Reduction In Lift Coefficient Research Articles

Deep reinforcement learning-based active flow control of an elliptical cylinder: Transitioning from an elliptical cylinder to a circular cylinder and a flat plate

We study the adaptability of deep reinforcement learning (DRL)-based active flow control (AFC) technology for bluff body flows with complex geometries. It is extended from a cylinder with an aspect ratio Ar = 1 to a flat elliptical cylinder with Ar = 2, slender elliptical cylinders with Ar less than 1, and a flat plate with Ar = 0. We utilize the Proximal Policy Optimization (PPO) algorithm to precisely control the mass flow rates of synthetic jets located on the upper and lower surfaces of a cylinder to achieve reduction in drag, minimization of lift, and suppression of vortex shedding. Our research findings indicate that, for elliptical cylinders with Ar between 1.75 and 0.75, the reduction in drag coefficient ranges from 0.9% to 15.7%, and the reduction in lift coefficient ranges from 95.2% to 99.7%. The DRL-based control strategy not only significantly reduces lift and drag, but also completely suppresses vortex shedding while using less than 1% of external excitation energy, demonstrating its efficiency and energy-saving capabilities. Additionally, for Ar from 0.5 to 0, the reduction in drag coefficient ranges from 26.9% to 43.6%, and the reduction in lift coefficient from 50.2% to 68.0%. This reflects the control strategy's significant reduction in both drag and lift coefficients, while also alleviating vortex shedding. The interaction and nonlinear development of vortices in the wake of elliptical cylinders lead to complex flow instability, and DRL-based AFC technology shows adaptability and potential in addressing flow control problems for this type of bluff body flow.

On the conditions for absolute minimum fuel burn for turbofan powered, civil transport aircraft and a simple model for wave drag

Abstract In a recent series of papers, Poll and Schumann have been developing a simple model for estimating fuel burn for turbofan powered, civil transport aircraft for a given mass, Mach number and flight level and in a specified ambient temperature profile for all phases of flight. This paper focuses upon the combination of Mach number and flight level at which an aircraft cruises with the absolute minimum fuel burn. For a given aircraft type, the information necessary to determine these conditions must be specified and this poses a challenge. An initial attempt to obtain these data has been described previously by the first author. In this paper, the optimum conditions are found using a completely different approach. Starting from first principles and using established theory, the equations governing the situation where engine overall efficiency and airframe lift-to-drag ratio both have local maxima at the same flight condition are developed. This special case is termed the “design optimum” condition and, for a specified aircraft mass and a specified atmospheric temperature versus pressure profile, it gives the lowest possible fuel burn for any aircraft and engine combination. The design optimum occurs at a particular Mach number and Reynolds number, and it is a fixed characteristic of the aircraft. The analysis reveals the significance of Reynolds number variations, wave drag, including its derivatives with respect to both lift coefficient and Mach number, and the atmospheric properties. Whilst wave drag is notoriously difficult to determine accurately, it is found that solutions to the equations are not particularly sensitive to the accuracy of this quantity. Consequently, a simple, physically realistic model can give good results. An appropriate model is developed and a complete, approximate solution is obtained. Taking the International Standard Atmosphere as the design atmosphere, results are presented for the 53 aircraft types previously considered by Poll and Schumann. Relative to the design optimum conditions, when Reynolds number is constant and wave drag is zero, compressibility alone reduces L/D by about 5%, reduces lift coefficient by about 1.5% and increases drag coefficient by about 3.5%. Reynolds number variation has little effect upon L/D, but it reduces lift coefficient and drag coefficient by a further 7% and 8% respectively. The reduction in lift coefficient has a significant impact on the optimum cruise flight level. In general, an aircraft’s operating optimum will not coincide with its design optimum, but deviations are expected to be small. Therefore, using the design optimum solution as a reference point, an improved version of the operating optimum estimation method described by Poll and Schumann in previous work is developed. This allows the estimation of the conditions for absolute minimum fuel burn for an aircraft of given mass flying thorough any atmosphere. Updated coefficients for the 53 aircraft types are given.

DESIGN, EXPERIMENTAL STUDIES, AND CFD INVESTIGATIONS OF BIO-INSPIRED SLOTTED REAR DIFFUSER ATTACHMENT ON CAR MODEL FOR ENHANCING AERODYNAMIC PERFORMANCE

The performance enhancement of vehicle design is still a challenging phenomenon for technologists. A suitable pathway for performance enrichment is naturally available in living species. One adaptable design solution arrived through the penguin microbubble concept, which was used to reduce drag. The present work attempts to design rear-slotted diffuser attachments on car models to reduce drag force through the diversion of air. The drag reduction enhances the vehicle’s aerodynamic performance and reduces lift force to increase stability. Normally, high-speed vehicle performance and stability are determined by aerodynamic forces exerted on the body surface. The higher level of drag and lift forces leads to more fuel utilization and, in the case of electric vehicles, more power consumption. The drag force on the vehicle is mainly due to the formation of the wake region behind the body in the downstream region. The above forces are reduced using a rearward-slotted diffuser at the rear end underneath the vehicle. The aerodynamic performance of the vehicle models was estimated through wind tunnel experiments and numerical simulation for various wind speeds. The experimental work was conducted on selected car models with and without rearward slotted diffusers to compare the forces and performances of vehicle models. Interestingly, increased pressure was observed downstream of the vehicle while providing a slotted diffuser, consequently diminishing the wake area behind the vehicle. The drag and lift coefficient reduction were attained as 4.74% and 11.02% at 50 m/s (180 km/hr) on the chosen vehicle model. Also, a significant reduction in fuel consumption was calculated at higher speeds while attaching a slotted diffuser at the rear end of the car.

An efficient method to assess effect of fin on the course stability of towing system

Some new-built ship-shaped offshore structures like FPSO or FPU are found to show insufficient course stability due to rough design, which makes the towing operation hazardous. This paper aims at investigating the stern fin as a proposed solution for improving the poor course stability of a FPSO in towing operation by creating a practical approach. The fin’s hydrodynamic lift coefficient is predicted by the vortex lattice method. The numerical towing system is of three degrees of freedoms and accounts for hydrodynamic forces, the interactions acting on the hull and other appendages, as well as the towing line. Various hydrodynamic derivatives are considered linear and estimated by the empirical method. The configurations of single fin, double fins, and twin fins, as well as the influence on towing stability, are studied. In particular, the hydrodynamic interaction of twin fins, lift coefficient reduction and the impact of spacing are analysed. Different stable and unstable behaviours in the time domain are observed and discussed in depth. The eigenvalue analysis is conducted based on the linearized towing system.

Wind Tunnel Experiment of UTM-LST Generic Light Aircraft Model with External Store

This paper discusses the impact of the external store on the aerodynamic performance of the light aircraft model in the subsonic region. Light aircrafts are commonly used for pilots training, survey, leisure and transportation. To date, there have been a lot of small aircrafts used for strategic purposes where an external store, either external fuel storage or armament, has been installed on its wing. Examples of such aircraft are KAI-KA1, A29 Super Tucano, and Beechcraft AT-6. Therefore, it is important to study the effect of this external store installation on the aerodynamic characteristics of a small aircraft. An available light aircraft model of UTM Low speed wind tunnel (UTM-LST) has been modified so that a generic external store can be mounted on the lower surface of the wing. Two set of experiments were carried out on the model which were; experimental with an external store followed by experimental without external store as a benchmark of tested configuration. The experiments were conducted at two different speeds of 26 and 39 m / s that correspond to Reynolds numbers 0.4 × 106 and 0.6 × 106respectively. Three measurement techniques were employed on each configuration. The first measurement was the 6 component forces and moments measurement technique. The second technique was the pressure measurement on the wing, and the final test was the tufts flow visualization. The result of steady balance indicated that the external store has no effect on the coefficient of lift at low attack angle. However, it showed that there was a reduction of lift coefficient by 2% at higher angle of attack. The data showed that the coefficient of drag increases by 4% when the external is installed. Surprisingly, the installation of the store has insignificant effects on the pitching moment coefficient. An interesting feature observed from surface pressure studies where, the results showed that the pressure coefficient increased when the external is mounted on the wing at a low angle of attack. Such changes, however, do not occur at high angle of attack.

Effects of blade design on ice accretion for horizontal axis wind turbines

This paper presents a numerical study to predict ice loads on a wind turbine blade section at 80% of blade span. Air flow, droplet impingement and ice accretion simulations are conducted in FENSAP ICE. The effect of low and high liquid water content (LWC) conditions on blade thickness is presented. All NREL airfoil families used for HAWTs are examined to study blade design's effect on ice accretion. Ice shapes are numerically predicted at different atmospheric temperatures and LWC conditions. The degradation in aerodynamic characteristics due to ice formation is predicted. The numerical predictions suggest a 10%–65% reduction in lift coefficient due to ice accretion, which is highly dependent on the accreted ice shape.

Impact of a Reverse Delta Type Add-on Device on the Flap-tip Vortex of a Wing

The effect of interactions of vortices produced by an outboard flap-tip of a half-span wing (NACA 23012 in landing configuration) and a slender reverse delta type add-on device, placed in the proximity of the outboard flap-tip, on the upper surface of the half-span wing is investigated using Particle Image Velocimetry in a closed loop low speed wind tunnel. Specifically the characteristics of the vortex interactions generated downstream in planes perpendicular to the free stream direction and their dependence on angles of attack at a chord-based Reynolds number of Rec=2.75×105 have been determined. It was found that the add-on device significantly reduces the tangential velocity magnitude and enlarges the vortex core of the resultant vortex by up to 36%. The aerodynamic performance of the half-span wing model was marginally affected by the use of a reverse delta type add-on device. The reduction in lift coefficient is 3.8% and the increase in drag coefficient is 14.9%.

Experiments on Rigid Wing Undergoing Hover-Capable Flapping Kinematics at Micro-Air-Vehicle-Scale Reynolds Numbers

The paper focuses on understanding the mechanism of force production on a hover-capable flapping-wing system by using a combination of direct force measurements and flowfield studies. The experiments were conducted in air at a Reynolds number of approximately 25,000 (based on mean chord and maximum tip speed), which is the typical operating regime of small flapping-wing micro air vehicles. The forces and moments were measured using a miniature six-component force transducer installed at the wing root. The wing was flapped in air and vacuum at the same frequency and wing kinematics, and the vacuum forces were subtracted from the total forces to obtain the pure aerodynamic forces. Flow visualization and particle image velocimetry were used to characterize the formation, strength, and structure of the leading-edge vortex on the flapping wing. A rapid increase in wing lift coefficient was associated with the growth of the leading-edge vortex and a progressive reduction in lift coefficient with the convection of the leading-edge vortex over the chord. The leading-edge vortex was observed to be stable for the spanwise locations closer to the root (i.e., the 25% span location) and burst for locations away from the root (50 and 75% span locations) just after the midstroke. High instantaneous lift coefficient values () were measured during the translational phase, clearly showing the role of leading-edge vortex in augmenting lift. However, lift-to-drag ratios were always less than 1. Aerodynamic efficiency during the translational phase was quantified in terms of figure of merit, which improved with decreasing translational wing pitch angle. The maximum figure of merit value measured was 0.36 at 40 deg translational pitch angle.

Aerodynamic Effects of Anti-Icing Fluids on a Thin High-Performance Wing Section

The Federal Aviation Administration has worked with Transport Canada and others to develop allowance times for aircraft operating in ice-pellet precipitation based upon wind-tunnel experiments with a thin high-performance wing. These allowance times are applicable to many different airplanes. Therefore, the aim of this work is to characterize the aerodynamic behavior of the wing section in order to better understand the adverse aerodynamic effects of anti-icing fluids and ice-pellet contamination. Aerodynamic performance tests, boundary-layer surveys, and flow visualization were conducted at a Reynolds number of approximately and a Mach number of 0.12. Roughness and leading-edge flow disturbances were employed to simulate the aerodynamic impact of the anti-icing fluids and contamination. In the linear portion of the lift curve, the primary aerodynamic effect is the thickening of the downstream boundary layer due to the accumulation of fluid and contamination. This causes a reduction in lift coefficient and an increase in pitching moment (nose up) due to an effective decambering of the wing. The stalling characteristics of the wing with fluid and contamination appear to be driven at least partially by the effects of a secondary wave of fluid that forms near the leading edge as the wing is rotated in the simulated takeoff profile. These results have provided a much more complete understanding of the adverse aerodynamic effects of anti-icing fluids and ice-pellet contamination on this wing.

Numerical Simulation of Laminar Flow over Slotted Airfoil

In this paper mainly focused effect of fixed slot on performance of NACA0012 airfoil. Using slot lower surface high pressurized air passes through slot to energized upper surface. Analysis has been done on NACA0012 of 1m cord length at 25C of air with 5m/s air velocity. First plain airfoil at different angle of attack has been analyzed to find out stall condition and Cl, Cd . After that same parameter and physics, only change in geometry of airfoil with 15%C leading edge slot. Flow separation is adverse effect for performance of airfoil. Due to flow separation adverse pressure gradient effect reverse flow is there, we cause reduction of lift coefficient and increment of drag coefficient so flow separation necessary to reduce. There are different techniques to reduce flow separation but effect of slot on airfoil has been studied. Keyword: Angle of attack (AOA), plain airfoil, slotted airfoil, SST model,

Numerical Investigation of Flow past a Wavy Airfoil

The effect of the wavy surface on the aerodynamic characteristics of an airfoil is studied using the large eddy simulations. A more gentle lift characteristic is obtained during stall. For angles of attack less than the baseline stall angle of a NACA0012 airfoil, a lift coefficient reduction was observed for the wavy airfoils, while the lift coefficient increases up to 23% greater than that of a NACA0012 airfoil when the angle of attack is larger than the baseline stall angle of the NACA0012 airfoil.

Preliminary Assessment of Wind Turbine Blade Lift Control via Plasma Actuation

This paper presents an experimental and computational study to assess the potential of plasma actuators to alter the lift of airfoils in view of controlling the output power of wind turbines under high wind conditions. Plasma actuators are electro-fluidic devices that create a jet adjacent to the surface. The proposed concept involves installing an actuator on the blade suction surface pointing in the upstream direction to induce and control boundary layer separation. Experiments have been carried in a subsonic wind tunnel, at angles of attack (AOA) in the range 0–15°, for velocities of 12.60 and 16.30 m/s, on a 6-inch chord untwisted 2–D blade. The aerodynamic balance force measurements show a reduction of lift coefficient for all the AOA. Flow fields obtained by PIV demonstrate that depending of the AOA, the actuator thickens the boundary layer, induces or amplifies its separation. The CFD simulations reproduce the experimental trends. Thus, plasma actuators are potentially efficient devices for lift control over a wind turbine blade.

A comprehensive numerical study of battle damage and repairs upon the aerodynamic characteristics of an aerofoil

AbstractA NACA 641-412 aerofoil with circle and star damage and also three repair configurations has been numerically investigated. Two different methods of mesh generation were employed: multi structured mesh for the star damaged aerofoil and unstructured mesh for the other aerofoils. The results show that the damage will cause a reduction in lift coefficient of the aerofoil and also a different stall angle relative to that of the undamaged aerofoil. Each kind of repair improves the aerodynamic characteristics of the aerofoil considerably. The flow Field inside the damage hole and the cavity caused by the repair sheets was also investigated. Finally, the numerical solution was qualitatively and quantitatively validated using the available experimental results.

Finite element analysis of flow control using porous media

A finite element model is established for simulating flow in and out a porous media. The extended Darcy equation inside the porous media and the Navier–Stokes equations in the fluid are coupled via the continuity condition at the interface between the two media. The model is firstly validated against the analytical and the numerical results available in literature. Then it is applied to simulate flow past a circular cylinder covered by a porous layer. The effect of the porous layer on the reduction of lift coefficient is investigated numerically. It is found that the lift reduction can be achieved by properly choosing the porous material. However, the amount of reduction greatly depends on the Reynolds number, the permeability and the Forchheimer coefficient.