# Question:Coupled PDEs:Error, (in pdsolve/numeric/animate) unable to compute solution for t>HFloat(0.0): Newton iteration is not converging

## Question:Coupled PDEs:Error, (in pdsolve/numeric/animate) unable to compute solution for t>HFloat(0.0): Newton iteration is not converging

Maple 2018

Analysis of the semiclassical (SC) momentum rate equations

Plotting the ICs and BCs and examining sensitivity to the Re and Im forces

MRB: 24/2/2020, 27/2/2020, 2/3/2020.

We examine solution of the SC version of the momentum rate equations, in which O terms for are removed. A high level of sensitivity to ICs and BCs makes solution finding difficult.

 > restart;
 > with(PDETools): with(CodeTools):with(plots):

We set up the initial conditions:

 > ICu := {u(x, 0) = .1*sin(2*Pi*x)}; ICv := {v(x, 0) = .2*sin(Pi*x)};  (1)
 > plot([0.1*sin(2*Pi*x),0.2*sin(Pi*x)],x = 0..2, title="ICs:\n u(x,0) (red), v(x,0) (blue)",color=[red,blue],gridlines=true); The above initial conditions represent a positive velocity field (blue) and a colliding momentum field (red).

Here are the BCs

 > BCu := {u(0,t) = 0.5*(1-cos(2*Pi*t))}; (2)
 > BCv := {v(0,t) = 0.5*sin(2*Pi*t),v(2,t)=-0.5*sin(2*Pi*t)}; (3)
 > plot([0.5*(1-cos(2*Pi*t)),0.5*sin(2*Pi*t),-0.5*sin(2*Pi*t)],t=0..1,color=[red,blue,blue],linestyle=[dash,dash,dot],title="BCs:\n u(0,t) (red-dash),\n v(0,t) (blue-dash), v(1,t) (blue-dot)",gridlines=true);
 > We can now set up the PDEs for the semiclassical case.

 > hBar:= 1:m:= 1:Fu:= 0.2:Fv:= 0.1:#1.0,0.2
 > pdeu := diff(u(x,t),t)+u(x,t)/m*(diff(u(x,t),x)) = Fu; (4)
 > pdev := diff(v(x,t),t)+u(x,t)/m*(diff(v(x,t),x))-hBar*(diff(u(x,t),x\$2))/(2*m)+v(x,t)*(diff(u(x,t),x))/m = Fv; (5)
 > ICu:={u(x,0) = 0.1*sin(2*Pi*x)}; (6)
 > ICv:={v(x,0) = 0.2*sin(Pi*x/2)}; (7)
 > IC := ICu union ICv; (8)
 > BCu := {u(0,t) = 0.5*(1-cos(2*Pi*t)), D(u)(2,t) = 0.1*cos(2*Pi*t)}; (9)
 > BCv := {v(0,t) = 0.2*(1-cos(2*Pi*t))}; (10)
 > BC := BCu union BCv; (11)

We now set up the PDE solver:

 > pds := pdsolve({pdeu,pdev},{BC[],IC[]},time = t,range = 0..2,numeric);#'numeric' solution (12)
 > Cp:=pds:-animate({[u, color = red, linestyle = dash],[v,color = blue,linestyle = dash]},t = 30,frames = 400,numpoints = 400,title="Semiclassical momentum equations solution for Re and Im momenta u(x,t) (red) and v(x,t) (blue) \n under respective constant positive forces [0.2, 0.1] \n with sinusoidal boundary conditions at x = 0, 1 and sinusoidal initial conditions: \n time = %f ", gridlines = true,linestyle=solid):Cp; (13)

Observations on the quantum case:

The classical equation for is independent of the equation for . (red) is a solution of the classical Burgers equation subject to a force 0.2, but is NOT influenced by .  On the otherhand, (blue) is a solution of the quantum dynamics equation subject to force 0.1 and is influenced by .   This one way causality (u )  is a feature of the semiclassical case, and it emphasises the controlling influence of the classical , which modulates the quantum solution for .  Causally, we have u .

The initial conditions are of low momentum amplitude: 0.1 for the classical (red) field and .2 for (blue)  but their influence is soon washed out by the boundary conditions and that drive the momentum dynamics.

The temporal frequency of the boundary condition on the -field is twice that of the classical -field. This is evident in the above blue transient plot. Moreover, the boundary condition on the classical -momentum (red), drives that field in the positive direction, initially overtaking the quantum field, as consistent with the applied forces [0.2, 0.1]. Although initially of greater amplitude than the classical field, the momentum field is asymptotically of the same amplitude as the field, but has greater spatial and temporal frequency, owing to the boundary conditions.

Referring to the semiclassical momentum rate equations, we note that the classical field (red) modulates the quantum momentum rate equation for .

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