Ask HN: How to go about modelling/simulating prototypes for products

I have several ideas and want to design some physical products ? How can I go about creating or modeling these without spending money on costly software such as AutoDesk/AutoCad etc ? Here is basically what I want :

1) Draw the object in 3D - physical design

2) Model and simulate it. Example aerodynamic modelling

3) I need the design software to be flexible since many changes may be necessary before finalization.

I am familiar with circuit design and simulation but not with physical things.

Also, any advice from people who've been involved with product prototyping, design and development ?

Thanks!

Update: Ok, I found FreeCAD and freeCAD, thats a start. My further question is, is there anything available for aerodynamic modelling ?