why is my plot plotting blank?

g = 9.8 %m/s^2

rohp = 1800 %kg/m3

Dp = 0.208*10^(-3); %m

T = 298.15 %K

roh = 994.6 %kg/m3

vis = 8.931*10.^-4 %kg/m · s

Vt=0:0.1:60

Re=roh*Vt*Dp/vis

Cd=zeros(1)

if Re>0.1 & Re<=1000

Cd= (24./Re).*(1+0.14*(Re.^0.7));

end

thvis=@(Vt) Vt-((4.*g.*(rohp-roh).*Dp)./(3.*Cd.*roh)).^(0.5)

dthvis=@(Vt) 1

y=thvis(Vt);

plot(Vt,y);

Answer 1

[Jon]

The reason your plot is blank is because all of your y values are infinite. They are infinite because Cd is zero for all of your points and Cd is in the denominator of your calculation. Note that you initialize your Cd to zero, and then because you never satisfy the if statement on Reynolds number range it remains zero.

[Jon]

More fundamentally your if statement will not assign multiple elements of Cd. Instead you could do something like

idx = Re>0.1&Re<1000;
Cd(idx) = (24./Re(idx)).*(1+0.14*(Re(idx).^0.7))

[Jon]

Actually you probably just want to work with values that are in the correct Re range you could do it like this

g = 9.8  %m/s^2
rohp = 1800 %kg/m3
Dp = 0.208*10^(-3); %m
T = 298.15 %K
roh = 994.6 %kg/m3
vis = 8.931*10.^-4 %kg/m · s
Vt=0:0.1:60
Re=roh*Vt*Dp/vis

% just keep values that are in Reynolds range
idx = Re>0.1&Re<1000;
Cd = (24./Re(idx)).*(1+0.14*(Re(idx).^0.7))
Vt = Vt(idx); 

% calculate and plot function values 
thvis=@(Vt) Vt-((4.*g.*(rohp-roh).*Dp)./(3.*Cd.*roh)).^(0.5)
y=thvis(Vt);
 plot(Vt,y);

Answer 2

[Star Strider]

The value of ‘Cd’ is 0 in the code you posted, so ‘y’ is uniformly -Inf.

Answer 3

[David Hill]

g = 9.8;  %m/s^2
rohp = 1800;%kg/m3
Dp = 0.208e-3; %m
T = 298.15; %K
roh = 994.6; %kg/m3
vis = 8.931e-4; %kg/m · s
Vt=0:0.1:60;
Re=roh*Vt*Dp/vis;
Cd=zeros(size(Re));
Cd(Re>.1&Re<=1000)=(24./Re(Re>.1&Re<=1000)).*(1+0.14*(Re(Re>.1&Re<=1000).^0.7));
thvis=@(Vt) Vt-((4*g*(rohp-roh)*Dp)./(3*Cd*roh)).^(0.5);
y=thvis(Vt);
plot(Vt,y);