I take it you mean "Design Model" when you talk about "Low level Design". A Design Model is just an object model describing the realisation of use cases. The specifics of your project. will determine how you tailor the development process, and hence the level of detail.
I assume you mean "Detailed Design" as "Low Level Design". Yes, I've done it in several projects.
I usually deliver the Class Diagrams,Sequence Diagrams and Activity Diagrams in the Detailed Design while I deliver the Deployment Diagram,Component Diagram etc as part of High Level Design Document.
In Detailed Design, I first identify the list of Classes and interfaces first, try to apply OOAD principles to it and finally arrive at the list of Classes and interfaces.
then prepare the sequence diagram w.r.t to the use case document and the class diagrams. With Sequence diagram we can fully verify whether the user's requirement is met or not.
Activity Diagram will help the developers very much when they do the actual coding to understand the flow of the use case or business.
I mean detailed design here. Would you also require to give documentation for classes and the imp methods in terms of what they do.
In my current assigment class and sequence dia are nor enough to expalin the complex process, i guesss Activity diagram will help in there.
Also what is the role of an architect once he delivers the detailed design. Especially project is of short 2-3 months duration.
Like I said, this should be dictated by whatever software development process you are following, and how you've tailored that process to meet the specific needs of your project. The tasks you perform and the artifacts you produce in your role as software architect are defined by that process.
If you need some guidance then take at look at the OpenUP documentation, or better still buy a good book on RUP (most iterative development processes have their origins in, or are a subset of, RUP.
If the project is really short 2-3 months, then ofcourse Software Architect has to do Technical Lead Role once he completes the design. He can setup Code Review tools like PMD ,etc for his project,review the Code done by developers w.r.t to the design that you made, help the developers in fixing the technical issues, if any, etc....
As Jonathan pointed out it depends on the Company's policy that you work. In Some company they will ask the Architect to move to the next project even.
If you want to know the complete Roles and responsibilities of an Architect, Please read the Book " J2EE Architects Handbook" which is very useful.