Experiments in Fluids最新文献

This paper presents the development of fluorescent tracer particles for use in gas flows as a countermeasure for undesired strong light reflections on surfaces of channel walls or obstacles and as a label for the discrimination of multi-constituent flows. The employment of fluorescent dye-doped tracer particles with a wavelength-specific optical filter enables the separation of the Stokes-shifted particle light emission from reflections on surfaces and Mie scattering from non-fluorescing particles. The fluorescent particles were made of Pyrromethene 567 (P567) and Di-Ethyl-Hexyl-Sebacate (DEHS), and the addition of P567 was not found to alter the characteristics of the particles generated. Investigations in a low-speed wind tunnel revealed that the intensity of fluorescent emission is proportional to the dye concentration at least up to (2.0,hbox {g l}^{-1}). The efficacy of reflection removal was investigated in a setup with a metal turbine blade placed in the flow and a laser sheet oriented to impinge the blade surface. With the installation of an appropriate optical filter, undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Finally, the performance of the modified DEHS was compared to conventional DEHS with the measurement of a canonical turbulent boundary layer (TBL). The flow was globally seeded with conventional DEHS and the TBL was locally seeded with fluorescing DEHS; simultaneous imaging with a notch filter confirmed that the flow is accurately tracked by the modified DEHS without additional bias. Furthermore, this indicated the possibility of using the newly developed particles to segregate portions of a flow with multiple constituents.

We study the transient dynamics of three-dimensional detonation cells when the detonation front is subjected to weak expansion due to the diffraction from a straight channel to a diverging channel. We focus on the effect of the cross-sectional shape, namely square or round, using diverging channels with the same initial cross-sectional area of 16 cm (^{2}) as the straight channels and the same expansion rate. The reactive mixture is (2,hbox {H}_{2} + hbox {O}_{2} + 2,hbox {Ar}) at the initial pressure of 20 kPa and temperature of 294 K, and we use the sooted-foil technique to record the cellular dynamics. The mean cell widths first increase from different initial values, which depend on the cross-sectional shape and then decrease to stabilize at the same value independent of the shape but larger than the initial values. We use a relation of detonation dynamics between the velocity, total curvature and acceleration of the average detonation front to interpret successfully, albeit qualitatively, all the experimental trends. This sensitivity thus makes these experimental data a reliable basis for high-resolution numerical simulations capable of handling three-dimensionality and detailed chemical kinetics mechanisms. Defining a significative mean width of detonation cells requires constant cross-sectional tubes of size and length sufficiently large. Inductively, representing three-dimensional cells requires more statistical descriptors than a single mean width.

An important step in the application of Lagrangian particle tracking (LPT) or in general for image-based single particle identification techniques is the detection of particle image locations on the measurement images and their sub-pixel accurate position estimation. In case of volumetric measurements, this constitutes the first step in the process of recovering 3D particle positions, which is usually performed by triangulation procedures. For two-component 2D measurements, the particle localization results directly serve as input to the tracking algorithm. Depending on the quality of the image, the shape and size of the particle images and the amount of particle image overlap, it can be difficult to find all, or even only the majority, of the projected particle locations in a measurement image. Advanced strategies for 3D particle position reconstruction, such as iterative particle reconstruction (IPR), are designed to work with incomplete 2D particle detection abilities but even they can greatly benefit from a more complete detection as ambiguities and position errors are reduced. We introduce a convolutional neural network (CNN) based particle image detection scheme that significantly outperforms current conventional approaches, both on synthetic and experimental data, and enables particle image localization with a vastly higher completeness even at high image densities.

The Shake-The-Box technique was applied to experimentally quantify the time-resolved volumetric flow field around a free-flying quadcopter UAV with an overall span of about 0.5 m. State-of-the-art LED illumination and high-speed camera equipment was combined with modern Lagrangian tracer particle tracking and data assimilation techniques, facilitating a measurement volume larger than ({1.5},{hbox {m}^3}). The setup allowed for both hover and limited maneuvering of the quadcopter, while resolving even small details of the complex interactional aerodynamics. In hover out of ground effect, the four individual rotor wakes merged into a single jet within a few rotor radii below the rotor planes. Evaluating the mass and momentum fluxes over suitable control volumes yields accurate estimates for the quadcopter’s total thrust, the asymmetric thrust distribution between front and back rotors, and the entrainment of external flow through turbulent mixing. Hover in ground effect decreases the power requirement and induces recirculating flow in the center of the four rotors. The outwash pattern is non-uniform with jets developing between the rotors and pointing in radially outward directions. Forward flight cases result in a skewed, rapidly merging wake flanked by the roll-up of two “supervortices” similar to the wingtip vortices of fixed-wing vehicles.

In aircraft engines, thermoacoustic oscillations in the combustion chamber contribute significantly to noise emissions, which, like all other emissions, must be drastically reduced. Thermoacoustic oscillations are not only a concern, they can also be beneficial in hydrogen combustion. This work demonstrates that thermoacoustic density oscillations with amplitudes at least an order of magnitude smaller than those resulting from density gradients in a turbulent flame can be detected using laser interferometric vibrometry. This improvement was made possible by heterodyning a carrier fringe system in background-oriented schlieren (BOS) recordings, which were subsequently analyzed using techniques commonly used for holographic interferometry. In comparison with other BOS evaluation techniques, the filtering of the individual frames in the Fourier domain offers a more efficient computational approach, as it allows for phase averaging of a high number of single recordings to reduce noise from turbulence. To address fringe pattern distortions and cross talk in the Fourier domain, which both have been observed by other authors, we propose background subtraction methods and an optimized background pattern. Additionally, the procedure provides a visualization tool for marking the high turbulence regions of heat release by the variations in fringe amplitude. Finally, the line-of-sight data are reconstructed using the inverse Abel transform, with the data calibrated by laser interferometric techniques, resulting in local values for density oscillations.

Graphical abstract

This study investigates the impact of insufficient spanwise spatial resolution on the measurement accuracy of streamwise velocity fluctuations over rough walls. We use a direct numerical simulation (DNS) database of turbulent open-channel flow over three-dimensional sinusoidal roughness with varied wavelengths and roughness heights. Employing a triple decomposition, we investigate both the attenuation of the turbulent fluctuations (about the local mean), (u^prime) and the dispersive stresses (roughness-induced fluctuations of the time-averaged mean about the global mean), ({tilde{U}}). A boxcar filter on DNS data is applied to investigate the effects of spanwise spatial filtering on these quantities. Our analysis reveals the significance of two key length-scale ratios for velocity measurements over rough walls: the wire length relative to the spatially and temporally plane-averaged Kolmogorov scale at the roughness crest ((l/langle eta rangle _k)), and the wire length relative to the roughness spanwise wavelength ((l/Lambda _y)). We observe that maintaining (l/langle eta rangle _k) constant while increasing (l/Lambda _y) attenuates the variance of ({tilde{U}}) and (u^prime) within the roughness sublayer. When fixing (l/Lambda _y), an increase in (l/langle eta rangle _k) influences the turbulent fluctuations across all wall-normal locations. These findings highlight the necessity of considering both length scales when evaluating spanwise spatial resolution in turbulence measurements over rough walls.

Tip-vortex cavitation is among the first forms of cavitation to appear around ship propellers. In the present study, the time-resolved three-dimensional flow field around non-cavitating and cavitating tip vortices in the wake of a marine propeller is investigated with tomographic PIV. The advance ratio of the propeller and the Reynolds number of the flow are kept constant, while the cavitation number is varied by changing the pressure inside the cavitation tunnel. The importance of masking the tip-vortex cavities before performing the tomographic reconstruction is firstly demonstrated, followed by a description of the applied masking algorithm. From the three-dimensional velocity vector fields, coherent structures of vorticity are identified using the Q-criterion. Three types of coherent structures are observed to populate the wake of the propeller, i.e. tip vortex, hub vortex, and secondary vortical structures. The secondary vortical structures surrounding the tip vortex appear to be progressively smaller in size and more chaotically-organized for decreasing cavitation number. This can be attributed to the pressure fluctuations induced by the cavity, which strengthen when the cavity size grows.

Unsteady separated flow affects the aerodynamic performance of many large-scale objects, posing challenges for accurate assessment through low-fidelity simulations. Full-scale wind tunnel testing is often impractical due to the object’s physical scale. Small-scale wind tunnel tests can approximate the aerodynamic loading, with tufts providing qualitative validation of surface flow patterns. This investigation demonstrates that tufts can quantitatively estimate unsteady integral aerodynamic lift and pitching moment loading on a wing. We present computational and experimental data for a NACA0012 wing, capturing unsteady surface flow and force coefficients beyond stall. Computational data for varying angles of attack and Reynolds numbers contain the lift coefficient and surface flow. Experimental data, including lift and moment coefficients for a tuft-equipped NACA0012 wing, were obtained at multiple angles of attack and constant Reynolds number. Our results show that a data-driven surrogate model can predict lift and pitching moment fluctuations from visual tuft observations.

Zonal calibration algorithm is the most widely used method to extend the measurement range of five-hole probes. However, large measurement error will be aroused near the boundary between two neighboring zones and this is acknowledged as the inner boundary measurement problem of zonal calibration algorithm. To tackle this problem, a two-dimensional uniform flow model is developed in this paper to describe the relationship between pressure from holes and flow angles. Based on this model, a method to adjust the boundary between two neighboring zones automatically with respect to inlet flow conditions is developed. With this novel method, the data extrapolation of zonal calibration algorithm at measurement stations near the boundary between two neighboring zones is avoided, and the corresponding large measurement error is eliminated. According to the experimental data, maximum measurement error of total pressure and flow angle can reach 7.5% and 3.2°, and will be reduced to 0.89% and 0.12° by the novel method. Resultantly, the inner boundary measurement problem of zonal calibration algorithm is solved. Influences of several key parameters on the measurement accuracy of the novel method are investigated too, and criteria to adjust the boundary between two neighboring zones are given. Conclusions of this paper can be used to further improve the accuracy of five-hole probes in measuring large angle flows.

Graphic abstract

Understanding the fundamental structure of shock-containing elliptic jets is of great academic and engineering interest, but there are still many unknowns. The three-dimensional flow features of an underexpanded jet emerging from an elliptic convergent nozzle with an aspect ratio of 4.0 at the exit face are experimentally investigated by rainbow schlieren tomography (RST). The elliptic jet is discharged into quiescent air using an intermittent blowdown wind tunnel. The Reynolds number based on the equivalent diameter and flow properties at the nozzle exit is (3.0times 10^{5}). Multi-view rainbow schlieren images of the elliptic jet are taken by rotating the nozzle around its longitudinal axis, and the density field is reconstructed using the convolution back-projection (CBP) method. The three-dimensional density field of the elliptic jet is acquired with a nominal spatial resolution of approximately 13 (upmu)m. The flow characteristics of shock-containing elliptic jets, such as the shock-cell length, the supersonic length, the switchover location, and the axis-switching location, are quantitatively revealed from the streamwise density profiles, the density contour plots in the minor-axis and major-axis planes where a method is proposed to quantitatively estimate the switchover and axis-switching locations. The shock-cell and supersonic lengths are quantitatively compared with the recently introduced analytical solution and scaling law, respectively. In addition, the shock structures and topology showing the spatial evolution in the streamwise direction of the near-field shock system within the elliptic jet are experimentally demonstrated for the first time.