Rotor blade–vortex interaction noise

The most immediate impression of turbulent flow is, probably, that it is an exceedingly complicated, tortuous and chaotic phenomenon. The experimenter equipped with instruments capable of recording the parameters of a turbulent flow will soon conclude that all these parameters fluctuate in an irregular fashion.

Statistical Description of Turbulence

This paper reviews the state of the art in helicopter rotor loads prediction using Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupling. The application of CFD to rotorcraft problems has evolved, over the period 1990 to 2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). CFD/CSD coupling has the potential to meet the long term objective of a coupled rotor-fuselage analysis that can predict accurate loads and vibration at all critical flight conditions without empirical or semi-empirical inputs. This paper reviews the capabilities and limitations of current CFD analyses to capture the key aerodynamic phenomena driving critical rotor loads: wake, dynamic stall and tip transonic effects. Recent initiatives in CFD/CSD coupling, targeted to improve predictions of rotor loads, are discussed. The high speed (155 kts) forward flight condition of the UH-60A BlackHawk is studied as a test case. The flight condition falls in a critical high vibration regime, where the mechanisms of rotor vibratory loads were not clearly understood. Significant improvements in fundamental understanding and prediction of vibratory loads at this flight condition has been recently demonstrated by researchers using CFD/CSD coupling. The physical mechanisms involved, and the role of CFD in improving predictions is explained.

Review of Rotor Loads Prediction with the Emergence of Rotorcraft CFD

Rotorcraft Aeroacoustics Computations with Overset-Grid CFD Methods

Pressure, being one of the key variables investigated in scientific and engineering research, requires critical and accurate measurement techniques. With the advancements in materials and machining technologies, there is a large leap in the measurement techniques including the development of micro electromechanical systems (MEMS) sensors. These sensors are one to two orders smaller in magnitude than traditional sensors and combine electrical and mechanical components that are fabricated using integrated circuit batch-processing technologies. MEMS are finding enormous applications in many industrial fields ranging from medical to automotive, communication to electronics, chemical to aviation and many more with a potential market of billions of dollars. MEMS pressure sensors are now widely used devices owing to their intrinsic properties of small size, light weight, low cost, ease of batch fabrication and integration with an electronic circuit. This paper aims to identify and analyze the common pressure sensing techniques and discuss their uses and advantages. As per our understanding, usage of MEMS pressure sensors in the aerospace industry is quite limited due to cost constraints and indirect measurement approaches owing to the inability to locate sensors in harsh environments. The purpose of this study is to summarize the published literature for application of MEMS pressure sensors in the said field. Five broad application areas have been investigated including: propulsion/turbomachinery applications, turbulent flow diagnosis, experimentalaerodynamics, micro-flow control and unmanned aerial vehicle (UAV)/micro aerial vehicle (MAV) applications.,The first part of the paper deals with an introduction to MEMS pressure sensors and mathematical relations for its fabrication. The second part covers pressure sensing principles followed by the application of MEMS pressure sensors in five major fields of aerospace industry.,In this paper, various pressure sensing principles in MEMS and applications of MEMS technology in the aerospace industry have been reviewed. Five application fields have been investigated including: Propulsion/Turbomachinery applications, turbulent flow diagnosis, experimental aerodynamics, micro-flow control and UAV/MAV applications. Applications of MEMS sensors in the aerospace industry are quite limited due to requirements of very high accuracy, high reliability and harsh environment survivability. However, the potential for growth of this technology is foreseen due to inherent features of MEMS sensors’ being light weight, low cost, ease of batch fabrication and capability of integration with electric circuits. All these advantages are very relevant to the aerospace industry. This work is an endeavor to present a comprehensive review of such MEMS pressure sensors, which are used in the aerospace industry and have been reported in recent literature.,As per the author’s understanding, usage of MEMS pressure sensors in the aerospace industry is quite limited due to cost constraints and indirect measurement approaches owing to the inability to locate sensors in harsh environments. Present work is a prime effort in summarizing the published literature for application of MEMS pressure sensors in the said field. Five broad application areas have been investigated including: propulsion/turbomachinery applications, turbulent flow diagnosis, experimental aerodynamics, micro-flow control and UAV/MAV applications.

A review of principles of MEMS pressure sensing with its aerospace applications

Helicopter rotor simulation codes of different organizations are validated against experimental data obtained in the DNW. The comparison addresses rotor blade loading, wake geometry, blade motion and noise radiation. Although specific differences exist the general prediciton of rotor noise is reasonably well.

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

A survey of the available turbulent boundary layer models for acoustic analysis is presented. The empirical mean square pressure models predict a relatively large spread in the values; bounded on the high end by Kraichnan and on the low end by Bull, and Willmarth and Wooldridge. The single point wall pressure spectrum models are evaluated and the two more modern models of Smol’yakov and Goody seem to perform the best. Important features of the normalized wavenumber-frequency spectrum models are presented. Separable models in the Corcos class tend to over-predict the response for a range of cases.

Review of Turbulent Boundary Layer Models for Acoustic Analysis

A MEMS microphone array has been designed and applied to the measurement of wall pressure spectra under the turbulent boundary layer in flow duct testing at Mach numbers from 0 to 0.6. The array was micromachined onto a single chip in the PolyMUMPS surface micromachining process, allowing high spatial resolution and low surface roughness. The chip measures 1 cm by 1 cm, and is flush mounted into the wind tunnel wall. Individual elements are 0.6 mm in diameter, with element to element spacing of 1.11 mm in the crossflow direction and 1.26 mm in the flow direction. The 64 element array has 59 working elements, 58 of which are matched to within ± 2.5 dB at 1 kHz. Phase matching between the 59 elements is ± 6.5 degrees at 1 kHz. The array has been calibrated from 100 Hz to 4 kHz in a plane wave tube. The transducer bandwidth is greater than 400 kHz as determined by laser vibrometry measurements. Sensor nonlinearity of less than 0.36% is observed at a sound pressure level of 150 dB SPL. Board level electronics allow the array to be reconfigured on the fly using computer controlled CMOS switches. Multipoint wall pressure spectra were measured in 38 array configurations at the wall of a 6 inch by 6 inch flow duct at Mach numbers from 0.0 to 0.6. The array shows excellent agreement with Kulite and Bruel & Kjaer microphone measurements in the 300 Hz to 10 kHz band, and appears to be able to measure turbulent pressure spectra at frequencies as high as 40 kHz.

MEMS Microphone Array on a Chip for Turbulent Boundary Layer Measurements

An inlet for an aircraft engine nacelle having an acoustic barrel panel, a septum buried or sandwiched within the acoustic barrel panel, and a perforated face sheet. The inlet may be located forward of an engine and/or an engine fan housed within the aircraft engine nacelle. Both the septum and the perforated face sheet may have perforations or holes therein of varying sizes and varying distances apart from each other. The smaller the area of the perforations or holes and the further apart they are from each other, the higher the impedance of a given area of the septum or the perforated face sheet. The impedance of the septum and/or the perforated face sheet may progressively increase in a direction away from the engine fan.

Judith Gallman

Papers

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

Review of Turbulent Boundary Layer Models for Acoustic Analysis

MEMS Microphone Array on a Chip for Turbulent Boundary Layer Measurements

Review of Turbulent Boundary Layer Models for Acoustic Analysis

Engine inlet varying impedance acoustic liner section