Constant Temperature Anemometry Research Articles

Influence of side dilution jets on swirling and non-swirling pulverised biomass turbulent annular flows

The use of pulverised biomass in non-premixed combustion systems, featuring turbulent annular flows (swirling, non-swirling) with downstream air dilution jets, is of immense practical importance. The overall combustion performance and emissions heavily rely on the underlying flow dynamics, turbulence and dispersion behaviour of the pulverised biomass particles. Despite the practical significance of side dilution jets in aiding complete combustion and controlling pollutants, the fundamental understanding of side dilution jets influence on the biomass particle velocity field, turbulence, and dispersion characteristics is still lacking. However, the presence of a multi-phase (solid-gas) flow field, highly turbulent annular flows, swirl, and cross-wise turbulent dilution jets poses significant challenges to these investigations. This work employs a combination of two-dimensional planar Particle Image Velocimetry (PIV) experiments and three-dimensional multi-phase numerical modelling to resolve the influence of turbulent side dilution jets (Red = 18,000 and 27,000) on the particle flow and dispersion characteristics of turbulent annular jets equipped with a central pulverised biomass-laden jet. The numerical modelling uses Reynolds stress model (RSM) and discrete phase model (DPM) for the solution of gas and dispersed phases, respectively. Walnut flour based raw pulverised biomass is injected at a certain particle loading ratio (Φp=0.33 and 0.49) through a turbulent central jet (Rj = 4500) into non-swirling (S = 0) and swirling (S = 0.3) turbulent confined annular flows (Res = 35,500 and 17,800). Comprehensive Constant Temperature Anemometry (CTA) experiments are conducted to obtain well-defined boundary conditions for experiments as well as numerical simulations. Prior to numerical predictions, the models are duly validated against the experimental data.Results reveal that the characteristic flow feature of side dilution jets, i.e., peripheral recirculation zones (PRZ) significantly entrains pulverised biomass particles from the turbulent particle-laden (central jet) annular flows (both swirling and non-swirling) and alters the particle flow and turbulence characteristics. Under non-swirling conditions (S = 0), side dilution jets reduce mean particle axial velocity by 22.4% and increase turbulent fluctuating velocity by 52.6%. These figures elevate to 32.5% and 100% respectively for swirling conditions (S = 0.3), depicting enhanced turbulent mixing between solid particles and the gas phase. In terms of particle dispersion, side dilution jets significantly enhance lateral dispersion of biomass particles, particularly under swirling flow conditions. In the PRZ region, side dilution jets led to a 108% and 279% increase in normalised particle count for non-swirling and swirling cases, respectively. These findings suggest that swirl doubles the particle dispersion in the PRZ. Furthermore, a 50% increase in Red enables side dilution jets to entrain more biomass particles and disperse them into the PRZ. Conversely, a 50% increase in ṁp leads to a significant increase in particle volume fraction along the central axis of the nozzle.

A Comparative Study on Numerical Flow Simulations of a Centrifugal Electronic Cooling Fan Using Four Different Turbulence Models

In this study the flow field of a centrifugal electronic cooling fan operating at an off-design point of 0 Pa static fan pressure is investigated by means of Computational Fluid Dynamics. The results obtained by four different turbulence models, the realizable k-ϵ model, the SST k-ω model, a Reynolds Stress Model, and Scale-Adaptive Simulation are analyzed and compared. The focus lies on describing how the flow through impeller and volute influences the fan outlet flow field, and velocity profiles and velocity fluctuations at the outlet are compared to previously published measurements. All models tend to underpredict the measured outlet flow rate, but are capable of producing the characteristic C-shaped profile of high velocities, previously determined in Constant Temperature Anemometry measurements. However, the realizable k-ϵ model is significantly too diffusive, leading to blurred velocity contours. The other models exhibit reasonable agreement with the measured flow field, but show differences in a number of aspects. The SST k-ω model, for instance, even produces local inflow in a confined area. The SAS approach overpredicts the length of the lower lobe of the C-shape. The research is relevant to improve simulation results of impingement cooling and heat sink optimization using centrifugal fans.

Insights into the intrinsic asymmetry of bluff-body stabilised swirling and non-swirling annular flows

Bluff-body stabilised annular jets possess simple, well-defined, axisymmetric geometry, yet exhibit highly complex flow fields, are susceptible to asymmetry at larger blockage ratios, and embody both complex zones and shear layers. Practical annular combustors feature swirl and a central jet which further complicates the flows. Despite the importance of asymmetry in practical combustor design, a systematic investigation for the role of flow regime (transitional, turbulent), Reynolds number, swirl number, or central jets on asymmetry remains lacking. In this study, two-dimensional time-averaged planar particle image velocimetry measurements are conducted to resolve the intrinsic asymmetry associated in transitional (Res = 2700) and turbulent (Res = 17,800 and 35,500) bluff-body stabilised confined annular jets. Experiments are conducted on non-swirling (S = 0) and swirling annular jets (with S = 0.3), both operating independently and with the addition of a turbulent central jet (Res = 10,400). Constant temperature anemometry measurements resolve boundary conditions at the exit of annulus and an Asymmetry Index is presented.Results show that swirl significantly mitigates asymmetric vortex shedding and restores annular jet symmetry. Quantification (at x/D = 1, r/D = 0.5) reveals that turbulent swirling annular jets demonstrates a 42 % (for Res = 17,800) and 78.6 % (for Res = 35,500) reduction in asymmetry compared to non-swirling annular jets. Whereas, swirling annular jets under transitional flow regime show a negligible impact on the flow asymmetry. In addition to this, increasing the Reynolds number from Res = 2,700 to 17,800 results in a 36.2 % and 58.7 % decrease in the asymmetry for non-swirling and swirling annular jets, respectively. A further increase to Res = 35,500 brings an additional 68.6 % decrease in the asymmetry for swirling annular jets, showing that swirl in combination with high turbulence restores and maintains symmetry in bluff-body stabilised annular jets. Moreover, at medium turbulence (Res = 17,800), the non-swirling annular jet with a superimposed turbulent jet exhibits a nearly axisymmetric flow behaviour compared to stand-alone non-swirling annular jet.

Experiments and modelling of biomass pulverisation in swirling and non-swirling bluff body-stabilised turbulent annular flows

Deeper insights into particle loading have practical significance in applications particularly when this affects fuel-oxidiser mixing and flame stabilisation, as occurs in pulverised fuel (biomass) combustion systems. The underlying particle flow and dispersion characteristics not only play a crucial role in controlling overall combustion performance but can also impact emissions. A fundamental understanding of particle flow dynamics and dispersion behaviour for raw pulverised biomass, under non-swirling and swirling conditions need further systematic investigation to better understand the interplay between swirling and non-swirling bluff-body stabilised recirculation zones and particles emitted from a centralised jet. This work uses Particle Image Velocimetry (PIV) with three-dimensional multi-phase simulations based on the Discrete Phase Model (DPM) and Reynolds Stress Model (RSM) to investigate the flow and dispersion characteristics in confined flows typical of many practical combustors. Raw pulverised biomass-laden is introduced through a central turbulent jet (Rej = 4500 and 7800) and also subjected to turbulent annular flows (Res = 35,500), both non-swirling (S = 0) and swirling conditions (S = 0.3). Simulations are first validated against Constant Temperature Anemometry (CTA) resolved inlet boundary conditions and flow field PIV data under similar conditions.Results show that when a pulverised biomass-laden central jet interacts with a surrounding turbulent annular flow (non-swirling, swirling) the presence of a bluff-body based recirculating zone (BB-RZ) leads to biomass particle entrainment (pick up) and their recirculation over the bluff-body before being dispersed further downstream. Under non-swirling conditions, a significant 35% decrease in the mean particle axial velocity is measured coupled with an even more substantive 177% increase in turbulent fluctuation along the centreline. These findings are indicative of intense upstream (x/D ≈ 0.64) turbulent mixing and more intense particle dispersion into a BB-RZ. For swirling annular flow conditions near the end of the BB-RZ, the interaction between a biomass-laden central jet and the annular flow is comparatively weaker in the upstream region relative to non-swirling conditions. However, swirl significantly enhances downstream particle dispersion and lateral spread as reflected by a 254% hike in the mean particle radial velocity. Numerical predictions show that for the same particle loading ratio, but different (higher and lower) Reynolds number of a central particle-laden jet (i.e., the carrier gas), the conditions at relatively lower central jet Reynolds number allow better particle recirculation in BB-RZ as well as enhancing downstream lateral particle dispersion and entrainment, compared to a higher Reynolds number in the central jet. Outcomes from this investigation may have implications on the design and operation of pulverised (solid) fuel combustors if operated on renewably sourced biomass rather than traditional fossil fuels.

Secondary air induced flow structures and mixing in a fixed bed combustor

Air staging features widely in biomass combustion from small space heaters to industrial-scale moving grate systems. Whilst studies have been conducted into the impact of air staging on emissions and combustion performance, there is little or no insight into the exact flow dynamic and physical mechanisms that are induced by secondary air under such conditions. This paper uses experimental data to validate numerical predictions before investigating the flow field structures and mixing characteristics in a fixed bed air staged combustor. The study utilizes Constant Temperature Anemometry (CTA) experiments to establish boundary conditions for a lab-scale fixed bed combustor. Results from numerical simulations obtained using [Formula: see text] l- 𝜔 model were in good agreement with experimental data. Conditions tested cover five different secondary to total air ratios (Qs/Qt) and two different locations of secondary air injection. Results show that at Qs/Qt = 0.50, 0.25 and 0.18 one strong recirculation zone is induced upstream of the secondary air injection and one downstream. Varying the point of injection of secondary air from h/D = 0.64 to h/D = 0.40 also had an effect on the size of the upstream recirculation zone. Qs/Qt = 0.50 had the highest values of mixing index among all spatially located planes at the upstream of secondary air injection. The overall findings shed light on the possible flow interactions between secondary air and the top layers of fuel bed. They also highlight the significance of secondary air on inducing both upstream and downstream flow structures.

Experimental Study of Turbulence Response in a Slowly Accelerating Turbulent Channel Flow

AbstractAn investigation of flow acceleration from initial statistically steady turbulent flow to final statistically steady turbulent flow is conducted experimentally using particle image velocimetry (PIV) and constant temperature anemometry (CTA). The turbulence response is investigated as the acceleration periods and acceleration rates are varied in a controlled fashion. This work expands the research by Mathur et al. (2018, “Temporal Acceleration of a Turbulent Channel Flow,” J. Fluid Mech., 835, pp. 471–490.) studying slower and longer transient flows. It also complements the numerical studies of a step increase in the flowrate of (He and Seddighi, 2013, “Turbulence in Transient Channel Flow,” J. Fluid Mech., 715, pp. 60–102. and He and Seddighi, 2015, “Transition of Transient Channel Flow After a Change in Reynolds Number,” J. Fluid Mech., 764, pp. 395–427.). The results obtained from the current investigations are qualitatively similar to those obtained previously. Consistent with previous studies, the response of turbulence in the current slow transient flow is again characterized by a laminar-turbulent transition. The initial increase of the flow development among the cases investigated can be categorized as faster, medium, and slower responses. Modifications are made to the equivalent Reynolds number and the initial turbulence intensity proposed earlier in order to account for the slow accelerating flow rates and the continuous change of the bulk velocities of the cases investigated. It has been shown that the critical equivalent Reynolds number based on these modifications and the initial turbulence intensity are well correlated for all cases investigated and a power-law relation is established.

Experiments into the interaction of side dilution jets with bluff-body stabilised annular jets

Fundamental insights into the effects of side dilution jets on the flow behaviour and turbulence field in gas turbine combustors further advances the ability to use these jets to control (upstream) flame stability and (downstream) emissions. Despite this, the interaction of side dilution jets with typical flame anchoring mechanisms, such as bluff-bodies, and annular flows, both swirling and non-swirling, remains elusive due to the challenges of undertaking highly resolved laser diagnostics in such complex geometries. In this work, Particle Image Velocimetry (PIV) is used to investigate the interaction of turbulent side dilution jets (Red = 18,000 and 11,300) with bluff-body stabilised annular jets (Res = 35500 and 17800), under both swirling (S = 0.3) and non-swirling (S = 0) conditions. Boundary conditions for the cases studied are well-defined through Constant Temperature Anemometry, which in conjunction with the PIV resolved flow field means the results presented additionally serve as good benchmark cases for modelling (not undertaken here). The experiments are conducted in an optically accessible 700 mm long square-like (310 mm × 310 mm) enclosure equipped with four peripheral side dilution jets on the rounded corner supports.Results show that irrespective of the flow configuration used (swirling or non-swirling), side dilution jets considerably modify the overall flow field and lead to the development of characteristic recirculation zones on the periphery of the annular jet, named as Peripheral Recirculation Zones (PRZ). In the upstream region, PRZ significantly promote turbulence in the outer shear layer of the bluff-body stabilised annular jet, leading to a massive 176% (at x/D = 1.4) and 218% (x/D = 1.8) increase in peak turbulent kinetic energy. These high levels of turbulence further magnify closer to the location of the side jets. In the downstream (beyond x/D > 2) of the bluff-body stabilised annular jet, noticeable increases in centreline velocity decay (13%) and jet spreading (32%) are observed in the presence of a PRZ, which indicates better mixing and flow entrainment. The overall effects of side dilution jets vary directly and inversely with the Reynolds number of side dilution jets and annular jets, respectively. To resolve the flow dynamic behaviour independent of heat release, all tests are undertaken isothermally (non-reacting).

Automotive cabin vent: Comparison of RANS and LES approaches with analytical-empirical equations and their validation with experiments using Hot-Wire Anemometry

The velocity field downstream of an automotive vent is one of the key parameters of passenger comfort. Two theoretical approaches (using analytical-empirical equations, and based on computational fluid dynamics) were applied to calculate the velocity of a jet emerging from a real rectangular benchmark ventilation outlet with adjustable blades. The computational simulations were performed by solving the Reynolds-averaged Navier–Stokes equations (RANS) with the realizable k-ε turbulence model and by Large Eddy Simulation (LES). The results were validated by experimental data acquired by constant temperature anemometry (CTA). The validation comprised a comparison of axial velocity decay, scalar velocity field, angles of jet inclination, and profiles of velocity and turbulence intensity.The study was performed for the isothermal free jet and attached jet, where surrounding walls simulated confinement in a car cabin. The analytical empirical equation by Rajaratnam can be successfully used also to determine the throw of the jet, which is favourable, especially in light of the fact that both computational methods were not very accurate in velocity decay predictions. Root mean square errors for the free jet, and attached jet (expressed for calculations made according to Rajaratnam, and by LES and RANS with respect to the experimentally measured values) were 0.50, 0.85, 0.87 m s−1, and 0.52, 0.30, 0.65 m s−1, respectively. The LES method was more accurate than RANS in predicting the velocity profiles. The average percentage error of LES, and RANS is 6.3%, and 17.4%, respectively however, the calculation time was almost 27 times higher for LES.

Experimental and Numerical Investigation of Bio-Inspired Riblet for Drag Reduction

Abstract Flow alteration using the bio-inspired riblet structure is a fascinating field of study resulting in drag benefits. Riblets have no power requirement being a passive method. This work aims to study the effect of riblets on flow and drag behavior using both experimental and numerical analysis. The experiments are performed using a flush mount shear stress probe (FMSSP) and constant temperature anemometry (CTA). FMSSP is a novel technique to measure stress without obstructing the flow. The study is done on longitudinal streamwise sawtooth-shaped riblet with a maximum Reynolds number (Re) of 1.68 × 105. Three-dimensional numerical modeling of the riblet structure over a smooth wall is analyzed to study the mechanism responsible for drag-reducing behavior. A maximum reduction of 13.2% in shear stress is observed in the study. The result infers an upward shift in the velocity profile relative to the smooth wall in the near-wall region. Due to riblets, large-scale structures breakdown near the wall and better mixing are observed above the surface. Near-wall vortices are imparted a movement away from the wall due to the riblet tip, thus mitigating the near-wall fluctuations. Along with this, drag-reducing riblets hamper the cross-flow near the wall, thus further decreasing the turbulence intensity. Results suggest that turbulent kinetic energy (TKE) has a similar trend to the drag reduction characteristic near the wall. The finding ascertains the potential application of the riblets for real-life settings.

Experimental study of laminar-to-turbulent transition in an adverse pressure gradient flow

Experimental results on laminar-to-turbulent transition in a separated laminar boundary layer formed over a flat plate in an adverse pressure gradient are provided. Experiments have been performed for a range of Reynolds numbers (1.4 − 2.4 · 105) and for two freestream turbulence levels (Tu =3.6 and 5.3%). The measurements of mean and fluctuating velocity profiles have been carried out at inlet to the test section using Constant Temperature Anemometry (CTA). The data might be useful for validation and development of transition models. In order to characterize the dynamics leading to the transition process in the separated boundary layer on the plate surface, the Particle Image Velocimetry (PIV) measurements have been performed. The results show that the transition process in the separated boundary layer is influenced by breakdown of the Klebanoff streaks.

Experimental investigation of flow characteristics over a surface-modified structured torpedo model

The present work aims to improve the aerodynamic performance of navigating objects using bio-inspired surface modification. Inspired by the denticles present over sharkskin, this work investigates the study of a surface-modified structured torpedo model. Different modified surface structures (riblets) are used to study their effect on the drag and flow characteristics. The modified structure placed in the shear layer modifies the flow above as well as behind the torpedo. A detailed flow field is studied using PIV (Particle Image Velocimetry) and CTA (Constant Temperature Anemometry) over the torpedo model. The experiments are performed in a subsonic wind tunnel for the Reynolds numbers (Re) range of 1.02–5.08 × 105, thus covering the laminar-turbulent transition range. The study reveals that riblets structure enhances the momentum flux, thus enabling the surface to reduce drag by suppressing the vorticity generation and reducing the turbulent dissipation. Different turbulent and wake characteristics are measured in the work. The velocity and fluctuation analysis shows the variation in wake width and wake velocity for different cases. The result showed that riblet effectively reduces drag coefficient by 11.7% using an optimal structure. The maximum reduction in formation length of 4.76% is observed at Re 3.05 × 105. The decrease in the formation length reduces the adverse pressure gradient zone and decreases the power requirement. The wake half-width at the 2D downstream location in the streamwise direction shows a reduction of 9% for the drag-reducing case at Re 3.05 × 105. The vorticity reduction is observed in the wake zone of drag-reducing riblets and suggests less dissipation of the eddy. The results show the perspective to improve the current research status by using riblet surface for marine application. The study could effectively reduce drag, thus improving the operating performance by reducing the energy consumption and increasing the speed of the navigating vessel.

Effect of side dilution jets on the velocity field and mixing in swirl and bluff-body stabilised annular confined flows

Stationary and mobile power generation systems, as well as those for aero propulsion, continue to rely heavily on gas turbines. Operational envelopes which critically depends on (flame) jet stabilisation mechanisms, such as swirl, are also affected by considerations for emissions control. In this context, the evolving swirling flow domains feature injection of side (peripheral) dilution air. Despite this, the role of side dilution jets on modifying the flow, turbulence, and mixing fields of turbulent bluff-body (BB) stabilised (swirling) annular flows has not been investigated. This study uses three-dimensional simulations, supported with experimental data to establish boundary conditions, to investigate the interaction of side dilution jets (Red = 18,000) with both non-swirling (S = 0) and swirling (S = 0.3) turbulent BB stabilised annular flows (Res = 35,500). Constant Temperature Anemometry (CTA) measurements are conducted to resolve inlet boundary conditions. The Reynolds Stress Model (RSM) and species transport model (isothermal) are used with validations against benchmark datasets. Results show that the introduction of side dilution jets into a confined domain, whether swirling or not, induces (asymmetrical) Peripheral Recirculation Zones (PRZ). However, the strength, length, and location of PRZ varies with swirl and the downstream positioning of side dilution jets (H = 2.6D, 6.6D). Additionally, side dilution jets increase centreline velocity decay for both, non-swirling and swirling BB flows. In terms of turbulence characteristics, addition of side dilution jets leads to significant gains in shear layer turbulence with an increase of 72% and 35% in turbulent kinetic energy for non-swirling and swirling BB flows, respectively. Analysis of the mixing characteristics shows negligible impact of side dilution jets in the upstream region, however, from x/D > 3 considerable improvement in central jet (fuel) mixing is observed.

Spray response on a model prefilmer under unsteady airflows of various frequencies

An appealing concept for jet engine combustors is the Lean Premixed Prevaporized (LPP) combustor, which operates at high pressures. The low NOx emissions achieved by lean combustion are one of the targets for modern aircraft engines. However, these types of combustors can introduce thermoacoustic instabilities that can potentially damage the engine and reduce its lifespan. Since the potential instabilities on the fuel spray characteristics, i.e. the spray mass flux, can affect the flame stability, the need arises to investigate the spray response under an unsteady airflow. For this study, a model prefilmer was experimentally investigated to produce a two-dimensional droplet flow without swirl flow. An acoustic forcing in the range of 100–500 Hz was introduced into the airflow, characterized by a hot wire Constant Temperature Anemometry (CTA) setup. Droplet characteristics, namely the droplet diameter distribution and velocity, were determined using a Phase Doppler Anemometry (PDA) setup, while the acquired data were phase-averaged in one period of the airflow oscillation. The influence of the excitation frequency and the air-to-liquid ratio (ALR) on the spray was studied: the spray responded to the acoustic excitation and therefore critical performance parameters, such as the spray mass flux, oscillated indicating potential problems regarding the flame stability.

Time-series analysis of differential pressure and flow velocity signals by means of weighted statistics

Turbulence intensity levels are assigned different values according to set a probability distribution. Many researchers do not consider probability distributions when determining turbulence intensity levels. This is one cause of the disagreement between numerical simulations of turbulence modeling and experimental data. This study focuses on the use of weighted statistics to evaluate turbulence intensity levels through three types of examples. The first is a hands-on example with a Pitot-static tube. The second utilizes numerical data and the Monte Carlo method. The third relies on a dataset by constant temperature anemometry and laser Doppler anemometry, available from the literature. This study finds that turbulence intensity evaluations differ depending on the selection of the probability distribution. The turbulence intensity according to a Gaussian distribution is reduced by more than 30% in comparison with that set with a rectangular distribution. The notion of the true and engineering turbulence intensity is introduced.

Analysis of velocity profile measurements obtained by different methods in low-speed, small-scale wind tunnel

A low-speed small-scale wind tunnel was designed for spray investigation under cross and co-flow conditions. Precisely defined profiles of mean velocity and turbulent intensity in the wind tunnel test section are crucial parameters for any rigorous flow study. Different velocity measurement techniques were used to evaluate the velocity and turbulent intensity profiles in the test section. Two non-intrusive techniques, Phase-Doppler Anemometry and Laser-Doppler Anemometry (PDA, LDA), and two intrusive techniques, Constant Temperature Anemometry (CTA) and pitot static tubes of S and L-type were applied. The velocity was measured in 19 equidistantly spaced positions in a centrally placed horizontal plane. The data were obtained for four different mean velocities in the test section (7, 14, 21, 28 m/s). Results of different measurement techniques were mutually compared, and repeatabilities and uncertainties of PDA and CTA measurements were assessed. Turbulent velocity spectra measured by the CTA were analysed. The effect of declination of the pitot static tubes (L-type and S-type) was briefly discussed and compared with an industrial velocity probe QuadraTherm® 640i In-Line Mass Flow Meter with a measuring range of 5–300 m/s. Velocity and declination of pitot static tubes were analysed only in the central point of the test section. The results show that a fully turbulent and uniform flow is developed 15 mm upstream the test section area. Mean velocity and turbulent intensity profiles obtained by different techniques are in a good agreement. Uncertainties of type B of PDA and CTA measurement method are below 5%. Turbulent intensity in the main stream is under 5%. Advantages and drawbacks of the presented measurement techniques were discussed. The PDA was found to be the most suitable measurement technique due to its precision and non–intrusive flow probing.

Experimental investigation of the wake behind a cooled cylinder

An experimental channel intended for investigation of a wake behind a cooled cylinder was designed and constructed in this work. The setup is designed for measurements with air at low Reynolds numbers. The measuring techniques based on the standard visualization with a fog generator and the constant temperature anemometry (CTA) were used. The flow performance was investigated behind two cylinders placed inside a channel with square cross section. One cylinder was kept at a constant flow temperature, while the second cylinder could be cooled using a water circuit with a thermostatic bath. The dependency of the Strouhal number on the Reynolds number was evaluated and compared to the literature data and the empirical correlations. The concept of an effective temperature in a non-isothermal flow was also applied. Unfortunately, a considerable difference between the preliminary measurements and the literature data was detected. The air-flow was found to have unsatisfactory high turbulence intensity. Consequently, some modifications of the current channel design are proposed.

Wake Unsteadiness and Tip Vortex System of Full-Scale Helicopters in Ground Effect

The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical takeoff, and forward flight. A high-speed schlieren system tracked the blade tip vortices at about 60 images per revolution. In addition, a constant temperature anemometry system utilized arrays of fiber film sensors, providing velocity statistics and spectra in the rotor flow. The overall wake structure agreed to preceding studies, but the velocity profiles and tip vortex trajectories were sensitive towards the environmental wind conditions. The tip vortices were observed in the schlieren images up to an age corresponding to about two revolutions below the rotor plane, before developing instabilities and falling below the detection limit. Systematic vortex pairing was found for the Bo105 but not for the EC135. The remnants of the tip vortices were identified further downstream in the wake by means of rotor-harmonic velocity signals, but they play a minor role in comparison to broad-banded turbulent fluctuations with a Kolmogorov-like spectrum. For vertical takeoff cases, the rotor wake had a hover-like structure until breaking down into low-frequency oscillations when exceeding a hub height of approximately 1.4 rotor radii. In forward flight, different types of wake velocity footprints were categorized on the basis of the normalized advance ratio. Blade–vortex interactions were found in the frontal area of the main rotor planes and between the main rotor tip vortices and the Bo105's tail rotor. The interactions prevent a further evolution of the tip vortices.

Local Shielding Gas Supply in Remote Laser Beam Welding

The use of shielding gases in laser beam welding is of particular interest for materials interacting with ambient oxygen, e.g., copper, titanium or high-alloy steels. These materials are often processed by remote laser beam welding where short welds (e.g., up to 40 mm seam length) are commonly used. Such setups prevent gas nozzles from being carried along on the optics due to the scanner application and a small area needs to be served locally with inert gas. The article provides systematic investigations into the interaction of laser beam processes and parameters of inert gas supply based on a modular flat jet nozzle. Based on the characterization of the developed nozzle by means of high-speed Schlieren imaging and constant temperature anemometry, investigations with heat conduction welding and deep penetration welding were performed. Bead-on-plate welds were carried out on stainless steel AISI 304 for this purpose using a disc laser and a remote welding system. Argon was used as shielding gas. The interaction between Reynolds number, geometrical parameters and welding/flow direction was considered. The findings were proved by transferring the results to a complex weld seam geometry (C-shape).

Possible Limitations of the Particle Image Velocimetry Method in the Presence of Strong Electric Fields

While examining the airflow generated between the asymmetrical electrodes connected to high voltage, the authors investigated the possible limitations of the particle image velocimetry (PIV) method in the presence of strong electric fields. The tracer particles used in the PIV method in these conditions are affected by electromagnetic forces; therefore, it is necessary to determine whether these forces have any non-negligible negative influence on the measurement results. For this purpose, the authors theoretically analyzed all the possible forces and measured the generated airflow using PIV and constant temperature anemometry methods. The experimental and theoretical results clearly show the viability of the PIV measurement method even in these very specific conditions.

The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow

Cavity flows occur in a wide range of low-speed applications (Mach number ≤0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 kg/ms2, 0.44 kg/m·s2, and 0.96 kg/m·s2). The investigation was performed for an open cavity with length to depth ratio of 4 at the Reynolds number based on a cavity depth of approximately 50,000. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed in this investigation. The aim of the jet blowing is to separate the cavity separated shear layer from the recirculation zone to reduce the cavity return flow, and hence stabilise the cavity separated shear layer. It was found that increasing the jet momentum flux causes an increase in the cavity return flow due to the increase in the thickness of the cavity separated shear layer. The study also found that the jet populates the separated shear layer with a large number of small-scale disturbances. These disturbances increase the broad band level of the pressure power spectra and Reynolds shear stress in the cavity separated shear layer. On the other hand, the jet disturbances make the shedding of the large vortical structures more intermittent.