Acoustics and Fluid Mechanics

Drs. Cattafesta and Sheplak focus on experimental acoustics and fluid dynamics research, with particular emphasis on the modeling, development, and implementation of MEMS sensors and actuators for fundamental studies and active control research. Sensor development activities include microphones for directional acoustic arrays and unsteady pressure transducers for turbulent boundary layer studies as well as for feedback sensing for flow control. We are also developing shear stress sensors for time resolved shear stress measurements. Actuator technology includes piezoelectric and electrodynamic zero-net mass-flux (i.e., synthetic jets) and plasma type devices. Our active flow control research focuses on adaptive feedback flow control with applications to circulation control, flow separation control, and cavity flow oscillations. Our aeroacoustics research focuses on acoustic liner technology and fundamental studies and mitigation of airframe noise studies (e.g., trailing edge noise, landing gear, circulation control, etc.) in our 29 in. x 44 in. x 72 in. (75 m/s max) open-jet anechoic wind tunnel located inside a 100 Hz cutoff anechoic chamber. Finally, we are developing ultrasonic transducers for proximity sensing, parametric acoustic arrays, and biomedical imaging.

An Experimental Investigation of Wing Tip Vortex Attenuation

A detailed study of the effect of blowing from a rounded wing tip on the attenuation of wing tip vortices generated by a NACA0012 semi-span airfoil is being conducted in two wind tunnels at IMG.  The first, the UF Closed Circuit Wind Tunnel, will be used for measuring lift and drag of the airfoil and characterizing the wing tip flowfield using one or more flow measurement techniques such as particle image velocimetry, laser Doppler velocimetry and a fast-response Cobra probe.  The second facility, the UF Aeroacoustic Flow Facility (UFAFF), is an open jet test section wind tunnel inside an anechoic chamber that will be primarily used to characterize the acoustics of the airfoil with far-field microphones and phased acoustic arrays.  Lift and drag will also be measured in the UFAFF to determine the effect, if any, of the differing wind tunnel configurations on aerodynamic performance.

An Integrated Study of Separation Control: Flow Physics, Nonlinear Dynamics and Effective Control Strategies

Motivation

  • Separated flow past an airfoil is characterized by multiple natural frequencies associated with various instabilities.

  • Traditional separation control forces at a single frequency and often neglects the influence of actuator dynamics (resonance).

Questions & Objectives

  • Is the effectiveness of a control strategy related to the natural flow instabilities and/or interactions?

Objective 1: Identify and analyze the various global dynamics in a separated flow configuration

  • Can we detect and leverage natural instabilities to understand, model, and control separated flow in a more efficient and systematic manner?

Objective 2: Investigate the effectiveness and efficiency of targeting the instabilities in open-loop experiments

Objective 3: Apply observations to design and implement effective closed-loop control strategies

Challenges & Approach

1.     Traditional experimental methods for detection of flow frequency content are limited

-  Localized dynamic sensors (location sensitive)

-  High-rate global measurements ($$$)

Approach: Use modal analysis methods amenable to experimental techniques to obtain low-order, global estimates

2.     Characteristics of 2D flow separation on airfoils is highly dependent on angle of attack and surface curvature

Approach: A canonical flat plate model retains the essential separation characteristics, eliminates curvature effects, and is amenable to both simulations and experiments

 

 

 

 

 

 

 

Experimental Objectives

1.     Estimate global flow dynamics (time-resolved velocity fields)

2.     Identify characteristic frequencies/instabilities

3.     Perform open-loop control

Target characteristic frequencies by modulating actuator resonance. The extent and height of the time-averaged separation bubble is reduced during control.

Forcing at or near the natural shear layer frequency "feeds" the characteristic roll-up of the shear layer, which increases mixing between the high-momentum freestream and the low-momentum separated flow. This ultimately reattaches the flow further upstream with little control effort, or cost, described by the coefficient of momentum Cμ.

Conclusions

  • DMD extracts modal structures of the characteristic shear layer and wake components and results reveal interaction between the wake and separation bubble dynamics

  • Most efficient OL control scheme targets multiple flow instabilities rather than a single instability

  • CL control reattaches the flow (time average) by reducing the controlled shear layer oscillations

An Experimental Study of Aircraft Landing Gear Noise

A series of aerodynamic and acoustic wind tunnel experiments are being performed on a 1/4-scale high fidelity replica of a Gulfstream G550 aircraft nose landing gear at the University of Florida Aeroacoustic Flow Facility (UFAFF). Experiments consist of the analysis of model surface pressure measurements and flowfield velocity via laser Doppler velocimetry (LDV), noise source localization maps via beamforming using a large-aperture microphone array, and the determination of far-field noise level spectra using a linear array of free-field microphones. The objective of this study is to determine the mechanisms of nose landing gear noise and to explore potential noise reduction schemes.

Optical Shear Stress Sensor

Design, fabricate, calibrate, and test time-resolved, direct shear stress sensors capable of measuring stresses in harsh environments using miniaturized optics.  Optical gratings on the floating element sensor generate Moire fringe patterns for optical amplification of the floating element displacements due to applied shear stress.

A Grazing Flow Impedance Facility For Engine Nacelle Acoustic Liners

Acoustic liners remain the standard for providing a method to reduce environmental noise from aircraft engine nacelles. To aid in their development, facilities are required that are capable of accurately educing the acoustic impedance in the presence of mean flow. The Grazing Impedance Tube at NASA LaRC possesses these capabilities and was donated to the University of Florida. Improvements have been made to enable optical flow diagnostics, provide an increased speed range, and reduce turbulence levels. This facility provides a testbed to improve upon current liner impedance eduction methods as well as facilitate development of novel design approaches and studies into fundamental liner flow physics.

Grazing Flow Impedance Tube Profile

Metric Value
Dimensions 2" x 2"

Maximum Mach Number

 0.6
Maximum SPL

130 dB @ Ma = 0.5

MEMS-based Optical Sensors for High Temperature Applications

The goal of this research is to develop a pressure sensor and shear stress sensor that are able to provide continuous, time-resolved flow measurements within high temperature environments such as those seen in hypersonic wind tunnels and turbines.  A primary focus of this research is on the micromachining of sapphire using a picosecond laser.  Sapphire’s mechanical and thermal properties make it an ideal material for high temperature measurements.  Each sensor operates by mechanically deflecting under either a pressure or shear stress.  These deflections can then be detected using an opto-mechanical transduction scheme.  The completed sensors will be tested in flow cells and wind tunnels at UF as well as in other transonic and hypersonic facilities.

J-355PS Picosecond Laser Micromachining Workstation from Oxford Lasers

Development of a MEMS Piezoelectric Microphone for Aeroacoustic Applications

Description

Boeing Dreamliner

Passenger expectations for a quiet flight experience [1] coupled with concern about long-term noise exposure of flight crews [2] drive aircraft manufacturers to reduce cabin noise in flight. Cabin noise has traditionally been limited using insulating panels and skin dampers on the fuselage. Unfortunately, these thin panels are not effective at reducing low frequency (long wavelength) noise and cannot be made thicker due to weight concerns [1]. Treating the noise at its source shows potential for reduction of low frequency noise and weight savings compared to insulating panels. With fuel costs rapidly increasing, reduction of excess weight and subsequent maximization of the "revenue-generating payload" [3] is more important than ever.

In order to identify noise sources and assess the impact of noise reduction technologies during the design process, aircraft manufacturers require robust, low cost microphones. Measuring primary sources of cabin noise, such as shockcell noise, is difficult under simulated cruise conditions in test facilities [1] and establishes the need for microphones that can be used in full-scale tests at altitude. Their use on the fuselage exterior requires extremely small packaged sizes, in addition to the ability to withstand moisture and freezing conditions at flight altitudes. Microelectromechanical systems (MEMS) microphones show promise for meeting the stringent performance requirements of aircraft manufacturers at reduced size and cost, made possible using batch fabrication technology [4-9].