Experiments in Fluids最新文献

Recovering pressure fields from image velocimetry measurements has two general strategies: (i) directly integrating the pressure gradients from the momentum equation and (ii) solving or enforcing the pressure Poisson equation (divergence of the pressure gradients). In this work, we analyze the error propagation of the former strategy and provide some practical insights. For example, we establish the error scaling laws for the pressure gradient integration (PGI) and the pressure Poisson equation. We explain why applying the Helmholtz–Hodge decomposition (HHD) could significantly reduce the error propagation for the PGI. We also propose to use a novel HHD-based pressure field reconstruction strategy that offers the following advantages or features: (i) effective processing of noisy scattered or structured image velocimetry data on a complex domain; (ii) using radial basis functions (RBFs) with divergence/curl-free kernels to provide divergence-free correction to the velocity fields for incompressible flows and curl-free correction for pressure gradients; and (iii) enforcing divergence/curl-free constraints without using Lagrangian multipliers. Complete elimination of divergence-free bias in measured pressure gradient and curl-free bias in the measured velocity field results in superior accuracy. Synthetic velocimetry data based on exact solutions and high-fidelity simulations are used to validate the analysis as well as demonstrate the flexibility and effectiveness of the RBF-HHD solver.

In the present study, flow over a circular cylinder under compressible low-Reynolds-number conditions was investigated using the low-density wind tunnel. The Reynolds number (Re) based on cylinder diameter was set in the range of (100; le ;{text{Re}}; le ;1000), and the Mach number (M) was set in the range of (0.1; le ;M; le ;0.7). The Schlieren visualization and force measurement were conducted under pressure below 10 kPa (0.81 kPa for the lowest case) with the circular cylinder with 1.2, 3.0, and 5.0 mm in diameter. Although the signal-to-noise ratio of the Schlieren image is very low because of the low-pressure condition, the fluctuation components originating from the flow phenomena were successfully extracted using the denoising technique based on the modal decomposition. As a result, the Mach number effects on the length of the recirculation region and the Strouhal number of the vortex shedding were revealed. The drag coefficient obtained at (M=0.1) and 0.2 for (100; le ;{text{Re}}; le ;1000) was in good agreement with that under the incompressible conditions, and the drag coefficient increases as the Mach number increases.

We present a novel experimental technique for characterizing the free surface of capillary flows using the spatiotemporal phase shifting profilometry (ST-PSP) method. This study specifically addresses various regimes of capillary flows over inclined surfaces, including drops, rivulets, meanders, and braided films. The technique is explained step by step with a detailed discussion of the calibration process, which is carried out on a solid wedge to determine the optical distances required for the phase-to-height relationship. In addition, the minimal dye concentration for accurately reconstructing the free surface of a dyed water flow is investigated. The ST-PSP method is then applied to profile different liquid flows, achieving large signal-to-noise ratios in all experiments. Notably, the analysis of a sessile droplet shows excellent agreement between the ST-PSP results and side-view visualizations, as demonstrated by the precise recovery of its apparent contact angle. Moreover, free surface reconstructions of rivulet flows align well with previous theoretical predictions. These findings suggest that the ST-PSP method is highly effective for obtaining precise height maps of capillary flows, offering a valuable tool for future validation of theoretical models.

A method for converting spatial data from high-speed schlieren visualizations into upsampled temporal data, previously limited to applications involving fluid flows with uniform-velocity disturbances, is extended to flows with variable-velocity disturbances. Schlieren-based velocimetry is first employed to derive disturbance propagation speeds and directions throughout the schlieren field of view. The velocity field is then integrated to determine disturbance streamlines through discrete pixel locations, permitting reconstructions of the temporal signal at times between schlieren images. The resulting temporal signals have an effective sampling rate governed by the spatial resolution of the images, rather than the camera frame rate, with the local propagation speeds providing the necessary conversion to temporal units. The efficacy of the method is demonstrated by applying it to a schlieren dataset capturing Mach 6 flow over a cone–flare model with a frame rate of 824 kHz. The frame rate is sufficient to resolve the relevant second-mode disturbances from the raw pixel times series, allowing for quantitative comparisons between the raw and reconstructed signals. The reconstruction process enhances the information extractable from the temporal signals by eliminating aliased content previously constrained by the camera’s Nyquist frequency and enabling the analysis of additional high-frequency content. The accuracy and robustness of the reconstruction are validated by introducing known errors into the signals. Increased camera frame rates correlate with improved robustness, with errors in propagation speed of up to (pm 10%) having minimal impact on the spectral characteristics of the signal for frame rates as low as approximately half the primary disturbance frequency.

Backward-facing steps (BFS) can have a detrimental impact on laminar flow lengths because of their strong effect on boundary layer transition. BFS with normalized step heights in the range of (h/delta _1 approx) 0.1 to 0.6 (corresponding to height-based Reynolds numbers of (hbox{Re}_h = (U_infty h / nu ) approx) 230 to 2430) were installed in a two-dimensional wind tunnel model and tested in the Cryogenic Ludwieg-Tube Göttingen, a blow-down wind tunnel with good flow quality. The influence of BFS on the location of laminar-turbulent transition was investigated over a wide range of unit Reynolds numbers from (hbox{Re}_1 = {17.5times 10^{6},{text {m}^{-1}}}) to (80times 10^{6},hbox{m}^{-1}), three Mach numbers, (M= 0.35), 0.50 and 0.65, and various streamwise pressure gradients. The measurement of the transition locations was accomplished non-intrusively by means of temperature-sensitive paint. Transition Reynolds numbers, calculated with the flow length up to the location of laminar-turbulent transition (x_{T}), ranged from (hbox{Re}_{rm tr}approx) 1 × 10⁶ to 11 × 10⁶, and were measured as a function of step height, pressure gradient, Reynolds and Mach numbers. Incompressible linear stability analysis was used to calculate amplification rates of Tollmien–Schlichting waves; transition N-factors were determined by correlation with the measured transition locations. In parallel to earlier investigations with a similar setup, this systematic approach was used to identify functional relations between non-dimensional step parameters ((h/delta _1) and (hbox{Re}_h)) and the relative change of the transition location. Furthermore, the change of the transition N-factor (Delta N) due to the installation of the steps was investigated. It was found that the installation of backward-facing steps with (h/delta _1 lesssim 0.15) and (hbox{Re}_h lesssim 300) does not lead to a reduction of (hbox{Re}_{rm tr}) and to (Delta N > 0). However, increasing the step size results in a decreasing laminar flow length and thus an increasing (Delta N). The reported results are in general agreement with earlier investigations at significantly lower Mach and Reynolds numbers.

Oil-flow visualizations represent a simple means to reveal wall streamline patterns. Yet, the evaluation of such images can be a time-consuming process and is subjective to human perception. In this article, we present a fast and robust method to obtain quantitative insight based on qualitative oil-flow visualizations. Specifically, the local wall streamline direction is predicted by a convolutional neural network. The supervised training of this network was based on an extensive dataset involving approximately one million image patches that cover variations of the flow direction, the wall shear-stress magnitude and the oil-flow mixture. For a test dataset that is distinct from the training data, the mean prediction error of the flow direction is as low as three degrees. A reliable performance is also noted when the model is applied to oil-flow visualizations obtained from the literature, demonstrating the generalizability required for an application in diverse flow configurations. The trained model is available at https://github.com/AeroTUBerlin/OilFlowCNN.

Wind turbine blades in standstill or parked conditions often experience large angles of attack (AoA), where vortex-induced vibrations (VIV) may occur that increase the risk of structural damage. To better understand the VIV of airfoils at high AoA from an aerodynamic perspective, we conducted experimental investigations into the vortex dynamics of a surging airfoil at a (90^circ) incidence undergoing forced vibrations. Experiments were conducted at two reduced frequencies (k) to demonstrate the lock-in effect, where the vortex shedding frequency aligns with the motion frequency. Results indicate distinct vortex shedding behaviors: at higher k value of 0.38, downstream wake vortices form when the airfoil is moving upwind, while upstream vortices emerge during the downwind motion, interacting with the downstream vortices and leading to an outward flow. At lower k value of 0.19, the wake remains directed to the downwind side, regardless of the airfoil’s motion direction. Lock-in is evident in both cases, with one vortex pair shed per cycle at lower k and two pairs at higher k. Furthermore, the study examines the influence of vortex dynamics on unsteady aerodynamic loads. The results show that drag peaks when the airfoil moves upwind near the center position of its trajectory; at higher k, negative drag occurs as the airfoil moves downwind near the center, driven by the interactions among convection, turbulent momentum, pressure, and viscous forces. A reduced-order load estimation model for a flat plate is applied to the experimental data, showing good agreement during the upwind motion of the airfoil, which is the design condition for the original flat plate model. However, during the downwind motion, as the flow condition does not match the original flat plate design condition, the circulatory part of the model is modified to account for the presence of two pairs of vortices in the flow field, yielding improved agreement with the drag values determined from the measured flow field. The findings highlight distinct flow patterns and vortex interactions for the two motion cases, offering insights into their impact on aerodynamic loads.

Stereo-PIV measurements performed in a refractive index-matched facility examine the mean flow and normal Reynolds stresses over the entire axial, radial, and circumferential extents of an axial compressor rotor at two operating conditions, including pre-stall. In the tip region, we follow the backward leakage jet and double leakage across the tip gap as well as the rollup, evolution, and breakdown of tip leakage vortex (TLV). With decreasing flowrate, these phenomena shift closer to the blade leading edge (LE). The TLV is surrounded by a region with elevated circumferential velocity, which expands once vortex breakdown occurs, especially at pre-stall. Conditional averaging highlights the effects of transient pre-stall features, which are ‘smeared’ by averaging, such as LE spillage, circumferential velocity exceeding the blade speed, backflow vortices (BFVs), and blade boundary layer separation. Cavitation-based flow visualization under pre-stall and stall shows the prevalence and evolution of BFVs and their role in the formation of high circumferential velocity regions. The operating conditions and transients affect the spatial distributions of normal Reynolds stresses and turbulent kinetic energy (TKE). Far from stall, the TKE peaks near the TLV center and is dominated by the radial stress. At pre-stall, the TKE increases rapidly following the TLV breakup along the periphery of high circumferential velocity regions, where the BFVs form. It is dominated by the circumferential stress within the rotor and by the axial stress downstream of the trailing edge. Many, but not all, of the observed trends can be elucidated based on the turbulence production, advection, and diffusion terms.

The current work discusses the demonstration of an event-based (EB) camera for time-resolved imaging (10,000 frames/sec) of the primary atomization of a canonical air-assist atomizer. Experiments were performed simultaneously with conventional high-speed and event-based cameras, enabling us to quantitatively assess the performance of event-based cameras in spray imaging (particularly near-field liquid jet breakup) applications. Three atomization breakup regimes are considered: columnar, bag, and multimode. Dynamic mode decomposition (DMD) was implemented to analyze the acquired instantaneous time-dependent images from both cameras and assess their performance in extracting turbulence statistics of the primary atomization. The computed DMD frequency spectrum and spatial modes of liquid breakup characteristics from the images recorded from both cameras are comparable, highlighting the potential of event-based cameras in extracting coherent structures in the primary atomization zone and their spectral contents. However, in some instances, the EB camera underpredicts the DMD modes compared to high-speed cameras, and the reasons for these discrepancies were explained. Finally, the limitations (e.g., event saturation) of event-based cameras in the context of primary atomization imaging were also discussed.

Time-resolved stereoscopic particle image velocimetry is employed to analyze the behavior of elliptic synthetic jet vortex rings impinging onto a solid wall. Reconstruction of three-dimensional flow field is achieved using a phase-locked method. Three jet Reynolds numbers (Re_sj = 318, 477, and 636) are investigated while maintaining a constant orifice-to-wall distance (H₀/D₀ = 5) and orifice aspect ratio (AR = 3). The results show that the elliptic vortex ring with non-uniform distribution of the circulation induces asymmetric secondary vortex, which is different from circular ring-wall interaction. The process of impingement is divided into three stages: strong interaction, weak interaction, and stable expansion. During the stable expansion stage, the elliptic vortex ring exhibits two scenarios: into a circle and into an ellipse. The difference can be explained as follows: in the strong interaction stage, the expansion of the primary vortex ring after the impingement is mainly influenced by both the self-induction of the noncircular vortex ring and the vortex strength. However, in the weak interaction stage, it is primarily affected by the latter effect owing to the reduced three-dimensionality of the vortex ring. Under different Reynolds numbers, the vortex rings undergo different phases of the axis switching process before the vortex-wall interaction, resulting in their different final shapes. In addition, the time-averaged flow characteristics are investigated by considering azimuthally averaged velocity fields. With increasing Reynolds number, the maximum radial velocity, turbulent kinetic energy, radial mass flow rate, and momentum flux increase. In particular, the maximum radial velocity distribution can match well with the final shapes of the vortex rings.