Peter D. Lysak

Unsteady Lift of Thick Airfoils in Turbulent Flow

Measurements of the unsteady lift forces acting on airfoils in turbulent flow were made to determine the effect of thickness on the gust response and validate a previously developed analytical model. A family of NACA 65-series uncambered airfoils with a range of thickness-to-chord ratios were tested in a water tunnel with grid-generated turbulence. Piezoelectric force gages were used to measure the spectral density of the unsteady lift, and the system was calibrated using an impulse force hammer. An accelerometer-based multiple coherence noise removal technique was employed to eliminate background noise contamination from the facility. The experimental results are shown to agree well with an analytical model of the unsteady lift based on turbulence ingestion theory and an incompressible gust response which accounts for airfoil thickness.Copyright

Effect of Blade Thickness on High Frequency Gust Response

Turbomachinery rotor blades experience gust loading due to both inflow turbulence and circumferential variation in the mean velocity. The unsteady lift forces that result from these velocity disturbances can be a source of unwanted vibration and radiated noise. For incompressible flows, the blade gust response is often modeled using the well-known Sears function, which acts as a transfer function between a sinusoidal component of the gust and the fluctuating lift. However, the Sears function has a relatively slow high frequency roll-off and overpredicts the unsteady lift when the gust wavelength becomes much smaller than the blade chord. A more accurate model can be obtained by including the effect of blade thickness, which causes the gust to become distorted as it approaches the leading edge. This distortion results in attenuation of the higher-frequency components of the gust near the leading edge, which subsequently leads to reduced unsteady lift. In this paper, a model for the thickness effect is developed based on rapid distortion theory. Numerical calculations are made for a step-function gust encountering an elliptical leading edge with several thickness-to-chord ratios. The unsteady lift is calculated in the time domain, and a Fourier transform is used to obtain the frequency response. The results indicate that the gust response of a thick blade can be closely approximated by modifying the Sears function to include an exponential decay factor based on the thickness.Copyright

Low Wavenumber Models for Turbulent Boundary Layer Excitation of Structures

When the spatial correlation length of the turbulent boundary layer (TBL) pressure fluctuations is small compared to the structural wavelengths, the vibration response can be determined by forming an equivalent point drive from the effective correlation area. This approach is equivalent to using the zero wavenumber component of the TBL pressure spectrum, so it only works for TBL models that are wavenumber white at low wavenumbers. In this work, a similar simplification is developed for TBL models with a wavenumber-squared dependence, that works for structural modes with a low-pass cutoff wavenumber. This introduces a boundary layer thickness dependence that results in significantly different predictions for structures excited by a developing boundary layer. Based on the analysis, an experimental setup is proposed that may help resolve some of the controversy surrounding the low wavenumber TBL spectrum.

Aeroacoustic Analysis of an Unmanned Aerial Vehicle

An acoustic analysis of a ducted fan unmanned aerial vehicle (UAV) was conducted to identify the primary aeroacoustic sources and to determine the potential for reducing the radiated noise levels. Computational fluid dynamics was used to determine the three-dimensional flow field through the ducted fan in hover and maneuvering configurations. The flow solutions provided information about the blade relative velocities, spatially non-uniform inflow, inflow turbulence, boundary layer turbulence, and blade wake velocity deficits for use in acoustical models of broadband and blade passing frequency noise. The computational results were in good agreement with experimentally measured noise levels, and showed that the tonal noise was produced primarily by unsteady forces resulting from the non-uniform inflow, while the broadband noise resulted from the inflow turbulence. Based on these findings, design modifications were recommended which offer the potential to reduce the noise by more than 10 dB.Copyright

Modeling the Wall Pressure Spectrum in Turbulent Pipe Flows

An important source of vibration and noise in piping systems is the fluctuating wall pressure produced by the turbulent boundary layer. One approach to calculating the wall pressure fluctuations is to use a stochastic model based on the Poisson pressure equation. If the model is developed in the wavenumber domain, the solution to the wavenumber-frequency spectrum can be expressed as an integral of the turbulent sources over the boundary layer thickness. Models based on this formulation have been reported in the literature which show good agreement with measured pressure spectra, but they have relied on adjustable “tuning” constants to account for the unknown properties of the turbulent velocity fluctuations. A variation on this approach is presented in this paper, in which only well-known “universal” constants are used to model the turbulent velocity spectrum. The resulting pressure spectrum predictions are shown to be in good agreement with canonical data sets over a wide range of Reynolds numbers.Copyright

Benefits of Rake and Skew for Turbulence Ingestion Noise

Turbulent flow through a fan or pump produces a broadband spectrum of unsteady lifting forces on the blades. These forces cause undesirable blade stresses, blade vibration and sound. Jonson (1994) has shown that broadband improvements can be obtained by adding rake and skew to the stacking line of rotor blade sections. This paper adds understanding to those experimental results, and shows that benefits can be reliably predicted using analytical methods.Copyright

Smart materials for turbomachinery quieting

As part of the Defense Advanced Research Projects Agency (DARPA) SAMPSON program, a team has been developing and testing the use of smart materials for quieting turbomachinery. The team is composed of representatives form Pennsylvania State University, General Dynamics Electric Boat, GTE BBN Technologies, and the Naval Surface Warfare Center Carderock Division. Four concepts for quieting were proposed and wind tunnel testing, water tunnel testing, as well as computational fluid dynamic analysis were performed to down select two of the concepts for further consideration: protuberance and gap control. The wind tunnel testing was performed to determine the optimum shape of the protuberance. Water tunnel testing was performed at Penn State University/Applied Research Laboratory to establish the performance of the protuberance and gap control elements. Piezoelectric inchworm actuators, developed by PSU/Center for Acoustics and Vibration, were utilized for the evaluation of the two concepts. GTE BBN Technologies developed the control system simulation for the ultimate concept, the General Dynamics Electric Boat was responsible for hydrodynamic and hydroacoustic analysis. Naval Surface Warfare Center/Carderock Division performed hydrodynamic analysis and developed the rotary component design for the water tunnel test fixture. Successful testing in the twelve- inch diameter water tunnel at PSU/ARL demonstrated superior performance with the gap control concept over the protuberance control concept, and efforts are on-going to develop the final large scale demonstration. This paper summarizes the result of these activities.