NO NEED FOR A LONG INTRO JUST A FEW SENTENCES AND THEN THE BODY AND SMALL CONCLUSION AND LIST THE SOURCES YOU USE.
HERE IS A SOURCE I THINK WILL HELP YOU:
USE IT IF POSSIBLE.
Fluid Dynamics Engineering: Shadowgraph
Flow visualization is a cardinal aspect of fluid mechanics studies. Fundamentally, conducting flow visualization serves as the initial step towards a more detailed analysis of flow. Typically, flow visualization can start with a basic structure that employs an illumination system and a camera to elucidate the structure and behavior of the flow. Once the appropriate flow information is obtained, other sophisticated systems can be used to give the analytical studies of flow fields. Essentially, three methods are used in experimental fluid dynamics. These approaches include optical methods, particle tracer techniques, and surface flow visualization (Merzkirch). The shadowgraph is one of the optical methods commonly employed and it is on this technique that the paper focuses.
In general, optical flow visualization methods are centered on the principle that a change in density elicits a commensurate change in the refractive index of light (Post and Walsum 6). Accordingly, they can only be used for fluids whose densities are not constant (Post and Walsum 6). Shadowgraphs are applied to reveal non-uniformities in transparent media. Hypothetically, it is impossible to directly note temperature differences, shock waves, or a different gas in the transparent air. However, it is imperative to appreciate the fact that all these forms of disturbances precipitate the refraction of light rays and thus cast shadows. A shadowgraph is obtained by passing a parallel beam of light through a fluid in motion. Appropriately, the variations in density will lead to the refraction of some of the light rays. As such, the creation of shadowgraphs depends on the differences in refractive indices in transparent media and the resulting effect of such variation on the beam of light transiting through the test unit.
Technically, the shadowgraph system needs no other optical component besides a source of light and a photographic plate to serve as a recording plane onto which the shadow of the varying density fields is projected. The shadow effect is generated since a ray is refractively deflected such that the position on the photographic plate where the undeflected ray would hit is left dark. Simultaneously, the point hit by the deflected ray appears lighter than the uninterrupted section of the medium. Consequently, an appreciable pattern of contrasted is yielded on the recording plane. An analysis of the shadow effect optics points to the fact that the perceptible signal is contingent on the second derivative of the medium’s refractive index. Accordingly, shadowgraph systems as optical diagnostic techniques respond to variations in the second derivatives of fluid densities.
The most significant advantage of the shadowgraph systems is that they are relatively cheap and easy to set up owing to the fact that they require few resources to implement. Also, the systems can deal with subjects that are quite large (Davidhazy 5). Their simple optical setups allow the shadow effect stemming from the heterogeneous density fields to be observed even outside a laboratory where the sun can serve as a light source. This technique also confers advantages when dealing with big density gradients as in flame phenomena and shock waves. By way of its double differentiation, shadowgraphy renders fine-scale images of turbulent flows (Settles 29). While it mostly finds use in qualitative flow visualization, a shadowgraph can be applied theoretically to the determination of temperature, density, and pressure. Additionally, the method is non-invasive and can be applied in various cases to measure characteristics including object sizes as well as interface speed and direction of flow (Castrejon-Garcia et al. 267).
Besides these exceptional features, being an optical technique, shadowgraphy requires transparent media and a flow confinement of optimal optical quality (Kovasznay et al. 322). Evidently, the shadowgraph is not a suitable method for the quantitative analysis of fluid density. This is because shadowgrams are mere shadows and not focused optical images (Settles 29). As such, there is no conjugate optical relationship between the shadow and the object. The technique has a relatively low sensitivity and attempts to improve this aspect invariably increases the cost. Shadowgrams are often interpreted subjectively due to the other properties of light such as reflection and absorption. As such, most interpretations are equivocal (Kovasznay et al. 322). Consequently, it is largely ambiguous and downplays the fine details of the object. Besides, improved versions of shadowgraph systems tend to be very similar to Schlieren systems (Davidhazy 5).
Shadowgraphy has a broad application in the field of engineering as it facilitates the study of low fields and hence the hydrodynamic properties of many objects. Over time, the technique has been used in the investigation of fluid mechanics in various industrial processes to analyze the behavior of aerosols, turbine burners, fuel injectors, droplet generators and liquid dispensers (Klapp et al. 130). It is used by aeronautical engineers to determine the flow of missiles and high-speed aircrafts. The technique is also used widely in combustion studies, research into explosions, and ballistics as it is optimal for the analysis of flow patterns.
Overall, shadowgraphy is one of the simplest flow visualization methods. Shadowgraph systems are cheap to set up and do not require sophisticated apparatus. However, the method is relatively less sensitive and is subject to the observer’s bias. However, the utility of shadowgraphy in engineering is unquestionable as it provides crucial information on fluid mechanics and dynamics.