Vortex shedding frequency

VoHex shedding The vortex-shedding frequency of the fluid in cross-flow over the tubes may coincide with a natural frequency of the tubes and excite large resonant vibration amplitudes. 

Pagni, P.J., Pool fire vortex shedding frequencies, Applied Mechanics Review, 1990, 43, 153-70. 

Based on the initial studies of unforced flows described above, inflow was selectively perturbed to investigate if the amount of particle dispersion and the location of enhanced dispersion within the combustor can be shifted as desired. Calculations were performed in which an acoustic perturbation was imposed from the back wall of the combustor with an amplitude of 0.5% of the initial chamber pressure and a frequency of 1380 Hz, 690 Hz, or 145 Hz, the characteristic frequencies of the system under study. The vortex-shedding frequency was 1380 Hz, the first-merging frequency was 690 Hz, and 145 Hz was the quarter-wave mode of the inlet. In addition, simulations were also performed with forcing at a frequency unrelated to the system, 1000 Hz. The particle size chosen for these simulations was 15 pm in diameter (St = 0.97), since this size particles were found to be optimally dispersed in the unforced flow case. All other parameters remain unchanged from the unforced case discussed above. 

Strouhal number St L vortex shedding frequency x characteristic flow Vortex shedding, von Karman vortex... 

Wall thickness Channel width Acoustic velocity Friction coefficient Conductance Capillary number Discharge coefficient Drag coefficient Diameter Diameter Dean number Deformation rate tensor components Elastic modulus Energy dissipation rate Eotvos number Fanning friction factor Vortex shedding frequency Force... 

The vortex shedding frequency /v for a tubebank is calculated from the Strouhal number... 

Determine the vortex shedding frequency using Eq. 17.157 and turbulent buffeting frequency using Eq. 17.158. Also compute the natural frequency/ of the tubes by using Eq. 17.159 or 17.160. 

The vortex-induced oscillation contributes mainly to the lower frequency range of the spectrum. This phenomenon becomes more notable as the wind speed increases since the vortex shedding frequency is proportional to the wind speed and it will be closer to the fundamental frequency of the East Asian Hall. However, self-excited motion is not expected since the vortex-shedding oscillation frequency is far from the resonant frequencies of the building [203]. 

On the other hand, the flow in the region downstream of the cylinder and in the wake, where vortex formation occurs has only been scetchily investigated. Measurement of the detailed flow structure for this region is extremely difficult since the diameter of the cylinder should be very small (0.1-0.5 mm). Even measurements of Strouhal number, St=fd/V were accomplished in a small number of works [4,8,9], and in very limited range of Reynolds numbers, Re=dV/v, where f is the vortex shedding frequency, d-cylinder, diameter, V-velocity of the undisturbed flow, v-kinematic viscosity. The... 

The experimental set-up (Fig. 1) was arranged in order to measure vortex shedding frequencies from very thin circular cylinders, perpendicular to the flow. A 500 liter constant-head tank was used to supply a steady flow of water or of. polymer solution. The flow was controlled by a valve at the extreme downstream end of the conduit and measured by an accurate magnetic flowmeter. 

A Thermo-Systems (TSI) Laser-Doppler Anemometer was used to measure the resulting vortex shedding frequencies. This was accomplished by measuring the velocity fluctuations in the direction of flow at some distance downstream from the cylinders. This type of measurements, being inherently optical in nature, did not disturb the flow field and was not influenced by the presence of the polymer. 

Because of the small cylinder diameters used in these experiments, high vortex shedding frequencies resulted, up to 2 Khz. Thus, in order to obtain a continuous signal of the velocity fluctuations, it was necessary to increase the number of particles in the flow. This was accomplished by mixing a small amount, 200 gr. of milk into the head tank. 

In Figures 3-5 the results are presented for three cylinder diameters 0.17, 0.25 and 0.7mm, separately, for various polymer concentrations. Each plot contains all the points resulting from several repetative runs. It should be noted that the vortex shedding frequencies were sensitive to the method of preparation of the polymer solution in the tank. 

All the plots clearly demonstrate a decrease in vortex shedding frequency resulting from the use of polymer additives as compared to the Newtonian case. This decrease became greater with increasing polymer solution concentrations or with decreasing cylinder diameters. At concentrations of 100 ppm (Fig. 6) no significant decrease in vortex shedding frequency was measured for cylinder of 2 mm in diameter, however, a small decrease was observed for a cylinder of diameter 0.7 mm. 

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