Hot Wire Laboratory

THE UNIVERISTY OF MANCHESTER school OF robotlike, AEROSPACE AND CIVIL engineering LABORATORY REPORT INSTRUMENTATION AND MEASUREMENT crack SHEDDING FROM A CYLINDER & DATA ACQUISITION NAMEMANISH PITROLA assimilator ID75050320 COURSEMEng MECHANICAL ENGINEERING DUE DATE27TH NOVEMBER 2012 1) What be the main advantages and injustices of victimisation a hotwire to amount of money pay heed velocities?There atomic hail 18 many advantages and disadvantages of use a hotwire to measure ascend velocities, one of the main advantages is the hotwire put outs a ceaseless analogue output of the bettingness at a particular point, and whence information nigh the upper foot be obtained for any specific time. Another advantage of development a hotwire wind gage is the ability to take fluctuating velocities to a high accuracy. Also some other advantage of development a hotwire wind gage is the sensor is able to reach the voltage and the amphetamine victimisation hotwire the ory. However even though hotwire anemometer is an adequate dig to obtain info it has its drawbacks.Read thisRespiratory Activity ane disadvantage of using a hotwire is that it has to be calib appreciated callable to the theory not coinciding with real entropy and the hotwire sess scarcely obtain the order of magnitude of the give and not the direction. Another disadvantage of using a hotwire is the disorganized effect that come out such as contamination and try out vibration. virtually systematic effects that affect the entropy argon the ambient temperatures and whirl round shedding from the wire. One of the main disadvantages of using a hotwire is the output depends on both pep pill and temperature, so when the temperature of a fluid increases the mensural fastness obtained ar withal low and adjustment is required. ) Why is setting the be consume run classic in digital entropy acquisition? What look intoal parameters or requirements tail assembly be a pply to establish the best taste identify? What may happen if the wrong sampling graze is social occasiond? utilize the correct sampling estimate is important because if the ridiculous sampling number is employ some aliasing effects may occur, presenting inadequate data where important data is ignored if the sampling regularize is on a lower floor the best, and if the sampling treasure is preceding(prenominal) the optimum to a great extent accurate data is obtained which carries the same gallery as the optimum with few distortion which are not required.This rear end cause inadequacy of the data, where recording is not general enough or too frequent. The optimum sampling rate sewer be naturalised using the Nyquist theory which states that the maximum measures oftenness is half(a) the sampling absolute frequency, yet the band comprehensiveness of the signal needs to considered, the rule for obtaining the sampling frequency of any probe must be at least 2. 5 mu ltiplication greater than the maximum frequency present. 3) Show how the sampling rate was determined for this sample.What was the sampling rate? For a fall down around a cylinder an experiential relation between the vortex shedding frequency and Reynolds number (Re) is used to find the sampling rate. The relationship below is used to find the frequency in the flow where the Strouhal number is 0. 2, diameter (d) is 15mm and the let go stream focal ratio (U0) is 10m/s. St=fdU0=0. 1981-19. 7Re? 0. 2 Then by simple algebraic rearranging the frequency is rig to be 133. 3Hz. Therefore the maximum frequency experienced is 2f = 2*133. 3 = 266. 6Hz.To obtain the optimum sampling frequency we obviously by using Nyquist theory multiply the maximum frequency by 2. 5 providing an optimum sampling rate of 666. 5Hz. The values for the sampling rate were taken as 330Hz, 660Hz and 1320Hz for essayal purposes to study the over and under sampling of data. 4) In the experiment the hotwire w as fine-tune in terms of swiftness vs (E-E0)2. spot out the standardisations for U = B((E-E0)2)n and the mingled polynomials. Compare the several(predicate) lines. Which is the best to use? rule 1 find 2 work up 3 pulp 4 From the to a higher place graphs is sight be seen that the best calibration to use is the cubic calibration ( formula 2) as this fits the actual speed line more accurately. 5) If the velocity higher than the ones calibrated foer was measured, which calibration is probably to give the best extrapolated data? move into 5 shape 6 Figure 7 Figure 8 From the above graphs it lowlife be seen that the worse extrapolated data is implant using the quartic calibration and the best extrapolated data foot be found using the linear calibration of A(V-Vo2)n.Also higher order polynomial extrapolation green goddess produce invalid values and as a consequent the hallucination go forth magnify as high order of polynomials are used, so thence the linear relationship is recommended. 6) In a fast Fourier transform (FFT) the data in the time domain is converted to the same data in the frequency domain. The original data can because be considered as the sum of a series of sine waves of regularly spaced frequencies, with different magnitudes and phases. How is the frequency legal separation in the FFT determined? How can the frequency musical interval in an FFT be reduced?What impact could this gull on an experiment? The frequency interval can be obtained by dividing the sampling rate by the number of samples used. For 660Hz the number of samples is 1024, so wherefore the frequency interval is 660/1024 = 0. 6445. The frequency intervals can be reduced by increasing the number of samples used this is advantageous as it gives a more accurate internal representation of the original signal. 7) Considering the FFT data, what can be done in an experiment to single out genuine signals from random fluctuations in the data? o wn an example of this in graphical form.Figure 9 Figure 10 From grade 9 it can be seen that the peak is unobtainable as the data is very creaky which could be due to disturbances. However this can be overcome by averaging the FFT which allows us to easily identify peaks which can be seen from figure 10. 8) In this experiment, why are 2 frequency peaks seen on the FFT when the hotwire is near the subject matter line? 2 frequency peaks can be seen on the FFT at the centreline due to the 2 vortices induced by the cylinder but as you move onward(p) from the centre line notwithstanding one of the vortices is predominant.The 2 peaks occur at 129Hz and 250Hz. 9) With increasing distance from the centreline, how does the FFT dispersion change? allow in graphs to illustrate this for various locations across the wake. From the below figures it can be seen that as you move away from the centre line the peaks in the FFT dissemination disappear. Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 10) spell the probability diffusion histograms of velocity for various positions across the wake.What does the histogram show and how can the variation in the histograms be explained in terms of the properties of the flow? Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 By comparing the above probability dissemination figures it can be seen that with distance away from the centreline the flow velocity develops a more uniform velocity. It can be seen that at bottom the 40mm distance away from the centreline, the probability distribution of the velocity produces wide distribution of velocities this is due to the various velocities inside the wake and excitement.For distance more than 40mm away the probability distribution of velocity sprains more uniform, which implies the vortices play no parting in bear upon the flow at these distances away from the centreline. It can overly be seen that the flow speed at these d istances increases as the flow diverges and accelerates around the cylinder. 11) Plot a graph showing the variation of mean velocity, RMS velocity and turbulence fervency with distance across the wake. What physical phenomena in the flow are do the distribution to be the shape they are?What do the results say about the size of the wake compared to the size of the cylinder? Figure 23 Figure 24 Figure 25 The vortices in the flow cause turbulence to occur behind the cylinder which causes the distributions to change. It can be seen from figure 23 that the velocity changes instantaneously as you move away from the centreline, it can also be observed that from 45mm away and more the velocity start to become more uniform and fluctuate around the free stream velocity. From figure 25 and 25 from 45mm and onwards the RMS and RTI decrease.From the above graphs it can be deduced that the size of the wake is 45mm from the centreline or a total width of 90mm, which is 6 times the diamet er of the cylinder. 12) What are the major sources of error likely to be in this experiment? Try and give a numeral estimate to the executable error(s) in the data. Some of the likely sources of error that may occur during this experiment are the calibration process as the hotwire was only calibrated at the centreline and as the hotwire was lowered using screw appliance which it not altogether accurate, there was no calibration of the at the saucy position.Another source of error can be due to pressure fluctuations, and due to the velocity creation measured using the pressure differences, these fluctuation can cause the velocity to vary. Another source of error could be the supposition of the flow creation 2-d as turbulence is a 3-d. To calculate the error, I used the measured velocity table and the theoretical linear calibration velocity. Taking the medium error, the percentage error in the experimental data was 5. 8%. inwardly a range Can not measure supersonic velocitie sHot Wire LaboratoryTHE UNIVERISTY OF MANCHESTER SCHOOL OF MECHANICAL, AEROSPACE AND CIVIL ENGINEERING LABORATORY REPORT INSTRUMENTATION AND MEASUREMENT VORTEX SHEDDING FROM A CYLINDER & DATA ACQUISITION NAMEMANISH PITROLA STUDENT ID75050320 COURSEMEng MECHANICAL ENGINEERING DUE DATE27TH NOVEMBER 2012 1) What are the main advantages and disadvantages of using a hotwire to measure flow velocities?There are many advantages and disadvantages of using a hotwire to measure flow velocities, one of the main advantages is the hotwire produces a continuous analogue output of the velocity at a particular point, and hence information about the velocity can be obtained for any specific time. Another advantage of using a hotwire anemometer is the ability to follow fluctuating velocities to a high accuracy. Also another advantage of using a hotwire anemometer is the sensor is able to relate the voltage and the velocity using hotwire theory. However even though hotwire anemometer is an adequate to ol to obtain data it has its drawbacks.Read thisRespiratory ActivityOne disadvantage of using a hotwire is that it has to be calibrated due to the theory not coinciding with actual data and the hotwire can only obtain the magnitude of the flow and not the direction. Another disadvantage of using a hotwire is the unsystematic effects that occur such as contamination and probe vibration. Some systematic effects that affect the data are the ambient temperatures and eddy shedding from the wire. One of the main disadvantages of using a hotwire is the output depends on both velocity and temperature, so when the temperature of a fluid increases the measured velocity obtained are too low and adjustment is required. ) Why is setting the correct sampling rate important in digital data acquisition? What experimental parameters or requirements can be used to establish the optimum sampling rate? What may happen if the wrong sampling rate is used? Using the correct sampling rate is important beca use if the incorrect sampling rate is used some aliasing effects may occur, presenting insufficient data where important data is ignored if the sampling rate is below the optimum, and if the sampling rate is above the optimum more accurate data is obtained which carries the same trend as the optimum with few distortion which are not required.This can cause inadequacy of the data, where recording is not frequent enough or too frequent. The optimum sampling rate can be established using the Nyquist theory which states that the maximum measures frequency is half the sampling frequency, however the bandwidth of the signal needs to considered, the rule for obtaining the sampling frequency of any probe must be at least 2. 5 times greater than the maximum frequency present. 3) Show how the sampling rate was determined for this experiment.What was the sampling rate? For a flow around a cylinder an empirical relation between the vortex shedding frequency and Reynolds number (Re) is used to f ind the sampling rate. The relationship below is used to find the frequency in the flow where the Strouhal number is 0. 2, diameter (d) is 15mm and the free stream velocity (U0) is 10m/s. St=fdU0=0. 1981-19. 7Re? 0. 2 Then by simple algebraic rearranging the frequency is found to be 133. 3Hz. Therefore the maximum frequency experienced is 2f = 2*133. 3 = 266. 6Hz.To obtain the optimum sampling frequency we simply by using Nyquist theory multiply the maximum frequency by 2. 5 providing an optimum sampling rate of 666. 5Hz. The values for the sampling rate were taken as 330Hz, 660Hz and 1320Hz for experimental purposes to study the over and under sampling of data. 4) In the experiment the hotwire was calibrated in terms of velocity vs (E-E0)2. Plot out the calibrations for U = B((E-E0)2)n and the various polynomials. Compare the different lines. Which is the best to use? Figure 1 Figure 2 Figure 3 Figure 4 From the above graphs is can be seen that the best calibration to use is the cubic calibration (figure 2) as this fits the actual velocity line more accurately. 5) If the velocity higher than the ones calibrated foer was measured, which calibration is likely to give the best extrapolated data? Figure 5 Figure 6 Figure 7 Figure 8 From the above graphs it can be seen that the worse extrapolated data is found using the quartic calibration and the best extrapolated data can be found using the linear calibration of A(V-Vo2)n.Also higher order polynomial extrapolation can produce invalid values and as a result the error will magnify as high order of polynomials are used, so therefore the linear relationship is recommended. 6) In a fast Fourier transform (FFT) the data in the time domain is converted to the equivalent data in the frequency domain. The original data can therefore be considered as the sum of a series of sine waves of regularly spaced frequencies, with different magnitudes and phases. How is the frequency interval in the FFT determined? H ow can the frequency interval in an FFT be reduced?What impact could this have on an experiment? The frequency interval can be obtained by dividing the sampling rate by the number of samples used. For 660Hz the number of samples is 1024, so therefore the frequency interval is 660/1024 = 0. 6445. The frequency intervals can be reduced by increasing the number of samples used this is advantageous as it gives a more accurate representation of the original signal. 7) Considering the FFT data, what can be done in an experiment to isolate genuine signals from random fluctuations in the data? Give an example of this in graphical form.Figure 9 Figure 10 From figure 9 it can be seen that the peak is unobtainable as the data is very noisy which could be due to disturbances. However this can be overcome by averaging the FFT which allows us to easily identify peaks which can be seen from figure 10. 8) In this experiment, why are 2 frequency peaks seen on the FFT when the hotwire is near the centre line? 2 frequency peaks can be seen on the FFT at the centreline due to the 2 vortices induced by the cylinder but as you move away from the centre line only one of the vortices is predominant.The two peaks occur at 129Hz and 250Hz. 9) With increasing distance from the centreline, how does the FFT distribution change? Include graphs to illustrate this for various locations across the wake. From the below figures it can be seen that as you move away from the centre line the peaks in the FFT distribution disappear. Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 10) Plot the probability distribution histograms of velocity for various positions across the wake.What does the histogram show and how can the variation in the histograms be explained in terms of the properties of the flow? Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 By comparing the above probability distribution figures it can be seen that with distance away from t he centreline the flow velocity develops a more uniform velocity. It can be seen that within the 40mm distance away from the centreline, the probability distribution of the velocity produces wide distribution of velocities this is due to the various velocities inside the wake and turbulence.For distance more than 40mm away the probability distribution of velocity becomes more uniform, which implies the vortices play no role in affecting the flow at these distances away from the centreline. It can also be seen that the flow speed at these distances increases as the flow diverges and accelerates around the cylinder. 11) Plot a graph showing the variation of mean velocity, RMS velocity and turbulence intensity with distance across the wake. What physical phenomena in the flow are causing the distribution to be the shape they are?What do the results say about the size of the wake compared to the size of the cylinder? Figure 23 Figure 24 Figure 25 The vortices in the flow cause tur bulence to occur behind the cylinder which causes the distributions to change. It can be seen from figure 23 that the velocity changes instantaneously as you move away from the centreline, it can also be observed that from 45mm away and more the velocity start to become more uniform and fluctuate around the free stream velocity. From figure 25 and 25 from 45mm and onwards the RMS and RTI decrease.From the above graphs it can be deduced that the size of the wake is 45mm from the centreline or a total width of 90mm, which is 6 times the diameter of the cylinder. 12) What are the major sources of error likely to be in this experiment? Try and give a numerical estimate to the possible error(s) in the data. Some of the likely sources of error that may occur during this experiment are the calibration process as the hotwire was only calibrated at the centreline and as the hotwire was lowered using screw mechanism which it not totally accurate, there was no calibration of the at the new pos ition.Another source of error can be due to pressure fluctuations, and due to the velocity being measured using the pressure differences, these fluctuation can cause the velocity to vary. Another source of error could be the assumption of the flow being 2-d as turbulence is a 3-d. To calculate the error, I used the measured velocity table and the theoretical linear calibration velocity. Taking the average error, the percentage error in the experimental data was 5. 8%. Within a range Can not measure supersonic velocities