Showing papers in "Experiments in Fluids in 2019"

Dense motion estimation of particle images via a convolutional neural network

Abstract: In this paper, we propose a supervised learning strategy for the fluid motion estimation problem (i.e., extracting the velocity fields from particle images). The purpose of this work is to design a convolutional neural network (CNN) for estimating dense motion field for particle image velocimetry (PIV), which allows to improve the computational efficiency without reducing the accuracy. First, the network model is developed based on FlowNetS, which is recently proposed for end-to-end optical flow estimation in the computer vision community. The input of the network is a particle image pair and the output is a velocity field with displacement vectors at every pixel. Second, a synthetic dataset of fluid flow images is generated to train the CNN model. To our knowledge, this is the first time a CNN has been used as a global motion estimator for particle image velocimetry. Experimental evaluations indicate that the trained CNN model can provide satisfactory results in both artificial and laboratory PIV images. The proposed estimator is also applied to the experiment of turbulent boundary layer. In addition, the computational efficiency of the CNN estimator is much superior to those of the traditional cross-correction and optical flow methods.

Screech-tone prediction using upstream-travelling jet modes

Abstract: The purpose of this paper is to characterise and model the A1 and A2 screech modes in supersonic jets operating at off-design conditions. The usual screech-modelling scenario involves a feedback loop between a downstream-travelling Kelvin–Helmholtz instability wave and an upstream-travelling acoustic wave. We review state-of-the-art screech-frequency prediction models and associated limitations. Following the work of Edgington-Mitchell et al. (J Fluid Mech 855, 2018), a new prediction approach is proposed where the feedback loop is closed by the upstream-travelling jet modes first discussed in Tam and Hu (J Fluid Mech 201:447–483, 1989) in lieu of the free-stream sound waves. The Kelvin–Helmholtz and upstream-travelling jet modes are obtained using a cylindrical vortex-sheet model. The predictions provide a better agreement with experimental observations than does the classical screech-prediction approach. Screech dynamics associated with the staging process is explored through a wavelet analysis, highlighting that staging involves mutually exclusive switching that is underpinned by non-linear interactions.

Accurate turbulence level estimations using PIV/PTV

Abstract: The estimation of the turbulence level of flow facilities is very important for the comprehensive description of experimental results. While for low flow velocities various measurement techniques can be used (for example hot-wire, LDV, PIV) the task becomes difficult in the case of compressible flows as temperature and density fluctuations bias the measurement of the velocity fluctuations. In this work, we analyze the free-stream flow of the trisonic wind tunnel Munich (TWM) by means of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). The goal is to determine the flow quality, i.e. the turbulence level, over the operating range of the facility without bias due to temperature and density variations. The capability of PIV/PTV for the estimation of small velocity fluctuations is investigated in detail. It is shown that a small particle shift on the measurement plane in combination with a large particle image displacement on the image plane allows for precise velocity measurements. Furthermore, a variation of the time separation between the PIV double images, $$\varDelta t$$ , enables the measurement uncertainty to be determined, which was estimated to be as low as $$0.04\%$$ of the mean displacement for a mean displacement of $$\varDelta x=100\,$$ pixel and an interrogation window size of $$32\times 32\,$$ pixel. Regarding the wind tunnel turbulence, it was found that the turbulence level generally decreases with increasing Mach number for the TWM facility, starting with $$1.9\%$$ at $$Ma=0.3$$ and reaching $$0.45\%$$ at $$Ma=3.0$$ . With this analysis, a methodology exists to perform accurate turbulence measurements in incompressible and compressible flows.

Active control of circular cylinder flow with windward suction and leeward blowing

Abstract: This study is an experimental investigation of the effectiveness and mechanism of a concept for bluff-body control characterized by combined windward suction and leeward blowing (WSLB). Wind tunnel tests are performed at a subcritical Reynolds number (Re) of 3.33 × 104. Open-loop WSLB is used to control the flow around a cylindrical test model. Distributed suction and blowing nozzles are symmetrically arranged at the front and rear stagnation points. Steady suction and blowing are implemented simultaneously on both surfaces of the cylinder. WSLB control is characterized by the dimensionless momentum of the suction/blowing relative to the incoming airflow. Instantaneous pressure distributions on the midspan of the cylinder surface for the baseline and controlled cases are measured to quantify the modifications of WSLB control to the aerodynamic coefficients of the cylinder. In addition to surface pressure measurements, a particle image velocimetry (PIV) system is used to describe the wake flow structures of the baseline and controlled cylinders. Experimental results demonstrate that WSLB control decreases sectional drag at the midspan and reduces the fluctuating amplitudes of dynamic wind loads acting on the cylinder. The Strouhal number to characterize the vortex shedding frequencies of the controlled cylinders is also found to deviate from that of the natural cylinder. PIV measurement results show that the active blowing positioned at the leeward stagnation point forms a pair of vortices into the cylinder wake that modify the original vortex shedding process. As the blowing vortices convect into the wake, they stretch the unsteady shear flows from the upper and lower sides of the cylinder, increase the vortex-formation length and push the alternative vortex shedding further downstream. It is also shown that a steady and symmetric perturbation imposed on the periodic cylinder flow can reduce the von Karman vortices and modify the mode of wake vortex shedding significantly. With active control of windward suction and leeward blowing (WSLB), blowing positioned at the leeward stagnation point forms a pair of vortices in the cylindrical wake that modifies the typical vortex shedding process. As the blowing vortices convect downstream, they contribute to detaching the unsteady shear layers from the cylinder. It is shown that a steady and symmetric perturbation imposed on the periodic cylinder flow can lead to a symmetric mode of wake vortex shedding.

Towards reliable turbulence estimations with phase-detection probes: an adaptive window cross-correlation technique

Abstract: Air–water flow turbulence was derived from pseudo-instantaneous velocities measured with a dual-tip phase-detection probe. This new technique is proposed based upon adaptive time windows for cross-correlation analysis combined with robust filtering criteria, allowing computation of velocity time series in highly aerated flows. Each velocity estimation corresponded to a small group of bubbles or droplets. Stochastic synthetic velocity fields were generated to assess the limitations and uncertainties related to the proposed analysis. Subsequently, capabilities of the technique were demonstrated through an application to a real two-phase flow on a large-size stepped spillway.

Multimodal partial cavity shedding on a two-dimensional hydrofoil and its relation to the presence of bubbly shocks

Abstract: The shedding dynamics of partial cavitation forming on a NACA0015 hydrofoil is known from the previous studies to be multimodal, exhibiting abrupt changes in Strouhal number as the cavitation number is reduced (Arndt et al. in Instability of partial cavitation: a numerical/experimental approach. National Academies Press, Washington, D.C., Retrieved from the University of Minnesota Digital Conservancy. http://hdl.handle.net/11299/49781 , 2000). The present study aims to understand the underlying shedding mechanisms responsible for this abrupt change in dynamics. As was observed in Ganesh et al. (J Fluid Mech 802:37–78, 2016), the shedding process can result from both re-entrant liquid flow and the formation of propagating bubbly shock waves within the cavity. Time-resolved X-ray densitometry and high-speed videography are combined with time synchronous measurements of acoustic noise produced by the cavity to examine the formation and shedding of the partial cavitation on a NACA0015 hydrofoil. From the experiments on the hydrofoil, it was observed that the mean cavity length increased with decreasing cavitation number, as expected. At higher pressures, stable partial cavities formed that shed smaller vapor clouds mainly due to the re-entrant liquid flow in the cavity closure. With a reduction in cavitation number, the partial cavity grew in length, and shedding often occurred when the cavity was pinched-off from its leading edge. Once a large region of vapor was shed, the pressure pulse caused by its subsequent collapse could suppress the growth of the newly forming partial cavity. This feedback led to complex, multistep cavity shedding dynamics. At still lower cavitation numbers, bubbly shocks were found to be responsible for the strongest periodic shedding of vapor clouds. Spectral analysis of the acoustic signal revealed a multimodal nature, which was more pronounced at lower cavitation numbers, and similar dynamic behavior was also found for the time varying local void fraction measurements in the vicinity of cavity closure. The occurrence of strongest multimodal behavior was also characterized by the presence of bubbly shock waves as the dominant mechanism of shedding. From the measurements and analysis, it is concluded that at least four different types of shedding modes occurred over a range of cavitation numbers at a fixed attack angle (10°).

Skin friction and coherent structures within a laminar separation bubble

Abstract: We study the Laminar Separation Bubble (LSB) which develops on the suction side of a NACA 0015 hydrofoil by means of a Temperature-Sensitive Paint (TSP), at a Reynolds number of $$1.8\times 10^5$$ and angles of attack $$\mathrm{AoA} = [3^{\circ }$$ , $$5^{\circ }$$ , $$7^{\circ }$$ , $$10^{\circ }$$ ]. The thermal footprints $$T_\mathrm{w}(x,y,t)$$ of the fluid unveil three different flow regimes whose complexity in time and space decreases when $$\mathrm{AoA}$$ increases, up to $$10^{\circ }$$ where the LSB-induced spatial gradients are linked to quasi-steady positions in time. At $$\mathrm{AoA} =7^{\circ }$$ the LSB system undergoes a 3D destabilization, that induces C-shaped arcs at separation and weak bubble-flapping at reattachment. Structural changes occur at $$AoA=5^{\circ }$$ and $$3^{\circ }$$ : bubble-flapping raises homogeneously at reattachment while intermittent, wedge-shaped events alter the LSB shape. The relative skin-friction vector fields $$\varvec {\tau }_\mathrm{w}(x,y,t)$$ , extracted from $$T_\mathrm{w}(x,y,t)$$ by means of an optical-flow-based algorithm, provide the topology of the flow at the wall and feed a physics-based criterion for the identification of flow separation $${\mathfrak {S}}(y,t)$$ and reattachment $${\mathfrak {R}}(y,t)$$ . This criterion fulfills, in average, a novel skin-friction ground-truth estimation grounded on the determination of the propagation velocity of temperature fluctuations. The obtained $${\mathfrak {S}}(y,t)$$ is composed of several manifolds that extend spanwise from saddle points to converging nodes via the saddles unstable manifold, while, at least at higher AoA, manifolds that compose $${\mathfrak {R}}(y,t)$$ move from diverging nodes to saddle points via the saddles stable manifolds. The triggering of a wedge-shaped event by a rising $$\varOmega$$ -shaped vortex in the reverse LSB flow is captured and described in analogy to a simplified model.

Pressure from 2D snapshot PIV.

Abstract: In this study, we quantify the accuracy of a simple pressure estimation method from 2D snapshot PIV in attached and separated flows. Particle image velocimetry (PIV) offers the possibility to acquire a field of pressure instead of point measurements. Multiple methods may be used to obtain pressure from PIV measurements, however, the current state-of-the-art requires expensive equipment and data processing. As an alternative, we aim to quantify the efficacy of estimating instantaneous pressure from snapshot (non-time resolved) two-dimensional planar PIV (the simplest type of PIV available). To make up for the loss of temporal information, we rely on Taylor’s hypothesis (TH) to replace temporal information with spatial gradients. Application of our approach to high-resolution 2D velocity data of a turbulent boundary layer flow over ribs shows moderate to good agreement with reference pressure measurements in average and fluctuations. To assess the performance of the 2D TH method beyond average and fluctuation statistics, we acquired a time-resolved measurement of the same flow and determined temporal correlation values of the pressure from our method with reference measurements. Overall, the correlation attains good values for all measured locations. For comparison, we also applied two time-resolved approaches, which attained values of correlation similar to our approach. The performance of the 2D TH method is further assessed on 3D time-resolved velocity data for a turbulent boundary layer and compared with 3D methods. The root-mean-square (RMS) pressure fluctuations of the 2D TH, 3D TH and 3D pseudo-Lagrangian methods closely follow the pressure fluctuation distribution from DNS. These observations on the RMS pressure estimates are further supported by similar analysis on synthetic PIV data (based on DNS) of a turbulent channel flow. The values of spatial correlation between the 2D TH method and the DNS pressure fields in this case, are similar to the temporal correlations achieved in the turbulent flow over the ribs. Finally, we discuss the accuracy of instantaneous pressure estimates and provide a rule of thumb to determine regions where the pressure fluctuation estimate from the 2D TH methods is likely to fail.

Suppressing tip vortex cavitation by winglets

Abstract: Despite the numerous remedies prescribed so far, tip vortex cavitation (TVC) remains a major issue in design and operation of diverse applications. In this paper, we experimentally investigate the effectiveness of winglets in suppressing TVC. An elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at ± 45° and ± 90° dihedral angles. To better focus on the physics of the problem, we have intentionally avoided any optimization on the geometries and our winglets are only smooth non-planar extensions of the original cross-section. Modifying no more than 3.7% of the lifting surface, lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not substantially different from the baseline. Nevertheless, cavitation inception–desinence tests reveal that undeniable advantages are achieved by the winglets in TVC alleviation. It is found that the 10%-bent 90° winglets are more effective than the 45° cases, with − 90° (bent down toward the pressure side) performing superior to + 90°. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14° incidence angle. In addition, the study on the bending length effect conducted for the 90° configurations shows that the 5%-bent winglets are not as striking as the 10% ones. Employing Stereo-PIV technique, the influence of winglets on non-cavitating flow structures is examined. For the most effective winglet (10%-bent 90°-downward), we observe that the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC.

On the physical mechanism of tip vortex cavitation hysteresis

Abstract: Inception and desinence thresholds of tip vortex cavitation (TVC), generated by an elliptical NACA 16-020 hydrofoil, are measured at different flow conditions for various gas contents. It is observed that TVC often disappears at cavitation indices significantly higher than the inception thresholds introducing large hystereses. Our measurements reveal that TVC desinence pressure increases with gas content and, under specific flow conditions, may reach to atmospheric pressure. When the pressure of the cavitating core is below the initial saturation pressure of the dissolved gases, water flowing adjacent to the interface becomes supersaturated, which leads to the diffusion of air molecules into TVC. To estimate the outgassing rate, a simple diffusion model is proposed and analytically solved. In addition, we demonstrate that the extent of the delay in desinence due to outgassing is also dictated by the bulk flow parameters, i.e., the incidence angle and freestream velocity. Owing to flow visualizations, we assert that formation of a laminar separation bubble of appropriate size and shape at the hydrofoil tip is a necessary condition for a delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core by forcing the incoming flow to wrap around the axis. By roughening the hydrofoil tip, we demonstrate that the hysteresis is completely suppressed once the laminar separation bubble is destroyed. Moreover, our velocity measurements show that at near-wake, the incidence angle associated with delayed desinence is accompanied by a jet-like axial velocity profile while a wake-like profile is observed for the low-hysteresis case.

Multi-exposed recordings for 3D Lagrangian particle tracking with Multi-Pulse Shake-The-Box

Abstract: The recent introduction of the Multi-Pulse Shake-The-Box (MP-STB) method opened the possibility of extending 3D Lagrangian particle tracking (LPT) to the investigation of high-speed flows, where long time-resolved sequences of recordings are currently not available due to the limited acquisition frequency of high-speed systems. The MP-STB technique makes use of an iterative approach to overcome the limitations posed by the short observation time offered by a multi-pulse recording sequence. Multi-pulse sequences are typically obtained by synchronizing multiple illumination systems to generate bursts of laser pulses where the time separation can be freely adjusted down to less than a microsecond. Several strategies can be adopted for the recording of multi-pulse sequences; a dual camera system can be adopted to separate the single pulses onto the camera frames (either by means of polarization or timing), while the use of multi-exposed frames allows for the employment of a single imaging system, largely reducing the complexity and cost of the experimental setup. The main strategies to generate multi-pulse recording sequences are presented here; the application and performances of the MP-STB method are discussed based on the analysis of experimental data from the investigation of three turbulent boundary layer flows at velocities ranging from 10 to approximately 30 m/s. Results show the capability of the MP-STB technique in reconstructing accurate track fields which can be exploited both to describe instantaneous flow structures and to produce highly spatially resolved statistics by means of ensemble average in small bins. The iterative reconstruction and tracking strategy for MP-STB can be successfully adapted to the case of multi-exposed frames. Results suggest that, despite the increase in particle image density resulting from the double-exposed particle images, the adoption of multi-exposed recordings has the potential to become the technique of choice for the recording of multi-pulse sequences suitable for Lagrangian particle tracking in high-speed flows.

Singular value decomposition of noisy data: noise filtering

Abstract: The singular value decomposition (SVD) and proper orthogonal decomposition are widely used to decompose velocity field data into spatiotemporal modes For noisy experimental data, the lower SVD modes remain relatively clean, which suggests the possibility for data filtering by retaining only the lower modes Herein, we provide a method to (1) estimate the noise level in a given noisy dataset, (2) estimate the root mean square error (rmse) of the SVD modes, and (3) filter the noise using only the SVD modes that have low enough rmse We show through both analytic and PIV examples that this method yields nearly the most accurate possible SVD-based reconstruction of the clean data Moreover, we provide an analytic estimate of the accuracy of this reconstruction

High-resolution velocimetry from tracer particle fields using a wavelet-based optical flow method

Abstract: A wavelet-based optical flow method for high-resolution velocimetry based on tracer particle images is presented. The current optical flow estimation method (WOF-X) is designed for improvements in processing experimental images by implementing wavelet transforms with the lifting method and symmetric boundary conditions. This approach leads to speed and accuracy improvements over the existing wavelet-based methods. The current method also exploits the properties of fluid flows and uses the known behavior of turbulent energy spectra to semi-automatically tune a regularization parameter that has been primarily determined empirically in the previous optical flow algorithms. As an initial step in evaluating the WOF-X method, synthetic particle images from a 2D DNS of isotropic turbulence are processed and the results are compared to a typical correlation-based PIV algorithm and previous optical flow methods. The WOF-X method produces a dense velocity estimation, resulting in an order-of-magnitude increase in velocity vector resolution compared to the traditional correlation-based PIV processing. Results also show an improvement in velocity estimation by more than a factor of two. The increases in resolution and accuracy of the velocity field lead to significant improvements in the calculation of velocity gradient-dependent properties such as vorticity. In addition to the DNS results, the WOF-X method is evaluated in a series of two-dimensional vortex flow simulations to determine optimal experimental design parameters. Recommendations for optimal conditions for tracer particle seed density and inter-frame particle displacement are presented. The WOF-X method produces minimal error at larger particle displacements and lower relative error over a larger velocity dynamic range as compared to correlation-based processing.

Unsteady pressure-sensitive-paint (PSP) measurement in low-speed flow: characteristic mode decomposition and noise floor analysis

Abstract: Pressure-sensitive-paint (PSP) measurement was conducted for unsteady phenomena at various frequencies up to the order of kHz in low-speed flow to evaluate measurement accuracy of PSP. Pressure fluctuations on the floor surface induced by the Karman vortex were measured by PSP and unsteady pressure transducer. The dominant frequency of the pressure fluctuations is varied from 0.15 to 1.7 kHz by changing the size of the square cylinder. While regions with large pressure fluctuations could be visualized by calculating root mean square of pressure fluctuations from PSP images, the values significantly differed from those measured by pressure transducer. By applying Fast Fourier Transform (FFT), the power spectral density (PSD) at peak frequencies could be obtained within an error of 20%. Singular-value decomposition (SVD) yields a remarkable improvement in signal-to-noise ratio. However, amplitude of pressure fluctuations is changed depending on the way how to select modes. Three mode-selection methods for SVD filtering/reconstruction analysis are proposed in this study which show good improvement compared with convection method and are proved capable of extracting characteristic behaviors of the flow phenomena even below the noise floor.

Generation and control of helium-filled soap bubbles for PIV

Abstract: The operating regimes of an orifice-type helium-filled soap bubbles (HFSB) generator are investigated for several combinations of air, helium and soap flow rates to establish the properties of the production process and the resulting tracers. The geometrical properties of the bubbles, the production regimes and the production rates are studied with high-speed shadowgraphy. The results show that the bubble volume is directly proportional to the ratio of helium and air volume flow rates, and that the bubble production rate varies approximately linearly with the air flow rate. The bubble slip velocity is measured along the stagnation streamline ahead of a cylinder via particle image velocimetry (PIV), yielding the particle time response from which the neutral buoyancy condition for HFSB is inferred. The HFSB tracing capability approaches that of an ideal tracer (i.e., minimum slip and shortest response time) when the volume flow rate of helium is approximately one thousandfold the soap flow rate. This study provides guidelines for operating HFSB generation systems, intended for PIV experiments. Graphical abstract: [Figure not available: see fulltext.].

Single-camera 3D PTV using particle intensities and structured light

Abstract: We use structured monochromatic volume illumination with spatially varying intensity profiles, to achieve 3D intensity particle tracking velocimetry using a single video camera. The video camera records the 2D motion of a 3D particle field within a fluid, which is perpendicularly illuminated with depth gradients of the illumination intensity. This allows us to encode the depth position perpendicular to the camera, in the intensity of each particle image. The light intensity field is calibrated using a 3D laser-engraved glass cube containing a known spatial distribution of 1100 defects. This is used to correct for the distortions and divergence of the projected light. We use a sequence of changing light patterns, with numerous sub-gradients in the intensity, to achieve a resolution of 200 depth levels.

Detailed experiments on weakly deformed cavitation bubbles

Abstract: We present high-precision experiments conducted with the aim to better characterise weak deformations of single cavitation bubbles. Using two needle hydrophones and a high-speed photodetector, we record the timings of shock waves and luminescence emitted at the collapse of laser-induced bubbles and are able to thereby obtain a precise measurement of their displacement during their lifetime. The bubbles are primarily deformed by variable gravity reached aboard parabolic flights, but we additionally take into account the effect of the nearest surfaces. A time shift of approximately 60 ns is found between the bubble lifetimes measured by the hydrophones and the photodetector for spherically collapsing bubbles, which we believe to be a result of different initial shock wave propagation speeds at the bubble’s generation and at collapse. The normalised bubble displacement is found to follow a $$\zeta ^{2/3}$$ scaling law for $$\zeta>0.001$$ , where $$\zeta$$ is the dimensionless anisotropy parameter quantifying the bubble deformation (analogous to Kelvin impulse). Additionally, we quantify the asymmetry of the shock wave generated at the collapse of bubbles with various levels of deformations by comparing the hydrophone signals at two different locations, and find significant variations between the shock peak pressures and energies at $$\zeta>0.001$$ . These results consolidate the suggestion to consider $$\zeta \sim 0.001$$ as a practical limit between spherical and deformed bubbles. This limit is probably sensitive to the bubble’s initial sphericity, which is exceptionally high in our mirror-based aberration-free setup.

Wall-pressure fluctuations induced by a compressible jet flow over a flat plate at different Mach numbers

Abstract: The wall-pressure fluctuations induced by a compressible subsonic jet over a flat plate are experimentally investigated. Measurements were performed in a semi-anechoic environment, where a compressible jet facility, whose nozzle diameter (D) was 12 mm, is installed. The position of the flat plate was fixed at H/D = 2, where H is the radial position of the flat plate from the jet axis. The study was carried out for several jet Mach numbers spanning the range from 0.5 to 0.9. An overall aerodynamic characterization of the plate effect on the jet plume is provided by means of Pitot tube measurements. The wall-pressure fluctuations acting on the flat plate were measured by a couple of cavity-mounted pressure transducers, providing pointwise pressure signals in the streamwise and in the spanwise directions. A statistical and spectral characterization of the wall-pressure fluctuation field is provided addressing the effect of the jet Mach number variation and of the spatial evolution along the streamwise and spanwise directions. Implications for wall-pressure fluctuations modelling are discussed by the application of the Corcos’ model. Scaling laws with respect to the different jet flow conditions for the wall-pressure spectra are finally presented.

Experimental study of secondary vortex structures in a rotor wake

Abstract: The operation and aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake, in particular regarding vortex–vortex interactions of hovering rotors. Rotor simulations using modern computational methods have the potential to capture high levels of detail, which recently triggered discussions of secondary vortex braids entangling the primary tip vortices. These structures are highly dependent on the numerical settings and need experimental validation. The current work investigates the wake of a subscale rotor in ground effect by time-resolved and volumetric flow-field measurements using the “Shake-The-Box” technique. Both the Lagrangian tracks of the flow tracers and the derived gradient-based vortex criteria clearly verify the existence of secondary vortices. A post-processing scheme is applied to isolate these vortices in larger data sets. No distinct spatial organization of the structures was observed, but a slightly preferred sense of rotation which agrees to the shear of the wake swirl. The secondary structures were created shortly downstream of the rotor blades, starting at wake ages of approximately $$75^\circ $$.

Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits

Abstract: Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° angles of attack. At 0° angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at positive angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract: [Figure not available: see fulltext.].

GPU-based, parallel-line, omni-directional integration of measured pressure gradient field to obtain the 3D pressure distribution

Abstract: The introduction of 3D time-resolved velocity measurement techniques enables calculation of the instantaneous pressure distribution by spatially integrating the material acceleration. This paper introduces an efficient method for 3D integration of the acceleration, which does not require prescribed Dirichlet boundary condition on one of the surfaces, minimizes the propagation of errors in acceleration, and can be easily utilized in flows with complex boundaries. This parallel-line, omni-directional integration procedure (Omni3D) calculates the pressure at every point by integration from all directions, while avoiding regions with large acceleration errors. To reduce the computational costs, the calculations are performed by a GPU-based algorithm, which determines the 3D pressure field from tomographic PIV data in 1 min. The accuracy of Omni3D is compared to that of several techniques, including procedures based on solving the Pressure Poisson Equation (PPE) with different Dirichlet boundary conditions. The error analysis is based on Direct Numerical Simulation (DNS) data for isotropic turbulence, synthetic 3D PIV images for turbulent channel flow generated from DNS data, and experimental data. It examines the effects of spatial resolution, propagation, and avoidance of embedded local errors, boundary conditions, method for calculating the velocity, as well as viscous and sub-grid stresses on the calculated pressures. For acceleration fields with low errors and properly specified boundary conditions, Omni3D and PPE give similar results. However, Omni3D is more effective in suppressing the effects of acceleration errors. Sample experimental results including instantaneous plot of pressure, pressure statistics, and pressure–velocity correlations based on tomographic PIV data are also provided.

Effects of droplet shape on impact force of low-speed droplets colliding with solid surface

Abstract: Experimental studies of low-speed droplets colliding with a flat solid surface are performed to record the impact force and deformation by using a highly sensitive piezoelectric transducer and a high-speed camera. The experimental data are used to verify the accuracy of a 3D numerical model via the smoothed particle hydrodynamics method, and the numerical method is used to explore the effects of the droplet morphology on the collision force. As the horizontal–vertical ratio of the droplets increases, the peak impact force increases by a power function trend and the time to reach the collision force peak decreases. The relationship between the equivalent volume of a spherical droplet and the volume of an ellipsoid droplet with different horizontal–vertical ratios under the same peak impact force is obtained. Self-similar theory is also suitable for droplets with ellipsoid shape. Finally, the stresses inside the material after large-sized spherical and small-sized oblate droplets hit the wall surface are compared. Results indicate that the curvature radius of droplets is a key factor that affects initial impact force and material erosion.

Hypersonic boundary-layer separation detection with pressure-sensitive paint for a cone at high angle of attack

Abstract: A measurement technique for identifying lee-side crossflow-induced boundary-layer separation on a blunt $$7^{\circ }$$ half-angle circular cone at high angle of attack has been developed and tested. Previous work has shown that local minima in root-mean-squared (rms) pressure fluctuations on the surface are good identifiers of separation. These surface pressure fluctuations are measured with a temperature-corrected, high-frequency-response anodized-aluminum pressure-sensitive paint (AA-PSP). This AA-PSP was made in-house to provide the high frequency response required for this work. The sensor’s frequency response of 3 kHz proved to be fast enough to detect lines of local minimum rms pressure fluctuations indicative of separation on the lee side of the cone for angles of attack from $$9.8^{\circ }$$ to $$15.8^{\circ }$$ . A shift in the separation location towards the windward side of the model was observed as angle of attack increased; however, the separation location converged to a constant azimuth for angles of attack greater than or equal to $$1.8\times$$ the cone’s half angle.

Volumetric calibration enhancements for single-camera light-field PIV

Abstract: This work presents a volumetric calibration method for single-camera light-field particle image velocimetry (light-field PIV or LF-PIV). The proposed technique makes use of the unique point-like feature of particle light-field images to accurately determine affected pixels for a spatial voxel over a relative large measurement volume. A calibration model is derived based on Gaussian optics, which relates a spatial point light source with its confusion circle produced on microlens array (MLA), and optical distortions are accounted for by introducing five calibration parameters. By taking lens defects and misalignment between MLA and image sensor into account, the calibration method can calculate weighting coefficient for particle image reconstruction more accurately than the theoretical ray-tracing method, especially for regions further away from focal plane where light ray deflections are significant due to optical distortions. The volumetric calibration method was validated by simulation tests using synthetic light-field images, and has been successfully applied to a classic vortex-ring LF-PIV measurement, where the measurable range in depth direction was successfully extended and the quality of reconstructed volumetric velocity field was greatly improved.

Turbulent flow and vortex characteristics in a blocked subchannel of a helically wrapped rod bundle

Abstract: The study of flow and heat transfer through a bundle of rods is important, as this type of configuration is widely used in many engineering applications, for example, in heat exchangers, clusters of support structures, steam generators, and nuclear power reactors. This work experimentally investigated the flow-field characteristics of rod bundles with helically wrapped wires (wire-wrapped rod assemblies) proposed for advanced burner reactors. Subchannel blockage is one of the most interesting accident scenarios for liquid metal fast reactors as different mechanisms can lead to a partial or total subchannel blockage. It is of paramount important to understand the thermal-hydraulic behavior of the flow in the vicinity of such blockages. The flow mixing characteristics in two interior subchannels located near the core of the bundle, one without and one with the presence of a blockage, were experimentally investigated. The velocity fields within the spatial gaps of two subchannels were obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) technique for Reynolds numbers of 4000 and 17,000. The first- and second-order statistics, including the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress, were computed from the obtained TR-PIV velocity vector fields. In the blocked sub-channel, the TR-PIV results revealed a large recirculation flow region in the vicinity of the blockage that was created by the interaction between inflows from neighboring subchannels. Spectral analysis was performed and the vortex shedding frequency indicated by the Strouhal number was found to be $$St=0.16$$ for the two Reynolds numbers investigated. The flow characteristics of the interior subchannels were investigated via the two-point cross-correlation of fluctuating velocities. In addition, proper orthogonal decomposition (POD) analysis was applied to the instantaneous vorticity fields to extract the statistically dominant flow structures. Finally, the statistical characteristics of the vortex, such as the vortex populations, spatial distributions, and vortex strengths, were acquired by combining POD analysis and the vortex identification algorithm.

Volumetric measurement and vorticity dynamics of leading-edge vortex formation on a revolving wing

Abstract: Leading-edge vortex (LEV) is a hallmark of insect flight that forms and remains stably attached on high angle-of-attack (AoA), low aspect ratio (AR) wings undergoing revolving or flapping motion. Despite the efforts on explaining the stability of LEV when it reaches steady state in revolving wings, its formation process remains largely underexplored. Here, we investigate the LEV formation on a revolving wing (AoA = 45°, AR = 4 and Re = 1500), starting with a constant acceleration in the first chord (c) length of travel and then rotating at a constant velocity. The ‘Shake-the-box’ (STB) Lagrangian particle tracking velocimetry (PTV) system together with a volumetric patching process were performed to reconstruct the entire time-resolved flow field. Results show that LEV reaches steady state after approximately 4c of travel, within which its formation can be separated into three stages. In the first stage, a conical LEV structure begins to form with a tangential downstream convection of (negative) radial shear vorticity. In the second stage, the radial vorticity within the LEV is further convected downwards by the developed downwash, which limits its growth, whereas vorticity stretching increases the LEV strength. In the third stage, vorticity tilting and spanwise convection reduce the LEV strength at its rear end next to the trailing edge and, therefore, preventing it from growing. Our results suggest that insect wings with AR ~ 4, Re ~ 103 and flapping amplitude between 2c and 4c may barely or even not reach the steady state of LEV, indicating an indispensable role of transient LEV dynamics in understanding insect flight.

Reconstructing the structural parameters of a precessing vortex by SPIV and acoustic sensors

Abstract: This paper presents an experimental study of a strongly swirling turbulent flow with the formation of a precessing vortex core (PVC) that emerges at the outlet of a tangential swirler nozzle. The studies were carried out using a stereoscopic particle image velocimetry (SPIV) and two acoustic pressure sensors. An analysis of the velocity fields measured by the proper orthogonal decomposition (POD) showed that the precession motion of the vortex makes a significant contribution (more than 34%) to the turbulence kinetic energy, making it possible to consider the PVC effect as a prominent and convenient object for testing theoretical models describing precessing vortex motion. Estimates of the model parameters of the precessing vortex such as the vortex core radius, vortex precession radius, and vortex intensity based on statistical data obtained from uncorrelated PIV images are presented in the paper. The estimated parameters were compared with the parameters obtained by phase averaging the PIV images, and as a result, three-component velocity distributions were obtained relative to the vortex position. The analysis showed that the precession radius, vortex core size, and vortex circulation are normally distributed. The conditional averaging technique made it possible to determine the structural parameters of the PVC, which was confirmed to be a left-handed helical vortex. Due to the rapid disintegration of the PVC above the nozzle, only a portion of the spiral vortex was observed. Therefore, the helical vortex pitch was estimated in a local sense. Basically, all of the approaches gave similar results for the vortex parameters, providing access to the vortex dynamics. In particular, these cross-checked parameters were used to calculate the precession frequency on the basis of the available helical vortex model. The obtained frequency was found to be in good agreement with the experimentally measured precession frequency, confirming the adequacy of the theory.

Oscillation dynamics of a bubble rising in viscous liquid

Abstract: In the present work, we study the oscillation dynamics of 3 mm-diameter bubbles generated through an orifice submerged in viscous liquids. The viscosity of those liquids is varied to change the behavior of the rising bubble. The details of the rising motion and shape oscillation of the bubbles are measured using a combination of high speed, high-resolution imaging, and an accurate digital image processing technique. Direct Numerical Simulations that mimic the experimental conditions are also performed using a front-tracking technique, called the Local Front Reconstruction Method. The predictions of the bubble shape and rising velocity obtained by the numerical simulations show good agreement with the experimental results. Our experimental and numerical results show that the oscillation frequency and the damping rate at lower modes can be predicted using available theoretical models found in the literature. However, discrepancies arise between our results with the theoretical predictions at higher order oscillation modes. We conclude that the discrepancies are due to the influence of rising motion and the vortex wave, which is not considered in the theoretical models.

Ethanol droplet formation, dynamics and combustion mode in the flame of the SpraySyn-nozzle

Abstract: The synthesis of nanoparticles via flame spray pyrolysis (FSP) is based on a couple of physicochemical steps such as precursor atomization, droplet evaporation, fuel combustion, particle nucleation and growth. Most recent studies on FSP are focused on the particle formation without taking the antecedent precursor atomization and spray formation into account. In the present work, the atomization characteristics under burning and non-burning conditions are investigated by means of laser sheet Mie-scattering images and phase Doppler anemometry. The influence of the gas to liquid volume ratio on the spray formation, jet morphology and scales of turbulence in the nozzle far-field are studied to assess the performance of the external-mixing gas-assisted SpraySyn-nozzle. The turbulent flame spray is characterized by the Kolmogorov length scale and the Kolmogorov shear rate. The droplet–droplet interaction and group combustion behaviour, enabling clouds of merely vaporizing droplets surrounded by a flame sheath, are investigated. The experimentally determined droplet velocities, turbulence length scales and the analysis of group combustion modes allow a deeper insight into the spray flame dynamics and droplet mixing zones.

Phase-resolved PIV measurements of flow over three unevenly spaced cylinders and its coupling with acoustic resonance

Abstract: The characteristics of acoustic resonance excitation by flow over the arrangement of three unevenly spaced inline cylinders in cross flow were experimentally investigated. Phase-resolved particle image velocimetry (PIV) measurements were conducted to demonstrate the role of the separated shear layers around the cylinders in the excitation mechanism of acoustic resonance. The Strouhal number of self-sustained flow oscillations around the investigated arrangement is presented. Before the onset of acoustic resonance excitation, the location of the middle cylinder has a significant effect on the shear layer separation and impingement mechanism. At flow velocities that caused coincidence between an acoustic mode frequency and the intrinsic vortex shedding frequency that would occur under free-field condition, severe acoustic resonance corresponding to acoustic particle velocities of up to one-tenth of the main flow velocity was observed. For certain arrangements, acoustic resonance was detected at lower flow velocities than necessary for frequency coincidence. The Strouhal number of these pre-coincidence oscillations corresponds to that of a cavity formed between two successive cylinders. Phase-resolved PIV measurements show significant differences between flow field during and in the absence of acoustic resonance. Most importantly, acoustic resonance is excited when vortices roll up and impinge on the middle cylinder and the downstream cylinder, and no flow passes through the two gaps. The acoustic mode frequency and the Strouhal number of the Rossiter-like modes are decreased when resonance takes place at a higher Mach number. The aerodynamic interference between the two successive gaps formed by the three cylinders seems essential to explain the variation in the amplitude of resonance excitation.