Laser Doppler Anemometer Measurements Research Articles

Laser-Doppler anemometer measurements of turbulent structure in non-Newtonian fluids

A laser-Doppler anemometer was used to measure the radial variation of axial turbulent velocity in the flow of water and aqueous polymer solution through a round pipe. Results were obtained for the mean velocity the turbulent intensity, and the one-dimensional energy spectrum. The polymers used were polyacrylamide (Separan AP 273; in a concentration range 25-200 WPPM). Both polymers are well known as effective additives to reduce turbulent drag, when dissolved in water to give a solution concentration of a few PPM. The mean velocity and turbulent intensity were both found to be increased by the polymer additives, with the magnitude of the effect depending on the polymer concentration This was in agreement with the results of other investigations. At low additive concentrations, the energy spectra were found to be identical to those for water flows; this also was in agreement with previous results. At higher concentrations, energy spectra showed noticeable attenuation in the largest (dissipation) range of wavenumbers. There was a corresponding enhancement of the small-wavenumber (energy-containing) range. This seemed to be a threshold effect with onset at a concentration of between 50 PPM and 100 PPM for Polyox WSR 301 and between 25 PPM and 50 PPM for Separan AP 273.

Modeling and Experimental Determination of the Axisymmetric Flow in a Diesel Engine Cylinder

Internal air motion in a Detroit Diesel 6V-92 two-stroke engine cylinder was determined by an idealized axisymmetric finite difference model under steady flow conditions. This model was used to compute axial and tangential velocities with entrance angles of 25 and 40 deg. Laser Doppler anemometer measurements were made in one plane for comparison under steady flow bench test conditions using actual engine hardware. Calculations and measurements showed that the swirl induced by the angled inlet flow was responsible for the nonuniform axial velocity profiles. These profiles exhibited large peaks in the midradius region (between cylinder center and wall). Increasing the inlet angle from 25 to 40 deg sharpened the axial velocity peak and shifted it toward the cylinder wall. The measurements indicated nearly isotropic turbulence across the measurement plane. A better understanding of the relationship between design and internal flow is expected to lead to improved combustion, scavenging tradeoffs.

Design Considerations for the Omniscience Pivoting Disc Cardiac Valve Prosthesis

Parametric studies are conducted in an effort to determine the optimum curvature and eccentricity for concave-convex pivoting disc prosthetic heart valves. Steady-state pressure measurements with prototype valves and laser-Doppler anemometer measurements of the velocity field with a two-dimensional model are presented. These results suggest that an optimum curvature exists and that the minimum eccentricity that promotes full opening should be used. Estimates of the shear stress distribution in the wake of the valve are computed. The maximum shear stress for both the major and minor outflow areas appear to be: 1) fairly evenly distributed, 2) well below the level for damage to the formed elements of blood as they pass through the valve, and 3) above the level required for the formation of a stable thrombus.

The dynamics of confined vortices

Referring to flow-visualization and laser-Doppler anemometer measurements of swirl and axial velocity profiles, we discuss the physics of the flow in a cylindrical vortex tube as various independent parameters are varied. Three main classes of flow occur, depending upon the location of a vortex jump within the vortex tube. We present evidence to suggest a connection between vortex breakdown and the criticality and stability of the vortex core upon which it occurs and attempt to reconcile the various explanations that have been proposed for the breakdown phenomenon. Similarities and differences between the present experiments and those of previous investigators are also pointed out. Finally, as an Appendix, we present the results of a hydraulic analogue of our vortex experiment.

High-Resolution LDA Measurements of Reynolds Stress in Boundary Layers and Wakes

The turbulent character of the boundary layer and wake associated with an airfoil has been studied at a Reynolds number of 10(exp 6) and a Mach number of 0.1. To accomplish these measurements, a unique laser Doppler anemometer (LDA) has been developed that is capable of sensing two velocity components from a remote distance of 2.13 m. Using special simultaneity logic and counter-type signal processors, the geometrical features of the LDA have been exploited to provide variable spatial resolution as low as 0.2 mm. By combining the LDA with an on-line computerized data acquisition and display system, it has been possible to measure mean velocity and Reynolds stress tensor distribution at several locations along the upper surface of a 0.9 m chord, flapped airfoil installed in the Ames 7- by 10-Foot Wind Tunnel.

Steady flow in a model of the human carotid bifurcation. Part II—Laser-Doppler anemometer measurements

The evidence for hypothesizing a relationship between hemodynamics and atherogenesis as well as the motivation for selecting the carotid bifurcation for extensive fluid dynamic studies has been discussed in Part I of this two-paper sequence. Part II deals with velocity measurements within the bifurcation model described by Fig. 1 and Table 1 of the previous paper. A plexiglass model conforming to the dimensions of the average carotid bifurcation was machined and employed for velocity measurements with a laser-Doppler anemometer (LDA). The objective of this phase of the study was to obtain quantitative information on the velocity field and to estimate levels and directions of wall shear stress in the region of the bifurcation.

Reflection suppression by polarization in backscatter LDA measurements near walls and in two-phase flows

A method is proposed and verified by experiments to suppress disturbing reflections in laser Dodppler anemometer (LDA) measurements. Disturbing reflections occur in a variety of practical applications of LDA, such as backscatter measurements near walls or in two-phase fluid or particle velocity measurements. The method uses circularly polarized light that changes the handedness when reflected and is quenched by a suitably oriented polarization analyzer. Scattered light, however, is partially transmitted by the analyzer to give undisturbed seeding particle signals. Verification experiments have been performed to demonstrate the method; systems for practical applications are proposed.

The application of computer graphics in the interpretation of laser Doppler and hot-wire anemometer measurements

The availability of dedicated minicomputers in fluid engineering laboratories has led to a rapid development of graphical techniques as an aid to the analysis of a flow in the time domain. This paper describes two experiments in which flow measurements made with a hot-wire and laser Doppler anemometer (LDA) were conditionally sampled and displayed as a time sequence on a VDU. In the case of the hot-wire anemometer a cine film was made by animating the display of both the velocity and vorticity fields.

Mean Flow Measurements Near the Plane of an Open Rotor Operating with an Inlet Velocity Gradient

As part of a study on the structure of a trailing vortex, laser doppler anemometer (LDA) measurements were made of the flow field near an open rotor having an inlet velocity gradient. The measurements were made in the 1.22 m dia water tunnel of the Applied Research Laboratory at The Pennsylvania State University. Velocity data were obtained for rotor inlet and outlet flow fields for several different inlet velocity gradients. Velocity data were also obtained downstream of the rotor plane that shows the vortex structure. Flow field measurements show the development of the downstream vortex motion. Small variations in the inlet velocity gradient near the rotor wall caused large differences in the structure of the trailing vortex. In addition, a measured downstream velocity profile is compared with a calculated velocity profile.

Unsteady laminar separation: an experimental study

Abstract : The design of most aerodynamic surfaces, as for example the helicopter rotor, is based essentially on quasi-steady theories. However the dynamics of a rotating blade introduce unexpected fluctuations and overshoots of properties like lift, drag, etc. The phenomenon of unsteady stall is intimately connected with the development of an oscillating boundary layer and separation. Experimental investigation of such flows was undertaken by a method of visualization developed especially for the study of laminar or turbulent boundary layers and separation. The method captures the instantaneous 2-D flow field, including regions of separated flow and provides accurate quantitative information. Laser doppler anemometer measurements complement the optically received data. Results reveal that separation responds with time-lag to external disturbances, in agreement with unsteady stall data. Oscillating outer flows result in displacement of the point of separation and under certain conditions, the Despard and Miller criterion was found to hold. Earlier theoretical models of separation are confirmed qualitatively and for the early stages of the transient phenomena. The findings provide physical insight and quantitative data that may help understand the phenomenon of unsteady stall and unsteady separation. (Author)

Spectral characteristics of vortex breakdown flowfields

Laser-Doppler anemometer measurements upstream and in the wakes of vortex breakdowns of bubble and spiral types are described. Spectral analysis of the data indicates prominent oscillations in the wakes at less than 10 Hz. These oscillations correspond closely to theoretical predictions of the linearly most unstable normal modes of the time-averaged mean flow profiles. The oscillations are more energetic, and vortex core expansions are greater for flows with a bubble form of vortex breakdown, which is therefore regarded as the stronger form of breakdown.

Disrupted states of vortex flow and vortex breakdown

Flow visualization studies and laser Doppler anemometer measurements on swirling water flows reveal six distinct types of very large amplitude disturbance modes of the vortex core. Three, ’’axisymmetric’’ and spiral vortex breakdowns, and the ’’double helix,’’ have been described by others. A definite order of evolution in parameter space (Reynolds number and circulations) occurs, and is described. Puzzling responses of the axisymmetric and spiral vortex breakdowns to imposed flow transients reported previously are confirmed here, and are traced to the shedding of starting and stopping vortices from swirl vanes. Conclusions bearing upon the validity of some theories of vortex breakdown are possible from the data.

Laser-Doppler anemometer measurements in drag-reducing channel flows

The objective of this study was to make velocity measurements in drag-reducing flows which would be sufficient in scope and accuracy to test proposed models of drag-reducing flows and to yield new information about the mechanisms of drag reduction. Consequently, measurements of the mean and turbulence intensity of the streamwise velocity component were made in fully developed, turbulent, drag-reducing flow in a two-dimensional channel with a laser-Doppler anemometer. The anemometer was operated in the individual-realization mode and corrections were made to eliminate statistical biasing of the data. Two polyacrylamides and a polyethylene oxide were used to produce seven flows which had drag reductions ranging from 24 to 41 %. Measurements were also made in water to establish the standard characteristics of the flow channel.The data show that the drag-reducing mean velocity profile can be divided into three zones: a viscous sublayer, a buffer or interactive region and a logarithmic region. There is no evidence that the viscous sublayers of the drag-reducing channel flows are thicker than those in the solvent flows. In addition the normalized streamwise fluctuations are essentially the same in both the solvent and drag-reducing sublayers. The changes caused by the polymer addition occur in the buffer region. The drag-reducing buffer region is thicker and the velocity profile in the outer flow region adjusts in order to accommodate this buffer-region thickening. The measurements of the streamwise velocity fluctuations also show that the polymer additives redistribute the primary turbulent activity over a broadened buffer region. The normalized magnitude of these fluctuations is, however, considerably lower in these two-dimensional drag-reducing channel flows than in those previously reported by Rudd (1972), Logan (1972) and Kumor & Sylvester (1973). Moreover, the mean velocity profiles in the buffer region do not confirm the hypothesis of Virk, Mickley & Smith (1970) that the data will follow their proposed ‘ultimate profile’ when the drag reduction is less than that given by the maximum asymptote. The mean velocity measurements also show that the proposed methods for predicting the upward shift in the outer portion of the mean velocity profile are inconsistent and lack universality. However, these results do confirm the previous suggestions of Virk (1971), Tomita (1970) and Lumley (1973) that the buffer region is the area of importance and change in drag-reducing flows.