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Numerical Model of Variable Surface Roughnesses for Parasite Drag Estimation


OBJECTIVE: To develop a numerical model for variable surface roughness distributions that can be implemented into computational fluid dynamics simulations for the estimation of the parasite drag of an aircraft. DESCRIPTION: Surface roughness can be a significant contributor to the drag of an immersed body such as an aircraft [1]. Current Army aircraft can employ a number of materials, paints and protective coatings over the wetted surfaces of the vehicle that can have significantly different surface compositions and textures. Exposure to harsh environments and sunlight can cause oxidation of paints and protective coatings and corrosion of metals which roughen the exposed surfaces. In addition, the effects of roughening and scouring due to rain and sand abrasion can alter the roughness characteristics of leading edge surfaces. The resulting differences in roughness height result in changes in parasite drag [1], which ultimately affects the performance and operational capability of the aircraft through engine power required for flight. Current computational fluid dynamics (CFD) methodologies are not able to geometrically simulate surface roughness with respect to the time and computational resources typically available for engineering drag estimation of complex shapes. Rather, the fluid dynamic effects of surface roughness are approximated through turbulence models [2,3], transition models [4], or wall functions [5,6]. These roughness models typically assume a single characteristic surface roughness applies over the entire body [7]. This assumption can result in an over- or under-prediction of the aircraft"s true aerodynamic drag, depending on the cumulative relative differences between the assumed roughness and the actual distributed roughnesses. The objective of this topic is to advance the state-of-the-art CFD in aerodynamic drag estimation due to parasite drag contributions from variable surface roughnesses, such as glasses, bare metals, protective coatings and paints. The desired ultimate end-product will be a numerical model that can be implemented into CFD software to estimate the parasite drag for an Army aircraft for flight within its design envelope. Equivalent sand-grain size roughness for the applicable surfaces would range from 0 to 200 microns. The ideal numerical model would cover a range in Mach number from the subsonic to the transonic regime, and would be applicable for a range in Reynolds number up to the tens of millions (O(10^7)). In the ideal implementation, the model would interact with a CFD solver through a boundary condition interface rather than requiring highly refined computational meshes that approximate the rough surfaces. This boundary condition interface could involve such physical characteristics as equivalent sand-grain sizes and concentrations. PHASE I: Identify innovative methods for modeling variable surface roughness distributions in computational fluid dynamics simulations utilizing current engineering-practice CFD meshes. Provide preliminary verification and validation approaches to support the activity. Identify the experimental data sets that will be used for validation. PHASE II: Develop and implement software modules implementing the new roughness model consistent with a component or system level flow simulation tool. Demonstrate the level of accuracy improvements in aerodynamic drag estimation and additional simulation cost in applying the model. Incorporate the model into current state-of-the-art CFD simulation tools useful for complete air vehicle aerodynamic drag estimation. Phase II deliverables may be subject to International Traffic in Arms Regulation (ITAR) control for the algorithmic implementation of the numerical model, dependent upon any previously existing distribution limitations for the CFD software into which the numerical model is integrated. PHASE III: If successful, the end product will be a numerical model for variable surface roughness distributions that can accurately estimate parasite drag for purposes of engineering analysis. For both military and civilian applications, this numerical model will be applicable for implementation into production-level CFD codes for estimating the total drag of a complete aircraft, as well as the constituent components of the aircraft. The accuracy of the aircraft drag estimates will be such that the results from the CFD simulations can be used as the basis for total aircraft aerodynamic performance estimation, estimation of engine power required, and to provide aerodynamic force and moment inputs to other engineering disciplinary tools such as computational structural mechanics and dynamics analysis software packages. In addition, military application of the model will allow evaluation of aircraft performance degradation due to deployment in regions of the world with high probabilities of surface roughening due to sand, rain and other environmental factors. The model will then allow for trade-off studies on the effect of different protective coatings versus operational capability impacts from aerodynamic drag.
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