Laser Doppler Anemometry Technique Research Articles

Flow distribution at the bottom region of a steam generator tube bundle

An experimental study was conducted using a 1:3 scale model of the bottom region of a U-tube steam generator to examine the flow patterns in the tube bundle. In addition to visual observations of flow patterns, vertical and horizontal tube gap velocities and turbulence intensities were measured using the laser Doppler anemometry technique. The results indicated a pronounced variation of the tube gap velocities with much lower vertical and horizontal velocities in the inner middle portion of the tube bundle compared to the outer portion close to the shroud entrance. The turbulence intensities generally varied from about 5 to 20%. Different wake patterns were observed behind the tubes depending upon the orientation of the tube arrays with respect to the incident flow.

PIV and LDA velocity measurements near walls and in the wake of a delta wing

Velocity measurements in flow regions with high turbulent intensities can be performed with high accuracy by means of optical methods. Despite the validity ranges of these methods, great care must be used in taking measurements near solid walls, where high noise levels are present due to the scattering of the light on the wall, and in flows with high vorticity regions like the ones in the wake of a finite wing. In this paper, Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) velocity measurement techniques are used in these two experimental situations and their results compared. This comparison shows that good measurements can be obtained from both techniques and that the resulting data sets do not provide alternative but rather complementary information.

Study of two-phase flow for a ramjet combustor

A reactive two-phase flow is studied experimentally by a special laser Doppler anemometry (LDA) technique and numerically by solving the elliptic governing equations. The relation between the liquid fuel injection and the aerodynamic flameholding mechanism of a dump combustor was investigated. The flow geometry chosen was an axisymmetric sudden expansion with small diameter ratio. The measurements included pressure, temperature, gas species concentration, as well as velocities of both phases and droplet size. A special processing technique enabled the simultaneous measurement of the gas velocity represented by the smallest droplets and of the size-velocity correlation of the droplets in the range of 5-500 |xm. The specific geometry of the sudden expansion combustor creates an annular flame stabilized just downstream of the step. The geometry of the flame depended very much on the equivalence ratio, with larger values creating longer and thicker flames. The combustion shortened the recirculation length from about 11 times the step height to about four times. There was no direct influence of the droplets on the average gas velocity in cold flow, whereas the size distribution of the droplets had significant influence on the combusting flow pattern. Two computational case studies were performed for isothermal and reactive two-phase flow situations. The numerical results indicated the fast dynamic and thermal response of the small droplets to the surrounding gas flow. Moreover, the droplets did not penetrate into the fuel-rich recirculation zone. The shear layer, which separates it from the core, was characterized by a local equivalence ratio close to unity. The numerical predictions and the experimental results exhibited fairly good agreement, provided that the initial conditions of the spray and the thermal boundary conditions are determined experimentally.

A combined laser Doppler anemometry and electrical probe diagnostic for bubbly two-phase flow

A review of the literature on laser Doppler anemometry (LDA) techniques to discriminate liquid from bubble velocities reveals that the techniques are unsuitable for large bubbles at moderate void fractions. A statistical technique, based on the separation of the combined gas and liquid velocity distribution function, was investigated. It was found to be appropriate for uniformly sized bubbles, a condition which is most easily achieved with small bubbles ( <4 mm dia ). A combined LDA and electrical probe (EP) technique was also developed to simultaneously measure the velocities from both the liquid and gaseous phase in unconfined two-phase bubbly plumes. The simultaneous time series of LDA velocities and EP signals from a chain of bubbles permitted separation of gas and liquid velocities at high volume fractions and for large, irregular bubbles.

Dynamics of fluid mixing in separated flows

Separated flows at high Re (&amp;gt;103) are highly turbulent. In some situations the turbulence generation and mixing processes associated with flow separation are desirable, e.g., in heat exchangers or in many chemical engineering applications. In others, e.g., stalled airfoils, separation must be avoided as it causes loss in pressure and kinetic energy. To control the phenomenon effectively, physical mechanisms of flow separation and related aspects, such as the growth of flow instabilities in shear layers, the process of vortex formation, and the dynamics of fluid mixing in recirculating flow regions, must be understood. In many cases numerical procedures, e.g., Navier–Stokes calculations including k-ε turbulence modeling, fail to predict real physical mechanisms in separated flows.1,2 Separated flows in the lee of bluff bodies have been studied for many years.3,4 However, accurate measurements of the magnitude and direction of velocities and the magnitude of the terms of the Reynolds stress tensor have been restricted by the unsuitability of the hot-wire anemometer in recirculating flows. The development of the pulsed-wire anemometer, flying hot-wire anemometer, and laser-Doppler anemometry (LDA) allows more reliable measurements also in turbulent separated flows.5–8 The aim of this paper is to investigate the dynamics of undisturbed fluid mixing in separated regions of 2-D, incompressible flows with visualization techniques and LDA. Measurements were performed with a vertical flat plate model, mounted in a closed-circuit wind tunnel at low blockage ratio. Because of the noninvasive character, optical techniques like LDA are more suitable to analyze complex fluid motions than pulsed-wire and flying-wire anemometry. The LDA system used to investigate turbulent flow structures consists of a two-channel version operating in backscatter mode and a specifically developed phase detector to extract phase-averaged information from recorded measurement ensembles.9 Endplates were used to get nearly 2-D behavior. Downstream of the plate the flow field exhibits a distinct periodic component at Strouhal frequency S=0.14. In the investigated Re number range, 103–105, S remains constant. The flow dynamics were visualized by the smoke-wire technique.10 Mixing between laminar external and turbulent separated fluid is evident. The process is initiated by entrainment of external fluid into the turbulent recirculating region. Corresponding quantitative results of LDA measurements are discussed. The plots show the developing of flow structures in a section from x/D=1.0 – 4.5 and y/D=−1.0 – 1.0, where D is the dimension of the plate model. The time difference between consecutive phases is t=0.02 sec. The saddle points are emphasized by short arrows indicating streamlines. Flow visualization and LDA measurements are in good agreement concerning the overall flow motions of the turbulent structures. Dynamic behavior of the Reynolds stress terms (u′2, v′2, and u′v′) are discussed. From these distributions the fields of phase-averaged turbulent energy production 〈P〉 could be derived. The isoline diagrams clarify that the location of extremum values in 〈P〉 are dependent on time and the position of the saddle point. In conclusion: (1) The dynamics of fluid mixing can be investigated with phase-locked LDA technique; (2) qualitative results of flow visualizations and quantitative LDA measurements support each other and shed new light upon the physics of fluid mixing; (3) extreme values of phase-averaged turbulent energy production coincide with the occurrence of saddle points in the flow field; and (4) mixing is initiated by entrainment of laminar external fluid into the turbulent recirculating region.

Simulation of the flow patterns in a disc and doughnut column

This paper is devoted to the simulation of the flow patterns generated in a disc and doughnut column. A recent calculation method, including a k-ϵ turbulent model, was used. The calculated values of the mean velocities were compared with experimental results at Re = 6000, obtained by the laser doppler anemometry technique. The simulation results are displayed under the form of the flow velocity directions, stream lines and turbulent isokinetics lines. In addition, the turbulent characteristics of the flow (turbulent kinetic energy, dissipation rate and integral scale) are discussed with reference to the Reynolds number, the distance between a disc and a doughnut and the open free area.

The effect of compliant coatings on coherent structure in turbulent boundary layers

The velocity profile, turbulence intensity profile, streaky structure and bursting frequency in turbulent boundary layers over a flat plate with compliant coatings were investigated by Laser Doppler Anemometry and conditional sampling techniques. This experiment led to the conclusions that in boundary layer flows on a compliant wall, as compared with that on a rigid wall, the log law region was extended further away from the wall, and that the maximum value of each turbulence intensity profile in the near wall region was reduced and the bursting frequency obviously decreased with the compliant coatings. One point worthy of notice was that the above results were very much like those of polymer drag reduction experiments.

Axial flow velocity patterns in a normal human pulmonary artery model: Pulsatile in vitro studies

It has been clinically observed that the flow velocity patterns in the pulmonary artery are directly modified by disease. The present study addresses the hypothesis that altered velocity patterns relate to the severity of various diseases in the pulmonary artery. This paper lays a foundation for that analysis by providing a detailed description of flow velocity patterns in the normal pulmonary artery, using flow visualization and laser Doppler anemometry techniques. The studies were conducted in an in vitro rigid model in a right heart pulse duplicator system. In the main pulmonary artery, a broad central flow field was observed throughout systole. The maximum axial velocity (150 cm s −1) was measured at peak systole. In the left pulmonary artery, the axial velocities were approximately evenly distributed in the perpendicular plane. However, in the bifurcation plane, they were slightly skewed toward the inner wall at peak systole and during the deceleration phase. In the right pulmonary artery, the axial velocity in the perpendicular plane had a very marked M-shaped profile at peak systole and during the deceleration phase, due to a pair of strong secondary flows. In the bifurcation plane, higher axial velocities were observed along the inner wall, while lower axial velocities were observed along the outer wall and in the center. Overall, relatively low levels of turbulence were observed in all the branches during systole. The maximum turbulence intensity measured was at the boundary of the broad central flow field in the main pulmonary artery at peak systole.

Velocity and Drop Size Measurements in Fuel Sprays in a direct injection diesel engine

AbstractHigh‐pressure injection fuel sprays have been investigated in a single‐cylinder research Diesel engine using the laser and phase Doppler anemometry techniques to measure fuel droplet velocities and sizes and their spatial and temporal distributions within the spray. The measurements relate to the determination of the droplets' axial velocity component and diameter on the spray centre line between 75 and 100 nozzle diameters downstream from a single‐hole injector nozzle. The experimental program covered the use of both Diesel and a non‐combusting alcohol fuel. Measurements at radial locations in the spray and at different fuel loads were also taken for the alcohol fuel. Temporally resolved measurements of droplet velocity and size are presented together with time — averaged values in such a way as to isolate the contributions from the leading and trailing regions of each spray and identify two different break — up mechanisms occurring as suggested by a conceptual model of the spray behaviour.

A comparison of the velocity and scalar spectra in premixed turbulent flames

Frequency spectra for scalar (progress variable c ̄ ) and two velocity components in premixed methaneair turbulent v-flames and stagnation flow-stabilized flames have been measured using laser light Mie scattering and laser Doppler anemometry techniques. To assist in interpreting the influence of flame crossing phenomenon on the scalar spectra, theoretical predictions of the c ̄ spectra developed by Bray-Libby-Moss and by Bray-Libby were evaluated. The spectral slopes and most energetic frequencies of the scalar spectra were compared with those of the velocity components normal and tangential to the turbulent flame brush. In both configurations, the spectra of the normal velocity components are similar to the corresponding scalar spectra, demonstrating the coupling between velocity and scalar spectra. However, the most energetic frequencies, which are related to the flame crossing frequencies, are about an order of magnitude higher in the v-flame than in the stagnation flames. Because this difference does not seem to be proportional to the difference in the mean velocities, the results imply that the spatial scale distribution in the two flame zones are not the same.

Experimental Investigation of Confined Turbulent Jets Part 11: Particle-Laden Flow Data

Spherical glass beads were added to the central jet of the injector. Mean and fluctuation velocities were measured for the gas phase and particles employing a laser Doppler anemometry technique. The results were compared to the single-phase flow data at comparable conditions

Bursting frequency in turbulent boundary layers

Employing laser Doppler anemometry and VITA techniques, the bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 320 to 1470. The result indicates that the mean and non-dimensional bursting frequency scaled with the variables appropriate for the wall region was constant and independent of Reynoids number. When the same data are plotted using the outer variables of boundary layer to normalize the bursting frequency, the non-dimensional frequency increases as the Reynolds number increases. This is in agreement with the results of Blackwelder et al. (1983) who used hot wire anemometry and VITA technique.

Laser Doppler anemometry under plasma conditions. Part I. Measurements in a d.c. plasma jet

A study is presented on the use of laser Doppler anemometry (LDA) techniques for the measurement of the gas and particle velocities under plasma conditions. Experimental data is presented for a d.c. plasma jet in which alumina particles are injected under different operating conditions. The results reveal that the plasma velocity at the exit of the jet is of the order of 200–300 m/s. The intensity of turbulence is as high as 30 to 40% in the free shear layer and the particle velocity distribution is shown to be asymmetric, with particle dispersion in the plane of injection considerably more important than that in the perpendicular direction. The average particle velocity depends on the composition of the plasma gas, the torch current, and power.

Fluid mechanics of aortic stenosis.

In vitro qualitative and quantitative flow-mapping studies were conducted in an adult size aortic flow chamber, using bioprosthetic valves (0.5-5.0 cm2) to mimic varying degrees of aortic stenosis. The studies were performed under physiologic conditions in a left heart stimulator using: flow visualization, laser Doppler anemometry (LDA), continuous-wave (CW) Doppler and colour Doppler flow mapping (CDFM) techniques. Pressure gradients in the range 15-150 mmHg were accurately predicted by CW Doppler using the Bernoulli equation (r = 0.99). The flow visualization and CDFM studies revealed that all degrees of aortic stenosis led to jet-type flow fields, in which jet orientation was not necessarily symmetric and was skewed to varying degrees. Therefore, in aortic stenosis, Doppler measurements should be conducted in multiple views in order to visualize the flow field properly. Measurements with cross-sectional LDA revealed that as aortic stenosis increased: jet size narrowed; the peak velocity and turbulent intensities of the jet increased; jet instability increased; and acceleration of the jet proximal to the valve increased. Peak velocities as high as 4-7 ms-1 with turbulence levels (i.e. root mean square axial velocities) of 1.0-2.3 m s-1 were measured, with the moderately and severely stenotic valves. These elevated levels of turbulence could cause damage to the formed elements of blood and the walls of the ascending aorta. The high velocities and turbulence levels created by the moderately and severely stenotic valves, made quantitative interpretation of CDFM recordings very difficult, if not impossible.

Measurement of particle movement in a spouted bed using a new microprocessor based technique

AbstractA new experimental technique for measuring particle cycle times, times of occurrence of particles at different levels, and particle velocities in the spout of a spouted bed is described. A magnetized marker particle and search coil or coils are employed together with a microprocessor system for ease of data collection and processing. The magnetic particles have the same diameter, density and similar surface texture to bed particles to ensure proper representation.Data is presented on distributions of cycle time and particle velocity and implied particle mass circulation rates at different levels in a spouted bed. Comparisons are made with photographic and laser–doppler anemometry techniques.

Conditional Reynolds stress in a strongly heated turbulent boundary layer with premixed combustion

A two-color laser Doppler anemometry technique has been used to measure velocity statistics and Reynolds stress in a turbulent boundary layer over a strongly heated wall (Tw=1100 K) with premixed ethylene–air combustion. Measurements in the isothermal and nonreacting heated wall turbulent boundary layers were also made for comparison. Conditional velocity covariance −∼(uv) are deduced using the quadrant and ‘‘hole’’ analysis methods. The conditional data obtained in the isothermal layer are in exellent agreement with those deduced by others based on hot-wire data. The results obtained in the heated and reacting layers indicate that the decrease in mean Reynolds stress can be attributed to a reduction in the −∼(uv) contributions associated with a burst of low momentum fluid form the wall. The reduction is much more significant in the reacting layer than in the heated layer. Comparison with previous density shows that the changes in conditional −∼(uv) contributions occur mostly within the constant density sublayer of combustion products adjacent to the wall. In the region where combustion reaction takes place, no significant change in the turbulence intensities or Reynolds stress is found.

Morphological adaptations of benthic invertebrates to stream flow - An old question studied by means of a new technique (Laser Doppler Anemometry).

The generally accepted concept that dorsoventral flatness and/or small size of benthic stream invertebrates staying on the surface of the bottom substratum allows a current-sheltered life in the boundary layer (Ambühl 1959) is checked by means of the new technique of Laser Doppler Anemometry (LDA). With LDA measurement of flow can be done nearly punctually without any mechanical disturbance. Mapping the current velocities around the body of Ecdyonurus cf. venosus (Insecta, Ephemeroptera) and Ancylus fluviatilis (Gastropoda) gives evidence that boundary layer separation occurs above the animals' bodies. Our results indicate that the velocities around the body of benthic stream invertebrates and probably the forces acting on them are much more complicate than is suggested by the currently accepted boundary layer concept.

Use of the Doppler effect in visible laser light to assess tissue viability by capillary blood flow.

The noninvasive technique of laser Doppler anemometry is briefly reviewed and its application in the assessment of tissue viability described. Calibration studies using the model of controlled skin burns in rats are presented and discussed. A correlation between the damage of burned tissue and the anemometry readings has been established.

An LDA technique for in situ simultaneous velocity and size measurement of large spherical particles in a two-phase suspension flow

A relatively simple optical scheme using the reference-mode laser Doppler anemometry for the in situ measurement of flow properties of a dilute particle-fluid two-phase suspension having a predominant flow direction is hereby proposed. It is an extension of the established technique of optical gating for particle sizing which is fully integrated into the established technique of laser Doppler anemometry for velocity measurement. Particles that can be measured by this scheme are limited to those with sizes greater than the smaller dimension of the optical measuring volume. Inherent in the methodology is a procedure for providing information on the local particle number density and velocity distributions for each size range of the particles and the local velocity distribution of the continuous phase. The accompanying electronics and interfaces are also established for data processing and analysis in a mini computer. Validation of the scheme has been accomplished by controlled experiments using stainless steel balls and water droplets of 1 mm and greater in diameter.