Wind velocity, critical

In the firework field, there are many cases where we must shoot burning pieces at a high speed, like the stars of a chrysanthemum or of Kyokudo. Accordingly it is important to know how the burning of such pieces bear the wind. In this book the maximum velocity of the wind, in which the pieces can continue the burning, is called "critical wind velocity". 

The above discussion is confirmed by an experiment (Table 5) Potassium nitrate composition shows a very different character from others the combustion is quite unstable, and we cannot pick up general rules from these data. We have no experimental data here concerning black powder type compositions (mixture of potassium nitrate, charcoal and sulphur) and metal powder compositions, but the burning of such compositions is quite stable and their critical wind velocities seem to be very high. 

Tabic 5. Experimental values of critical wind velocity (n>/scc)... 

Classification of firework light sources 2 Standard diameters of round flowers 3. Experimental values of critical wind velocity Detonation velocities of report compositions 5. Observed flame temperature for representative fuels... 

Most of the literature concerns sway crosswise to wind direction. This occurs in theory because of the shedding of vortex streets at so-called critical wind velocities, but observations often show the columns swaying or vibrating along with the wind or in random directions. 

It is sometimes su ested that the design be such that the critical wind velocity of the column does not coincide with the prevailing wind velocity at the plant site. However, most plant sites are not fortunate enough to have a prevailing wind velocity and there is reason to suspect that little good would be done by such design. 

Another suggestion is to make the critical wind velocity greater than design wind velocity, but this is prohibitively expensive. Such design reflects itself in large diameters and heavy wall thickness. The sway caused by so-called critical wind velocity is a real thing and has a theoretical basis — but to attempt to compute the magnitude of the sway of real columns on this basis is virtually impossible in spite of empirical coefficients developed to be used with standard wind pressure equations. 

The deflections resulting from vortex shedding are perpendicular to the direction of wind flow and occur at relatively low wind velocities. When the natural period of vibration of a stack or column coincides with the frequency of vortex shedding, the amplitude of vibration is greatly magnified. The frequency of vortex shedding is related to wind velocity and vessel diameter. The wind velocity at which the frequency of vortex shedding matches the natural period of vibration is called the critical wind velocity. 

Once a vessel has been designed statically, it is necessary to determine if the vessel is susceptible to wind-induced vibration. Historically, the rule of thumb was to do a dynamic wind check only if the vessel L/D ratio exceeded 15 and the POV was greater than 0.4 seconds. This criterion has proven to be unconservative for a number of applications. In addition, if the critical wind velocity, V,., is greater than 50 mph, then no further investigation is required. Wind speeds in excess of 50 mph always contain gusts that will disrupt uniform vortex shedding. 

V = basic wind speed, mph Vc = critical wind velocity, mph Vj,j,Vc2 = critical wind speeds for modes 1 and 2, mph or fps... 

Step I Calculate critical wind velocity, V. . No analysis need be perfonned if V. > V. 

Critical wind velocity The velocity at which the frequency of vortex shedding matches one of the normal modes of vibration. 

Fundamental Period. . Seconds Figure 6-15. Graph of critical wind velocity, Vc. 

Critical wind velocity is the wind speed in which dynamic resonance occurs, Vi. This is defined as follows ... 

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