The Acoustic Measurement Capabilities in the Acoustic Linear Research Laboratory at University of Cincinnati

Transcription

1 The Acoustic Measurement Capabilities in the Acoustic Linear Research Laboratory at University of Cincinnati Asif Syed Research Professor, School of Aerospace Systems College of Engineering and Applied Science University of Cincinnati. 1

2 Test Facilities for Research in Acoustic Liners 1. Wave Tube of 1.5-inch square cross section Sound attenuation testing up to 4300 Hz. Normal incidence impedance testing up to 8600 Hz. 2. New Wave Tube of 2.0-inch square cross section* Max frequency of Impedance and Suppression tests Hz. Compatible with Facility at NASA La RC Capable of conducting tests of transmission loss through materials 3. The Large Wave Tube (3-inch by 5-inch cross section) for testing 5inch by 12 inch acoustic liner panels. 4. Acoustic Impedance Measurement System (AIMS) 1.25 inch diameter impedance tube Normal incidence impedance testing up to 6000 Hz. 5. Flow Duct Apparatus * Acoustic attenuation in flow using modal measurements Frequency range: 500 Hz. to 4000 Hz. Flow speeds up to Mach Steady (DC) Flow Resistance Test Stand Characterization of flow resistance of porous materials 10 cm (4.0 inch) diameter for flow area. * The Flow Duct Apparatus and the new wave tube are in the development phase. 2

3 The small wave tube (SWT) of 1.5-inch square cross section The current wave tube is of 1.5-inch square cross section It uses 3 acoustic transducers in the upstream and 3 transducers in the downstream locations to measure the plane wave amplitudes A U, B U, A D, and B D. Suppression (db) testing up to the maximum frequency 4300 Hz. Normalized Impedance testing up to the maximum frequency 8600 Hz. 3

4 14 Comparison of the Measured Acoustic Suppression data for Porous versus Impervious Partitions 12 Suppression of the Plane Wave Mode (db) * The baseline configuration had partitions at 3/8 inch spacing. Baseline configuration with Impervious Partitions* Test configuration with Porous Partitions at 3/8 inch spacing Test configuration with Porous Partitions at 1/4 inch spacing Test configuration with Porous Partitions at 3/16 inch spacing Frequency (Hz.) Examples of measured suppression (db) data in the small (1.5-inch square cross section) wave tube (SWT) 4

5 The New Wave Tube This tube will also be used to test the acoustic transmission loss characteristics of materials. 5

6 Schematic diagram of the large wave tube (LWT) apparatus for testing acoustic liner panels. A IN. The Acoustic Liner Panel A 12.0 IN. Array of 20 acoustic transducers In two axial planes The details of the acoustic liner panels need to be worked out so that it may also be tested in the flow duct apparatus IN. Section A-A 1.50 IN. The Acoustic Liner Panel 6

7 The large wave tube (LWT) transducer distribution. Distribution of 10 acoustic transducers in one of the two cross-sectional planes X-ducer # x (inch) y (inch) z (inch) 1 X X X X X X X X X X The locations of the first 10 transducers in the plane: x = x 0 7

8 Acoustic Impedance Measurement System (AIMS) with Circular Tube PC computer Measuring Head NI Data Acquisition System Rubber Gasket 8

9 Schematic diagrams showing the use of AIMS in the Impedance Tube and the Plunker modes. Nondestructive Plunker mode Impedance Tube Mode Acoustic Driver Acoustic Driver Brass Tube Rubber Gasket Acoustic Pressure Transducers Acoustic Liner The diameter of the tube in the measuring head is 1.25 inch (3.175 cm) Two AIMS systems, supplied by UC to MRAS and to P&W Auto-Air, are in constant use since

10 The Flow Duct Facility for Research in Acoustic Liners Diffuser Liner Test Section Circular to Rectangular Inlet Nozzle Provision For Acoustic Liners 10

11 Acoustic transducers Liner test section 11

12 Cross section of the flow duct apparatus 3.0-inch 5.0-inch 5-inch wide and up to 24-inch long liner test panels on one or two opposite sides Designed for mean flow velocities up to Mach 0.7 Acoustic modal measurements with two arrays of acoustic transducers. Each array consists of 16 transducers in two planes. One array is located upstream and the second array is located downstream of the liner test segment of the flow duct. Capability to measure boundary layer velocity profiles in the test section and for controlling the boundary layer thickness. Capability to measure the flow resistance of porous materials in the presence of high speed grazing flow. 12

13 The research capabilities of the Acoustic Liner Research Laboratory at UC The Flow Resistance Test Stand (a.k.a the Raylometer) is a stand alone system. It can test porous test samples which are 4.0 inch (10 cm.) in diameter. 13

14 Schematic diagram of the steady flow resistance test stand or the Raylometer Test Sample T a, P a ΔP s P f, T f T s, P s ΔP fm Laminar Element Flow Meter 14

15 An example of test data measured on the Steady (DC) Flow Test Stand 30 R(U) = A + B U Resistance, R (cgs Rayl) A B R (cgs Rayl) = U R 105 = (A +105 B) NLF = (A B) (A + 20 B) Flow Velocity, U (cm/s) R(U) - is the flow resistance (cgs Rayl) at mean flow velocity, U (cm/s) R is the flow resistance at flow velocity of 105 cm/s NLF is the non-linearity factor A - is the intercept and B is the slope of the straight line fit through the measured test data. All the flow resistance test data are computed in cgs Rayl units. 15