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Coupled CFD/CSD Analysis of an Active-Twist Rotor in a Wind Tunnel with Experimental Validation
Abstract and Figures
An unsteady Reynolds averaged Navier-Stokes analysis loosely coupled with a comprehensive rotorcraft code is presented for a second-generation active-twist rotor. High fidelity Navier-Stokes results for three configurations: an isolated rotor, a rotor with fuselage, and a rotor with fuselage mounted in a wind tunnel, are compared to lifting-line theory based comprehensive rotorcraft code calculations and wind tunnel data. Results indicate that CFD/CSD predictions of flapwise bending moments are in good agreement with wind tunnel measurements for configurations with a fuselage, and that modeling the wind tunnel environment does not significantly enhance computed results. Actuated rotor results for the rotor with fuselage configuration are also validated for predictions of vibratory blade loads and fixed-system vibratory loads. Varying levels of agreement with wind tunnel measurements are observed for blade vibratory loads, depending on the load component (flap, lag, or torsion) and the harmonic being examined. Predicted trends in fixed-system vibratory loads are in good agreement with wind tunnel measurements. NOTATION Latin Symbols C p coefficient of pressure M Mach number M β flapwise bending moment, in-lb M θ torsional bending moment, in-lb M ξ chordwise bending moment, in-lb Q second invariant of the velocity gradient tensor r normalized rotor radius U ref X-component of velocity at boundary layer edge (0.995u ∞), ft/sec u X-component of velocity, ft/sec X axial CFD direction y + dimensionless, sublayer-scaled wall coordinate of first node away from surface Y horizontal CFD direction, wall normal direction in boundary layer plots Z vertical CFD direction
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A framework of numerical formulations for the aeroelastic analysis of helicopter rotor is presented in this paper. The blade structural dynamics are modeled by an open source multibody dynamic software MBDYN, which solves finite element equation of elastic bodies in general motions. Then the structural deformation is transformed to blade surface grid by radial base function (RBF) interpolation, and volume grids are regenerated by RBF and TFI methods. Lastly, the fluid governing equations are solved. By integrating the above methods, S76 hovering rotors are simulated and compared to the test data. Results show that elastic torsion decreases local angle of attack. For status at [Formula: see text] and [Formula: see text], the shock and shock-induced separation are reduced on the outboard blade, which has remarkable effects on the prediction of rotor hovering performance.
A general formulation for the reduction of the three-dimensional problem of electrothermoelasticity in slender solids to an arbitrarily defined reference line is presented. The dimensional reduction is based on a variational- asymptotic formulation, using the slenderness ratio as small parameter. In the proposed scheme, the coupled linear electroelastic equations are solved at the cross-sectional level using the finite element method. Furthermore, modal components of the displacement field are added to introduce arbitrary deformation shapes into the one-dimensional analysis, and arbitrary electric modes are used to define applied electric fields at the cross section. This results in a general definition of a coupled electroelastic stiffness, which can be used in virtually all composite and active beam formulations, as well as in the development of new low-order high-accuracy reduced models for active structures. Finally, the formulation also yields recovery relations for the elastic and electric fields in the original three-dimensional solid, once the one-dimensional problem is solved. The method has been implemented in a computer program (UM/VABS) and numerical results are presented for active anisotropic beam cross sections of simple geometries, which are shown to compare very well with three-dimensional finite element analysis.
The use of an Active Twist Rotor system to provide both vibration reduction and performance enhancement has been explored in recent analytical and experimental studies. Effects of active-twist control on rotor noise, however, had not been determined. During a recent wind tunnel test of an active-twist rotor system, a set of acoustic measurements were obtained to assess the effects of active-twist control on noise produced by the rotor, especially blade-vortex interaction (BVI) noise. It was found that for rotor operating conditions where BVI noise is dominant, active-twist control provided a reduction in BVI noise level up to about 3 dB. However, low frequency noise was found to increase up to 7 dB and vibration levels were usually adversely affected, in some cases doubling the amplitude, for operating conditions favoring minimum BVI noise. Conversely, operating conditions favoring minimum vibration levels affected BVI noise levels, with both increases and reductions on the order of 3 dB, and also incurred low frequency noise increases up to 7 dB. Minimum vibration levels representing a 90% reduction from baseline loads were observed.
Boundary-conforming coordinate transformations are used widely to map a flow region onto a computational space in which a finite-difference solution to the differential flow conservation laws is carried out. This method entails difficulties with maintenance of global conservation and with computation of the local volume element under time-dependent mappings that result form boundary motion. To improve the method, a differential ″geometric conservation law″ (GCL) is formulated that governs the spatial volume element under an arbitrary mapping. Numerical results are presented for implicit solutions of the unsteady Navier-Stokes equations and for explicit solutions of the steady supersonic flow equations.
Analysis and Testing of Active Twist Rotor Blades, Master's thesis
- A R Kreshock
- Design
Kreshock, A. R., Design, Analysis and Testing of Active
Twist Rotor Blades, Master's thesis, Old Dominion University, Norfolk, VA, 2013.