Experiments in Fluids最新文献

In the present study, the effect of various two-dimensional surface defects (forward-facing steps and ramps, backward-facing steps and ramps, gaps, and steps and gaps) on boundary layer transition was experimentally investigated in the compressible, subsonic regime. A laminar profile was specifically designed and manufactured by ONERA to allow for a maximum number of defects to be tested simultaneously, and to include resin pockets to accurately monitor laminar–turbulent transition using infrared thermography. Transition was also characterized using the (Delta N) model based on linear stability calculations. Relatively good agreement with existing (Delta N) models for forward-facing steps as well as gaps was found, indicating that these models, which were mostly developed for incompressible flows, can still be used as an initial estimate for compressible flows. One particular case of interest included a critical step and gap (for which transition occurred immediately downstream of the defect) where neither the gap nor the step component could be identified as mainly responsible for triggering transition. Steps and gaps should therefore be included whenever possible to the canonical shapes of defects investigated in transition experiments to further refine the different types of defect encountered in industrial application, and provide appropriate criteria for their allowable tolerances.

The flow field around tandem heated cylinders is a subject of ongoing research due to its relevance in various engineering applications. This study investigates the influence of wall temperature and incoming flow velocity on the flow regime and heat transfer characteristics of tandem cylinders. Using a combined technique of Schlieren imaging and particle image velocimetry (PIV), the flow field was characterized over a range of incoming flow velocities (0.4 to 0.9 m/s) and cylindrical wall temperatures (423 to 673 K). The results indicate that higher wall temperatures promote a transition from the co-shedding regime to the shear layer reattachment regime. Conversely, increasing the incoming flow velocity leads to a transition from the shear layer reattachment regime back to the co-shedding regime. Beyond the flow regimes, the convective heat transfer was also quantified in the study, and a strong positive correlation was found between both wall temperature and incoming flow velocity and the convective heat transfer coefficient. These results demonstrate the significant influence of thermal and velocity conditions on the flow behavior and heat transfer in tandem cylinder arrangements.

A plug/aerospike nozzle is a promising concept as a propulsion system for space launchers and space planes. The inherent ability to adapt the nozzle jet to the ambient pressure level improves the thrust performance under overexpanded operating conditions compared to conventional bell nozzles, which is of great interest for future single-stage-to-orbit vehicles. This experimental study investigates the topology and aerodynamics of a cold flow linear plug nozzle jet in an outer flow environment. PIV and high-speed schlieren measurements are utilized to understand the mutual aerodynamic interaction between each other. The jet flow is studied for a variety of nozzle pressure ratios in combination with an outer flow at sub-, trans-, and supersonic Mach numbers. The flow is examined for two plug lengths, which are 72% and 24% of an ideal contour. It is found that the combination of nozzle pressure ratio and outer Mach number strongly influences the flow pattern and local velocity magnitudes. Backflow regions are measured, mainly emerging through the integration of the nozzle in a bluff aft body. The strength and frequency of aerodynamic modes are found to be highly dependent on the operating conditions as well. The most relevant ones are jet screeching, alternating vortex shedding of the outer flow, and vortex shedding in the base wake of the plug with strong truncation. The latter causes strong fluctuations in the flow, which are transmitted to the shear layer and induce acoustic wave emission. In addition, the flow locally accelerating in the plug base region results in increased shock strength in the jet structure. At trans- and supersonic outer flow, however, the aerodynamic modes of the jet flow are strongly suppressed. The impact of plug truncation on the velocity field becomes less for higher nozzle pressure ratios and outer flow Mach numbers.

Lagrangian defocusing particle tracking velocimetry (defocusing PTV, DPTV) measurements are performed in a thin volume above a plasma actuator array that is applied to mimic the effect of wall oscillations by inducing alternating, wall-parallel forcing in opposite directions into the air above the actuator surface for flow control purposes. The aim of the experiments is to capture the plasma-induced flow topology in otherwise quiescent air throughout the oscillation cycle within the measurement volume of 14 mm (times) 1 mm (times) 14 mm, immediately adjacent to the wall-mounted actuator. For this purpose, particle image velocimetry equipment for time-resolved measurements with one camera is used in a DPTV setup, where the out-of-plane particle coordinate is obtained through the diameter of a defocused particle image. Three-dimensional, three-component velocity and acceleration data is extracted by introducing a continuous particle tracking approach and an extended ex situ calibration procedure based on the detection of solid particles directly applied to a wall boundary, for which no prior knowledge of the flow topology or velocity data in the direct vicinity of the wall is required. A novel method for estimating measurement uncertainty in this context is introduced, and the influencing factors are discussed from an application perspective. Through the analysis of Lagrangian particle tracks, both individual flow events and statistical effects within the oscillation cycle can be evaluated. The extraction of phase-resolved flow fields with adaptable spatial resolution shows the forcing effect to be regular across different discharge zones on the plasma actuator array, indicating well-balanced voltage settings and precise manufacturing. Furthermore, the relation between the forcing-induced velocity and acceleration fields is quantitatively assessed, revealing the spatio-temporal transmission characteristics of the applied forcing. In summary, the obtained results demonstrate the applicability of DPTV measurement technique for the flow characterization above a plasma actuator array using the presented modifications.

A series of experiments were performed in a tornado simulator to study tornado-like vortices, with a focus on the surface pressure fields beneath the vortices. In the experiments, the pressures on the floor beneath a single-celled vortex and a two-celled vortex were measured. The measurements were used to characterize the surface pressure fields, which revealed, among others, the spatially varying non-Gaussian distribution of the surface pressures, the existence of narrowband components in the pressure fluctuations, and the differences between the characteristics of the surface pressure fields beneath the two types of vortices. A further analysis of the time and spatial variations of the surface pressure fields enabled the identification of the phenomena that cause these major characteristics of the surface pressure fields and the specific differences in these phenomena that result in the differences in the characteristics of the surface pressure fields beneath the two types of vortices. The findings from the study of the surface pressure fields provided insights into the nature of the tornado-like vortices aloft.

This paper presents a method for measuring the wall shear stress of a flat plate in incompressible flow based on optical measurement of the viscous liquid surface deformation in a cylindrical cavity. The liquid surface deformation is measured by detecting the liquid-surface-deformation-induced displacement field of white dots randomly distributed on the dark bottom of the cavity in images taken by a camera viewing the cavity perpendicularly. Numerical simulations of the flow over a simplified surface model indicate that the liquid surface deformation is caused by the elevated dynamic pressure in the cavity. Further, an analysis based on the similarity law of velocity in a turbulent boundary layer shows there is a functional relationship between the wall shear stress and the light deflection angle charactering the liquid surface deformation. Subsequently, in experiments in a low-speed wind tunnel, the liquid surface deformation is measured using the background-oriented Schlieren (BOS) technique and local velocity profiles are obtained via hot-wire anemometry to infer the wall shear stress in a range of the incoming flow velocities. Therefore, the relationship between the wall shear stress and the light deflection angle induced by the liquid surface deformation is established. The potential error sources are discussed.

Flow characteristics of inclined jet in crossflow (JICF), especially in-hole fields, remain insufficiently explored in experimental observations of near-wall properties and turbulence statistics, due to strong optical distortions and blind spots caused by refractive indices mismatch at solid–fluid interfaces. This study represents the first application of the refractive index matching (RIM) technique in JICF research, enabling in-hole measurements through time-resolved particle image velocimetry (TR-PIV). Experiments were conducted on a round hole at four velocity ratios (VR = 0.4, 0.8, 1.2, and 1.5). Focusing on the in-hole and near-exit mean flow field, this study identified a low-speed separation zone on the downstream side near the hole inlet and a jet acceleration zone on the upstream side near the hole exit. Near the hole inlet, vortex is formed due to the high-speed shear effects on upstream sidewall. Within the low-speed zone, flow characteristics were associated with strong vorticity, high turbulent kinetic energy, and low Reynolds stress components. In contrast, turbulence in the jet acceleration zone is higher for two intermediate VRs, which depends on the momentum balance between jet and mainstream. At VR = 0.4, large-scale vortex structure was formed inside the hole. The mainstream blockage led to a counter vortex in the leading edge of hole exit, which caused strong oscillation and contributed to hairpin vortex downstream. As VR increased, more complex axial vortical structures were observed, and dominant frequencies were converted. At VR = 1.5, the high-speed jet was more stable to show more regular vortical structures inside the hole and induced shear vortices with strong K-H instabilities in external field. By clarifying in-hole flow characteristics and establishing correlations with external JICF behaviors, this study aims to enrich the data of experimental benchmark for in-hole JICF validation and provides insights for optimizing film cooling strategies.

In this study, the effects of separated flow over a melting paraffin slab on atomization is considered. Experiments are conducted using an optical chamber with a paraffin wax slab exposed to high shear flows of 71–234 kg/m(^2-) s with non-vitiated heated air at 84–199 (^{circ })C. The formation of a lobe structure and leading edge instability responsible for wax atomization is imaged and quantified. The lobe height and receding angle are measured, and a simplified theory is developed to predict the steady-state shape using a sinusoidal profile. A Weber number of the leading edge is defined (We(_textrm{LE})) to non-dimensionalize and correlate the experimental data to the theory. A linear correlation is observed with an (R^2) of 0.94 between the shear correction factor and We(_textrm{LE}). The overall agreement between theory and measurements is good for the lobe height and width. Measurements of entrainment from the lobe account for (approx 20%) of the total mass loss. The size of the droplets entrained from the leading edge are recorded and normalized. Droplet size distributions for varying air fluxes and temperature are shown to collapse to the same distribution when normalized using their z-scores.

The effects of edge geometry and compressibility on the wake of a slanted afterbody model with rounded edges, relevant to cargo aircraft and high-speed train applications, are investigated through detailed experimental observations of the flowfield. The wake flow features are examined using centerline planar PIV measurements and novel Scanning-SPIV measurements to reconstruct the full mean volumetric velocity field. Planar PIV measurements at the model centerline reveal that increasing the Mach number reduces the shear layer growth rate, leading to decreased entrainment within the recirculation region. Consequently, the recirculation region increases in both length and height. Further downstream, the vortex circulation for the rounded-edge model remains nearly constant across both incompressible and compressible Mach numbers. Additional vortex properties are examined through the Reynolds-averaged vorticity transport equation applied to the volumetric flowfield measurements, revealing an increase in x-vorticity compression within the recirculation region. By Helmholtz’s vortex theorem, this increased vortex compression contributes to the growth of the recirculation region between Mach number conditions. Additionally, the dilatation term was explored, allowing for the delineation of compressibility effects on the recirculation region.

Traditional methods for directly measuring aerodynamic forces are particularly challenging at low Reynolds numbers due to the low dynamic pressures. This becomes even more challenging when rapid motions of the test articles are present, with inertial forces often larger than the aerodynamic forces. Existing methods for calculating pressure fields from experimental vector fields, such as those measured using particle image velocimetry (PIV), have constraints that make them difficult or impossible to apply to data sets that do not meet certain conditions, such as boundary condition requirements or restrictions on the grid shape of the data. This paper describes a new method of determining surface pressures and aerodynamic forces using experimentally collected velocity field data. This method leverages field erosion to constrain a point-stepping spatial integration of the pressure gradient field. A systematic method for dividing the flow field into zones based on the vorticity of the flow and the known geometry of the experiment allows for pressure in less-disturbed portions of the flow to be calculated and used as the boundary conditions for more unsteady flow regions. Surface pressures are then extracted from on or near the surface and integrated to calculate lift and drag. Two data sets are used as validation cases: a pitch-and-hold dynamic stall and static lift around an airfoil, both at low Reynolds number. The pressure-derived lift curves compare favorably with the reference data sets, demonstrating the accuracy of the new method.