% Takes one time step of size k using the rungekutta method
% starting from v_0 and where the function F(v,t) gives the
% time derivatives.
function v = rungekutta_4RV(v, t , k, F, RV)    
    k1 = F(v, t, RV.getViscosity());
    
    RV.update(v+0.5*k*k1, v, 0.5*k);
    k2 = F(v+0.5*k*k1, t+0.5*k, RV.getViscosity());
    
    RV.update(v+0.5*k*k2, v, 0.5*k);
    k3 = F(v+0.5*k*k2, t+0.5*k, RV.getViscosity());
    
    RV.update(v+k*k3, v, k);
    k4 = F(v+k*k3,t+k, RV.getViscosity());
    
    RV.update(v + (1/6)*(k1+2*(k2+k3)+k4)*k, v, k);
    v = v + (1/6)*(k1+2*(k2+k3)+k4)*k;
end