Skin Friction Drag Research Articles

Annealing Effect on Temperature Coefficient of Resistance of Microflexible Thermal Shear Stress Sensor

Knowledge of wall shear stress is of primary importance in aerodynamic and hydrodynamic designs because it determines a major part of the skin friction drag. The sensitivity of microflexible thermal shear stress sensor is positively correlated with the temperature coefficient of resistance (TCR) of thermistor film, but the TCR of thermal sensor is extremely low due to the lattice defects of deposited films. A high-sensitivity microflexible thermal shear stress sensor with TCR of 5100 ppm/°C after annealing treatment is present in this letter. The annealing effect on TCR of magnetron sputtered thermistor films was investigated. The TCR increases with increasing annealing temperature and time until annealing time exceeds 6 h. The microstructural characterization indicates that of the TCR of thermistor films is dominated by electron grain boundary scattering. The TCR improvement after annealing is attributed to the growth of grain and reduction of the grain boundary reflection coefficient.

Surface temperature effects of solar panels of fixed-wing drones on drag reduction and energy consumption

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.

Machine-learning-based feedback control for drag reduction in a turbulent channel flow

Abstract

Characteristics of array of distributed synthetic jets and effect on turbulent boundary layer

An array of distributed round synthetic jets was used to control a fully developed turbulent boundary layer. The study focused on the related skin friction drag reduction and mechanisms involved. The control effects were analyzed by measuring the streamwise velocities using a hot-wire anemometer downstream of the array. A reduction in the skin friction was observed both in the regions downstream of the orifices and in the regions between two adjacent orifices. A statistical analysis with the variable-interval time-averaging (VITA) technique demonstrated a weakened bursting intensity with synthetic jet in the near-wall region. The streamwise vortices were lifted by the upwash effect caused by synthetic jet and induced less low-speed streaks. The control mechanism acted in a way to suppress the dynamic interaction between the streamwise vortices and low-speed streaks and to attenuate the turbulence production in the near-wall region. The forcing frequency was found to be a more relevant parameter when synthetic jet was applied in turbulent boundary layer flow control. A higher forcing frequency induced a higher reduction in the skin friction. The power spectral density and autocorrelation of the fluctuating velocities showed that the synthetic jets gradually decayed in the streamwise direction, having an effect as far as 34.5 times the displacement thickness that was on the trailing edge of the distributed synthetic jets array.

Aviation Impacts on Fuel Efficiency of a Future More Viscous Atmosphere

AbstractAircraft cruising near the tropopause currently benefit from the highest thermal efficiency and the least viscous (sticky) air, within the lowest 50 km of Earth’s atmosphere. Both advantages wane in a warming climate, because atmospheric dynamic viscosity increases with temperature, in synergy with the simultaneous engine efficiency reduction. Here, skin friction drag, the dominant term for extra aviation fuel consumption in a future warming climate, is quantified by 34 climate models under a strong emissions scenario. Since 1950, the viscosity increase at cruising altitudes (∼200 hPa) reaches ∼1.5% century‒1, corresponding to a total drag increment of ∼0.22% century‒1 for commercial aircraft. Meridional gradients and regional disparities exist, with low to midlatitudes experiencing greater increases in skin friction drag. The North Atlantic corridor (NAC) is moderately affected, but its high traffic volume generates additional fuel cost of ∼3.8 × 107 gallons annually by 2100, compared to 2010. Globally, a normal year after 2100 would consume an extra ∼4 × 106 barrels per year. Intermodel spread is <5% of the ensemble mean, due to high inter–climate model consensus for warming trends at cruising altitudes in the tropics and subtropics. Because temperature is a well-simulated parameter in the IPCC archive, with only a moderate intermodel spread, the conclusions drawn here are statistically robust. Notably, additional fuel costs are likely from the increased vertical shear and related turbulence at NAC cruising altitudes. Increased flight log availability is required to confirm this apparent increasing turbulence trend.

Effects of Localized Micro-blowing on a Spatially Developing Flat Turbulent Boundary Layer

Direct numerical simulation (DNS) is used to investigate the turbulent flat-plate boundary layer with localized micro-blowing. The 32 × 32 array of micro-holes is arranged in a staggered pattern on the solid wall, located in the developed turbulent region. The porosity of the porous wall is 23%, and the blowing fraction is 0.0015. The Reynolds number based on the inflow velocity is set to be 50,000. The structures of the turbulent boundary layer are carefully analyzed to understand the effects of micro-blowing and its drag reduction mechanism. The DNS results show that the drag reduction is efficient, and the local maximum rate of drag reduction achieves 40%. A low-speed “turbulent spot” near the micro-blowing region thickens the boundary layer. Some turbulent properties, such as the mean velocity profile, stream-wise vorticity and stream-wise velocity fluctuation are lifted up. Particularly, the tilting term of vorticity transport is significantly increased. Meanwhile, the visualization of 3-dimensional vortex displays several concave marks on the surface of the near-wall vortices, which is caused by the micro-jets, leading to more broken vortices and isotropic small scales. This impact travels downstream with a small distance due to the accumulation of the micro-jets, while the uplift effect will gradually disappear. In addition, FIK identity reveals that the spatial development term and mean wall-normal convection term play opposite roles in the contribution to the skin friction drag.

The effect of a systematic change in surface roughness skewness on turbulence and drag

Previous work by the authors (Flack and Schultz, 2010) has identified the root-mean-square roughness height, krms, and the skewness, Sk, of the surface elevation distribution as important parameters in scaling the skin-friction drag on rough surfaces. In this study, three surfaces are tested in turbulent boundary layer flow at a friction Reynolds number, Reτ = 1600–2200. All the surfaces have similar root-mean-square roughness height, while the skewness is varied. Measurements are presented using both two-component LDV and PIV. The results show the anticipated trend of increasing skin-friction drag with increasing skewness. The largest increase in drag occurs going from negative skewness to zero skewness with a more modest increase going from zero to positive skewness. Some differences in the mean velocity and Reynolds stress profiles are observed for the three surfaces. However, these differences are confined to a region close to the rough surface, and the mean velocity and Reynolds stress profiles collapse away from the wall when scaled in outer variables. The turbulence structure as documented through two-point spatial correlations of velocity is also observed to be very similar over the three surfaces. These results support Townsend’s (1976) concept of outer-layer similarity that the wall boundary condition exerts no direct influence on the turbulence structure away from the wall except in setting the velocity and length scales for the outer layer.

Heading control of a novel finless high-speed supercavitating vehicle with an internal oscillating pendulum

High-speed supercavitating vehicles are surrounded by a huge cavity of gas and only a small portion of the nose and the tail of the vehicle are in contact with the water which leads to a considerable reduction in skin friction drag and reaching very high speeds. High-speed supercavitating vehicles are usually controlled by the cavitator at the nose which controls the pitch and depth of the vehicle and the control surfaces or fins which control the roll and heading angle of the vehicle using the bank-to-turn maneuvering method. However, control surfaces have disadvantages such as the high drag force and ineffectiveness due to the supercavity. Therefore, the purpose of the present study is to eliminate the fins from high-speed supercavitating vehicles and propose a new bank-to-turn heading control of this novel finless high-speed supercavitating vehicle which is composed of the cavitator at the nose and an oscillating pendulum as the internal actuator. Sliding mode control as a robust method is used for the six-degrees-of-freedom model of this finless high-speed vehicle against exposed disturbances. Some design criteria for the design of the internal pendulum in this finless supercavitating vehicle are presented for the damping coefficient, pendulum mass, and radius.

Transportation and analysis of hybrid nanomaterial (graphene oxide, copper) in radiated Darcy–Forchheimer flow with entropy optimization

Entropy optimization or entropy plays vital roles in our understanding of numerous various diverse phenomena running from cosmology to science. Their significance is shown in regions of immediate practical interest like provision of global energy as well as in others of a progressively essential flavor, such as the source of order and unpredictability in nature. The purpose of this communication is to investigate some of ongoing and significant outcomes in a way that not only appeals to the entropy expert but also makes them available to the nonexpert looking for an outline of the field. This communication addresses the entropy optimized flow of hybrid nanofluid between two plates accounting Darcy–Forchheimer porous medium. Energy equation is developed through implementation of first law of thermodynamics subject to radiative flux, dissipation and Joule heating. MHD fluid is rotating with angular frequency [Formula: see text]. Total entropy rate obtained is subject to thermal irreversibility, friction or dissipation irreversibility, magnetic or Joule heating irreversibility and Darcy–Forchheimer irreversibility via second law of thermodynamics. The nonlinear ordinary system (differential equations) is tackled via homotopy method for series solutions. Behaviors of sundry variables on the velocity, skin friction, temperature, Nusselt number and entropy generation rate are discussed and presented through various plots. Schematic flow diagram is presented. Furthermore, skin friction (drag force) and Nusselt number are discussed numerically. Obtained results analyzed that the entropy rate increases subject to higher radiation parameter and Hartmann and Brinkman numbers.

3-dimensional particle image velocimetry based evaluation of turbulent skin-friction reduction by spanwise wall oscillation

The reduction of turbulent skin-friction drag and the response of vortical structures in a zero-pressure gradient, turbulent boundary layer subjected to spanwise wall oscillation is investigated using planar and tomographic particle image velocimetry (PIV). The experiments are conducted at a momentum based Reynolds number of 1000, while the range of spanwise oscillation amplitude and frequency is chosen around the optimum reported in previous studies. A high-resolution planar PIV measurement is employed to determine the drag reduction directly from wall shear measurements and to analyze the accompanying modifications in the turbulent vortical structures. Drag reduction of up to 15% is quantified, with variations following the trends reported in the literature. The analysis of the turbulence structure of the flow is made in terms of Reynolds shear stresses, turbulence production, and vortex visualization. A pronounced drop of turbulence production is observed up to a height of 100 wall units from the wall. The vorticity analysis, both in the streamwise wall-normal plane and in the volumetric results, indicates a reduction of vorticity fluctuations in the near-wall domain. A distortion of the hairpin-packet arrangement is hypothesized, suggesting that the drag-reduction mechanism lies in the inhibition of the hairpin auto-generation by the spanwise wall oscillations.

Effect of the Air Duct System of a Simplified Vehicle Model on Aerodynamic Performance

In this numerical study, ducted Ahmed model is used to study aerodynamic characteristics of ducted models and how ducting would contribute to the energy consumption reduction effort from aerodynamic resistance. Three-dimensional, incompressible, and steady governing equations were solved by commercial software PHOENICS (version 2018) with extended turbulent model proposed by Chen-Kim (1987). The study can be considered as a stepping stone to bring a new approach to design aerodynamically efficient vehicle by ducting target vehicles. To investigate the feasibility of the ducting for aerodynamic energy efficiency improvement, fundamental study has been conducted numerically on a simple body before applying to appropriate vehicle model. It is observed from the study that ducting would reduce over 19% of the pressure drag, to the contrary the increase in skin friction drag is noted although its significance to the total drag is very less compared to pressure drag. Therefore, study of ducting and its effect on aerodynamic performance is expected to contribute to improvement of electric vehicle aerodynamic performance.

Non-similar solution of Sisko nanofluid flow with variable thermal conductivity: a finite difference approach

PurposeThe main purpose of this study is to present a non-similar analysis of two-dimensional boundary layer flow of non-Newtonian nanofluid over a vertical stretching sheet with variable thermal conductivity. The Sisko fluid model is used for non-Newtonian fluid with an exponent (n* > 1), that is, shear thickening fluid. Buongiorno model for nanofluid accounting Brownian diffusion and thermophoresis effects is used to model the governing differential equations.Design/methodology/approachThe governing boundary layer equations are converted into nondimensional coupled nonlinear partial differential equations using appropriate transformations. The resultant differential equations are solved numerically using implicit finite difference scheme in association with the quasilinearization technique.FindingsThis analysis shows that the temperature raises for thermal conductivity parameter and velocity ratio parameter while decreases for the thermal buoyancy parameter. The thermophoresis and Brownian diffusion parameter that characterizes the nanofluid flow enhances the temperature and reduces the heat transfer rate. Skin friction drag can be effectively reduced by proper control of the values of thermal buoyancy and velocity ratio parameter.Practical implicationsThe wall heating and cooling investigation result in the analysis of the control parameters that are related to the designing and manufacturing of thermal systems for cooling applications and energy harvesting. These control parameters have practical significance in the designing of heat exchangers and solar thermal collectors, in glass and polymer industries, in the extrusion of plastic sheets, the process of cooling of the metallic plate, etc.Originality/valueTo the best of authors’ knowledge, it is found from the literature survey that no similar work has been published which investigates the non-similar solution of Sisko nanofluid with variable thermal conductivity using finite difference method and quasilinearization technique.

Influence of curvature on drag reduction by opposition control in turbulent flow along a thin cylinder

Opposition controlled fully developed turbulent flow along a thin cylinder is analyzed by means of direct numerical simulations. The influence of cylinder curvature on the skin-friction drag reduction effect by the classical opposition control (i.e., the radial velocity control) is investigated. The curvature of the cylinder affects the uncontrolled flow statistics; for instance, skin-friction coefficient increases while Reynolds shear stress (RSS) and turbulent intensity decrease. However, the control effect in the case of a small curvature is similar to that in channel flow. When the curvature is large, the maximum drag reduction rate decreased. However, the optimal location of the detection plane is the same as that in a flat plate. Further, the drag reduction effect is achieved even on a high detection plane where the drag increases in the flat plate. Although a difference in the drag reduction effect can be observed with a change in the curvature, its mechanism considered in this analysis based on the transport of the Reynolds stress is similar to that of the flat plate.

Turbulent drag reduction over curved walls

This work studies the effects of skin-friction drag reduction in a turbulent flow over a curved wall, with a view to understanding the relationship between the reduction of friction and changes to the total aerodynamic drag. Direct numerical simulations (DNS) are carried out for an incompressible turbulent flow in a channel where one wall has a small bump; two bump geometries are considered, that produce mildly separated and attached flows. Friction drag reduction is achieved by applying streamwise-travelling waves of spanwise velocity (StTW). The local friction reduction produced by the StTW is found to vary along the curved wall, leading to a global friction reduction that, for the cases studied, is up to 10\% larger than that obtained in the plane-wall case. Moreover, the modified skin friction induces non-negligible changes of pressure drag, which is favorably affected by StTW and globally reduces by up to 10\%. The net power saving, accounting for the power required to create the StTW, is positive and, for the cases studied, is one half larger than the net saving of the planar case. The study suggests that reducing friction at the surface of a body of complex shape induces further effects, a simplistic evaluation of which might lead to underestimating the total drag reduction.

Wall-bounded turbulence control: statistical characterisation of actions/states

The present paper reports the results of a Monte Carlo experiment using a turbulent channel flow. Different actions are proposed, varying the size, duration and sign of a localised volumetric force that acts near one wall of a turbulent channel flow, running at a small Reynolds (ReT = 165) in a small computational domain. The effect of each action is evaluated comparing the evolution of the flow with and without the action, gathering statistics over 1700 repetitions of the experiment for each action. The analysis of the results show that small/short forcings are equally likely to increase or decrease the skin friction drag, independently of the sign of the forcing (i.e., towards or away from the wall). When the size or the duration of the forcing increases, so does the probability of increasing the skin friction. Then, an a priori analysis is performed, evaluating the state of the flow just before the action, conditioned to actions that result in a decrease of the skin friction over a period of one eddy turn-over time. The resulting fields of velocity, wall shear stresses and wall pressure are consistent with an opposition control strategy, where the forcing is opposing the vertical motions near the wall. Finally, a preliminary analysis of the performance of actuation triggered by pressure or wall shear stresses sensors is evaluated (i.e., a posteriori analysis). Our results show that the actuation triggered by a wall shear sensor seems to be more effective than the actuation triggered by a wall pressure sensor, at least for the preliminary definitions of sensors and thresholds used here.

AN INVESTIGATION OF EFFECT OF CONTROL JETS LOCATION AND BLOWING PRESSURE RATIO TO CONTROL BASE PRESSURE IN SUDDENLY EXPANDED FLOWS

The drag force is an essential factor in any projectile, from road vehicles to rocket or aircraft. The total drag includes skin friction drag, wave drag, and base drag. The base drag is the drag due to low pressure in the base region of the projectile. In the case of suddenly expanded flows, due to the sudden expansion of flow from the nozzle into the enlarged duct, the low pressure is created in the base region of the enlarged tube, which results in base drag and hence overall thrust reduced. In this paper, Computational Fluid Dynamic (CFD) analysis is used to analyze the effect of secondary air blowing jets called control jets to control base pressure in the base region of suddenly enlarged duct. These control jets are placed at different Pitch Circle Diameters (PCD) on the base face of the enlarged pipe. The objective of this work is to increase the base pressure up to atmospheric pressure and hence reduces the base drag. Mach number 3.0 is considered for analysis. The CFD analysis is done for different combinations of Area Ratios (AR) (2, 5 and 8), Nozzle Pressure Ratios (NPR) (2, 5 and 8), and PCD (d1, d2, and d3). Further analysis is done for different air blowing pressure ratios (BPR) to optimize air blowing pressure. The analysis results are plotted for different area ratios, nozzle pressure ratios, and PCD of control jets. By observing results, it can be concluded that the base pressure is strongly influenced by AR, NPR, and PCD of control jets. The air blowing pressure should be optimum to save energy, and the optimum values can be selected from the results.

Effect of Flexible Flaps on Lift and Drag of Laminar Profile Flow

Experiments with elastic flaps applied on a common airfoil profile were performed to investigate positive effects on lift and drag coefficients. An NACA0020 profile was mounted on a force balance and placed in a wind tunnel. Elastic flaps were attached in rows at different positions on the upper profile surface. The Reynolds number of the flow based on the chord length of the profile is about 2 × 10 5 . The angle of attack is varied to identify the pre- and post-stall effects of the flaps. Polar diagrams are presented for different flap configurations to compare the effects of the flaps. The results showed that flaps generally increase the drag coefficient due to the additional skin friction and pressure drag. Furthermore, a significant increase of lift in the stall region was observed. The highest efficiency was obtained for the configuration with flaps at the leading and trailing edges of the profile. In this case, the critical angle was delayed and lift was increased in pre- and post-stall regions. This flap configuration was used in a gust simulation in the wind tunnel to model unsteady incoming flow at a critical angle of attack. This investigation showed that the flow separation at the critical angle was prevented. Additionally, smoke–wire experiments were performed for the stall region in order to visualize the flow around the airfoil. The averaged flow field results showed that the leading-edge flaps lean the flow more towards the airfoil surface and reduce the size of the separated region. This reduction improves the airfoil performance in the deep stall region.

Natural Convection in Power-Law Fluids from a Pair of Two Attached Horizontal Cylinders

Laminar natural convection in unconfined power-law fluids from two attached horizontal cylinders has been investigated numerically to elucidate the non-Newtonian behavior of the fluid over a wide range of pertinent parameters: Grashof number (10 to 105), Prandtl number (0.71 to 100), and power-law index (0.2 to1.8). The heat transfer characteristics are elucidated in terms of isotherms, local Nusselt number distributions, and average Nusselt number values, whereas the flow characteristics are interpreted in terms of streamlines, local distribution of the pressure drag, and skin friction drag coefficients along with the total drag coefficient values. Under identical conditions, the average Nusselt number shows a positive dependence on both Grashof and Prandtl number, whereas it shows an adverse dependence on power-law index. Overall, shear-thickening fluid behavior impedes the convection heat transfer, whereas shear-thinning fluid behavior promotes it with reference to the Newtonian fluids. The heat transfer from an individual cylinder in attached condition has been degraded compared to the case of a single horizontal cylinder. Furthermore, the heat transfer rate of the top cylinder is reduced by 29%–41% relative to that of the bottom cylinder due to preheating of the thermal plume.

Evaluation and Effect of Micro Bubble Lubrication in Drag Reduction

This paper aims to study the reduction of skin friction coefficient (Cf) of a flat plate by injecting micro bubbles onto the surface of the plate. Numerical simulation of flow of water past a flat plate was carried out at different Reynolds numbers. Skin friction coefficient (Cf) was found and compared with research papers and suitable empirical formula. The numerical solution was solved in ANSYS. Subsequently, micro bubbles were injected and the new Cf found. The velocity of injection of the bubbles was then varied. Drag reduction was observed for flat plate post micro bubble injection and injection velocity was varied, corresponding results were noted. The results agree well. This paper verifies effect of micro bubble injection in skin friction drag reduction computationally.

Active Control for Wall Drag Reduction: Methods, Mechanisms and Performance

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.