Determination of the measuring position in laser-Doppler anemometry

The present paper is concerned with the determination of the measuring position of a laser-Doppler anemometer (LDA) relative to a wall. The proposed method is based on the finding that the output of a hot-wire anemometer increases when the wire, which is mounted in quiescent air parallel to the wall, is brought closer than 800 m to the wall. For given hot-wire anemometer parameters, the hot-wire anemometer output voltage depends on the wall material and the wire distance from the wall. After suitable calibration for the wall material of the test section, the anemometer reading in a test rig can be used to find the wire position. Moving the measuring volume of a LDA-system across the wire yields an output voltage variation of the LDA-photomultiplier showing a Gaussian shape. When the maximum output voltage is reached, the centre of the measuring control volume is located at the centre of the wire and, hence, the location of the LDA-measuring position is known. All position measurements for the LDA-system are then taken relative to this point using the scale of the LDA-traversing system. If optical effects of transparent test section walls are eliminated by employing refractive index matched liquids, there are other ways to find the measuring position of a laser-Doppler anemometer relative to a wall. One such method and its application to the study of the turbulent near wall flow in a pipe is described in this paper.     

Instantaneous visualization of surface flows

An experimental system is described for visualizing the surface flow of a wing, using an oil smoke tracer technique. The method leads to the determination of the instantaneous velocity direction at the output of surface injectors. A preliminary investigation is made on a flat plate to optimize the conditions of oil smoke injection. Then, the visualization is performed on the upperside of a sweptback wing in the vicinity of the reattachment of the vortex flow. This visualization technique can be applied to other types of wall flows — separated or not — around various bodies.List of symbols b wing span - c _n normal (to leading edge) chord - c _r streamwise (or root) chord - d diameter of the injectors - distance from the apex along the leading edge - relative distance from the apex along the leading edge ( = /C _d) - sweep angle - e injector geometric parameter (e = d/l) - angle of attack - K injection parameter - l length of the injectors - v kinematic viscosity - P _t, P_s total and static pressure of the flow - P _inj injection pressure - P _r reduced pressure (P _r = (P_inj – P_t)/(P_t – P_s)) - Re flow Reynolds number (Re = V ·c _n/v) - Re _i injector Reynolds number (Re = V ·d/v) - s curvilinear distance along c _d - s relative curvilinear distance along c _d(s = s/c _d) - V infinite upstream flow velocity     

Experimental investigation of the interaction of two flows on the axial fan hollow blades by flow visualization and hot-wire anemometry

Experimental investigation of the interaction of internal flow with external flow around hollow airfoil NACA series in a low-speed wind tunnel was conducted and is presented in the paper. The region near the trailing edge of the hollow airfoil was studied in detail and measurements of velocity and turbulence intensities were performed with hot-wire anemometry. Determination of flow structure on the hollow airfoil was performed with computer-aided visualization. It can be concluded from the measurement analysis that higher values of velocities, lower turbulence intensities and a significant decrease of circulation effects on the suction side of the hollow blade were achieved, due to the introduction of internal flow. The results obtained on the hollow airfoil were applied on the rotating axial fan. Influence of the internal flow of the hollow blade on the flow field of the axial fan was studied. With the introduction of the internal flow a reduction of circulation effects on the fan hollow blade was achieved. Aerodynamic characteristic of the axial fan reached higher degree of total pressure difference and normalized efficiency through the entire fan working conditions.     

Application of photogrammetry to surface flow visualization

The construction of three-dimensional surface flow fields is an extremely difficult task owing largely to the fragmented information available in the form of 2D images. Here, the method of photogrammetric resection based on a comprehensive camera model has been used to map oil flow visualization images on to the surface grid of the model. The data exported in the VRML format allow for user interaction in a manner not possible with 2D images. The technique is demonstrated here using the surface oil flow visualization images of a simplified landing gear model at low speed in a conventional wind tunnel without any specialized rigs for photogrammetry. The results are not limited to low-speed regimes and show that this technique can have significant impact on understanding the flow physics associated with the surface flow topology of highly three-dimensional separated flows on complex models.     

Development and application of streakline visualization in hypervelocity flows

A method for visualizing streaklines in hypervelocity flows has been developed. The method uses the high temperatures produced in hypervelocity flows to ablate small amounts of sodium deposited onto a wire stretched across the flow and to broaden the lines in the sodium spectrum. By using a dye laser, tuned to a wavelength close to one of the sodium D-lines, as the light source in shadowgraph or Schlieren visualization, streaklines seeded with sodium become visible through absorption and/or enhanced refractivity. The technique has been used to investigate the stability of the shear layer produced by the curved bow shock on a cylindrically blunted wedge. The results suggest that the shear layer is unstable, exhibiting structures with a wavelength that is comparable to half the nose radius of the body.     

On interpretation of flow visualization of unsteady shear flows

Flow visualization of an unsteady wake is simulated by computer-generated streaklines and timelines, the two most commonly employed markers in laboratory flows. The resultant visualization patterns are compared with prescribed vorticity fields having constant or decreasing strength. Effects on the visualized flow of thickness and location of the injected marker, discrete (as opposed to continuous) marker elements, and different reference frames are addressed.     

Application of hot-wire anemometry in shock-tube flows

Several techniques associated with the use of hot wire anemometry in compressible turbulence measurements are described and tested in shock tube flows. These techniques include 1. in-situ calibration of the hot-wire probe by firing several shock waves of different strengths in the shock tube; 2. on-line analog frequency compensation or off-line digital compensation of the temperature-wire; 3. simultaneous acquisition of time-dependent flow velocity and temperature of the flow without invoking Morkovin's hypothesis of strong Reynolds analogy. The techniques were tested in two different shock tube facilities, where a grid generated turbulent flow interacting with a moving shock was set up.The financial support provided by National Science Foundation and NASA is greatly acknowledged.     

Experimental investigation of the angular dependence of laser light scattered in laser-Doppler anemometry

In laser-Doppler anemometry (LDA) the light scattered from small particles is generally observed either in on-axis or sligthly off-axis forward or backscattering mode. The present paper reports on experimental results achieved with a LDA-system, which clearly demonstrates the distinct superiority of the forward scattering mode over backscattering on the bases of scattering diagrams. These diagrams are compared to Mie's theory of the diffraction of electromagnetic waves on dielectric spheres. Good agreement could be found for all the observation angles which are mostly used in LDA-arrangements.     

A displaced-line velocity diagnostic and its application in a visualization engine

A technique for measuring three-dimensional velocity by imaging the displacement of a marked fluid line is described, together with its use in an automotive visualization engine. In a flow seeded with 2–3 μ phosphorescing particles, a line is excited by a UV laser beam, deformed by the local velocity field, and detected by stereo low-light-level video cameras. The derivation of velocity from digitized images is discussed and capabilities of the diagnostic are assessed. Some image data taken in the engine are shown and quantitative two-component velocity plots along the line are presented.     

Error estimation of laser-Doppler anemometry measurements in fluids with spatial inhomogeneities of the refractive index

We report a combined experimental and theoretical investigation of the influence of spatial non-uniformities of the refractive index on the accuracy of laser Doppler anemometry (LDA) measurements in transparent fluids. One LDA beam is guided through heated air of a thermal boundary layer near a heated vertical flat plate. It is found that the hot air is deflecting the beam because of a modification of the refractive index n in the fluid. This deflection causes three effects: (1) spatial displacement of beam intersection, (2) waist mismatch in the measurement volume and (3) variation in interference fringe distance. With the help of a rotating disk calibration system the resulting displacement of the LDA measurement volume and the Doppler frequency variation is systematically studied at different temperatures. Using a simple model of beam propagation under the influence of well-defined temperature inhomogeneities, the displacement of measurement volume and change in Doppler frequency are calculated and are found to be in agreement with the experimental observations. The results provide a rational framework for an assessment of the accuracy of LDA data in arbitrary transparent fluids with non-uniform refractive index.     

In-cylinder tumble flows and performance of a motorcycle engine with circular and elliptic intake ports

The temporal and spatial evolution processes of the flows in the cylinder of a four-valve, four-stroke, single cylinder, reciprocating motorcycle engine installed with the elliptic and circular intake ports were experimentally studied by using the particle image velocimetry (PIV). The engine was modified to fit the requirements of PIV measurement. The velocity fields measured by the PIV were analyzed and quantitatively presented as the tumble ratio and turbulence intensity. In the symmetry plane, both the circular and elliptic intake ports could initiate a vortex around the central region during the intake stroke. During the compression stroke, the central vortex created in the cylinder of the engine with the circular intake port disappeared, while that in the engine cylinder with the elliptic intake port further developed into the tumble motion. In the offset plane, weak vortical structures were initiated by the bluff-body effect of the intake valves during the intake stroke. The vortical structures induced by the elliptic intake port were more coherent than those generated by the circular intake port; besides, this feature extends to the compression stroke. The cycle-averaged tumble ratio and the turbulence intensity of the engine with the elliptic intake port were dramatically larger than those of the engine with the circular intake port. The measured engine performance was improved a lot by installing the elliptic intake port. The correlation between the flow features and the enhancement of the engine performance were argued and discussed.     

The role of flow visualization in the study of afterbody and base flows

The role of the flow visualization in providing the necessary insight for the development of theoretical models of complex afterbody and base flows is described. Methods of calculating the turbulent base pressure for axisymmetric configurations are discussed. Emphasis is placed on the treatment of supersonic flows for cylindrical, boattail, and flare bodies as well as sharp and blunt cones and base flow nozzles. The guidance provided by flow visualization in the development of theoretical models for the transonic case is also discussed. The current interests in applying finite difference techniques to these-problems indicate that the extensive use of flow visualization data will continue.     

Creeping flow analyses of free surface cavity flows

Two industrially important free surface flows arising in polymer processing and thin film coating applications are modelled as lid-driven cavity problems to which a creeping flow analysis is applied. Each is formulated as a biharmonic boundary-value problem and solved both analytically and numerically. The analytical solutions take the form of a truncated biharmonic series of eigenfunctions for the streamfunction, while numerical results are obtained using a linear, finite-element formulation of the governing equations written in terms of both the streamfunction and vorticity. A key feature of the latter is that problems associated with singularities are alleviated by expanding the solution there in a series of separated eigenfunctions. Both sets of results are found to be in extremely good agreement and reveal distinctive flow transformations that occur as the operating parameters are varied. They also compare well with other published work and experimental observation.     

Study of the intake tumble motion by flow visualization and particle tracking velocimetry

The purpose of this work is to characterize the in-cylinder tumbling flow generated by an engine head during the induction process using flow visualization and particle tracking velocimetry (PTV). The study was carried out for a 4-valve engine head with shrouded intake valves in a special single cylinder transient water analog. This shrouded intake valve configuration was used to obtain a prototypical pure tumble flow suitable for fundamental combustion studies. The results revealed that the shrouded intake valves generate a strong, well-behaved tumble vortex on the axial plane between the cylinder head and the piston face. This vortex dominates the entire flow field and seems to be highly repeatable from cycle to cycle. The effect of engine speed on this tumbling flow was studied. An equivalent tumble ratio was defined and evaluated using the measured velocity fields at BDC (bottom dead center).List of symbols ABDC after bottom dead Center - ATDC after top dead center - BBDC before bottom dead center - BDC bottom dead center - BTDC before top dead center - dm mass of the volume element - M total angular momentum - PTV particle tracking velocimetry - r radial distance from the reference point - t total pulse duration - TDC top dead center - U instantaneous velocity - v velocity of the center point of the element - X streak length     

Alignment and application of the conical shadowgraph flow visualization technique

An optical bench study has been carried out to assess the effects of both beam alignment and nonconical disturbances in the application of conical shadowgraphy for flow visualization. Conical and quasiconical plastic test models were immersed in a refractive index matching fluid and then examined by shadowgraphy with a conical light beam. The results show that problems of interpretation may arise due to both axial and transverse beam misalignment. Among these difficulties, axial misalignment with positive vertex displacement is the least serious. Also, the effect of a particular nonconical disturbance field was found to introduce fewer difficulties of interpretation than that of beam misalignment.     

Volumetric intake flow measurements of an IC engine using magnetic resonance velocimetry

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of $9.21$  mm to simulate the induction flow at crank-angle $270^{\circ }$ bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., $y \ge -20$  mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.     

Characterization of particle-laden, confined swirling flows by phase-doppler anemometry and numerical calculation

The particle dispersion characteristics in a confined swirling flow with a swirl number of approx. 0.5 were studied in detail by performing measurements using phase-Doppler anemometry (PDA) and numerical predictions. A mixture of gas and particles was injected without swirl into the test section, while the swirling airstream was provided through a co-flowing annular inlet. Two cases with different primary jet exit velocities were considered. For these flow conditions, a closed central recirculation bubble was established just downstream of the inlet.

The PDA measurements allowed the correlation between particle size and velocity to be obtained and also the spatial change in the particle size distribution throughout the flow field. For these results, the behaviour of different size classes in the entire particle size spectrum, ranging from about 15 to 80 μm, could be studied, and the response of the particles to the mean flow and the gas turbulence could be characterized. Due to the response characteristics of particles with different diameters to the mean flow and the flow turbulence, a considerable separation of the particles was observed which resulted in a streamwise increase in the particle mean number diameter in the core region of the central recirculation bubble. For the lower particle inlet velocity (i.e. low primary jet exit velocity), this effect is more pronounced, since here the particles have more time to respond to the flow reversal and the swirl velocity component. This also gave a higher mass of recirculating particle material.

The numerical predictions of the gas flow were performed by solving the time-averaged Navier-Stokes equations in connection with the well known kε turbulence model. Although this turbulence model is based on the assumption of isotropic turbulence, the agreement of the calculated mean velocity profiles compared to the measured gas velocities is very good. The gas-phase turbulent kinetic energy, however, is considerably underpredicted in the initial mixing region. The particle dispersion characteristics were calculated by using the Lagrangian approach, where the influence of the particulate phase on the gas flow could be neglected, since only very low mass loadings were considered. The calculated results for the particle mean velocity and the mass flux are also in good agreement with the experiments. Furthermore, the change in the particle mean diameter throughout the flow field was predicted approximately, which shows that the applied simple stochastic dispersion model also gives good results for such very complex flows. The variation of the gas and particle velocity in the primary inlet had a considerable impact on the particle dispersion behaviour in the swirling flow and the particle residence time in the central recirculation bubble, which could be determined from the numerical calculations. For the lower particle inlet velocity, the maximum particle size-dependence residence time within the recirculation region was considerably shifted towards larger particles.     

Measurement of velocity in rotational flows using ultrasonic anemometry: the flowmeter

In this paper a previously developed theoretical model of the measurement process performed by a transit-time ultrasonic anemometer is applied to a fluid flowing through a circular section pipe. This model considers the influence of the shift of the acoustic pulse trajectory from straight propagation due to the flow on the measured speed. The aim of this work is to estimate the errors induced in the measured velocity by the shift of the acoustic pulse trajectory. Using different duct’s flow models, laminar and turbulent regimes have been analyzed. The results show that neglecting the effect of shift of the acoustic pulse trajectory leads to flow rate measurement underestimation.     

Investigation of turbulent flows via pseudo flow visualization part I: Axisymmetric jet mixing layer

Hot-wire anemometer measurements obtained in the near-field axisymmetric jet mixing layer by Glauser and George [1] are examined using a pseudo flow visualization (PFV) technique. Pseudo flow visualization is a visualization procedure used to manipulate data obtained from an array of probes to create a graphical representation of the instantaneous and fluctuating velocity components of a flow field. An indicator function was employed to identify the frequency content of each velocity-time trace, giving insight into the analysis of the visualizations. From this application, the natural shedding frequency, or preferred mode, of the large-scale structures was determined and compared with the conventional streamwise and radial spectral measurements acquired by Glauser and George [1]. Furthermore, the wavelength of the preferred mode, nondimensionalized by the jet exit diameter, was determined to be approximately 2.4, a result consistent with the work of Crowe and Champagne [2]. In Part 1 the technique is developed and discussed for the fundamental and fairly well-researched mixing layer of the axisymmetric jet. Our aim is to verify the effectiveness of PFV in the context of a well-documented flow. In Part 2, this technique is then applied to an industrial flow field, namely, the mixing region of a lobed mixer     

Combined laser-doppler and cold wire anemometry for turbulent heat flux measurement

 combined laser-doppler and cold wire anemometry technique for determining turbulent heat flux is described. The system can be used in flows of arbitrarily high turbulent intensity and large temperature variations. Its potential is demonstrated via measurements in a simulated stable atmospheric boundary layer, for which the Monin-Obukhov length scale was about 70% of the boundary layer depth. Mean and turbulence properties were obtained throughout the boundary layer and the results are shown to be both internally consistent and similar to corresponding field data. Measurements in the highly turbulent, separated flow behind a bluff body mounted in the stable boundary layer are also presented. Received: 9 May 1997 / Accepted: 2 September 1997