Drew Wetzel

An Experimental Study of Circulation Control on an Elliptic Airfoil

Circulation control uses the Coanda effect to generate high lift but can produce significant acoustic self-noise. The purpose of the study is therefore to gain insight on the flow mechanisms responsible for the observed tonal and broadband noise reported in Ref. 2. The results of high-resolution particle image velocimetry measurements of a 2-D, 20% thick elliptical circulation control airfoil with single-slot blowing are described. Data are presented for the following subset of test conditions: zero angle of attack, Rec =0 .65M, h/c =0 .0019,Cµ = 0, 0.015, and 0.057. Together with complementary Cp data, local flow scales are extracted from the velocity data to aid in identifying the contribution of each broadband noise source to the overall circulation control acoustic spectrum.

Shear layer time-delay correction using a non-intrusive acoustic point source

Microphone array processing algorithms often assume straight-line source-to-observer wave propagation. However, when the microphone array is placed outside an open-jet test section, the presence of the shear layer refracts the acoustic waves and causes the wave propagation times to vary from a free-space model. With a known source location in space, the propagation time delay can be determined using Amiets theoretical method. In this study, the effects of shear layer refraction are examined using a pulsed laser system to generate a plasma point source in space and time for several different test section flow speeds and configurations. An array of microphones is used to measure the pulse signal, allowing for the use of qualitative beamforming and quantitative timing analysis. Results indicate that Amiets method properly accounts for planar shear layer refraction time delays within experimental uncertainty. This is true both when the source is in the inviscid core of the open-jet test section, as well as when the source is located in different model wakes of varying complexity. However, the method breaks down where the thin layer assumption fails, such as in the region where the tunnel test sections open jet interacts with the facility jet collector.

Acoustic Characteristics of a Circulation Control Airfoil

The sound generated by a 2-D, 20% thick elliptical circulation control airfoil with single slot blowing is experimentally investigated in the University of Florida Aeroacoustic Flow Facility. Experiments are conducted at a variety of freestream and slot jet velocities for three different slot heights h. The airfoil’s lift characteristics compare favorably with data from a previous research study in a conventional closed-wall wind tunnel. A large aperture microphone array reveals low noise contamination in the UFAFF test section. Data from farfield microphones indicate both tonal and broadband noise sources. Vortex shedding from the rounded trailing edge occurs when jet momentum levels are insufficient to prevent flow separation. High frequency tonal noise is also identified due to vortex shedding, where fD/Ujet = 0.21, from the D = 0.28 mm thick slot lip. For large jet velocities, high frequency broadband slot jet noise is identified as the dominant noise source for fh/Ujet > 0.1. Jet velocity amplitude and frequency non-dimensionalization collapse the high-frequency portion of the spectra reasonably well when freestream velocity is low or zero. For curvature and slot noise generated by the external boundary layer flow, local mean velocities at the trailing edge and above the slot are not more effective than freestream and jet velocities when nondimensionalizing frequency. Finally, freestream amplitude and Strouhal number scaling show a remarkable collapse of spectra for a variety of freestream velocities, jet velocities, and slot heights for a constant momentum coefficient.

A Theoretical and Experimental Study of Broadband Circulation Control Noise

Circulation control can greatly enhance lift on a xed wing. However, the introduction of a high-speed Coanda jet is theorized to signi cantly increase noise, which is a growing concern for a variety of potential circulation control applications. The sound produced by a circulation control airfoil is measured using a phased microphone array and freeeld microphones in an open-jet anechoic wind tunnel. The phased array maps reveal dominant high-frequency noise sources at the sidewall-trailing edge interfaces. These three-dimensional sources prohibit the accurate measurement of two-dimensional circulation control trailing edge noise by a single microphone. Hence, an integrated phased array spectrum is computed and compared with a theory that identi es three potential noise sources: the interaction of the freestream boundary layer with the slot lip edge, the interaction of boundary layer turbulence with the rounded trailing edge, and the interaction of the jet with the surrounding slot geometry. The theory requires detailed ow eld information, including friction velocities, boundary layer thicknesses, displacement thicknesses, and local mean velocities. These parameters are estimated using particle image velocimetry measurements. A quantitative comparison of the measured and theoretical spectra reveal large di erences. The model predicts that the interaction of the freestream boundary layer with the slot lip is the dominant noise source mechanism over most frequencies.

An Experimental Investigation of Unsteady and Steady Circulation Control for an Elliptical Airfoil

c K = . The experimental results demonstrate that unsteady circulation control with phase shifts of 45° and 90° at an appropriate frequency (102.4 Hz ) near the baseline vortex shedding frequency of the uncontrolled model and duty cycle ( 50% ) can improve lift generation in comparison with steady blowing and with reduced mass flow rates. However, the noise generated by the present actuators is significant, and unsteady circulation control produce considerably more noise than its steady counterpart for similar lift. Finally, substantial noise reduction is demonstrated via appropriate phase shift operation of the valves. Nomenclature

Design-optimization of a broadband phased microphone array for aeroacoustic applications

Phased microphone arrays are commonly used in acoustic beamforming applications. While numerous beamforming algorithms have been proposed to alleviate deficiencies of the delay-and-sum approach, few studies have focused on the array design itself. In aeroacoustic applications, the most common designs are based on circularly symmetric spiral arrays devised by Underbrink (1995). The design of an array using such a method is complex and tedious due to the numerous design variables and corresponding trade-offs between resolution, sidelobe suppression, size, and cost. In this paper, a systematic design-optimization approach is described that offers several objective functions and constraints. Candidate arrays for use in the University of Florida Aeroacoustic Flow Facility (UFAFF) are designed for a broadband frequency range of 1 to 80 kHz. The results of these different cases will be compared to those of an existing array design currently used in the UFAFF. An optimized design is selected and fabricated for ch...

An aeroacoustic microelectromechanical systems microphone phased array.

Phased microphone arrays are useful tools for noise source localization using a process known as beam‐forming. In scale‐model wind tunnel experiments, the frequency range of interest can extend as high as 90 kHz. In both open and closed wall wind tunnels, microphones with high dynamic range are required to sense large turbulent pressure fluctuations from the open jet shear layer and the tunnel wall boundary layer. Microphones that meet the frequency and dynamic range demands of such experiments are readily available but expensive. When considering the high‐sensor counts typically needed for phased array measurements, total sensor cost can be a limiting factor. The presentation will discuss a proof‐of‐concept phased array consisting of 25 piezoelectric microelectromechanical systems (MEMS) microphones arranged in a log‐spiral pattern on a single printed circuit board. The microphones were designed in‐house and have a dynamic range from 40–160‐dB SPL and possess a resonant frequency greater than 100 kHz. A ...