Numerical Database Of Stall Flutter Gust Response - AGREE Project

NUMERICAL DATABASE OF STALL FLUTTER GUST RESPONSE AGREE Project – Deliverable #10 Authors: Konstantinos Rekoumis National Technical University of Athens The research project is implemented in the framework of H.F.R.I call “Basic research Financing (Horizontal support of all Sciences)” under the National Recovery and Resilience Plan “Greece 2.0” funded by the European Union – NextGenerationEU (H.F.R.I. Project Number:016749) K. Rekoumis 1 Introduction This document contains comprehensive documentation for the numerical database created to characterize gust-induced airfoil stall flutter. The aim of the present report is to present the data in a clear and concise manner for others to use. The present work investigates the numerical modeling of gust induced stall flutter oscillations on a NACA 64-418 airfoil. The computational framework aims to numerically represent the experimental apparatus installed in the NTUA wind tunnel [1]. In section 2, the examined cases will be described. In section 3 the geometry and the modeling conventions of the computational model will be discussed, then in section 4 the numerical methodology employed will be discussed. Finally, in section 5 the data organization protocol will be presented. 2 Examined cases In the present study, the gust-induced stall flutter oscillations of a NACA 64-418 are numerically investigated. This numerical investigation on the gust induced stall flutter is based on experiments performed in the NTUA Wind Tunnel. To perform the present study a segmented approach was followed. First, the gust was studied in an empty wind tunnel (without the target airfoil). To produce the studied gust, the gust generation vanes motion followed a ”1-cos” profile (see eq. 1) with an Amplitude A= 34 deg and an oscillation frequency f= 7 Hz. The U∞= 17.94 m/s and is kept constant throughout this study. In the gust generation analysis, both 3D and 2D simulations were performed to capture any 3D effects during the gust propagation. For this purpose, simulations were run both on a 2D grid and 2 3D grids, where the 3D grids have ARgg =zmax chordgg = 1 & 3.5. The grid with ARgg = 3.5→z= 700 mm is equal to the half span of the wind tunnel. θ=(A 2[1 −cos(2πfδt)] , t ∈[t0, t0+1 f] 0, else (1) where δt =t−t0,fthe excitation frequency, and Athe target gust angle. Figure 1: Monitor point location on the xy plane and spanwise arrangement 2 K. Rekoumis Figure 2: The angle timeseries of the gust generation foils Our analysis concluded that no-significant 3D effects take place, as good agreement was found between the 2D and 2 3D simulations. However for completeness’s sake, the data from the 3D simulation for a ARgg = 3.5were uploaded. Therefore the velocity field was measured in several stations along the span of the computational domain at the same xy coordinate. The x is 1295 mm downstream of the gusts’ TE, and the y is +140 mm off the center-line of the tunnel. The z coordinates distribution is given in the table 3. To acquire better context of the arrangement, advise the figure 1. Then, the gust response simulation was performed. In the gust response simulation, the airfoil is coupled with a torsional spring-dampener system and is able to freely oscillate around its elastic axis while having all its other Degrees of freedom constrained (the particulars of the foil are found in table 2). Note that, the elastic axis is parallel to the Global Z axis and consequently there are no gravitational effects to take into consideration in the simulations. Initially, the airfoil was stabilized in the flow and then subjected to four gusts according the timeseries in the diagram 2. For the aero-elastic simulations a 2D simulation was run for a total simulation time of 30 sec (23 of which the foil performed stall flutter LCOs). Wind Tunnel Working Length 3.3 m Geometry Breadth 1.8 m Height 1.4 m Gust Generation Number 4 Vanes Vane chord 200 mm Vane Profile NACA 0015 c Center of Rotation 25 % chord Table 1: Wind Tunnel & Gust Generator dimensions Foil Profile NACA 64-418 c Foil chord 500 mm Elastic Axis - x 35 % chord Elastic Axis - y 1.6 % chord Moment of Inertia 0.7282 kg m2 Damping ratio 0.21 Spring Coefficient 254 Nm/rad Table 2: Aero-Elastic (target) Foil particulars 3 Geometrical Configuration The computational grid is modeled after the physical Wind tunnel (for dimensions advise table 1). More specifically, the xy plane in the region of interest is dimensionally accurate with respect to the real world. After the region of interest, the grid is coarsened progressively to emulate the farfield region of the wind tunnel. The section we study is uniform along the z axis up to the Height of the Tunnel. The dimensions of the xy plane in the zone of interest can be seen in figure 3. 3 K. Rekoumis Figure 3: Top down view of the computational domain with dimensions for the gust response case The dimension are given for the xy section of the gust response case computational domain to relay all the available information. The gust generator foils (vanes hereafter) are created after the NACA 0015 foil, with a chord of 200 mm (advise table 1). The vanes are placed inside the computational domain as seen in figure 3. Again in figure 3, the x axis distances of the vane placement can be seen. Finally, in the gust response case the target foil has a NACA 64-418 profile, with a chord of 500 mm (advise table 2). As seen in figure 3, the LE is placed 1295 mm downstream of the gust generator foils’ TE and on the Wind Tunnel’s Center Line1. For the structural part, the elastic axis from which the target foil’s motions are performed is located 165 mm from the LE and +8 mm off the chord of the foil. 4 Numerical Setup In figure 4, the different boundary elements of the computational domain are color coded and presented in the included table. The bounding walls element are modeled as Inviscid walls and the inflow & outflow elements with Inflow and Outflow Boundary Conditions respectively. Both the gust generation foils and the target foil elements are modeled with viscous wall boundary conditions and the y+ value is kept ≤1. In the present work, all numerical simulations were done using the in-house URANS code MaPFlow [2], [3]. In both gust generation and aero-elastic scenarios MaPFlow resolved the Incompressible Unsteady Reynolds Averaged Navier-Stokes equations, employing an artificial compressibility scheme. To model turbulence the Menter’s K-omega SST model [4] along with the γ−Reθ transition model was used in both examined cases. MaPFlow has the ability to resolve Fluid-Structure Interaction problems internally, via its strongly coupled RBD solver [3]. Therefore, both the forces applied to the target foil and the responses of the target foil, can be extracted from the same piece of software and ensure consistency of the data at each time instance. 1The Center Line along with the frame of reference axis are shown in figure 1 4 K. Rekoumis Figure 4: Geometrical Configuration of the Aero-elastic setup Each different element of the setup is color - coded differently. Notice that each solid boundary (gust generation foils & target foil) is treated as a separate entity, hinting at the versatility of our setup. Finally, the code has the ability to insert probes in the flowfield and extract at a certain point the velocity field. 5 Data Organization In the present section, the data naming and structure conventions for using the provided data will be analyzed. All data provided are in a csv (comma-separated values) text file for ease of use. The first row will contain the name of the respective column at each cell. In the subsections that follow, we will give some instructions on how to read the uploaded data as long as the context of the numerical values. 5.1 Gust Generation Case Data for the gust generation case are uploaded in the gust generation directory. The monitor point X.csv (X takes values from the ”Monitor Point” column of table 3) files contain the velocity vector at the respective point in space for each time instance. As the x-y coordinates as stated above, the mapping of header names to z coordinate follows the table 4. 5 5.2 Gust Response Case K. Rekoumis Monitor Point Z [m] A 0.1 B 0.2 C 0.3 D 0.4 E 0.5 F 0.6 Table 3: Spanwise arrangement of the monitor points 1 time 7−→ Simulation Time [sec] 2 velx 7−→ Flow Velocity x component [m/s] 3 vely 7−→ Flow Velocity y component [m/s] 4 velz 7−→ Flow Velocity z component [m/s] Table 4: Mapping of header names to physical context for monitor point files 5.2 Gust Response Case The files for the gust response case can be found under the gust response directory. To give insight in the movement performed by the foil, files containing the movement and loading timeseries are uploaded. Inside the directory of each aero-elastic case you find the following files: 1. hist rbd.csv : The file containing the motion history of the target foil as it is resolved by the RBD solver. 2. hist loads.csv : The file containing the raw loads developed on the global orthogonal axes. These loads are the direct result of integrating the pressure field and the shear stresses on the solid boundary. To ensure that the information of the files is relayed properly, in tables 5 and 6 an explanation about each columns data is presented. 1 time 7−→ Simulation Time [sec] 2 aoa 7−→ Angle of Attack [deg] 3 rot speed 7−→ Rotation Speed [rad/s] 3 rot accel 7−→ Rotational Acceleration [rad/s2] 4 moment 7−→ Rotational Moment around local z-axis [Nm] Table 5: Columns’ header explanation for hist rbd.csv file 1 time 7−→ Simulation Time [sec] 2 Fx 7−→ Force at the Global x-axis [N] 2 Fy 7−→ Force at the Global y-axis [N] 2 Mz 7−→ Moment around the Global z-axis [Nm] 3 rot speed 7−→ Rotation Speed [rad/s] 3 rot accel 7−→ Rotational Acceleration [rad/s2] 4 moment 7−→ Rotational Moment around local z-axis [Nm] Table 6: Columns’ header explanation for hist loads.csv file References [1] Marinos Manolesos, Christos Ampatis, Dimitris Gkiolas, Nikolaos Papakonstantinou, Andreas Alexandris-Galanopoulos, Konstantinos Rekoumis, and George Papadakis. Design of a wind tunnel set up to measure airfoil aeroelastic gust response, July 2025. 6 REFERENCES K. Rekoumis [2] Georgios Papadakis. Development of a hybrid compressible vortex particle method and application to external problems including helicopter flows. PhD thesis, National Technical University of Athens School of Mechanical Engineering, 2014. [3] Dimitris Ntouras. Adaptation of the Artificial Compressibility Formulation for Free Surface Flows with Applications in Ship & Marine Hydrodynamics. PhD thesis, National Technical University of Athens School of Naval Architecture & Marine Engineering, 2023. [4] F. R. Menter. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8):1598–1605, 1994. 7