llnl / llnl/libROM

dmd prediction of initial condition

Open
#210 2 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Dominant language
C++
Stars
236
Forks
47
PR merge metrics
No merged PRs in 30d

Description

My DMD code seems to be working, but it seems to make a large error at the starting time. Shouldn't DMD be able to reproduce the initial condition exactly? Looking at

https://www.librom.net/features.html

I can see the last equation in the DMD section

$$
\mathbf{u}(t) = \mathbf{\Phi\Lambda}^{{t}/{\Delta t}}\mathbf{b}_0
$$

using $\mathbf{b}_0=\mathbf{\Phi}^\dagger\mathbf{u}_0$ we get

$$
\mathbf{u}(t) = \mathbf{\Phi\Lambda}^{{t}/{\Delta t}}\mathbf{\Phi}^\dagger\mathbf{u}_0
$$

At $t=0$ we have $\mathbf{\Lambda}^{{t}/{\Delta t}}=\mathbf{1}$ and hence,

$$
\mathbf{u}(0) = \mathbf{\Phi}\mathbf{\Phi}^\dagger\mathbf{u}_0
$$

Since $\mathbf{\Phi}\mathbf{\Phi}^\dagger=\mathbf{1}$ we get

$$
\mathbf{u}(0)=\mathbf{u}_0
$$

Hence, the initial condition is faithfully reproduced. Is my understanding correct?

Contributor guide

No contributing guide indexed for this repository

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reviewing the DMD documentation and equations referenced in the issue, then locate the existing DMD implementation and reproduce the reported initial-condition error. Done would require a clearly defined resolution explaining whether the observed behavior is expected or identifying the implementation change needed.

Written by the indexing model from the issue text.

Assessment

Tech stack
cpp
Domain
data
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
20/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.