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.
Local pressure must be fed into the other manometer inlet. In principle its separation in a parallel pipe face is simple (Figs. C-4, C-5), but practical realisation is often associated with complicating problems, as noted in the text associated with Fig.C-6 below. Special pressure probes were developed, of which the Prandtl probe (case C in Fig.C-6) is probably the most common. It may be combined with the Pitot tube into the single velocity proble shown in Fig.C-7.
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.

Note: Evren though there is zero flow through the detour branch leading downwards in the case A in Fig.C-5 (which represents the basic principle of measuring pressure by a pressure tap drilled in in a wall), it should be said that some motions inside this branch will be, after all, present. It is driven by the tangential stress acting on the interface. This motion will result in pressure measurement error. The error increases with the dimensions of the presure tap inlet. This is why the practical pressure taps in walls (such as shown e.g. in Fig.C-4 above or in the probe wall in Fig.C-6) are made of as small diameter as possible - usually of the order of tenths of a millimetre.

New principles and methods of velocity measurement became recently abundant - such as e.g. laser measurements - the main advantage of which is direct availability of their output signal in electric form, suitable for computer data logging or processing. Laser methods also bring the advantage of being non-invasive: the investigated flow is not influenced by the presence of the probe. Nevertheless, piezzometric probes are and will be important - because of their advantages: simplicity, low cost and reliability, and - in basic arrangement - easy manufacture.They are usually suitable for measuring steady or slowly varying flows. Although there are variants with miniature pressure transducers built directly into the probe, capable of fast response, to reach really high frequency (to study details of turbulent fluctuations), rquires different methods. There is an extensive literature already available about piezzometric methods and this should be consulted for further details. Let us just note that among the numerous variants of Pitot probes an important position occupy those which employ the - otherwise rather unwellcome - directional sensitivity for measuring flow direction, and others with several pressure taps which can resolve the individual velocity components.
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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This is page Nr. C04 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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