XFOIL vs CFD performance predictions for high lift low Reynolds number airfoils

Abstract

Blade Element Momentum (BEM) theory is an extensively used technique for calculation of propeller aerodynamic performance. With this method, the airfoil data needs to be as accurate as possible. At the same time, Computational Fluid Dynamics (CFD) is becoming increasingly popular in the design and optimization of devices that depend on aerodynamics. For fixed and rotary wing applications, the airfoil lift over drag coefficient is the dominant airfoil performance parameter. Selecting a suitable computational tool is crucial for the successful design and optimization of this ratio. The XFOIL code, the Shear Stress Transport k−ω$$ turbulence model and a refurbished version of k−kl−ω$$transition model were used to predict the airfoil aerodynamic performance at low Reynolds numbers (around 2.0×10⁵$\mathrm{}{\mathrm{}}^{\mathrm{}}$). It has been shown that the XFOIL code gives the overall best prediction results. Also, it is not clear that CFD turbulence models, even with boundary layer transition detection capability, can compute better airfoil performance predictions data.

Keywords

• XFOIL; 
• Airfoil analysis; 
• k−kl−ω$$ modified transition model; 
• k−ω$$ SST turbulence model

Design of a low Reynolds number airfoil for small horizontal axis wind turbines

Abstract

A low Reynolds number airfoil was designed for applications in small horizontal axis wind turbines to achieve better startup and low wind speed performances. Experiments were performed on the improved airfoil (AF300) in an open circuit wind tunnel at Reynolds numbers of 38,000, 75,000, 128,000 and 205,000. Pressure distributions were obtained over the surface of the airfoil and the lift and drag forces were measured with a dynamometer at different angles of attack, α. A CFD analysis was also performed to get additional information on the flow characteristics. Particle Image Velocimetry (PIV) together with smoke flow visualization were used to study the flow around the airfoil. At the Reynolds numbers of 75,000, 128,000 and 205,000, maximum lift coefficients of 1.72, 1.81 and 1.86 respectively were obtained at the stall angle of 14°. The lift coefficient increased from 0.41 to 1.05 at Re = 38,000 in the α range of 0–18°, in which no stalling was documented. The results from PIV and smoke flow visualization showed that the flow stayed fully attached to the airfoil surface from Re as low as 56,000 at an angle of attack of 8° and maintained a fully attached flow up to 14° angle of attack for Re as low as 75,000.

Keywords

• Horizontal axis wind turbine; 
• Airfoil; 
• Wind tunnel testing; 
• Particle image velocimetry;
• Adverse pressure gradient; 
• Flow characteristics
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Numerical simulation of flow over an airfoil in heavy rain

Numerical simulation of flow over an airfoil in heavy rain via a two-way coupled Eulerian–Lagrangian approach

Highlights

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A new two-way momentum coupled Eulerian–Lagrangian approach is developed.

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Scaling laws are implemented for raindrop particles.

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A random walk dispersion approach is adopted.

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Raindrop impacts, splash-back and formed water film are modelled.

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Airfoil aerodynamic performance degradation in heavy rain is studied.

Abstract

Airfoil performance degradation in heavy rain has attracted many aeronautical researchers’ eyes. In this work, a two-way momentum coupled Eulerian–Lagrangian approach is developed to study the aerodynamic performance of a NACA 0012 airfoil in heavy rain environment. Scaling laws are implemented for raindrop particles. A random walk dispersion approach is adopted to simulate raindrop dispersion due to turbulence in the airflow. Raindrop impacts, splash-back and formed water film are modelled with the use of a thin liquid film model. The steady-state incompressible air flow field and the raindrop trajectory are calculated alternately through a curvilinear body-fitted grid surrounding the airfoil by incorporating an inter-phase momentum coupling term. Our simulation results of aerodynamic force coefficients agree well with the experimental results and show significant aerodynamic penalties at low angles of attack for the airfoil in heavy rain. An about 3° rain-induced increase in stall angle of attack is predicted. The loss of boundary momentum by raindrop splash-back and the effective roughening of the airfoil surface due to an uneven water film are testified to account for the degradation of airfoil aerodynamic efficiency in heavy rain environment.

Keywords

• Multiphase flow; 
• Eulerian–Lagrangian; 
• Airfoil; 
• Heavy rain; 
• Water film; 
• Aerodynamic degradation.

If a pilot jumps from a plane over the Pacific Ocean without a parachute, say 40,000 feet, and lands vertically into the sea, what would happen?

If a pilot jumps from a plane over the Pacific Ocean without a parachute, say 40,000 feet, and lands vertically into the sea, what would happen? Will he be able to survive?

ANSWER:- First of all, if you don't have a protective gear, once thrown off the plane, you will be in -42 degree temperature and very low oxygen as well as very low air pressure. You will instantly feel hypoxia and will become unconscious and because of pressure difference, you may end up rupturing your body. Also -42 degrees without any protection could be outright killer.

Anyway assume that you survive that and drop down to 22000 ft, where air is breathable and you are actually conscious. Then you have some chance of survival. It's now the impact that will kill you and not the fall itself. You have about 2 minutes to the landing from 22000 feet. Instantly turn your chest towards ground and arch your body with hands and legs spread. With that you fill further reduce the speed of fall, now when you are about 300 to 500 ft from impact go full vertical and clinch you butt and cross your legs. If you hit the water any other angle, its like hitting concrete and you will be dead. If you go perfectly vertical, there is 2% chance that you will survive. But you also have to remember to instantly start moving your hand rapidly to reduce the speed of drowning. Remember water pressure double at every 30 ft or 10 meters. If you drown too much, you can simply die because of water pressure.It's a pretty safe bet that the extremely rapid deceleration upon entering the water will fatally scramble one's brains, and other organs, too. If you succeed to stay alive there, your last struggle is to have enough oxygen till you surface. If you manage to do that you should write a book :)

First flight of the Aero L-29 Delfín

This Day in Aviation History April 5th, 1959
First flight of the Aero L-29 Delfín.

The Aero L-29 Delfín (English: Dolphin, NATO reporting name: Maya) is a military jet trainer aircraft that became the standard jet trainer for the air forces of Warsaw Pact nations in the 1960s. It was Czechoslovakia's first locally designed and built jet aircraft.

Source:
Wikipedia, Aero L-29 Delfín:http://gstv.us/1q1wSJE

FLUID MECHANICS PROJECTS

Numerical study of the steady-state uniform flow past a rotating cylinder

Results from the numerical simulation of the two-dimensional incompressible unsteady Navier–Stokes equations for streaming flow past a rotating circular cylinder are presented in this study. The numerical solution of the equations of motion is conducted with a commercial computational fluid dynamics package which discretizes the equations applying the control volume method. The numerical set-up is validated by comparing results for a Reynolds number based on the free stream of Re = 200 and dimensionless peripheral speed of q˜ = 3, 4 and 5 with results from the literature. After the validation stage, various pairs of Re and q˜ are specified in order to carry out the numerical experiments. These values are Re = 200 with q˜ = 4 and 5; Re = 400 with q˜ = 4, 5 and 6, and Re = 1000 with q˜ = 3. In all these cases, gentle convergence to fully developed steady state is reached. From the numerical vorticity distribution, the position of the outer edge of the vortical region is determined as a function of the angular coordinate. This position is found by means of a reasonable criterion set to define the outmost curve around the cylinder where the vorticity magnitude reaches a certain cut-off value. By considering the average value of this profile, a uniform vortical region thickness is specified for every pair of Re and q˜.           

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Inviscid, Incompressible Flow Past Circular Cylinders and Joukowski Airfoils

 Considered steady, 2-D, inviscid, incompressible, adiabatic and irrotational flow, also called potential flow. The influence of gravity will be neglected.

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Lift of a Rotating Circular Cylinder in Unsteady Flows 

A cylinder rotating in steady current experiences a lift known as the Magnus effect. In the present study, the effect of waves on the Magnus effect has been investigated. This situation is experienced with the novel, floating offshore vertical axis wind turbine (VAWT) concept called the DEEPWIND concept, which incorporates a rotating spar buoy and thereby utilizes seawater as a roller bearing. The a priori assumption and the results suggest that the lift in waves, to a first approximation, may be represented by a formulation similar to the well-known Morison formulation. The force coefficients are experimentally found to depend primarily on the ratio between the surface speed of the cylinder and the outer flow velocity. 

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CFD_cylinder

First flight of the Grumman XF5F Skyrocket

April 1st, 1940
First flight of the Grumman XF5F Skyrocket.

The Grumman XF5F Skyrocket was a prototype twin-engined shipboard fighter interceptor. The U. S. Navy ordered one prototype, model number G-34, from Grumman Aircraft Engineering Corporation on 30 June 1938; its designation was XF5F-1. The aircraft had a unique appearance: The forward "nose" of the fuselage did not extend forward of the wing. Provisions were included for two 23 mm (0.906 in) Madsen cannon as armament.

Source:
Wikipedia, Grumman XF5F Skyrocket:http://gstv.us/1MFiJfE