Ki-Hoon Chung

Numerical Investigation on a Rotor Tip-Vortex Instability in Very Low Advance Ratio Flight

Helical tip vortex is known as stable vortex structure, however the specific frequency component of far wake perturbation induces the vortex pairing in hover and axial flight. It is expected that the tip vortex pairing phenomena may happen in transition flight and very low advance ratio flight so that inflow may be most nonuniform in the low advance ratio flight. The objectives of this paper are that a tip-vortex instability during the transition from hover into very low advance ratio forward flight is numerically predicted to understand a physics by using a time-marching free-wake method. To achieve the objectives, numerical method is firstly validated in typical axial and forward flights cases. Present scheme with trim routine can predict airloads and inflow distribution of forward flight with good accuracy. Then, the transition flight condition is calculated. The rotor used in this wake calculation is a small-scale AH-1G model. By using a tip-vortex trajectory tracking method, the tip-vortex pairing process are clearly observed in transient flight(μ=0.03) and disappears at a slightly higher advance ratio(μ=0.05). According to the steady flight simulation at μ=0.03, it is confirmed the tip-vortex pairing process is continued in the rear part of rotor disk and not occurs in the front part. Time averaged inflow in this case is predicted as smooth distribution.

Development and application of fan noise prediction method to axial and centrifugal fan

Numerical predictions of fan noise have not been studied extensively. This is due to the scattering effect of the fan casing, duct and the difficulty in obtaining aerodynamic acoustic source. New method to predict the fan noise and performance is developed and used to calculate various fan noise problems. A vortex method is used to model the fan and to calculate the flow field. Acoustic pressures are obtained from the unsteady force fluctuations of the blades using an acoustic analogy. But the acoustic analogy can be applied only in the free field in general. In order to consider the solid boundary effects of the casing, the newly developed Kirchhoff-Helmholtz BEM (Boundary Element Method) is introduced. With the above-mentioned method, the flow field and sound field of centrifugal and axial fan were calculated. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. Also, in the predicted sound field, we can see the scattering effect of duct and casing.Copyright

Comparisons of Rotor Performance and Noise between Candidate Light Civil Helicopters

The rotor blade of helicopter is the core component determining helicopter performance and requiring low noise and low vibration because the blade becomes the major source of noise during flight. The performance analysis of candidates rotor blades is very critical because LCH(Light Civil Helicopter) will be developed parallel with LAH(Light Armed Helicopter) as an international upgrade program based on the existing platform of foreign civil helicopter. This research was aimed to recognize the performance of the candidates rotor blades compared with the newly developed foreign rotor blades and to investigate the feasibility about developing korea unique shape rotor blades by analysis the rotor performance and noise. The result of this research can be used for the target performance index during negotiation with foreign helicopter company and developing korea unique shape rotor blades.

Aerodynamic Analysis of Rotor Blades using Overset Grid with Parallel Computation

The helicopter aerodynamics is simulated in hovering and forwarding flight using the unsteady Euler equations. As the steady condition, flight test of DLR-F6 and hovering flight test data of Caradonna & Tung’s rotor blades were used, and as the unsteady condition, non-lift forwarding flight test data of the rotor blades were used. The parallelized numerical solver was validated with the two of data above. By using this solver, AH-1G rotor blades to forwarding flight numerical test were conducted. In the test of forwarding flight, the numerical trim was applied to decide cyclic pitching angles using the Newton-Raphson method, and the results were good well match to the experimental data, Especially, the BVI effects were well simulated in advancing side in comparison with other numerical results. To consider the blade motion and moving effects, an overset grid technique is applied and for the boundary, Riemann invariants condition is used for inflow and outflow.

Unsteady Aerodynamic Analysis for Helicopter Rotor in Hovering and Forward Flight Using Overlapped Grid

In this paper, the helicopter aerodynamics is simulated in hovering and forward flight. Also, an overlapped grid technique is applied in this simulation to consider the blade motion and moving effects. The Caradonna & Tung`s rotor blade was selected to analyze the unsteady aerodynamics in hovering and non-lift forward flight. Also, the AH-1G rotor blade was selected in forward flight. In forward flight case, the numerical trim was applied to determine the cyclic pitching angles using Newton-Raphson method, and the numerical results were in good agreement with experimental data, especially, the BVI effects were well simulated in advancing side in comparison other numerical results. The governing equation is a three dimensional unsteady Euler equation, and the Riemann invariants condition is used for inflow and outflow at the boundary.

Aerodynamic Optimization of Helicopter Blade Planform (II): Applications to Design Optimization

ABSTRACT This paper focuses on the application of the proposed aerodynamic optimization techniques to design the blade planform of helicopter rotors. The design problems are formulated to maximize the hover figure of merit and the equivalent lift-to-drag ratio for high forward speed by optimally distributing airfoils, twist, and chord along the blade span. The numerical characters are investigated by solving various design problems. The advantages and limitations with the present design approach and the present modeling features for performance prediction are discussed. The recommendations for the required model refinements to get more accurate optimal configurations are addressed as future research areas. 초 록 본 논문은 헬리콥터 로터 블레이드 플랜폼 설계를 위해 개발된 공력 최적설계 기법의 응용에 대한 내용이다. 블레이드 스팬 방향에 따른 익형의 배치, 비틀림 및 시위 등의 최적 분포를 결정하여 호버링 figure of merit과 고속 전진비행 시 등가 양항비를 최대화하는 설계문제를 정식화 하였다. 다양한 설계문제에 대해 최적설계 기법의 특성을 파악하였으며, 본 연구의 설계기법과 성능예측에 사용된 모델링 특성과 관련된 장점과 제한점에 대해 검토하였다. 보다 정확한 최적형상의 설계를 위해 요구되는 모델링 개선방향에 대한 검토와 향후 연구분야를 정의하였다.Key Words : Helicopter Blade(헬리콥터 블레이드), Rotor Planform(로터 형상), Aerodynamic Optimization(공력 최적화)

Investigation on Prediction Methods for a Rotor Averaged Inflow in Forward Flight

Prediction methods for a rotor averaged inflow in forward flight are investigated in this study. The investigated methods are Drees linear inflow model, Mangler & Squire model and free vortex wake(FVW) method. Predictions have been performed for a four-blade rotor operating at three different advance ratios i.e. 0.15, 0.23 and 0.30, at which experimental data are available. According to results, Drees model has a limitation for the inflow non-uniformity prediction due to an inherent linear characteristics. Mangler & Squire model has a reasonable accuracy except the disk edge region. KARI FVW method has very good accuracy and has better accuracy than the other FVW method especially in inboard region. However, there are some discrepancies in retreating side due to the dynamic stall effect and in near hub region due to the fuselage upwash effect.

A Study on the Far-Field Boundary Condition of Tightly Coupled CFD/FreeWake Method in Hover

In this study, helicopter rotor flow is simulated by using a tightly coupled CFD/FreeWake method to describe wake characteristics and to calculate the flow field and rotor aerodynamics. In this tightly coupled CFD/FreeWake method, freewake model provides the boundary condition required in the CFD calculation and CFD provides the pressure distribution on blade surface used in freewake generation. To show the advantage of this method, the pressure distributions on blade surface of a hovering 2-bladed rotor are compared with other numerical methods. This tightly coupled CFD/FreeWake method shows good accuracy in the predicted results and efficient computation time.

Sound Characteristics and Performance of Even and Uneven Blade Spacing Helicopter Anti-Torque System

This paper described the performance test and sound characteristics of helicopter `Tail-Fan` anti-torque system. In this research, Korea Aerospace Research Institute(KARI) developed `Tail-Fan` anti-torque system for a helicopter and carried out performance and sound capturing tests of even and uneven tail fans. The performance test is carried out and the noise signals which are generated during the test are saved with microphones at the same time. The performance test`s results meet the design requirements. Tone-corrected perceived noise level is reduced by replacing the even product with the uneven one.

Helicopter BVI Noise Prediction Using Acoustic Analogy and High Resolution Airloads of Time Marching Free Wake Method

The BVI(blade vortex interaction) noise Prediction has been one of the most challenging acoustic analyses in helicopter aeromechanical Phenomenon. It is well known high resolution airloads data with accurate tip vortex positions are necessary for the accurate prediction of this phenomenon. The truly unsteady time-marching free-wake method, which is able to capture the tip vortices instability in hover and axial flights, is expanded with the rotor flapping motion and trim routine to predict unsteady airloads in forward and descent flights. And Farassat formulation 1-A based on the FW-H equation is applied for the noise prediction considering the blade flapping motion. Main objective of this study is to validate the newly developed prediction code. To achieve the objective, the descent flight condition of AH-1 OLS(operational loads survey) configuration is analyzed using present code. The predicted sectional thrust distribution and sectional airloads time histories show the present scheme is able to capture well the unsteady airloads caused by a parallel BVI. Finally, the predicted noise data, observed in two different positions where are 3.44 times of rotor radius far from the hub center, are quite reasonable agreements with the experimental data compared to the other analysis results.