Using the resolvent analysis, we investigate how the near-wall mode primarily responsible for the friction drag is amplified or suppressed depending on the shape of the mean velocity profile of a turbulent channel flow. Following the recent finding by Kuhnen et al. (2018), who modified the mean velocity profile to be flatter and attained¨ significant drag reduction, we introduce two types of artificially flattened turbulent mean velocity profiles: one is based on the turbulent viscosity model proposed by Reynolds and Tiederman (1967), and the other is based on the mean velocity profile of laminar flow. A special care is taken so that both the bulk and friction Reynolds numbers are unchanged, whereby only the effect of change in the mean velocity profile can be studied. These mean velocity profiles are used as the base flow in the resolvent analysis, and the response of the wavenumber-frequency mode corresponding to the near-wall coherent structure is assessed via the change in the singular value (i.e., amplification rate). The flatness of the modified mean velocity profiles is quantified by three different measures. In general, the flatter mean velocity profiles are found to result in significant suppression of near-wall mode. Further, increasing the mean velocity gradient in the very vicinity of the wall is found to have a significant importance for the suppression of near-wall mode through mitigation of the critical layer.
Compared with conventional high-specific-speed axial flow pump, better cavitation performance and compact size have been achieved in contra-rotating axial flow pump, where the rear rotor is employed additionally to the front rotor to convert the swirling flow to the pressure rise. Meanwhile, significantly deteriorated performance has also been observed at well off-design flow rates with design rotational speed. The rotational speed control (RSC) of front and rear rotors has been experimentally proved to be effective to enhance the performance. However, thorough investigations are necessary to find the optimum rotational speeds of rotors. It may be done by computational fluid dynamics (CFD) simulations, whereas it is time-consuming to cover the wide ranges of rotational speeds. Therefore, in the present paper, a fast and effective performance prediction model is established by considering radial equilibrium condition, conservation of rothalpy and mass, empirical deviation angle, bladerows interaction and empirical losses. Experimental and CFD results are employed to validate the proposed prediction model. It is found that the proposed model shows good enough accuracy in predicting performances of contra-rotating axial flow pump under RSC in broad flow rate range. Furthermore, an energy saving application of the proposed model is also illustrated for two typical system resistances. Compared with the traditional valve control under the design rotational speed operation, the RSC method can well modify the pump head to satisfy the system resistance at wide flow rate range with the significantly improved energy performance.
The aerodynamic performance of the multi-rotor drone under the wall proximity has been investigated by experiment and numerical simulation. The propeller airflow along with the wall deflects toward the wall due to the Coanda effect, and it yields a negative impact on the aerodynamic performance. The present study aims to reveal the link between the propeller thrust and the propeller airflow under different wall proximity conditions. The deflection of the flow is confirmed by the flow visualization, and the wall pressure exhibits the signature of flow attachment both in the profiles of the mean and the fluctuation. The force measurement indicates that the degradation of the thrust is significant enough to affect the stability of the drone body. A possible reason of the decrease in thrust is found in the streamwise velocity distribution. The velocity distributions obtained by the numerical simulation indicate that the swirling motion is significantly suppressed due to the wall proximity effect. Moreover, the pressure distribution on the propeller surface explains the decrease of the thrust. The magnitude of the pressure difference becomes smaller when the propeller blade approaches very close to the wall.
A wind tunnel experiment was performed to investigate the requirements for providing a suitable environment for easily creating a localized turbulent region in a laminar boundary layer. A combination of a short-duration jet and suctions was used to prepare a potentially unstable region upstream, and another jet was ejected downstream against the region at various timings and different relative spanwise locations. Based on the results, the combination of the potentially unstable region and the short-duration jet promoted the transition to turbulence only under limited conditions, whereas in most cases, it worked negatively. However, the turbulent spot generation was enhanced when the downstream jet was used at the timing and location that enlarged the low-velocity area created upstream. Moreover, the locally disturbed region generated by the combination of the potentially unstable region and the short-duration jet did not directly grow into a turbulent spot; rather, the turbulent spot grew in the region following the disturbed region.
External forcing on a wing-tip vortex can affect its instability, and therefore an optimal perturbation can improve the aerodynamic performance of the wing. The present study examined the unsteadiness of the wing-tip vortex under periodic wing-tip vibration, and revealed its effect on the aerodynamic performance of the wing. A 3D-printed vibrating wing-tip model was prepared, which was driven by a sheet-type piezo actuator. Phase-averaged stereo particle image velocimetry (PIV) measurements clarified that the averaged position of the vortex depends on the phase of the wing-tip vibration, and the vortex shifted further from the wing as the actuation frequency increased. The phase-averaged velocity distributions indicate that the velocity deficit inside the vortex is significantly enhanced near the end of the downstroke of the wing-tip motion. The wing-tip vortex is weakened in the mid-upstroke, and its impact depends on the actuation frequency. This is because the motion of the wing is in the same direction as the flow rolling up from the pressure side, which prevents the formation of the vortex. In the mid-upstroke phase, the turbulence quantities, e.g., the turbulent kinetic energy and the Reynolds shear stress, are significantly suppressed; these effects depend monotonically on the actuation frequency. These arguments are supported by time-resolved recordings of the flow and the wing motion. The force measurements reveal that the vibration of the wing-tip brings a positive effect on the lift-to-drag ratio.