Laser Doppler Anemometry Data Research Articles

On the origin, frequency and magnitude of macro-instabilities of the flows in stirred vessels

The mean flow and turbulence fields in a fully baffled vessel stirred by a Rushton impeller at three clearances and a pitched-blade turbine at one clearance have been investigated with laser-Doppler anemometry (LDA) to characterise the macro-instabilities (MIs) present in such flows. Time-resolved velocity measurements were made and the frequency content of the velocity recordings was analysed with FFT techniques. The study aims to throw light into the frequency, magnitude, nature and origin of such flow variations, especially in view of the different findings that have been reported in the published literature. The frequency of the MIs was found to be linearly related to the rotational speed of the impeller and to be essentially independent of impeller design. A single fundamental frequency, around 0.015–0.02 N (Hz), where N is the impeller rotational speed, was present for all configurations, together with harmonic frequencies, the prominence of which depended on impeller geometry and/or clearance. The LDA data and direct observations made with laser-sheet flow visualisation indicated clearly that the macro-instability stems from a precessional motion about the vessel axis, similar to the precession encountered in most swirling flows. The results show that MIs, as a mean flow motion superimposed on the flow pattern in the vessel, can result in a broadening of the measured turbulence levels by up to 25%, and a mean velocity variation of up to 0.3 V tip. The findings indicate therefore that it might be necessary and indeed it could be advantageous for improved process prediction to take MI into account in models of the flows in stirred vessels.

Particle image velocimetry and laser Doppler anemometry experimental studies of a compressible short take-off and vertical landing ground vortex flow

A particle image velocimetry (PIV) and laser Doppler anemometry (LDA) study of a scaled short take-off and vertical landing (STOVL) ground vortex flow is presented. The scaled flow features a compressible impinging jet in cross-flow with a moving ground plane. Mean and transient PIV and LDA velocity data are recorded from the ground vortex about the jet centre-line over a range of nozzle pressure ratios (NPR s) from 2.3 to 3.7, nozzle height—diameter ratios ( h/dn) from 3 to 10 (where dn = 12.7 mm) and cross-flow velocities (V∞) from 10 to 20 m/s, corresponding to effective velocity ratios of 19 < Ve−1 < 38. For each condition, 72 PIV vector maps were taken from the ground vortex region to generate an instantaneous and time-average data set. From the instantaneous data, a cinematic sequence was used to track the ground vortex position, which was found to fluctuate longitudinally by a root mean square distance of up to 4.47 dn and vertically by up to 2.18 dn. From the time-averaged PIV measurements, selected LDA pointwise data were taken at the average ground vortex core. Subsequent spectral analysis of the PIV time series showed the ground vortex position to fluctuate at dominant frequencies of between 2.5 and 5 Hz while the LDA data showed the velocity to fluctuate by dominant frequencies ranging between 1 and 30 Hz.

A direct transform method for the analysis of laser Doppler anemometry engine data

A new, computationally efficient method of fitting a specific parametric stochastic model of in-cylinder flow to real laser Doppler anemometry (LDA) internal combustion engine data, obtained either at a single position in space or over a range of spatial positions at effectively one point in the engine cycle (scanning data) is proposed. The model, which is fully specified in terms of a set of parameters, assumes that the velocity variations can be modelled as the sum of an ensembleaveraged mean component, a non-stationary ‘turbulence’ component and a random ‘cycle-to-cycle’ component, the last of these being phase locked to the engine cycles. The fitting technique is based on a direct transform of the velocity data obtained in each cycle and leads to an identification of the mean-square magnitudes of the turbulence and cycle-to-cycle components. A unique advantage of this approach is that it is possible to determine, theoretically, confidence limits for the estimates of the statistics of the fluctuation components. This provides a theoretical means of designing experiments to achieve a prescribed level of accuracy. The estimation method, and the theoretical determination of confidence limits, is validated initially through application to some simulated data which fully capture all the characteristics inherent in real data. Finally some results of analysing real in-cylinder LDA data, of the scanning type, are presented. Although the approach proposed here is applied to a particular model of in-cylinder flow it is, in principle, extendable to other models.

An investigation of in-cylinder tumbling motion in a four-valve spark ignition engine

The formation and break-up of the tumble in the cylinder were studied in a single-cylinder four-valve spark ignition engine using laser Doppler anemometry (LDA) measurements and multidimensional numerical simulations. The flow structure generated by the tumble break-up was also analysed using the cycle-resolved LDA data processing method. These results show that, during the intake stroke, two counter-rotating vortices are generated in the cylinder by the intake flow along the two sides of the cylinder. They then gradually evolve into the tumble vortex at the initial stage of the compression stroke. Tumble motion can be strengthened by increasing the intake flow going along the surface of the exhaust valves and/or decreasing the intake flow descending directly along the cylinder wall on the side of intake valves. Although a partially decayed tumble vortex still exists in the central part of the combustion chamber near the end of compression, in other parts of the combustion chamber the tumble distorts and breaks up into small vortices and eddies so that the root mean square velocity fluctuation increases. The flow structure generated by the tumble break-up has a characteristic of lower frequency and larger eddy scale.

Aerodynamics of Gurney Flaps on a Single-Element High-Lift Wing

The trailing-edge region of a single-element wing fitted with Gurney flaps has been studied. Measurements include surface pressure, force, and velocity by laser Doppler anemometry (LDA). The mean-velocity vectors and streamlines suggest a twin vortex structure downstream of the Gurney flap. Spectral analysis of the LDA data indicates that the wake consists of a von Karman vortex street of alternately shed vortices, and this flow structure is confirmed by smoke visualization of the flow downstreamof the Gurney flap. The vortex shedding increases the trailing-edge suction of the aerofoil, whereas the upstream face of the device decelerates the flow at the trailing edge of the pressure surface. These two changes result in a pressure difference acting across the trailing edge, and it is this that generates the increase in circulation.

Impinging jets confined by a conical wall: Laminar flow predictions

AbstractA laminar jet flow confined by a conical wall and an impinging plate was investigated. The Navier‐Stokes equations were numerically solved by a finite difference technique and the results compared with laser Doppler anemometry data, the latter also covering the transition and turbulent flow regimes. Transition was found to start in the impingement region at a jet Reynolds number of around 1,600. The investigation concentrated on assessing the effects of nozzle‐to‐plate distance (always less than one nozzle diameter), jet Reynolds number, and nozzle outlet conditions. The shape of the velocity profile at the nozzle outlet determines the entire cell flow field, whereas nozzle‐to‐plate distance affects the flow in the expansion region. Under certain flow and geometric conditions a recirculation zone appears in the expansion region, in the vicinity of the plate.

Evaluation of LDA temporal and spatial velocity structure functions in a low Reynolds number turbulent channel flow

Measurements have been made using Laser Doppler Anemometry (LDA) in a fully developed turbulent channel flow with the aim of determining second-order and third-order temporal and spatial structure functions of the longitudinal velocity fluctuation. A reliable determination of these moments requires the data to be corrected for the effect of noise. Correction procedures are outlined, based on the behaviour of temporal or spatial correlation functions in the limit of small time intervals or small separations. No a priori assumptions about the nature of the noise are made so that the procedure should be quite general. The corrected LDA data indicate that, especially for spatial separations, the effect of noise can be felt even within the inertial range. The corrected structure functions should allow an unambiguous assessment to be made of Taylor’s hypothesis and of the extended self-similarity (ESS) method; examples are given in each case. Temporal structure functions obtained by hot wire anemometry (HWA) are much less affected by noise than the LDA data.

Fluid temperature distributions in a rotating cavity with an axial throughflow

Radial and circumferential variations of cavity air temperature are presented for a rotating cavity with an axial throughflow of cooling air. Results show that the air temperatures are influenced significantly by the cavity surface temperature distribution. Strong (around 5°C) circumferential air temperature variations are observed showing the flow to be three-dimensional. Temperature time traces are used to infer flow structure features and also the variation of the fluid to cavity angular velocity ratio with temperature. Features and trends are found to be consistent with flow visualisation evidence, Laser Doppler Anemometry data, and also computations of other workers.

Estimation of turbulence power spectra from randomly sampled data by curve-fit to the autocorrelation function applied to laser-Doppler anemometry

The development of a novel slotting technique for the estimation of the autocorrelation function (ACF) of randomly sampled data has eliminated the major disadvantages of the Mayo algorithm. The reduced variance of the estimators in the vicinity of and the elimination of estimators greater than 1 has opened the possibility to approximate the ACF with a curve-fit, leading to a noise-free estimator of the power spectrum. In this paper we describe an analytical function, based on known properties of turbulence power spectra, which is suited to the approximation of the ACF from randomly sampled laser-Doppler anemometry (LDA) data with noise. Using simulated LDA data, based on both simulated and hot-wire measured turbulence signals, it is shown that the method estimates the power spectrum with an error of less than 25% over six decades of power. Application to various LDA data sets shows that the analytical function, derived from the spectral properties, is sufficiently flexible to describe ACFs from data sets of pipe flow, mixing layer flow, flow in stirred tanks and flows around airfoils. An extension of the technique could be in automatic incorporation of periodic components and compensation for data sets with a high velocity bias (turbulence intensities > 40%).

On the evolution of laminar vortex rings

Using Laser Doppler Anemometry (LDA) and Digital Particle Image Velocimetry (DPIV), the physical properties of laminar vortex rings are investigated in the Reynolds-number range 830 ≤ Re ≤ 1650. The measured initial circulations of the vortex rings are found to agree well with corrected versions of the vorticity-flux (slug-flow) model proposed by Didden and Pullin. The DPIV and LDA data show excellent agreement regarding local velocities and vortex-ring circulations. The DPIV data depict the distribution of the vorticity and circulation in the core regions, where the resulting vorticity distributions are found to be self-similar Gaussian profiles. The propagation velocity of the vortex rings is well approximated by an analytical model of Saffman for large core sizes. In the asymptotic limit t → ∞, the trajectories are in excellent agreement with the exact Stokes-dipole solution of Cantwell and Rott.

Laser Doppler anemometry data simulation - Application to investigate the accuracy of statistical estimators

Autoregressive models of order 1 are used to generate three-dimensional flowfields with specified probability distributions, spectra, and shear stress tensor. These known flowfields are then used to simulate the particle arrival statistics in laser Doppler anemometry for homogeneous particle densities. Further simulation steps allow modeling of the processor sampling and data acquisitions, providing all necessary data for investigating the bias and variance of statistical estimators. Results are presented for estimators of the first two moments of the velocity distribution function. Experimental data are obtained for verification purposes. The results demonstrate the technique`s potential for studying novel ot theoretically intractable estimators.

An investigation of the influence of the size distribution of seeding particles on LDA velocity data in the vicinity of a large velocity gradient

The influence of the properties of the size distribution of seeding particles on the results of laser Doppler anemometry (LDA) velocity measurements in the vicinity of a large velocity gradient downstream of an aerodynamic shock is studied numerically. Accounting for the differing velocity lag of particles of different sizes the average velocity and the pertaining rms values are determined in the relaxation region within which the particles adjust towards the changed flow velocity downstream of the shock. It is shown how the average velocity and the rms value depend on the mean particle size and the density of the particle material. It is demonstrated in a numerical simulation how the LDA system’s ‘‘amplitude limit,’’ which can offer a means of excluding the contribution of large particles towards the experimental results, affects the average velocity and the rms value to be measured in the relaxation region. Experimental LDA data of velocity measurements across an oblique shock are presented. A good qualitative agreement of the numerical and the experimental results is found.

Spectral estimation and analysis of LDA data in pulsatile flow through heart valves

The Laser Doppler Anemometry (LDA) measurement technique presents an inherent difficulty when spectral analysis is applied to it. The random nature of the LDA signal prohibits sampling at regular, equi-spaced, time instants. Irregular sampling presents additional variability of the spectral estimator. In order to reduce this variability, spectral analysis of LDA data is performed according to the method of direct Fourier transform of short blocks of data, as suggested by Gaster and Roberts. The LDA data is measured in a flow field distal to prosthetic heart valves with varying degrees of stenosis. The spectral estimates of velocity data sampled during the rapid closure stage of the valve are achieved with excellent frequency resolution. Important and useful information about dominant frequency peaks and preferred modes which exist in the flow, otherwise smeared or concealed in the spectral contents, are then derived from the spectral information. These modes are quantitatively analysed in light of vortex formation and related flow mechanisms. Comparative studies of normal and stenosed valves show that the preferred modes are governed by the valve geometry and dynamic behavior and are correlated to the severity of the stenosis.

Fluid-Dynamic Investigation and Optical Characterization of Particulate to Reduce Diesel Emissions

ABSTRACT This paper reports recent studies on air motion in a new combustion chamber geometry that reduces the exhaust emission and an on-line optical characterization of the particulate emitted at the exhaust for a development of an after treatment device. A direct injection (d.i.), heavy-duty, single cylinder diesel engine with an optical access, operating at 1000 rpm under motored conditions, was used. Laser Doppler Anemometry (LDA) measurements of cycle resolved instantaneous tangential velocity, performed in different locations of the conventional straight sided and new generation of re-entrant combustion chamber, were reported. The mean motion and turbulence intensity were computed, during the compression stroke, using a filtering procedure of LDA data In addition, simultaneous multiwavelength scattering and extinction measurement from the near ultra-violet (u.v.) to visible were carried out at the exhaust of a d.i. diesel engine operating at 2200 and 3000 rpm. Size, concentration, optical properties and mass of particulate were evaluated versus different engine parameters. A good agreement was observed on comparing the data with those obtained by conventional techniques

Time and spatial correlation measurements in the turbulent wake of a circular cylinder using Laser Doppler Anemometry

The present paper reports Laser Doppler Anemometry (LDA) measurements in the turbulent (R d =1300) wake of a circular cylinder (d=2 mm). On several typical locations in the wake, at cross-section x/d=125, temporal and lateral space velocity correlations have been measured. Temporal autocorrelation functions were determined from the randomly sampled LDA data by means of the “Slotting Technique”. The autocorrelation functions which were obtained were too low, the discrepancy giving an indication of the influence of noise in the measurements. The influence of noise generated by the photomultiplier tubes, which appeared to be the most important noise source, could be reduced by a cross-correlation technique. A partly new LDA system for two-point velocity correlation measurements was developed, consisting of an elongated measuring volume and a two-point detection system including a semireflecting mirror to divide the scattered light. The results of this investigation clearly demonstrate that LDA is a powerful tool for measuring time and spatial correlations and related properties of turbulence.

Versatile microprocessor data logging and processing system for laser Doppler anemometry

A description is given of the design and performance of a versatile microprocessor data-acquisition system used for the logging and processing of counter-type laser Doppler anemometry data. The system with a 32-K of RAM can handle up to 10 000 data points. The statistical analysis carried out involved the determination of Vmin, Vmax, V̄, and σ together with the complete probability density distribution function. In order to allow for the repeated analysis of the same data file using different numbers of data points and velocity limits, care was taken not to destroy the original data files during the analysis. The use of an arithmetic processor unit reduced the processing time of the data by almost one order of magnitude. Under typical operating conditions, the system performed a complete statistical analysis of 10 000 data in about 28 s.