Measuring Turbulent Flows

JoVE Science Education
Mechanical Engineering
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JoVE Science Education Mechanical Engineering
Measuring Turbulent Flows

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10:05 min
April 30, 2023

Genel Bakış

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI

Turbulent flows exhibit very high frequency fluctuations that require instruments with high time-resolution for their appropriate characterization. Hot-wire anemometers have a short enough time-response to fulfill this requirement. The purpose of this experiment is to demonstrate the use of hot-wire anemometry to characterize a turbulent jet.

In this experiment, a previously calibrated hot-wire probe will be used to obtain velocity measurements at different positions within the jet. Finally, we will demonstrate a basic statistical analysis of the data to characterize the turbulent field.

İlkeler

Prosedür

Measure the width of the slit, W, and record this value in table 1. Set the hot-wire anemometer at a distance from the exit equal to x = 1.5W along the centerline. Record this streamwise position in table 2. The centerline is the origin of the spanwise coordinate (y = 0). Start the data acquisition program for traversing the jet. Set the sample rate at 500 Hz for a total of 5000 samples (i.e. 10s of data). Record the current spanwise position of the hot-wire in table 3. Acquire data. The data acquisition system will calculate the average velocity and turbulence intensity of that dataset using equations (1) and (4). Record those two values in table 3. Move the hotwire to the next (positive) spanwise position ( mm). Repeat steps 5 to 8 until there is not any noticeable change on both the average velocity and the turbulence intensity. Move the hot-wire back to the centerline. Move the hotwire to the next (negative) spanwise position ( mm). Acquire data. The data acquisition system will calculate the average velocity and turbulence intensity of that dataset using equations (1) and (4). Record those two values in table 3. Repeat steps 11 to 14 until there is not any noticeable change on both the average velocity and the turbulence intensity. Move the hot-wire back to the centerline of the jet. Move the hot-wire along the centerline of the jet in the downstream direction to a new position (e.g. x = 3W). Repeat steps 4 to 17 for as many streamwise positions as wanted (e.g. x = 1.5W, 3W, 6W, 9W). Table 1 . Basic parameters for experimental study. Parameter Value Slit width (W) 19.05 mm Air density (r) 1.2 kg/m3 Transducer calibration constant (m_p) 76.75 Pa/V Calibration constant A 5.40369 V2 Calibration constant B 2.30234 V2(m/s)-0.65 Figure 4. Flow control in the flow system. The stack on top of the plenum serves the purpose of diverting flow from the jet slit allowing to control the jet ' s exit velocity. Please click here to view a larger version of this figure.

Sonuçlar

Figure 5 shows the distribution of average velocity across the jet at the downstream position x = 3W. And Figure 6 shows the distribution of turbulence intensity across the jet at the same downstream position. Table 3 has the results for the local values of average velocity and turbulence intensity at the streamwise position x = 3W. The last column of this table is the ratio between the local v…

Applications and Summary

This experiment demonstrated the application of hot-wire anemometry for characterizing turbulent flows. Given that turbulence exhibits high frequency velocity fluctuations, hot-wire anemometers are suitable instruments for its characterization due to their high time-resolution. With this in mind, we used a calibrated hot-wire anemometer to characterize the average local velocity and turbulence intensity at different positions within a planar jet. These quantities were determined using statistical descriptors for turbulen…

Referanslar

  1. Chapra, S.C. and R.P. Canale. Numerical methods for engineers. Vol. 2. New York: McGraw-Hill, 1998.
  2. King, L.V. On the convection of heat from small cylinders in a stream of fluid: determination of the convection constants of small platinum wires with applications to hot-wire anemometry. Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character 214 (1914): 373-432.
  3. White, F. M. Fluid Mechanics, 7th ed., McGraw-Hill, 2009.
  4. Munson, B.R., D.F. Young, T.H. Okiishi. Fundamentals of Fluid Mechanics. 5th ed., Wiley, 2006.
  5. Buckingham, E. Note on contraction coefficients of jets of gas. Journal of Research,6:765-775, 1931.

DEŞİFRE METNİ

Measure the width of the slit, W, and record this value in table 1. Set the hot-wire anemometer at a distance from the exit equal to x = 1.5W along the centerline. Record this streamwise position in table 2. The centerline is the origin of the spanwise coordinate (y = 0). Start the data acquisition program for traversing the jet. Set the sample rate at 500 Hz for a total of 5000 samples (i.e. 10s of data). Record the current spanwise position of the hot-wire in table 3. Acquire data. The data acquisition system will calculate the average velocity and turbulence intensity of that dataset using equations (1) and (4). Record those two values in table 3. Move the hotwire to the next (positive) spanwise position ( mm). Repeat steps 5 to 8 until there is not any noticeable change on both the average velocity and the turbulence intensity. Move the hot-wire back to the centerline. Move the hotwire to the next (negative) spanwise position ( mm). Acquire data. The data acquisition system will calculate the average velocity and turbulence intensity of that dataset using equations (1) and (4). Record those two values in table 3. Repeat steps 11 to 14 until there is not any noticeable change on both the average velocity and the turbulence intensity. Move the hot-wire back to the centerline of the jet. Move the hot-wire along the centerline of the jet in the downstream direction to a new position (e.g. x = 3W). Repeat steps 4 to 17 for as many streamwise positions as wanted (e.g. x = 1.5W, 3W, 6W, 9W). Table 1 . Basic parameters for experimental study. Parameter Value Slit width (W) 19.05 mm Air density (r) 1.2 kg/m3 Transducer calibration constant (m_p) 76.75 Pa/V Calibration constant A 5.40369 V2 Calibration constant B 2.30234 V2(m/s)-0.65 Figure 4. Flow control in the flow system. The stack on top of the plenum serves the purpose of diverting flow from the jet slit allowing to control the jet ' s exit velocity. Please click here to view a larger version of this figure.