Experiments in Fluids

The ability to track vortices spatially and temporally is of great interest for the study of complex and turbulent flows. A methodology to solve the problem of vortex tracking by the application of machine learning approaches is investigated. First a well-known vortex detection algorithm is applied to identify coherent structures. Hierarchical clustering is then conducted followed by a unique application of the Hungarian assignment algorithm. Application to a synthetic flowfield of merging Batchelor vortices results in robust vortex labelling even in a vortex merging event. A robotic PIV experimental dataset of a canonical Ahmed body is used to demonstrate the applicability of the method to three-dimensional flows.

The authors have carried out a study to investigate and clarify the characteristics of purely oscillating pipe flows over the developing region. The main objective of this study is to establish the method of time-dependent velocity profiles obtained by the ultrasonic velocity profile (UVP) measurement method. First, the relationship between the test fluids and the micro-particles, as reflectors of ultrasonic pulses, was investigated, and also the relationship between the sound speeds of the test fluids and wall materials was studied. Second, the UVP was used to obtain the instantaneous velocity profiles in oscillating pipe flows, and the developing characteristics of the flows were analyzed. Finally, the "entrance length" (analogy with a unidirectional pipe flow) required for oscillating pipe flows were analyzed examining the amplitude of the harmonic spectral components of the oscillating frequency. The fast Fourier transform (FFT) is proposed as the applicable method to estimate the "entrance length". From the Fourier transform of the velocity on the centerline, nonlinear oscillation of fluid occurs in the "entrance length" of the oscillating flows, and the viscous dissipation of the higher order velocity harmonics determines the entrance region. The "entrance length" can be obtained from the dissipation length of the third order harmonic. These results prove that the UVP method is highly applicable for carrying out flow measurements in the "entrance length" of the oscillating pipe flow.

This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m−1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96 $$\delta_{i,0}^*$$ . The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80–90 $$\delta_{i,0}^*$$ downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43 $$\delta_{i,0}^*$$ ), and transition is found to be accelerated over the separation bubble.

We investigated the wettability and impact dynamics of water droplets on rice leaves at various leaf inclination angles and orientations. Contact angle, contact angle hysteresis (CAH), and roll-off angle (α roll) of water droplets were measured quantitatively. Results showed that droplet motion exhibited less resistance along the longitudinal direction. Impact dynamic parameters, such as impact behaviors, maximum spreading factor, contact distance, and contact time were also investigated. Three different impact behaviors were categorized based on the normal component of Weber number irrespective of the inclination angle of the rice leaf. The asymmetric impact behavior induced by the tangential Weber number was also identified. Variation in the maximum spreading factor according to the normal Weber number was measured and compared with theoretical value obtained according to scaling law to show the wettability of the rice leaves. The contact distance of the impacting droplets depended on the inclination angle of the leaves. Along the longitudinal direction of rice leaves, contact distance was farther than that along the transverse direction. This result is consistent with the smaller values of CAH and α roll along the longitudinal direction.

In relation to the development of the interfacial area transport equation, local flow measurements of vertical downward air–water flows in a pipe with an inner diameter of 50.8 mm were performed at three axial locations of z/D=6.50, 34.0, and 66.5 as well as ten radial locations from r/R=0 to r/R=0.9 using a multi-sensor probe. In the experiment, the superficial liquid velocity and the void fraction ranged from −0.620 m/s to −2.49 m/s and from 0.21% to 8.4%, respectively. The dependence of the interfacial area transport on the liquid velocity, void fraction, and bubble size is discussed in detail.

This work presents a method for measuring fluctuating quantities such as temperature or velocity using a constant current hot wire anemometer. The scope of this method has been extended to include not only supersonic flows, but also transonic flows with low Reynolds numbers and transonic or supersonic heated flows. After examining the dependence of the different coefficients of sensitivity to aerodynamic and thermal parameters, the result of the study was applied to a turbulent boundary layer using a suitable processing method.

Accurately determining the wall-shear-stress, $$\tau _\textrm{w}$$ , experimentally is challenging due to small spatial scales and large velocity gradients present in the near-wall region of turbulent flows. To avoid these resolution requirements, several indirect iterative fitting methods, most notably the Clauser chart method, exist for determining $$\overline{\tau }_{w}$$ by fitting the mean velocity profile further away from the near-wall region in the log-law layer. These methods often require proper selection of fitting constants, assumptions of a canonical flow state, and other empirical-based generalizations. To reduce the amount of ambiguity, determining the near-wall velocity gradient by assuming a linear relationship between the mean streamwise velocity and wall normal distance in the viscous sublayer can be used. However, this requires an accurate unbiased measurement of the near-wall velocity profile in the region below five viscous spatial units, which can be less than 50 µm for high Reynolds number flows. Therefore, in this study a method for a volumetric defocusing microparticle tracking velocimetry method is presented that is capable of resolving the flow in the viscous sublayer of a turbulent boundary layer up to $$U_{\textrm{e}}=44.7\,$$ m/s ( $$Re_{\theta }=27250$$ ). This method allows for the measurement of the near-wall flow through a single optical access for illumination and imaging and serves as an excellent complement of larger scale measurements that require near-wall information. The $$\overline{\tau }_\textrm{w}$$ values determined from the defocusing approach were found to be in good agreement values obtained from a simultaneous parallax PTV measurement. Furthermore, analysis of the diagnostic plot and cumulative distribution of measured fluctuations in the near-wall region, showed that both methods are capable of accurately determining mean velocity and fluctuation profiles in the self-similar viscous sublayer region.