Localized fluid flow measurements with an He-Ne laser spectrometer

Two-point measurements of the streamwise velocity in a turbulent channel flow are performed using laser-Doppler anemometry. High spatial and temporal resolutions (about 1 Kolmogorov microscale in space and time) are achieved. Data are obtained at several distances from the wall forRe

 δ
 in the range 1500–5000. Results from correlation functions are compared with the hypotheses of Taylor and Tennekes: they reproduce the experimental data even at low Reynolds numbers and small distances from the wall, providing that convection rather than mean velocity is used. Convection velocities are computed from transfer function phase diagrams: the ratio of convection to mean flow velocity is found to decrease with increasing Reynolds number and distance from the wall. Large flow structures are convected with the local mean velocity rather than the test section mean velocity; data at small Reynolds and close to the wall exhibit convection velocities lying between the two. The good agreement between the time evolution of the envelope of space-time-correlation function with the corresponding Lagrangian correlation over one integral time scale confirms the existence of a strict relation between the Eulerian and Lagrangian descriptions of turbulence.

Analysis of two-point velocity measurements in near-wall flows

A new configuration for the transmitting optics of a laser Doppler anemometer has been developed in order to measure the velocity at two different points at the same time. From the simultaneous measurements at two points along the mean flow direction it is possible to evaluate the spatial correlations and to compare them with the temporal correlation to verify the validity limits of Taylor's hypothesis also known as the frozen turbulence hypothesis. The transfer function between the velocity signals at two different points has been introduced to better explain the differences between Taylor's hypothesis and non frozen flow. The analysis is carried out in a flow with high turbulence levels.

Experimental testing of Taylor's hypothesis by L.D.A. in highly turbulent flow

Lorentz force velocimetry (LFV) is a contactless technique for the measurement of liquid metal flowrates. It consists of measuring the force acting upon a magnetic system and arising from the interaction between an external magnetic field and the flow of an electrically conducting fluid. In this study, a new design is proposed so as to make the measurement independent of the fluid’s electrical conductivity. It is made of one or two coils placed around a circular pipe. The forces produced on each coil are recorded in time as the liquid metal flows through the pipe. It is highlighted that the auto- or cross-correlation of these forces can be used to determine the flowrate. The reliability of the flowmeter is first investigated with a synthetic velocity profile associated with a single vortex ring, which is convected at a constant speed. This configuration is similar to the movement of a solid rod and enables a simple analysis of the flowmeter. Then, the flowmeter is applied to a realistic three-dimensional turbulent flow. In both cases, the influence of the coil radii, coil separation, and sign of the coil-carrying currents is systematically assessed. The study is entirely numerical and uses a second-order finite volume method. Two sets of simulations are performed. First, the equations of motion are solved without accounting for the effect of the magnetic field on the flow (kinematic simulations). Second, the Lorentz force is explicitly added to the momentum balance (dynamic simulations), and the influence of the external magnetic field on the flow is then quantified.

Lorentz force velocimetry based on time-of-flight measurements

Time-of-flight Lorentz force velocimetry is a non-invasive electromagnetic measurement technique that can be used to determine both the flow rate and/or the local velocities in electrically conducting fluids like liquid metals. Using this technique, two identical so-called Lorentz force flow meters—each consisting of a permanent magnet system and an attached digital force sensor—are arranged in a row and separated by a defined distance. Each flow meter measures the Lorentz force that is generated within the melt when the electrically conducting liquid metal passes the magnetic field. This time-of-flight technique can be exploited for the flow measurement by purely cross-correlating the two force signals. Hence, the measurement becomes independent of any fluid properties and magnetic field parameters. We present results of two model experiments that demonstrate that time-of-flight Lorentz force velocimetry is feasible for non-contact measurement of both global flow rates and local surface velocity in turbulent liquid metal flow. In these experiments, we use the low-melting eutectic alloy Ga68In20Sn12 as a test melt. Moreover, to support these experimental findings, we present results of numerical simulations using the commercial codes FLUENT and MAXWELL. The numerical predictions are in good agreement with the experimental findings.

https://iopscience.iop.org/article/10.1088/0957-0233/23/7/074021/pdf

Electromagnetic flow measurements in liquid metals using time-of-flight Lorentz force velocimetry

Two-point velocity correlations, mean velocities and turbulence intensities were measured for turbulent flow of water in a 2.5 cm diameter pipe for Reynolds numbers ranging from slightly above transition to 18100. By using two laser anemometers it was possible to make direct measurements of both longitudinal and lateral correlation coefficient functions for the axial velocity component. All two-point correlations were measured with one point fixed at the pipe centerline while the other point was displaced both radially and axially with respect to the fixed point. From the measured correlation coefficient functions the longitudinal and lateral micro and integral scales were determined for the range of Reynolds numbers investigated. The results of this investigation indicate that the laser anemometer is a very good means of measuring correlation functions and related properties of turbulence.

https://iopscience.iop.org/article/10.1088/0022-3735/4/11/004/pdf

W H Clark

Papers

Measurements of two-point velocity correlations in a pipe flow using laser anemometers