Collaborative Design Exploration

Optimal search methods can be applied to the design problem, but simply understanding that one design is better than another is not sufficient to make an informed design decision. In fact, the concept of a single optimal design is not applicable to rotorcraft because of the many objectives they must meet. There are many optima depending on how the objectives are prioritized. Design decision makers must be able to defend their position which requires understanding risks and trade-offs. Specifically, decision makers must understand a broad design space in terms of:

• How the choice of a design feature impacts all relevant engineering disciplines • How the design performs at off-design conditions

• How slight perturbations might impact design performance

• Why a design feature is helpful or hurtful to each discipline and the overall design metric. The rotorcraft MDO architecture must be able to provide this information—in other words, it must provide a map of the design space. Response surfaces lend themselves nicely for displaying this kind of

map. Not only does it provide information on how the design will perform, it will show how neighboring designs will perform and how relaxing or tightening constraints can impact the design.

Consider for example the design information shown in Figure 52. Two important design variables are plotted as independent variables along the horizontal and vertical axes. These represent two variables that are shared between two (or more) disciplines or subsystems because of their strong impact on both. Contours of a critical design metric are constructed by interrogating the design space at the green dots and fitting a surface to the results. The interrogation of the design space is a flow down to the disciplines or subsystems (low level) of the shared variables (high level). At the low level there is freedom to find a feasible optimal design that uses the shared variables as fixed parameters; any appropriate optimal search method can be used. Thus, response values at the high level not only represent the value of critical design metric, they represent the best that the low level can achieve. If a feasible design cannot be found at the low level with the prescribed shared variables, the shared variables represent an infeasible design as shown in the figure.

A DV₁ DV₂ Design Objective Infeasible Region Increasing value B 1 4 3 2 6 5 7 11 10

Figure 52. A view of a response surface map of a multidisciplinary design space

In this example the response surface clearly shows increasing values in the two design variables benefits the design objective. The second response surface for the constraint indicates boundaries in the design variables beyond which the design becomes infeasible.

• There are two important design points; Design A is a global optimum and design B is a local optimum.

• For high values of the second design variable, the design objective becomes less sensitive to the first design variable

• Design A and B give nearly the same performance though they are very different designs. • Small relaxation of the constraint near the region of Design A can lead to a relatively big

payoff

The benefits of a global view of the design space are very important to the decision maker. However, constructing the response surface needed to create the design space roadmap is a non-trivial task for rotorcraft MDO. Generation of response surfaces suffers from what has been called the curse of dimensionality [42]. As the dimension of the problem increases, the number of function evaluations needed to construct the surface rises rapidly. If, for example, we choose to discretize the n-dimensional design space in a grid of dimension m, the number of evaluations if every grid point were analyzed would

be mn_{. For a modest design dimension of 10 and m = 3, the total evaluations would be 59,049. Adding}

another design dimension increases the number of function evaluation to 177,147. For high fidelity analyses where compute times can be long, the maximum number of dimensions can be severely limited. Luckily, there are more efficient methods, such as orthogonal arrays and fraction factorials, to select designs with which to populate the design space, though they are still subject to the curse of dimensionality. In addition there are response-model-based optimization methods that carefully select region in which new data are obtained [26].

Response surface methods limit the number of design variables that can be used in a high-fidelity rotorcraft MDO. The limit can be increased, though, with some innovative methods that must be built into the rotorcraft optimization architecture. One way is a variable fidelity approach whereby much of the design space is populated with low-order analysis and only select cases are analyzed with high-fidelity approaches [43].

The multi-level approach that is described here is inspired by the collaborative optimization method whereby the design space is expanded on the single disciplinary level and only a limited number of design variables are used at the global level. In collaborative optimization variables are separated into global and local categories. Global variables affect two or more disciplines; local variables on the other hand affect only one discipline, or at the most, weakly affect other disciplines. For example, rotor twist is assuredly a global variable.

In typical collaborative optimization, a gradient-based approach drives the search while a variety of optimization procedures can be pursued at the local level for each step of the global optimization process. More general methods have been described for decomposition into multi-level systems [44]. Here the global search is replaced by a global response surface, which is the surface along which collaborative optimization moves during search. A form of global collaborative optimization has also been developed and applied to the optimal design of a tiltrotor [45], but this approach retains and presents a picture of the global design space. Instead of optimizing at the global level, the rotorcraft optimization architecture will utilize a Design of Experiments approach for a handful of important global variables. Each candidate design in the DOE will be optimized on the local level. The DOE will lend itself to design exploration instead of optimization to provide designers with information to make educated decisions. The process is

drawn in Figure 53.

Geometry Engine & Common Database

Common Optimization Architecture

For Collaborative Design Exploration

Design of Experiments Individual Discipline Optimization Framework Development Rotor Aerodynamics Airframe Aerodynamics Rotor Structure Airframe Structure Propulsion & Drive System

Individual Discipline Optimization with Local Design

Variables Rotor Aerodynamics Airframe Aerodynamics Rotor Structure Airframe Structure Propulsion & Drive System Global Design Variables

G L L L L L Set of Candidate Designs for Analysis

Responses of Localized Optimized Designs

Design Exploration Tool Set Including Optimization and Trade

Studies

Figure 53. Common Optimization Architecture for Collaborative Design Exploration

The single disciplinary design problems have been identified in Section 4, and specifically design variables of the disciplinary design problems have been identified. Those variables that are common to two or more discipline are pulled out in the Common Optimization Architecture and are categorized as the rotorcraft MDO design variables. These, or more than likely, a subset of these will be used for collaborative design exploration. The disciplinary design variables which did not get identified as global variables are set aside for local sub-optimization.

In the Common Optimization Architecture, a set of candidate designs is chosen for analysis. Each of the disciplines will return an analysis of the best possible performance for that design given the latitude of modifying only local variables. The analyses will be surface fit and send to the design exploration tool set to produce plots like Figure 52 and to conduct optimization and trade studies.