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87 CHAPTER 10 Engineered Aggregate Arrestor Concept 10.1. Concept Description fabric layer, and finally topped with reinforced turf. Pre- cipitation would not seep through the aggregate in this 10.1.1. System Overview case, but would run off to the perimeter of the arrestor An engineered aggregate arrestor concept has been pro- bed. Hence, the bed would be kept dry and the material posed. Its primary material is a spherical engineered aggre- response during an overrun would likely prove more gate that has excellent flow properties and resists settling and consistent. compaction that are more typical for angular gravels (Fig- ure 10-1). This material would reside in a shallow bed and be 10.1.2. Performance Considerations covered with a reinforced turf layer. However, the engineered aggregate may also be used without a turf layer, which has The dynamic performance issues for the engineered aggre- been done at four airports in the UK. Other top layer materi- gate concept are different from those of the crushable mate- als are possible, such as a thin asphalt skim coat. Figure 10-2 rial candidates. During the assessment process, the following illustrates the two major variants of the engineered aggregate aspects were considered: arrestor concept. The difference between the two concepts is the existence of · Dynamic behavior and energy absorption, a confining top layer, which can serve several purposes: · The degree of non-linearity of the landing gear loading and danger of failing the nose gear, 1. Prevent aggregate dispersion due to jet blast; · Performance in a short-landing situation that involves an 2. Mitigate aggregate spraying during overrun by aircraft aircraft touchdown inside the arrestor bed, tire, thus limiting engine ingestion hazard; · Applicability to arresting a wide range of aircraft types, 3. Regulate water drainage and potential ice crust formation · Vulnerability to ice crust formation in severe winter envi- in winter; and ronments, and 4. Act as a structural component to prevent lightweight land · Effect of a cover layer on the performance of the aggregate vehicles from penetrating the arrestor bed. system. Because the engineered aggregate materials are not greatly 10.2. Modeling and Testing affected by water, the bed can be designed to handle precipita- Approach tion in two different ways (Figure 10-3). As with the crushable systems, the goal for the performance 1. Drainage Approach. This approach would allow water evaluation was to perform testing that would allow cali- drainage downward through the bed. In this case, the bed bration of high-accuracy computer models of the engi- would be designed to prevent standing water within it using neered aggregate concept. Determining the necessary physical normal civil engineering design practices. tests was driven by the requirements of the modeling soft- 2. Waterproof Approach. In the second approach, a compos- ware. Therefore, a modeling method capable of high- ite top layer could be employed to prevent water drainage fidelity aggregate simulations was determined first, and through the bed. It would likely be composed of a channel- the required tests for material characterization followed ized or cuspated plastic layer, overlaid with a geo-textile thereafter.
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88 Drainage Approach Water-Proof Approach Layers of Plastic Keep Precipitation Drains Aggregate Dry Through Bed Figure 10-3. Aggregate bed methods for handling precipitation and drainage. particles move and interact with one another in fundamen- tally the same way that actual aggregate does, with friction Figure 10-1. Engineered aggregate. between the particles, compaction, fluid-like spraying behav- ior, and momentum transfer. 10.2.1. Discrete Element Modeling (DEM) Method 10.2.2. Required Parameters Modeling aggregates presents a number of complexities for Material Models when compared with crushable materials because they have both solid-like and fluid-like regions of behavior. Bell et al. EDEM defines aggregate pieces as rigid spheres, or lumps describe the complexities from a modeling and simulation of rigid spheres grouped together to form irregular pieces. standpoint: the fluid-like behaviors cannot be adequately The rigid sphere assumption was valid here since the spheres simulated with solid material models (Lagrangian), nor can behave as fairly incompressible particles with respect to the the solid-like behaviors be adequately simulated with a fluid aircraft tire. Particle size, size variation, density, and so on material model (Eulerian) (39). LS-DYNA, which can simu- were all defined as expected. late both Lagrangian and Eulerian behaviors, is still not ade- The main effort in characterizing the engineered aggregate quate for modeling this particular material. behavior was in determining the particle interactions with The current state of the art for modeling aggregate behav- each other and with solid surfaces (i.e., the aircraft tire, bed ior involves the use of DEM codes, which model the aggregate walls, etc.). This was a fundamental contrast to the crushable on a particle-by-particle basis. The EDEM software package, material modeling in LS-DYNA, where the main effort was produced by DEM Solutions, was selected from among the expended in calibrating the material models. commercially available DEM codes. Using EDEM, the bed of Four interaction parameters required definition in EDEM, aggregate was modeled using individual aggregate pieces. The which are briefly described here: Aggregate Arrestor Concept 1: Open Bed Aggregate Bed Arrestor Basin Aggregate Arrestor Concept 2: Covered Bed Cover Layer of Engineered Turf Aggregate Bed Figure 10-2. Engineered aggregate arrestor concepts.