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The task of modeling the surface atmospheric boundary layer (ABL) in the wind turbine zone at the location of the Ulyanovsk wind farm is set. Reliable and accurate prediction of the evolution of ABL interacting with a wind farm over a wide range of spatial and temporal scales provides valuable quantitative information about its potential impact on the local meteorological situation and is of great importance for optimizing both the design (placement of turbines) and the operation of wind farms. The main problems of modeling and numerical investigation of the atmospheric boundary layer in combination with a wind turbine are considered. The main modeling problems include: multiscale, accounting for a highly rough inhomogeneous surface, wind irregularity in amplitude, direction and frequency, accounting for convection, solar radiation, stratification and phase transitions and precipitation, turbulence generation, and choice of modeling method and tool. The problem of multiscale research of the ABL-wind turbine system is considered and an overview of computational technologies for solving aerodynamic problems on the scale of one installation and wind farms is given. An analytical review of methods for modeling ABL and its interaction with a wind turbine is carried out. Approaches to the study of ABL based on systems of equations averaged by Reynolds, eddy-resolving models, and direct numerical modeling are considered; their advantages and limitations are given for solving the problem of studying the ABL–wind generator system. The mathematical model of the ABL–wind turbine system is described. The results of mathematical modeling and numerical study of the aerodynamics of the ABL–wind turbine system of the Ulyanovsk wind farm are presented, and numerical data on the attenuation of the aerodynamic wake behind the wind turbine and the restoration of the velocity profile, as well as on the friction resistance on the surface of the wind turbine blade, are obtained and analyzed. The analysis of the results of mathematical modeling of ABL in the wind turbine zone is carried out. [ABSTRACT FROM AUTHOR]

The development of partial-pressure field (PPF) theory (Schmitz, S "Drag Decomposition Using Partial-Pressure Fields in the Compressible Navier-Stokes Equations," AIAA Journal, Vol. 57, No. 5, 2019, pp. 2030-2038) and the application of the concept to conditions relevant to commercial transport aircraft, operating in both the subsonic and transonic regimes (Hart, P. L., and Schmitz, S., "Drag Decomposition using Partial-Pressure Fields: ONERA M6 Wing," AIAA Journal, Vol. 60, No. 5, 2022 pp. 2941-2951), have shown promise as complements to classical far-field methods. In the present work, a further comprehensive analysis of drag decomposition methods is applied to wing and aircraft configurations. First, the Ventus-3 sailplane wing case demonstrates the ability of PPFs to successfully deconstruct drag on industry-relevant cases including transition modeling and winglets. A quantitative comparison to classical aerodynamic theory and far-field methods supports PPF theory and provides additional insight to viscous-inviscid interaction by means of a viscous interactional aerodynamics term in PPF theory. Second, the NASA Common Research Model is simulated in transonic flow to showcase drag decomposition of full aircraft configurations where PPF applications are used to provide further insight into wave drag sources acting on the aircraft in comparison to classical far-field decomposition methods. [ABSTRACT FROM AUTHOR]

The engineers are fascinated in fluid flows and heat transfer in engineering optics and devices. For such purposes, fluid flows are computed in different geometries. The engineers normally look for reduced heat transfer, enhanced cooling rate and reduced friction drag. Thus, the focus of the present article is to undertake the flow of viscous or Newtonian fluid through joined cavities which is very important in many engineering fields. The Navier-Stokes equations are operated for studying behavior of flow, while Fourier's law of heat conduction is employed for the analysis of heat flow in connected cavities. The consequences show that high pressure, lowest velocity and high temperature are observed at the thin connection of cavities.

Improving marine vehicles performance by reducing energy consumption, decreasing environmental pollution, increasing speed and range has always been the goal of engineers for optimal designs. Nowadays, inspiration from nature is one of the scientists' approaches to solve many engineering problems. The researchers, by mimicking the skin of some marine species, such as fast swimming sharks, dolphins and sailfishes have been succeeded to design and fabricate drag reducing microstructured surfaces. In this research, after describing various types of fish scales, a Ctenoid-shape microstructure is designed by inspiration from bony fish scales. Then, the designed microstructures with various dimensions are implemented on an underwater hydrodynamic model to evaluate its ability to reduce drag force. Models numerically analyzed at different Reynolds numbers for turbulent flow regime utilizing the k-ω SST turbulence model. Furthermore, the effects of Ctenoid-shape microstructure on the different drag components including form drag and friction drag have been examined separately. The results indicated that the Ctenoid-shape microstructured surfaces applied to the underwater hydrodynamic model in the optimum condition have reduce the total drag force on average 20% in turbulent flow range. [ABSTRACT FROM AUTHOR]

The improvement of sailing speed and endurance of microunderwater vehicle can be achieved by reducing the wall friction drag. We proposed a piezoelectric drag reducer (PDR), which can be embedded into the wall of the underwater vehicle without affecting the original streamline. We discussed the drag reduction characteristics of the PDR with different arrangement angles based on the simulation analyses, and a maximum friction drag reduction rate of 45.3% can be achieved when the arrangement angle between the direction of PDR and the incoming flow is 60°. The influences of key parameters on the turbulence characteristics are obtained with a view to select control parameters for meeting different drag reduction requirements. In order to verify the simulation results, we carried out experimental tests, the experimental data have been analyzed using turbulence statistical method. The relative errors of friction drag reduction results between simulation and experiment are less than 12%, which proved the effectiveness of the proposed arrangement and control parameter selection methods. Then, the feasibility of achieving high-efficiency active drag reduction of microunderwater vehicle, based on the proposed PDR, is verified to be effective according to the test results of total drag experiments. [ABSTRACT FROM AUTHOR]

This paper presents a computational study of unsteady transonic effects on a supercritical laminar airfoil using high-order implicit large-eddy simulation. The unsteady transitional flow behavior on a static CAST10-2 airfoil operating at a chord-based Reynolds number of Rec=2×106, a transonic freestream Mach number of M∞=0.73, and several small angles of attack (0 deg ≤α≤1.5  deg) was examined alongside corresponding inviscid simulations. Shock- and boundary-layer behaviors were found to be rather sensitive, producing significant change in shock structure, flow transition location, and laminar separation bubbles over this small range of α. One intermediate case exhibited a buffeting phenomenon in which flow transition alternated between two positions, causing pronounced oscillations in the pitching moment. [ABSTRACT FROM AUTHOR]

The effect of the Reynolds number on the flying wing common research model with a large sweep angle and small aspect ratio is studied by numerical simulation. The helicity cross-flow transition correction of the Langtry–Menter model is established in view of the deficient prediction accuracies of existing transition prediction models in simulating cross-flow transition, and the helicity parameters are calibrated to improve the prediction accuracy of the numerical method. The transition prediction method is verified by the test results of the standard model transition of the DLR-F5 wing and small-aspect-ratio flying wing. At the same time, the numerical method is effectively verified by comparing it to the aerodynamic/torque results of the standard model of the small-aspect-ratio flying wing. On this basis, the variation laws with the Reynolds number are analyzed for the aerodynamic and flow field characteristics of the flying wing common research model with a large sweep angle and small aspect ratio. The numerical simulation is mainly carried out in the high subsonic speed range (M = 0.9), and the simulated Reynolds number range is from 2 × 106 to 80 × 106, which includes the test Reynolds number and flight Reynolds number. The influence of turbulence on the surface flow pattern and drag characteristics of the small-aspect-ratio standard model is analyzed. Calculation results show that when the turbulence is greater than 0.4%, it has a considerable influence on the surface laminar flow range and total drag coefficient. The results reveal that the Reynolds number mainly affects the magnitude of friction in the cruise state (small angle of attack). The problem of the Reynolds number in the self-aligning region of the flying wing standard model with a small aspect ratio is further studied, and the Reynolds number is found to be greater than 10 × 106. The drag and lift coefficients after the approach are close to the Reynolds number in the self-aligning region of the flying wing standard model with a small aspect ratio. [ABSTRACT FROM AUTHOR]

Drag decomposition has classically been undertaken using a far-field approach both from a wind-tunnel testing and computational fluid dynamics standpoint. It has been suggested by Schmitz ("Drag Decomposition Using Partial-Pressure Fields in the Compressible Navier-Stokes Equations," AIAA Journal, Vol. 57, No. 5, 2019, pp. 2030-2038) that a decomposition of static pressure into partial pressure fields can allow one to decompose a computational fluids dynamics solution in the near field. This paper covers the implementation process of partial pressure fields on the ONERA M6 wing at conditions relevant to commercial transport aircraft. Relations between partial pressure field decomposition, the classical far-field theory, and physical drag sources are discussed. It is demonstrated that the near-field partial pressure field decomposition is equivalent to classical far-field analysis. In transonic flow, a new method to compute wave drag using a combination of both partial pressure fields and the far-field method is demonstrated. [ABSTRACT FROM AUTHOR]

The present study investigates the flow behavior due to three different vertical canard locations on the scaled‐down model aircraft by using tuft flow visualization and boundary layer measurements. The aircraft is fabricated with wing airfoil S1223 modified and canard with N22 airfoil using laser cutting technique. Three different color tufts are pasted in wing and canard at 30%, 60%, and 90%. The assembled model is placed inside the subsonic wind tunnel with flow chord Reynolds number 3.8*106, which corresponds to 35 m s−1 of the flow velocity and zero‐degree angle of attack. The boundary layers are measured at 70% of the wing chord with 00 incidence angle at three different span locations, i.e., 30%, 60%, and 90% of the wingspan with varying vertical canard positions. The results are compared with wing alone and it is found that the high wing and high canard configuration are outperformed by providing minimum velocity gradient and hence minimum skin friction drag. The tuft flow visualization also shows the identical results as the boundary layer measurements results. The aerodynamic performance of the high wing high canard configuration is found to be promising, as the skin friction drag coefficient is found to be minimum. [ABSTRACT FROM AUTHOR]

Friction between the drill string and the well wall is one of the key factors restricting the rate of penetration (ROP) in the process of sliding drilling in a horizontal well. The problem is effectively solved by applying the vibrating drag-reduction technology. In this paper, the authors propose a new type of mechanical rotary percussion tool driven by the positive displacement motor (PDM) and simulate the drag reduction effect of the tool in a horizontal well. The results show that the drag reduction effect depends on how the weight on the bit (WOB) is transmitted to the bit. When the excitation frequency is high, it causes severe fluctuation of the WOB. To maximize the overall WOB, the excitation frequency needs to be optimized. The WOB fluctuation contains not only the excitation frequency but also the double-excitation frequency and triple-excitation frequency. The research results can provide theoretical guidance for the application of the rotary percussion tool in horizontal wells. [ABSTRACT FROM AUTHOR]

Active flow control for drag reduction in turbulent boundary layer is a hot research field nowadays. In order to explore the effect of the synthetic jet through a spanwise slot on the friction drag reduction of the turbulent boundary layer of a flat plate, the streamwise velocity are measured by a constant temperature hot-wire anemometer with and without synthetic jet. According to the measured experiment data, the mean velocity profiles, fluctuating velocity profiles, skewness and flatness are obtained respectively. Typical combinations of different frequencies and intensities of synthetic jet are selected to study the variation of the forcing effect in the streamwise direction. The experimental results show that the skin friction drag reduction in the turbulent boundary layer is related to the characteristics of the imposed synthetic jet and the distance between the local station and the slot. The synthetic jet leads to an increase of the skin friction drag in the station close to the slot, and as the flow moved downstream away from the slot, a reduction in the skin friction occurred with the tendency of firstly increasing and then decreasing. The reduction of skin friction caused by the synthetic jet of a higher frequency is more than that of a lower frequency. The power spectral density and autocorrelation of the fluctuating velocities indicate that the effect of the synthetic jet on the turbulent boundary layer decays in the streamwise direction.

Active reducing the skin friction drag of vehicles (such as missiles, rockets, aircraft, high-speed train, submarines, etc.) during the navigation process can improve the respond speed effectively, save the energy consumption and increase the endurance time. As lots of active drag reduction methods were proposed for different applications, a review article is urgently needed to guide researchers to choose suitable methods for their pre-research. In this review, the generation mechanisms of skin friction drag under the action of disturbing the turbulent boundary layer are discussed, and the main active drag reduction methods are summarized. According to the actuation modes, we divided the active drag reduction methods into: drag reduction based on wall motion; drag reduction based on volume force control; drag reduction based on wall deformation and drag reduction based on micro vibration generated by piezoelectric actuator. The development status, drag reduction mechanisms, typical structures and drag reduction performance of each active drag reduction method are discussed and summarized in this work, which can provide preliminary research references for those who are engaged in the research of active wall drag reduction.

Variation in flow direction requires extensive consideration in the practical application of riblet surfaces. However, studies scarcely examine the impact of flow angle α for riblet, which is usually adopted to reduce flow drag. Accordingly, this research conducted large eddy simulation for a wide range of flow angles. We explored the effect of 0° to 90° flow angle on the surface drag change of triangular riblet. The time-averaged statistical data and instantaneous flow details indicated that skin friction is decreased with the increase in α. However, pressure drag increased much faster than the friction decrease. Result revealed that skin friction reduction by 4.537% is obtained when α=0°, and it inhibits turbulence in the spanwise direction. When α≈20°, the total drag reduction disappeared. Within this range, the deviation angle showed little influence on the total drag reduction. When α=90°, skin friction is reduced by 73.3%; however the pressure drag and total drag increased, accompanied by an increased turbulence. The flow must be nearly parallel to the riblet to achieve drag reduction. Otherwise, the transverse riblet is an effective method to increase the drag.

The effect of the Reynolds number on the flying wing common research model with a large sweep angle and small aspect ratio is studied by numerical simulation. The helicity cross-flow transition correction of the Langtry–Menter model is established in view of the deficient prediction accuracies of existing transition prediction models in simulating cross-flow transition, and the helicity parameters are calibrated to improve the prediction accuracy of the numerical method. The transition prediction method is verified by the test results of the standard model transition of the DLR-F5 wing and small-aspect-ratio flying wing. At the same time, the numerical method is effectively verified by comparing it to the aerodynamic/torque results of the standard model of the small-aspect-ratio flying wing. On this basis, the variation laws with the Reynolds number are analyzed for the aerodynamic and flow field characteristics of the flying wing common research model with a large sweep angle and small aspect ratio. The numerical simulation is mainly carried out in the high subsonic speed range (M = 0.9), and the simulated Reynolds number range is from 2 × 106 to 80 × 106, which includes the test Reynolds number and flight Reynolds number. The influence of turbulence on the surface flow pattern and drag characteristics of the small-aspect-ratio standard model is analyzed. Calculation results show that when the turbulence is greater than 0.4%, it has a considerable influence on the surface laminar flow range and total drag coefficient. The results reveal that the Reynolds number mainly affects the magnitude of friction in the cruise state (small angle of attack). The problem of the Reynolds number in the self-aligning region of the flying wing standard model with a small aspect ratio is further studied, and the Reynolds number is found to be greater than 10 × 106. The drag and lift coefficients after the approach are close to the Reynolds number in the self-aligning region of the flying wing standard model with a small aspect ratio.

The aerodynamic resistance induced by a high-speed metro train entering and operating in a tunnel with a speed of 120 km/h is simulated using a three-dimensional, compressible turbulence model. An overset mesh method is adopted to solve the moving boundary problem, and the flow field around the train is simulated with a k-omega SST turbulence model to obtain accurate stress values at the walls. The selected model is verified with experimental and numerical data from the literature. Then, numerical simulations are performed to analyse the formation mechanisms of the pressure and friction drags. The aerodynamic drag basically stabilizes after the expansion wave passes the train head. The results reveal that the variations in friction resistance are related to the direction of the Mach wave, and this trend differs from that observed for pressure resistance. Mach wave influences the velocity where it passes, and further influences the friction drag. Result shows that friction drag of the train increases when encounter with compression wave propagated from the front or expansion wave propagated from the back, and decreases otherwise. The effects of the blockage ratio on the maximum and average pressure and friction resistance values of each train section are evaluated based on fitting functions. The predicted aerodynamic drag varies with the blockage ratio and for each train section, and the results are summarized and compared with predictions and experimental data from the literature. The variations in tunnel resistance and the overall trend are in good agreement with the previous results. Therefore, the findings presented in this study may provide a reference for the design of high-speed subway tunnels. [ABSTRACT FROM AUTHOR]

Although the hull of a recently dry-docked large ship is expected to be relatively smooth, surface scanning and experimentation reveal that it can exhibit an "orange-peel" roughness pattern with an equivalent sand-grain roughness height k s = 0. 101 mm. Using the known k s value and integral boundary layer evolution, a recently cleaned and coated full-scale ship was predicted to experience a significant increase in the average coefficient of friction % Δ C ¯ f and total hydrodynamic resistance % Δ R ¯ T during operation. Here the report also discusses two recently reported empirical estimations that can estimate ks directly from measured surface topographical parameters, by-passing the need for experiments on replicated surfaces. The empirical estimations are found to have an accuracy of 4.5 − 5 percentage points in % Δ C ¯ f. [ABSTRACT FROM AUTHOR]

In order to investigate the applicability of the skin-friction reduction technique using hydrogen injecting into turbulent boundary layer, three-dimensional numerical simulation was carried out for a constant-cross-confined-space with rearward facing steps. The flow characteristics near wall surface and development of wall shear stress were analyzed and compared under different coming flow and injection conditions. The simulation results show that the hydrogen injection can achieve around 13.5% skin-friction drag reduction under the coming flow Mach number of 2.3Ma or 2.8Ma. At 2.8Ma, the optimal reduction profit is 13.5% which is obtained when the equivalent ratio is 0.06. The gases mixings are gradually enhanced along the flow path. At the positions of shock wave-boundary-layer interactions, the mixings are first strengthened and then suppressed, and meanwhile, the wall shear stress and density changes with similar law that first decreases and then rebounds at the positions. The declines of skin-friction drag decrease along the flow direction, the best reduction area can profit nearly 60%.

The key purpose of the existing article is to discuss the effects of various hybrid nanofluids and a simple nanofluid over the heat transfer and friction drags along a stretched surface. The various kinds of hybrid nanofluids and a simple nanofluid together with the effects of aligned magnetic field, nonlinear radiation and suction have been taken into consideration. These hybrid nanofluids are prepared by suspending a couple of distinct nanoparticles Cu and A l 2 O 3 into the base fluids H 2 O and C 2 H 6 O 2 . The comparison of various graphical results of skin friction coefficient, rate of heat transfer, velocity and temperature for two different hybrid nanofluids C u - A l 2 O 3 / H 2 O , C u - A l 2 O 3 / H 2 O - C 2 H 6 O 2 and a simple nanofluid A l 2 O 3 / H 2 O is considered. Moreover, the impact of surface stretching, aligned magnetic field and thermal radiation over the velocity, temperature, skin friction coefficient and local Nusselt number are also considered. The outcomes drawn from this modern research is that the hybrid nanofluid C u - A l 2 O 3 / H 2 O - C 2 H 6 O 2 is quite effective in cooling and heating in comparison to the other hybrid nanofluids C u - A l 2 O 3 / H 2 O , C u - A l 2 O 3 / C 2 H 6 O 2 and a simple nanofluid A l 2 O 3 / H 2 O . Based on these findings we could say that the suspension of multiple particles in the composition of two or more base fluids provides a better rate of heat transfer and limits the friction drag. [ABSTRACT FROM AUTHOR]

This manuscript presents a successful application of the inverse feed-forward control (IFFC) technique for control of the Tollmien–Schlichting (TS) waves over a wing profile placed in an open-circuit wind tunnel. Active cancellation of two-dimensional broadband TS disturbances is performed using a single dielectric barrier discharge (DBD) plasma actuator. The measurements required for the IFFC are performed with microphones, instead of hot wires often used for this purpose, in order to reduce the space occupied by the sensors and assess the suitability of simpler and cheaper devices. An attenuation of the TS-wave amplitude of one order of magnitude is achieved. Direct numerical simulations (DNS) are also performed and compared to the outcome of the experiments. The plasma-actuator model used in DNS is a mapping of the force field used by Fabbiane et al. (In: Proceedings of TSFP-9, Melbourne, 2015a) to the actual geometry, whereas the sensors (microphones) are modeled as pressure probes. Despite these modelling choices, a good agreement between the results of DNS and the experiments is achieved. However, the control performance is better in the DNS, with attenuation of three orders of magnitude of TS-wave amplitude. Further analysis of experiments and simulations shows that the limiting factor in the experiments is the ambient low-frequency acoustic waves in the wind tunnel. These waves are sensed by the microphones and act as noise in the analysis of TS-wave evolution and thus leading to lower coherence between sensors and actuators. This in turn leads to a suboptimal control kernel in the experiment.Please confirm if the inserted city and country are correct in Affiliations [Aff1, Aff2]. Amend if necessary.Confirmed. It is correct.Please confirm if the corresponding author is correctly identified. Amend if necessary.Confirmed. The corresponding author is Pedro P. C. Brito. [ABSTRACT FROM AUTHOR]

Drag reduction is vitally important for marine applications, and a promising means of reducing the turbulent frictional drag in external flow is polymer injection. In this paper, the integrated drag reduction rate (DR) of polymer injection in turbulent flow is investigated using pressure measurements. As the inlet speed varies 1.0–3.5 m/s, the speed dependence of DR is derived, DR increases with speed at an equivalent normalized polymer flux, and a maximum value of 50.0% is obtained. It is postulated that the injected polymer interacts with the turbulence much more efficiently as the speed increases, which is similar to the drag-reducing behavior of homogeneous polymer flows. The DR performance in polymer-injected flows reported in this paper not only indicates the existence of speed dependence at moderate speeds, but also supplies the reported speed dependence in higher speed tests. [ABSTRACT FROM AUTHOR]

In this paper, the thermal effects of solar panels are investigated experimentally and computationally on the efficiency of an Unmanned Air Vehicle (UAV) in laminar and turbulent flows. At first, the impact of temperature on output power and efficiency of an eFlex 30 Wp solar panel is studied. Then, the surface temperature and output voltage of two different types of solar panels, a flexible and a solid panel, are measured under a heat lamp. The heat lamp provides the radiation and raises the temperature of the solar panels. A thermal camera and laser thermometer are used to measure the surface temperature of the solar panels. Considering a tilt-rotor UAV as a case study, an energy balance is modeled for the wing of UAV, which is assumed as a flat plate. Applying the Blasius boundary layer for laminar flow and 1/5 power law for turbulent flow, it is shown that there is skin friction drag changes on the top surface of the solar panel due to its dark blue color. In order to validate the results of the proposed model, a thermal-fluid study is carried out on the NACA 2412 airfoil through COMSOL to see whether changing the surface temperature on the solar panel relates to skin drag reduction. The results indicate that an increase in the surface temperature of the solar panel will decrease the output power and efficiency to a maximum of 8%; while this increase in temperature reduces drag by up to 10% in laminar flow. This research shows that despite the reduction of efficiency and generated power by solar panels with increasing the surface temperature on top of a UAV, the aerodynamic efficiency can be improved with drag reduction in laminar flow. [ABSTRACT FROM AUTHOR]

Copyright of Oil Drilling & Production Technology / Shiyou Zuancai Gongyi is the property of Shiyou Zuancai Gongyi Bianjibu and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The current paper investigates the flow and turbulence characteristics over dune bedforms by means of laboratory experiments, where spatially dense and temporally high frequency velocity measurements were done. Although similar studies are available in the literature, the focus and novelty of the current study is to assess the influence of surface roughness of the dune bedforms on the nearbed flow. For direct comparison, two different surface roughness heights over idealized, fixed-shaped, high-angled dune bedforms were tested; one with a hydraulically-smooth surface, and the other with a fully-rough surface. Spatial variation of time-averaged flow as well as turbulence statistics were examined, which was complemented by streamline plots and spectral analyses. The results are interpreted from sediment entrainment and sediment transport points of view. The results show that increased dune surface roughness reduces the nearbed flow velocity, but increases the flow velocities at upper regions. The upward directed flow near the dune crests becomes stronger in the case of smooth surface, while the re-attachment point moves further downstream compared to the rough wall case. It is concluded that the roughness of the dune surface affects the nearbed flow and turbulence characteristics qualitatively and quantitatively, which is shown to have direct consequences on sediment entrainment characteristics. • The flow and turbulence characteristics over idealized dune bedforms are experimentally investigated. • To assess the influence of surface roughness of dunes on the nearbed flow, two different roughness conditions were tested. • Spatial variation of time-averaged flow as well as turbulence statistics were examined. • Results show that roughness of the dune surface affects the nearbed flow and turbulence, and also, sediment entrainment. [ABSTRACT FROM AUTHOR]

To develop a better understanding of 'in-service' performance of modern marine coatings, this study explored the combined effects of different roughness ranges of foul-release coating (FRC) and light biofouling (slime) on the surface, boundary layer and drag characteristics under a range of 'in-service' conditions. Natural and laboratory biofilms were grown dynamically on FRC panels by exposing panels in facilities dedicated to realistic fouling culture. The boundary layer experiments were conducted in a circulating water tunnel. Boundary layer similarity-law scaling was used to predict the combined effects of coating roughness and biofilms on the added frictional resistance (% Δ C F) and added required effective power (% Δ P E ) for a benchmark KRISO container ship (KCS) and a bulk carrier. The increase in % Δ P E due to the presence of biofilms on commercial FRC is estimated to be between 7% and 16% depending on the biofilm type, biofilm thickness and percentage coverage. Significant increases in effective power are estimated for non-fouling control primers with heavy fouling. Moreover, the paper suggests updated roughness allowances ( Δ C F ) for two vessel types assuming FRCs on their hulls with more representative hull roughness ranges and fluffy biofilms. [ABSTRACT FROM AUTHOR]

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%.

Variation in flow direction requires extensive consideration in the practical application of riblet surfaces. However, studies scarcely examine the impact of flow angle a for riblet, which is usually adopted to reduce flow drag. Accordingly, this research conducted large eddy simulation for a wide range of flow angles. We explored the effect of 0° to 90° flow angle on the surface drag change of triangular riblet. The time-averaged statistical data and instantaneous flow details indicated that skin friction is decreased with the increase in a. However, pressure drag increased much faster than the friction decrease. Result revealed that skin friction reduction by 4.537% is obtained when a=0°, and it inhibits turbulence in the spanwise direction. When a -20°, the total drag reduction disappeared. Within this range, the deviation angle showed little influence on the total drag reduction. When a=90°, skin friction is reduced by 73.3%; however the pressure drag and total drag increased, accompanied by an increased turbulence. The flow must be nearly parallel to the riblet to achieve drag reduction. Otherwise, the transverse riblet is an effective method to increase the drag. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A unique approach to evaluate the reduction of skin friction drag by riblets was applied to boundary layer profiles measured in wind tunnel experiments. The proposed approach emphasized the turbulent scales based on hot-wire anemometry data obtained at a sampling frequency of 20 kHz in the turbulent boundary layer to evaluate the skin friction drag reduction. Three-dimensional riblet surfaces were fabricated using aviation paint and were applied to a flat-plate model surface. The turbulent statistics, such as the turbulent scales and intensities, in the boundary layer were identified based on the freestream velocity data obtained from the hot-wire anemometry. Those turbulent statistics obtained for the riblet surface were compared to those obtained for a smooth flat plate without riblets. Results indicated that the riblet surface increased the integral scales and decreased the turbulence intensity, which indicated that the turbulent structure became favorable for reducing skin friction drag. The proposed method showed that the current three-dimensional riblet surface reduced skin friction drag by about 2.8% at a chord length of 67% downstream of the model's leading edge and at a freestream velocity of 41.7 m/s (Mach 0.12). This result is consistent with that obtained by the momentum integration method based on the pitot-rake measurement, which provided a reference dataset of the boundary layer profile. [ABSTRACT FROM AUTHOR]

Turbulent boundary layer (TBL) over various area coverages of rough surfaces have been studied via computational fluid dynamics using the steady Reynolds Averaged Navier–Stokes (RANS) technique. The rough surfaces are modelled by randomly distributed hemispheres with a 1 mm radius, covering 5%, 10%, 20%, 30%, 40%, and 50% of the wall surface area designed to represent a typical three-dimensional roughness and also replicate the biofouling growth of a ship's hull. The results of this study show the relationships between the roughness area coverage and the skin friction drag within a fully rough regime. The flow experiences maximum drag and the highest equivalent sand grain roughness height at 30% roughness area coverage. However, beyond this area coverage value, the drag gradually decreases. Further analysis indicates that the 30% roughness area coverage corresponds to a roughness frontal density of λ f = 0. 15. Recent reports indicate that when λ f ≲ 0. 15 , the pressure drag to total drag ratio increases with increased roughness frontal density. • CFD simulations of random distribution hemispheres roughness at varied area coverage. • The RANS method and SST k- ω turbulence model were used for simulations. • The roughness area coverage effects on the skin friction drag were investigated. [ABSTRACT FROM AUTHOR]

Nanofluids are composed of nano-sized particles dispersed in a carrier liquid. The present investigation’s aim is to examine theoretically the magneto-thermomechanical coupling phenomena of a heated nanofluid on a stretched surface in the presence of magnetic dipole impact. Fourier’s law of heat conduction is used to evaluate the heat transmission rate of the carrier fluids ethylene glycol and water along with suspended nanoparticles of a cobalt–chromium–tungsten–nickel alloy and magnetite ferrite. A set of partial differential equations is transformed into a set of non-linear ordinary differential equations via a similarity approach. The computation is performed in Matlab by employing the shooting technique. The effect of the magneto-thermomechanical interaction on the velocity and temperature boundary layer profiles with the attendant effect on the skin friction and heat transfer is analyzed. The maximum and minimum thermal energy transfer rates are computed for the H2O-Fe3O4 and C2H6O2-CoCr20W15Ni magnetic nanofluids. Finally, the study’s results are compared with the previously available data and are found to be in good agreement.

For most vessels, the frictional resistance of the hull in the water has the largest share in the total resistance of the ship, so in the past few decades much research have been done on reducing the frictional resistance of ships. Among these, stepped vessels with a significant reduction in the amount of wetted surface have a special place. In this study, the effect of a single transverse-step on a flat plate is investigated in calm water with computational fluid dynamics methods. First, the vessel is modeled as a planing flat plate in a fixed trim and draft. By adding a transverse step to the geometry of the model, the proportion of frictional drag to the lift is calculated and compared with the other results. Also, changing the amount of frictional drag and wetted surface due to the changing of the longitudinal position and height of the step are investigated.

The flow around cylinder open the path for studying more complex shape bodies like the ship’s hulls. The hydrodynamic properties of the ship’s hulls can be decomposed as combinations of the flow properties of simpler bodies like flat plates, cylinders, ellipses, spheres and ellipsoids. The aim of this study is to describe the flow around a cylinder based on simulations with platforms like Comsol and Ansys that further can be compared with experimental and analytical results. The drag force caused by the flow around cylinders can be combined with the drag force of simple elements like flat plates, ellipses in order to correlate with the drag force of a specific hull of ship. Cylinder is a case that offers with its simplicity the possibility to check the results in all three ways: analytical, computational and experimental. An exhaustive analysis of this shape offers a beginning path for generalizing the external flows like the application of the superposition theory for complex geometries. [ABSTRACT FROM AUTHOR]

Highlights • Wall-resolved LES adapted to the simulation of heated turbulent flow in a ribbed tube. • Detailed and direct comparison with experiment. • The simulation is accurate both in terms of flow dynamics and heat transfer. • Pressure drag is the main contribution to the total drag in the studied ribbed tube. • Heat transfer is enhanced by a factor 2.3 compared to a smooth tube. Abstract This work presents a study of the turbulent flow in a single-started helically ribbed tube with low blockage ratio. The Large Eddy Simulation (LES) approach is used in a wall-resolved periodic configuration. Both an adiabatic and a wall-heated simulations are performed and validated against experiment. Velocity profiles and wall temperatures were measured at the Von Karman Institute (VKI) using Stereoscopic Particle image Velocimetry (S-PIV) and Liquid Crystal Thermography (LCT) by Mayo et al. (2018). Comparisons show that the numerical methodology gives accurate results in terms of mean and fluctuating velocity fields as well as the correct friction drag. The wall temperature profile is also in good agreement with the experiment. The rib induces a large recirculation zone immediately downstream, with a reattachment point occurring a few rib heights farther downstream. The helical shape of the rib also induces a strong swirling motion close to the wall. The pressure drop is found equal to 3.37 Pa/m and is mostly due to the pressure drag. Maximum heat transfer is found just upstream of the reattachment point and on top of the ribs, which is in good agreement with experimentally obtained values. The mean Nusselt number in the ribbed tube is found 2.3 times higher than in a smooth tube confirming the positive impact of such geometry on heat transfer. [ABSTRACT FROM AUTHOR]

Abstract Prediction of mean flow and turbulence structure of boundary layer on surfaces with roughness is still not fully understood in fluid mechanics. Specific characteristics of roughness in terms of shape, size and distribution vary from case to case. Current paper shows a method for obtaining the roughness function of any arbitrary rough surface by using resolved RANS simulations in channel flows, which are considerably cheaper in terms of time and resources when compared with experiments and other modeling approaches (LES/DNS). Drag characterization and roughness function determination were studied for a particular antifouling paint with different roughness used in marine applications. Reynolds numbers based on channel height were in the range 84000-280000. Roughness functions from resolved RANS simulations were compared with those obtained for the same coating from two of the most commonly used experimental techniques: a rotating disc rig and towing tank. Data obtained from all three methods were successfully fitted on same roughness function curve. Furthermore, to check consistency between the obtained roughness function and drag prediction by using this function, CFD simulations of flat plate flow using standard wall functions were performed. Total drag of a plate from computational results deviated by around 7% with results from experiments. Highlights • Drag characterization of antifouling paints was carried out by means of resolved RANS CFD simulations. • RANS CFD can be an inexpensive alternative to find the roughness function for any arbitrary roughness. • Improved accuracy of CFD is required to better match experiments. Agreement is seen between the experimental approaches. • The equivalent sand-grain roughness could also be easily determined. [ABSTRACT FROM AUTHOR]

Due to the importance of copper and its alloys in marine applications, the main objective of this research is to provide a simple, effective and low cost manufacturing approach to fabricate a superhydrophobic riblet copper surface with high drag reduction capability in laminar and turbulent flow regimes. Therefore, the riblets are produced by wire cut technique on the copper substrate and then by using a wet chemical method, a superhydrophobic coating is produced on the riblet surface. A pressure drop measurement system consists; pump, closed channel flow with a fabricated surfaces on the lower wall, connections and pressure drop transmitter is employed to measure the pressure drop in the close channel flow, for Reynolds number from 300 to 2769, in order to evaluate the ability of the fabricated surface to reduce the friction drag. The experimental results revealed that combining the abilities of the riblet and superhydrophobic surfaces increases the surface's ability to reduce friction drag. In addition, the riblet surface and superhydrophobic riblet surface on average decreased the friction drag by 10.33% and 42.65% correspondingly in water flow ranging from laminar to turbulent flow regime. Finally, according to the experimental results, the drag reduction performance of riblet surface is improved from 18.9% to 56.9% after superhydrophobic coating. [ABSTRACT FROM AUTHOR]

There are limited scientific data on contributors to the added drag of in-service ships, represented by modern-day coating roughness and biofouling, either separately or combined. This study aimed to gain an insight into roughness and hydrodynamic performance of typical coatings under in-service conditions of roughened ships' hull surfaces. Comprehensive and systematic experimental data on the boundary layer and drag characteristics of antifouling coating systems with different finishes are presented. The coating types investigated were linear-polishing polymers, foul-release and controlled-depletion polymers. The data were collected through state-of-the-art equipment, including a 2-D laser Doppler velocimetry (LDV) system for hydrodynamic data in a large circulating water tunnel. Three coating systems were first applied on flat test panels with 'normal' finishes in the first test campaign to represent coating applications under idealised laboratory conditions. In order to address more realistic roughness conditions, as typically observed on ships' hulls, 'low' and 'high' roughness densities were introduced into the same types of coating, in the second test campaign. The data collected from the first test campaign served as the baseline to demonstrate the effect on the surface roughness and hydrodynamic drag characteristics of these coating types as a result of 'in-service' or 'severely flawed' coating application scenarios. Data collected on coatings with a range of in-service surface conditions provided a basis to establish correlation between the surface roughness characteristics and hydrodynamic performance (roughness function). The findings of the study indicate that the estimations of drag penalties based on well-applied, relatively smooth coating conditions underestimate the importance of hull roughness, which although undesirable, is commonplace in the world's commercial fleet. [ABSTRACT FROM AUTHOR]