The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

Summary

Here, we present a protocol to measure, with high spatial resolution, the unsteady surface pressure in turbulent flows. This method demonstrates the construction of a remote microphone probe (RMP) and the determination of its frequency-dependent, complex transfer function. An analytical determination of the dynamic response is presented and validated.

Abstract

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various characteristic length scales can result in pressure fluctuations over a wide frequency range. This property of turbulence requires sensing devices to have sufficient sensitivity over a wide range of frequencies. Furthermore, the small characteristic length scales of turbulent structures require small sensing areas and the ability to place the sensors in very close proximity to each other. The complex geometries of the solid bodies, often including large surface curvatures or discontinuities, require the probe to have the ability to be set up in very limited spaces. The development of a remote microphone probe, which is inexpensive, consistent, and repeatable, is described in the present communication. It allows for the measurement of pressure fluctuations with high spatial resolution and dynamic response over a wide range of frequencies. The probe is small enough to be placed within the interior of typical wind tunnel models. The remote microphone probe includes a small, rigid, and hollow tube that penetrates the model surface to form the sensing area. This tube is connected to a standard microphone, at some distance away from the surface, using a "T" junction. An experimental method is introduced to determine the dynamic response of the remote microphone probe. In addition, an analytical method for determining the dynamic response is described. The analytical method can be applied in the design stage to determine the dimensions and properties of the RMP components.

Introduction

Fluid flow over surfaces generally leads to unsteadiness and turbulence that result in unsteady surface pressure (USP). Flow-induced sound and vibration are often a direct result of this unsteadiness. The radiated sound generated by cooling fans, propellers, and wind turbines are dominated by sources related to USP¹. Measurements of the spatial and temporal characteristics of USP in turbulent flows are generally required in order to predict the radiated sound.

The statistical characterization of USP is generally given in the form of auto-spectral density, two-point cross-spectral densities, and spatial correlation functions^2....

Protocol

1. Preparation of Experiments

1. Select a proper microphone to build the RMP. Use a frequency range of the microphone within the frequency range of interest.
   NOTE: In this experiment, pressure fluctuations between 100 and 10,000 Hz are of interest. The measurement frequency range of the selected microphone is from 100 to 10,000 Hz. The size of the microphone should be as small as possible, although there are no specific criteria for the size.
2. Estimate the sensitivity and frequency response of the RMP system using the analytical method described in the appendix. Adjust the sensitivity and frequency response of the RMP by varying the dime....

Results

Calibration results from two representative RMP designs are shown in this section. The first one used a 5.35 cm primary tube, and the second one used a 10.4 cm primary tube. The dissipative terminations are 4.75 m long for both RMPs.

The coherence between the pressure fluctuations measured by the microphone in the RMP and by the reference microphone is shown in Figure 3. The data show a near-unity coherence valu.......

Discussion

The measurement of USP in wind tunnel experiments is needed for many applications related to aeroacoustics and flow-induced vibrations. Compared to existing methods, such as flush-mounted imbedded sensors, PSP, or vibrated membranes, the method described here allows for accurate measurements with a high sensitivity to large-magnitude fluctuation over a wide range of frequencies. More importantly, it also provides a method for USP measurements using a small sensing area that minimizes the spatial averaging effects describ.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Microphone	ACO Pacific (http://www.acopacific.com/)	7016	Used to measure the sound pressure and calibrate the RMP as a reference.
Microphone	Knowles (http://www.knowles.com/eng)	FG-23629-C36	Used to measure the pressure fluctuation as a part of the RMP.
Microbore Tubing	Saint-gobain (http://www.biopharm.saint-gobain.com/en/index.asp)	Tygon ND 100-80	Used to dissipate the sound waves as a dissipation termination.
Hypodermic Tubing	MicroGroup (http://www.microgroup.com/)	304H21RW	Used to connect the surface tap and allow the surface pressure fluctuation to convect to the microphone in the RMP in the form of sound.
Hypodermic Tubing	MicroGroup (http://www.microgroup.com/)	304H14H	Used to reduce the dissipative effect and allow the surface pressure fluctuation to convect to the microphone in the RMP in the form of sound.
plexiglass	Plaskolite (http://www.plaskolite.com/)	1X76204A	Used to make cradles which can connect the tubing and the microphone for the RMP.
Data acquisition chassis	National Instruments (http://www.ni.com/)	PXI-1006	For data acquisition.
Data acquisition channel	National Instruments (http://www.ni.com/)	PXI-4472	For data acquisiton.
Function generator	thinkSRS (http://www.thinksrs.com/)	DS360	To generate white noise signal.
Pistonphone	B&K (http://www.bksv.com/)	4228	To generate sine waves with constant frequency which will be used to calibrate the reference microphone.
Loudspeaker	Mackie (http://www.mackie.com/index.html)	HD1531	Used to convert the electrical white noise signal into sound. It is the sound source for calibrating the RMP.
MatLab	Mathworks (http://www.mathworks.com/)		Used to process experimental data.
LabVIEW	National Instruments (http://www.ni.com/)		Used control the hardware for data acquisition and record the data.