Spring 2012

 Laboratory #4: The Hot Wire Anemometer

A data report  is required for this laboratory. Please follow the instructions for data report formats carefully, and include the items listing in the Additional Instructions.


Additional Instructions

In step 14 of the procedure, pick your sampling frequency based on the assumption that important fluctuations in the flow exist up to only 7 kHz.  Make sure to change the filter settings accordingly.  Save all four types mean and rms (final), raw, histograms, and power spectra.  Also, you should make sure the power spectra is set to decibel (dB) scaling before you acquire the data.

 

The jet flowfield studied in this Lab will be measured again in a following lab.  You will be asked to compare hot wire data from this Lab with the measurements in the laser Doppler velocimeter (LDV) lab.  Therefore, make sure you keep a copy of your data for this comparison.  Also, you will need to be careful how you set the motors for the wind tunnel and jet, so that you can attempt to repeat the same flow in the LDV lab.  At the end of your data acquisition session (but before you recheck the hotfilm resistance), test your ability to repeat the flow by placing the probe at an appropriate point in the flow and taking a velocity measurement - then change the speed of the tunnel/jet (don't turn them off or you will have to turn off the hotwire) and then return the controls to the original values (as best you can).  Repeat this procedure as much as you wish to check repeatability.

 

In the Results section, remember to include any calculations along with the results of your measurements.  In addition to the results required in the lab manual, you also need to present velocity histograms for each of the ten locations in your profile, and power spectra from the two repeated locations.  For the spectra, first get an average power spectrum at each of the two locations by finding the average power at each frequency using the repeated data.  Then plot the averaged results as power (in dB) versus log10 of the frequency (Hz).  Don't forget the important narrative description section required when you present data. 

 

 

Questions
Your report must also include answers to the following questions in the appropriate section:

  1. If you had chosen to take your data with a sampling rate of 1500 Hz (and adjusted the filter appropriately to prevent aliasing), would you expect to have measured different mean or rms velocities.  Be sure to explain the reasoning behind your answer.
  2. Your colleague is planning to use a hot wire system to measure the velocity distribution in the recirculating flow behind a backward facing step.  What do you think is the most serious problem she is liable to encounter?  How will this affect the data?  To explain your answer, you will find it useful to make a sketch that shows how you think the mean velocity in the x-direction actually varies with height above the floor in the recirculation zone, and a second sketch showing the velocity distribution you think would be measured using the hot wire.