Name: the measurement of unsteady surface pressure using a remote microphone probe
Uploaded: 2016-12-03T13:00:00
Description: Watch this Scientific Journal Video about The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe at JoVE.com

This research was made possible through funding from the U.S. Office of Naval Research under Grant No. N000141210337, Deborah Nalchajian and Ronald Joslin.

1. Preparation of Experiments

<ol>
	<li>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.</li>
	<li>Estimate the sensitivity and frequency respon...

<ol>
	<li>Blake, W. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Mechanics of Flow-induced sound and vibration. , (1986).</li><li>Schloemer, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+pressure+gradients+on+turbulent-boundary-layer+wall-pressurefluctuations.">Effects of pressure gradients on turbulent-boundary-layer wall-pressurefluctuations.</a> J Acoust Soc Am. 42 (1), 93-113 (1967).</li><li>Willmarth, W., Wooldridge, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+fluctuating+pressure+at+wall+beneath+a+thick+turbulent+boundary+layer.">Measurements of fluctuating pressure at wall beneath a thick turbulent boundary layer.</a> J Fluid Mech. 14 (2), 187-210 (1962).</li><li>Gautschi, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Piezoelectric Sensorics: Force, Strain, Pressure, Acceleration and Acoustic Emission Sensors, Materials and Amplifiers. , (2002).</li><li>Blake, W. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> A statistical description of pressure and velocity fields at the trailing edges of a flat strut. , (1975).</li><li>Huettenbrink, K. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Lasers in Otorhinolaryngology. , (2005).</li><li>Bell, J. H., Schairer, E. T., Hand, L. A., Mehta, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+Pressure+Measurement+using+Luminescent+Coatings.">Surface Pressure Measurement using Luminescent Coatings.</a> Annu Rev Fluid Mech. 33, 155-205 (2001).</li><li>Englund, D., Richards, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+infinite+line+pressure+probe.">The infinite line pressure probe.</a> ISA Transactions. 24 (2), 11-19 (1985).</li><li>Bergh, H., Tijdeman, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theoretical+and+Experimental+Results+for+the+Dynamic+Response+of+Pressure+Measuring+Systems.+NRL+Report+TR+F+238.">Theoretical and Experimental Results for the Dynamic Response of Pressure Measuring Systems. NRL Report TR F 238.</a> National Aero-and Astronautical Research Inst. , (1965).</li><li>Perennes, S., Roger, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aerodynamic+noise+of+a+two-dimensional+wing+with+high-lift+devices.">Aerodynamic noise of a two-dimensional wing with high-lift devices.</a> , (1998).</li><li>Leclercq, D., Bohineust, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigation+and+modeling+of+the+wall+pressure+field+beneath+a+turbulent+boundary+layer+at+low+and+medium+frequencies.">Investigation and modeling of the wall pressure field beneath a turbulent boundary layer at low and medium frequencies.</a> J Sound Vibrat. 257 (3), 477-501 (2002).</li><li>Franzoni, L. P., Elliott, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+innovative+design+of+a+probe-tube+attachment+for+a+half+in+microphone.">An innovative design of a probe-tube attachment for a half in microphone.</a> JASA. 104, 2903-2910 (1998).</li><li>Arguillat, B., Ricot, D., Robert, G., Bailly, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+the+wavenumber-frequency+spectrum+of+wall+pressure+fluctuations+under+turbulent+flows.">Measurements of the wavenumber-frequency spectrum of wall pressure fluctuations under turbulent flows.</a> Collection of Technical Papers - 11th AIAA/CEAS Aeroacoustics Conference. 1, 722-739 (2005).</li><li>Yang, H., Sims-Williams, D., He, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Unsteady pressure measurement with correction on tubing distortion. Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines. , 521-529 (2006).</li><li>Hoarau, C., Boree, J., Laumonier, J., Gervais, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+the+wall+pressure+trace+downstream+of+a+separated+region+using+extended+proper+orthogonal+decomposition.">Analysis of the wall pressure trace downstream of a separated region using extended proper orthogonal decomposition.</a> Phys Fluids. 18 (5), 055107 (2006).</li><li>Bilka, M. J., Paluta, M. R., Silver, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spatial+correlation+of+measured+unsteady+surface+pressure+behind+a+backward-facing+step.">Spatial correlation of measured unsteady surface pressure behind a backward-facing step.</a> EXIF. 56 (2), (2015).</li><li>Probsting, S., Gupta, A., Scarano, F., Guan, Y., Morris, S. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tomographic+PIV+for+Beveled+Trailing+Edge+Aeroacoustics.">Tomographic PIV for Beveled Trailing Edge Aeroacoustics.</a> , (2014).</li><li>Wong, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Microphones and Their Calibration. Springer Handbook of Acoustics. , (2007).</li><li>Bendat, J. S., Piersol, A. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Random data: analysis and measurement procedures. , (1986).</li><li>Goody, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Empirical+spectral+model+of+surface+pressure+fluctuations.">Empirical spectral model of surface pressure fluctuations.</a> Am Instit Aero Astronaut. 42 (9), 1788-1794 (2004).</li><li>Corcos, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resolution+of+pressure+in+turbulence.">Resolution of pressure in turbulence.</a> J Acoust Soc Am. 35 (2), 192-199 (1963).</li><li>Tijdeman, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+propagation+of+sound+waves+in+cylindrical+tubes.">On the propagation of sound waves in cylindrical tubes.</a> J Sound Vibrat. 39 (1), 1-33 (1975).</li><li>Iberall, A. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Attenuation+of+oscillatory+pressures+in+instrument+lines.">Attenuation of oscillatory pressures in instrument lines.</a> J Res Natl Bureau Stand. 45 (1), 85-108 (1950).</li><li>Zwikker, C., Kosten, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Sound Absorbing Materials. , (1949).</li></ol>

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...

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 functions2, 3. The frequency response required can vary depending on the application. In many wind tunnel applications, a response of 10 kHz to 20 kHz is sufficient. The small scales of turbulent motion often require sensing areas and sensor spacing to be less than 1 mm.
Extensive experimental studies have been conducted in order to obtain turbulence-induced pressure fluctuations. A direct method uses flush-mounted embedded sensors. This method often employs large arrays of microphones, because each sensor can only measure the pressure fluctuation at one discrete point. Typical sensors utilized in this method are piezoelectric transducers, suggested by Gautschi4. Arrays of piezoelectric sensors can be expensive, and the frequency range of measurement is often less than 10 kHz.
Direct surface-mounted microphones are often used as inexpensive USP sensors5. Microphones have high sensitivity, which is a substantial benefit for low-speed flows. However, this also leads to the risk of sensor saturation when large amplitude fluctuations in pressure are present. This method is not suitable for surfaces with large curvatures, discontinuities, or geometries that are too thin to contain the entire sensor.
An indirect method for obtaining both spectral and spatial information is to use thin membranes flush-mounted to a surface6. The time- and space-dependent vibration motions are measured and then converted to surface pressure statistics using known mechanical properties of the membrane. This method requires careful design, implementation, and accurate calibration of the membrane&#39;s dynamic response. Additionally, the vibration measurement equipment, such as laser Doppler vibrometers, are expensive. Lastly, this method can only be applied to flat surfaces.
Pressure-sensitive paint (PSP) is another technique that can be used to measure the unsteady surface pressure. This technique requires the surfaces to be coated in a transparent polymer binder, which causes the molecules within to be excited to a higher energy state as they are illuminated by light of a specific wavelength. As the molecules undergo oxygen quenching, energy is released as light at a rate proportional to the oxygen partial pressure, resulting in luminescence that is inversely proportional to the surface pressure7. The major drawback to PSP methods is the relatively low sensitivity of the measurement when compared to microphones. This limits the application of PSP to relatively high-speed flows.
The present communication describes a method for USP that uses a remote microphone probe (RMP). This method was first described by Englund and Richards8. The concept uses a standard miniature microphone that is connected to the surface pressure tap with a hollow tube. The unsteady pressure at the model surface will travel into the tubing in the form of sound waves. The tubing acts as a &#34;wave-guide&#34; to allow the microphone, which is mounted perpendicularly to the tubing, to measure the sound waves. The waves then continue into another tube that is long enough to eliminate large-amplitude acoustic reflections.
Englund and Richards applied an analytical approach outlined by Bergh and Tijdeman9 to determine the dynamic response of the RMP. Perrenes and Roger10 utilized an RMP to measure surface pressure over a two-dimensional airfoil with high-lift devices. They developed a probe with a 0.5 mm diameter capillary tube at the surface that was connected to a 27-cm-long rigid tube that expanded from 0.7 mm to 2.5 mm via two separate step changes. Each step change caused a relatively large change in the acoustic impedance of the tube. Leclercq and Bohineust11 studied the wall pressure field beneath a turbulent boundary layer. They used a constant-diameter RMP, as suggested by Franzoni and Elliott12. However, the dynamic response was high enough only in a limited frequency range. Arguillat et al.13 designed an RMP to study the noise transmitted to the interior of a vehicle compartment. They tested various tubes to conduct the pressure fluctuation to the microphones. Yang et al.14 corrected for the tubing distortion by using a tubing transfer function approach that is similar to the method introduced in this report. Hoarau et al.15 studied the wall pressure trace downstream of a separated region. The RMPs that they designed had constant inside diameters, and the tubing was entirely non-rigid.
According to previous studies, the accuracy of surface pressure measurements obtained using RMPs is mainly dependent upon the determination of the frequency-dependent transfer function of the probe that relates the surface pressure to the microphone pressure. The following sections will describe an RMP geometry that is both simple and effective. Experimental and analytical methods will be introduced and validated in order to accurately determine the dynamic response of the RMP. The analytical model allows for an RMP to be optimized in the design stage for a potentially wide range of applications.
RMPs can be used to measure pressure fluctuations over a wide range of frequencies. The relatively high spatial resolution can offer detailed information on characteristics of the spatially-distributed unsteady pressure field16. As the probe is small, RMPs can be utilized to measure pressure fluctuations over complex geometries, such as large curvatures or limited spacing17. In addition, the tube connecting the surface tap and the microphone sensor can reduce the magnitude of the induced pressure fluctuation at the microphone. Thus, proper design of RMP sensor geometry and parameters yields a method for obtaining USP characteristics that are significantly less restrictive when compared to flush-mounting the microphone directly to the model surface.
Structure of the RMPThe general structure of the RMP is shown in Figure 1. The RMP consists of one tube leading from the model surface to an expansion section and a second tube extending from the expansion section to a &#34;cradle.&#34; A third tube is then connected to act as an anechoic termination. The cradle is a machined plastic component used for housing the microphone and the tube connections. The details of the RMP structure can be adjusted for various experimental conditions. The purpose of the second, larger-diameter tube is to allow the relatively bulky microphone and cradle to be placed further from the point of the USP measurement without significantly reducing the measurement sensitivity. This second tube can be eliminated if it is not necessary, and the expansion section can be built in the cradle. The anechoic termination was made of soft plastic that was approximately 2 to 3 m in length.
For this demonstration, the design of the RMP was optimized for the measurement of surface pressure fluctuations under a turbulent boundary layer without a streamwise pressure gradient, as shown in Figure 2. The second tube was eliminated. The effects of the two different lengths of the first tube were observed. The first tube was constructed from stainless steel with an inner diameter of 0.5 mm and an outer diameter of 0.81 mm. The lengths of the first tube were 5.35 and 10.40 cm, respectively. The inner diameter of the inlet of the expansion section, which was incorporated into the cradle, was 0.5 mm, and the inner diameter of the exit was 1.25 mm, which was identical to the inner diameter of the dissipation termination. The angle of the expansion section was 7&#176;. There was a hole in the cradle with a 1.25 mm diameter in order to smoothly connect the expansion section with the anechoic termination. The sensing area was connected to the 1.25 mm hole through a perpendicular 0.75 mm hole.

<table border="0" cellpadding="0" cellspacing="0" width="1651">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Microphone</td>
			<td style="width:513px;">ACO Pacific (http://www.acopacific.com/)</td>
			<td style="width:137px;">7016</td>
			<td style="width:739px;">Used to measure the sound pressure and calibrate the RMP as a reference.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Microphone</td>
			<td style="width:513px;">Knowles (http://www.knowles.com/eng)</td>
			<td style="width:137px;">FG-23629-C36</td>
			<td>Used to measure the pressure fluctuation as a part of the RMP.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Microbore Tubing</td>
			<td style="width:513px;">Saint-gobain (http://www.biopharm.saint-gobain.com/en/index.asp)</td>
			<td style="width:137px;">Tygon ND 100-80</td>
			<td>Used to dissipate the sound waves as a dissipation termination.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Hypodermic Tubing</td>
			<td style="width:513px;">MicroGroup (http://www.microgroup.com/)</td>
			<td style="width:137px;">304H21RW</td>
			<td>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.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Hypodermic Tubing</td>
			<td style="width:513px;">MicroGroup (http://www.microgroup.com/)</td>
			<td style="width:137px;">304H14H</td>
			<td>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.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">plexiglass</td>
			<td style="width:513px;">Plaskolite (http://www.plaskolite.com/)</td>
			<td style="width:137px;">1X76204A</td>
			<td>Used to make cradles which can connect the tubing and the microphone for the RMP.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Data acquisition chassis</td>
			<td style="width:513px;">National Instruments (http://www.ni.com/)</td>
			<td style="width:137px;">PXI-1006</td>
			<td>For data acquisition.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Data acquisition channel</td>
			<td style="width:513px;">National Instruments (http://www.ni.com/)</td>
			<td style="width:137px;">PXI-4472</td>
			<td>For data acquisiton.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Function generator</td>
			<td style="width:513px;">thinkSRS (http://www.thinksrs.com/)</td>
			<td style="width:137px;">DS360</td>
			<td>To generate white noise signal.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Pistonphone</td>
			<td style="width:513px;">B&#38;K (http://www.bksv.com/)</td>
			<td style="width:137px;">4228</td>
			<td>To generate sine waves with constant frequency which will be used to calibrate the reference microphone.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">Loudspeaker</td>
			<td style="width:513px;">Mackie (http://www.mackie.com/index.html)</td>
			<td style="width:137px;">HD1531</td>
			<td>Used to convert the electrical white noise signal into sound. It is the sound source for calibrating the RMP.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">MatLab</td>
			<td style="width:513px;">Mathworks (http://www.mathworks.com/)</td>
			<td style="width:137px;"></td>
			<td>Used to process experimental data.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:263px;">LabVIEW</td>
			<td style="width:513px;">National Instruments (http://www.ni.com/)</td>
			<td style="width:137px;"></td>
			<td>Used control the hardware for data acquisition and record the data.</td>
		</tr>
	</tbody>
</table>

the measurement of unsteady surface pressure using a remote microphone probe

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.

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...

Watch this Scientific Journal Video about The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe at JoVE.com

The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

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.

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various ...

The overall goal of this experimental method is to perform high spatial resolution measurement of the turbulence-induced pressure fluctuations over surfaces of bodies immersed in fluid flows. This method can help answer key questions in the field of fluid dynamics, aeroacoustics, and flow-induced structure vibrations, such as spatial and temporal characteristics of the unsteady surface pressure induced by turbulent motions with small-length scales. The remote microphone probe can provide detailed information of characteristics of the unsteady surface pressure fields, with relatively high spatial resolution over a wide range of frequencies.Prepare for the experiments by constructing the remote microphone probe. This requires several components. Primary among these is a small microphone that is sensitive to the frequencies of interest.The size of the remaining components depends on the desired sensitivity and frequency response of the probe. This probe uses a length of stainless steel tubing with a half millimeter inner diameter. It also requires a long piece of soft tubing, with a 1.25 millimeter inner diameter.A custom cut and drilled piece of Plexiglass will serve as the microphone cradle. On one side of the cradle, is a hole to accommodate the stainless steel tube. On the opposite end of the cradle, is another hole.It is along the same axis and accommodates the soft tube. These two holes are connected through the Plexiglass. On top, there are two concentric holes.The larger outer hole will hold the microphone. The smaller inner hole accesses the cavity. This cross section provides a guide to the construction of the cradle.Note that the taper needed to match the side hole diameters is near the entrance for the stainless steel tube. Have epoxy ready to attach the components to the cradle. Get the microphone and orient it with its diaphragm pointing toward the cradle, and seat it in its position.When it is in position, apply epoxy to fix it in place. Next, insert the stainless steel tube into the opening made for it before securing with epoxy. Attach the soft tubing in its position in the same way.At the other end of the soft tubing, use a plug to seal the tube. With all the elements attached to the cradle, this probe is ready to be used in an experiment. Next, turn attention to the model surface for the experiment.This demonstration uses a flat plate with prepared measurement points. At a point of measurement, there should be a through hole perpendicular to the surface. The hole should have the outer diameter of the stainless steel tube in the probe.For the experiment, mount the model surface in the wind tunnel. Next, get a microphone probe to attach to the model surface at the measurement hole. From the rear of the surface, thread the stainless steel tube of the microphone probe into the hole.The tube opening should be flush with the measurement surface. Apply epoxy to the side opposite the measurement site to secure the tube in place. With the microphone probe in position, surround it with acoustic foam to prevent parasitic noise from contaminating the system.Next, work with the soft anechoic tube from the probe. Move the closed end out of the test section of the wind tunnel. Surround it with acoustic panels to isolate it from its surroundings.Back at the probe, connect the microphone to the inputs of a low-noise amplifier, which is outside the tech section of the wind tunnel. Next, move to the electronics outside of the tunnel. The low-noise amplifier is connected to the data acquisition system.Set the gain of the amplifier to ten. Calibration of the remote microphone probe requires several pieces of equipment. For the first phase, use a high quality reference microphone with frequency independent response, along with the piston phone.In addition, there should be an amplifier with its output connected to the data acquisition system. Connect the microphone to the input of the amplifier, and set the input and the output gain to ten decibels. Next, get the piston phone.Insert the microphone into the piston phone for the measurement. When ready, turn on the piston phone. Turn to the data acquisition system to set the data acquisition frequency, and number of samples.Begin acquiring and saving the voltage from the reference microphone to calculate the calibration constant. Once the calibration constant is determined, remove the microphone from the piston phone. Now, move the microphone to the model's surface, use a holder to position the microphone perpendicular to the surface over the probe tube, where the pressure fluctuation will be measured.Align the microphone with the center of the probe tap, and one millimeter above it. The next steps require a loud speaker with an internal amplifier connected to a function generator. Mount the loud speaker near the model's surface and 2.5 meters from the microphone.The speaker should be off and the cone should be directed toward the model's surface. Outside the wind tunnel, work with the function generator. Turn it on and use the white noise option to provide the acoustic signal.Return to the loud speaker and ensure the amplification is at minimum. Then, turn the loud speaker on and adjust its volume as high as possible without causing damage. Return to the data acquisition system.Acquire and save time series data from the voltage outputs of the probe and reference microphones. Calibration is the most critical step of this procedure. To ensure success, use a model to predict the transfer function and compare it with the experimental results.In addition, use high quality devices, including the reference microphone and loud speaker. These are plots of the coherence between the pressure fluctuations as measured in the remote microphone probe and the reference microphone as a function of frequency. This probe system has a 5.35 centimeter long primary tube.For comparison, this probe has a 10.4 centimeter long primary tube. The coherence is above 0.97 in the frequency range of interest. These plots are of the magnitude of the transfer functions from the same two probes.The blue curves are experiment results. The green curves are theoretical predictions. The predictions of the dynamic response are accurate across most of the frequency range.Small aberrations in the probe may explain disagreements at middle and high frequencies. Comparison of the data and the predictions for the slope of the face shift reveal the predictions are slight overestimates. This may be due to small errors in measurements of tube lengths, or by temperature variations.In this plot, the light grey band represents the range of previous surface pressure spectra data at various Reynolds numbers. The data points represent measurements using remote microphone probes. The measurements are within the spread of measurements observed in the published literature.Once mastered, calibration of a single remote microphone probe can be done in five minutes if it's performed properly. While attempting this procedure, it's important to repeat the calibration with various loud speaker volumes to obtain the highest coherence for the remote microphone probe. After this development, this technique paved the way for researchers to explore the characteristics of the pressure fluctuations over surfaces with large curvature and limited spacing under complex flow conditions.Working with loud speakers generating high volume white noise can be extremely hazardous. Don't forget precautions, such as using noise-muting headphones while you're performing this procedure. After watching this video, you should have a good understanding of how to build, set up, and calibrate a remote microphone probe.

Research

JoVE Journal

Engineering

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