Fig.C-4 Pitot probe for fluid velocity measurement. In a tube with its inlet oriented against the oncoming flow pressure rises above the ambient level. This impact overpressure corresponds to decrease in kinetic energy (brought down to zero because flow into the tube is ihibited). Since there is a direct and relatively simple relationship between the velocity stopped down to zero at the tube tip and the resultant impact overpressure, the tube might be used in connection with a manometer to measure local flow velocity. |
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![]() | Fig.C-5 Although in the present text we work with one-dimensional approach which does not permit taking into account spatial orientations - and in our basic equation the essential quatities are components of energy, which are scalar quantities - it should not be forgotten that velocity has in fact vector character. This means that although velocity enters the present equation in the form of the kinetic energy scalar, it possesses direction as an important attribute. In the inlet according to A, parallel to velocity direction only the pressure component of fluid energy will act - while in the perpendicular orientation B there will be also full kinetic energy action. |
Fig.C-6 In the caseA the pipe, in spite of its mouth parallel to flow direction, will not tap exact value of the local pressure. This is because flow past the end of a pipe generates complex vortex motions. A help might be souht in a disk according to B, which forces the flow to become parallel at the tap hole - as long, of course, as the velocity vector direction is known ant the disk is accurately position in line with it. If the vector direction is not precisely known or if it is not easy to align the probe with it, the is advantageous to use the probe C with pressure taps around the cylindrical surface. | ![]() |
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Vaclav TESAR : "BASIC FLUID MECHANICS"