Question

b) A three-dimentional flow velocity can be measured by using various method. Please name two devices that use a probe and two sophisticated devices without a probe usage for the three-dimentional flow velocity measurement.

Answer 1

1)

The instrument is used for the accurate determination of the current velocity in water ways, channels, rivers and the sea. The meter can also be applied in polluted water currents. The measurements are executed with the propeller mounted on the rod(s) or connected to a cable. The current velocity meter has a measuring range of 0.025 to 10 m/sec.

Benefits of Current meter

Universal current meter with highest accuracy

Propeller covers most velocities

Extremely low friction assures high accuracy even in slow running water

2)

Hot Wire Anemometer

Definition: The Hot Wire Anemometer is a device used for measuring the velocity and direction of the fluid. This can be done by measuring the heat loss of the wire which is placed in the fluid stream. The wire is heated by electrical current.

The hot wire when placed in the stream of the fluid, in that case, the heat is transferred from wire to fluid, and hence the temperature of wire reduces. The resistance of wire measures the flow rate of the fluid.

The hot wire anemometer is used as a research tool in fluid mechanics. It works on the principle of transfer of heat from high temperature to low temperature.

3)

The use of the multihole pressure probes has become common to determine total and static pressures, flow velocity, and flow directions in three-dimensional flow fields with suitable calibrations. Multihole pressure probes make accurate and simultaneous measurement of total and static pressures and flow direction when pressure and velocity gradients are small. Pressure probes have some advantages over other methods as their maintenance, relatively low cost, and simplicity in operation. Hence these are preferred in research and industrial purposes. In principle, any aerodynamic body such as cylinder, sphere, wedge, or prism, with a number of holes can be used to measure three-dimensional flows. A minimum of four holes on an aerodynamic body is required to measure the four unknowns, namely, total and static pressures and two angles in mutually perpendicular planes, in three-dimensional flows. However for the sake of symmetry and extended range of measurement capability, five-hole and seven-hole probes are preferred. Because of their simplicity in operation and low cost, extensive investigations are carried out on multihole probes, particularly on five-hole probes, on their calibration and data reduction methods and their application to complex three-dimensional flow measurements.

4) Flow Velocity Measurement Using a Spatial Averaging Method with Two-Dimensional Flexural Ultrasonic Array Technology

5) Force reaction probes

These probes respond to the force exerted onto them by the flowing medium, which is in principle a pressure. Presumably, the best-known mechanical anemometer is the vane type used in weather stations in order to determine wind speed. It usually consists of a few hemispheres or cups attached to radial spokes. The rotation speed can be measured by a number of different mechanisms. Often a magnet, affixed to the shaft, traversing past a fixed coil induces a pulse for each revolution, or a digital shaft encoder is used.
6) Another category of velocity probes makes use of directly measuring pressures, thus avoiding any moving mechanical part. The principle of operation of all these tubes is based on Bernoulli’s law p+ρ2v2=p0,where p0 denotes the total pressure, which is a constant, p the static pressure, and ρ and v the fluids density and velocity, respectively. In a stationary incompressible flow, the sum of the dynamic pressure ρ2v2and palways results in the pressure within the resting fluid, which is that of the ambient atmosphere, plus the hydrostatic contribution ρgh of the fluid. Tube anemometers comprise basi-cally a bend with one end directed in such a way that it faces the flow. As the kinetic energy is converted into potential one at the stagnation point, all tubes measure at least the total pressure p0. Once the static pressure is known, the simpler Pitot tube allows the determination of the velocity according to Bernoulli’s law. The static pressure pcan only be determined accurately by measuring it in a manner such that the velocity pressure has no influence on the measurement at all. This is achieved by measuring it at right angle to the streamlines. The Prandtl tube sketched in Fig. 2 is an example of this, where pis determined through several static taps arranged circumferentially in the outer tube. A differential manometer thus allows for a direct measurement of the fluid velocity.

7) More promising way to measure local velocities is offered by the ultra-sound Doppler method, often called ultrasound Doppler velocimetry (UDV)or ultrasonic velocity profile (UVP) monitor. The origin of this techniquecan be retraced to the medical branch it has also been established in physics and fluids engineeringThe measuring principle is based on the pulsed echo technique. Ultrasound pulses of a few cycles are emitted from the transducer and travel along themeasuring line. If such a pulse hits microparticles suspended in the liquid, apart of the ultrasonic energy is scattered. It can be received using a secondtransducer or by the same transducer working in the listening mode betweentwo emissions. In the majority of cases the second variant is realized. The entire information of the velocity profile along the ultrasonic beam is contained in the echo. If the sound velocity of the liquid is known, the spatial position along the measuring line can be determined from the detected time delay between the burst emission and its reception. The movement of an ensemble of scattering particles inside the measuring volume will result in a small time shift of the signal structure between two consecutive bursts. The velocity is obtained from a correlation analysis between consecutive bursts. The measuring principle is sketched in Fig. 6. Owing to the Nyquist theorem, the product of measurable maximum velocity and penetration depth is limited by the sound velocity and the ultrasonic frequency.

Hope it will help you.. please please give thumbs up if you are satisfied with the answer...all the very best...thanks