Laser Particle Image Velocimetry Research Articles

Computational fluid dynamics as a modern tool for engineering characterization of bioreactors

Since design, construction and evaluation of bioreactors for large-scale production is costly and time consuming, computational methods may give some insights into the fluid mechanics within bioreactors. Thus, critical limiting factors, such as insufficient mixing as well as inhomogeneous nutrient and oxygen mass transfer, may be identified early in the design process. Although advanced experimental techniques such as laser Doppler anemometry and particle image velocimetry are also reliable, they are too time consuming to characterize the complete flow pattern in industrial scales and rely on optical accessibility. Therefore, the knowledge of flow characteristics provided by computational fluid dynamics (CFD) is indispensable for the rational design of bioreactors. Based on previously published reviews, the present work summarizes the latest publications on the usage of CFD to characterize and scale-up bioreactors used in biotechnological processes. Selected models that are used to predict the fluid flow pattern and key engineering parameters of commonly used bioreactors are described. Related issues, such as grid dependency of CFD results and the requirement for experimental verification are also addressed. Finally, an overview of proposed but not yet feasible CFD applications is presented, including fluid–structure interaction, the use of direct numerical simulation and the coupling fluid flow and chemical reactions.

Particle image velocimetry in viscoelastic fluids and particle interaction effects

Two main assumptions in laser doppler velocimetry and particle image velocimetry (PIV) are that tracer particles are following the flow faithfully and that they are not interacting hydrodynamically or affecting the flow field. It has been shown, however, that particles in polymeric fluids even in low volumetric concentrations (ϕ < 1 % v/v) can interact profoundly because of the viscoelastic nature of the background fluid, leading even to particle chain or string formation. The objective of this study is to determine how these interactions can affect the flow conclusions drawn from PIV measurements. We chose a highly viscoelastic and shear thinning fluid and confirmed that particle string formation does occur rapidly in a simple shear flow of this fluid. Then, we measured via PIV, the fully developed velocity profile in the mid-plane of a square channel over a wide range of particle loadings 0.005–0.2 % v/v. The measurements confirm that despite the chaining process, there is no discernible effect of particle interaction on PIV velocity measurements.

Experimental Investigation of Parameters, Influencing Velocity Fields during Beer Fermentation

In the course of the research aiming on the optimisation of beer fermentation, a large number of fermentations have been performed under various boundary conditions. We carried out different measurements, including temperature and velocity investigations. Due to turbidity, the latter cannot be performed easily by using common techniques like laser Doppler anemometry or particle image velocimetry. Therefore the ultrasound Doppler velocimetry got utilised. It permits measurements in opaque fluids and provides velocity fields for any time during the fermentation.

Subsonic jet pressure fluctuation characterization by tomographic laser interferometry

This paper describes the application of a nonconventional experimental technique based on optical interferometry for the characterization of aeroacoustic sources. The specific test case studied is a turbulent subsonic jet. Traditional experimental methods exploited for the measurement of aerodynamic velocity fields are laser Doppler anemometer and particle image velocimetry which have an important drawback due to the fact that they can measure only if the flow is seeded with tracer particles. The technique proposed, by exploiting a laser Doppler interferometer and a tomographic algorithm for 3D field reconstruction, overcomes the problem of the flow seeding since it allows directly measuring the flow pressure fluctuation due to the flow turbulence. A laser Doppler interferometer indeed is sensitive to the density oscillation within the medium traversed by the laser beam even though it integrates the density oscillation along the entire path traveled by the laser. Consequently, the 3D distribution of the flow density fluctuation can be recovered only by exploiting a tomographic reconstruction algorithm applied to several projections. Finally, the flow pressure fluctuation can be inferred from the flow density measured, which comprehends both the aerodynamic pressure related to the turbulence and the sound pressure due to the propagation of the acoustic waves into the far field. In relation to the test case studied in this paper, e.g., the turbulent subsonic jet, the method allows a complete aeroacoustic characterization of the flow field since it measures both the aerodynamic “cause” of the noise, such as the vortex shedding, and the acoustic “effect” of it, i.e., the sound propagation in the 3D space. The performances and the uncertainty have been evaluated and discussed, and the technique has been experimentally validated.

Experimental Acoustic Flow Analysis Inside a Section of an Acoustic Waveguide

Abstract Noise propagation within ducts is of practical concern in many areas of industrial processes where a fluid has to be transported in piping systems. The paper presents experimental data and visualization of flow in the vicinity of an abrupt change in cross-section of a circular duct and on obstacles inside where the acoustic wave generates nonlinear separated flow and vortex fields. For noise produced by flow wave of low Mach number, laminar and turbulent flows are studied us- ing experimental sound intensity (SI) and laser particle image velocimetry (PIV) technique adopted to acoustics (A-PIV). The emphasis is put on the development and application of these methods for better understanding of noise generation inside the acoustic ducts with different cross-sections. The intensity distribution inside duct is produced by the action of the sum of modal pressures on the sum of modal particle velocities. However, acoustic field is extremely complicated because pressures in non-propagating (cut-off) modes cooperate with particle velocities in propagating modes, and vice versa. The discrete frequency sound is strongly influenced by the transmission of higher order modes in the duct. By under- standing the mechanism of energy in the sound channels and pipes we can find the best solution to noise abatement technology. In the paper, numerous methods of visualization illustrate the vortex flow as an acoustic velocity or sound intensity stream which can be presented graphically. Diffraction and scattering phenomena occurring inside and around the open-end of the acoustic duct are shown.

Flow field measurements of a series of turbulent premixed and stratified methane/air flames

This paper presents flow field measurements for the turbulent stratified burner introduced in two previous publications in which high resolution scalar measurements were made by Sweeney et al. [1,2] for model validation. The flow fields of the series of premixed and stratified methane/air flames are investigated under turbulent, globally lean conditions (ϕg=0.75). Velocity data acquired with laser Doppler anemometry (LDA) and particle image velocimetry (PIV) are presented and discussed. Pairwise 2-component LDA measurements provide profiles of axial velocity, radial velocity, tangential velocity and corresponding fluctuating velocities. The LDA measurements of axial and tangential velocities enable the swirl number to be evaluated and the degree of swirl characterized. Power spectral density and autocorrelation functions derived from the LDA data acquired at 10kHz are optimized to calculate the integral time scales. Flow patterns are obtained using a 2-component PIV system operated at 7Hz. Velocity profiles and spatial correlations derived from the PIV and LDA measurements are shown to be in very good agreement, thus offering 3D mapping of the velocities. A strong correlation was observed between the shape of the recirculation zones above the central bluff body and the effects of heat release, stoichiometry and swirl. Detailed analyses of the LDA data further demonstrate that the flow behavior changes significantly with the levels of swirl and stratification, which combines the contributions of dilatation, recirculation and swirl. Key turbulence parameters are derived from the total velocity components, combining axial, radial and tangential velocities.

Flow field measurements of pulverized coal combustion using optical diagnostic techniques

This paper demonstrates the application of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) techniques to a particle-laden reacting flow of pulverized coal. A laboratory-scale open-type annular burner is utilized to generate velocity profiles of coal particles and micrometric alumina particles. Pair-wise two-component LDV measurements and high-speed stereo PIV measurements provide three-dimensional velocity components of the flow field. A detailed comparison of velocities for alumina and coal particle seeding revealed differences attributed to the wide size distribution of coal particles. In addition, the non-spherical shape and high flame luminosity associated with coal particle combustion introduces noise to the Mie scatter images. The comparison of mean and RMS velocities measured by LDV and PIV techniques showed that PIV measurements are affected by the wide size distribution of coal particles, whereas LDV measurements become biased toward the velocity of small particles, as signals from large particles are rejected. This small-particle bias is also reflected in the spectral characteristics for both techniques, which are in good agreement within the range of frequencies accessible. PIV measurements showed an expected lack of response of large coal particles to the turbulence fluctuations. The overall good agreement between LDV and PIV measurements demonstrates the applicability of the high-speed PIV technique to a particle-laden, high luminosity coal flame while highlighting some of its limitations.

An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall

The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.

Experimental Investigation of a Synthetic Jet Array in a Laminar Channel Flow

The paper deals with an impinging synthetic jet, namely on the case of a synthetic jet array interacting with a laminar channel flow. This arrangement can be useful in many micro-scale applications, such as cooling of micro-electronics. The flow regime in micro-scale is usually laminar with very small Reynolds numbers; therefore synthetic jet array can be used for the profile disturbance and heat transfer enhancement. The paper focuses on the low Reynolds number (in order 102). The working fluid is water and a piezoceramic transducer is used as a moving membrane in the synthetic jet actuator. Experiments are performed with four experimental methods (tin ion visualization, hot wire anemometry in constant temperature mode, laser Doppler vibrometry and particle image velocimetry) in three laboratories (at the Eindhoven University of Technology, Netherlands, at the Institute of Thermodynamics CAS, v.v.i. and Technical University of Liberec, both Czech Republic).

Heat transfer processes in parallel-plate heat exchangers of thermoacoustic devices – numerical and experimental approaches

This paper addresses the issues of heat transfer in oscillatory flow conditions, which are typically found in thermoacoustic devices. The analysis presented concerns processes taking place in the individual “channels” of the parallel-plate heat exchangers (HX), and is a mixture of experimental and numerical approaches. In the experimental part, the paper describes the design of experimental apparatus to study the thermal-fluid processes controlling heat transfer in thermoacoustic heat exchangers on the micro-scale of the individual channels. Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques are applied to obtain spatially and temporally resolved temperature and velocity fields within the HX channels. The temperature fields allow obtaining the local and global, phase-dependent heat transfer rates and Nusselt numbers, and their dependence on the Reynolds number of the oscillating flow. The numerical part of the paper deals with the implementation of CFD modelling capabilities to capture the physics of thermal-fluid processes in the micro-scale and to validate the models against the experimental data. A two-dimensional low Mach number computational model is implemented to analyse the time-averaged temperature field and heat transfer rates in a representative domain of the HXs. These are derived by integrating the thermoacoustic equations of the standard linear theory into a numerical calculus scheme based on the energy balance. The comparisons between the experimental and numerical results in terms of temperature and heat transfer distributions suggest that the optimal performance of heat exchangers can be achieved when the gas displacement amplitude is close to the length of hot and cold heat exchanger. Heat transfer coefficients from the gas-side can be predicted with a confidence of about 40% at moderate acoustic Reynolds numbers.

Flow over riblet curved surfaces

The present work studies the mechanics of turbulent drag reduction over curved surfaces by riblets. The effects of surface modification on flow separation over steep and smooth curved surfaces are investigated. Four types of two-dimensional surfaces are studied based on the morphometric parameters that describe the body of a blue whale. Local measurements of mean velocity and turbulence profiles are obtained through laser Doppler anemometry (LDA) and particle image velocimetry (PIV).

Unsteady velocity measurements in a realistic intracranial aneurysm model

The initiation, growth and rupture of intracranial aneurysms are intensively studied by computational fluid dynamics. To gain confidence in the results of numerical simulations, validation of the results is necessary. To this end the unsteady flow was measured in a silicone phantom of a realistic intracranial aneurysm. A flow circuit was built with a novel unsteady flow rate generating method, used to model the idealised shape of the heartbeat. This allowed the measurement of the complex three-dimensional velocity distribution by means of laser-optical methods such as laser doppler anemometry (LDA) and particle image velocimetry (PIV). The PIV measurements, available with high temporal and spatial distribution, were found to have good agreement with the control LDA measurements. Furthermore, excellent agreement was found with the numerical results.

Shallow-flow visualization analysis by proper orthogonal decomposition

The identification of the spatial characteristics and the shedding frequencies of coherent structures in shallow flows usually involves the use of sophisticated equipment for velocity measurements such as laser Doppler anemometry or particle image velocimetry. In this work, a simple and low-cost alternative for the quantitative characterization of quasi two-dimensional shallow coherent structures is presented. The technique is based on the image pre-processing of flow visualizations and post-processing by means of the proper orthogonal decomposition. As an illustration of the method, the analysis of a shallow flow in the wake of a cylinder with a vortex-street pattern is presented. The method is able to discriminate concentration patches attached to the vortices and their advection, providing frequencies in excellent agreement with a previous study.

Near Valve Flows and Potential Blood Damage During Closure of a Bileaflet Mechanical Heart Valve

Blood damage and thrombosis are major complications that are commonly seen in patients with implanted mechanical heart valves. For this in vitro study, we isolated the closing phase of a bileaflet mechanical heart valve to study near valve fluid velocities and stresses. By manipulating the valve housing, we gained optical access to a previously inaccessible region of the flow. Laser Doppler velocimetry and particle image velocimetry were used to characterize the flow regime and help to identify the key design characteristics responsible for high shear and rotational flow. Impact of the closing mechanical leaflet with its rigid housing produced the highest fluid stresses observed during the cardiac cycle. Mean velocities as high as 2.4 m/s were observed at the initial valve impact. The velocities measured at the leaflet tip resulted in sustained shear rates in the range of 1500-3500 s(-1), with peak values on the order of 11,000-23,000 s(-1). Using velocity maps, we identified regurgitation zones near the valve tip and through the central orifice of the valve. Entrained flow from the transvalvular jets and flow shed off the leaflet tip during closure combined to generate a dominant vortex posterior to both leaflets after each valve closing cycle. The strength of the peripheral vortex peaked within 2 ms of the initial impact of the leaflet with the housing and rapidly dissipated thereafter, whereas the vortex near the central orifice continued to grow during the rebound phase of the valve. Rebound of the leaflets played a secondary role in sustaining closure-induced vortices.

Transient response of the Delft jet-in-hot coflow flames

The ignition of impulsively started jets of natural gas in a hot coflow has been studied, with the aim of gaining insight into the processes that occur in a furnace equipped with regenerative burners. The development of the velocity field was measured in a time resolved manner with Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) at 10 kHz. Furthermore, simultaneous PIV and Planar Laser Induced Fluorescence of the OH-radical (OH-PLIF) were employed in combination with high-speed recordings of flame luminescence to map the ignition sequence. A characteristic feature of the impulsively started jets was that the turbulent phase was preceded by a laminar phase, during which no significant amount of combustion took place. After the sudden transition towards a turbulent state, significant ignition delay times were observed. As demonstrated in earlier work Oldenhof et al. (2011) [1], the radial temperature gradient of the coflow makes the entrainment of coflow fluid into the jet region very important to the jet stabilisation, influencing strongly the lift-off height. Using the time-resolved PIV data, the transport of fluid parcels in the coflow region was monitored during the transition towards the turbulent state. This transport is governed by the low velocities of the coflow and as a result, ignition delay times and the time until a steady state of combustion is attained are not solely related to jet time scales. It is thus concluded that in the presence of large-scale inhomogeneities, ignition delay times are several times greater than the time needed for the velocity field of the jet to reach its steady turbulent state.

The upstream flow of a wind turbine: blockage effect

AbstractThe flow upstream a wind turbine is studied in order to investigate blockage effects. We use rotating wind turbine models in a wind tunnel, where velocity measurements have been made both with hot‐wire anemometry up to approximately 4.5 diameters (D) upstream the turbine, as well as laser particle image velocimetry measurements close to the turbine rotor. Also, numerical simulations have been carried out by means of a finite volume code. The measurements show, among other things, that the flow is affected more than 3 D upstream the rotor plane. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Optical investigation of a long throated Venturi conveying inert spherical particulate with size range similar to pulverised coal

This experimental and numerical calculation work demonstrates the behaviour of a two-phase gas–solid Venturi. The type of two-phase flow studied is that encountered in conveying coal to the furnaces in power stations - a light to moderate powder loading at near atmospheric pressure. A glass Venturi with a long throat (three throat diameters) is inserted into a pneumatic conveying test rig, which has similar pipe configuration, and powder and air dynamic similarity to a coal fired power station. The Reynolds number (Re) based on throat diameter is 460,000 and the particles have a size range up to 140 μm. Laser sheet visualisation and particle image velocimetry (PIV) reveal the behaviour of the fine powder in the Venturi. The PIV registers velocities less than the gas velocity in the throat with scatter in the order of 5% around the mean. There is a region next to the walls on the entire circumference in the throat section and diffuser which contains few particles. The particle lean region appears to correspond to the velocity boundary layer in the throat (according to standard flat-plate boundary layer theory) and the limit of the wake in the diffuser based on a numerical calculation. The experimentally determined particle velocities are compared with a numerical calculation, which includes a calculation of the wake using initial boundary layer assumptions at the end of the throat. The initial condition which corresponds best to the PIV results uses the assumption of a turbulent flat plate boundary layer on the length of the throat. The PIV velocity corresponds approximately to the calculated velocity of the mass mean diameter particle. This particle size is approximately 50% of the pixel resolution size on the recorded images. The observation of Payne and Crowe (1984) that the pressure relationship with mass flow rate is unaffected by particle size, provided that the Stokes number for the particles is greater than 7, is supported here. Particles with St<7 achieve the throat velocity, and the proportion that has St>7 continues accelerating beyond the end of the throat. It is likely that in power station operation there is a significant proportion of coal particles having St<7 over the operational range.

The stabilization characteristics of turbulent lifted diffusion flames of CH4/CO blended fuels

The outstanding stabilization characteristics of lifted turbulent jet diffusion flames of CH4/CO blended fuels in still air were investigated experimentally by igniting the flame from downstream in the initial condition of a nonreacting fuel jet. In this study, we concentrated on the effects of the shift of outstanding reaction paths on the stabilization characteristics as the composition of the blended CH4/CO is varied. Systematic experiments were conducted using the nonintrusive diagnostics of shuttered laser particle image velocimetry (PIV), OH∗ chemiluminescence, and digital image processing to study the flame stabilization characteristics in terms of the key parameters such as the instantaneous flame-base location, lift-off height, and streamline velocity distribution in the vicinity of the edge flame leading point by varying the exit velocity and blended-fuel concentration. A conditional leading point from shutter PIV measurement is defined to calculate the instantaneous streamline velocity profile at the flame base. For the CO concentration in fuel blends lower than 80%, the stabilization is similar in characteristics to that proposed by the triple flame model. As the CO concentration is increased to 90%, the flame stabilizes on the fuel-rich side and the stabilization characteristics may be related to the maximum burning velocity at the rich equivalence ratio. This is confirmed by the location of the instantaneous flame stabilization point defined by the instantaneous leading point of the lifted flame with respect to the fuel concentration profiles.

Theoretical and Experimental Studies of Hydraulic Characteristics of Discharge Tunnel with Vortex Drop

The velocities at given points in the volute chamber, the contracted section and the vertical dropshaft of a discharge tunnel with vortex drop were measured by a small specially designed L-shaped tube, as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) would not work there due to the special structure of the discharge tunnel with vortex drop. Hydraulic empirical formulas were proposed to predict the velocities and the angles of the velocities made with the vertical direction θ. The theoretical analysis results were in good agreement with experimental data. Therefore, the method proposed in this paper can be used to analyze related characteristics of discharge tunnels with vortex drop. Additionally, different model scales were considered to predict the cavitation characteristics on the wall of a dropshaft in practical engineering.

Two-Dimensional Intraventricular Flow Mapping by Digital Processing Conventional Color-Doppler Echocardiography Images

Doppler echocardiography remains the most extended clinical modality for the evaluation of left ventricular (LV) function. Current Doppler ultrasound methods, however, are limited to the representation of a single flow velocity component. We thus developed a novel technique to construct 2D time-resolved (2D+t) LV velocity fields from conventional transthoracic clinical acquisitions. Combining color-Doppler velocities with LV wall positions, the cross-beam blood velocities were calculated using the continuity equation under a planar flow assumption. To validate the algorithm, 2D Doppler flow mapping and laser particle image velocimetry (PIV) measurements were carried out in an atrio-ventricular duplicator. Phase-contrast magnetic resonance (MR) acquisitions were used to measure in vivo the error due to the 2D flow assumption and to potential scan-plane misalignment. Finally, the applicability of the Doppler technique was tested in the clinical setting. In vitro experiments demonstrated that the new method yields an accurate quantitative description of the main vortex that forms during the cardiac cycle (mean error for vortex radius, position and circulation). MR image analysis evidenced that the error due to the planar flow assumption is close to 15% and does not preclude the characterization of major vortex properties neither in the normal nor in the dilated LV. These results are yet to be confirmed by a head-to-head clinical validation study. Clinical Doppler studies showed that the method is readily applicable and that a single large anterograde vortex develops in the healthy ventricle while supplementary retrograde swirling structures may appear in the diseased heart. The proposed echocardiographic method based on the continuity equation is fast, clinically-compliant and does not require complex training. This technique will potentially enable investigators to study of additional quantitative aspects of intraventricular flow dynamics in the clinical setting by high-throughput processing conventional color-Doppler images.