Dynamics pressure

Although the destructive effects of the blast wave are usually related to values of the peak overpressure, there is another quantity of equivalent importance called the dynamic pressure . 

The dynamic pressure is proportional to the square of the wind velocity and to the density of the air behind the shock front. For very strong shocks the dynamic pressure is larger than the overpressure but below 480 kN/m (4.7 atmospheres) overpressure at sea level the dynamic pressure is smaller. The 

Some indication of the corresponding values of peak overpressure, peak dynamic pressure and maximum blast wind velocity for an ideal shock front in air at sea level are given in Table 9.1. 

Peak overpressure, KN/M (atoms) Peak dynamic pressure, KN/M (atoms) Maximum wind velocity, Km/h 

When the shock front reaches a given point, both the overpressure and the dynamic pressure increase almost immediately from zero to their maximum values and then decrease. The dynamic pressure (and wind velocity) will fall to zero some what later than the overpressure beeause of the momentum of the air in motion behind the shock front, but for the purposes of estimating damage the difference is not significant. During the negative (suetion) phase of the blast wave the dynamic pressure is very small and aets in the opposite direetion. 

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Solid Desiccants. The sohd desiccants used in dynamic appHcations fad into a class caded adsorbents (see Adsorption). Because they are used in large packed beds through which the gas or Hquid to be treated is passed, the adsorbents are formed into soHd shapes that adow them to withstand the static (fluid plus sohd head) and dynamic (pressure drop) forces imposed on them. The most common shapes are granules, extmded pedets, and beads. 

Strain-gauge pressure transducers are manufactured in many forms for measuring gauge, absolute, and differential pressures and vacuum. Full-scale ranges from 25.4 mm of water to 10,134 MPa are available. Strain gauges bonded direc tly to a diaphragm pressure-sensitive element usually have an extremely fast response time and are suitable for high-frequency dynamic-pressure measurements. 

Dynamic pressure may be measured by use of a pitot tube that is a simple impact tube. These tubes measure the pressure at a point where the velocity of the fluid is brought to zero. Pitot tubes must be parallel to the flow. The pitot tube is sensitive to yaw or angle attack. In general angles of attack over 10° should be avoided. In cases where the flow direction is unknown, it is recommended to use a Kiel probe. Figure 10-3 shows a Kiel probe. This probe will read accurately to an angle of about 22° with the flow. 

Chile [Prog. Aerosp. Sc7, 16, 147-223 (1975)] reviews the use of the pitot tube and allied pressure probes for impact pressure, static pressure, dynamic pressure, flow direction and local velocity, sldn friction, and flow measurements. 

In most flat-plate impact experiments, the direction of motion of the impacting plate is normal to its surface, such that only a planar compressive shock is introduced into the specimen. Within the last fifteen years, however, techniques have been developed for dynamic pressure-shear loading of specimens (Abou-Sayed et al., 1976 Chhabildas and Swegle, 1980). These involve an oblique impact, as illustrated in Fig. 3.6, in which the impact surface on the... 

Hoover, W.G., in Behavior of Dense Media Under High Dynamic Pressure (edited by Berger, J.), Gordon and Breach, New York, 1968, pp. 397-406. 

Nellis, W.J., Properties of Condensed Matter at Ultrahigh Dynamic Pressures, in High Pressure Measurement Techniques (edited by Peggs, G.N.), Applied Science, London, 1983, pp. 69-89. 

Batsanov, S.S., Inorganic Chemistry of High Dynamic Pressures, Russian Chem. Rev. 55(4), 297-315(1986). 

Graham, R.A., in 3rd International Symposium on High Dynamic Pressure (edited by Cheret), Assoc. Francaise de Pyrotechnic, Paris, 1989, pp. 175-180. 

Let s consider now a system with dynamic pressures and a constant elevation. A classic example of this would be where a pump feeds a sealed reactor vessel, or boiler. The fluid level in the reactor would be more or less static in relation to the pump. The resistances in the piping, the Hf and Hv, would be mostly static although they would go up with flow. The Hp, pressure head would change with temperature. Consider Figure 8-14. 

The use of dynamic pressure transducer in the combustor section, especially in the Low NOx Combustors ensures that each combustor can is burning evenly. This is achieved by controlling the flow in each combustor can till the spectrums obtained from each combustor can match. This technique has been used and found to be very effective and ensures smooth operation of the turbine. 

The use of dynamic pressure transducers gives early warning of problems in the compressor. The very high pressure in most of the advanced gas turbines cause these compressors to have a very narrow operating range between surge and choke. Thus, these units are very susceptible to dirt and... 

Implementation of advanced performance degradation models, necessitate the inclusion of advanced instrumentation and sensors such as pyrometers for monitoring hot section components, dynamic pressure transducers for detection of surge and other flow instabilities such as combustion especially in the new dry low NO combustors. To fully round out a condition monitoring system the use of expert systems in determining fault and life cycle of various components is a necessity. 

When designing air supply through a filter ceiling, one should ensure that the dynamic pressure in the supply air does not affect the static pressure distribution above the filter ceiling too much. 

The calculation of the pressure drop for a chosen exhaust depends on the calculation method (Chapter 9). Pressure drop is usually calculated as the product of a hood entry loss factor, and the dynamic pressure in the connecting duct, p,/. The is expressed a.s p v-/l, where p is the air density and 1/ IS the air velocity in the duct. Some common hood entry loss factors are given in Table 10.4. 

In such openings, bidirectional flow driven by temperature differences between the two rooms can occur. If a total pressure (static pressure plus dynamic pressure) is specified, this phenomenon can be accounted for. For higher accuracy in the neighborhood of this opening, it is, however, recommended to expand the calculation domain beyond this opening. 

Usually, modified Newton-Raphson methods with relaxation are applied. Additional iteration loops are necessary for the determination of the dynamic pressure losses in ducts and duct fittings. 

Dynamic pressure The pressure equivalent of a fluid velocity at a given point. 

Static The fan total pressure minus the dynamic pressure corresponding to the mean air velocity at the fan outlet. The fan static pressure is the bursting or collapsing pressure on the enclosure,... 

Velocity (or dynamic) Pressure associated with the kinetic energy in the air stream in the fan exerted in the direction of flow. 

System-powered Deriving its energy from the dynamic pressure in the air-stream to maintain its constant flow rate function and can be either a constant or variable type. 

Static head The difference between the total fluid pressure and the dynamic pressure. 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

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