Attempt to decrease timestep just adds more steps

Question

Tom Oud 2018-12-13

0
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https://ww2.mathworks.cn/matlabcentral/answers/435620-attempt-to-decrease-timestep-just-adds-more-steps

回答： Walter Roberson 2018-12-13

Writing a script which is supposed to simulate the trajectory of a water rocket. Having some trouble conrtolling the timestep though. When I make dt smaller, all that seems to happen is more steps being added instead of the actual increment decreasing. What am I doing wrong?

This is my code so far:

close all
clear all
P0 = 6e5; %Pa
Pout = 1e5; %Pa
V0 = 2e-3; %m^3
V(1) = 1.5e-3; %m^3
gamma = 1.4;
rho_w = 998;  %kg/m^3
Rn = 0.002; %m
dt = 0.01;
t = 1:dt:10;
Ve(1) = 0; %m/s
Pin(1) = 6e5; %Pa
SP0 = 0; %m/s
SP = 0; %m/s
M0 = 0.1; %kg
M(1) = M0 + (V0-V)*rho_w;
for i = 1:numel(t)
    k = t(i);
    
    Pin(i+1) = Pin(i) - P0*(V(i)/V0).^-gamma;
    Ve(i+1) = sqrt(2*(Pin(i) - Pout)/rho_w);
    M(i+1) = M(i) - Ve(i)*pi*Rn.^2*rho_w;
    V(i+1) = V0*(Pin(i)/P0).^gamma;
    if V(i) > V0 - 1e-18
        V(i+1) = V(i);
    end
    if M(i) < M0 + 1
        M(i+1) = M0;
    end
    if Pin(i) < Pout
        Pin(i+1) = Pout;
        Ve(i+1) = Ve(i);
    end
    %SP = Ve*log(M0/M) + SP0;
end