Total Drag Reduction Research Articles

Ship Drag Reduction by Air Bottom Ventilated Cavitation in Calm Water and in Waves

The goal herein is ship drag reduction by air bottom cavitation in the moderate range of Froude number Fr (0.4 < Fr < 0.65) in both calm water and in waves. A ship hull with a bottom niche terminating in a cavity locker/seal (suppressing cavity tail oscillations and reducing the air escape from the cavity) was designed using nonlinear ideal fluid theory. The wave impact on the cavity shape and drag reduction was estimated with a novel analytical approach that takes into account the air compressibility in the cavity and air entrainment by the water. The model drag was measured in the Naval Surface Warfare Center linear tow tank at different drafts in calm water and in waves. The baseline configuration was with the niche closed by a flat cover. The attained total drag reduction at 0.45 < Fr < 0.63 was up to 25%, whereas the air supply power was under 4% of the gain in the required propulsion power. The air cavity was stable in waves (up to sea state 5 for a 90 meter ship) and the effectiveness of drag reduction by cavitation in seaway was greater than in calm water due to smaller wave-induced additional drag of the ship with air bottom cavity. Two identical models were built and tested also as a seatrain. However, the percentage drag reduction due to cavity ventilation in the seatrain configuration was less than for a single hull. The need for fine tuning the air supply distribution between the hulls was found.

Experimental investigation on the drag reducing efficiency of the outer-layer vertical blades

An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins and Choi (Proceedings of ASME FEDSM’02 2002 ASME Fluids Engineering Division Summer Meeting, Montreal) and Hutchins (An investigation of larger-scale coherent structures in fully developed turbulent boundary layers, Hutchins N (2003), PhD thesis, University of Nottingham). The reported drag reduction efficiency, which was as much as 30%, was quantified only in terms of the reduction in the local skin-friction coefficient. The assessment of the drag reducing efficiency did not take the side effects of the inclusion of the blades into considerations. Those effects are the increase in the wetted surface area and the flow disturbances due to the presence of the blades. In the present study, a series of drag force measurements in towing tank has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades. It was found that for the case of h 4.0 × z 4.0 (h/δ = 1.04), the outer-layer vertical blades array achieved about 9.6% drag reduction without considering the increase in the wetted surface area. A proper scaling method to give collapsed plot of drag reduction efficiency CF/CF0 was attempted, but the correlation remained limited. Of the two scaling methods, the outer scaling is found to be relevant one.

Active Transonic Aerofoil Design Optimization Using Robust Multiobjective Evolutionary Algorithms

The use of adaptive wing/aerofoil designs is being considered, as they are promising techniques in aeronautic/ aerospace since they can reduce aircraft emissions and improve aerodynamic performance of manned or unmanned aircraft. This paper investigates the robust design and optimization for one type of adaptive techniques: active flow control bump at transonic flow conditions on a natural laminar flow aerofoil. The concept of using shock control bump is to control supersonic flow on the suction/pressure side of natural laminar flow aerofoil that leads to delaying shock occurrence (weakening its strength) or boundary layer separation. Such an active flow control technique reduces total drag at transonic speeds due to reduction of wave drag. The location of boundary-layer transition can influence the position and structure of the supersonic shock on the suction/pressure side of aerofoil. The boundarylayer transition position is considered as an uncertainty design parameter in aerodynamic design due to the many factors, such as surface contamination or surface erosion. This paper studies the shock-control-bump shape design optimization using robust evolutionary algorithms with uncertainty in boundary-layer transition locations. The optimization method is based on a canonical evolution strategy and incorporates the concepts of hierarchical topology, parallel computing, and asynchronous evaluation. The use of adaptive wing/aerofoil designs is being considered, as they are promising techniques in aeronautic/ aerospace since they can reduce aircraft emissions and improve aerodynamic performance of manned or unmanned aircraft. This paper investigates the robust design and optimization for one type of adaptive techniques: active flow control bump at transonic flow conditions on a natural laminar flow aerofoil. The concept of using shock control bump is to control supersonic flow on the suction/pressure side of natural laminar flow aerofoil that leads to delaying shock occurrence (weakening its strength) or boundary-layer separation. Such an active flow control technique reduces total drag at transonic speeds due to reduction of wave drag. The location of boundary-layer transition can influence the position and structure of the supersonic shock on the suction/pressure side of aerofoil. The boundarylayer transition position is considered as an uncertainty design parameter in aerodynamic design due to the many factors, such as surface contamination or surface erosion. This paper studies the shock-control-bump shape design optimization using robust evolutionary algorithms with uncertainty in boundary-layer transition locations. The optimization method is based on a canonical evolution strategy and incorporates the concepts of hierarchical topology, parallel computing, and asynchronous evaluation. Two test cases are conducted: the first test assumes the boundary-layer transition position is at 45% of chord from the leading edge, and the second test considers robust design optimization for the shock control bump at the variability of boundary-layer transition positions. The numerical result shows that the optimization method coupled to uncertainty design techniques produces Pareto optimal shock-control-bump shapes, which have low sensitivity and high aerodynamic performance while having significant total drag reduction.

경계층 외부 수직날의 마찰저항 저감 기구에 대한 PIV 관측

An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins. The drag reduction efficiency of the blades was reported to reach as much as 30%. However, the drag reduction efficiency was quantified only in terms of the reduction in the local skin-friction coefficient. In the present study, a series of drag force measurements in towing tank has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades. A maximum 9.6% of reduction of total drag was achieved. The scale of blade geometry is found to be weakly correlated with outer variable of boundary layer. In addition, detailed flow field measurements have been performed using 2-D time resolved PIV with a view to enabling the identification of drag reduction mechanism.

Supersonic aerodynamics of a projectile with slot cavities

AbstractThis paper presents the results of experimental and computational investigations on the effect of slot cavities on a supersonic projectile. Experimental work was carried out to show the effects of the slots on the drag at Mach numbers M = 1·36, 1·65, 1·83. The computational analysis was done at M = 1·36. A single configuration of the slot pattern was used with two different slot widths (0·5mm and 2·0mm). Flow features were investigated in the slotted area and at the base. The analysis presented includes the pressure distribution, the supersonic cavity flow and the effect of the slots on the overall aerodynamic drag. Unlike the case of slots at transonic speeds, the suction and blowing mechanism is not found at supersonic Mach numbers. Streamwise cavity slots cause a small base drag reduction. The reduction in total drag is modest when the width is 0·5mm. However, the experiments showed that with the wider slots (2mm) the drag actually increases at Mach numbers from 1·36 to 1·83.

Pressure loss reduction in hydrogen pipelines by surface restructuring

This paper concerns the reduction of pressure losses during pipeline hydrogen transportation, as the cost of hydrogen compression is a significant obstacle for efficient hydrogen pumping on a large-scale basis. The use of organized micro-structures on pipeline walls is proposed to obtain lower values of pressure losses with respect to smooth walls. Three-dimensional micro-structures of a sinusoidal shape are investigated as potentially more efficient counterparts to conventional two-dimensional structures (riblets) developed in aerospace industry. Aerodynamic performance of three-dimensional structures is investigated computationally in terms of both skin friction and pressure drag, two constituents of the total drag. Three-dimensional structures are shown to provide larger total drag reduction than two-dimensional structures for some range of geometrical parameters (14.5% versus 11%). Parametric dependence of both pressure and skin friction drag on structure geometry is analyzed, and an optimum configuration maximizing the total drag reduction is proposed.

Frictional drag reduction by wavy advection of deformable bubbles

Bubbles can reduce frictional drag in wall turbulence, and its effect is expected to use for ships and pipelines to save their power consumptions. A number of basic experiments have been carried out to date for finding out the best condition for enhancing the drag reduction. One issue that remains at present is the difference of the performance between steady and unsteady status in terms of bubble concentration. All the experiments in the past deal with the steady effect, i.e., the drag reduction is evaluated as a function of mean void fraction or given gas flow rate of continuous injection. Despite to this, the actual phenomena highly depend on local interaction between two phases upon unsteady manner. We focus on this point and elucidate the influence of time-fluctuating void fraction on the total response to the drag reduction. This view is in fact important to estimate the persistency of the bubble-based drag reduction in the flow direction since bubbles formulate wavy advection during their migration.Our experiments are designed to measure the above-mentioned effect from laminar, transitional, and turbulent flows in a horizontal channel. For avoiding the contamination effect that worsens the reproducibility of the experiment, Silicone oil is used as carrier fluid. The oil also simulates the high Weber number bubble condition because of low surface tension. The unsteady interaction between the wavy advection of bubbles and the local skin friction, a synchronized system is constructed to connect the high-speed camera with the shear transducer, which can evaluate the interaction at 1000 fps. From the results, we confirm that the drag reduction is provided at Re>3000 in the turbulent flow regime, and also the total drag reduction is enhanced by the presence of the waves.

외부경계층 수직 날의 저항저감효과에 대한 실험적 연구

Abstract An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins (2003). The drag reduction efficiency of the blades was reported to reach as muc h as 30%. The assessment of the drag reducing efficiency is mainly restricted to the downstream region of the blades. Indeed, sufficient care has not been taken to such adverse effects as the increase in the wetted surface area and the flow disturbances due to the presence of the blades. In the present study, a series of drag force measurements in towing tank and circulating water channel has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades. ※Keywords: Flow control(유동제어), Drag reduction(저항저감), Turbulent boundary layer(난류경계층), Outer-layer vertical blades(외부경계층 수직 날) 1. 서론 난류경계층으로 인한 마찰저항의 저감은 선박의 연료효율 제고에 있어 매우 중요하다. 선형최적화 기술의 급격한 발달로 인하여 잉여저항이 선박 접수일: 2008년 7월 8일, 승인일: 2008년9월 10일 g교신저자: inwon@pusan.ac.kr, 051-510-2764

Three-dimensional contour bumps for transonic wing drag reduction

Two-dimensional and three-dimensional contour bumps are designed and optimized for substantial wave drag reduction for an un-swept natural laminar flow (NLF) wing (RAE5243 aerofoil section) at transonic speeds. An NLF aerofoil wing is chosen in this study, as shock control is more crucial for such wings due to the requirement of favourable pressure gradients on a substantial part of the wing. For the validation purpose and to focus on the wave drag issues, the boundary layer is assumed to be fully turbulent from the leading edge. Key bump geometrical parameters including the maximum height, the length, and the crest position have been chosen for the parameterization of the two-dimensional and three-dimensional shock control bumps. For the three-dimensional bumps, an array of the contour bumps is installed spanwise on the transonic wing and their width and spanwise spacing are chosen as additional design parameters. Both the two-dimensional and the three-dimensional bump shapes are optimized using a discrete adjoint-based optimization method. The performance of the three-dimensional contour bumps are compared in detail with the similarly optimized two-dimensional bumps both at and around the design point. The results show that, for the NLF wing studied, the optimized three-dimensional bumps are as effective as the optimized two-dimensional bump in terms of total drag reduction at the given design point, despite the significant difference in their geometrical shapes. More importantly, in terms of the operational range for varying lift conditions for practical applications, the three-dimensional bumps outperform the two-dimensional bump by a substantial margin.

Drag Reduction From Square Base Afterbodies at High Speeds

Experiments have been carried out in the 0.5-m base flow wind tunnel at high speeds evaluating the drag reduction potential of a family of square base afterbodies including jet flow at the base. Direct afterbody total drag measurements have been made on square bases as well as conventional axisymmetric afterbodies with conical and circular arc boat tails having the same annular base area and jet flow parameters. The results show conclusively that, in the Mach-number range of 0.95‐1.60, the square-base afterbodies have globally minimum drag in the range of jet pressure ratio studied; the total drag reduction observed is about 10‐12% relative to the circular arc afterbodies, which can be of significant value in design applications. Certain broad flow features on square-base afterbodies are discussed based on surface-pressure measurements and surface flow-visualization studies. Nomenclature Ab = annular base area Ab/A f = ratio of annular base area to forebody area, 0.23 A f = forebody cross-sectional area A j = area of jet at exit A j/A f = ratio of jet exit area to forebody area, 0.30 C D = afterbody total drag coefficient; drag force/(q∞∗ A f ) CDb = base-drag coefficient; base force/(q∞∗ A f ) C Dβ = boat-tail profile drag coefficient; boat-tail drag force/(q∞∗ A f ) C p = afterbody surface-pressure coefficient; ( p − p∞)/(0.5γ p∞ M 2 ∞ ) Cpb = base-pressure coefficient; ( pb − p∞)/(0.5γ p∞ M 2 ∞) D = forebody diameter of the model, 127 mm db = base diameter d j = nozzle-exit diameter M∞ = freestream Mach number Poj/ p∞ = jet pressure ratio; ratio of stagnation pressure of jet to the freestream static pressure pb = base pressure p∞ = freestream static pressure q∞ = dynamic pressure (0.5γ p∞ M 2 ∞) β = boat-tail angle, see Fig. 3

Quantitative Analysis of Drag Reduction in Horizontal Slug Flow

Summary Calculations have been made to predict the components of pressure drop in slug flow. This analysis is aimed at understanding, in a quantitative manner, the contributions of both frictional and accelerational components to total pressure drop in horizontal slug flow and the effect of drag reducing agents (DRAs) on each component. Experimental results were in good agreement with predicted values. The DRA used in this study was effective in reducing both components of the pressure drop. The accelerational component was found to be dominant and formed more than 80% of the total pressure drop. It increased dramatically with increasing superficial gas velocity. With the addition of 20 ppm DRA, the accelerational and frictional components were noticed to decrease by a factor of 70%. At DRA concentration of 50 ppm, they decreased by a factor of 80%. Total drag reduction was found to generally decrease at higher superficial gas velocities. In sharp contrast with expectations, the drag reduction was recovered mainly from the accelerational component, indicating that the DRA worked not only in the buffer zone but also in the mixing zone in the slug body. The accelerational drag reduction reached values as high as 89% of total drag reduction.

FLOW PAST A BLUFF BODY WITH A WAVY STAGNATION FACE

Numerical investigations have been performed to study the flow past square-section cylinders with a spanwise geometric deformation leading to a stagnation face with a sinusoidal waviness. The computations were performed using a spectral/hp element solver over a range of Reynolds numbers from 10 to 500. Starting from fully developed shedding past a straight cylinder at a Reynolds number of 100, a sufficiently high waviness is impulsively introduced resulting in the stabilization of the the near-wake to a time-independent state. The steady nature of the near-wake is associated with a reduction in total drag of about 16% at a Reynolds number of 100 as compared with a straight, non-wavy cylinder. Further increases in the amplitude of the waviness lead to the emergence of hairpin vortices from the near-wake region, similar to the wake of a sphere at low Reynolds numbers. At higher Reynolds numbers, the drag reduction increases substantially, e.g. at a Reynolds number of 500 it is 34%, principally due to the increase in drag of the nonwavy cylinder. Alternative methods based on three-dimensional forms of bleed are investigated to suppress the von-Kármán vortex street of a straight, non-wavy cylinder.

Flow past a square-section cylinder with a wavy stagnation face

Numerical investigations have been performed for the flow past square-section cylinders with a spanwise geometric deformation leading to a stagnation face with a sinusoidal waviness. The computations were performed using a spectral/hp element solver over a range of Reynolds numbers from 10 to 150.Starting from fully developed shedding past a straight cylinder at a Reynolds number of 100, a sufficiently high waviness is impulsively introduced resulting in the stabilization of the near wake to a time-independent state. It is shown that the spanwise waviness sets up a cross-flow within the growing boundary layer on the leading-edge surface thereby generating streamwise and vertical components of vorticity. These additional components of vorticity appear in regions close to the inflection points of the wavy stagnation face where the spanwise vorticity is weakened. This redistribution of vorticity leads to the breakdown of the unsteady and staggered Kármán vortex wake into a steady and symmetric near-wake structure. The steady nature of the near wake is associated with a reduction in total drag of about 16% at a Reynolds number of 100 compared with the straight, non-wavy cylinder.Further increases in the amplitude of the waviness lead to the emergence of hairpin vortices from the near-wake region. This wake topology has similarities to the wake of a sphere at low Reynolds numbers. The physical structure of the wake due to the variation of the amplitude of the waviness is identified with five distinct regimes. Furthermore, the introduction of a waviness at a wavelength close to the mode A wavelength and the primary wavelength of the straight square-section cylinder leads to the suppression of the Kármán street at a minimal waviness amplitude.

Drag Reduction of Afterbodies by Controlled Separated Flows

The zero-lift drag characteristics of a class of multistep afterbodies that utilize the concept of controlled separated flows are documented at transonic and supersonic speeds. The important geometrical and flow parameters affecting the drag of such afterbodies are identified, and their effects are examined through a parametric study. The results show that multistep afterbodies can be designed that provide significant total drag reduction (as high as 50%) compared to (unmodified) blunt bases; however, compared to axisymmetric boattailed afterbodies of a given base area, the multistep afterbodies have relatively higher drag. Finally, the certain flow features involving separation and reattachment on multistep afterbodies are discussed based on flow visualization studies

Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere

Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 × 103 to 6.5 × 105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics. In the subcritical range of Re (<2 × 105), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a small change in the vortex shedding frequency. In the supercritical range (Re > 4 × 105), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes. In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.

Universal drag reduction characteristics of saline water-soluble poly(ethylene oxide) in a rotating disk apparatus

Polymer-induced turbulent drag reduction in a rotating disk apparatus was investigated using nonionic poly(ethylene oxide) (PEO) in a synthetic saline solution with novel application to ocean thermal energy conversion technology. A maximum total (skin friction plus form) drag reduction of 30% was obtained with 50 wppm of PEO with molecular weight 5.0 × 106. The concentration dependence of the percentage drag reduction for the PEO/saline solution system is found to fit Virk's empirical correlation, and a universal correlation for various molecular weights and Reynolds numbers is also presented. Furthermore, hydrodynamic volume fraction was introduced to correlate drag reduction efficiency with molecular parameters in this PEO/saline solution system.

Drag Reduction Due to Riblets on a GAW(2) Airfoil

THE study of turbulent drag reduction by use of riblets has been an area of significant research during the past de cade. Riblets with symmetric v grooves with adhesive-backed film manufactured the 3M Company (U.S.) have been widely used in earlier studies. The effectiveness of riblets in reducing the drag of a simple wo-dimensional configuration is fairly we 11 established now. Although there has been some effort to assess the effectiveness riblets on airfoils, the results reported by Sundaram et al on a NACA 0012 airfoils at low speeds have been particularly noteworthy.13; Their studies showed that both total and viscous drag reduction increased monotonically with an angle of attack up to 6 deg; it was also shown3 that the higher drag reduction resulted primarily from airfoil upper (or suction) surface, suggesting increased effectiveness of riblets in adverse pressure gradients. In a subsequent study Subaschandar et al., who extending the work of Sundaram etal to higher angles of attack (by using the same NACA 0012 model13; and the same wind tunnel), it was observed that the drag reduction decreased rapidly beyond a = 6 deg with virtually no drag reduction a =12 deg. The present study is an attempt to assess the total drag reduction that is due to riblets on a cambered airfoil up to high angles of attack low speeds. The 13% thickness General Aviation Wing [GAW(2)]13;

Jet methods of gas injection into fluid boundary layer for drag reduction

Two jet methods for saturating the fluid boundary layer with microbubbles for drag reduction in contrust with gas injection through porous materials are considered. The first method is the gas injection through the slot under a special fluid wall jet. The second method is the saturation of boundary layer by microbubbles via the gas-water mixture injection through the slot. Experimental data, reflecting the skin friction drag reduction on the flat plate and total drag reduction of axisymmetric bodies, are presented. The comparison between a jet methods of gas injection and gas injection through porous materials is made.

Subsonic drag reduction of the Space Shuttle Orbiter

Various near-wake flow-modifying devices were experimentally evaluated for their effectiveness in increasing base pressure of the Space Shuttle Orbiter at low subsonic speed. The results confirmed the strong three-dimensional character of the Orbiter near wake. A base cavity was found to be the most effective mechanism for increasing base pressure. However, for this mechanism to be effective, the cavity had to be longer than the main engine nozzles. Surface characteristics of the base cavity exposed to freestream had a strong influence on the base pressure. The trapped-vortex mechanism due to a back step was found to be effective in increasing the base pressure only in the region of the orbital-maneuvering-system pods. A combination of base-cavity and trapped-vortex mechanisms increased the base pressure by 25%, and the reduction in total drag was approximately 6%.

A NUMERICAL STUDY OF OPTIMAL DRAG REDUCTION FOR TURBULENT TRANSONIC PROJECTILES USING A PASSIVE CONTROL

Abstract The purpose of this research is to numerically study a drag reduction method—passive control of shock/boundary layer interaction, which is applied to the boattail portion of a secant-ogive-cylinder-boattail projectile in turbulent transonic flows. The flow pattern and the components of aerodynamic drag computed from numerical data are analyzed. The effectiveness of this method is studied by varying the values of parameters such as porosity distribution, maximum porosity factor and size of porous region. The conditions for optimal drag reduction are investigated and reported. The present results show that the use of this passive control method can not only reduce the boattail drag but also the base drag, and results in an additional 8% total drag reduction compared to that without the passive control technique. This passive control method can be an effective approach for the design of high-performance projectiles in the transonic regime.