Experiments in Fluids最新文献

Ammonia–methane (NH₃–CH₄) mixtures have potentials to serve as low-carbon fuels for gas turbines. Recent studies demonstrated that secondary air injection was an effective strategy to reduce NOx emissions from the combustion of NH₃–CH₄–air mixtures. However, the effects of secondary air injection on the performance of thermoacoustic instability of NH₃–CH₄–air flames remain unknown. To this end, this study experimentally investigated the thermoacoustic instability performance of a premixed, swirl-stabilized NH₃–CH₄–FF air combustor without and with secondary air injection. Without secondary air injection, thermoacoustic instabilities were widely detected at NH₃ ratios below 50% (by volume). It was discovered that such instabilities were significantly suppressed by secondary air injection over a wider range of operating conditions. In conjunction with the analysis of the flame and flow dynamics, it was revealed that the secondary air injection suppressed the large amplitude axial oscillations of the flame and the flow field. Combined with the experimental results, it was inferred that the introduction of secondary air changed the velocity and pressure distribution downstream of the primary combustion zone, which in turn affected the formation and evolution of the vortex structures, thereby mitigating thermoacoustic coupling. Finally, emission measurements were discussed and the results indicated that the secondary air injection strategy reduced unburnt NH₃ and CO emissions to a certain extent as well as decreased NOx emissions at specific equivalence ratios.

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This work investigated the influence of crosswinds and leg positions on the aerodynamics of an articulated cycling mannequin on a track bicycle. Force, wake total pressure and wake velocity measurements were made in a low-speed sports wind tunnel of closed-circuit type. The freestream velocity and wheel speeds were kept at (15, hbox {m/s}). The crank angle of the mannequin was varied across a pedal cycle. Yaw angles from (0^{circ }) to (20^{circ }) were examined. The experimental results reveal that the leg position significantly affects the aerodynamic performance of a cyclist. At high yaw angles, the aerodynamic drag on the cyclist showed noticeable deviations between most leg positions and their (180^{circ })-apart pairs. A wake analysis technique effectively captured the influence of leg positions on drag. The total pressure deficit contributes dominantly to the overall drag. The wake pressure contours demonstrate how leg-wheel interaction affects the total pressure distribution and drag under crosswinds. This study offers valuable insights into the flow behavior and drag generation around a cyclist with varying leg positions under crosswinds.

Studying turbulent heat transfer over rough surfaces is vital for enhancing heat transfer efficiency in various practical applications. This research presents an in-depth examination of the commissioning of a heated floor boundary layer wind tunnel facility, specifically focussing on addressing the uncertainties in measuring heat transfer over rough walls. Our findings show that minor variations in the slope of the inner-scaled mean temperature profile ((kappa _h)) on a heated smooth wall have a marginal effect on the estimates of friction temperature and heat transfer coefficients across a range of friction Reynolds numbers (({900 lesssim textrm{Re}_tau lesssim 3700})) when using the Clauser fit method. Direct heat transfer measurements using power metres validate this conclusion. Temperature measurements over a three-dimensional sinusoidal roughness indicate constant (kappa _h) within uncertainty limits across the examined range ({2300 lesssim textrm{Re}_tau lesssim 10{,}400}), contingent on prior knowledge of the roughness’s virtual origin. Nevertheless, measuring heat transfer coefficients and roughness functions entails large uncertainty due to challenges in estimating heat losses and applying the modified Clauser method. Recommendations for enhancing accuracy in heated rough wall measurements include direct measurement of wall shear stress and heat flux, selecting low emissivity heated plates and ensuring precise control of heated wall conditions. This work also emphasises the significance of conducting a comprehensive uncertainty analysis as a valuable tool for identifying and addressing any shortcomings in the measurement facility and equipment.

Gas flow visualization is an essential technique for understanding the gas flow characteristics. Various quantitative distribution measurement methods have been proposed, each with its own advantages and disadvantages. For example, the background-oriented schlieren method provides the quantitative density distribution for wide areas with a simple optical setup, but it disadvantageously requires the appropriate boundary conditions need to be set when integrating the Poisson equation. The laser Rayleigh scattering method also provides quantitative density distribution, but it requires a high-power laser for wide-area measurements because laser intensity directly influences measurement accuracy. This study proposes a method that complements the weak points of the above two methods. First, a wide area is measured using the background-oriented schlieren method, and then, the laser Rayleigh scattering method is applied only for the boundary region to obtain the boundary condition. For a heated turbulent air jet with Reynolds number 3000, the results of the proposed method are compared with the numerical analysis and thermocouple temperature measurements. The results well match, indicating the applicability and usefulness of the proposed method. Furthermore, these results contribute to demonstrating the significance of boundary conditions in the background-oriented schlieren method and the establishment of setting guidelines.

The characterization of thin liquid films is relevant to many engineering applications, ranging from oil and chemical industry to refrigeration systems, to cooling of light water nuclear reactors. The total internal reflection method (TIRM) is an optical method known for decades for being able to non-intrusively measure film thickness of a wide range of fluids flowing over a transparent wall, but systematic studies on the accuracy of the method are still missing. In this work, TIRM is presented and all the main potential error sources related to the application of such measurement are thoroughly characterized. The analysis includes the potential impact of variation of the refractive index on the measured thickness, the extension of the experimental calibration range to a broader set of measurable thicknesses and the effect of the inhomogeneity of the film free surface on the measured thickness. This latter aspect was never investigated in detail before because of the inherent complexity of the involved physical phenomena, but an in-house developed ray-tracing simulation allows new insights into the problem. Overall, the present paper redefines the utilization limitations and the accuracy of TIRM.

We propose a simple boundary condition regularization strategy to reduce error propagation in pressure field reconstruction from corrupted image velocimetry data. The core idea is to replace the canonical Neumann boundary conditions with Dirichlet ones obtained by integrating the tangential part of the pressure gradient along the boundaries. Rigorous analysis and numerical experiments justify the effectiveness of this regularization.

In this study, an experimental analysis is conducted on the settling of straight and curved cylindrical rods, which are used to replicate a Reynolds number range applicable to atmospheric settling of microplastic fibres. The rods are dropped in a chamber filled with a quiescent water–glycerin mixture, and their settling velocity is determined from the images of two cameras arranged with perpendicular views. It is shown that a curved cylindrical rod settles faster than a straight cylindrical rod with the same diameter and length. As the rod radius of curvature decreases, the terminal velocity increases, and the corresponding drag coefficient decreases. The maximum difference in the terminal velocity between the straight and curved rods depends on the rod aspect ratio, curvature index, and Reynolds number. A new semi-empirical model is also developed to estimate the drag coefficient and terminal velocity of both straight and curved cylindrical rods studied in this research. The results of the new model are significantly more consistent with the experimental data compared to the previous models, with a low RMS error of 6.8%. This novel model has been utilized to predict the terminal velocity of realistic fibres in the atmosphere.

This paper takes a new look at how the aerodynamic interactions of multiple bodies in high-speed flow affect their motion behaviors. The influence of the body shape and orientation on aerodynamic and stability behavior in the case of shock–shock and wake–shock interactions is the focus of this publication. Experiments were performed in the hypersonic wind tunnel H2K at the German Aerospace Center (DLR) in Cologne. Free-flight tests with tandem arrangements of spheres and cubes were performed with a synchronized dropping of both objects at various initial conditions of relative streamwise and vertical distance as well as pitch angle. A high-speed stereo-tracking captured the model motions during free-flight, and high-speed schlieren videography provided documentation of the flow topology. Based on the measured 6-degrees-of-freedom (6DoF) motion data, aerodynamic coefficients were determined. As a result, the final lateral velocity of trailing cubes is found to be many times greater than that of spheres regarding shock-wave surfing. For rotating cubes, the results showed that stable shock-wave surfing can become possible over an increasingly wide range of initial positions. This study has identified that the trailing drag coefficient of two axially aligned objects varies strongly with their relative streamwise distance. Furthermore, it was shown that the wake is a region of stability for downstream objects.

Graphical abstract

A magnetic field is generated to modify the effective gravity acting on settling particles in a laboratory experiment. When applied to a magnetized spherical particle settling in water-glycerol mixtures, the magnetic field produces a vertical force that counteracts the gravitational field, hence allowing for the magnetic tuning of the settling properties of the particle. While doing so, the spin of the particle around the direction perpendicular to the applied magnetic field is blocked, thus allowing spin solely around the direction of the magnetic field. This method of magnetic modification of the effective gravity is tested on the settling of spherical magnets in quiescent fluids over Galileo numbers in the range [100, 300] and a fixed particle density of 8200 kg/m(^3). The results obtained by varying the Galileo number via the magnetic modification of effective gravity are compared to those obtained with non-magnetic spheres when the Galileo number is modified by varying the fluid’s viscosity. We show that the same taxonomy of settling regimes with nearly identical geometrical properties (in terms of planarity and obliqueness) of the trajectories is recovered. In addition to proving that it is possible to magnetically tame the settling of particles in fluids preserving the features of the non-magnetic case, this also reveals that blocking the spin of the particles does not produce any significant effect on its settling properties in a quiescent fluid. This novel experimental methodology opens new possibilities to experimentally explore many other subtle aspects of the coupling between settling particles and fluids (for instance, to disentangle the effects of rotation, inertia, and/or anisotropy of the particles) in more complex situations including the case of turbulent flows.