جستجوی مقالات مرتبط با کلیدواژه « drag reduction » در نشریات گروه « مکانیک »
تکرار جستجوی کلیدواژه «drag reduction» در نشریات گروه «فنی و مهندسی»-
سطوح فوق آب گریز به عنوان روشی اساسی جهت کاهش پسای اصطکاکی اجسام غوطه ور در آب مورد توجه زیادی قرار گرفته اند. ارزیابی دقیق و پیش بینی مقدار کاهش پسای ناشی از به کارگیری این سطوح نیازمند اندازه گیری های هزینه بر، شبیه سازی های عددی و یا توسعه مدل ها و روابط قابل اعتماد می باشد. در این مقاله یک مدل برای محاسبه ضریب اصطکاک پوسته ای و کاهش پسای سطوح تخت فوق آب گریز ارائه می شود. از داده های پیشین مربوط به ضریب اصطکاک پوسته ای سطوح تخت با شرایط مرزی عدم لغزش استفاده شده و مدلی ارائه می شود که به کمک آن بتوان کاهش پسای اصطکاکی و ضریب اصطکاک پوسته ای این سطوح را پس از اعمال پوشش های فوق آب گریز محاسبه نمود. با استفاده از شبیه سازی عددی، نتایج مدل با نتایج شبیه سازی جریان سیال روی صفحه تخت در سرعت های مختلف مقایسه شده و اعتبار مدل تایید شده است. نتایج مدل و شبیه سازی نشان دهنده آن است که در سرعت های ورودی 1، 5 و 25 متر بر ثانیه و طول لغزش 50 میکرون، کاهش پسای اصطکاکی به ترتیب 15، 41 و 77 درصد انتظار می رود. همچنین، با افزایش عدد رینولدز جریان، کاهش اصطکاک پوسته ای افزایش می یابد. مدل توسعه داده شده برای سطوح تخت اعتبارسنجی شده و توانایی آن در محاسبه ضریب اصطکاک پوسته ای و نیروی پسای این سطوح به دقت مورد بررسی قرار گرفته است. با این حال برای بررسی اعتبار مدل برای سطوح با انحنا و طول لغزش متغیر، تحقیقات بیشتری نیاز می باشد.
کلید واژگان: صفحه تخت, کاهش پسا, ضریب اصطکاک پوسته ای, فوق آب گریز, طول لغزش}Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.
Keywords: Flat Plate, Drag Reduction, Skin Friction Coefficient, Superhydrophobic, Slip Length} -
Modern underwater warfare necessitates the development of high-speed supercavitating torpedoes. Achieving supercavitation involves integrating a cavitator at the torpedo's front, making cavitator design a critical research area. The present study simulated supercavity formation by cavitators of various shapes attached to a heavyweight torpedo. The study involves simulations of thirteen cavitator designs with various geometrical configurations at different cavitation numbers. The simulations employ the VOF multiphase model along with the Schnerr and Sauer cavitation model to analyze supercavitation hydrodynamics. The study examines the supercavity geometry and drag characteristics for individual cavitator designs. The results reveal a significant reduction in skin friction drag by a majority of cavitators. Notably, a disc cavitator at a cavitation number of 0.09 demonstrates a remarkable 92% reduction in the coefficient of skin friction drag. However, the overall drag reduces when incorporating a cavitator, but it introduces additional pressure drag. The study found that the cavitators generating larger supercavities also yield higher pressure drag. Therefore, the supercavity should just envelop the entire torpedo, as excessively small supercavities amplify skin friction drag, while overly large ones elevate pressure drag. Ultimately, the study concludes that selecting the ideal cavitator entails a comprehensive evaluation of factors such as supercavity and torpedo geometry, reductions in skin friction drag and increments in pressure drag.Keywords: Multiphase Flow, Supercavitation Simulation, Flow Control, Drag Reduction, Under-Water Vehicle, SS Cavitation Model, Shape Optimization}
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To reduce the fluid resistance on the surface of flow-through components and improve energy utilization efficiency, a biomimetic fitting structure model is constructed based on the ridge-like features of beluga skin. The SST k-ω model is employed to numerically simulate the drag reduction characteristics of three biomimetic structures (fitting structure, V-shaped structure, and arc structure) included in the design. The variations of the fitting structure’s viscous resistance and pressure drop resistance with different widths and depths are compared. The drag reduction mechanism of the fitting structure surface is studied based on the pressure stress, velocity field, and shear stress. The results demonstrate that the fitting structure exhibits the best drag reduction performance. The fitting structure with a width of 30 mm and a depth of 0.7 mm achieves an optimal drag reduction effect of 4.18%. The fitting structure exhibits a large low shear stress region, which increases the thickness of the bottom boundary layer, thereby reducing surface velocity and viscous resistance.Keywords: Fitting Structure, Drag Reduction, Numerical Simulation, Surface Flow Field}
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This study examined the drag reduction properties of cylindrical flows across various asymmetric notched structures through numerical simulation and particle image velocimetry. The focus was on investigating the influence of the number of asymmetric grooves on the drag characteristics, including the mean drag, spectral characteristics, time-averaged streamlines, separation point prediction, time-averaged pressure, wake vortex strength, Reynolds stress, and turbulent kinetic energy. The results showed that the presence of asymmetric grooves significantly influenced these flow parameters. Notably, the improvement was optimal in the four-groove configuration, evidenced by the lowest mean drag coefficient (0.804), vortex shedding frequency (2.74 Hz), recirculation area length (1.208D), and pressure difference across the cylinder (81.76). Moreover, this configuration resulted in the weakest trailing vortex, a 45% reduction in the maximum Reynolds stress (0.011), and a 40.5% decrease in the maximum turbulent kinetic energy (0.05). Thus, the presence of asymmetric grooves had a significant positive effect on the cylindrical flow properties, though the degree of improvement decreased with further increase in the number of grooves.Keywords: Asymmetrically grooved cylinder, Flow around a cylinder, Particle image velocimetry, Drag reduction, Recirculation region}
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Superhydrophobic surfaces have garnered attention for their ability to decrease fluid resistance, which can significantly reduce energy consumption. This study aims to accurately capture critical flow phenomena in a microchannel and explore the internal drag-reduction mechanism of the flow field. To achieve this, the three-dimensional (3D) superhydrophobic surface flow field with conical microstructure is numerically simulated using the gas–liquid two-phase flow theory and Volume of Fluid (VOF) model, combined with a Semi-implicit method for the pressure-linked equation (SIMPLE) algorithm. The surface drag-reduction effect of the conical microstructure is investigated and compared it to that of the V-longitudinal groove and V-transverse groove surfaces. Additionally, the changes in the drag-reduction effect during the wear of the conical microstructure were explored. The numerical results reveal that the drag-reduction effect improves with a larger period spacing of the conical microstructure, the drag reduction rate can reach 25.23%. As the height of the conical microstructure increases, the aspect ratio (ratio of width to height) decreases, and the dimensionless pressure drop ratio and the drag-reduction ratio increase. When the aspect ratio approaches 1, the drag reduction rate can reach over 28%. indicating a more effective drag-reduction. The microstructure is most effective in reducing drag at the beginning of the wear period but becomes less effective as the wear level increases, when the high wear reaches 10, the drag reduction rate decreases to 3%. Compared to the V-shaped longitudinal groove and V-shaped transverse grooves, the conical microstructure is the most effective in reducing drag.Keywords: Superhydrophobic surface, Drag reduction, Numerical simulation, Conical microstructure, 3D flow field}
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Flow control has a tremendous technological and economic impact, such as aerodynamic drag reduction on road vehicles which translates directly into fuel savings, with a consequent reduction in greenhouse gas emissions and operating costs. In recent years, machine learning has also been used to develop new approaches to flow control in place of more laborious methods, such as parametric studies, to find optimal parameters with few exceptions. This paper proposes an intelligent passive device generator (IPDG) that combines computational fluid dynamics (CFD) and genetic algorithm, more specifically, the Non-dominated Sorting Genetic Algorithm II (NSGA II). The IPDG is not application specific and can be applied to generate various devices in the given design space. In particular, it creates three-dimensional passive flow control devices with unique shapes that are aerodynamically efficient in terms of the cost function (i.e., aerodynamic drag and lift). In this paper, the IPDG is demonstrated using a rear flap and an underbody diffuser as passive devices. The three-dimensional Reynolds-averaged Navier-stokes (RANS) equations were used to solve the problem. Relative to the baseline, the IPDG generated flap-only, and diffuser-only provide drag reductions of 6.3% and 5.4%, respectively, whereas the flap-diffuser combination provides a drag reduction of 7.4%. Furthermore, the increase in the downforce is significant from 624.4% in flap-only to 4930% and 4595% in the diffuser and flap-diffuser combination. The proposed method has the potential to evolve into a universal passive device generator with the integration of machine learning.
Keywords: Machine learning, Flow control, Shape optimization, Genetic Algorithm, Drag reduction} -
An experimental investigation has been undertaken to determine the effects of plain and notched base flaps on the drag performance of simplified tractor-trailer combination without any intermediate gap, Generalized European Transport System (GETS). Both plain and notched base flaps are rigid, made up of three identical flaps whose length is equal to the width of the GETS model, and not angled inward or outward. The experiments examined three-sided flap configurations corresponding to various combinations of seven heights of the plain part (from 10 to 40 mm in steps of 5 mm), four notch amplitudes (from 2.5 to 10 mm in steps of 2.5 mm), and five notch wavelengths (from 10 to 50 mm in steps of 10 mm). It is shown that the drag performance of the plain flap at zero yaw highly depends on the height of the plain flap. The maximum drag reduction occurs for e/w=0.1 yielding a drag reduction of 1.9% when compared to the GETS model without flap (baseline GETS). It was shown that the time-averaged drag coefficient increased slightly until a maximum was reached at e/w=0.3 but then decreased slightly with increasing e/w. Under zero yaw angle conditions, GETS model with a notched base flap, e10-a05.0-λ20, gives the lowest drag. The addition of this base flap to the GETS model resulted in a 2.8% drag reduction. This notched base flap was shown to be more effective not only at reducing under yawed flow conditions tested but also at reducing time-averaged side coefficient under yawed flow conditions tested, compared to the e10-a00.0-λ00 flap.Keywords: Passive flow control, Drag reduction, Plain base flap, Notched base flap, Simplified heavy vehicle, Cross-wind}
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Skin friction drag can be reduced through the application of bio-inspired riblet surfaces. Numerical simulations were performed using Large Eddy Simulation (LES) to investigate the effect of using riblets on reducing skin friction drag. In this study, three different riblet configurations were used; scalloped, sawtooth and a new design, hybrid, riblet. To validate the effect of using the proposed hybrid riblet design compared with other riblets used in the literature; before applying to complex geometries, they were initially applied to a flat plate in parallel arrangement. Results showed skin friction coefficient reduction of 14% using the proposed hybrid riblet. This reduction was 9.2 times and 1.2 times more compared to sawtooth and scalloped configurations, respectively. The hybrid riblet was then applied partially and fully to NACA 0012 airfoil. Skin friction coefficient reduction of 34.5% was obtained when the hybrid riblet fully applied on the airfoil surface. Furthermore, the Convergent-Divergent (C-D) arrangement was studied, where the riblets were placed fully on the NACA 0012 and aligned with a yaw angle with respect to the flow direction. The convergent lines are inspired by the sensory part of the shark skin, whereas the divergent lines or herringbone are found on the bird feather. The two different riblet configurations, sawtooth and hybrid were modeled with the C-D arrangement and the hybrid riblet with C-D arrangement contributed to higher skin friction coefficient reduction, 34.5%, than the sawtooth riblet shape, 26.75%. Moreover, the C-D arrangement was compared to the parallel arrangement and shown that the C-D arrangement increased the lift coefficient (cl) of the airfoil, the flow separation was delayed and the overall performance of the airfoil was enhanced.Keywords: Computational fluid dynamics, Large eddy simulation, Riblets, Drag reduction, Aerodynamic efficiency NACA0012}
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Because the helical axial flow gas-liquid mixing pump has the great advantage of conveying gas-liquid two-phase mixed medium, it has become the main core equipment for deep-sea oil and natural gas exploitation. The gas phase aggregation and bubble movement trajectory in the impeller channel have been widely studied, but the increase of medium flow resistance caused by flow separation has not been deeply discussed. Combined with the Euler multiphase flow model and the SST k-ω turbulence model, the numerical calculation of the helical axial flow gas-liquid mixed pump is carried out. Under design flow conditions Q = 100 m3/h, head H = 30 m, speed n = 4500 r/min, specific speed ns =213.6 r/min, and under different inlet gas content conditions, the influence of the bionic waveform leading edge blade on drag reduction characteristics of the helical axial flow gas-liquid mixed pump was investigated. By designing the blade with a leading-edge structure with different heights and pitches, the separation of the mixed medium and the suction surface is effectively suppressed, and the flow resistance of the medium in the 1/10 area of the inlet end of the blade is reduced. The results show that when the height A is 0.25%L and the pitch λ is 12.5%h, the maximum drag reduction rate in this region is 52.6%, the maximum increase in efficiency of the mixed pump is 2.2%, and the maximum increase in head is 4.8%. This study can provide technical support for flow drag reduction in gas-liquid mixed pump.Keywords: Helical axial flow gas-liquid mixing pump, Waveform edge, Separation, Drag reduction, Bionic waveform}
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در این مطالعه با بهره گیری از کنترل غیرفعال جریان ضریب پسای استوانه کاهش داده شده است. با نصب یک صفحه ی کنترل در دو ارتفاع و در فواصل طولی مختلف در بالادست جریان، ممنتوم جریان ورودی به استوانه و در نتیجه مقاومت جریان لایه ی مرزی در برابر گرادیان فشار نامطلوب افزایش می یابد که باعث تعویق در جدایش جریان، افزایش فشار در پشت استوانه و در نهایت باعث کاهش نیروی پسا می شود. در حالتی که جریان بین استوانه و صفحه از نوع حفره (کویتی) باشد، سطح بالادستی استوانه از سطح فشاری به سطح مکشی تغییر می یابد. در نتیجه فشار خالص روی استوانه و نیروی پسا بیشترین کاهش را پیدا می کند که در حالت پیکربندی بهینه، ضریب پسای استوانه تا 90\٪ درصد استوانه تنها در شرایط جریان یکسان، کاهش می یابد. کاهش ضریب پسا در رینولدزهای زیر بحرانی روند مشابهی دارد. در نهایت با بررسی ترمودینامیک سیال مشخص شد تولید آنتروپی با ضریب پسا رابطه ی مستقیم دارد.
کلید واژگان: ضریب پسا, کنترل غیرفعال, جاری شدن گردابه ها, تولید آنتروپی}In this study, the cylinder drag coefficient is reduced by using passive flow control. Installing a flat plate in two heights and different longitudinal distances in upstream flow increases the upstream flow momentum of the cylinder, leading to the higher boundary layer flow resistance against adverse pressure gradient which delays the flow separation. The flow separation delay enhances the pressure on the cylinder downstream. Then, the net pressure on the cylinder in the flow direction and, consequently, the cylinder drag coefficient are decreased. In case that the higher flat plate is utilized, the pressure on the upstream side is reduced more, leading to lower drag coefficient. However, for both heights of the flat plate at specific longitudinal distances from the cylinder due to the cavity flow formation between the cylinder and the flat plat, the vortex shedding is suppressed and the cylinder upstream is changed from the pressure side to suction side, leading to lower net pressure on the cylinder in the flow direction and as a result, less drag coefficient. At the optimal flat plate configuration at and , the minimum cylinder drag coefficient reached 90% reduction in comparison to the single cylinder case in the same flow condition. Results show that the drag coefficient reduction behavior is similar for different sub-critical Reynolds numbers due to the constant flow pattern and no considerable variation of the separation point. The entropy generation for the single cylinder and the case where the flat plate is located in its optimal configuration were investigated. The single cylinder has the highest entropy value, while the entropy of the optimal flat plate configuration with the cylinder reaches the lowest value, the same as the drag coefficient. Then, the drag coefficient is reduced by decreasing entropy generation, indicating the direct relation between drag coefficient and entropy generation.
Keywords: Drag reduction, Vortex shedding, passive flow control, entropy generation} -
در تحقیق حاضر، جدیدترین ایده پیشنهاد شده به منظور کاهش نیروی پسا در پرواز ماوراء صوت با استفاده از شبیه سازی عددی مورد مطالعه دقیق قرار گرفته است. اساس روش مذکور، استفاده از ماده ای انرژی زا به عنوان پوشش سطح دماغه جسم بوده که تحت تاثیر دمای بالای هوا در اطراف ناحیه سکون در حین پرواز، طی واکنش با هوا و با آزادسازی انرژی، منجر به دورتر شدن موج ضربه ای از دماغه و کاهش نیروی پسا خواهد شد. اطلاعات ارایه شده در مورد این روش محدود بوده و بر اساس بررسی های انجام شده قبل، بعضا تناقضاتی در این اطلاعات مشاهده و گزارش شده است. بررسی های قبل، متمرکز بر فیزیک این فرایند و تاثیر روش در میزان افزایش فاصله موج ضربه ای از دماغه بوده و به تاثیر این روش در میزان کاهش نیروی پسا نپرداخته اند. در این تحقیق، تمرکز اصلی بر روی تغییرات نیروی پسای جسم بوده و با استفاده از فرضیات مناسب، تاثیر روش در میزان کاهش نیروی پسا مورد مطالعه قرار گرفته است. نتایج این تحقیق نشان می دهد که بر خلاف ادعای ارایه دهندگان این روش، میزان کاهش نیروی پسا قابل توجه نبوده و لذا در کنار اثرات منفی جنبی آن، باید بر اساس ملزومات طراحی و به منظور کاربرد، با دقت بیشتری مورد ارزیابی قرار گیرد.
کلید واژگان: کاهش نیروی پسا, پرواز ماوراء صوت, شبیه سازی عددی, پوشش انرژی زا, روش نوین, جریان آشفته}The most recent proposed technique for drag reduction in hypersonic flow speeds is using the exothermic coating on the blunt nose. This leads the heat addition into the flow field behind the shock wave, and increases the shock-stand-off distance. The variation of the drag force on the flying body using this technique is numerically investigated in the present study. The compressible turbulent flow is simulated around the body to compute the drag force, and to estimate the drag reduction capabilities of this technique. This quantitative analysis makes better insight to evaluate it in operational applications
Keywords: Drag reduction, hypersonic flight, numerical study, exothermic coating, new technique, turbulent flow} -
Superhydrophobic surfaces have attracted great attention owing to their capacity of reducing fluid resistance. Most of the previous numerical simulations on drag reduction of the superhydrophobic surfaces have concentrated on the rectangular microstructures, whereas few studies have focused on the continuous V-shaped microstructures. Based on the gas–liquid two-phase flow theory and volume-of-field model, combined with the semi-implicit method for pressure-linked equations algorithm, the effects of laminar drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were numerically studied. Three different sizes of superhydrophobic microchannels with continuous V-shapes were simulated according to the experimental data. Results showed that the drag reduction effects of continuous V-shaped microstructures were mainly determined by the width of adjacent microstructures, with the height of the microstructures only having minimal influence. At the same time, the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were compared with those with V-shaped and rectangular microstructures. The results indicated that the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were obviously better than for those with V-shaped microstructures, whereas the superhydrophobic surfaces with rectangular microstructures were more effective in reducing their drag than those with V-shaped microstructures under the condition of the same shear-free air–water ratios. Therefore, in the preparation of superhydrophobic materials, the continuous V-shaped microstructures are recommended; in addition, increasing the microstructure width should be emphasized in the preparation of superhydrophobic materials with continuous V-shaped microstructures.Keywords: Superhydrophobic surface, Laminar flow, Drag reduction, V-shaped microstructure, Flow field}
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امروزه، با توجه به کم بودن منابع انرژی ، صنایع توجه بیشتری به مساله بهره وری انرژی نشان می دهند. در بخش های کاهش مصرف انرژی و افزایش سرعت کشتی ها و زیردریایی ها، بحث مقاومت اصطکاکی سطح نقش بسیار مهمی دارد. سطوح ریبلتی، یک روش منفعل هستند که توانایی کاهش درگ (پسا) سطوح را دارا می باشند. این مطالعه، به بررسی عددی کاهش پسای 4 هندسه مختلف و جدید ریبلتی در ابعاد و سرعت های مختلف می پردازد. جریان در محدوده رژیم جریان آرام بوده و ریبلت ها به صورت عمود بر جهت جریان قرار گرفته اند. همچنین توزیع تنش برشی و فشار استاتیکی بر روی سطوح ریبلتی مورد بررسی قرار گرفته اند. نتایج نشان دادند که هندسه های 1 و 4 میزان کاهش پسای بالاتری را (در حدود 10/7% و 10/2%) نسبت به هندسه های 2 و3 (در حدود 7/7% و 4/4%) به ترتیب، ارایه می دهند. برخلاف هندسه های1 و 4 ، هندسه های 2 و 3 نیروی پسای اصطکاکی بالاتری را نسبت به نیروی پسای فشاری بر روی سطوح خود دارند. همچنین مطالعه تغییرات سرعت نشان داد که هندسه های 1 و 4 کاهش درگ بالاتری را به ترتیب در هرکدام از سرعت ها دارند.
کلید واژگان: ریبلت, کاهش درگ, شبیه سازی عددی}Nowadays, due to the shortage of energy resources, the industries pay more attention to the energy efficiency. in terms of transportation, reducing of energy consumption and increasing the speed of marine vessels and submarines, the surface friction resistance holds an important role. riblets surfaces are a passive method that have ability of surface drag reduction. This study, investigates numearically, the drag reduction of 4 different and new geometries of riblets in different dimensions and velocities. the flow is in the laminar regime range and riblets are aligned perpendicular to the flow direction. Also the distribution of shear stress and static pressure on the riblet surfaces are examined. The Results showed that geometries 1 and 4 have a higher drag reduction (about 10/7% and 10/2%) relative to the geometries 2 and 3 (about 7/7% and 4/4%) Respectively. Unlike geometries 1 and 4, geometries 2 and 3 have a higher viscose force relative to the pressure force on their surfaces. Also the study of speed changes showed that geometries 1 and 4 have a higher drag reduction in each speed respectively.
Keywords: Riblet, Drag Reduction, Numerical Simulation} -
چالش اصلی پیش روی طراحان اجسام پرنده ماوراء صوت، میزان قابل توجه پسای فشاری و گرمایش آیرودینامیکی است. اگرچه دماغه های پخ برای توزیع بهتر گرما ترجیح داده می شوند لیکن نیروی پسای زیادی تولید می کنند. اسپایک ها و آیرودیسک ها ابزار کارآمدی برای کاهش پسا و گرمایش هستند. در این تحقیق اثرات هندسه آیرواسپایک در کاهش پسای سه نوع دماغه پخ نیم کروی، تریدنت و HB1، در یک تونل باد ماوراء صوت ارزیابی شده است. آزمایش ها بر روی دماغه های پخ در شرایط با و بدون آیرواسپایک در عدد ماخ 6.4 با اندازه گیری نیروی پسا و مشاهده امواج ضربه ای با استفاده از روش شیلیرین انجام شده است. برای این مطالعه دو آیرواسپایک نیم کروی با نسبت طول به قطر 1 و 2 در نظر گرفته شد. نتایج نشان می دهند که در هر سه دماغه، آیرواسپایک با تبدیل موج کمانی قوی به امواج ضعیف و به دنبال آن ناحیه چرخشی یا منطقه هوای مرده، موجب کاهش فشار در جلوی دماغه و اعمال پسای کمتر می گردد. عامل اصلی کاهش پسا، دماغه آیرواسپایک بوده و نسبت بهینه طول به قطر اسپایک با هندسه دماغه پخ تغییر می کند. بیشترین کاهش پسا به میزان 74.8 درصد برای آیرودیسک با نسبت طول به قطر 2 در دماغه پخ نیم کروی مشاهده گردید.
کلید واژگان: تونل باد ماوراء صوت, دماغه های پخ, آیرواسپایک, کاهش پسا}The main challenges facing the designers of hypersonic flying objects are the significant amount of pressure drag and aerodynamic heating. However, blunt noses are preferred for better heat distribution, but they produce a lot of drag force. Spikes and aero disks are effective tools for reducing drag and heating. In this study, the effects of aerospike geometry on the drag reduction of three types of hemispherical, Trident and HB1 noses, in a hypersonic wind tunnel have been evaluated. Experiments were performed on blunt noses in the two conditions of with and without aerospike at Mach number of 6.4 by measuring the drag force and observing the shock waves using the Schlieren technique. For this study, two hemispherical aerospikes with length to diameter ratios of 1 and 2 were considered. The results show that in all three noses, the aerospike converts a strong bow shock into a weak shock, and followed by a rotational zone or dead air zone, reduces the pressure in front of the nose and hence, leads to less drag exertion. The main cause of drag reduction is the aerospike nose and the optimal spike length to diameter ratio is with the blunt nose geometry. The highest drag reduction of 74.8% is observed for an aero disk with the length to diameter ratio of 2 in the hemispherical blunt nose.
Keywords: Hypersonic Wind Tunnel, blunt noses, aerospike, Drag Reduction} -
Flow optimization and drag reduction are of great importance in industrial applications. However, most of the structural optimization and drag reduction in pipe flows are based on industrial experience or a large number of experiments, and there is a lack of general theoretical guidance. In the present work, a general approach for flow optimization and drag reduction in turbulent pipe flows is developed based on the irreversibility of flow process and the principle of minimum mechanical energy dissipation. Considering that the effective viscosity coefficient is related to the space coordinates, the field synergy equation of turbulent flow is derived. The reliability and performance of the field synergy principle of turbulent flow as well as the general approach are then evaluated and validated in a turbulent parallel flow conduit, and finally applied to industrial pipe flows. It demonstrates that the present approach is able to optimize flow field for different purposes by adding speed splitter or deflector as an interface at proper locations to alter the interactions between fluid and wall. It is robust and easy to implement, which provides general theoretical guidance for flow optimization and drag reduction in turbulent pipe flows.Keywords: Field Synergy Principle, Minimum mechanical energy dissipation principle, Effective viscosity coefficient, pipe flow, Drag reduction, Flow optimization}
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The objective of this work is to investigate the effect of the bionic microstructure surface on flow field structure of the slab. The motivation behind this study is to investigate the effect of the bionic microstructure parameters including the height and intersection angle of microstructure in order to improve the drag reduction characters. The numerical simulation is performed on the bionic microstructure model of the V-shaped and serrated bionic microstructures using the RNG k-ε model. The drag reduction rates of two bionic microstructures under different dimensionless sizes are ob13.79 tained. The drag reduction efficiency is up to 8.76% when the dimensionless height of microstructure h + is at and the intersection angle = 40 for V-shaped microstructure. In addition, combined with wall temperature control of drag reduction technology, the influence of wall temperature on the drag reduction effect is also analyzed. Compared with the flow field structure of the surface boundary layer of the smooth plate, the wall microstructure divides the surface boundary layer into two parts: the bottom and the tip. The average velocity profile is moved up and the thickness of the linear bottom layer is increased. A large number of "quiet" fluids are gathered at the bottom of the surface boundary layer. In addition, the existence of wall microstructure can weaken the momentum exchange in the boundary layer and restrain the spreading vortex motion of the fluid in the near-wall region. The "secondary vortex pair" on both sides of the tip of the microstructures can effectively limit the lateral pulsation of fluid So as to achieve a good drag reduction effect
Keywords: Bionic microstructure, RNG model, Numerical simulation, Drag reduction} -
This paper explores the use of shark-skin inspired two-dimensional forward facing steps to attain laminar flow control, delay boundary layer transition and to reduce drag. Computation Fluid Dynamics (CFD) simulations are carried out on strategically placed forward facing steps within the laminar boundary layer using the Transition SST model in FLUENT after comprehensive benchmarking and validation of the simulation setup. Results presented in this paper indicate that the boundary layer thickness to step height ratio (d/h), as well as the location of the step within the laminar boundary layer (x/L), greatly influence transition onset. The presence of a strategically placed forward facing step within the laminar boundary layer might damp disturbances within the laminar boundary layer, reduce wall shear stress and energize the boundary layer leading to transition onset delay and drag reduction as compared to a conventional flat plate. Results presented in this paper indicate that a transition delay of 20% and a drag reduction of 6% is achievable, thereby demonstrating the veracity of biomimicry as a potential avenue to attain improved aerodynamic performance.Keywords: Laminar Flow Control, Drag Reduction, Forward Facing Steps, CFD, Shark Skin, Biomimetics}
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This study aims to explore equivalence between active and passive flow control techniques in reducing the wave drag and surface heat flux over a blunt cone model kept in Mach 8 stream. Computational investigations were carried out by using finite volume-based compressible flow solver. Throughout the study, the solution of governing equations is sought by assuming two dimensional-axisymmetric nature of the flowfield. Both counter flow-stagnation point injection and forward facing-physical spike are considered to mitigate the excess drag and heat flux experienced by a blunt body representing the nose cone section of a hypersonic vehicle. Eventually, based on identified drag reductions, the present study proposes equivalence cases between these two methods. It is shown that a pointed spike of L/D=1 provides almost the same drag reduction as the counterflow injection jet with a pressure ratio of 8.25. Similarly, other equivalence cases are identified and the physics behind them is explored. The identified equivalence is expected to help the designers in effectively replacing one technique with another according to the requirement. Equivalence matrix is presented for different spike cases in terms of injection ratios of counterflow injection.
Keywords: Counterflow injection, Physical spike, Blunt body, Hypersonic, Drag reduction, Aerodynamicdrag, Shock interaction, Recirculation region} -
The present study investigates the flow around two tandem spheres and their aerodynamic optimization. In a systematic view, the downstream sphere is regarded as the projectile and the upstream sphere is the sensor. The aim of this study is to find the most appropriate configuration with lowest drag force. Therefore, the results of the effects of the center-to-center (CC) distance of the spheres, and the reduction of the sensor’s diameter were investigated in 15 different cases. The results show that as the distance between the spheres decreases, the drag force of the spheres decreases too; reduction in the sensor’s diameter would increase the projectile’s drag while decreasing the sensor’s drag. The highest effects on drag reduction were induced by constant distance between spheres and a change in sensor’s diameter. Consequently, in the last stage of the study, the adjoint solution of the FLUENT software was used to reduce the drag of the whole set through optimization of the sensor frontal hemisphere. However, due to systematic limitations, only the shape of the forepart of the sensor can be changed. Since the sphere is a bluff body, efficient options are needed for the adjoint optimization algorithm and it’s worth noting that the optimized shape in each case is different from other cases. The highest drag reduction happened in the case with a CC distance of 2.5 m and sensor diameter of 0.75 m. Furthermore, the case with CC distance of 1m and sensor diameter of 0.25 is the only case after optimization in which simultaneously the drag force of both spheres has been reduced.Keywords: Bluff Body, adjoint optimization, tandem spheres, Drag Reduction}
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هدف اصلی در پژوهش حاضر معرفی روشی کارآمد و بدون نیاز به ابزارهای بیرونی جهت کاهش پسای وارد بر اجسام متحرک در سیالات است. رژیم جریان سیال آرام در نظر گرفته شده است. ابزار مورد نظر ریبلت های مستطیلی می باشند که به صورت عمود بر حرکت جریان سیال قرار گرفته اند. میدان جریان روی سطوح ریبلت دار با استفاده از دینامیک سیالات محاسباتی در فضای دوبعدی به دست آمده است. اثر پارامترهای هندسی ریبلت ها شامل عرض و ارتفاع و هم چنین اندازه سرعت جریان آزاد بر میزان کاهش پسای وارد بر سطوح ریبلت دار نسبت به سطح صاف بدون ریبلت مورد بررسی قرار گرفته است. نتایج به دست آمده نشان از کارآمدی روش مذکور در کاهش پسای وارد بر سطح در رژیم جریان آرام دارد. حداکثر کاهش پسا در عرض ریبلت 1/0 میلی متر و سرعت جریان آزاد 10 متر بر ثانیه برابر با 7/8 درصد محاسبه شده است.
کلید واژگان: ریبلت, آب گریز, کاهش درگ, شبیه سازی عددی, دینامیک سیالات محاسباتی, کاهش پسا}Numerical Investigation of Drag Reduction Mechanism in Laminar Flow Regime using Rectangular RibletsThis paper aims to introduce an efficient and passive method in order to reduce the amount of drag exerted on the surfaces of the objects moving in the water. The flow regime is considered as laminar. The desired method is to use rectangular riblets being perpendicular to the flow direction. Two-dimensional computational fluid dynamics is utilized to resolve the flow field around the ribbed surfaces. The effects of geometrical parameters of riblets including width and height as well as free-stream velocity on the amount of drag reduction are numerically calculated. The numerical results certify the efficiency of rectangular riblets as a drag reduction tool in the laminar flow regime. Maximum amount of the drag reduction is about 8.7% achieved for the riblets width of 0.1 millimeter and at free-stream velocity of 10 m/s.
Keywords: Riblet, , Hyrophobic, Drag reduction, Numerical simulation, Computational Fluid Dynamics}
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