];

qs = pi*(ds/2)^2*vs;

%% Symbolic

tau_c_laminar = zeros(size(qs));

tau_s_laminar = zeros(size(qs));

ind = zeros(size(qs));

qcsolution_lam = zeros(size(qs));

qcsolutionlmin_lam = zeros(size(qs));

qcsolution_d_lam=[];

qclmin = qc*60000;

syms qcsyms qcsolution_lam

cont = 0;

range=[];

for i = 1:length(qs)

if Re_c(i) < 2300

fprintf("Re: %f \n",Re_c(i))

tau_c_laminar = (mwall*cp_al)/((rho_w*cp_w*qcsyms)*(1-exp((-hl_c(i)*pi*lc*dc)/(rho_w*cp_w*qcsyms))));%s

fprintf("talsym: %s \n", char(tau_c_laminar))

tau_s_laminar(i) = (mcv*cp_al)/((rho_w*cp_w*qs(i))*(1-exp((-hl_s(i)*pi*ls*ds)/(rho_w*cp_w*qs(i)))));%s

fprintf("taunum: %f \n",tau_s_laminar(i))

qcsolution_lam(i) = vpasolve (tau_c_laminar == tau_s_laminar(i));

fprintf("vpasolve: %f \n",qcsolution_lam(i))

end

end

The output is

Re: 433.464423

talsym: Inf % here I have no ideia why is like this

taunum: 1.437457

Error using sym/vpasolve (line 145)

Unable to find variables in equations.

Error in RASCUNHO (line 41)

qcsolution_lam(i) = vpasolve (tau_c_laminar == tau_s_laminar(i));

Answer 1

[Guru Mohanty]

Hi, I understand you are trying to solve these equations using vpasolve. In your code there are some undeclared variables as 'Re_c', 'qc', 'mwall', 'hl_c', 'lc', 'dc'. After declaring these variables the code is working. I have used random value for these variables. Here is the code for it.

Re_c=[100 200 300 433.464423 ];
qc=1;
mwall=1;
hl_c=1;
lc=1;
dc=1;

ds= 3;
rho_w= 3;
cp_w = 5;
hl_s = [2 , 2, 2, 2];
ls= 3;
mcv=9;
cp_al=7;
vs = [1, 2, 3, 4];
qs = pi*(ds/2)^2*vs;
%% Symbolic
tau_c_laminar = zeros(size(qs));
tau_s_laminar = zeros(size(qs));
ind = zeros(size(qs));
qcsolution_lam = zeros(size(qs));
qcsolutionlmin_lam = zeros(size(qs));
qcsolution_d_lam=[];
qclmin = qc*60000;
syms qcsyms qcsolution_lam 
cont = 0;
range=[];
for i = 1:length(qs)
 
    if Re_c(i) < 2300    
        
    fprintf("Re: %f \n",Re_c(i))
    
    tau_c_laminar = (mwall*cp_al)/((rho_w*cp_w*qcsyms)*(1-exp((-hl_c*pi*lc*dc)/(rho_w*cp_w*qcsyms))));%s
        fprintf("talsym: %s \n", char(tau_c_laminar)) 
    
    tau_s_laminar(i) = (mcv*cp_al)/((rho_w*cp_w*qs(i))*(1-exp((-hl_s(i)*pi*ls*ds)/(rho_w*cp_w*qs(i)))));%s
        fprintf("taunum: %f \n",tau_s_laminar(i))
    
    qcsolution_lam(i) = vpasolve (tau_c_laminar == tau_s_laminar(i));
    fprintf("vpasolve: %f \n",qcsolution_lam(i))
    end
end

[Lucas Teixeira]

Mohanty, Thanks so much for your time. Sorry for the undeclared variables I tried to take this part from the original script and I forgot to declare those variables before post it. Sorry again!

Yeah, you got my intention. It is working now...in the original script the problem the solution is that for the first element of the input vector the symbolic equation is Inf.

Anyway, thanks so much for your help.