Many computer simulators used in engineering applications are based on solving a set of partial differential equations, or a mix of differential and algebraic equations, derived from physical laws and principles. Finite element analysis in mechanics is a frequently mentioned example of this type of computer simulators. Running computer simulators is to reduce cost by not conducting too many physical experiments, either too expensive, or too time consuming, or unrealistic. The surrogate models are models of models, because computer simulators are themselves mathematical models of a physical reality, rather than the physical reality itself. The popularity of the Gaussian process model as an emulator arises from its modeling of deterministic computer simulators. Computer simulators, including the turbine load simulators, are considered black boxes because an output is numerically computed by going through thousands of lines of computer codes. The development of turbine load simulators is indeed based on aerodynamic and aeroelastic physical principles.