Problem mit DSolve
- To: mathgroup at smc.vnet.net
- Subject: [mg43204] Problem mit DSolve
- From: "Stefan Braun" <stefan.braun at smartcae.de>
- Date: Wed, 20 Aug 2003 22:24:49 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Hello,
i have the following problem.
If i solve a differential equation with numerical parameters i get in a few
cases wrong results.
If i Rationalize the differential equation i get the correct results.
What is the reason for this?.
Enclosed a typical example:
dgl1 = \[Rho]1*V1*cp1*Derivative[1][T1][t] == A*\[Alpha]1*(Ta -
T1[t]) + A*\[Alpha]2*(T2[t] - T1[t])
dgl2=\[Rho]2*V2*cp2*T2'[t]==A*\[Alpha]2*(T1[t]-T2[t])
dglsys={dgl1,dgl2,T1[0]=,T2[0]=}
dglsysn=dglsys/.{cp1->2430,\[Rho]1->1261,cp2->4200,\[Rho]2->1000,V1->dicke*L*2*radius*Pi,A->2*radius*Pi,V2->100*10^(-6)*10^(-3)}/.radius->0.00265/2/.dicke->0.0003/.Ta->40/.L->0.021/.\[Alpha]1->20/.\[Alpha]2->20
lsg={T1[t],T2[t]}/.DSolve[dglsysn,{T1[t],T2[t]},t][[1]]
Plot[Evaluate[lsg[[1]]],{t,0,100}]
Plot[Evaluate[lsg[[2]]],{t,0,100}]
If i change dglsyn with
dglsysn=Rationalize[dglsys/.{cp1->2430,\[Rho]1->1261,cp2->4200,\[Rho]2->1000,V1->dicke*L*2*radius*Pi,A->2*radius*Pi,V2->100*10^(-6)*10^(-3)}/.radius->0.00265/2/.dicke->0.0003/.Ta->40/.L->0.021/.\[Alpha]1->20/.\[Alpha] 2->20,0]
all results are o.k
Best regards,
Stefan
SmartCAE Stefan Braun
Marthastr. 9a
D-81825 Munchen
Tel: 089/43738805
Fax: 089/43738061
Mobil: 0179/5173483
e-mail stefan.braun at smartcae.de
internet: www.smartcae.de