The methods of measuring the velocity of liquids or gases can be classified into three main groups: kinematic, dynamic and physical.
In kinematic measurements, a specific volume, usually very small, is somehow marked in the fluid stream and the motion of this volume (mark) is registrated by appropriate instruments. Dynamic methods make use of the interaction between the flow and a measuring probe or between the flow and electric or magnetic fields. The interaction can be hydrodynamic, thermodynamic or magnetohydrodynamic.
For physical measurements, various natural or artificially organized physical processes in the flow area under study, whose characteristics depend on velocity, are monitored.
The main advantage of kinematic methods of velocity measurements is their perfect character, and also their high space resolution. By these methods, we can find either the time the marked volume covers a given path, or the path length covered by it over a given time interval. The mark can differ from the surrounding fluid flow in temperature, density, charge, degree of ionization, luminous emittance, index of refraction, radioactivity, etc.
The marks can be created by impurities introduced into the fluid flow in small portions at regular intervals. The mark must follow the motion of the surrounding medium accurately. The motion of marks is distinguished by the method of their registration, into nonoptical and optical kinematic methods. In the probe (nonoptical) method which traces thermal nonuniformities, a probe consisting of three filamants located in parallel plates (see Amemometers, Thermal) is used. The thermal trace is registered by two receiving wires located a distance 1 from the central wire. By registering the time Δt between the pulse heat emitted from the central wire and the thermal response of the receiving wire, we can determine the velocity u = l/Δt. Depending on which receiving wire receives a thermal pulse, we can define the direction of flow.
Marks consisting of regions of increased ion content are also widely used. To create ion marks, a spark or a corona discharge or an optical breakdown under the action of high-pulse laser radiation is used. In tracing by radioactive isotopes, the marks are created by injecting radioactive substances into the fluid flow; the times of passing selected locations by the marks are registered with the help of ionizing-radiation detectors.
Optical kinematic methods use cine and still photography to follow the motion of marks. Three main types of photography are used: cine photography, still photography with stroboscopic lighting and phototracing. In cine photography, to determine the velocity, successive frames are aligned and the distance between the corresponding positions of the mark is measured. In the stroboscopic visualization method, several positions of the mark are registered on a single frame (a discontinuous track), which correspond to its motion between successive light pulses. Two components of the instantaneous velocity vector are determined by the distance between the particle positions. Typical of the marks used are 3-5 mm aluminum powder particles or small bubbles of gas generated electrolytically in the circuit of the experimental plant. Of vital importance in this method is the accuracy of measurement of the time intervals between the flashes.
In the phototracing method, the motion of the mark is recorded by projecting the image of the mark through a diaphragm (in the form of a thin slit oriented along the fluid flow) onto a film located on a drum rotating at a certain speed. The mark image leaves a trace on the film whose trajectory is determined by adding the two vectors: the vector of mark motion and the vector of film motion. The slope angle of a tangent to this trajectory is proportional to the velocity of mark motion. Further information on the photographic technique is given in the article on Tracer Methods.
Laser Doppler anemometers can also be classified as kinematic techniques (see Anemometers, Laser Doppler).
Among the dynamic methods the most generally employed are, because of the simplicity of the corresponding instruments, the methods based on hydrodynamic interaction between the primary converter and the fluid flow. The Pitot tube is used most often (see Pitot Tube) whose function is based on the velocity dependence of the stagnation pressure ahead of a blunt body placed in the flow.
The operating principle of fibre-optic velocity converters is based on the deflection of a sensing element, in the simplest case, made in the form of a cantilever beam of diameter D and length L and placed in the fluid flow between the receiving and sending light pipe, depends on the velocity of fluid flowing around it. The change in the amount of light supplied to a receiving light-pipe is measured by a photodetector.
The upper limit of the range of velocities measured umax is limited by the value of Re = umax D/ν 50 and the frequency response is limited by the natural frequency f0 which depends on the material, diameter and length of the sensing element. However, by varying L and D, we can change the velocities over a wide range. Depending on the fluid in which measurements of umax are made, the dimensions of the sensing elements vary within the limits of 5 D 50 μm, 0.25 L 2.5 mm.
The tachometric methods use the kinetic energy of flow. Typical anemometers using this principle consist of a hydrometric current meter with several semi-spherical cups or an impeller with blades situated at an angle of attack to the direction of flow (see Anemometer, Vane).
The physical methods of velocity measurements are, as a rule, indirect. This category includes sputter-con methods, which use the dependence of the parameters of an electric discharge on velocity; ionization methods which depend on a field of concentrated ions, produced by a radioactive isotope in the moving medium on the fluid flow velocity; the electrodiffusion method which uses the influence of flow on electrode-diffusion processes; the hot-wire or hot-film anemometer; magnet to acoustic methods.
The hot-wire method is derived from the dependence of convective heat transfer of the sensing element on the velocity of the incoming flow of medium under study (see Hot-wire and Hot-film Anemometer). Its main advantage is that the primary converter has a high frequency response, which allows us to use it for measuring turbulent characteristics of the flow.
The electrodiffusion method of investigation of velocity fields is based on measuring the current of ions diffusing towards the cathode and discharging on it. The dissolved substances in the electrolyte must ensure the electrochemical reaction occurring on electrodes. Two types of electrolytes are most often used: ferrocyanidic, consisting of the solution of potassium ferri and ferrocyanide K3Fe(CN)6, K4Fe(CN)6, respectively, with concentration 10−3 − 5 × 102 mole/1) and of caustic sodium NaOH (with concentration 0.5-2 mole/1) in water; triodine, consisting the iodide solution I2 (10−4 − 10−2 mole/1) and potassium iodide KI (0.1-0.5 mole/1) in water. Platinum is used as the cathode in such systems. In velocity measurement, a sensor which is made of a glass capillary tube 30-40 μm in diameter with a platinum wire (d = 15-20 μm) soldered into it is used. The sensing element (the cathode) is the wire end facing the flow, and the device casing is the anode. The dependence between the current in the circuit and the velocity is described by the relation I = A + B , where A and B are transducer constants defined in calibration tests.
The magnetohydrodynamic methods are based on the effects of dynamic interaction between the moving ionized gas or electrolyte and the magnetic field. The conducting medium, moving in a transverse magnetic field, produces an electric force E between the two probes placed at a distance L in the fluid flow, proportional to the magnetic field intensity H and to the flow velocity u: E = μ . The disadvantage of the method is that it can only be used to measure a velocity averaged over the flow section, nevertheless it has found use in investigating hot and rarefied plasma media.
Among direct methods the most abundant are the acoustic, radiolocation and optical methods. In using acoustic methods for determining the velocity of the medium, we can measure either the scattering of a cluster of ultrasound waves by the fluid flow perpendicular to the cluster axis, or the Doppler shift of the frequency of ultrasound scattered by the moving medium, or the time of travel of acoustic oscillations through a moving medium. These methods have found application in studying the flows in the atmosphere and in the ocean, where the requirements for the locality of measurement are less stringent than in laboratory model experiments. To carry out precision experiments with high space and time resolution, optical methods are used—the most refined method used is laser Doppler anemometry. (see Anemometers, Laser Doppler). Laser Doppler anemometry depends on scattering from small particles in the flow and can also be considered a kinematic method (see above).
For measuring the mean-mass velocity of flows, differential pressure flowmeters, rotameters, volumetric turbine, vortices, magnetic induction, thermal, optical and other flowmeters are used in which we can define the mass velocity = = (see Flowmetering) as the flow rate measured of the substance and by the known section of the flow S.
Fluid Meters. (1971) Their Theory and Application. Report of ASME, Research Committee on Fluid Meters, New York, Publishers by ASME.