Fidelity Enhancement of a Multirotor Dynamic Inflow Model via System Identification

Multirotor analytical dynamic inflow models in the literature, such as pressure potential superposition inflow model or velocity potential superposition inflow model (VPSIM), have been shown to capture the fundamental inflow interference effects between the rotors. Some of the differences in inflow predictions seen between these analytic models and high-fidelity wake models are attributed to missing real flow effects such as wake distortion, contraction, decay, swirl, etc. As such, correction terms are needed in the analytically derived multirotor finite-state inflow models, because of the potential flow and rigid wake assumptions they are based on, in order to capture some of the missing real flow effects in them. This paper develops a systematic methodology for arriving at the needed correction terms in the VPSIM through comparisons of its inflow predictions with those of a viscous vortex particle model (VVPM). Also, a procedure is developed to assess the relative importance of individual real flow effects and the associated corrections needed for improving the overall fidelity of the VPSIM. The developed methodology is applied to the Harrington coaxial rotor using its geometric and aerodynamic data from the literature. It is shown that the addition of swirl coupling correction terms to the VPSIM significantly improves its correlations with the VVPM. Further, it is shown that the required corrections are reasonably insensitive to thrust sharing ratio conditions between the rotors.

Finite State Inflow Flow Model for Coaxial Rotor Configuration

An analytical coaxial rotor inflow model has been developed from potential flow theory using the pressure potential superposition approach. The coaxial rotor pressure potential superposition inflow model (PPSIM) is formulated in statespace form with structure similar to the Peters–He model, except that additional off-diagonal blocks are included in the apparent mass (M-matrix) and influence coefficient matrices (L-matrix). These off-diagonal blocks take into account mutual interference effects present in a coaxial rotor system by relating the rotor's inflows due to other rotor's pressure loadings. Induced inflow distributions on both upper and lower rotors are computed using PPSIM for comparison against predictions from high-fidelity models such as GT-Hybrid and the viscous vortex particle method (VVPM). Good agreement between PPSIM-induced inflow results and GT-hybrid as well as VVPM data has been shown for hover flight condition. At low advance ratio, there are differences in fore-to-aft inflow states between PPSIM and the high-fidelity models. This is because PPSIM assumed rigid, skewed cylindrical wake geometries for both upper and lower rotors during forward flight. But in GT-Hybrid and VVPM, wake structures are allowed to move freely in space and are mainly affected by rotor-induced velocities at low advance ratios. Owing to the close proximity between upper and lower rotors, mutual interference-induced velocities significantly distorted the rotors' wake geometries. The rigid rotor wake geometry assumptions in PPSIM and the distortion captured in higher fidelity models are the reasons behind differences in rotor-induced inflows. At higher advance ratios, wake distortion effects are less prominent since free-stream inflows are significantly larger than rotorinduced velocities. Hence, smaller differences between PPSIM inflow states and those extracted from GT-Hybrid as well as VVPM are observed at high advance ratios.

High-Fidelity Simulations of Low-Pressure Turbines: Effect of Flow Coefficient and Reduced Frequency on Losses

Large eddy simulations validated with the aid of direct numerical simulation (DNS) are used to study the concerted action of reduced frequency and flow coefficient on the performance of the T106A low-pressure turbine profile. The simulations are carried out by using a discretization in space and time that allows minimizing the accuracy loss with respect to DNS. The reference Reynolds number is 100,000, while reduced frequency and flow coefficient cover a range wide enough to provide valid qualitative information to designers. The various configurations reveal differences in the loss generation mechanism that blends steady and unsteady boundary layer losses with unsteady wake ingestion losses. Large values of the flow coefficient can alter the pressure side unsteadiness and the consequent loss generation. Low values of the flow coefficient are associated with wake fogging and reduced unsteadiness around the blade. The reduced frequency further modulates these effects. The simulations also reveal a clear trend of losses with the wake path, discussed by conducting a loss-breakdown analysis that distinguishes boundary layer from wake distortion losses.

Vehicle Modeling Additions for Off-Axis Improvements in a Real-Time Helicopter Model

An ongoing concern with helicopter simulations is that they often have an incorrect off-axis response to cyclic control inputs when compared with the corresponding flight-test data. The more commonly suggested contributing factors for this discrepancy include the influence of dynamic wake distortion, rotor blade elasticity, and unsteady blade airfoil aerodynamics. A rotor model was developed using a Ritz expansion approach with constrained elastic modes to account for the blade elasticity, as it is computationally compact and efficient and therefore suitable for use in a real-time simulation. The effect of including this blade flexibility on the dynamic response, and in particular the on-axis and off-axis response, of the University of Toronto Institute for Aerospace Studies helicopter models is examined. In addition, the combined effects of dynamic wake distortion, unsteady blade section aerodynamics, and blade flexibility on the dynamic response are examined. The various features were successful in altering the off-axis response, with improvements in some areas, while not disrupting the on-axis response. In some conditions, the magnitude of the resulting change due to flexibility was greater than the differences noted due to the addition of dynamic wake distortion or unsteady aerodynamics.

High-Fidelity Simulations of Low-Pressure Turbines: Effect of Flow Coefficient and Reduced Frequency on Losses

Large Eddy Simulations validated with the aid of Direct Numerical Simulation are used to study the concerted action of reduced frequency and flow coefficient on the performance of the T106A low-pressure-turbine profile. The simulations are carried out by using a discretization in space and time that allows minimizing the accuracy loss with respect to Direct Numerical Simulation. The reference Reynolds number is 100,000, while reduced frequency and flow coefficient cover a range wide enough to provide valid qualitative information to designers. The various configurations reveal differences in the loss generation mechanism that blends steady and unsteady boundary layer losses with unsteady wake ingestion losses. Large values of the flow coefficient can alter the pressure side unsteadiness, and the consequent loss generation. Low values of the flow coefficient are associated with wake fogging and reduced unsteadiness around the blade. The reduced frequency further modulates these effects. The simulations also reveal a clear trend of losses with the wake path, discussed by conducting a loss-breakdown analysis that distinguishes boundary layer from wake distortion losses.

Assessment of Computational Models for the Effect of Aeroelasticity on BVI Noise Prediction

This paper deals with the computational analysis of acoustic fields generated by helicopter rotors when Blade-Vortex Interactions (BVI) occur. The prediction procedure starts from the determination of the steady periodic blade deformations. Then, the BVI-affected, unsteady aerodynamics solution is obtained by a potential-flow boundary integral formulation suited for aeronautical configurations experiencing blade-wake impingements. It is applicable to blades with arbitrary shape and motion and evaluates both wake distortion and blade pressure field. Finally, the noise field radiated by the rotor is computed through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation examines the sensitivity of BVI noise prediction on the aeroelastic model applied for the calculation of blade deformations, and assesses the accuracy of the results through correlation with experimental data concerning a helicopter main rotor in descent flight. Noise predicted is examined in terms of both acoustic pressure signatures and noise radiation characteristics.