# Towards Hierarchical Workflows in SysML to Support Virtual Validation of Technical Systems
## 6 Discussion  
The case study shows that the proposed workflow design framework can be applied to support the virtual validation process of complex technical systems. The virtual validation process based on the function-oriented system architecture by using a standardized designed virtual validation workflow has advantages over the classical validation process mentioned in the previous literature. First of all, a nested virtual validation workflow is designed and implemented in an activity diagram, which contains sub-virtual validation workflows based on a hierarchical functional architecture. Therefore, designers can easily use automated workflows to fully validate the functions at all hierarchy levels. Compared to classical methods that rely on the manual work and experience of engineers, using the proposed virtual workflow allows early and reliable detection of the risk of technical system failure due to redesign. This work shows that since the nested framework of the virtual validation workflows corresponds to the system's functional architecture, virtual validation workflows can be created synchronously as new functions are added and related sub-virtual validation workflows can also be created as functions are decomposed. Secondly, each function includes a corresponding standardized designed virtual validation workflow, such as fatigue analysis and smearing analysis. The results of these virtual tests can be used as validation criteria. This work finds that the functions can be validated based on these virtual tests, which contain the related simulation activities of the domain models at different hierarchy solution levels. Compared to classical methods, the  
Figure 12: The verification & validation results of the WT system under different design alternatives. standardized workflow design in this work ensures that the virtual validation workflow can be easily redesigned, with related functions being reused in different systems. At last, the modular design based on model classification greatly improves the design agility of the virtual testing workflows. A virtual testing workflow consisting of modular simulation activities allows testers to select appropriate models. Additionally, the workflow with modular design can be easily adapted in different system contexts. Compared to classical methods, the virtual testing workflows proposed in this work can automatically and sequentially execute the corresponding simulation activities through iterations according to the step information provided by the engineer. This work uses a WT system as an example to demonstrate the application of workflow in a technical system. Although the scope of the system displayed by the WT system is very specific, as the modeling method of the system architecture is function-oriented, and the workflow design is also modularized based on the system architecture, the conceptual design of the workflow is considered to be scalable in theory. However, the current workflow still has limitations. While engineers can use pre-designed workflows to automatically perform the virtual validations of system solutions, the workflow still relies on experienced engineers to manually determine the order of a series of simulation activities in a workflow. Therefore, to further automate the virtual validation process while reducing potential errors from manual steps, a mechanism should be developed to automatically arrange simulation sequences based on dependencies between parameters in the proposed workflow and ensure data matching between models. After the virtual validation process, engineers need to optimize and redesign the system based on the validation results and perform the re-validations. However, this method does not give a solution that would provide an adaptive system design after virtual validation is failed. Therefore, it is necessary to extend the workflow to support optimization mechanisms.