With computational fluid-dynamics engineers analyze liquid and gasses flows inside or outside parts. Starting from the knowledge of fluid properties, boundary conditions (flow rate, pressure, temperature, etc.) and wall properties, it is possible to simulate the kinematic and thermodynamic behavior of fluids and their interaction with solids. Readers interested in techniques for deducing the governing equations and solution strategies can refer to referenced book [37].
Let us consider a practical application in order to illustrate how the augmented reality can support this kind of engineering simulation. The purpose is to simulate an external air flow around a cylinder on a table. Both table and cylinder are physically present in the real world (Figure 15). The stream line and the pressure field on the surfaces have to be added as virtual contents. The first step is to built the virtual parts. This task can be done using the modeling
techniques illustrated in the previous section, using the I-Pen to acquire the geometry of the cylinder and of the table. The second step is to define all the bounding conditions for the simulation. Because there are many values to be defined, it is convenient to complete the model outside the augmented reality environment. Then we have to perform the numerical computation using an external CFD solver. The last step is the projection of the numerical results in the real scene. Generally, the main results of such simulations concern with several stream lines describing the fluid trajectories which are coloured with a palette indicating the velocity, pressure or temperature value. The information has to be added to the real world. Once again, we have to collimate the two main reference frames of the real world and the virtual one. This operation can be made using communication reference frames in the same way of the previous discussions. For the specific example of the cylinder, it can be convenient to put a visible patterned marker locate near the physical cylinder (Figure 15), defining the real world communication frame. Similarly, we can define the virtual communication frame at the corresponding location in the virtual world. By this way it is possible to locate the virtual stream lines and surface plots in the real scene with a simple coordinate transformation (translation and rotation) using OpenGl operators glRotated and glTranslated.
Figure 15. Simulation of the air flow around a cylinder on a table in augmented reality. Starting from a real environment and adding collimated virtual objects (on the top), an augmented animation can be built (at the bottom) .
As a result, the augmented scene includes the real world with the virtual surfaces touched by the fluid (cylinder and table) (Figure 15, at the bottom). The scene can be also enriched including an animation of the stream lines. By this way the user can visualize from different points of view, the fluid stream around the cylinder and the pressure acting on the boundary surfaces directly in the real world.
The implementation can be summarizes in the following six steps:
1. The real scene has to contain information for collimating the real world to the virtual objects. A patterned marker or a coordinate system defined by picking points with I- Pen can be used;
2. The geometry of component(s) under investigation has to be acquired;
3. Boundary conditions (fluid flows, pressure openings, wall properties, etc.) have to be defined too;
4. The computation of fluid field and pressure on surfaces can be performed using an external FEM solver;
5. The results coming from step 4 has to be converted into coloured .wrml entities (i.e. stream lines, surfaces coloured according to pressure values);
6. The .wrml entities have to be collimated with the real world and rendered on the augmented scene.
The augmented video can be also enriched with other visual information on the exact numerical values of fluid or surface parameters adding a virtual legend (i.e for describing pressure, velocities, temperature, etc.).
4. Conclusion
The integration between computer aided engineering tools and augmented reality has revealed to be a valid instrument in supporting designers and users in modeling, testing and reviewing their products. With the integration of specific hardware devices as trackers and sensors, the user can interact with the scene in an immersive way. For the modeling of shapes, a magnetic sensor can be useful to acquire a precise position in space of a virtual pen in order to allow the user to sketch virtual objects directly on the real world. Moreover the comprehension of the results of engineering simulations as motion analyses, structural investigations, fluid dynamics computations can be improved by enhanced visualization and interaction with real and virtual objects.
The discussed instrument and methodologies can be also useful for the collaborative design. Very often, scientists or engineers teams work on the same project at different locations. In this case, all the designers can wear an AR sub-system and all these sub-systems can communicate among them. Imagine that a group of designers is working on the model of a complex device for their customers. The designers and customers want to perform a joined design review even though they are physically separated. If each of them is equipped with an augmented reality display this could be accomplished. The physical prototype that the designers have mocked up is imaged and displayed in the client’s AR system in 3D. The client may look at different aspects of it, testing engineering performances and checking its integration to the real world.
The future of this combination between AR and Virtual Engineering is very promising. The discussed examples are only a small part of the capabilities of such integration. The main future challenges are about the manipulation of objects, the capabilities for virtual assembling and the fully integration of numerical methodology without requiring external solvers.
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