Impact pressure

The pressure exerted on a plane at right angles to the direction of flow of the fluid consists of two components  

Considering a small filament of liquid which is brought to rest at section 2, and applying the energy balance equation between the two sections, since gAz, W, and F are all zero  

If the fluid is incompressible or if the change in the density of the fluid between the sections is negligible, then (since 112 — 0)  

Little error is introduced if this expression is applied to the flow of a compressible fluid provided that the velocity is not greater than about 60 m/s. When the velocity is high, the equation of state must be used to give the relation between the pressure and the volume of the gas. For non-isothermal flow, Pi/ = a constant, 

Equations 6.11 and 6.12 can be used for the calculation of the fluid velocity and the impact pressure in terms of the static pressure a short distance upstream. The two sections are chosen so that they are sufficiently close together for frictional losses to be negli ble. Thus P will be approximately equal to the static pressure at both sections and the equations give the relation between the static and impact pressure — and the velocity — at any point in the fluid. 

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Fig. 4. (a) Type-S pitot tube and thermocouple (b) side view of the correct pitot tube configuration when used ia conjunction with a sampling nozzle, where A is the static pressure opening plane and B is the impact pressure opening plane and (c) bottom view. 

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. 

Pitot Tubes The combination of pitot tubes in conjunction with sidewall static taps measures local or point velocities by measuring the difference between the total pressure and the static pressure. The pitot tube shown in Fig. 10-5 consists of an impact tube whose opening faces directly into the stream to measure impact pressure, plus one... 

Pressures substantially lower than true impact pressures are obtained with pitot tubes in turbulent flow of dilute polymer solutions [see Halliwell and Lewkowicz, Phys. Fluids, IS, 1617-1625 (1975)]. 

The objective in these gauges is to measure the time-resolved material (particle) velocity in a specimen subjected to shock loading. In many cases, especially at lower impact pressures, the impact shock is unstable and breaks up into two or more shocks, or partially or wholly degrades into a longer risetime stress wave as opposed to a single shock wave. Time-resolved particle velocity gauges are one means by which the actual profile of the propagating wave front can be accurately measured. 

You ean see, ba.sed on the veloeity head formula, a eavitation bubble impacts the impeller and other pump parts at about 155,069 psi. Other experiments in test laboratories using a more preeise rHv, have calculated the impact pressure at 1 Gigapascal, or 147,000 psi. This is the reason that the damage from eavitation appears like someone was beating on your impeller with a large ball pein hammer. 

Fig. 1.1. The pressure range over which shock-compression events are of interest is very broad. Quite different and distinctive behaviors are to be expected at the various pressures. The figure shows pressures produced by impact and detonation as well as physical (p), mechanical (m), and chemical (c) events at selected pressures. The indicated impact pressures are those for impactor and target materials which are the same.

Baanfschlagung, /. impact, pressure, stress admission (of steam). 

The pitot tube is used to measure the difference between the impact and static pressures in a fluid. It normally consists of two concentric tubes arranged parallel to the direction of flow the impact pressure is measured on the open end of the inner tube. The end of the outer concentric tube is sealed and a series of orifices on the curved surface give an accurate indication of the static pressure. The position of these orifices must be carefully chosen because there are two disturbances which may cause an incorrect reading of the static pressure. These are due to ... 

For the flow not to be appreciably disturbed, the diameter of the instrument must not exceed about one-fiftieth of the diameter of the pipe the standard instrument (diameter 7.94 mm) should therefore not be used in pipes of less than 0.4 m diameter. An accurate measurement of the impact pressure can be obtained using a tube of very small diameter with its open end at right angles to the direction of flow hypodermic tubing is convenient for this purpose. The static pressure is measured using a single piezometer tube or a piezometer ring upstream at a distance equal approximately to the diameter of the pipe measurement should be made at least 50 diameters from any bend or obstruction. 

The measured impact pressures and static pressures were converted to gas velocities using the following equation ... 

Determine the worst-case gas mixture combustion characteristics, system pressure, and permissible pressure drop across the arrester, to help select the most appropriate element design. Not only does element design impact pressure drop, but also the rate of blockage due to particle impact, liquid condensation, and chemical reaction (such as monomer polymerization) can make some designs impractical even if in-service and out-of-service arresters are provided in parallel. 

The total pressure P+ exerted by the gas on the disc or valve face is the sum of the static pressure and the impact pressure rise AP ... 

Wright and Humberstone (1966) presented a brief description of water impact studies on molten aluminum. The experiments were carried out in vacuo with moving water columns striking a molten aluminum surface with an impact pressure of 1.4 MPa (200 psi). The aluminum pool was about 7 mm deep. The result was a violent disruption with the generation of high pressures, the highest recorded being 20 MPa (2900 psi) when the initial aluminum was at 1220 K. 

Table 4.5 Impact pressures of average raindrops at various wind speeds...

Wind speed (km h ) Impact pressure of average raindrop of 0.5 g mass (cm head of water)... 

In the barrier test, a cylindrical sample of explosive is separated from a charge of expl known as donor by a harrier, which is a thick disk of Al, br ass or other metal. In this test, the donor creates a shock wave in the barrier, which in turn gives rise to an impact pressure at the contact surface... 

M.A. Cook et al, "Measurements of Detonation Shock and Impact Pressure , 3rd-SympDeton (i960), 357-85 9) J.L. Austing... 

A necessary but not sufficient condition for initiation by impact is that impact pressure (stress) be sufficiently high so that the melting point of the explosive is raised above some critical temperature, TCr- For T>Tcr the explosive, in the hot spot, will decompose adiabati-cally in times of the order 10/usec, which has been observed experimentally. The relation between Tcr the critical stress is then expressed... 

We can now determine the impact pressure on the walls of the vessel. On the average, a single wall of the cube will be struck by a particular molecule each time it has traveled a distance equal to the round trip across the inside of the cube, 2L. 

FIGURE 16.2 Three possible modes through which inertial cavitation may enhance SC permeability, (a) Spherical collapse near the SC surface emits shock waves, which can potentially disrupt the SC lipid bilayers, (b) Impact of an acoustic microjet on the SC surface. The microjet possessing a radius about one tenth of the maximum bubble diameter impacts the SC surface without penetrating into it. The impact pressure of the microjet may enhance SC permeability by disrupting SC lipid bilayers, (c) Microjets may physically penetrate into the SC and enhance the SC permeability. (From Mitragotri, S., and Kost J., Adv. Drug Deliv. Rev., 56, 589, 2004. With permission.)... 

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