Experiments in Fluids最新文献

Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements of nitric oxide (NO) using a Quantum Cascade Laser (QCL) in the vicinity of 5.26 μm were conducted in a hypervelocity flow generated in the Texas A&M Hypervelocity Expansion Tunnel (HXT). The nascent NO was produced downstream of symmetric Mach reflections generated in Mach 8.5 flows with stagnation enthalpies from 6.9 to 11.1 MJ/kg. Path-averaged flow parameters of rotational and vibrational temperatures and NO concentration at a measurement rate of 30 kHz were obtained. By probing the R-branch of the fundamental absorption band in NO, thermal nonequilibrium and NO concentration levels in the post-shock region were measured. Measurements are compared to equilibrium calculations. NO equilibrium values during the 1 ms test period differ from the experimental rotational and vibrational measurements across the same time period. The experimental measurements of the rotational temperature show a consistent value around 3000 K larger than the recovered vibrational temperature across any run. The NO concentrations in all runs are near to the reported equilibrium value; often beginning higher than, and over time decaying to, the equilibrium concentration value of the specific tunnel run.

A novel test setup called the asymmetric shock tube for experiments on nonideal rarefaction waves (ASTER) has been commissioned at Delft University of Technology. The ASTER, which works according to the principle of Ludwieg tubes, is designed to generate and measure the speed of small and finite amplitude waves propagating in the dense vapors of fluids formed by complex organic molecules, therefore in the nonideal compressible fluid dynamics regime. The ultimate goal of the associated research is to prove the existence of nonclassical gasdynamics. The setup consists of a high-pressure charge tube and a vacuum tank separated by a glass disk equipped with a breaking mechanism for rarefaction waves experiments. When the glass disk is broken, an expansion wave propagates into the tube in the direction opposite to the fluid flow. The propagation speed of this wave is measured using a time-of-flight method with the help of four fast-response pressure sensors placed equidistantly in the middle of the tube. The charge tube can withstand pressures and temperatures of up to 15 bar and 400(^{circ }mathrm{C}). Preliminary rarefaction experiments were successfully conducted using dodecamethylcyclohexasiloxane, (hbox {D}_{6}), as the working fluid and at pressures and temperatures of up to 9.4 bar and 372(^{circ }mathrm{C}) , respectively. The results of an experiment featuring the initial state for which a theoretical model predicts the nonclassical acceleration of rarefaction waves show that the propagation is qualitatively different from that put into evidence by experiments for which the propagation is classic. Upcoming setup improvements and experimental campaigns are planned with the objective of experimentally verifying the existence of nonclassical gasdynamics.

Graphical abstract

An isooctane spray from a high-pressure multihole GDI injector (Bosch HDEV6) was characterised by means of optical extinction tomography, relying on collimated illumination by a focused shadowgraph setup. The tests were carried out in air under ambient conditions at an injection pressure of 300 bar. Spray images were acquired over a 180-degree angular range in 1-degree increments. The critical issues of optical extinction tomography of sprays, related to the strong light extinction by the dense liquid core of fuel jets, were addressed. To mitigate artefacts arising from the reconstruction process, the extinction data were subjected to spatially-variant filtering steps for both raw and post-log data before being analytically inverted through the inverse Radon transform. This approach made it possible to process extinction data at very large optical depths. A nearly complete three-dimensional reconstruction of the spray was obtained, providing significant details of the spray morphology and the internal structure of the jets throughout spray development. Different phases of the atomization process, from the near-field to the far-field regions of the spray, were observed.

This work studies the link between the bursting process of a flat plate laminar separation bubble and the modification of the stability characteristics of the separated shear layer due to changes in the flow parameters. A vast population of short and long laminar separation bubbles was surveyed by means of Particle Image Velocimetry instrumentation for different values of the Reynolds number, the free-stream turbulence intensity and the streamwise pressure gradient. A fine-step variation of the free-stream velocity allowed us to determine the critical Reynolds number at which bursting occurs. Successively, the most amplified wavelength and frequency were computed for both the short and the long bubble regimes. Once scaled with the boundary layer displacement thickness at separation, the average wavenumber of the vortices shed by the bubble was found to be constant and equal to about 0.9 in the short regime, accordingly to previous studies. Differently, this quantity reduces to about 0.6 in the long bubble regime, and a marked change in the Strouhal number of vortex shedding occurs. Also, the temporal growth of spanwise vortices was seen to occur in the recirculation region of long type bubbles, being linked to an absolute instability of disturbances. The currently acquired data demonstrate the existing link between the bursting process of a laminar separation bubble and a marked change in the instability mechanisms driving the transition process of the boundary layer. A simplified correlation for the prediction of bursting is provided in this work as a function of the free-stream turbulence intensity and the streamwise pressure gradient.

Abstract

A magnetic suspension system is developed for free-motion wind tunnel testing. An external magnet-yoke open magnetic circuit produces a levitation force acting on the disk magnet in the model, and stable suspension of the model is achieved by PID feedback control using two coils. The suspended models are free from support interference and move in three degrees of freedom in the wind tunnel. The pitch rotation around the y-axis and the translational motion in the xz plane under the influence of unsteady aerodynamic forces are observed using the motion capture technique. Parameter identification methods using Fourier analysis of the motion capture data are developed to determine the moment slope (C_{textrm{M}alpha }), lift slope (C_{textrm{L}alpha }) and drag coefficient (C_{textrm{D}}). The standard deviations of the identified values of (C_{textrm{M}alpha }), (C_{textrm{L}alpha }) and (C_{textrm{D}}) are less than 5%, 8% and 6% of the respective means.

Graphical abstract

An optically based experimental approach for estimating detonation cell size of premixed gas phase fuel–oxidizer mixtures in an optically accessible linear detonation tube is presented. Detonation wave fronts propagating through undiluted near-stoichiometric ethylene–oxygen mixtures in the circular detonation tube were visualized and recorded using CH* chemiluminescence imaging near 430 nm at 100 kHz for initial mixture pressures up to 22 kPa. The chemiluminescence imaging, coupled with high-speed videography, is shown to capture cellular detonation structures as small as 1.6 mm in width. The measured cell sizes increase as the initial fill pressure decreases, corroborating well-established relationships between detonation cell sizes and initial reactant pressures. The optically based method is validated against conventional soot foil measurements performed simultaneously with multiple detonations at various initial fill conditions. Both chemiluminescence images and soot foil measurements are compared to previously published cell size trends for undiluted fuel–oxygen detonations, demonstrating reasonable agreement with the established methods. Paired with the optically accessible detonation channel, the high-speed chemiluminescence imaging technique offers a passive estimation of detonation cell size for the range of conditions investigated with a faster experimental turnaround time relative to conventional methods.

Fast pressure-sensitive paint (PSP) was applied to an inlet/isolator designed using the Osculating Internal Waverider Inlet with Parallel Streamlines (OIWPS) method. The dorsal isolator surface pressure was measured using anodized-aluminum PSP through transparent cast acrylic that makes up the ventral portion of the isolator. Temperature-sensitive paint was utilized to correct for the PSP’s temperature sensitivity. The model was tested under Mach 5.7 flow at Re (=) 8.5 (times 10^6) /m and 10.2 (times 10^6) /m in the AFOSR–Notre Dame Large Mach-6 Quiet Tunnel (ANDLM6QT) under conventional noise conditions. Flow phenomena, such as shocks originating in the inlet and flow separation at the throat, were visualized with high spatial resolution. The dynamics measured by the PSP and pressure transducers matched well where the spectral signal-to-noise ratio was above unity. Power spectral densities showed significant frequency content at (approx)1 kHz in the shock-wave/boundary-layer interaction (SWBLI) regions. Coherence analysis showed a linear relationship between the unsteady pressures at locations underneath different SWBLI in the isolator, with the exception of the Busemann throat shock. Temporal correlation of shock positions indicated that disturbances propagated downstream at 114% of the core-flow velocity; however, improved calculations of the core-flow velocity are needed to refine this assessment. The surface pressure fields at Re = 8.5 (times 10^6) /m and 10.2 (times 10^6) /m were quantitatively very similar, and the results in the ANDLM6QT were qualitatively similar to previous studies in the Boeing/AFOSR Mach-6 Quiet Tunnel under noisy flow.

Accurate reconstruction of particle acceleration requires post-processing of Lagrangian particle trajectories to limit noise amplification by differentiation. Over the past two decades, many studies have used a convolution filter based on a truncated Gaussian kernel. The present work evaluates the performance of Gaussian kernels truncated at varying standard deviations. It is shown that, compared to the truncation typically used in Lagrangian particle tracking, a stronger truncation has a similar frequency response, but is superior in terms of overall noise reduction. For kernels of equal width, particle accelerations calculated using a kernel with stronger truncation have up to 20% lower noise. Alternatively, for a specified reduction in noise a shorter kernel can often be used compared to a Gaussian kernel at the commonly used truncation, resulting in less loss of data at trajectory endpoints. It is shown that at the optimal truncation, a Gaussian kernel is mathematically approximated by a second-order polynomial. In this limit, the use of a polynomial kernel has equal results at reduced computational expense compared to the Gaussian kernel.

Polymer droplets subjected to a heated environment have significance in several fields ranging from spray drying, powder formation, and surface coating. In the current work, we study the evaporation of an acoustically levitated high-viscoelastic aqueous polymeric droplet under radiative heating. Depending on the irradiation intensity, we observe bubble nucleation in dilute regime of polymer concentration, contrary to previously observed nucleation in semi-dilute entangled regime for low-viscoelastic polymer droplets. After bubble nucleation, a quasi-steady bubble growth occurs depending on the irradiation intensity and polymer concentrations. Our scaling analysis reveals that initial bubble growth follows Plesset–Zwick criteria, independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the irradiation intensity. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. At high polymer concentrations and irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin of significant strength. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low-viscoelastic polymer droplets.

Abstract

Pressure-sensitive paint (PSP) was applied to the surface of a NACA0012 airfoil to investigate pressure fluctuations associated with trailing edge (TE) noise under low-velocity flow conditions. The primary focus is to assess the feasibility of employing laser pulses exposed at the airfoil surface to mitigate TE noise. However, the weak pressure fluctuations accompanying TE noise pose a challenge, as they are overshadowed by image sensor noise in high-speed cameras capturing PSP emission changes. To address this issue, a novel time-resolved phase-locking technique was introduced, utilizing the signal from a semiconductor pressure transducer at the trailing edge as a phase-lock trigger source. By repetitively conducting phase-locked measurements (1150 times), time series ensemble averaged data based on PSP emission images were obtained, enabling the capture of these subtle pressure fluctuations. Quantitatively, fluctuations with a dominant frequency of 679 Hz and an amplitude of 50 Pa are resolved within an accuracy of about 15 Pa, achieved at a recording rate of 19.2 kHz. Both the suppression and subsequent redevelopment of the pressure field with the TE noise offer valuable insights into the dynamics of TE noise and open avenues for targeted noise reduction strategies in aerodynamic applications.

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