TURBULENCE SIGNAL ANALYSIS ; ';

first suppression of 25%is found at Std = 0.154 followed by a second suppression of 30%at Std

=

0.616. For location X/D

=

0.75, maximum turbulence amplification of 65%occurs at Std

=

0.078. First suppression of 10%occurs at Red

=

^{0.065 and}

second suppression of 28% occurs at Std = 0.477. These two locations are sufficiently close compared to centerline points, but the turbulence characteristics on excitation are different. So, it can be concluded that, though the pattern of excitation characteristics are similar but they are function of geometric locations.

Fig. 5.6.31 shows the effect of the location of excitation source on centerline turbulence intensity at X/D

=

^{2.50, Red}

=

^{3.49 x 104•} The comparison shows that turbulence amplification occurs at much earlier frequencies in case of surface excitation and the relative amplification is much higher (maximum450%)than the upstream excitation (maximum 48%). However the suppression characteristics is more stable in case of upstream excitation though there location is almost the

same.

AtX/D = 5.0, surface excitation shows earlier and greater turbulence amplification but similar and unstable suppression and is presented in Fig. 5.6.32.

So, surface excitation can cause much more turbulence amplification in the near field of a jet but have similar hut unstable suppression characteristics. Hence for stable turbulence suppression upstream excitation can be recommended.

The effect of the location of excitation source at X/D

=

0.25 on a YID

=

0.50 line is presented in Fig. 5.6.33. With negligible suppression characteristics surface excitation can amplify turbulence about twice the upstream excitation. As presented in Fig. 5.6.34, downstream traverse reduces the effect of surface excitation but still remain iIi prestigious position than upstream excitation.

However, the combined excitation i.e. upstrp.am and surface excitation together, plays a mediocre role.

57 suppression are clearly understood from the FFT signals when compared to unexcited traces.

Fig. 5.7.l.A and Fig. 5.7.1.B shows the effect of surface excitation on centerline turbulence intensity at X/D " 2.5. Excitation increases turbulence fluctuation and reaches maximum at f " 5 Hz. Then suppression is found at f " 9 Hz. At f " 15 Hz, second peak amplification is found, followed by a suppression at f " 20 Hz.

First and Second turbulence amplification and suppression frequencies are simple multiples. These signals confirm the results obtained in Fig. 5.6.22.

Fig. 5.7.2 shows the effect of surface excitation on centerline turbulence intensity at X/D " 5.0. Turbulence amplification is found at f " 0.5 Hz and suppression at f " 20 Hz and f " 60 Hz, which confirms the previous finding of Fig. 5.6.22.

Fig. 5.7.3 shows the effect of surface excitation on centerline turbulence intensity at X/D " 7.5, which corresponds to previous Fig. 5.6.22. Weak suppression at f

" 70 Hz is found, however amplification is not clearly understood.

Fig. 5.7.4.A and Fig. 5.7.4.B shows the effect of upstream excitation at X/D " 0.25 on a YID " 0.50 line (Corresponding previous figures are Fig. 5.6.23 and Fig 5.6.33). Turbulence amplification of different pattern at different frequencies is observed but at even f " 200 Hz, suppression is not visible.

Fig. 5.7.5.A, Fig. 5.7.5.B, Fig. 5.7.5.C, and Fig. 5.7.5.D shows the effect of surface excitation at X/D " 0.25 on a YID " 0.50 line (Corresponding previous figures are Fig. 5.6.24 and Fig 5.6.33). Turbulence amplification is observed much higher than tbe previous upstream excitation and at lower frequencies but noticeable turbulence suppression is not observed.

I

Fig. 5.7.6.A and Fig. 5.7.6.B shows the effect of upstream excitation at X/D

=

^0.50

on a Y

ID

= 0.50 line (Corresponding previous figures are Fig. 5.6.23 and Fig 5.6.34). Turbulence amplification is found at f = 6.25 Hz, 10 Hz, 40 Hz and 100 Hz.

At f " 100 Hz, the signal was found to be locked and is considered as dominating excitation wpere the original fluctuation can play no role. Suppression is observed at

f =

?O Hz and 50 Hz.

Fig. 5.7.7.A and Fig. 5.7.7.B shows the effect of surface excitation at X/D

=

^0.50

on a Y/D = 0.50 line (Corresponding previous figures are Fig. 5.6.24 and Fig 5.6.34). Here the effect is more than upstream excitation. Turbulence amplification is found at f = 2.0 Hz, 4.0 Hz, 8.0 Hz, 20 Hz and 80 Hz, however suppression is found at f

=

10 Hz and 40 Hz.

Fig. 5.7.8 shows the time traces of unexcited jet in four different Y/D locations at X/D = 0.50. It is found that the amplitude and frequency of fluctuation at different Y/D locations are different. The peak is found at Y/D

=

0.52. The frequency of turbulence fluctuation is higher in inner edge than outer edge. Fig.

5.7.9 and Fig. 5.7.10 shows the turbulence fluctuation in the same locations at f

=

3 Hz and f

=

40 Hz. When excited at f

=

3 Hz the turbulence is amplified except at Y/D

=

0.52, however excitation with f

=

40 Hz shows suppression at Y/D

=

^0.48

and 0.68 but not at Y/0

=

0.52. From these patterns of fluctuation it can be concluded that with on single frequency it may not be possible to amplify or suppress turbulence intensities at every locations.

Fig. 5.7.11.A and Fig. 5.7.11.B the time traces of unexcited jet in different Y/D loclltions at X/D = 5.0. These signals, starting from centerline, gives a total picture of turbulence fluctuation at this location. Turbulence intensity gradually rises from centerline and reaches maximum at Y/D = 0.91 and then decreases outer wllrds. Fig. 5.7.12 shows the turbulence fluctuation at four different Y/V locations when excited at f = 0.9 Hz for amplification. Same locations are presented in Fig. 5.7.13 when excited f

=

60 Hz for suppression.

Time traces on a Y/0

=

0.50 line in four different X/D locations are presented in Fig. 5.7.14. The gradual development of turbulence fluctuation on the line .is observed.

The frequency spectrum of unexcited jet in four X/D locations on centerline is presented in Fig. 5.7.15 which is analyzed in the following table (Table 5.7.1).

59 TABLE 5.7.1

Analysis Table Centerline Frequency Spectrum.

Location Dominating Frequency Hange Peak Instant Frequency

X/D = 2.5 40 Hz

-

^{70 Hz 48.50 Hz}

X/D ₌ 5.0

o

^Hz

-

^{50 Hz 11.50 Hz}

X/D = 7.5

o

^Hz

-

^{70 Hz 24.00 Hz}

X/D = 10.0

o

^Hz

-

^{20 Hz 13.00 Hz}

The frequency spectrum shows that the dominating frequency ranges are different in different X locations. Peak instant frequencies changes every instant and they are also different. It is observed in the downstream (5.0 < ^X/D < 10.0) that there are multiple instant peak frequencies which invariably suggests that excitation with multiple frequencies and amplitude may be a good approach.

The frequency spectrum of unexcited jet in four X/D locations on a Y/D = 0.50 line is presented in Fig. 5.7.16, which is analyzed in the following table (Table 5.7.2).

TABLE5.7.2

Analysis Table : Wall Frequency Spectrum.

Location Dominating Frequency Range Peak Instant Frequency

X/D = 0.25

o

Hz

-

^{10 Hz 2.00 Hz}

X/D = 0.50 100 Hz

-

^{250 Hz 226.25 Hz}

X/D = 0.75 50 Hz

-

^{150 Hz 126.25 Hz}

X/D _{= 1.00}

o

Hz

-

^{150 Hz 86.25 Hz}

Frequency spectrum analysis of Y/D = 0.50 line in the near field shows similar results of Y/D = 0.0 line (centerline) in the total field. So, it can be decided that a particular frequency which causes turbulence amplification or suppression in

the near field 9f a jet may cause suppression or amplification at far field of jet.

CONCLUSIONS AND RECOMMENDATIONS

60 CHAPTER VI

CONCLUSIONS AND RECOMMENDATIONS