摘要 :
Reducing the ship's drag is an effective technique for reducing emissions, operating expenses, and improving EEDI. For slow-moving vessels, frictional resistance has been reported to contribute up to 80% of total resistance, deman...
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Reducing the ship's drag is an effective technique for reducing emissions, operating expenses, and improving EEDI. For slow-moving vessels, frictional resistance has been reported to contribute up to 80% of total resistance, demanding a thorough investigation in reducing it. Air or the gas has been used as a lubricant, to reduce the frictional resistance known as Micro Bubble Drag Reduction, which is the need of a present era. In this paper, the current research scenario on the technique is presented, which suggests a plausible reduction in frictional resistance of 80% for ships. The review suggests that reduction in drag depends on void fraction, coalescence and breaking of injected bubbles, the salinity of water and type of gas used, depth of water in which the bubbles are injected, and of course, on the location of injection points. In the end, recommendations have been provided to improve the drag reduction.
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Pressure losses and mixing in vibrating channels were analysed. The vibrations in the form of long-wavelength travelling waves were considered. Significant reduction of pressure losses can be achieved using sufficiently fast waves...
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Pressure losses and mixing in vibrating channels were analysed. The vibrations in the form of long-wavelength travelling waves were considered. Significant reduction of pressure losses can be achieved using sufficiently fast waves propagating downstream, while significant increase of such losses is generated by waves propagating upstream. The mechanisms responsible for pressure losses were identified and discussed. The interaction of the pressure field with the waves can create a force which assists the fluid movement. A similar force can be created by friction, but only under conditions leading to flow separation. An analysis of particle trajectories was carried out to determine the effect of vibrations on mixing. A significant transverse particle movement takes place, including particle trajectories with back loops. The downstream-propagating out-of-the phase waves provide a large reduction of pressure gradient and significant potential for mixing intensification. Analysis of energy requirements demonstrates that it is possible to identify waves which reduce power requirements, i.e. the cost of actuation is smaller than the energy savings associated with the reduction of pressure gradient. The fast forward moving waves provide an opportunity for the development of alternative propulsion methods which can be more efficient than methods based on the pressure difference.
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The effect of surface vibrations on the propulsion augmentation and resistance in the relative movement of parallel plates has been studied. The analysis was focused on monochromatic waves and laminar flows. The effectiveness of t...
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The effect of surface vibrations on the propulsion augmentation and resistance in the relative movement of parallel plates has been studied. The analysis was focused on monochromatic waves and laminar flows. The effectiveness of the vibrations was gauged by determining the external force required to maintain the movement of one of the plates at a prescribed velocity. It is shown that waves propagating upstream always increase the resistance but the flow response to waves propagating downstream is more intricate and is a function of the flow Reynolds number. In general, waves must be sufficiently fast to reduce the flow resistance. This leads to a natural division between slow and fast waves; a characterization that is helpful for flows at a sufficiently small Reynolds number Re. An increase in Re brings into play the complication of possible resonances with the natural flow frequencies. Resonances are not possible with waves faster than the plate velocity and these supercritical waves generally decrease the flow resistance. More complex flow responses can occur with slower (subcritical) waves which tend to increase the flow resistance. A complete elimination of the resistance is possible if the waves are of sufficiently short wavelength and travel quickly. This suggests that our mechanism has great potential in the development of propulsion augmentation systems. None of the waves produced net energy savings.
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The potential of frequency-tuned surfaces as a passive control strategy for reducing drag in wall-bounded turbulent flows is investigated using resolvent analysis. These surfaces are considered to have geometries with impedances t...
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The potential of frequency-tuned surfaces as a passive control strategy for reducing drag in wall-bounded turbulent flows is investigated using resolvent analysis. These surfaces are considered to have geometries with impedances that permit transpiration and/or slip at the wall in response to wall pressure and/or shear and are tuned to target the dynamically important structures of wall turbulence. It is shown that wall impedance can suppress the modes resembling the near-wall cycle and the very-large-scale motions and the Reynolds stress contribution of these modes. Suppression of the near-wall cycle requires a more reactive impedance. In addition to these dynamically important modes, the effect of wall impedance across the spectral space is analysed by considering varying mode speeds and wavelengths. It is shown that the materials designed for suppression of the near-wall modes lead to gain reduction over a wide range across the spectral space. Furthermore, a wall with only shear-driven impedance is found to suppress turbulent structures over a wider range in spectral space, leading to an overall turbulent drag reduction. Most importantly, the present analysis shows that the drag-reducing impedance is non-unique and the control performance is not sensitive to variations of the surface impedance within a favourable range. This implies that specific frequency bandwidths can be targeted with periodic material design.
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Pressure losses in laminar, pressure-gradient-driven channel flows modified by wall transpiration have been analysed in the range of Reynolds numbers guaranteeing flow stability. It was found that these losses were affected by the...
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Pressure losses in laminar, pressure-gradient-driven channel flows modified by wall transpiration have been analysed in the range of Reynolds numbers guaranteeing flow stability. It was found that these losses were affected by the reduction of the effective channel opening due to formation of transpiration 'bubbles', by nonlinear streaming and by the elimination of direct contact between the stream and the bounding walls. It was further found that pressure losses can be reduced by properly selecting the transpiration pattern. It was determined that nonlinear streaming is the dominant effect as the transpiration wave number resulting in the largest reduction of pressure losses corresponds to the maximization of this streaming. Using transpiration at both walls further decreases pressure losses, but only when both transpiration patterns are in a proper relative position. The largest reduction of losses is achieved by concentrating transpiration at a single wave number. It is shown that the performance of finite-slot transpiration is well captured using just a few leading modes from the Fourier expansions describing the transpiration distribution. The analysis of energy expenditure shows that the use of transpiration increases the energy cost of the flow. Conditions leading to the minimization of this cost represent the most effective use of transpiration as a propulsion augmentation system.
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One of the successful feedback controls for skin-friction drag reduction designed by Choi et al. (J. Fluid Mech., vol. 262, 1994, pp. 75-110), called `opposition control', has a limitation in application because the sensors need t...
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One of the successful feedback controls for skin-friction drag reduction designed by Choi et al. (J. Fluid Mech., vol. 262, 1994, pp. 75-110), called `opposition control', has a limitation in application because the sensors need to be placed slightly away from the wall, i.e. at y(+) = 10, and measure the instantaneous wall-normal velocity. In the present study we train convolutional neural networks using the database of uncontrolled turbulent channel flow at Re-tau = 178 to extract the spatial distributions of the wall shear stresses and pressure that closely represent the wall-normal velocity at y(+) = 10. The correlations between the predicted wall-normal velocities at y(+) = 10 from the wall-variable distributions and true ones are very high, and they are 0.92, 0.96 and 0.96 for the streamwise and spanwise wall shear stresses and pressure, respectively. We perform feedback controls of turbulent channel flow with instantaneous blowing and suction determined by the trained convolutional neural networks from the measured wall-variable distributions. The predicted wall-normal velocities during the controls have higher energy at small to intermediate scales than the true ones, which degrades the control performance in skin-friction drag reduction. By applying a low-pass filter to the predicted wall-normal velocities to remove those scales, we reduce skin-friction drag by up to 18% whose amount is comparable to that by opposition control. The convolutional neural networks trained at Re-tau = 178 are also applied to a higher Reynolds number flow (Re t = 578), and provide a successful skin-friction drag reduction of 15%.
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Flows in channels exposed to spatially distributed heating were investigated. Such flows are of interest as theoretical analyses suggest that heating leads to the reduction of pressure losses. A special apparatus providing the mea...
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Flows in channels exposed to spatially distributed heating were investigated. Such flows are of interest as theoretical analyses suggest that heating leads to the reduction of pressure losses. A special apparatus providing the means for the creation of well-controlled spatially periodic heating with the desired intensity as well as precise control of the flow rate in flows with small Reynolds numbers was constructed. The apparatus works with air and provides optical access to the flow interior. The relevant theory has been generalized to handle the temperature fields measured in the experiments. The experiments were carried out for Reynolds numbers $Re<20$ and at a single Rayleigh number based on the peak-to-peak temperature difference and channel half-height of $Ra_{p}=3500$ . Flow visualization and particle image velocimetry measurements demonstrated the formation of two-dimensional steady rolls whose size was dictated by $Re$ , with the largest rolls observed for the smallest $Re$ and the roll size being gradually reduced as $Re$ increased until their complete elimination at the largest $Re$ used in the experiment. An excellent agreement between the theoretically and experimentally determined complex flow fields was found. Wall shear stresses extracted from the velocity measurements agree with their theoretical counterparts within the expected accuracy. The agreement between the experimental and theoretical velocity fields and their unique relation with the corresponding pressure fields indirectly verify the heating-induced pressure-gradient-reducing effect.
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Abstract The hydrodynamic mechanism of drag reduction by a flexible hairy coating was explored using the penalty immersed boundary method. A two-dimensional flexible hairy coating is constituted by multiple flexible filaments. A s...
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Abstract The hydrodynamic mechanism of drag reduction by a flexible hairy coating was explored using the penalty immersed boundary method. A two-dimensional flexible hairy coating is constituted by multiple flexible filaments. A simulation of a cylinder without a hairy coating at a Reynolds number of 100 was also performed for comparison. The results of the simulations show good agreement with the experimental data by Niu & Hu (Phys. Fluids, vol. 23, 2011, 101701), where maximum drag reduction of 22% was attained at a particular length, bending rigidity, coating density and coating angle of the hairy coating. The hydrodynamic mechanism of drag reduction was characterized in terms of the wake pattern, shape deformation and kinetic energy of the hairy coating. The effect of a non-uniform bending rigidity of the hairy coating on drag reduction was explored. A stable streamline shape of the hairy coating was found to delay the vortex formation and stabilize the recirculation zone, resulting in decreased form drag. Active flapping of the hairy coating with enhanced vortex shedding is adverse to drag reduction. A hairy coating with a stiff base and flexible trailing edge is beneficial to maintaining a stable shape.
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The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2- 10 m/s. It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. Accordin...
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The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2- 10 m/s. It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments, the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three dif- ferent methods were proposed and tested to generate smaller bubbles.
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The addition of elastic polymers to turbulent liquids is known to produce significant drag reduction. In this study, we prove that the drag in pipe and channel flows of an unforced laminar fluid constitutes a lower bound for the d...
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The addition of elastic polymers to turbulent liquids is known to produce significant drag reduction. In this study, we prove that the drag in pipe and channel flows of an unforced laminar fluid constitutes a lower bound for the drag of a fluid containing dilute elastic polymers. Further, the addition of elastic polymers to laminar fluids invariably increases drag. This proof does not rely on the adoption of a particular constitutive equation for the polymer force, and would also be applicable to other similar methods of drag reduction, which are also achieved by the addition of certain particles to a flow. Examples of such methods include the addition of surfactants to a flowing liquid and the presence of sand particles in sandstorms and water droplets in cyclones.
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