Domain size limit for external flows
 

Dear CFD Folks,

Defining the domain size for internal flow is hassle free as compared to external flows;

Does anyone knows if there is any rule of thumb in defining the size of the domain (characteristic length) when we are simulating external flows by taking into considerations the following scenarios:

1. There are a number of trucks lining up on the highway traveling at around 110km/hr. There are also several trucks traveling along lands side by side.

2. factory chimney releasing fumes into the atmosphere. There are high rise buildings within 500m of radius together with some residential buildings

3. Modeling chemical dispersion on a city. There are building blocks scattered all over the area.

For the above scenarios, I am wondering what would be the best practice in terms of defining the size of the simulation domain (length X width x height). Can anything experienced CFD guru please provide some comments on the above.

Appreciate with thank you.

regards,
Simon