# Towards Hierarchical Workflows in SysML to Support Virtual Validation of Technical Systems
## 2 State of the Art
### Virtual Validation Process  
The virtual validation process can be viewed as an orchestrated and repeatable set of activities. This process is able to systematically organize the simulation activities of the domain models and update critical parameters during the simulation activities. The common approaches to designing a workflow in the current state of the art in both the literature and in practice are either to design a descriptive workflow or to apply an executable workflow based on one or more independent simulation calculations to satisfy specific design requirements. Regarding management workflow designs, such as those presented by [PERSON] et al. [15] and [PERSON] [16], these workflow designs focus on describing the workflow of entire system development processes at the conceptual level. Although these workflows clearly define the developer's tasks and specific operations at each step, they do not dive into the parameter level for simulations of the physical behavior of a system and do not consider the qualitative analysis of the system. Therefore, it is still challenging for the designer to directly obtain a reliable virtual validation conclusion regarding engineering issues by executing these management workflows. Regarding executable workflow designs, the commercial workflow builders (e.g., ModelCenter) provide a way to design virtual testing workflows at the parameter level [17]. Although these workflow designs are based on quantitative analysis, they are still not enough to judge whether the system solution fulfills the function. This is because the workflows and the function-oriented system architecture are built independently in different tools, which means the changes in the system architecture need to be propagated to the workflow design by using specific interfaces. Due to the lack of a direct link between system functions and workflows, these individual quantitative analyses are not sufficient to support virtual validation against functional requirements. To solve this problem, a set of virtual testing workflows was created within the system model based on Motego [12; 18]. In Motego, there is a clear mapping between solutions and functions. Thus, the system engineer can state that the function can be satisfied by the corresponding designed solutions by testing all virtual tests in the relevant solutions. Ref. [19] proposed the structured virtual testing workflows based on the classification of domain models to increase the reusability of the workflows, so that the engineer can easily use the workflow to meet different test purposes. However, the current workflows proposed in [19] still have limitations in achieving the virtual validation process. First of all, the current workflows cannot be easily used for virtual validation across the system function level. Specifically, functions often contain sub-functions. The failure of any sub-function may lead to failures of the entire function. Therefore, a comprehensive virtual validation of a system at different hierarchical levels is required, which means engineers do not only need to virtually validate a system solution, but also each of its sub-solutions. As system complexity and the number of functions increase, where the dependencies between system designs increase exponentially, manually managing virtual testing workflows for the virtual validation of the specific functions increases manual efforts. Secondly, virtual testing workflows employ a predetermined simulation sequence. When a solution is reused in another system, the workflows must be manually reorganized and reordered according to the system context to guarantee accurate testing. Thus, it is crucial to structure semi-automated workflows to facilitate the efficiency of the virtual verification process across the entire functional hierarchy. To achieve this goal, the virtual testing workflow should be easily adjusted and redesigned when related solutions are reused or the test purpose and accuracy demands have changed.