Nomenclature c = chord CD =drag coefficient, D/qA CL =lift coefficient, L/qa CP = pressure coefficient, P-PW /q CM = pitching-moment coefficient, M/qA D =drag, Ib k = reduced frequency, wc/2U L =lift, Ib M =Mach number; pitching moment, ft-lb P = pressure, lb/in q = dynamic pressure, 1 /2p U R, Re, Re, Rn = Reynolds number t time U,V = freestream velocity, ft/s x = distance a = angle of attack, deg f =nondimensional chord length, x/c $tr transition location co = rotational frequency, rad/s p = density, lb/ft A = sweep angle, deg

Progress in analysis and prediction of dynamic stall

Results are presented of measurements on turbulent round jets of air and of helium of the same nozzle momentum efflux, using, for the air jets, x-wire hot-wire probes mounted on a moving shuttle and, for He jets, a composite probe consisting of an interference probe of the Way-Libby type and an x-probe. Current models for scalar triple moments were evaluated. It was found that the performance of the model termed the Full model, which includes all terms except advection, was very good for both the air and the He jets.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112093000096

Turbulence measurements in axisymmetric jets of air and helium. I - Air jet. II - Helium jet

Turbulence measurements in axisymmetric jets of air and helium

Nonreacting turbulent mixing flows

The compressible dynamic stall flowfield over a NACA 0012 airfoil transiently pitching from 0 to 60 deg at a constant rate under compressible flow conditions has been studied using real-time interferometry. A quantitative description of the overall flowfield, including the finer details of dynamic stall vortex formation, growth, and the concomitant changes in the airfoil pressure distribution, has been provided by analyzing the interferograms. For Mach numbers above 0.4, small multiple shocks appear near the leading edge and are present through the initial stages of dynamic stall. Dynamic stall was found to occur coincidentally with the bursting of the separation bubble over the airfoil. Compressibility was found to confine the dynamic stall vortical structure closer to the airfoil surface. The measurements show that the peak suction pressure coefficient drops with increasing freestream Mach number, and also it lags the steady flow values at any given angle of attack. As the dynamic stall vortex is shed, an anti-clockwise vortex is induced near the trailing edge, which actively interacts with the post-stall flow.

/pdf/interferometric-investigations-of-compressible-dynamic-stall-1ulexj3yuy.pdf

Interferometric investigations of compressible dynamic stall over a transiently pitching airfoil

The present paper discusses the transient aerodynamic characteristics of a two-dimensional low-speed airfoil whose angle of attack is varied impulsively. The study is mainly an experimental one with observations made of the three dynamic loads, the static pressure distribution, and flow on the airfoil surface, following the airfoil motion. The changes in the characteristics and their aerodynamic causes are investigated in terms of the ultimate angle of attack and the rise time. foil other than the two-dimensional motion to be discussed in this paper. For instance, Refs. 7-10 deal with the effects of variations in the flow velocity and in the angle of attack. There are other studies11'12 that have analytically pursued the rela- tionship between dynamic stall and the ambient flowfield. Furthermore, little is known about the effects of Reynolds number, Mach number, airfoil profile, and airfoil motion.13 At this stage of information it is natural that the technology should be based on experimentation. In the present paper, in- stead of a conventional study of an oscillating airfoil, which has been often undertaken, the relations between the transient aerodynamic characteristics of an airfoil with stepwise varied angle of attack and the state of the ambient airflow are ex- perimentally investigated as a basic study leading to applica- tion. Another reason for adopting a stepwise varied input is the desirability of obtaining a simple motion for understand- ing the phenomena in a flow with prominent nonlinearity and hysteresis.

Transient Aerodynamic Characteristics of a Two-Dimensional Airfoil During Stepwise Incidence Variation

In a round helium jet with a small nozzle, and a Reynolds number of 2.95 × 103, with and without an air stream, mean profiles, instantaneous fluctuations, and correlations of velocity and matter are investigated using the hot‐wire technique. The characteristic difference in the behavior of the jet due to the presence of an air stream is extensively studied and summarized as follows: (1) Similarity in mean profiles is not reached at least in terms of r/z and the spreading and decay of the jet follow some slower laws, and (2) lateral turbulent diffusion is reduced while active axial turbulent diffusion is induced in the middle of the distribution of the jet. The correlations between fluctuations are established as a whole, but those between concentration and velocity fluctuations are reduced in the lateral direction. It seems that the turbulent motion of large eddies plays an important role in the transport mechanism of the flow field.

Effects of an air stream on turbulent diffusion of a helium jet from a small nozzle

Micro Air Vehicles (MAVs) are very small and fly at low speeds. Therefore, their wings are flat plates or membranes because the Reynolds number is very low. For MAVs, a flapping wing is proposed to generate lift and thrust simultaneously like birds and insects. In this study, the Immersed Boundary Method with the original ideas is used to analyze the airflow around stationary, heaving and feathering flat-plate numerically. The advantages of this method are easiness of the grid generation and less amount of calculation. The grid is not body-fitted but rectilinear. And the plate is expressed by the external force. The computational results are compared with the analytical solutions and the experimental photo. In the stationary plate, the drag coefficient and the velocity distribution agree with the Blasius solution. In the heaving plate, the lift and moment coefficients are close to the analytical solution with the Theodorsen function. And the pressure contours and the streamlines show the plate exists in the grid. In the feathering plate, the vorticity distribution and the experimental result show the similar trend.

/pdf/numerical-analysis-of-flapping-wing-36oprbmgnv.pdf

Numerical Analysis of Flapping Wing

Heat transfer from a circular cylinder in a rarefied gas stream.

The present paper discussesthe transient aerodynamic characteristics of a twodimensional low-speed wing whose angle of attack is varied impulsively. The study is mainly an experimental one with observations made of the three dynamic loads, the static pressure distribution and flow on the wing surface, following khe wing motion. The changesin the characteristics and their aerodynamic causes are investigated in terms of the ultimate angle of attack and the rise time.

Hisao Koyama

Papers

Transient Aerodynamic Characteristics of a Two-Dimensional Airfoil During Stepwise Incidence Variation

Effects of an air stream on turbulent diffusion of a helium jet from a small nozzle

Numerical Analysis of Flapping Wing

Heat transfer from a circular cylinder in a rarefied gas stream.

Transient aerodynamic characteristics of a two-dimensional 84-2076 low-speed wing at several angles of attack