Reynolds-averaged Navier–Stokes simulations (RANS) of flows around a Clark-Y airfoil with uniform blowing (UB) and uniform suction (US) are performed aiming at improvement of the airfoil performance. First, the control effect in the case with single UB or US applied on the airfoil surface is investigated at the various control locations. The magnitude of UB/US is 0.14% of the free-stream velocity, and the control region is set at four different locations on the upper and lower surfaces. The Reynolds number based on the chord length and the angle of attack are 1.5 × 106 and 0°, respectively. It is found that the friction drag is decreased/increased by single UB/US control. It is also found that UB on the lower surface or US on the upper surface improves the lift-to-drag ratio, while UB on the upper surface or US on the lower surface worsens it. In the combined control of UB and US having the equal flow rate, the magnitude of blowing and suction is set at 0.14% or 0.26% of the free-stream velocity. The locations of blowing/suction and flow conditions are the same as those in the cases with either UB or US only. The simulation result suggests that the lift-to-drag ratio is improved by the combined control of UB on the lower surface and US on the upper surface. In particular, the lift-to-drag ratio is most improved by a combination of UB on the lower rear surface and US on the upper rear surface. In contrast, a combined control of UB on the upper front surface and US on the lower rear surface is identified as the most effective case for the friction drag reduction only.
The output power coefficient of the Savonius rotor needs to be improved in attaining better practical applications. Up until now, to improve the output power coefficient, the newly developed Savonius rotor with semi-elliptical sub-buckets has been introduced. However, some of the parameters on the semi-elliptical bucket have not yet been properly determined. Therefore, the influence of the additional semi-elliptical bucket’s shape in the newly developed Savonius rotor on the output power coefficient was investigated. The flow around the rotor was simulated by using the regularized lattice Boltzmann method. The virtual flux method was used to describe the shape of the rotor on Cartesian grids, and the multi-block method was used for the local fine grids around the rotor. The rotational speed of the Savonius rotor was maintained as a constant, and its performance was evaluated by the output power and torque coefficients. As a result, the additional semi-elliptical bucket successfully generated a positive torque during the advancing bucket period. While, it did not generate a large negative torque during the returning bucket period owing to its position behind the main bucket in the wind flow direction. Through a cycle, the semi-elliptical bucket only generated a positive torque with the interaction of the main bucket. The output power coefficient of the newly developed Savonius rotor was improved when compared to that of the traditional or Bach-type ones. The maximum output power coefficient of the newly developed Savonius rotor was 50.7% higher than that of the traditional rotor and 16.9% higher than that of the Bach-type rotor.
A single impinging jet exhibits high heat transfer performance around an impingement point on a wall. However, the heat transfer performance deteriorates as it moves away from the impingement point. Consequently, multiple impinging jets are commonly introduced to overcome the shortcomings of a single jet: inhomogeneous heat distribution on the wall and a narrow heating area. However, inhomogeneous heat transfers still occur. Therefore, a new jet control is required to improve the uniformity of heat transfer. Meanwhile, blooming jets are produced by appropriate combinations of axial and helical excitations at the nozzle exit. Using appropriately selected excitations, a jet can split into two separate jets (bifurcating jet) or spread into a shower of toroidal vortex rings. Blooming jets exhibit good performances of mixing and diffusion, suggesting possible applications in flow control. However, studies regarding the heat transfer performance of blooming jets are non-existent. In this study, we conducted direct numerical simulations of blooming jets impinging upon a wall and investigated their flow characteristics and heat transfer performances. As control parameters, the impingement distance (the distance from the nozzle to the wall) and frequency ratio (the axial excitation frequency to the helical frequency) are varied. The vortex structures and velocity magnitude reveals flow modulations due to blooming control. With the time-averaged local Nusselt number, the heat transfer performance of the blooming jets is evaluated quantitatively. Compared with uncontrolled jets, the uniformity of heat transfer of blooming jets is better, suggesting their potential application for leveling the heat transfer of impinging jets.
Friction drag reduction effect of a passive blowing on a Clark-Y airfoil is investigated. Uniform blowing, conducted in a wall-normal direction on a relatively wide surface, is generally known as an active control method for reduction of turbulent skin friction drag. In the present study, uniform blowing is passively driven by the pressure difference on a wing surface between suction and blowing regions. The suction and the blowing regions are respectively set around the leading edge and the rear part of the upper surface of the Clark-Y airfoil in order to ensure a sufficient pressure difference for passive blowing. The Reynolds number based on the chord length is 0.65×106 and 1.55×106. The angle of attack is set to 0° and 6°. The mean streamwise velocity profiles on the blowing region and the downstream, measured by a traversed hot-wire anemometry, are observed to shift away from the wall by passive blowing. This behavior qualitatively suggests reduction of local skin friction on the wing surface. A quantitative assessment of the friction drag is performed using the law of the wall accounting for pressure gradients (Nickels, 2004), coupled with a modified Stevenson’s law (Vigdorovich, 2016) to account for the weak blowing. From this assessment, the local friction drag reduction effect of passive blowing is estimated to reach 4%–23%.
An experimental study is conducted to investigate the flow characteristics of multiple elliptic jets issuing from a 6×6 nozzle array at a relatively low-Reynolds number (Re (= Ub·de/ν) = 4.3×103). Two aspect ratios of the multiple elliptic nozzles (equivalent diameter, de, of a nozzle was 6 mm), namely a/b = 2.25 and 6.25, where a and b are the radii of the major and minor axes of an elliptic nozzle, respectively, and two nozzle azimuthal orientations, namely the same and alternate azimuthal orientation arrangements, were used. The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer. The multiple jets located at the side of the ambient fluid were stretched due to interactions between the self-induced flow of an elliptic vortex ring and the secondary flow caused by the entrainment of the ambient fluid. For a/b = 2.25, axis switching occurred only once in the range of 1 < x/de ≤ 3 for both nozzle azimuthal orientations. For a/b = 6.25 and the same azimuthal orientation arrangement, axis switching occurred only once at 3 < x/de ≤ 5; axis switching did not occur for the alternate azimuthal orientation arrangement. Thus, the flow characteristics of multiple elliptic jets are influenced by the azimuthal orientation of adjoining nozzles.
This work addresses the numerical study of wave-piercing planing hull and related hydrodynamic performance as the appendages. From the half century ago, the interest in high-speed planing crafts has been advanced toward maintaining performance stably. The main reasons to make it hard are instability motion occurring from porpoising and wave condition. Porpoising is mainly due to overlap the heaving and pitching motion with certain period, which is caused by instable pressure distribution and changing longitudinal location of center of gravity. In addition, in wave condition, encountering wave disturbs going into planing mode. This paper presents numerical results of wave-piercing planing hull in porpoising and wave condition. Numerical simulation is conducted via Reynolds Averaged Navier-stokes (RANS) with moving mesh techniques (overset grid), performed at different wave condition. The numerical results reveal motion characteristics overlapping porpoising and wave condition. At first, motions on low wavelength region show resonance on this condition, and some appendages enhance the motion amplitude larger than original values. Finally, this resonance was suppressed by stern appendages. However, momentum generated from stern appendages increases motions in high wavelength region. This effective motion corresponds with vertical accelerations from CG.