# Towards Hierarchical Workflows in SysML to Support Virtual Validation of Technical Systems
## 3 Wind Turbine System Virtual Validation  
Examination of energy scenarios reveals that wind energy is set to become increasingly vital in the future energy supply system [20]. The production of energy from wind is typically achieved by the WT system [20]. For wind turbine manufacturers to remain competitive and meet the increasing demands of customers, WT systems need to be constantly redesigned and developed to capture more wind energy. However, as complexity grows in the later stages of system development, the number of interdependencies between designs also increases significantly. Therefore, ensuring that the redesigned WT system and its sub-systems meet functional requirements is still a challenge today. Before introducing the method of this work, this section describes a classical virtual validation process of a technical system (i.e., WT) to identify the research object. In classical virtual validation processes, developers often focus on their domains. In the case described below, it is assumed that \"Engineer A\" considers WT as a whole system and focuses on the annual energy production (AEP) testing of the WT, while \"Engineer B\" considers the development of WT component, i.e., a bearing system, and focuses on fatigue analysis related to bearing lifetime. Figure 2 illustrates a joint development of a WT system and its components by the two engineers. It was assumed that the customer's requirement for AEP of the WT increased from 8 GWh/year to 12 GWh/year (Step 1). In order to meet the higher power generation requirement, the WT was redesigned with larger blades, increasing length from 61.5 to 80 m, and a higher power (from 3.2 MW to 6 MW) generator (Step 2). After the WT AEP testing, the AEP of the redesigned WT increased from 8.25 GWh/year to 13.38 GWh/year. Therefore, as the relevant requirement was satisfied, it was a successful redesign by \"Engineer A\". However, in highly coupled complex systems, a design change often leads to unsatisfied design requirements in other fields due to physical interdependencies. In the case of these redesigns, larger blades and higher power generators will lead to increased loads on the bearing system and shaft system, which then leads to potential failure risks (Step 3). Based on the experience of engineers, a foreseeable failure risk is that the lifetime of the bearings will be reduced and the rate of utilization of the shaft will be improved. For the bearing system, the bearing lifetime requirements are set to be greater than 20 years (i.e., 175,200 h) and the requirement of the shaft rate of utilization should be less than 1. In order to ensure the reliability of the virtual validation of the bearing, \"Engineer B\" needs to obtain the latest data of loads on the bearing system from \"Engineer A\" (Step 4). The executable workflow proposed in [19] can save these updated simulation results and transmit them to the following related tests. After the bearing fatigue analysis, it was found that the bearing lifetime was reduced to 82,014 h, which does not meet the requirements anymore (Step 5). From the perspective of \"Engineer B\", this was a failed redesign, which may have conflicted with the conclusion of \"Engineer A\". As a result, \"Engineer B\" needed to inform \"Engineer A\" that there was a risk of failure in the new design; this is a process that would normally rely on manual effort rather than being seamless. In complex multi-level systems, the failure of components may lead to the failure of the overall system. Therefore, the first problem is that the current virtual validation process still lacks a systematic organization that can handle the virtual testing and verification results at different hierarchy levels to achieve comprehensive and automated virtual validation. To fulfill all requirements, \"Engineer B\" needed to redesign the bearing system as well. In order to adapt to the higher loads, \"Engineer B\" chose bearings with larger diameters, which allow for a relatively long service life.