Field Velocity Approach Research Articles

Implementation and verification of gust modeling in an open-source flow solver

A gust simulation and modeling capability will support the aircraft certification process and therefore reduces the development cost. Two different gust modeling techniques are considered in this article: 1) a user-defined boundary condition available in the commercial flow solver of Cobalt and 2) a field-velocity method implemented in the open-source flow solver of FlowPsi. Specifically, this article describes implementation of gust models in the FlowPsi code including a new time-stepping scheme and a new grid-motion capability that could be used in the simulation of responding vehicles to wind gusts. In more detail, the three stage time-stepping scheme, as here developed, provides much better convergence properties than the Crank–Nicolson scheme available in the code. The grid motion using an external code, as here implemented, is a part of larger development plans to make FlowPsi communicate with external processes such as a finite element code. In addition, the FlowPsi code uses a field-velocity approach to simulate the wind gust which offers a reduced computational expense for simulating wind gust responses when compared to Cobalt. Instead of propagating the gust perturbation from the far inflow boundary as used in Cobalt, an artificial velocity profile is imposed on the grid cells to induce the gust effects on the vehicle. This article aims to determine the accuracy of FlowPsi in predicting gust responses when compared to Cobalt which has already been verified to produce accurate results. The two methods in different codes are compared for a number of gust profiles such as one-minus-cosine, step (sharp-edged), and a random gust profile. FlowPsi shows close convergence and agreement to Cobalt predictions for smooth gust profiles such as the one-minus-cosine, however, the code produces a numerical oscillation pattern at low free-stream Mach numbers when modeling gusts with a rapid change of velocity such as the step gust.

Nonlinear Gust Response Analysis of Free Flexible Aircraft

Gust response analysis plays a very important role in large aircraft design. This paper presents a methodology for calculating the flight dynamic characteristics and gust response of free flexible aircraft. A multidisciplinary coupled numerical tool is developed to simulate detailed aircraft models undergoing arbitrary free flight motion in the time domain, by Computational Fluid Dynamics (CFD), Computational Structure Dynamics (CSD) and Computational Flight Mechanics (CFM) coupling. To achieve this objective, a structured, time-accurate flow- solver is coupled with a computational module solving the flight mechanics equations of motion and a structural mechanics code determining the structural deformations. A novel method to determine the trim state of flexible aircraft is also stated. First, the field velocity approach is validated, after the trim state is attained, gust responses for the one-minus-cosine gust profile are analyzed for the longitudinal motion of a slender-wing aircraft configuration with and without the consideration of structural deformation. RBA), while the second aims mainly to the analysis of elastic aircraft experiencing relatively small deformations. The idea to not consider cross-coupling effects between these two disciplines has been commonly justified by the large frequency separation of the characteristic motion which is typical of conventional structures. Nowadays, the focus on weight minimization for aircraft, leads toward more and more flexible vehicles. The resulting underlying structures may not exhibit the usual wide frequency separation among the rigid body degrees of freedom and the remaining elastic modes. So that the approach mentioned above can lead to mistakes/errors in analyses of flight performance, flying qualities, and control systems design. In these cases, an integrated analysis of flight mechanics and aeroelasticity is necessary from the very early stages of preliminary design (6).

향상된 자유후류 기법을 이용한 비정상 로터-동체 상호작용 시뮬레이션

This study is to investigate the aerodynamic effects of the Rotor-Fuselage Interactions in forward flight, and is conducted by using an improved time-marching free-wake panel method. To resolve the instability caused by the close proximity of the wake to the blade surface, the field velocity approach is added to the prior unsteady panel code. This modified method is applied to the ROBIN(ROtor Body Interaction) problem, which had been conducted experimentally in NASA. The calculated results, pressure distribution on fuselage surface and induced inflow ratio without and with the rotor, are compared with the experimental results. The developed code shows not only very accurate prediction of the aerodynamic characteristics for the rotor-fuselage interaction problem but also the rotor wake development.

Efficient Prediction of Helicopter BVI Noise under Different Conditions of Wake and Blade Deformation

Predictions of helicopter BVI noise using three-dimensional Euler code with a single blade grid are conducted under three different conditions: BVI noise caused by (1) interaction between rotating blades and vortex shed from fixed wing vortex generator, (2) interaction between rotating blades and tip vortices shed from preceding blades, and (3) interaction between rotating blades with elastic deformation and shed tip vortices. In the CFD calculation, the Field Velocity Approach (FVA) and Scully’s vortex model are used to import the wake information into the calculation grid and to determine the induced velocity made by tip vortices, respectively (cases 1–3). Beddoes generalized wake model is used to prescribe the tip vortices position in the wake (cases 2 and 3). Information about blade elastic deformation is imported from HART II project experimental data into the calculation (case 3). Acoustic analyses based on Ffowcs-Williams and Hawkings (FW-H) equation are conducted subsequently in each case. The results from the calculations show good agreement with experiments in all three cases, indicating that application of FVA, Scully’s model, and Beddoes generalized wake model is effective for BVI noise prediction in this study, which is intended for low calculation cost using a single blade grid. Also, use of blade elastic deformation data in the calculation shows marked improvement in calculation precision. Consequently, the method used in this study can predict BVI noise under various conditions of wake or blade deformation with acceptable precision and low calculation cost.

Numerical investigation on the directionality of nonlinear indicial responses

An unsteady Euler solver is modified to investigate the directionality of nonlinear indicial response to a step change in the angle of attack. An impulsive change in the angle of attack is incorporated by using the field velocity approach, which is known to decouple the step change in the angle of attack from a pitch rate. Numerical results are thoroughly compared against analytical results for two-dimensional indicial responses. The same method is applied to investigate the directionality of nonlinear indicial responses. It is found that directionality is mainly due to the asymmetry of initial shock locations. Since the directionality of the pitching moment responses is significant in the critical Mach number region, it is also shown that consideration of the directionality is crucial for accurate modeling of the nonlinear indicial functions.

Loose Coupling Approach of CFD with a Free-Wake Panel Method for Rotorcraft Applications

As a first step toward a complete CFD-CSD coupling for helicopter rotor load analysis, the present study attempts to loosely couple a CFD code with a source-double panel method. The far-field wake effects were calculated by a time-marching free vortex wake method and were implemented into the CFD module via field velocity approach. Unlike the lifting line method, the air loads correction process is not trivial for the source-doublet panel method. The air loads correction process between the source-doublet method and CFD is newly suggested in this work and the computation results are validated against available data for well-known hovering flight conditions.

Field Velocity Approach and Geometric Conservation Law for Unsteady Flow Simulations

A technique to model unsteady flow phenomenon which involve simultaneous elastic motions of the boundaries and interaction with gust fields is discussed. The field velocity approach is used to include the effect of the gust fields caused by a vortex wake in to the flow computations. Elastic deformations of the boundaries causes changes in grid cell volumes which requires the rigorous enforcement of the Geometric Conservation Law. Mathematical modeling required to enforce conservation when performing such unsteady flow computations is detailed. The application of the technique developed to a variety of problems from simple 2-D model problems to more complex realistic problems are detailed. The predictions obtained are validated with exact analytical results/ experimental data as appropriate. Overall, the field velocity approach together with a technique to enforce the Geometric Conservation Law is found to provide a powerful tool for unsteady flow simulation.

Computational-Fluid-Dynamics-Based Enhanced Indicial Aerodynamic Models

Direct Euler calculations were performed to verify the accuracy of linearized airload models and to generate a numerical database for the determination of generalized sharp-edged vertical gust response functions. The field velocity approach was used in the computational fluid dynamics (CFD) code to incorporate the impulsive boundary conditions that were characteristic of the problems studied. Linearized airload models were constructed in three dimensions by coupling the indicial models obtained in two dimensions with a Weissinger-L vortex panel method. The vortex panel method was suitably modified to include the shed near wake effects and impulsive boundary conditions. Studies conducted on step change in angle of attack and step change in pitch rate for a finite wing showed that the linearized models are sufficiently accurate in predicting the sectionwise air loads. Generalized gust functions were determined with a constrained minimization of least-square error with a CFD-generated database