Experiments in Fluids最新文献

The study investigates the liquid film thickness-dependent laser-induced fluorescence of a dye in a heat transfer oil affected by wall reflections at different solid surface materials (aluminum, copper, steel) and surface textures (polished and sandblasted). A specially designed fluorescence cell allows a precise adjustment of the film thickness at a fixed temperature and allows the investigation of various substrate materials and textures. Photo-dissociation free measurements are ensured due to a closed-loop circuit, driven by a pump. The LIF signal was generated by admixture of the fluorescent dye Nile red to the heat transfer oil Marlotherm LH. A CW laser at 532 nm was applied for excitation, and emissions were recorded by using a spectrometer. The use of a relatively low dye concentration (0.59 mg/l) ensures negligible reabsorption of the fluorescence and thus minimal spectral changes due to a variation in film thickness, which is indispensable for precise temperature measurements. A comparison of the dye fluorescence affected by reflections at different solid materials and surface treatments for a 1-mm film thickness reveals a similar trend for all investigated materials, except for copper. Copper leads to a surface texture-dependent spectral shift of the peak emission (polished: 3.8 nm, sandblasted: 4.3 nm) toward larger wavelengths in comparison with the remaining materials (peak always at 586.4 nm). This is attributed to the more distinct wavelength-dependent reflection behavior of copper evaluated in a theoretical study. Since the fluorescence signal experiences a stronger reflection in comparison with the incident laser beam, this leads to a spectral shift of the emission spectra toward larger wavelengths. A model approach is developed describing effects of direct and non-direct reflection of fluorescence for different materials and textures. A diffusive reflection leads to an overall decrease of reabsorption. This is caused by the reduced direct reflection of laser light, which passes through the liquid film a second time (or multiple times) and consequently less emission signal. Temperature-dependent measurements in combination with a two-color measurement approach showed the significant influence of wavelength-dependent reflection behavior on the temperature determination on liquid films.

Flight vehicles can optimize their performance by sensing unsteady flow phenomena and leveraging this information to improve decision-making and actuation. This study experimentally investigates the use of surface pressure measurements as unsteady flow sensors during large-amplitude transverse wing–gust encounters. An instrumented wing model is developed to overcome difficulties associated with unsteady pressure measurements in water towing tank facilities. The measurement system is validated through steady and unsteady experiments and is used to study the unsteady pressure distributions associated with transverse wing–gust encounters. Concurrent analysis of the pressure distributions and flowfields yields the following flow event sequence for high gust ratio (GR) experiments: As the wing enters the gust, a large suction peak forms on the leading edge. The suction peak widens and eventually splits into two distinct peaks. The secondary suction peak is associated with the leading-edge vortex (LEV) and its suction strength is found to be proportional to the gusting flow dynamic pressure, (textrm{GR}^2 + 1). Integration of the sectional pressure distributions resulted in accurate estimates of the overall wing loads during the vortex formation stage of the dynamic stall process but not during the vortex separation stage. Dynamic stall initiation is shown to be associated with an inflection point on the leading-edge suction transient and an abrupt drop in the leading-edge pressure gradient. The timing of LEV formation is found to be associated with a maximum in leading-edge suction and an increase in leading-edge pressure gradient.

The air gap of an optically accessible model of a directly cooled radial flux electric motor is investigated using laser-induced fluorescence (LIF). The cooling oil enters the air gap and a film forms on the stator surface, which resembles a thin film in a shear flow. The flow phenomena at different rotational speeds (2000 rpm to 10,000 rpm) are described. An intensity-based LIF measurement technique is developed and used to measure the film thickness on the stator in a realistic air gap environment. The rotational speed influences the flow phenomena and the film thickness of the stator film. With increasing rotational speed, i.e., increasing gas Reynolds number, the film thickness probability density functions (PDFs) shift to lower film thicknesses and become narrower, which is in agreement with the characteristic behavior of films in shear flows as reported in the literature. Additionally, the velocities of the wave crests which move across the film surface are evaluated and used to calculate the film Reynolds number Re_F, which characterizes the investigated operating points.

The nanoscale thermal anemometry probe (NSTAP) has been widely used to measure turbulence levels in high Reynolds number flows. Here, we report that the probe measurements are geometrically sensitive to its particular orientation in the flow under certain conditions. The geometric sensitivity appears as an amplification of the turbulence signal but has no effect on the mean. Data are taken using an NSTAP in multiple configurations and compared to data from previous studies and a conventional hot-wire. Results suggest that geometric sensitivity is caused by a difference between the mean and dynamic calibrations of the NSTAP.

New modification of background-oriented schlieren (BOS) technique using periodic grayscale and color patterns and phase-shifting profilometry (PSP) image processing is proposed. Its accuracy, spatial resolution and robustness with respect to large displacement gradient are analyzed using synthetic images. Comparison with multi-pass direct cross-correlation, wavelet-based optical flow analysis and Fourier transform profilometry (FTP) is performed. Grayscale PSP BOS requires at least three phase-shifted images to be taken with the schlieren object remaining unchanged. Its accuracy improves with growing number of images. The required three grayscale images can also be coded as the channels of a single color image. Thus, single-frame color PSP BOS is realized, which can be used to measure the unsteady flows. The negative effects of the color channels cross-talk and imbalance are corrected. PSP BOS yields reliable results for displacement gradient far above 1 pix/pix, which is the limit for other image processing techniques. However, its accuracy is affected by the camera sensor noise. The ability of color PSP BOS to measure strongly refracting objects is confirmed by experimental measurements of the glycerol mass fraction in glycerol–water diffusion layer. Quantitative comparison between PSP, direct cross-correlation and FTP is also performed for the measurements of natural convection in water near a heated vertical plate.

We propose a robust method to detect colors, positions, and sizes of particles on color particle images. The method is free of color artifacts originating from the demosaic process of Bayer raw images. We test the method using synthetic color particle images to quantitatively evaluate its performance and demonstrate its capacity for precise extraction of colors while refining particle positions at sub-sub-pixel level. We demonstrate the method’s applicability by applying it to different types of particle images from laboratory experiments of color particle tracking velocimetry and liquid crystal thermometry. The method standardizes color detection on particle images and promises the improvement in final performances of these color-utilizing measurements.

To assess the performance of a flexible membrane wing in a harmonic transverse gust, the aerodynamic forces, membrane deformations and surrounding flow fields are synchronously measured. The gust is generated by two periodically pitching airfoils. First, the flexible wing outperforms its rigid counterparts in time-averaged lift, achieving a maximum lift increment of 44.6% at the angle of attack (α) of 16°. The lift-enhancement is attributed to the camber increase and membrane vibration to suppress flow separation—a mechanism analogous to the behavior in steady flow conditions. Second, for the unsteady lift response, the flexible and rigid cambered wings reduce lift fluctuations induced by the gust when 3° ≤ α ≤ 9°, implying a gust alleviation effect. The maximum alleviation of the flexible wing is 32.0% at α = 6°. To elucidate the gust alleviation mechanism, phase-averaged lift and flow fields at α = 6° are investigated. The flow over the flexible wing remains consistently attached, while the rigid plate wing exhibits the evolution of leading-edge vortices (LEVs). These LEVs provide additional lift that helps the rigid plate wing to match the maximum lift of the flexible wing. However, the minimum lift of the plate wing is much smaller. Consequently, the primary reason for the aerodynamic load alleviation is elaborated. However, the inherent resonant vibration of the membrane is a by-product that has a negative impact on the alleviation effect.

Graphical abstract

The droplet impact phenomenon onto liquid films is predominant in a variety of modern industrial applications, including internal combustion engines and cooling of electronic devices. These are characterised by heat and mass transfer processes, such as evaporation, condensation and boiling. However, studies regarding droplets and liquid films under non-isothermal conditions are scarce in the literature and do not explore temperature-dependent phenomena. Due to this, the main objective of this work is to evaluate the influence of temperature on the splashing occurrence of single droplets impinging onto liquid films under the presence of a heat flux. The crown evolution is evaluated qualitatively to provide insight regarding breakup mechanisms. Water, n-heptane and n-decane are the fluids considered for the current study, as these provide a wide range of thermophysical properties and saturation temperatures. The splashing dynamics are evaluated by varying the droplet impact velocity and dimensionless temperature of the liquid film. Qualitative results show that an increase in the liquid film temperature leads to the transition from spreading to splashing, which is less evident for fuels in comparison with water. For water and n-heptane, the formation of cusps on the crown rim is promoted, which is associated with ligament breakup. For n-decane, the crown rims are relatively homogeneous in terms of shape and size, whereas the atomisation process varies a function of the liquid film temperature. Visually, the secondary droplets exhibit a greater size in comparison with lower temperatures. Transitional regimes display some irregularities, such as splashing suppression/reduction, which require further attention. In terms of splashing correlation, the authors propose to develop a non-splash/splash boundary for both iso- and non-isothermal conditions. Results show that the splashing threshold is dependent on the thermophysical properties and the dimensionless temperature of the liquid film.