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Multidisciplinary Design Optimization

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A presentation (PPT) summarising MDO activities of CASDE as on February 2003 (1 MB file).

Current design processes in the aerospace industry rely on sequential decision making through the traditional route starting with conceptual design, going on to the preliminary design phase and then to the detailed design, wherein each subsequent phase sees a significantly reduced design freedom as a consequence of decisions frozen in the earlier phase. When interdisciplinary conflicts arise, as they will in a complex coupled system, current design practice relies on non-automated means to resolve these, which might lead to sub-optimal designs in a global sense. MDO combines analysis and optimizations in several individual disciplines with those of the entire system concurrently through formal mathematical processes. It puts into place a formal integrated system design process for better product quality by effectively exploiting the synergism of interdisciplinary couplings. MDO, as a discipline, itself comprises of many areas of research such as the formal MDO problem formulation, architectures and methodologies for decomposition; approximation concepts and design oriented disciplinary analyses; software frameworks for implementation of integrated analysis, optimization, data management and communications; interfaces for human expertise and heuristic knowledge; etc.

During the discussion at the 2-Day Discussion Meeting-cum-Workshop on Integrated Modeling and Simulation organized by CASDE at Kalsa, Chickmaglur on August 14-15, 1999, many senior delegates from leading aerospace organizations of the country emphasized the pressing need to look at Multi-disciplinary Design Optimization (MDO) as part of a Systems Engineering approach. It was noted that formal MDO practice has not been initiated in the country as far as aerospace systems are concerned. With CASDE showing an interest to initiate some work in this direction, the meeting endorsed the inclusion of MDO as an additional dimension to CASDE's activities. CASDE's role was envisaged as a facilitator to the Indian aerospace industry in the following

  • Act as a repository of information on MDO and create/increase awareness about MDO amongst the industry;
  • Study the optimization and software aspects of MDO;
  • Study issues of integrating conceptual design with some aspects of preliminary design by bringing in higher levels of analysis fidelity along with formal optimization at the early design phase.

Literature search of important papers/theses/Internet sites has been carried out and a good amount of information on various aspects of MDO has been collected. A library of optimization related software packages as well as analysis tools for structural, aerodynamic and propulsion disciplines has been established. Towards the aim of creating awareness and sharing information on work going on in the country, a Special Interest Group on MDO (SIG-MDO) was formed and a number of workshops and meetings have been organized. CASDE has also supported a number of student projects to initiate studies on some of the optimization aspects of MDO including a comparative study of various MDO architectures, response surface approximations and computational evaluation of gradients. A study was carried out to implement and evaluate the computational issues related to three MDO approaches, namely All-At-Once, Concurrent Sub-Space and the Collaborative approach considering a simplified wing structural and aerodynamic optimization problem. Another study related to the use of a complex variable approach to the evaluation of gradients in comparison with the standard method of finite differences used by most optimizers, especially for the case when the disciplinary analyses involve a recursive or iterative computation. A third study considered a pure aerodynamic optimization problem and worked upon the issues related to single and multi-point response surface approximation models for the lift and drag as a function of the camber, geometric pre-twist and angle of attack variables based on a D-Optimality criterion for sampling the design space and a Vortex Lattice method for the aerodynamic analysis. Next on the cards is the use of an Euler code in design optimization. Other studies include the use of automatic differentiation for gradient computations, engine cycle and sizing optimization based on response surfaces.

In a series of post-graduate student projects, a multi-disciplinary design optimization problem for a medium size transport aircraft design with structures, aerodynamics, performance as disciplines and focussed on aeroelasticity has been formulated. An additional aim of this exercise was also, in the long run, to create a group of software modules incorporating sufficient interdisciplinary complexities and representing a sufficiently large aerospace optimization problem, which could be used as a basic platform to study optimization related aspects of MDO in greater detail, as well as become a test case for software framework related issues. The design problems being solved and compared are: (a) strength based structural design for fixed aerodynamic design coming from phase two; (b) aeroelastic structural design for fixed aerodynamic design from phase two; (c) simultaneous structural and aerodynamic design based on strength; (d) simultaneous structural and aerodynamic design based on aeroleasticity. A general code incorporating all the above options with re-configurable design problems has been implemented and tested. A number of variants and combinations of basic MDO architechtures, i.e. Multi-Discipline Feasible, Individual Discipline Feasible and All-at-Once, are implemented. Analysis fidelity levels incorporated include equivalent plate analysis and finite element analysis for structures and VLM for aerodynamics.

Apart from the above, specific studies of interest to aerospace organizations have also been taken up as collborative exercises. One of these is concerned with the MDO of an Airborne Early Warning System (with CABS, Bangalore). In this study the emphasis is basically on the top-down systems approach to the evolution of the AEWS design problem, which includes requirements capture, decomposition into subsystems, subsystem input-output models, identification of the candidate design variables and constraints, posing the optimization problem and its solution. The second such study is a feasibility study on the benefits and possibility of implementing Maneuvre Load Control on an existing fighter aircraft using its existing control surfaces(with ADA, Bangalore). The MLC problem is posed as a design optimization problem. This is a highly complex problem involving the aerodynamics, stability and control, structures and aeroelasticity. Like the AEWS study, the major part of the work is on the problem formulation, taking the realistic nature of the problem and supporting data and analyses requirement at hand. Another challenging problem taken up is the design optimization of a 3-D Intake Duct Design using high fidelity analysis (NS Solver). A composite team from CASDE and ADA Bangalore is attempting to solve this problem. MDO of a hypersonic vehicle has been taken up at DRDL, Hyderabad in collboration with CASDE. Collaborative work with VSSC in robust design optimization as well as concurrent optimization of launch vehicle system and its trajectory as an MDO problem is in its preliminary stage.

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