Laser Anemometer Measurements Research Articles

Modélisation semi-empirique des écoulements et des transferts dans un milieu poreux en régime turbulent

The model allows the prediction of turbulent fluid flow and heating transfers inside a stack of obstacles, which is considered as a porous media. The heat transfer intensity between fluid and particles is related to the local values of velocity and turbulence intensity. The turbulent kinetic energy is predicted by a transport equation inspired from literature, which can also be used for two-dimensional flow patterns. Model parameters were experimentally identified for a stack of spheres with a void fraction of 34%. Laser anemometer measurements were performed upstream the from stack. Heat transfer coefficients were measured at different positions in the stack and for different flow rates. The standard error between measured and predicted coefficients is 5%.

Wake Recovery Performance Benefit in a High-Speed Axial Compressor

Abstract Rotor wakes are an important source of loss in axial compressors. The decay rate of a rotor wake is largely due to both mixing (results in loss) and stretching (no loss accrual). Thus, the actual loss associated with rotor wake decay will vary in proportion to the amounts of mixing and stretching involved. This wake stretching process, referred to by Smith (1996) as recovery, is reversible and for a 2-D rotor wake leads to an inviscid reduction of the velocity deficit of the wake. It will be shown that for the rotor/stator spacing typical of core compressors, wake stretching is the dominant wake decay process within the stator with viscous mixing playing only a secondary role. A model for the rotor wake decay process is developed and used to quantify the viscous dissipation effects relative to those of inviscid wake stretching. The model is verified using laser anemometer measurements acquired in the wake of a transonic rotor operated alone and in a stage configuration at near peak efficiency and near stall operating conditions. Results from the wake decay model exhibit good agreement with the experimental data. Data from the model and laser anemometer measurements indicate that rotor wake straining (stretching) is the primary decay process in the stator passage. Some implications of these results on compressor stage design are discussed.

CFD Comparison to 3D Laser Anemometer and Rotordynamic Force Measurements for Grooved Liquid Annular Seals

A pressure-based computational fluid dynamics (CFD) code is employed to calculate the flow field and rotordynamic forces in a whirling, grooved liquid annular seal. To validate the capabilities of the CFD code for this class of problems, comparisons of basic fluid dynamic parameters are made to three-dimensional laser Doppler anemometer (LDA) measurements for a spinning, centered grooved seal. Predictions are made using both a standard and low Reynolds number κ-ε turbulence model. Comparisons show good overall agreement of the axial and radial velocities in the through flow jet, shear layer, and recirculation zone. The tangential swirl velocity is slightly under-predicted as the flow passes through the seal. By generating an eccentric three-dimensional, body fitted mesh of the geometry, a quasi-steady solution may be obtained in the whirling reference frame allowing the net reaction force to be calculated for different whirl frequency ratios, yielding the rotordynamic force coefficients. Comparisons are made to the rotordynamic force measurements for a grooved liquid annular seal. The CFD predictions show improved stiffness prediction over traditional multi-control volume, bulk flow methods over a wide range of operating conditions. In cases where the flow conditions at the seal inlet are unknown, a two-dimensional, axisymmetric CFD analysis may be employed to efficiently calculate these boundary conditions by including the upstream region loading to the seal. This approach is also demonstrated in this study.

A Numerical Investigation of Transonic Axial Compressor Rotor Flow Using a Low-Reynolds-Number k–ε Turbulence Model

We have developed a computer simulation code for three-dimensional viscous flow in turbomachinery based on the time-averaged compressible Navier–Stokes equations and a low-Reynolds-number k–ε turbulence model. It is described in detail in this paper. The code is used to compute the flow fields for two types of rotor (a transonic fan NASA Rotor 67 and a transonic axial compressor NASA rotor 37), and numerical results are compared to experimental data based on aerodynamic probe and laser anemometer measurements. In the case of Rotor 67, calculated and experimental results are compared under the design speed to validate the code. The calculated results show good agreement with the experimental data, such as the rotor performance map and the spanwise distribution of total pressure, total temperature, and flow angle downstream of the rotor. In the case of Rotor 37, detailed comparisons between the numerical results and the experimental data are made under the design speed condition to assess the overall quality of the numerical solution. Furthermore, comparisons under the part-speed condition are used to investigate a flow field without passage shock. The results are well predicted qualitatively. However, considerable quantitative discrepancies remain in predicting the flow near the tip. In order to assess the predictive capabilities of the developed code, computed flow structures are presented with the experimental data for each rotor and the cause of the discrepancies is discussed.

Blockage Development in a Transonic, Axial Compressor Rotor

A detailed experimental investigation to understand and quantify the development of blockage in the flow field of a transonic, axial flow compressor rotor (NASA Rotor 37) has been undertaken. Detailed laser anemometer measurements were acquired upstream, within, and downstream of a transonic, axial compressor rotor operating at 100, 85, 80, and 60 percent of design speed, which provided inlet relative Mach numbers at the blade tip of 1.48, 1.26, 1.18, and 0.89, respectively. The impact of the shock on the blockage development, pertaining to both the shock/boundary layer interactions and the shock/tip clearance flow interactions, is discussed. The results indicate that for this rotor the blockage in the endwall region is 2–3 times that of the core flow region, and the blockage in the core flow region more than doubles when the shock strength is sufficient to separate the suction surface boundary layer.

The Effect of Adding Roughness and Thickness to a Transonic Axial Compressor Rotor

The performance deterioration of a high-speed axial compressor rotor due to surface roughness and airfoil thickness variations is reported. A 0.025 mm (0.001 in.) thick rough coating with a surface finish of 2.54–3.18 rms μm (100–125 rms μin.) is applied to the pressure and suction surface of the rotor blades. Coating both surfaces increases the leading edge thickness by 10 percent at the hub and 20 percent at the tip. Application of this coating results in a loss in efficiency of 6 points and a 9 percent reduction in the pressure ratio across the rotor at an operating condition near the design point. To separate the effects of thickness and roughness, a smooth coating of equal thickness is also applied to the blade. The smooth coating surface finish is 0.254–0.508 rms μm (10–20 rms μin.), compared to the bare metal blade surface finish of 0.508 rms pm (20 rms μin.). The smooth coating results in approximately half of the performance deterioration found from the rough coating. Both coatings are then applied to different portions of the blade surface to determine which portions of the airfoil are most sensitive to thickness/roughness variations. Aerodynamic performance measurements are presented for a number of coating configurations at 60, 80, and 100 percent of design speed. The results indicate that thickness/roughness over the first 2 percent of blade chord accounts for virtually all of the observed performance degradation for the smooth coating, compared to about 70 percent of the observed performance degradation for the rough coating. The performance deterioration is investigated in more detail at design speed using laser anemometer measurements as well as predictions generated by a quasi-three-dimensional Navier–Stokes flow solver, which includes a surface roughness model. Measurements and analysis are performed on the baseline blade and the full-coverage smooth and rough coatings. The results indicate that adding roughness at the blade leading edge causes a thickening of the blade boundary layers. The interaction between the rotor passage shock and the thickened suction surface boundary layer then results in an increase in blockage, which reduces the diffusion level in the rear half of the blade passage, thus reducing the aerodynamic performance of the rotor.

An Improved Aspirating Probe for Total-Temperature and Total-Pressure Measurements in Compressor Flows

The aspirating probe originally designed by Epstein and Ng at MIT was modified by replacing the two platinum-coated tungsten hot wires normally used with platinum–iridium alloy wires. The resulting improved unsteady total pressure and total temperature resolution of the modified probe is demonstrated. Flowfield measurements were made downstream of NASA Rotor 37 for a part-speed operating condition to test the performance of the probe. Time-resolved blade-to-blade total temperature and total pressure as calculated from the two platinum–iridium hot-wire voltages are shown. The flowfield measurements are compared with independent measurements of total pressure with high response transducers and total temperature calculated from laser anemometer measurements. Limitations of a more often used unsteady temperature data reduction method, which involves only one aspirating probe hot-wire voltage and a high-response pressure measurement, are discussed.

A Viscous Flow Study of Shock-Boundary Layer Interaction, Radial Transport, and Wake Development in a Transonic Compressor

A numerical study based on the three-dimensional Reynolds-averaged Navier–Stokes equation has been conducted to investigate the detailed flow physics inside a transonic compressor. Three-dimensional shock structure, shock-boundary layer interaction, flow separation, radial mixing, and wake development are all investigated at design and off-design conditions. Experimental data based on laser anemometer measurements are used to assess the overall quality of the numerical solution. An additional experimental study to investigate end-wall flow with a hot film was conducted, and these results are compared with the numerical results. Detailed comparison with experimental data indicates that the overall features of the three-dimensional shock structure, the shock-boundary layer interaction, and the wake development are all calculated very well in the numerical solution. The numerical results are further analyzed to examine the radial mixing phenomena in the transonic compressor. A thin sheet of particles is injected in the numerical solution upstream of the compressor. The movement of particles is traced with a three-dimensional plotting package. This numerical survey of tracer concentration reveals the fundamental mechanisms of radial transport in this transonic compressor. Strong radially outward flow is observed inside a separated flow region and this outward flow accounts for about 80 percent of the total radial transport. The radially inward flow is mainly due to the traditional secondary flow.

Scanning laser anemometer measurements of a forced cylinder wake

It is demonstrated that a scanning laser anemometer is capable of measuring rapid and randomly occurring flow events. Mean and instantaneous velocity profiles in the near wake of a cylinder at Reynolds number of 155 are shown together with spatial correlations. In addition, the scanning laser anemometer has been used to show that the mean velocity defect behind the cylinder can be substantially reduced, when the wake is disturbed by unsteady forcing with a line of small pulsating jets emanating from the trailing edge of the cylinder.

Laser anemometer measurements of Reynolds stress in a turbulent channel flow with drag reducing polymer additives

Turbulence measurements with a high resolution laser Doppler anemometer in a channel flow with injection of a concentrated polyethylene oxide solution are reported. With polymer injection sufficient to yield a well mixed concentration of 10 ppm the total drag is reduced by 38%, but the sum of the Reynolds stress and the conventional molecular shear stress is only (2)/(3) of the force produced by the streamwise pressure gradient. Thus, the injected polymer solution has a significant effect on the turbulence and causes an additional retarding force equal to approximately (1)/(3) of the total drag.

Laser anemometer measurements in a compressor rotor flowfield at off-design conditions

The measurement of the flowfield at the peak pressure rise condition in an axial flow compressor rotor is reported in this paper. The inviscid flow measurement was carried out using a laser Doppler velocimeter and the blade boundary layers were measured using a two-sensor hot-wire pobe. The blade-to-blade distribution of velocities at several axial and radial locations are plotted, interpreted, and compared with the conditions existing at a lower (design) flow coefficient. The leading-edge effects on inviscid flow are found to be appreciable. The flowfield is dominated by inviscid flow at most radial locations and the blade boundary-layer thickness is found to be appreciable in the tip region. The flow near the tip is dominated by leakage and its influence is more severe at the peak pressure rise condition. Nomenclature local lift coefficient based on local inlet dynamic head of the relative flow static and stagnation pressure of absolute flow pressure and suction surfaces, respectively radius ratio, r/rt blade spacing streamwise, normal, and radial coordinate directions blade tip speed absolute and relative flow velocity normalized by the blade tip speed resultant velocity on the cylindrical surface, pfpQ

A new system for index-matched laser-anemometer measurements

An index-matching system was developed for laser-anemometer measurements in remote flow passages. The flow model was constructed of borosilicate glass and filled with a solution of tetrachloroethylene and Freon-113. Both the glass and liquid had a refractive index of 1.4708 at 632.8 nm.

Comparison of Two- and Three-Dimensional Flow Computations With Laser Anemometer Measurements in a Transonic Compressor Rotor

Two-and three-dimensional inviscid solutions for the flow within a transonic axial compressor rotor at design speed are compared to laser anemometer measurements at maximum flow and near stall operating points. Computational details of the two-dimensional axisymmetric stream function solution and the three-dimensional full Euler solution are described. Upstream of the rotor, the two and three-dimensional solutions for radial distribution of relative Mach number and total pressure agree well with the data. Within the bow wave system and the blade row, the axisymmetric two-dimensional solution shows only qualitative agreement with the data.

A new look at particle statistics in laser-anemometer measurements

Many algorithms have been proposed and evaluated for dealing with laser-anemometer measurements in low particle densities since the original paper of McLaughlin & Tiederman (1973). All of them describe the effect of making a measurement whenever a particle is visible to the measurement instrument. However, in many circumstances, one wishes to sample the velocity at a constant rate–for instance, in measurement of velocity fluctuation spectra. It is shown here that the measurement statistics for this case are different from those previously discussed in the literature. The product of the particle density and the sample interval is the controlling parameter for the statistical description of the measurements. The asymptotic forms for low and high particle density-sample time products are derived.

Laser Anemometer Measurements in a Transonic Axial Flow Compressor Rotor

A laser anemometer system employing an efficient data acquisition technique has been used to make measurements upstream, within, and downstream of the compressor rotor. A fluorescent dye technique allowed measurements within endwall boundary layers. Adjustable laser beam orientation minimized shadowed regions and enabled radial velocity measurements outside of the blade row. The flow phenomena investigated include flow variations from passage to passage, the rotor shock system, three-dimensional flows in the blade wake, and the development of the outer endwall boundary layer. Laser anemometer measurements are compared to a numerical solution of the streamfunction equations and to measurements made with conventional instrumentation.

An experimental study of the transformation zone of plunging breakers

A plunging breaker begins its characteristic deformation some distance seaward of the wave breaking point and its behaviour through this transformation has been investigated in the present study. It is shown that a modified version of the linear-theory equation may be used to predict the speed of the wave crest within the transformation zone. Laser anemometer measurements of water-particle velocities show that when the breaking point has been reached the horizontal particle velocity at the crest of a plunging breaker may still be less than the crest speed.

Instrumentation techniques for studying heterogeneous combustion

Diagnosis of heterogeneous combustion systems requires the measurement of physical and chemical properties in flames laden with liquid and solid particles. For liquid-fuelled combustors, the characteristics of the spray require to be determined, including the trajectories of individual droplets and their interaction with gas streams. Vaporization rates can be determined from measurement of the rate of change of diameter of individual droplets. Particulate matter emitted from combustion chambers is mainly in the solid phase and consists of both unburned fuel and non-burnable material. With heavy fuel oils, particles of the order of 100 μm can be emitted while, for the lighter fuels, particulate emissions are mainly in the sub-micron size region. The laser anemometer can be used for simultaneous measurement of velocity and particle size of individual droplets and particles in spray flames. Laser diffraction techniques can be used for measurement of over-all size distributions. Laser Raman spectroscopy offers the possibility of simultaneous measurement of temperature and species concentration in flames. Presence of particles results in laser irradiated particulate heating and incandescence, which can lead to the swamping of signals. The use of coherent anti-Stokes Raman and near-resonant Raman offers possibilities for making laser Raman measurements in particle laden flames. Solid probes for measuring temperature and species concentration require special adaptation when used in particle laden flows. Particles may damage probes by direct impingement and cause blockage of orifices. Thermocouples can be used in sprays, but the effects of direct droplet impingement must be considered. Diagnostic techniques have been developed in which thermocouple signals are digitized and recorded on a computer. This system permits the rapid measurement of time constants and, subsequently, digital frequency compensation of temperature time histories, providing a frequency response of the order of 1 kHz. Information on spray boundaries is provided by both laser anemometer and thermocouple measurements. Gas sampling probes, specially designed to separate liquid and solid particles from the gas, are used for removal of samples from within the flame. Size analysis of particles is made by microphotography and computer image analyser. Species concentration in the gas is measured by gas phase chromatography, flame ionization detectors, non-dispersive infrared and ultraviolet detectors and chemiluminescent analysers. All measurement diagnostic techniques are affected by the presence of large-scale coherent eddy structures in the turbulent flow systems. The need for increasing frequency response, recording and analysis of variations with time, and the use of conditional sampling in intermittent flows is emphasized. The coupling of high-speed movies with laser optical proves to give simultaneous recording of visual and signal events is recommended.

Laser anemometer measurements in flames with swirl

An experimental study has been made of flow fields in turbulent swirling jets under flame and no-flame conditions. Natural gas was supplied separately to a burner with a divergent exit of 20°. The recirculation zone penetrated into, the diffuser at a swirl number of 0.3. Time mean axial, radial and circumferential components of velocity and rms velocity fluctuations were measured. The laser anemometer operated in the double Doppler mode and frequency shifting was obtained with a rotating diffraction grating. Signal processing was carried out by an electronic single particle pulse counter. Substantial changes in flow patterns were detected as a consequence of combustion, and the kinetic energy of turbulence per unit mass under flame conditions was higher than in the corresponding cold conditions in almost all regions of the flame.

Laser anemometer measurements of trailing vortices in water

A series of measurements of trailing vortices behind lifting hydrofoils is described. These measurements were made in the Caltech Free-Surface Water Tunnel, using a laser-Doppler velocimeter to measure two components of velocity in the vortex wake. Two different model planforms were tested, and measurements were made at several free-stream velocities and angles of attack for each. Velocity profiles were measured at distances downstream of the model of from five to sixty chord lengths. These measurements are the first results of a continuing experimental programme.In § 3 of this paper, the theory of trailing vortices is discussed. The effects of ‘vortex wandering’ upon the measurements are computed, and the corrected results are seen to be in reasonable agreement with the theory.

Biasing correction for individual realization of laser anemometer measurements in turbulent flows

In turbulent flow situations the histograms constructed in the individual realization mode of laser anemometry are biased. The biasing occurs because a larger than average volume of fluid, and hence a larger than average number of scattering centers, pass through the probe volume during periods when the velocity is faster than the mean. Similarly, a smaller volume of fluid and a smaller number of scattering particles pass through the probe volume during periods when the velocity is slower than the mean. The proper weighting function needed to correct the biased data is the inverse of the instantaneous velocity vector. However, an analysis using turbulent flow models show that corrections based only upon the streamwise component of the velocity vector are adequate for many flow situations.