Experimentalists often need a sense for the overall flow before they can decide where to measure in greater detail. For such situations, flow visualization techniques are a powerful tool since they provide quick ways to see and compare flows.
Here, researchers paint a viscous oil atop their flying wing model and observe how the oil moves once the air flow starts up. This oil flow visualization shows the large-scale shifts in how air flows over the craft as the angle of attack increases. The disadvantage is that these techniques often give only a qualitative sense of the flow. But they can allow experimentalists to test many different conditions to decide which specific cases they should examine quantitatively. (Image and video credit: V. Kumar et al.)