Dynamic wind

For a building with a flat roof (pitch less than 10°) it is normally assumed that reflection does not occur when the blast wave travels horizontally. Consequently, the roof will experience the side-on overpressure combined with the dynamic wind pressure, the same as the side walls. The dynamic wind force on the roof acts in the opposite direction to the overpressure (upward). Also, consideration should be given to variation of the blast wave with distance and time as it travels across a roof element. The resulting roof loading, as shown in Figure 3.8, depends on the ratio of blast wave length to the span of the roof element and on its orientation relative to the direction of the blast wave. The effective peak overpressure for the roof elements are calculated using Equation 3.11 similar to the side wall. 

The basic dynamical wind quantities are M and v. What does the theory predict with respect to their dependence on the stellar parameters This has been investigated in the pioneering paper by Castor, Abbott, Klein (1975, "CAK"), who for the first time formulated the theory of radiation driven winds in a selfconsistent way. Besides some crucial simplifying assumptions (see below) the theory in its later version (Abbott, 1982) used a realistic line list of 250000 lines of H to Zn in ionization stages I to VI. The prediction of the theory were ... 

Table 1 and show for given stellar parameters the observed dynamical wind quantities M and v are well reproduced by the theory. This includes even such extreme objects like P Cygni, which has an extremely high mass-loss rate and a very slow wind. The dynamically improved theory of radiation driven winds therefore appears to be highly reliable. 

Dynamic Wind Stresses in Hyperbolic Cooling Towers Abu-Sitta, Salman H. Hashish, Mahmoud G. 

Dynamic Wind It has been observed by many researchers that when a cantilevered cylinder is subjected to a steady wind flow, there is a certain velocity at which the cylinder begins to oscillate in the direction transverse to the wind axis. This phenomenon has been attributed to the effect of vortex shedding. These vortices, commonly referred to as the Von Karman effect, are eddies of wind such as you d see in water when rowing a boat. (See Figure 48-6 and 48-7). 

Figure 48-6 The effect of vortex shedding on a stack subjected to a steady wind is oscillation of the cantilevered cylinder in a direction transverse to that of the wind (left). Theory says that vortices are shed intermittently from each side of the stack, causing the motion. Studies of such dynamic wind effects show only the first vibration mode to be significant...

Take dynamic wind pressure as 1280N/m, corresponding to 160 kph (100 mph) ... 

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. 

Staley, C. M., and Graven, G. G., The Static and Dynamic Wind Design of Steel Stacks, ASME technical paper 72-PET-30, 1972. 

Atmospheric features which are smaller than the mesoscale have pressure fields in which wind acceleration is a significant component (which is referred to as the dynamic wind). The pressure gradient which causes this dynamic wind is called the nonhydiostatic pressure. 

Computer program Dynamic wind pressure on cooling tower (NAZAM-4)... 

C Prepared by Liu, checked by Y. Bangash C THE SUBPROGRAM FINDS THE DYNAMIC WIND PRESSURE AT ... 

Davenport,A.G. and King,J.P.C. "Dynamic Wind Forces of Long-Span... 

Detonations in solid material are characterized by a sharp rise in pressure which expands from the centre of the detonation as a pressure wave impulse at or above the speed of sound in the transmission media. It is followed by a much lower amplitude negative pressure impulse, which is usually ignored in the design, and is accompanied by a dynamic wind caused by air behind the pressure wave moving in the direction of the wave. 

The forces on a structure associated with a blast wave resulting from an external detonation are dependent upon the peak values and the pressure-time variation of the incident and dynamic wind pressure action, including characteristics of the reflected blast wave caused by interaction with the structure.28... 

The effective loads on structures due to blast and associated dynamic wind loads are a function not only of the dynamic characteristics of the load but also the dynamic response characteristics of the structure, which should be... 

In the evaluation of blast damage to structures, a distinction should be made between local and global response of structures. Local response would be associated with response of wall elements relative to their supporting members (girt, purlin, beam and column). For local structural elements the blast and dynamic wind loads are typically associated with only their load on the local structure. 

II-5. When the blast wave impulse encounters an obstruction it results in a reflected wave typically two to four times the magnitude of the side-on peak pressure, but of shorter duration, impinging on obstructions perpendicular to the free field or side-on blast wave s direction of travel. As the positive blast wave traverses a building structure, in addition to the reflected pressure on the windward side, it exerts a positive pressure on all walls and the roof of the structure as it passes. Dynamic winds following the blast wave exert a positive pressure (inward) on the windward wall and negative pressures on the side and leeward walls and roof. 

FIG. II-4. Additional side-on blast parameters for TNT U shock front velocity (m/s) u particle velocity behind the shock wave (m/s) Q dynamic wind pressure (Pa) b decay constant. 

No.4, July 1999, p.249-65 NEW DYNAMIC WIND LOAD CYCLE TO EVALUATE MECHANICALLY ATTACHED FLEXIBLE MEMBRANE ROOFS... 

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