Rotorcraft thickness noise control

The paper explains further developments of a new concept called TNC (Thickness Noise Control) of the application of surface ventilation to the reduction of helicopter rotor low-frequency in-plane harmonic (LF-IPH) noise. The TNC method is based on introduction of four cavities covered by perforated plates (connected to low and high pressure reservoirs) and positioned symmetrically at the front and rear extremities of the blade tip. Two operational modes are analyzed: constant (steady) and periodically varying (unsteady) transpiration mass-fluxes. For exemplary two-bladed model helicopter rotor of Boxwell et al. (Bell UH-1H Iroquois helicopter) in hover conditions, the results of numerical simulations, based on the CFD code SPARC (Spalart-Allmaras turbulence and Bohning-Doerffer transpiration models), suggest that the acoustic pressure fluctuations are significantly reduced in the near-field of the blade tip. Moreover, the unsteady approach (designed for forward located observers) is almost equally efficient to the basic (steady) operation mode, while substantially lowering penalties in terms of the aerodynamic performance and required transpiration flow intensity. Finally, it is proven that when TNC is activated, the twisted blade (operating in low-thrust conditions) exhibits lower deterioration of the aerodynamic performance compared to its straight (untwisted) counterpart.

The article explains a new concept (called here ATNC – active thickness noise control) of the application of surface transpiration to the reduction of helicopter main rotor harmonic noise. The ATNC method is based on an introduction of four cavities covered by perforated plates (connected to pressure reservoirs) at the leading and trailing edges of the outer 20% of blade span. For an exemplary, two-bladed (NACA 0012 airfoil) model helicopter rotor of Purcell in low-lifting, hover conditions (tip Mach number MaT = 0.66 and collective θ = 1.5° ) the results of numerical simulations, based on the SPARC code (Spalart-Allmaras turbulence and Bohning-Doerffer transpiration models), prove that the acoustic pressure fluctuations are significantly reduced by ATNC in the near-field of blade tip. The peak amplitude is attenuated by more than 45%, with a reduction of the Overall Sound Pressure Level OASPL by 3.4 dB. It is shown also that the venting has not only a large impact on the acoustic radiation, but also on the aerodynamic performance of the rotor, expressed in terms of the torque penalty of 38%. The ATNC method proves to be a promising candidate as a mean of helicopter rotor thickness noise control, but not in the current arrangement.

In this effort, a new approach in aerodynamic modeling that accounts for unsteady wake effects as well as viscous friction drag, Leading edge suction effect and post stall behavior for rotary wings in forward flight is proposed. The adopted approach commingles the unsteadiness with the problem of helicopter rotor blade in forward flight. The results of the local normal force coefficients were compared with experimental results of the 7A rotor case study in high speed test point 312 at five non-dimensional radial positions. A CFD solver, HOST/elsA, results are compared with the obtained results at five radii locations. The results show a good agreement between the experimental results and the proposed model preserving the same pattern of variation along the azimuth angle with a slight discrepancy for amplitude and phase angle. Of particular interest, the presented model showed better agreement with the experimental for higher radii locations.

A parallel Navier-Stokes solver based on dynamic overset unstructured grids method is presented to simulate the unsteady turbulent flow field around helicopter in forward flight. The grid method has the advantages of unstructured grid and Chimera grid and is suitable to deal with multiple bodies in relatively moving. Unsteady Navier-Stokes equations are solved on overset unstructured grids by an explicit dual time-stepping, finite volume method. Preconditioning method applied to inner iteration of the dual-time stepping is used to speed up the convergence of numerical simulation. The Spalart-Allmaras one-equation turbulence model is used to evaluate the turbulent viscosity. Parallel computation is based on the dynamic domain decomposition method in overset unstructured grids system at each physical time step. A generic helicopter Robin with a four-blade rotor in forward flight is considered to validate the method presented in this paper. Numerical simulation results show that the parallel dynamic overset unstructured grids method is very efficient for the simulation of helicopter flow field and the results are reliable.

High fidelity CFD simulations of rotors with different anhedral tip angles are performed in isolated and tandem configurations at two different flight scenarios. The studied cases are hover, as well as a 25 m/s forward flight. Additionally, the convection of emitted noise of the rotors is simulated by using a Ffowcs Williams & Hawkings based CAA code. For comparison of the acoustic characteristics each rotor is simulated with the same RPM and pitch angle. Futhermore a flight mechanic trim is performed using the flight mechanic tool VFAST to compare selected anhedral rotors at the same thrust level. A NACA0012 rotor was used as a baseline rotor and all tip geometry changes are applied to this rotor blade. In hover, the isolated anhedral rotors show a drop in the Figure of Merit (FM) compared to the baseline rotor. Meanwhile the noise emission in the rotor plane increases, while the noise emission below the rotor plane decreased with rising anhedral angles. In contrast, the tandem configuration in hover showed increase FM due to changed inflow conditions. Further no clear acoustic benefits were found, with higher noise emission in the rotor plane. For the 25 m/s forward flight the efficiency of the anhedral rotors is increased, while the noise emission is also increased. This behavior was found for both the isolated and tandem rotors.

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Objectives Long-lasting Vortex Induced Motion(VIM)may cause the Spar platform mooring system fatigue and even the structure damage. Therefore, the adverse effects of VIM must be fully taken into account. Methods In this paper, the CFD solver naoe-FOAM-SJTU and dynamic grid method developed by ourselves are used to make numerical simulation of the characteristics of the vortex induced flow field around the Spar platform. The time Delayed Detached-Eddy Simulation(DDES) turbulence model based on shear-transport-stress equation is used to simulate the three-dimensional fine wake vortex structure of the Spar platform with helical strakes. The longitudinal motion, transverse motion and yawing of the Spar platform with helical strakes at different reduced velocities are studied. The platform mooring system is stimulated by using the linear spring system. The research also takes into account the effects that the platform coupling effect at different degrees has on VIM. Then the numerical simulation results are compared with the model test results, to analyze the time history, spectral characteristics and locking of longitudinal and transverse VIM of Spar platform with helical strakes and to reveal the intrinsic mechanism of VIM. Results The results show that the helical strakes can effectively reduce the response amplitude of Spar platform's VIM; the CFD solver naoe-FOAM-SJTU has good accuracy and reliability for the calculation and simulation of the offshore platforms' VIM. Conclusions The numerical simulation and analysis of the VIM of Spar platform with helical strakes is of great significance to the practical design of Spar platform.

In order to investigate the effect of the surface shape on the performance of perforated panels, three non-flat shapes were considered for perforated panel with their absorption performance compared with the usual shape of the (flat) perforated panel. In order to simulate the absorption coefficient of a non-flat perforated panel, the finite element method was implemented by the COMSOL 5.3a software in the frequency domain. Numerical simulation results revealed that all the shapes defined in this paper improve the absorption coefficient at the mid and high frequencies. A and B shapes had a higher performance at frequencies above 800 Hz compared to the flat shape. Also, shape C had a relative superiority at all frequencies (1–2000 Hz) compared to the reference shape; this superiority is completely clear at frequencies above 800 Hz. The maximum absorption coefficient occurred within the 400–750 Hz range. After determining the best shape in terms of absorption coefficient (shape C), a perforated panel of 10 m2 using fiberglass fibers and desired structural properties was built, and then it was also subjected to a statistical absorption coefficient test in the reverberation chamber according to the standard. The results of the statistical absorption coefficient measurement showed that the highest absorption coefficient was 0.77 at the frequency of 160 Hz. Also, to compare the experimental and numerical results, these conditions were implemented in a numerical environment and the statistical absorption coefficient was calculated according to the existing relationships. A comparison of the numerical and laboratory results revealed acceptable agreement for these two methods in most frequency spectra, where the numerical method was able to predict this quantity with good accuracy.

North Buzachi field is structurally located in the western part of Ustyurt Basin on the edge of Caspian Sea in Kazakhstan. It's a heavy oil reservoir with viscosity nearly 400 mPa·s at 33°C. After nearly 20 years of cold-water flooding, the oil production has fallen sharply. Low sweep efficiency and high water cut come to be the main severe challenges at this stage. Ways to improve efficiency and productivity need to be tried and evaluated. Water shut off tests, polymer flooding and adjusting perforations have been tried, but with little success. Thermal enhanced oil recovery (EOR) methods generally are effective options and cyclic steam stimulation (CSS) was considered. But the high-water saturation and water cut make CSS get rapid economic decline after 1 or 2 cycle. Therefore, the feasibility of Gas-Steam Composite Stimulation (GSCS) on this field is studied and evaluated. This paper shows the prospect of GSCS in this reservoir by studying flow mechanisms and numerical simulation analysis. Gas-Steam Composite is a mixture of steam, carbon dioxide and nitrogen, which is produced based on a high-pressure jet combustion mechanism. The mixed components are simultaneously injected into the formation at the same pressure and temperature. GSCS has been tried in other similar oil fields (Mortuk oil field in Kazakhstan) before, and the actual production results are better than predicted. The application prediction of North Buzachi field was based on the previous theory and experience. During the GSCS progress, gas overlap occurs underground, resulting in secondary gas cap drive and higher reservoir pressure. Along with other synergistic effects, the oil rate can theoretically increase to 4-5 times of water flooding in North Buzachi field. By the results of numerical simulation, the upper residual oil was swept and the sweep efficiency increase from 14% to 40%. The daily oil production per well grows from the initial 3.5 t/d to 15 t/d. Compared with the final recovery of 12% by water flooding, the recovery may reach 33.4% after 7 cycles of GSCS. Meanwhile, the water cut declines from 97% to 50% and the average cyclic oil-steam ratio reaches 4.24. Economical evaluation shows that cost per ton of oil can be reduced by 24%. With the remarkable increase in production, more economic benefits can be achieved with GSCS technology. This study not only lays the groundwork for future pilot of GSCS in this field, but also provides a significant reference for the development of similar heavy oil reservoirs.

Cycloidal rotor is a rotor whose blades pitch around the pitching axis and revolve around the rotor shaft that is parallel to blade. It can generate omni-directional thrust with high efficiency. In this paper, the numerical simulation models were validated by the wind tunnel experiments. Then physics of the cycloidal rotor in forward flight was studied. The effects of blade number and advance ratio were qualitatively discussed based on the numerical simulation results. Results of the analysis indicate that the rotor with more blades will result in smoother force curve, so that there will be lower vibration. However, the rotor with three or four blades will be the most efficient. The maximum forward flight efficiency is obtained from medium-to-high advance ratio. The efficiency is comparable with that of a screw propeller at the same Reynolds number. At low advance ratio, the peak lift on the blade can be observed when the blade is located at the lower left part of its trajectory. This is caused by positive blade pitch angle and relatively large inflow speed, which is similar to the downwash in the rotor cage under hovering status. From medium-to-high advance ratio, the thrust and lift primarily originated from the plunging motion of the blade. If the advance ratio is high enough, there will be negative horizontal force and positive torque, which means that the blade is taking energy from the inflow. Resultant force of the horizontal and vertical force does not vary too much with the advance ratio. But with the advance ratio approaching 1.0, the direction of the resultant force will point upwards and no horizontal force can be observed.

<div class="section abstract"><div class="htmlview paragraph">The main objective of this work is to investigate, by means of numerical simulations, the performance of the engine nacelle ventilation cooling system of a helicopter under hover and forward flight conditions, and to propose a simplified method of evaluating the performance based on rotor downwash flow by taking the synthetical effect of engine nacelle, exhaust ejector and external flow of a helicopter into account. For the engine nacelle of a helicopter, an integrated model of the nacelle and exhaust ejector was set up including the domain of external flow. The unstructured grid and finite volume method were applied for domains and control equations discreteness, and the standard k-ε model was applied for solving turbulent control equations. Using the business CFD software, the flow field and the temperature field in the nacelle were calculated for single inlet scheme and double inlets scheme, total up to 9 schemes. The performance of the exhaust ejector was computed. And the influence on the ventilation cooling performance of the nacelle was analyzed for different inlet number, inlet size and inlet position. The comparisons were completed between flight trials and numerical simulation results, and they show good agreement. The results show that the method can predict the performance of the ventilation cooling system of the nacelle. Moreover, the study indicates that, for the same inlet scheme, the temperature of the nacelle in forward flight is lower than that under hovering condition, but the pumping coefficient of the exhaust ejector in forward flight is higher. The inlet position should avoid being located on the underside of the cowl. It is also indicated that the simulation results can be regarded as a reference for the design and optimization of the system.</div></div>

A new explanation is advanced for the cause of vibrations in rotary wing systems in forward flight, based on the fact tha t the down-wash is greater in the rear part of the swept disc than in the front part . An estimate of the magnitude of the down-wash variation at different forward speeds is made with the aid of data previously published by the N.A.C.A. on tests of a 10-ft. gyroplane rotor. This nonuniform down-wash causes a periodic change in the drag force on a blade and is a cause of forced vibrations, especially noticeable in two-bladed rotors at low air speeds.

Numerical investigation on the use of various blade tips for the helicopter's main rotor blade in forward flight regarding aerodynamic performance and HSI noise considerations

Aeroelastic stability of complete rotors with application to a teetering rotor in forward flight

This paper gives an overview of CFD techniques developed and used at ONERA for rotorcraft applications. First, the complex multidisciplinary environment around helicopters, in which aerodynamics, flight dynamics, aeroelasticity and aeroacoustics strongly interact, is highlighted. Rotorcraft simulations are thus performed by comprehensive codes capable of dealing with the whole system efficiently, using integrated simplified models for each discipline, e.g. the aerodynamics. However, fast aerodynamic models cannot accurately represent the full complexity of rotorcraft aerodynamics, in particular as far as nonlinear phenomena are concerned, contrary to CFD. Nevertheless, helicopter problems are particularly demanding for numerical methods, requiring efficient simulation of unsteady flows with shock waves, massive flow separation, concentrated vortex structures and deforming bodies with large amplitude relative motion, while allowing fine description and analysis of local flow phenomena impacting the vehicle behaviour. Helicopter trim in the CFD solution is obtained by iterative coupling with comprehensive analysis, so that the global multidisciplinary simulation can be achieved with an advanced aerodynamic model. The approaches taken by ONERA for the comprehensive code and the CFD solvers are outlined in the paper. Examples of applications typical of rotorcraft problems are given to illustrate current possibilities and difficulties. They include an isolated rotor in hover, the dynamic stall of an oscillating wing, an isolated rotor in descent flight with Blade-Vortex Interactions, the dynamic-aerodynamic coupling of a rotor in high-speed forward flight and the simulation of a complete helicopter in forward flight. Finally, expected and needed developments are reviewed in order to make CFD a more efficient tool in the design office of helicopter manufacturers.