Matlab not returning 0 when every element is multiplied by 0.

Hi all. I'm currently trying to model couette flow using the following equation

u = @(x1,y1,dP_1) U*(y1/B) + (1/(2*mu))*dP_1*(y1^2-B*y1);

where x and y are a meshgrid of two row vectors

and dp_1 is given by a user specified function of constants, evaluated separately

dP = dP_1(X,Y)

The velocity field is evaluated with

U = u(X,Y,dP);

However, even though every though u depends on y1 and should be zero when y1 = 0, when the function plots at certain inputs y1 is non zero.

I need to know why there are non-zero values. Thanks. The full code is given below.

x=linspace(0,10,10); y=linspace(0,10,10);

%Set X,Y as a meshgrid of x,y [X,Y] = meshgrid(x,y);

%Receive constant inputs from user input boxes a = get(handles.a, 'String'); b = get(handles.b, 'String'); c = get(handles.c, 'String'); d = get(handles.d, 'String'); e = get(handles.e, 'String'); f = get(handles.f, 'String'); g = get(handles.g, 'String'); h = get(handles.h, 'String');

%Convert values from string to num a = str2num(a); b = str2num(b); c = str2num(c); d = str2num(d); e = str2num(e); f = str2num(f); g = str2num(g); h = str2num(h);

if exist(P0) if P0 == 0 a=0;b=0;c=0;d=0;e=0;f=0;g=0;h=0; end end

P = symfun( a*x1^3+b*y1^3+c*x1^2*y1+d*x1*y1^2+e*x1*y1+f*x1+g*y1+h, [x1,y1]); dP_1 = matlabFunction(diff(P,x1))

%Change to user defined height B = get(handles.plate_dist, 'String'); B = str2num(B); U = get(handles.U, 'String'); U = str2num(U); mu = get(handles.mu, 'String') mu = str2num(mu);

dP = dP_1(X,Y)

u = @(x1,y1,dP_1) U*(y1/B) + (1/(2*mu))*dP_1*(y1^2-B*y1);

U = u(X,Y,dP);

V = zeros(numel(x),numel(y));

quiver(handles.axes1,X,Y,U,V)

axis([0 10 0 10])

Answer 1

[Sean de Wolski]

Are your perhaps forgetting to convert a string '0' to a double?

>> '0'*pi
ans =
150.7964

>> 0*pi
ans =
   0
>>

The output from the edit boxes is a string.