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# Results Interpretation

In these topics, we examine the physical meanings of some of the results you will likely see in your simulations.

## Colors

The legend on the left shows the corresponding values of the colors:

1. The green region upstream of the car is the wind speed. Before it reaches the car, the wind moves in a uniform manner at a constant velocity.
2. At the front of the car, the blue spot is a "stagnation" region, and is where the wind velocity relative to the car goes to zero.
3. Above and below the car, the red means that the wind is moving faster. It has to do this because as the air passes over the car, there is less space for the air to occupy. Because the same volume of air must move through any given cross-section, it accelerates. (A similar situation is when you put your thumb over the end of a garden hose. The water jets out in a thin stream. The air in the wind tunnel is doing the same thing.)
4. The blue region just downstream of the car is called the "wake." The air that was jammed up above the car takes a little while to fill the entire wind tunnel volume after it passes the car. This region can be a little chaotic, and is an important part of managing drag. Designs that produce smaller and smoother wakes usually have less drag, and in many cases, improved energy efficiency.
5. The green and blue spots downstream of the wake are an example of "vortex shedding." This is a common occurrence in aerodynamics. It is a time-varying phenomenon in which the air oscillates in a regular pattern. (A common example is when driving behind a fast-moving tractor-trailer on the highway, you'll feel a cross-wind on the front on your vehicle, alternating side-to-side. As soon as you pass the truck, the cross-wind stops.)

## Changes in the Flow

As the solution progresses, you will see a lot of change. There are two primary causes:

1. The solution is not complete yet. When the flow field changes radically (big swings in color, for example), the analysis is still evolving. Simulations in 2D (Concept) evolve quite quickly, but bigger models will take more time to converge.
2. The solution contains a transient effect. (See the description of Vortex shedding in the previous section.) These are not uncommon, and appear in many aerodynamic situations.

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.

## Assess Design Quality

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:

Flow separation and recirculation

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.

Wake

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.

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

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.

## When is the Solution Complete?

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.