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Rotary Wing Aerodynamics & Aeroacoustics


  • Rotor-Fuselage Interaction

    The physics of the flow around the helicopter rotor is quite complex mainly because of the interference of the rotor wake with the blades. When a rotor-fuselage configuration is used instead of an isolated rotor, the mutual aerodynamic interaction among the wake, fuselage, and blade magnifies the complexity of the flow field. This interaction influences the helicopter’s dynamics, performance, and handling qualities, and also produces undesirable vibration and noise owing to steady/unsteady loadings on the blades and the fuselage. Therefore, this interactive phenomenon is regarded as one of the primary interests in helicopter design and has been investigated over the past 30 years. However, prediction of the aerodynamic rotor-fuselage interaction is still challenging to aerodynamics researchers owing to the absence of a computationally efficient tool to analyze interactional aerodynamics.
    In the present study, an integral solution of the Poisson equation was used to calculate the surface pressure on a fuselage. Vortex lattice and source-doublet panel methods were used for analyzing the rotor and the fuselage, respectively. The wake vorticity was modeled by using free wake and vortex particle methods. The GIT model was simulated to observe the increase in stagnation pressure and unsteady pressure fluctuations due to the tip vortex-fuselage interaction. In addition, the ROBIN model, which considers a generic helicopter fuselage, was solved and the blade-passing effect was observed in the unsteady pressure fluctuation on the fuselage surface.
  
                    Flowfield for GIT model                  Time-averaged pressure on top of GIT fuselage 
  
               Flowfield for ROBIN model                   Unsteady pressures on retreating side of ROBIN
                                                                      fuselage (x/L=0.90, z/L=0.13)


  • Simulation of Complete Helicopter Configuration

    A helicopter is subjected to various flight conditions such as hover, low- and high-speed cruises, and high-performance maneuvering. Among the flight conditions, all helicopters spend considerable time in hover flight, in which they are specifically designed to be efficient in terms of power requirement. However, hovering is not an easy maneuver for pilot because frequent pilot input correction is required on the controls. It is neither easy theme for helicopter researcher because accurate simulation of the helicopter rotor flow field in hover is one of the most complex and challenging problems. The hover flight has been studied extensively through theory, experiment, and numerical simulation. However, most of them are focused on hovering of an isolated rotor rather than hovering of a complete helicopter configuration. Besides, finding a trimmed input for hovering of the full helicopter configuration has been hardly investigated up to now. The purpose of the present research is to simulate the complete helicopter configuration in hover, and predict the trimmed pilot control inputs including the tail rotor pitch angle. The main rotor wake-tail rotor interaction, and tail rotor-vertical fin interference will be the primary concern. The converged solution is required to obtain the accurate stability derivatives which are used in the numerical trim procedure. Achieving the trimmed hover state and finding the corresponding pilot control input values are quite important because accurate free-flight simulation of maneuver is only possible if the perfect hovering is given as an initial condition.
 


Wake structure in hover flight of UH-60

  • Autogyro

      Autogiros have been developed for a better way to fly at low speeds. Many of the technical problems associated with rotary-wing flight had been discovered and rectified by early autogiro pioneers, most notably de la Cierva`s solution of installing flap hinges to accommodate asymmetric lift from the rotor blades. Many people studied aerodynamics of the autogiro using mathematical and experimental methods in 1920s and 1930s. The development of the autogiro receded as the helicopter became more popular and successful. In recent years, the autogiro has become a very popular vehicle for hobby flying and people have studied stability, dynamics, control characteristic and etc. of the autogiro.
     In this research, we calculate thrust of the autogiro using free wake method which is useful to analysis of helicopter. There are two kind of free wake methods. One of them is Iterative free wake method which calculates the induced velocity, lift and thrust from predicting the wake produced by spinning rotor of helicopter. The other is Time marching free wake method which calculates the induced velocity from formation of wake at each small time. Vortex lattice method of free wake method which is base on lifting line theory estimates wake well but it doesn’t contain airfoil thickness. But panel method estimates flow affected by airfoil thickness.
 
Autogyro aerodynamics analysis
 

  • Ducted Fan UAV

       In recent years, UAV has been developed and researched for a wide variety of applications such as aerial photography, crop dusting, fire fighting, etc. Key design considerations for the UAV are maximum payload and flight time. There are many type of UAVs, fixed wing, helicopter, tilt-rotor, quad-rotor, ducted fan UAV, etc. In U.S. Army, ducted fan UAV is developing to detect the enemy concealed in forests or hills, around buildings in urban areas, or in places where the soldier does not have a direct line-of-sight. It consists of duct, rotor, stator, control flap. Ducted fan UAV has satisfactory thrust characteristic in hover compared with helicopter and capability of vertical take-off and landing. Duct produces thrust which is leading edge suction force in hover and reduces tip loss of fan. And it also plays an important role in forward flight. It produces lift like fixed wing so it is called ring-wing. However, its absence of typical wing of conventional aircrafts causes difficulty of control of ducted fan UAV and short flight time since it needs more required power to produce enough lift in hover.
     The ring-wing UAV is designed to achieve high efficiency in hover. Momentum and blade element theories are used for the blade design and ideal twist and taper are adopted to reduce induced power. Static tests are performed to know the aerodynamic performance of ducted fan, stators and flaps respectively. Wind Tunnel test is performed to build the aerodynamic database in forward flight for control modeling of flight model.
 
Wind-tunnel test of ducted fan UAV



Flow and Acoustics of Turbo Jet & Rocket


  • Open Rotor

      Global CO2 emission and noise pollution by aircrafts has long been recognized as an environmental problem. This issue resulted in tighter environmental regulations limiting gas emissions and noise level of airliners. An Open Rotor is a turbofan engine without fan cowling, which is also referred as an Unducted Fan (UDF) Engine. The Open Rotor has considerably higher propulsion efficiency relative to modern high bypass turbofan engine of equivalent thrust, which satisfies the airlines’ request for low fuel consumption. NASA and GE investigated the Open Rotor during 1980’s in response to high fuel prices at that time. However, the development was not pursued further due to a fall in the oil price and noise problem. Now, the Open Rotor is considered as a strong candidate for the next generation propulsion system. Analysis of noise characteristics and understanding of noise source are necessary since the Open Rotor’s noise is currently a prominent issue. The objective of this research is to predict the aerodynamics and the noise of the Open Rotor, and suggest the method to reduce noise.
 
 (Walsh, “An Industry Perspective,” Advanced Open Rotor Aircraft Workshop, 2008)

 

     The aerodynamics of the Open Rotor of generic geometry was simulated using a coupled Navier-Stokes Solver/Source-Doublet Panel/Free-Wake Method. Since it uses the Free-Wake method, the physics of the wake and the blade-vortex interaction could be analyzed effectively. The tightly coupled method does not use a background grid, which means that it requires less computation time while maintaining high accuracy. With the obtained aerodynamic data, an acoustic analogy method was used to examine the noise characteristics of the Open Rotor.
     The flow around the Open Rotor is always unsteady and complex. The interaction between the front rotor wake and the aft rotor blade was the most significant factor affecting the aerodynamics and the acoustics of the open rotor. In the present study, the tip vortex of the front rotor, the propagation of the sound and the flow interaction with the nacelle were investigated in depth. If an efficient method to reduce the interaction between the front rotor wake and the aft rotor is applied, the optimized design could be obtained. The various operating conditions were simulated to see the effect of the distance between the counter-rotating rotors, the radius of the blades and their rotating speed.
 
       
 Wake strength and shape of coaxial rotor(contrary rotation)
 
 
 Thrust Comparison
 

  • International Space Station (ISS)

      One of the serious problems in the International Space Station (ISS) other than the micro gravity is the noise generated by machine inside the station. The noise level of ISS is about 70dBA which is similar to the noise level of a circumference of freeway traffic. The astronauts are exposed to the noise during not only the work day but also bedtime and suffer physically and psychologically. The noise is generated by ventilation systems, cooling fans of experiment equipment, etc. The ISS module is closed and the noise from equipment is not easily emitted or radiated from the space module to space. It is difficult to identify noise source in ISS using simple noise measurement equipment. Acoustic holography method is usually used to find noise source. Acoustic pressure on source plane is calculated from acoustic pressure measured by microphone array. It uses widely mechanic, aerospace and electronic fields. But it is unsuitable to use in ISS because it needs large space for array installation in ISS and is heavy for transportation.

     Noise measurement equipment is developed to measure noise sources in ISS-RS. It is light, small size and user friendly. The image and sound pressure is recorded and display measurement data on monitor. The Korean astronaut measured the noise and made sound map on three locations in ISS-RS. Noise level is similar to noise level of noisy laboratory. The noise sources are mechanical noise like fans, resonance noise related to size of ISS-RS and tonal noise. Astronauts who stay in ISS for a long time usually can suffer physically and psychologically from the noise. Noise would also affect science, animals and plants experiment in ISS-RS. To find exact noise sources, it need more experiment to get pressure data of whole inner surfaces of ISS-RS and noise characteristics of equipment in ISS-RS is also investigated.

 

                                                             ISS noise measurement
 

  • Turbocharger

 

Turbocharger Configuration 

(http://www.answers.com/topic/turbocharger)

 

     Turbocharger is the device for high performance of engine. Turbocharger operates with exhaust gas of engine. Exhaust gas of engine go into the turbine of turbocharger and make the turbine rotate. Rotational power of turbine transmits to the compressor through shaft which connect turbine and compressor. Using this power compressor works and delivers compressed air to the cylinder of engine. This compressed air increases the efficiency of combustion and reduces exhaust gas. So, using this turbocharger we can expect high performable and green engine.

 

            Sound spectra in compressor and inlet duct

 

     Using turbocharger, we can expect high performance but the turbocharger induces noise problem. Because the operating RPM of turbocharger goes up to 200,000RPM, high intensity noise occurs and this noise can induce passenger discomfort. And this high intensity noise can induce combustion instability in the engine intake also.

     To test turbocharger noise, we installed turbocharger test bench in anechoic wind-tunnel. This test bench uses compressed air to operate turbine and turbine outlet & inlet are connected with duct. Most noise of turbocharger come from compressor, so turbine is just operator. Noise from turbocharger is measured using condenser microphone.

 

Turbocharger test bench

 

     Through the test using test bench, we didn’t measure noise only, but also fluid variables to provide boundary condition for CFD calculation. Using this boundary condition, we conducted CFD calculation to see the detail of flow in turbocharger and to analyze noise from turbocharger using computational aeroacoustics.

 
                                                                Turbocharger CFD Results
 
 

  • Naro Rocket

      Naro rocket is the first rocket launcher which can deliver 100kg-class satellite. Naro rocket is composed by 2 stage. First stage is liquid engine and second stage is solid kickmotor. First stage is co-developed by Russia and Korea, and second stage is developed by Korea. Naro Rocket is launched at the Naro Space Center located in Oenaro island which is the 13th rocket launching facility in the world. Naro rocket has the mission to deliver STSAT-2 satellite to low orbital of earth.
 
 
 
                                             Naro Rocket & STSAT-2 (http://www.kslv.or.kr/)
 
     Rocket radiate high intensity noise during launching due to the jet and this high intensity noise can cause problems. Low frequency component of the jet noise can propagate to the upside of rocket and damage to the structure. This damage can't induce fatigue to the structure only, but combustion instability can be occured also. There are so many failure cases due to the damage induced by noise in many countries. So, noise analysis of rocket launching noise is very important for successful launching.
 
Low frequency noise propagation in near field
(Casalino, D.,Barbarino, M., Genito, M., and Ferrara, V. (2009) Hybrid Empirical/Computational Aeroacoustics Methodology for Rocket Noise Modeling, AIAA Journal, 47(6), pp. 1445-1460)


 

  • Supersonic Cavity Flow for Weapons Bay Design

   The physics of cavity flow has long been investigated. The importance of this lies in the abundance of imperfection on vehicle surface. For instance, aircraft has doors, landing gear bay, weapons bay, etc. and these cavities may generate changes to the flow field on the aircrafts.
 
    Especially in supersonic flight, there are recorded incidents of unsafe weapons bay separation. Despite being investigated for more than half a century ago, supersonic weapons bay design is still far from perfection. Cavities in high speed generates very high flow instability, which is coupled with acoustic phenomena. These phenomena include, pressure and temperature fluctuations, vortex-induced vibration, acoustic resonance, etc. Thus, there is a need further understanding of it. 


Numerical simulation of weapons bay





Wind Energy


  • Windturbine

 
     Windturbine is one of the best solution for global main issue, CO2 reduction & fossil fuel drain. So, many conturies are focusing on windturbine. Until now, most technology has developed to increase aerodynamic performance and noise was the second consideration. But as the noise restriction increases, noise has been one of the most important issues. So, we have to consider noise level generated from wind turbine in design stage.
     Most dominant noise source of windturbine is blade. Aerodynamically generated noise from blade propagate to the far-field and very noisy. Noise from windturbine blade can be classified to tonal noise and broadband noise. Tonal noise comes from harmonic motion of blade and broadband noise comes from turbulent boundary layer and trailing edge interaction. To reduce tonal noise, 3D blade shape has to be changed. For example, blade tip shape modifying. To reduce broadband noise, 2D airfoil shape has to be changed.  For example, blunt trailing edge. We are considering both cases.
 

Blunt Trailing Edge(NREL)
 
 

 Low noise blade(Enercon & Southwest)
 
 
 

  • Parawing

 
    • Wind power generation using a parawing on ships

 

     Generating clean and sustainable energy is in the very center of global attention. As the world population has risen steadily and almost all kinds of industries have developed rapidly, the demand for energy has increased incessantly. However, the most of natural resources for generating energy has been drained. It is proposed that wind power generation by using a parawing on ships can be a solution for current energy situation. It can produce a lot of energy and make little pollution.

      A parawing launched in high altitude pulls a ship. As it goes forward, electrical power is produced by turbines equipped below the ship. Since wind velocity becomes faster in proportional to the altitude up to approximately 12km, a parawing in higher altitude can produce more lift and tow a ship faster. Therefore, turbine installed below the ship generates still more energy.

 

 

Wind power generation by pulling a ship using a parafoil

(Kim, J., and Park, C., Wind Power Generation with a Parawing on Ships, a Proposal, AIAA paper 2009-3959, 2009) 

 

    • Flow separation control through by suction and blowing effects of a porous parafoil

 

     In order to produce far more wind power, it is obvious that a parawing should generate more lift and less drag. This can be achieved by delaying separation of flow on the parawing. In the boundary layer theory, suction and blowing are demonstrated as a mean of controlling flow separation. Inspiring by this, we are examining a way of using porous materials to improve performances of the parawing. Comparing to the general parafoils, which are made of non-porous ripstop nylons, suction and blowing through porous parafoils is expected to show better aerodynamic characteristics.

 

Wind tunnel tests of parawing having surfaces with different porosities