Design Optimization of a Weather Radar Antenna using Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD)

Transcription

1 Design Optimization of a Weather Radar Antenna using Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) Fernando Prevedello Regis Ataídes Nícolas Spogis Wagner Ortega Guedes Fabiano Armellini

2 Summary Introduction Objectives Geometric model Computational model Domain and Computational Mesh Boundary Conditions Results Conclusions Next steps

3 Introduction Weather antenna positioning is a very complex mechanism and some knowledge is required for the effective design and implementation; The air flow around the antenna can affect directly the positioning mechanism behavior; FEA and CFD models are useful for obtaining the knowledge about the plant without the need of physical construction of a model or prototype for experimentation; CFD analysis can be used to determine the air flow pressure field and the vortex separation frequency; FEA modal analysis is useful for determining the natural frequencies of the structure and for the qualification, validation and approval of the mechanisms after they are built.

4 Objective The aim of this work is to perform a design optimization study of a positioning mechanism for a S- band Doppler weather radar antenna; The specific objective of this study is to analyze and validate the prototype of the positioning mechanism designed for RMD700S-1M Radar, the first Weather Radar totally developed in Brazil.

5 Computational Model CFD Flow characteristics: Steady State with Mesh Adaptation using velocity variable to obtain the first mesh refinement; Transient simulation for the final mesh; Incompressible Flow; Turbulent Flow: Shear Stress Transport turbulence model; Advection scheme: High-resolution. FEA characteristics: Bearing modeled by Coupled DOF s; Static simulation using surface effect elements to apply the wind loads; Harmonic simulation with given rotation.

6 Computational Model Fluid properties: Air Density: [kg m^-3]; Dynamic Viscosity: 1.831e-05 [kg m^-1 s^-1] Material properties: Aluminum Alloy Density: 2800 [kg m^-3]; Young Modulus: 70 [GPa] Poisson Ratio: 0.3 Structural Steel Density: 7850 [kg m^-3]; Young Modulus: 200 [GPa] Poisson Ratio: 0.3

7 Computational Model Numerical Data (CFD) Mesh: Ansys ICEM CFD 10.0 Solver: Ansys CFX 10.0 Post-Processor: Ansys CFX Post Numerical Data (FEA) Mesh: Ansys ICEM CFD 10.0 Solver: Ansys 10.0 Post-Processor: Ansys 10.0

8 Geometric Model Positioning Mechanism Antenna Support

9 The CFD Model and Results

10 CFD Computational Model - Antenna Geometric simplifications Wind direction

11 CFD Computational Model Boundary conditions Boundary conditions Outlet Opening Far field Opening Monitor Points Inlet: Velocity = 100 km/h Ground - wall no slip Antenna

12 CFD Computational Model Mesh last adaptation refinement Mesh adaptation Side view nodes elements Ground Wall (Prism Layer)

13 CFD Computational Model Mesh last adaptation refinement Top view Vortex separation region

14 CFD Results Time average pressure field Top View Side View

15 CFD Results Time average velocity Top View Side View

16 CFD Results Transient Vorticity field 0.2s 0.4s 1.0s 1.2s 1.4s 1.6s

17 CFD Results Time average 3D streamlines Main vortex caused by antenna

18 CFD Results Vectors Movie Vorticity Movie

19 CFD Results Pressure distribution that will be used at Ansys Structural analysis

20 Frequency Analysis Reynolds Number D = [ mm] = [ m] ρ = [ kg m ^ 3] V = 100 [ k h^ 1] = [ m s^ 1] 2 µ = [ kg m ^ 1s ^ 1] Re = ρvd = µ 6 Considering the antenna as a flat plate: Strouhal Number nd St = V St = 0.14 ( Flat Plate) V = 100 [ k h^ 1] = [ ms^ 1] D = [ mm] = [ m] n= [ Hz] Analytical frequency

21 CFD Frequency Response (FFT) Four points were monitored. Pressure variation was calculated as a function of time at each point. A classical Fast Fourier Transform was applied on the results in order to obtain a frequency response function.

22 CFD Frequency Response (FFT) Monitor Point Main Frequency [Hz] INT [Pa] [Hz] Frequency [Hz]

23 CFD Frequency Response (FFT) Monitor Point Main Frequency [Hz] INT [Pa] [Hz] Frequency [Hz]

24 CFD Frequency Response (FFT) Monitor Point 3 7 Main Frequency [Hz] 5 INT [Pa] [Hz] Frequency [Hz]

25 CFD Frequency Response (FFT) Monitor Point 4 4 Main Frequency [Hz] INT [Pa] [Hz] Frequency influenced from antenna support vortex separation Frequency [Hz]

26 Results Frequency Analysis Monitor Point 1 Monitor Point 4

27 The FEA Model and Results

28 FEA Computational Model Structural Steel Aluminium Alloy

29 FEA Computational Model From original geometry, a mid-surface model was taken. The Finite Element Model has been built using SHELL181 on those surface, BEAM188 on bolts and bars, and SURF154 on concave surface of Antenna for input the wind loads. FEA Model Nodes Elements Coupled DOF`s were considered to simulate the bearing of Azimuth and Elevation axles.

30 FEA Model Static Analysis Boundary Condition Fixed Support U x = 0 mm U y = 0 mm U z = 0 mm G = 9810 mm s^-2 Pressure distribution from CFD Results

31 FEA Results Static Analysis Displacement (mm) Von Mises Stress (MPa)

32 FEA Model Modal Analysis Boundary Condition Fixed Support U x = 0 mm U y = 0 mm U z = 0 mm

33 FEA Results Modal Analysis Mode 1 : 5.43 Hz Mode 2 : 9.60 Hz

34 FEA Results Modal Analysis Mode 3 : Hz Mode 7 : Hz

35 FEA Model Harmonic Analysis Boundary Condition Fixed Support U x = 0 mm U y = 0 mm U z = 0 mm Given Displacement = One degree rotation about Azimuth Axis Damping = 2% Critical

36 FEA Results Harmonic Analysis Response Points Point 1 Point 2 Point 3

37 FEA Results Harmonic Analysis Point 1 Point 1 Point 2 Point 3

38 Conclusions CFD and FEA computational models were developed in order to study the flow around a weather radar antenna and its structural response; For the transient case, a preliminary CFD mesh adaptation study was performed in order to obtain an adequate mesh refinement; The CFD transient model was used to obtain the flow behavior and the vortex separation frequency; The comparison between the theoretical frequency and the result obtained in CFD model presented good agreement (~1.5% of difference);

39 Conclusions The static analysis, using pressure distribution from CFD analysis, showed that the structure is over dimensioned in terms of mechanical failure, therefore it is possible to reduce the systems total payload, specially at the structure s base; From the modal analysis, the first two modes are the most significant for the control system. The first mode is due to torsion of the axis of elevation and the second is due to torsion at the azimuth axis.

40 Conclusions The lowest natural frequency found was 5.4Hz (at elevation axis). Thus, the dynamic response and the specifications of the system (maximum speed of 36 o /s and maximum acceleration of 10 o /s 2 ) are not coincident. The excitation due to the wind flow along the parabolic antenna do not affect the control system. The frequencies obtained from the CFD analysis were considered.

41 Next Steps Use CFD response into a dynamic structural analysis (Fluid Structural Iteration); Design optimization using Workbench Design Xplorer.