The only technique available a half-century ago to describe coherent structures in turbulent flows was smoke and dye injection [Brown and Roshko (1974); Winant and Browand (1974)]. According to Hesselink (1988), flow visualization results from the interaction between matter and light. The definition was based on the classical methods available in those years, such as shadow photography and Schlieren photography. In the early 1980s, other techniques were developed through numerical processing of flow visualization pictures for experimentally estimating some of the principal physical variables of fluid flows. That classical visualization relied on variations of the index of refraction or spatial derivatives of pressure, density, and temperature integrated along the light path through the fluid [Hesselink (1988)]. The Visualization Handbook (by Hansen and Johnson (2011)) describes flow visualization as a relevant topic of scientific visualization, intending to provide more understanding of fluid flows. Where typically, nowadays, the data is originated from numerical simulations, such as those of computational fluid dynamics (CFD). Visualization techniques have substantially evolved along with the world’s technology, spanning all disciplines [Sherman et al. (1997)]. Today visualization leans greatly on computers and numerical approaches because, in 20 years, the computing capacity has increased over three orders of magnitude.​

Clarifying the difference between Virtual Reality (VR) and Augmented Reality (AR) requires an a priori explanation of the concept “virtuality continuum”, introduced 29 years ago by Milgram and Kishino (1994). They declared the continuum with four regions and argued that the taxonomy should be based on three sub-concepts, as follows: (i) “Extent of World Knowledge: The amount of information bleeding from the real environment into a virtual environment”; (ii) “Reproduction Fidelity: an attempt to quantify the image quality of the displays”; and (iii) “Extent of Presence Metaphor: a term to encapsulate the degree of immersion”. In other words, Milgram and Kishino (1994) described a continuum between the real world, which they call the “real environment”, and the “virtual environment” which is a fully synthetic world in which computer-generated signals are provided to the senses of a person in full replacement of the natural signals their senses normally perceive. This is what we are referring to as “Virtual Reality”. Between those two end points are various levels of mixing, the natural signals and the synthetic signals. In short, the difference between Virtual Reality and Augmented Reality is how the real world is treated. In Virtual Reality, the real (natural) world is hidden and the participant experiences only computer-generated stimuli to their senses, but in Augmented Reality, the participant fully experiences the real world with the addition of computer-generated signals to augment their real-world experience. There are different ways to achieve this mixing, the primary distinction is whether the real world is mediated through some type of video, or if the real world is perceived optically. Years later, Flavian et al. (2019) proposed a new taxonomy in five divisions, as shown in the Figure 3.1 below. Following the present interpretation, this continuum represents the classifications of the different modes a user can perceive sensory experiences and have their interactions.

Figure 3.1: Virtuality continuum, reproduced from Flavian et al. (2019).

From left to right in the Figure 3.1 above, the Real Environment is the physical world, where technology is not needed for its existence and interactions. Second is Augmented Reality (AR), where virtuality overlays reality yet does not interact with the whole physical world. AR usually requires a lens device so the user can visualize the virtual objects overlying the physical world. In the midway, it is the Pure Mixed Reality, where virtuality and reality are fused, explained as the generation of virtual objects with complete awareness of the real environment. The fourth is Augmented Virtuality (AV), where reality overlaps with virtuality, for example, a computer-generated display crowded with digital images but controlled by physical commands. The last taxonomy is the Virtual Environment, where the complete environment is digital and Virtual Reality is the medium used to access this mode [Bryson (2013)]. Any input from the real world has to be translated into a digital expression. The junction from AR to AV is called Mixed Reality (MR). At the same time, Extended Reality (XR) covers from AR to Virtual Environment, also commonly used as an unspecified reality mix.

Regarding flow visualization, such XR technologies benefit visual representation. It enables a researcher to perform scientific analysis and to gain a better insight into complex flows in complicated geometries that are hard to perceive with the naked eye at a fraction of the effort and cost required in a physical setup [Paeres et al. (2021b)]. For example, in the early 2000s, Aoki and Yamamoto (2000) proposed the design system for a 3-dimensional blade involving VR in turbomachinery. They showed how virtual reality could help a designer to visualize the 3-dimensional flow fields and modify the blade configuration to further calculate the results in an iterative approach. In a different project, Eissele et al. (2008) presented a system able to track position, orientation, and other additional aspects from a scenario used to influence the visualization. They observed automatically seeded streamlines rendered with halos to enhance the contrast to the underlying real-world image captured by a camera. Their work was focused on flow visualization in AR. However, they claimed that their approach could also be applied to other visualization tasks. In a more recent project, Walcutt et al. (2019) used virtual reality as a scalable and cost-efficient visualization mechanism to interpret large volumes of ocean data. They summarized ways where VR could be applied to the oceanography field and demonstrated the utility of VR as a 3-dimensional visualization tool for ocean scientists.