Transient flow structures, however, can indicate noise. In the example above, a vortex is oscillating off the back end of the car. A vortex coming off of a protrusion (such as a side-view mirror) can mean that the current design will cause some noise that will affect the occupant, and that perhaps it should be modified.
This is the most subjective area because every design project is different. The goals of the project drive the metrics with which the design is evaluated. However, there are some fundamental items that are universally important:
This is where the wind stops following the shape of the object and moves around in a circular pattern. These regions often occur in the wake, but can also occur downstream of any protruding object. The smoother the flow, the less drag forces you'll encounter, which translates to improved energy efficiency.
A large wake downstream of the object means higher drag. To minimize the wake, form the object so that the flow can follow it as smoothly as possible. Chaotic recirculation patterns behind objects tend to produce low pressures, and the result is energy-robbing drag.
Pay attention to the design of the front of the object, and plot Air Pressure to assess the wind resistance. A large, flat face resists the wind much more than a low, stream-lined design (as in the example pictured above). Obviously lowering the drag (wind resistance) is a good thing, but there are limits.
Noise is caused whenever air moves over a stationary object (or when an object moves through stationary air). While aerodynamic-induced noise cannot be eliminated, it can be managed within a design. Noise often occurs locally in areas that have rapid, high value oscillations. Use Wind Velocity to look for recirculation regions and transient vortices downstream of anything that protrudes from the model.
Falcon uses a transient flow solver, so you will see some variations as the simulation runs. Because the simulation is not time-averaged, Falcon shows transient (time-dependent) results that you might not otherwise see in a steady-state simulation. It is important, however, to assess results and make design decisions after the simulation has achieved some level of stability.
Early in every simulation, you will typically see the solution vary to a high degree. These early variations are part of the calculation process, and are a result of the solver computing a new results field. As the flow develops, the startup-up transient effects dissipate, and the simulation continues toward stability.
A good way to assess solution stability is to display the Drag plot. As the solution runs, the drag plot is a good indication of the stability of the solution. As the early simulation variations dissipate, you should see just the effects of the physical transient nature of the simulation. This is reflected in either a horizontal (or nearly horizontal) drag plot (if the solution is not physically transient) or in a repeatable, periodic drag fluctuation pattern (if it is physically transient). Either way indicates that the solution has stabilized and that you can start assessing results.