Free-stream turbulence refers to the random fluctuations in velocity and pressure of a fluid flow that occur in the absence of any obstacles or disturbances. This type of turbulence is typically found in open, unobstructed flows such as in the atmosphere or in large bodies of water. Research in free-stream turbulence aims to understand the characteristics and behavior of these turbulent flows, including their effects on the transport of momentum, energy, and mass. This research can have applications in various fields, such as aerodynamics, environmental science, and oceanography. Scientists and engineers study free-stream turbulence using experimental techniques, numerical simulations, and theoretical models to better predict and control the behavior of turbulent flows in natural and engineered systems.