Search Results (102)

Flow stabilization and velocity uniformity in a Göttingen-type closed-circuit subsonic aerodynamic wind tunnel with an expanded test section are investigated in this study. Both experimental and numerical approaches were employed. The experiments were performed by using Laser Doppler Anemometry, Pitot tubes, and thermal anemometry. For numerical simulations, Reynolds-averaged Navier–Stokes simulations with a standard k-ε turbulence model were employed to evaluate flow characteristics in the velocity range of 0.05–20 m/s. The study shows that a properly contoured contraction nozzle suppresses inlet turbulence and ensures stable Reynolds-independent core flow. The contraction nozzle significantly accelerates and redistributes the flow, allowing rapid hydrodynamic stabilization and ensuring velocity measurements with high repeatability. These characteristics are inherent in a benchmark facility. Additionally, the study shows that the outlet-to-inlet diameter has the most prominent role in longitudinal velocity distribution in the test section. An optimal ratio of 1.10 was identified, stabilizing the pressure distribution and providing the most uniform longitudinal velocity profile. These findings offer geometry-dependent design guidelines for achieving high-quality measurements in Göttingen-type wind tunnels with expanded test sections and support accurate velocity measurement instrument calibration and aerodynamic testing. Full article

This paper investigates the oscillatory effect of a single blade on the turbulence wake downstream of a low-pressure turbine cascade. Experimental investigations were conducted at a chord-based Reynolds number of $2.3\times {10}^5$ with an excitation frequency of 73 Hz. The experimental campaign encompassed two incidence angles (−3° and +6°) and three blade motion conditions: stationary, bending, and torsional vibrations. Turbulence characteristics were analyzed using hot-wire anemometry. The results indicate that the bending mode notably alters the wake topology, causing a 5% decline in streamwise velocity deficit compared to other modes. Additionally, the bending motion promotes the formation of large-scale coherent vortices within the wake, increasing the integral length scale by 7.5 times. In contrast, Kolmogorov’s microscale stays mostly unaffected by blade oscillations. However, increasing the incidence angle causes the smallest eddies in the inter-blade region to grow three times larger. Moreover, the data indicate that at −3°, bending-mode results in an approximate 13% reduction in the turbulence energy dissipation rate compared to the stationary configuration. Furthermore, the study emphasizes the spectral features of turbulent flow and provides a detailed assessment of the Taylor microscale under different experimental conditions. Full article

Overfall flow, characterized by high Froude numbers and intense turbulence, generates boundary shear stress on vertical surfaces, which is considered the direct cause of headcut erosion. This study aims to analyze the hydraulic characteristics of nappe flow over a vertical or near-vertical overfall. Detailed experiments using hot-film anemometry were conducted in an indoor flume to examine the shear stress distribution on vertical surfaces under varying flow rates, overfall heights, and backwater depths. The results show that when the jet dynamic pressure head is less than the backwater depth, the dimensionless relative shear stress and relative depth relationship can be fitted with a beta probability density function. When the dynamic pressure head exceeds the backwater depth, the distribution follows a cubic polynomial form. Dimensional analysis and flow trajectory calculation methods were used to establish shear stress distribution formulas, with determination coefficients of 0.829 and 0.652, and the mean absolute percentage error (MAPE) between the measured and predicted values being 0.106 and 0.081, respectively. The findings provide valuable insights into the effects of complex flow structures on shear stress and offer essential support for the development of scour models for overfall structures. Full article

Synthetic jet actuators are becoming increasingly popular for enhancing electronic heat transfer. However, their use is currently limited due to the high noise they generate. This article examines how actuator parameters (orifice diameter, orifice length and cavity height) affect synthetic jet velocity and noise generation. Hot-wire anemometry was used to measure velocity, and noise was measured with a sound meter. The actuator was supplied with constant power at different frequencies ranging from 50 to 500 Hz. Observation of the velocity showed that it decreased with an increasing orifice diameter and increased with a decreasing orifice length. No relationship was observed between cavity height and synthetic jet velocity. This article indicates that increasing the orifice diameter or reducing the orifice length causes an increase in the noise generated by SJAs, provided we remain in the vicinity of the characteristic frequency. It was demonstrated that higher actuator chambers produce higher noise levels, although this was not a consistent trend across the entire tested frequency range. Full article

Determination of nozzle quality ratings based on macroscopic and microscopic parameters generally requires the use of separate measurement methods in research. The guiding idea determining the direction of the conducted research was to use a 2D (two-dimensional) laser analyzer LDA/PDA (laser Doppler anemometry/phase Doppler anemometry) to evaluate the values of selected micro and macro parameters (microstructure characterization with simultaneous evaluation of lateral distribution) of the spray. The research was conducted for variable measurement times. The main issue of the research was an attempt to reduce the measurement cycle time, important in the case of point tests performed with an analyzer. The scope of the conducted research covered three areas. In the first stage of the research, the variability of the coefficients characterizing the spray spectrum as a function of variable measurement time was analyzed. In the next, the value of the coefficient of transverse volume distribution (for a single sprayer) was determined. The results were determined on the basis of the volume diameters obtained from measuring the droplets with a 2D LDA/PDA analyzer. In the third stage, an attempt was made to combine the volume distribution results obtained for single nozzles on the boom. The results obtained were compared with those determined using a groove table. Both measurement methods used a different representativeness in volume measurement (sampling method and significantly different amounts of liquid analyzed); nevertheless, the results of the transverse volume distribution were found to be consistent. Full article

This paper presents experimental results on unsteady turbulent flow in a low-pressure turbine blade cascade, specifically exploring the effects of blade vibrations on wake topology and turbulence structure. The study focused on comparing the flow patterns of a stationary blade to those observed during its bending and torsion vibrations. Hot-wire anemometry was used for the experimental analysis. The flow velocity was characterized by a chord-based Reynolds number of approximately $R{e}_c\approx 2.3\times {10}^5$, with the excitation frequency set at $f=72.8\mathrm{Hz}$. The findings reveal a strong effect of the bending mode on the wake topology, resulting in a 5% reduction in the streamwise velocity deficit compared to the stationary and torsional modes. Additionally, the bending mode encourages the active formation of large vortices in the wake region, which leads to a fivefold increase in the integral length scale. In contrast, the Kolmogorov microscale remains consistent across all scenarios, exhibiting a minimum in the wake region and a maximum in the inter-blade space. The paper also discusses the impact of blade oscillations on the energy dissipation rate. Various calculation methods yield consistent results, indicating that the lowest dissipation rate occurs during the bending mode. Furthermore, the paper emphasizes the spectral analysis of turbulent flow and provides a comprehensive assessment of the Taylor microscale under different experimental censorious. Full article

Both propeller slipstream and flap jet flow can significantly increase the aircraft lift coefficient. To establish design principles for efficient lift enhancement via jet flow under the influence of slipstream, wind tunnel experiments are conducted on a wing with propeller slipstream and jet flow. Force measurements using a balance and flow field measurements using hot-wire anemometry are employed to investigate the effects of different jet flow distribution methods on lift enhancement. The results indicate that the coupling of slipstream and jet flow effects can significantly increase wing lift. The stronger the slipstream effect, the more pronounced the lift enhancement under the same momentum coefficient. At the same thrust coefficient, a higher momentum coefficient is required in the slipstream-affected region to suppress airflow separation. Under the same jet flow rate, increasing the momentum coefficient in the slipstream-affected region can significantly improve lift enhancement. At the thrust coefficient of 0.46 and the momentum coefficient of 0.1, the optimized jet flow distribution method achieved a 52.6% greater lift enhancement compared to the spanwise uniform jet flow distribution method. Full article

The recent progress in the acoustic fractionation of particulate suspensions within microfluidic devices emphasizes the utility of the acoustic fractionation process also for gas-suspended particles as a significant advancement in the field of mechanical process engineering. In the literature, analytical and numerical studies have found the gas-based acoustic particle fractionation process to be suitable for particles in a size range below 10 µm. The viability remains experimentally unassessed. In this article, we present particle fractionation experiments conducted on gas-born particles suspended in high-intensity acoustic fields. A particle-size-dependent accumulation of particles in the acoustic sound velocity lobes and nodes could be observed, indicative of an acoustic fractionation process. Additionally, evidence of acoustic streaming and acoustic focusing has been found, both of which have the potential to impede the fractionation process. The experimental results align with the conclusions of numeric simulations. The in-process particle behavior is discussed in the context of the relevant literature and reinforces the notion of selective entrainment. Full article

This article presents an overview of airflow velocity measurement methods applied to renewable energy. Basic measurement methods used in this field are discussed: tachometric, ultrasonic, and calorimetric anemometry. The principle of operation and basic properties of anemometers are presented, and based on publications from the last decade, a systematic review of development directions and trends in this field is made. The aim of the article is to familiarize people dealing with renewable energy problems, in particular wind energy, with the current state of knowledge in the field of anemometric measurements, properties of individual types of measuring devices, as well as directions of development of measurement tools. This will allow for the optimization of processes in the field of wind energy, in particular in the selection of the location of the energy facility, implementation of investments and control, diagnostics, and monitoring during operation. The selection of metrological tools adequate to the problem also allows for ensuring an appropriate level of work safety and environmental and ecological harmony and supporting the process of sustainable development. Full article

The main objective of the current study is a detailed (both numerical and experimental) investigation of the highly unsteady and complex swirl flow in a straight conical diffuser (with a total divergence angle of 8.6°) generated by an axial fan impeller. Pressure, and axial and tangential velocity profiles along several cross-sections were measured by original classical probes in two different flow regimes at the inlet: the modified solid body type of moderate swirl and the solid body type of strong swirl and reverse flow; they were additionally confirmed/validated by laser Doppler anemometry measurements. Computational studies of spatial, unsteady, viscous, compressible flows were performed in ANSYS Fluent by large eddy simulation. The fan was neglected, and its effect was replaced by the pressure and velocity profiles assigned along the inlet and outlet boundaries. The two sets of data obtained were compared, and several conclusions were drawn. In general, the relative errors of the pressure profiles (2–5%) were lower than the observed discrepancies in the axial velocity profiles (5–40% for the first and 15–50% for the second flow regime, respectively). The employed reduced numerical model can be considered acceptable since it provides insights into the complexity of the investigated swirl flow. Full article

Cavity flows have a wide range of low-speed applications ($M\le 0.3$), such as aircraft wheel wells, ground transportations, and pipelines. They induce strong flow oscillations which can substantially increase noise, drag, vibration, and lead to structural fatigue. In the current study, a steady jet was forced from the cavity trailing edge with different momentum fluxes (J = 0.11 kg/m·s², 0.44 kg/m·s², and 0.96 kg/m·s²). The aim of this study was to investigate the impact of the steady jet on the time-averaged flow field and the cavity separated shear layer oscillations for an open cavity with a length-to-depth ratio of $L/D=4$ at $R{e}_{\theta }=1.28\times {10}^3$. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed. The study found that increasing the jet momentum flux caused a significant increase in thickness and deflection of the cavity separated shear layer. Due to the counterflow interaction between the jet and cavity separated shear layer, the growth rate ($d{\delta }_{\omega }/dx$) of the cavity separated shear layer increased significantly from 0.193 for the no-jet case to 0.273 for the J = 0.96 kg/m·s² case. As a result, the return flow rate increased, causing the separation point on the cavity floor to shift upstream from $x/L\approx 0.2$ for the no-jet case to $x/L\approx 0.1$ for the J = 0.96 kg/m·s² case. Furthermore, increasing the jet momentum flux increased the broadband level of the cavity separated shear layer oscillations. Full article

This research focuses on analyzing the aerodynamic characteristics of residual air currents generated by vehicle movement and evaluating their feasibility for energy generation, then designing a vertical axis wind turbine. The parameters assessed include the characteristic velocity profile, the average and maximum velocities, disturbance lifetimes, as well as the frequency and probability of recurrence of these disturbances. Using the data, projections are made on the electrical energy amount that can be produced by a wind turbine operating under such wind conditions. Measurements were taken at four locations: three within Mexico City (CDMX) and one on the outskirts. The measurement station, consisting of a 2.35 m vertical tower equipped with eight vertically aligned thermos-resistive anemometers, is installed on medians less than 0.50 m from moving vehicles. The data from within CDMX show maximum wind velocities ranging from 6 to 8 m/s at ground level, while measurements on the outskirts record velocities of up to 19.5 m/s. A probabilistic analysis reveals that usable air currents could be present 58% of the time. Based on electrical production calculations, it is estimated that harnessing this residual energy could power approximately 4500 homes, considering the national cost per kWh and the average electricity consumption of a four-person household. Full article

Despite major advances in the field of actuator technology for microsystems, miniaturized microfluidic actuation systems for mobile devices are still not common in the market. We present a micropump concept and an associated mass flow sensor design, which, in combination, have the potential to form the basis for an integrated microfluidic development platform for microfluidic systems in general and microdosing systems in particular. The micropump combines the use of active valves with an electrostatic drive principle for the pump membrane and the valves, respectively. With a size of only 1.86 mm × 1.86 mm × 0.3 mm, the first prototypes are capable of pumping gaseous media at flow rates of up to 110 μL/min. A specific feature of the presented micropump is that the pumping direction is perpendicular to the chip surface. The corresponding flow sensor combines the principle of hot-wire anemometry with a very small footprint of only 1.4 mm × 1.4 mm × 0.4 mm. The main innovation is that the hot wires are fixed inside a through-hole in the substrate of the microchip, so that the flow direction of the fluid to be measured is perpendicular to the chip surface, which enables direct integration with the presented micropump. Detection thresholds of around 10 μL/min and measuring ranges of up to 20 mL/min can be achieved with the first prototypes, without dedicated evaluation electronics. Full article

Conical diffusers are widely used in technical devices (gasifiers, turbines, combustion chambers) and technological processes (ejectors, mixers, renewable energy). The perfection of flow gas dynamics in a conical diffuser affects the intensity of heat and mass transfer processes, the quality of mixing/separation of working media and the flow characteristics of technical devices. These parameters largely determine the efficiency and productivity of the final product. This article presents an analysis of experimental data on the gas-dynamic characteristics of stationary air flows in a vertical, conical, flat diffuser under different initial boundary conditions. An experimental setup was created, measuring instruments were selected, and an automated data collection system was developed. Basic data on the gas dynamics of air flows were obtained using the thermal anemometry method. Experimental data on instantaneous values of air flow velocity in a diffuser for initial velocities from 0.4 m/s to 2.22 m/s are presented. These data were the basis for calculating and obtaining velocity fields and turbulence intensity fields of the air flow in a vertical diffuser. It is shown that the value of the initial flow velocity at the diffuser inlet has a significant effect on the gas-dynamic characteristics. In addition, a spectral analysis of the change in air flow velocity both by height and along the diffuser axis was performed. The obtained data may be useful for refining engineering calculations, verifying mathematical models, searching for technical solutions and deepening knowledge about the features of gas dynamics of air flows in vertical diffusers. Full article

In view of the necessity of measuring the air content in oil in two-phase flows in the context of general industry, a review of the most popular methods of measuring the air content in oil was carried out. This review includes an assessment of their advantages and disadvantages and of whether they meet criteria such as the degree of filling, the size and number of bubbles, verification, the absence of additional pressure drops, simplicity, and repeatability. In the review, the following methods were examined: the classic trapping method, a modified trapping method, a trapping method using hydrostatic pressure loss, the pressure loss due to frictional flow resistance, the pressure loss with a rapid increase in diameter, the pressure drop in a Venturi tube, the pressure drop in an orifice, a method using the Coriolis effect, the electrical resistance method, the electrical conductivity method, the electromagnetic method, the electrical capacitance method, the thermal anemometry method, the liquid–solid contact electrification method, the photographic method, holography, light scattering, sound dispersion, the ultrasonic transit-time method, X-ray radiation, gamma radiation, neutron radiation, and fiber-optic methods. Full article