Laser Doppler Anemometry Measurements Research Articles

Comparison of LDA and hot-wire measurements at moderate subsonic mach numbers

A detailed comparison of Hot-Wire-Anemometry (HWA) and Laser-Doppler-Anemometry (LDA) measurements in the wake of an airfoil is presented. An evaluation of the capabilities of both techniques in measuring three-dimensional turbulence in turbomachinery applications is made and potential application pitfalls are highlighted. It is shown that the HWA tends to underestimate the turbulence intensity due to damping effects, while the LDA tends to overestimate the turbulence intensity due to the optical setup. The wake flow of a single airfoil at inflow Mach numbers of 0.35 and 0.45 and corresponding Reynolds numbers of 480,000 and 630,000 are used as a reference case. Different HWA probe head designs and various wire diameter are considered, as well as different setups for the LDA measurement device. In a comparison of the two techniques, the turbulence intensity measured differs by up to <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mn>1.3</mml:mn></mml:math></inline-formula> percentage points (pp) in the freestream region and 4.0 pp within the wake. This is primarily due to the resolution of high-frequency fluctuations. For both measuring techniques, potential sources of errors are highlighted, especially regarding the application to turbomachinery flows.

An experimental and numerical study of turbulent oscillatory flow over an irregular rough wall

The hydrodynamics of turbulent oscillatory flow over a gravel-based irregular rough wall is investigated using laser-Doppler anemometry measurements of velocities in a large oscillatory flow tunnel and direct numerical simulation (DNS) of the Navier–Stokes equations. The same periodic irregular roughness was used for both experiments and DNS. Four flow shapes are investigated: sinusoidal, skewed, asymmetric and combined skewed–asymmetric. The experiments were conducted for target Reynolds numbers (based on the Stokes length and standard deviation of free-stream velocity) of$R_{\delta,\sigma }=800$and$R_{\delta,\sigma }=1549$; DNS was conducted for flows with target$R_{\delta,\sigma }=800$. Boundary layer thickness, bottom phase lead and friction factor are in good agreement with previous studies. For the first time, evidence of Prandtl's secondary flows of the second kind in oscillatory flow is presented. Turbulence structure is visualised using isosurfaces of$\lambda _{2}$(Jeong & HussainJ. Fluid Mech., vol. 285, 1995, pp. 69–94), revealing densely packed structures that grow stronger and weaker in correspondence with the free-stream velocity. Reynolds and dispersive stresses peak just below the highest roughness crest, with dispersive stress vanishing a short distance above the roughness. Bursts of turbulence kinetic energy and wake kinetic energy are generated each flow half-cycle, with variable behaviour depending on flow shape. Non-Gaussian turbulence statistics are observed that originate near the wall, becoming increasingly non-Gaussian far from the wall. Probability density functions of turbulence statistics can be closely approximated by a fourth-order Gram–Charlier distribution at most phases and elevations, though when statistics deviate more strongly from Gaussian, streamwise and wall-normal (spanwise) statistics are better described by a Pearson type IV (VII) distribution.

Pulsating flow velocity profile measurement at an acoustically reflecting and non-reflecting open pipe end using laser doppler anemometry (LDA)

Depending on the geometry and operating condition, occasionally strong and destructive acoustic resonances occur in the pipe spool piece between the high-pressure discharge of screw compressors and the connected pulsation damper/silencer inlet. To avoid these resonances, the use of a nozzle with defined cross-section reduction is investigated. By designing the contraction ratio as a function of the nozzle inlet Mach number, a non-reflecting termination/transition can be provided. For a defined operating point, acoustic reflection from the nozzle can be avoided in this way, thus preventing the acoustic resonance in the spool piece.Within this paper the pulsating flow field directly downstream an acoustically reflecting open pipe end is investigated using laser doppler anemometry (LDA) at the air test rig at the Chair of Fluidics at TU Dortmund University. In the modified experimental setup an operating point-specific designed nozzle is installed at the open pipe end to create a non-reflecting termination. By means of LDA measurements, the velocity profile is determined experimentally at the nozzle outlet for the designed operating condition. The 2D measurement results (radius dependant velocity and phase distribution) extend established 1D decomposition of pressure measurement signals into upstream and downstream travelling waves and allow a deeper understanding of the acoustic behaviour of non-reflecting nozzles.

Using Numerical Dissipation Rate and Viscosity to Assess Turbulence‐Related Data Accuracy – Part 2: Results

AbstractThis is the second part of a two‐part paper focusing on the assessment of accuracy of turbulence‐related data from CFD simulations using effective numerical dissipation rate and effective numerical viscosity. Experimental setup has been discussed in the first part of this series. Here, the relevant solution data obtained via CFD are compared to the values from laser Doppler anemometry measurements, and it is studied whether the accuracy of such data can be assessed using the two mentioned quantities. The overall outcome is that although judging mesh quality generally is possible, alone the two quantities are insufficient to draw conclusions regarding the actual solution data.

Turbulent drag reduction of viscoelastic wormlike micellar gels

Long-chained, viscoelastic surfactant solutions (VES) have been widely employed in the oil and gas industry, particularly in hydraulic fracturing and gravel-packing operations, where turbulence is commonly reached due to high pumping rates. With this motivation, we experimentally investigate the turbulent duct flow of an under-studied class of wormlike micellar solutions that forms a gel at room temperature. The fluid is characterized via rotational rheometry, and the turbulent velocity and Reynolds stress profiles are measured via Laser Doppler Anemometry (LDA). Three surfactant concentrations are investigated at increasing Reynolds numbers. The turbulent flow fields of water, and semi-dilute solutions of partially hydrolysed polyacrylamide (HPAM) and xanthan gum (XG) are used as comparisons. Our study reveals that the gel-like structure of the wormlike micellar gel is mostly broken down during turbulent flow, especially in the near-wall region where the results indicate the presence of a water layer. Turbulent flow at low concentrations of surfactant show a Newtonian-like flow field throughout most of the duct, where the energy spectra shows a − 5 / 3 power law scale with wavenumber, whereas higher concentrations lead to drag reduction and lower power spectral densities at large wavenumbers. A comparison of the flow of polymeric fluids and the wormlike micellar solutions at maximum drag reduction (MDR) shows comparable drag-reduction effects, with a large decrease in Reynolds shear stresses, and increased turbulence anisotropy in the buffer layer due to the large streamwise fluctuations and near-zero wall-normal fluctuations. At the centreline of the duct, the power spectral densities of polymers and VES are significantly reduced at all wavenumbers probed. Additionally, the MDR regime was bounded by Virk’s asymptote for both polymers and the micellar gel, which implies a similar mechanism of drag reduction at MDR. • Turbulent flows of wormlike micellar gels (VES) are experimentally studied. • Turbulent flows of VES and polymeric liquids are quantified via LDA measurements. • Steady flow curves of VES are not representative of its rheology in turbulent flow. • At MDR, the VES and polymer solutions show decreased energy spectra. • Our results show similar drag reduction mechanisms between VES and polymers at MDR.

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities.

Validated numerical simulation of airflow in child respiratory airways

Both pulmonary airway geometry and breathing pattern evolve from birth to adulthood. These gradual changes significantly influence airflow dynamics and subsequently particle transport and deposition. Regarding the airway structure, the differences between infants and adults are most profound till 5 years of age. Since that age the child airway geometry can be downsized from adult lungs. The objective of the present work was to investigate airflow patterns in child airways. The digital model of male respiratory airways was downsized to dimensionally correspond to a 5–year old child. Airflow simulations with properly validated RANS k–ω turbulence model were performed under steady inhalation boundary conditions. The inspiratory flow rate was 12.5 L/min. The velocity profiles just upstream of the first bifurcation were validated by Laser–Doppler anemometry (LDA) experiments. A replica of the same geometry as the one used in the simulation was 3D printed and part of the trachea was replaced by a glass tube. LDA measurements in several points upstream of the first bifurcation were carried out. The validated flow patterns were compared to the ones obtained in adult lungs.

LDA measurements in a one-sided ribbed square channel at Reynolds numbers of 50,000 and 100,000

Laser Doppler anemometry (LDA) measurements in the fully developed turbulent flow through a one-sided rib-roughened square channel with a rib-height-to-channel-height ratio of k/H = 0.0667 and a rib-pitch-to-rib-height ratio of p/k = 9 were carried out at Reynolds numbers (based on the channel height H and the mean bulk velocity u_B) of {Re}_H = 5.0 cdot 10^4 and 1.0 cdot 10^5. Mean velocities, Reynolds normal, and shear stresses were determined. The first- and second-order statistical moments of the velocity differed slightly between both Reynolds numbers when normalized by the bulk velocity and the channel height. Deviations were attributed to a minor elongation increase in the large recirculation region behind the rib with increasing Reynolds numbers. The crucial role of turbulent diffusion was reflected in the non-coincidence of the maximum primary mean shear and the Reynolds normal stress as well as outward drifts of the Reynolds stress peaks along the rib crest. Turbulent diffusion contributed significantly to the turbulence kinetic energy (TKE) budget near the rib crest and within the separated shear layer, while mean convection made a remarkable contribution only at the rib leading edge. The detailed first- and second-order moments provide a basis for the validation of computational methods and turbulence models.Graphic abstract

Urban Boundary Layers Over Dense and Tall Canopies

Wind-tunnel experiments were carried out on four urban morphologies: two tall canopies with uniform height and two super-tall canopies with a large variation in element heights (where the maximum element height is more than double the average canopy height, h_{max}=2.5h_{avg}). The average canopy height and packing density are fixed across the surfaces to h_{avg} = 80~hbox {mm}, and lambda _{p} = 0.44, respectively. A combination of laser Doppler anemometry and direct-drag measurements are used to calculate and scale the mean velocity profiles with the boundary-layer depth delta . In the uniform-height experiment, the high packing density results in a ‘skimming flow’ regime with very little flow penetration into the canopy. This leads to a surprisingly shallow roughness sublayer (depth approx 1.15h_{avg}), and a well-defined inertial sublayer above it. In the heterogeneous-height canopies, despite the same packing density and average height, the flow features are significantly different. The height heterogeneity enhances mixing, thus encouraging deep flow penetration into the canopy. A deeper roughness sublayer is found to exist extending up to just above the tallest element height (corresponding to z/h_{avg} = 2.85), which is found to be the dominant length scale controlling the flow behaviour. Results point toward the existence of a constant-stress layer for all surfaces considered herein despite the severity of the surface roughness (delta /h_{avg} = 3 - 6.25). This contrasts with the previous literature.

The near-field aerodynamic characteristics of hot high-speed jets

Abstract

Large-eddy simulation of flow turbulence in clarification systems

Prediction of turbulent flow is required for design and assessment of clarifier systems that have been implemented throughout history to treat water in the urban water cycle through physical clarification. Yet, turbulent flow modeling is a relatively new tool that has not existed until the last half-century and can be, and often is, a tenuous component in a computational fluid dynamics simulation of unit operations and processes. Common Reynolds-averaged Navier–Stokes equation (RANS) approaches can be inadequate to obtain consistent and accurate flow solutions. In contrast, this study presents an application of large-eddy simulations (LES) for a clarification system with a high-order spectral element method employing 48 million degrees of freedom. Turbulent and unsteady flow characteristics are investigated, and statistics are examined for such a system. Simulation results are compared with laser Doppler anemometry measurements for mean flow velocity, turbulence kinetic energy, and Reynolds shear stress. LES results agree well with measurements, and the differences between LES and measurements are generally less than the reported measurement error. LES results capture the transition behavior from a jet-like flow at the near-inlet region to an open-channel flow at the downstream end of the system. Furthermore, LES results reveal that the widely adopted log-law of a classical turbulent boundary layer is not established in the system even at the most downstream location. Preliminary examination of commonly used RANS models identifies the challenges in application of RANS to such systems. The results from this study provide a benchmark for turbulence modeling of common water clarification systems.

Assessment of algebraic subgrid scale models for the flow over a triangular cylinder at Re = 45000

The turbulent separated flow over an equilateral triangular cylinder at a Reynolds number 45000 (based on the triangle edge) is studied by means of large-eddy simulation (LES). The moderate Reynolds number is chosen in order to replicate available experimental data (Laser Doppler Anemometry measurements at the Volvo test rig). The OpenFoam CFD toolbox is used for the present numerical simulations. Several algebraic subgrid scale (SGS) closures, including the conventional Smagorinsky model (Cs=0.1 and Cs=0.053), the model by Vreman (Cs=0.1) and the dynamic version of the k-equation model are investigated. The spectral analysis of the vortex shedding and the convective instabilities as well as the turbulence kinetic energy dissipation rate is presented. Some aspects of the blockage effects are investigated and compared to experimental data available in the literature. It was shown that blockage affects significantly the mean drag and pressure coefficients of the triangular cylinder. At the same time the mean velocity field is tending to similarity. Overall, all SGS models reproduced most important aspects of the flow physics of the separated bluff-body flow, including integral flow parameters and spectral characteristics with dispersion about 10%, showing a reasonable consistency with the experimental data.

Sediment Morphology and the Flow Velocity Field in a Gully Pot: An Experimental Study

Urban runoff (re)mobilises solids present on the street surface and transport them to urban drainage systems. The solids reduce the hydraulic capacity of the drainage system due to sedimentation and on the quality of receiving water bodies due to discharges via outfalls and combined sewer overflows (CSOs) of solids and associated pollutants. To reduce these impacts, gully pots, the entry points of the drainage system, are typically equipped with a sand trap, which acts as a small settling tank to remove suspended solids. This study presents data obtained using Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA) measurements in a scale 1:1 gully to quantify the relation between parameters such as the gully pot geometry, discharge, sand trap depth, and sediment bed level on the flow field and subsequently the settling and erosion processes. The results show that the dynamics of the morphology of the sediment bed influences the flow pattern and the removal efficiency in a significant manner, prohibiting the conceptualization of a gully pot as a completely mixed reactor. Resuspension is initiated by the combination of both high turbulent fluctuations and high mean flow, which is present when a substantial bed level is present. In case of low bed levels, the overlaying water protects the sediment bed from erosion.

Nasal cavity airflow: Comparing laser doppler anemometry and computational fluid dynamic simulations

Objective parameters to assess the physical flow conditions of breathing are scarce and decisions for surgery, e.g. nasal septum correction, mainly rely on subjective surgeon judgment. To define decision supporting parameters, we compare laser Doppler anemometry (LDA) and numerical computational fluid dynamic simulations (CFD) of the airflow velocity vector fields in the nasal cavity, including lattice Boltzmann (LB) and finite volume methods (FVM). The simulations are based on an anonymous patient CT dataset with septal deviation. LDA measurements are preformed using a 3D printed model. Nasal airflow geometry is randomly deformed in order to approximate surgical changes. The root-mean-square velocity error near the nasal valve of laser Doppler anemometry and lattice Boltzmann simulations is 0.071. Changes in geometry similarly affect both measurement and simulation.

Numerical simulations and experimental measurements on flow features in a patient-specific upper airway model with obstructive sleep apnea

In this article, the turbulent flow features in a patient-specific upper airway model with obstructive sleep apnea before and after surgery are numerically and experimentally studied by applying the large eddy simulation and laser Doppler anemometry. Two three-dimensional anatomically accurate numerical models are reconstructed based on the CT-scan of one severe obstructive sleep apnea patient for both pre- and post-surgery. Two experimental models of the same upper airway are established by using the three-dimensional printing technique according to the numerical models. The first direct comparison of flow velocity profiles between measurement and simulation at different locations and cross-sections is conducted in two upper airway models. The simulated result indicates that the especially large axial velocity and negative pressure appear in the narrowest cross-section before surgery, but they either reduce or vanish after surgery. Both measurement and simulation can capture the “pharyngeal jet” flow and the reversed flow in two upper airway models, and the agreement between the measured and calculated velocity profiles at different locations and cross-sections is excellent, which gives us great confidence that the large eddy simulation can be a reliable method for predicting the flow features associated with obstructive sleep apnea airway. Although this work represents significant advance, there still exists some limitations, such as the discrepancies between measurement and simulation in the interface of main stream velocity and reverse velocity due to the flow complexity, in the reversed flow region owing to the uncertainty of laser Doppler anemometry measurement at low velocity range, and the incapability of capturing flow near the wall due to the intrinsic feature of laser Doppler anemometry measurement. Besides, it is very difficult to find the corresponding locations exactly between experimental and numerical models, which could also cause the deviation between experiment and simulation.

Validation for CFD Simulation in Rod Bundles With Split-Vane Spacer Grids Based on LDA Measurement

Laser Doppler anemometry (LDA) measurement of axial flow velocity and pressure drop measurement has been carried out in a 5x5 rod bundle installed with two split-mixing-vane grids. The measured results are utilized to validate the CFD simulation. The realizable and nonlinear k-ε turbulence model is utilized in the CFD computation, while the two-layer wall treatment is employed with both models. Mesh sensitivity investigation shows that the pressure drop is weakly affected by local mesh refinement in the spacer grid, while its effect on the velocity in the near wake of spacer grid is apparent. The validation shows that prediction on the mean axial velocity is relatively poor in the near wake of the spacer grid where the measured result shows relatively smooth distribution of axial velocity. Refinement of mesh in the spacer grid eliminates some peaks for the nonlinear model. Comparing with the effect of mesh refinement, the difference caused by the turbulence models is relatively weak in the near wake region. In the far-wake region it is still difficult to judge which model shows definite superiority to the other. For pressure drop, prediction of nonlinear k-ε is closer to the experiment.

Similarity of the asymmetric swirl generator and a double bend in the near-field range

An asymmetric swirl generator (ASG) is considered to replace the current swirl generator in the upcoming 2020 revision of the standard for heat meters EN 1434. While recent studies have shown its superiority with respect to a reproducible representation of the double bend out-of-plane (DB) flow disturbance in the far field, there are still open questions regarding the similarity in the near-field range and the determination of an optimum testing distance. In this paper, we examine the performance of an ASG in the potential testing range and investigate an increase of the segmental orifice plate. Laser Doppler anemometry (LDA) measurements downstream of the ASGs and a DB carried out at Reynolds numbers (Re) of 5×104 and 5×105 are evaluated based on a comparison of the flow patterns and a quantitative analysis by means of performance indicators. The results indicate, that the original version of the ASG does not reach the level of asymmetry and swirl provided by the DB in the near-field range. An increase of the orifice height yields higher swirl and asymmetry, hence, a better similarity of the flow characteristics. A maximum degree of resemblance with the crescent-shaped velocity patterns of the DB is found seven diameters downstream of the modified ASG. In contrast, a testing distance of two diameters or less does not represent the DB, as the early flow development bears the traces of the disturber's geometry. The results may be considered for the implementation of the test procedure in EN 1434 and the next revision of the standards for water meters.

Vibrations in a 7-rod bundle subject to axial flow: Simulations and experiments

Being able to quantify mechanical vibrations is of key importance for the safety of nuclear power plants, as they are able to induce damage. In this work, numerical simulations are used to compute water flow and vibration in a densely packed bundle of 7 rods, mimicking an experimental setup. This flow configuration is chosen to resemble the coolant flow through a nuclear reactor core. Because of the wall proximity, a considerable velocity difference between the narrow gaps and the subchannels exists, with an inflection point in the velocity profile. This yields an unstable situation, and large vortices are continuously created through a mechanism similar to the Kelvin–Helmholtz instability. The vortex streets in between the rods are associated with a fluctuating pressure field, causing vibrations of the rods.The experimental setup contains 7 steel cylinders, encased in a hexagonal duct. The central rod contains a section where the steel is replaced by a water-filled silicone tube, clamped at both extremes to the steel rod, and the vibrations of this section are examined. The numerical approach consists of coupled fluid–structure interaction (FSI) simulations, with the flow being modelled using computational fluid dynamics (CFD) and the structure using computational solid mechanics (CSM).The available experimental data consist of Laser Doppler Anemometry (LDA) measurements and high-speed camera footage of the wall movement of the silicone rod. Equivalent data is collected from the numerical simulations. The simulations are repeated for different flow rates. The frequency spectrum of the coherent structures, and the frequency and amplitude of the wall movement are compared for each operating point, as well as their trend as a function of the flow rate. The dominant frequencies found in the simulation results were similar to the experimental results, although slightly higher. They also showed a linear trend, just like the experiments. A larger mismatch was present for the structural response, the frequencies found using the FSI model being more than twice as high.

Thermal performance enhancement of non-premixed syngas combustion in a partial combustion unit by winged nozzle: Experimental and CFD study

This paper presents the effect of nozzle assembly design on the performance of a partial combustion unit (PCU) using the scale-adaptive simulation (SAS) and non-intrusive laser measurement techniques. Four different configurations were tested, namely, nozzle without wing and three nozzles with wing, i.e., flat surface wing, semi-sphere hollow wing and bent wingtip. The syngas-oxygen reaction chemistry was calculated using non-premixed flame model incorporated with GRI-MECH 3.0 mechanism. The radiative heat transfer was modelled using the discrete ordinates (DO) model. The simulation was compared with the particle image velocimetry (PIV) and a two-dimensional laser doppler anemometry (LDA) measurement on a scaled-down PCU model. A good agreement between the SAS prediction and experimental measurement was obtained. It was found that the modified nozzle assembly design with a semi-sphere hollow wing yielded the highest combustion temperature owing to the intense turbulence-induced recirculation mixing of oxy-fuel. The modified nozzle assembly design introduced in this work increased the peak outlet combustion temperature up to 18% higher compared to the original design. The finding in this work may useful for design retrofits of a combustion system.

Mars 2020 Rover Influence on Wind Measurements at Low Reynolds Number

The Mars 2020 rover is the new vehicle dedicated to the Martian surface investigation. This vehicle will transport Mars Environmental Dynamic Analyzer, the new meteorological station, including two wind sensors installed in the camera mast. An experimental characterization was conducted to investigate the influence of the Mars 2020 rover in the Mars Environmental Dynamic Analyzer wind measurements at low Reynolds numbers. Wind tunnel experiments were conducted using a 1:45th scaled model in a wind tunnel specially designed for these experiments. The velocity was measured using laser Doppler anemometry. A method is proposed in this investigation to calculate a correction factor for the data measurements of wind sensors embarked on rovers dedicated to planetary exploration missions. In particular, the method was applied to wind measurements taken by Mars Environmental Dynamic Analyzer in the Mars 2020 rover using the laser Doppler anemometry measurements, and corrections up to 40% in the velocity magnitude and 23 deg in the deflection angle were found.