The solution to the Navier-Stokes equation is the Holy Grail of fluid dynamics because it contains the velocity distribution at every point in time and space. From this, the shear force force everywhere can be known, since τ=μ*du/dy. Shear force is one component of drag because the car is transfering some of its kinetic energy to the air it through which it travels. Also, energy is lost because heat is generated by this "fluid friction." Unfortunately, due to an impossibly high number of input parameters, the Navier-Stokes equation cannot in our case be solved.
Since the infinately small pieces of fluid are too numerous, several methods use very small but still finite fluid particles. CFD software uses Finite Element Analysis in an attempt to predict the flow. This method is still limited by input parameters and assumptions, however. Only an experiment can show what truly happens. The lab trial must measure the speed of the fluid at all points. We used PIV for this lab. In this method, the velocity of smoke particles over a short time interval is calculated from the location of the particles at those instants.
Theory
The theory behind the velocity field is simple. The two-dimensional velocity vectors of the particles are simply displacement over time. The challenge lies in determining which particle moved to which location. Advances in software have recently made these measurements and calculations possible.