NACA Advanced Restricted Reports

Report presenting tests made in the 7- by 10-foot tunnel to determine the effect of direction of propeller rotation on the static longitudinal and lateral stability characteristics of the scale model of the North American B-28 airplanes. The results of the present investigation indicate that the mode of propeller operation has a considerable effect upon lateral and longitudinal stability and may have an effect upon propulsive efficiency

NACA Advanced Restricted Reports

Report presenting an investigation in the NACA 7- by 10-foot wind tunnel of several arrangements of a plug-type, spoiler-slot aileron on an NACA 23012 airfoil with a full-span slotted flap. The results indicated that a plug-type, spoiler-slot aileron probably has negligible drag in the neutral position and will provide satisfactory lateral control for airplanes equipped with full-span slotted flaps

Turbulence spectrum of a strongly conductive temperature field in a rapidly stirred fluid

In an earlier paper [Phys. Fluids 31, 2065 (1988)], a numerical simulation of a passive scalar field converted by a frozen velocity field, i.e., a velocity field with an infinite correlation time, was performed. In this paper, a simulation of a passive scalar field converted by a velocity field which is rapidly stirred at all scales of motion, i.e., a velocity field with near zero correlation time, is performed. For an energy spectrum of the form E(k) ∞k -5/3, the temperature spectrum G(k) is found to obey G(k) ∞k-11/3 when conductive effects are dominant. A theoretical model is proposed which obtains the above result by representing the transfer of scalar variance by an eddy conductivity, whose correlation time is limited by the correlation time of the velocity field. © 1989 American Institute of Physics

Coherent vortex extraction in three-dimensional homogeneous turbulence: Comparison between CVS-wavelet and POD-Fourier decompositions

International audienceThe coherent vortex simulation (CVS) decomposes each realization of a turbulent flow into two orthogonal components: An organized coherent flow and a random incoherent flow. They both contribute to all scales in the inertial range, but exhibit different statistical behaviors. The CVS decomposition is based on the nonlinear filtering of the vorticity field, projected onto an orthonormal wavelet basis made of compactly supported functions, and the computation of the induced velocity field using Biot-Savart's relation. We apply it to a three-dimensional homogeneous isotropic turbulent flow with a Taylor microscale Reynolds number Rλ=168, computed by direct numerical simulation at resolution N=2563. Only 2.9%N wavelet modes correspond to the coherent flow made of vortex tubes, which contribute 99% of energy and 79% of enstrophy, and exhibit the same k-5/3 energy spectrum as the total flow. The remaining 97.1%N wavelet modes correspond to a incoherent random flow which is structureless, has an equipartition energy spectrum, and a Gaussian velocity probability distribution function (PDF). For the same flow and the same compression rate, the proper orthogonal decomposition (POD), which in this statistically homogeneous case degenerates into the Fourier basis, decomposes each flow realization into large scale and small scale flows, in a way similar to large eddy simulation (LES) filtering. It is shown that the large scale flow thus obtained does not extract the vortex tubes equally well as the coherent flow resulting from the CVS decomposition. Moreover, the small scale flow still contains coherent structures, and its velocity PDF is stretched exponential, while the incoherent flow is structureless, decorrelated, and its velocity PDF is Gaussian. Thus, modeling the effect of the incoherent flow discarded by CVS-wavelet shall be easier than modeling the effect of the small scale flow discarded by POD-Fourier or LES