Component Laser Doppler Velocimetry Research Articles

Laser doppler velocimetry and confined flows

Finding the mode, in which two component laser Doppler velocimetry can be applied to flows confined in cylindrical tubes or vessels, was the aim of this study. We have identified principle issues that influence the propagation of laser beams in laser Doppler velocimetry system, applied to flow confined in cylindrical tube. Among them, the most important are influences of fluid and wall refractive indices, wall thickness and internal radius ratio and beam intersection angle. In analysis of the degrees of these influences, we have applied mathematical model, based on geometrical optics. The separation of measurement volumes, that measure different velocity components, has been recognized as the main drawback. To overcome this, we propose a lens with dual focal length ? primary focal length for the measurement of one velocity component and secondary focal length for the measurement of the other velocity component. We present here the procedure for calculating the optimal value of secondary focal length, depending on experimental set-up parameters. The mathematical simulation of the application of the dual focal length lens, for chosen cases presented here, confirmed the accuracy of the proposed procedure.

Laser–doppler–velocimetry measurements in a one and a half stage transonic test turbine with different angular stator–stator positions

A one and a half stage transonic turbine was tested using a two component laser–doppler–velocimetry system. The measurements were carried out in order to record rotor phase resolved velocity, flow angle and turbulence distributions upstream and downstream of the second stator row at several different angular stator–stator positions (“clocking” positions). Altogether, the measurements downstream of the second stator were performed for ten different clocking positions and upstream of the second stator for two different clocking positions. These different clocking positions have a significant influence on the flow field upstream and downstream of the second stator. Furthermore error estimation and a discussion of the tracer particle response are discussed.

Isolated Synthetic Jet in Crossflow: Experimental Protocols for a Validation Dataset

An overview of the data acquisition, reduction, and uncertainty estimates of experimental measurements made of the flowfield created by the interaction of an isolated synthetic jet and a turbulent boundary layer is presented. The experimental measurements were undertaken to serve as the second of three computational fluid dynamics validation databases for active flow control. The validation databases were first presented at the NASA Langley Research Center Workshop on CFD Validation of Synthetic Jets and Turbulent Separation Control (CFDVAL2004) in March 2004. Detailed measurements were made to document the boundary conditions for the flow and also for the phase-averaged flowfield itself. Three component laser-Doppler velocimetry, 2-D particle image velocimetry, and stereo particle image velocimetry were used to document the phase-averaged velocity field and the turbulent stresses.

Far Field Pressure Fluctuations and Coherent Structures in a Low Mach Number Turbulent Jet Flow

Simultaneous far field pressure/in-flow velocity fluctuations were measured in a turbulent jet flow at low Ma number. Measurements were conducted using 2 component Laser Doppler Velocimetry technique and standard microphones. The velocity signals, acquired in several positions within the flow field, were conditioned on the far field pressure peaks and signatures of ensemble averaged noise emitting events were retrieved. The results reported here definitely confirm previous outcomes obtained on the same apparatus showing that intermittent structures generated within the turbulent region of the jet, i.e. around 10 diameters downstream the jet exit, give a relevant contribution to the overall far field noise.

Control of backward-facing step flow using a flapping foil

The effect of oscillating a small foil in plunge on the reattachment of a separated shear layer in a two-dimensional backward-facing step flow has been studied using flow visualization and single component laser Doppler velocimetry (LDV) measurements. It has been shown that a jet instead of a wake is generated by the flapping action of the foil. Results indicate that this action induces strong mixing and entrainment when the foil is located within the recirculation flow region, thereby reducing the reattachment length by as much as 70%. Furthermore, it has been shown that the flapping foil is most effective in reducing the size of the separation zone when placed close to the wall and to the step. It is least effective when placed outside the separated shear layer or downstream of the reattachment zone.

Turbulent Flow Through Spacer Grids in Rod Bundles

The effects of the spacer grids with mixing vanes in rod bundles on the turbulent structure were investigated experimentally. The detailed hydraulic characteristics in subchannels of a 5 × 5 rod bundle with mixing spacer grids were measured upstream and downstream of the spacer grid by using a one component LDV (Laser Doppler Velocimetry). Axial velocity and turbulent intensity, skewness factor, and flatness factor were measured. The turbulence decay behind spacer grids was obtained from measured data. The trend of turbulence decay behaves in a similar way as turbulent flow through mesh grids or screens. Pressure drop measurements were also performed to evaluate the loss coefficient for the spacer grid and the friction factor for a rod bundle.

Vortical flow structures at a helicopter rotor model measured by LDV and PIV

AbstractFlowfield measurements of the blade tip vortices from a rotating helicopter rotor model were performed by three component laser-Doppler velocimetry (3D-LDV) and conventional (two component) particle image velocimetry (PIV). In general, the results are in good correspondence, but also illustrate the different properties of both techniques: LDV offers the capability of three-component measurements, whereas PIV captures the unsteadyness of the flowfield.

Reattachment studies of an oscillating airfoil dynamic stall flowfield

The reattaching flow over an oscillating airfoil executing large amplitude sinusoidal motion around a mean angle of attack of 10 degrees has been studied using the techniques of stroboscopic schlieren, two component laser Doppler velocimetry and point diffraction interferometry, for a free stream Mach number of 0.3 and a reduced frequency of 0.05. The results show that the dynamically stalled flow reattaches in a process that begins when the airfoil is very close to the static stall angle on its downward stroke and progresses over the airfoil through a large range of angles of attack as the airfoil decreases to about 6 degrees. The airfoil suction peak shows a dramatic rise as the static stall angle is approached and the velocity profiles develop such that the flow near the surface is accelerated. The process completes through the disappearance of a separation bubble that forms over the airfoil.

Flow Characterization in Particle‐Loaden Coal Burner Systems

AbstractIn pulverized coal combustion, one of the most important parameters for pollutant formation in the near‐burner zone is the flow and mixing process of fuel and oxygen. To study the near‐field aerodynamics of pulverized coal burners, non‐intrusive optical measurements with Laser Doppler Velocimetry (LDV) were performed. In order to provide a better understanding of the mixing process of the small coal particles with the surrounding combustion air, the unavoidable formation of ropes and the dispersion of the particles was first investigated in an idealized two‐phase flow with glass beads under isothermal conditions.Furthermore, non‐intrusive measurements of velocity and turbulence intensities in the complex flow situation of radiating pulverized coal flames have been performed using a two component LDV system with a 15 W Argon‐ion laser, operating in backscatter mode. It has been used for flow characterization in a semi‐industrial pulverized coal combustion facility with a maximal thermal load of 500 kW. LDV is an indispensable research instrument to study the interdependencies between fluid flow, combustion reactions and pollutant formation processes in reactive flows. Additionally, results of numerical simulation obtained with a pulverized coal combustion code are validated with the LDV measurements and the experimentally observed phenomena are further explained. The influence of the flow pattern in the near‐burner region on pollutant emission will be discussed.