Azemi Benaissa

Scaling range of velocity and passive scalar spectra in grid turbulence

Isotropic velocity and scalar fluctuations are closely approximated by slightly stretching a heated grid flow through a short (1.36:1) contraction. The heating is such that temperature serves as a passive scalar, and the velocity/scalar time scale ratio is about one. At small values of Taylor microscale Reynolds number (10 102) indicate that, to obtain a 5/3 scaling range, Rλ must exceed 103. The ratio (5/3 + mu)/mθ is approximately 2, in close conformity with the proposal of Danaila and...

Correction of cold-wire response for mean temperature dissipation rate measurements

Abstract This work is aimed at improving dissipation rate measurements using cold-wire anemometry. The mean dissipation rate of the temperature variance is measured on the axis of a heated turbulent round jet. Measurements are performed with a constant current anemometer (CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose uses a current injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the time constants obtained for two wire diameters ( d =1.2 and 0.58 μm) compare well with those measured at the same time using two other methods (laser excitation and pulsed wire method). Moreover, for these two wires, the estimated time constants are in good agreement with those obtained from theory. In the second part of the paper, a compensation procedure, involving post-processing filtering, is developed in order to improve the frequency response of the cold-wire probes. Measurements on the jet axis (Re D =16 500 , Re λ =167) show that the frequency response of the 1.2 μm wire is indeed significantly improved after compensation. The spectrum of the compensated signal for the 1.2 μm wire compares fairly well with that for the 0.58 μm wire. The results also indicate that the compensation should be applied when the cut-off frequency of the cold-wire f C is less than about 2 f K , where f K is the Kolmogorov frequency. When f C ≈0.6 f K , the compensation can reduce the error in the estimated mean dissipation rate by more than 20%. When f C =2 f K , the reduction is about 5%.

Effect of Freestream Turbulence on Airfoil Limit-Cycle Oscillations at Transitional Reynolds Numbers

Numerical simulations are performed to study the effect of freestream turbulence on small-amplitude limit-cycle oscillations of an airfoil at transitional Reynolds numbers. A one-degree-of-freedom aeroelastic model was coupled with the National Research Council Canada in-house computational-fluid-dynamics code INSflow to perform unsteady Reynolds-averaged Navier–Stokes simulations for flows around a rigid NACA 0012 airfoil in free-to-rotate conditions. Without coupling a transition model, unsteady Reynolds-averaged Navier–Stokes computations based on the commonly used shear-stress-transport turbulence model could not capture the limit-cycle oscillations. This was expected because it had been previously shown that the limit-cycle oscillations were fed by negative aerodynamic damping due to laminar boundary-layer separation. A correlation-based transition model was then implemented in the code and applied to investigate the turbulence effects. The computed results confirmed qualitatively the experimental ob...

Aerodynamic and Aeroacoustic Performance of a Skewed Rotor

This paper presents an experimental investigation and numerical simulation of the aerodynamic and aeroacoustic performance of an axial-flow fan with skewed rotating blades in the design and off-design operation. The blade is designed with a forward skew angle for which the stacking line is directed towards the rotating direction on the circumferential section. A detailed investigation of a three-dimensional flow field in the inter-blade row and passage using five-hole probes and a hot-wire anemometer at the upstream and downstream locations of the rotors has been carried out and compared with a fan with unskewed rotor blades. Noise testing was performed in the anechoic chamber. The experiments were performed at three rotating speeds. Aerodynamic curves show that the performance of the skewed blade increased at a higher pressure rise of 13.1% and gave a larger flow rate of about 5% and a higher efficiency of more than 3%. The higher efficiency in the skewed rotor was due to the practical and advantageous spanwise redistribution of aerodynamic parameters, a greater boundary movement into the main flow, a secondary flow reduction and the thinness of the rotor wake. Aeroacoustic performance and frequency spectra in almost the whole frequency domain showed a noise reduction of 2 to 4 dBA in the skewed fan. Lower noise in the skewed blade comes from the broadband noise reduction owing to a thinner wake layer, a phase difference in rotor radiation and tip leakage noise reduction. A wider stall margin for more than 20% is obtained in the skewed blade due to the proportional distribution of aerodynamic parameters. The three-dimensional Navier-Stokes approach is simulated in the inner blade flow.Copyright

Aeroelastic Dynamics of a NACA 0012 Airfoil in the Transitional Reynolds Number Regime

The work discussed herein is a focused extension of a series of studies that were carried out at the Aeroelasticity Laboratory of the Royal Military College of Canada in recent years. Initial work revealed the presence of self-excited oscillations over certain ranges of airspeed when a NACA 0012 airfoil was immersed in the laboratory’s wind tunnel and allowed to oscillate freely in both pitch and heave. The range of airspeeds tested corresponded to Reynolds numbers in the low-to-moderate regime. While the aeroelastic apparatus is capable of two-degrees-of-freedom motion, the present work concerns only the motion of the airfoil when it is constrained to rotate in pure pitch. A parametric investigation is presently being undertaken to more fully comprehend the airfoil’s pitch behaviour, specifically the amplitude and frequency of its oscillations which are observed in the following range of chord based Reynolds numbers: 5.0 × 104 ≤ Rec ≤ 1.2 × 105 . This paper focuses on the effect of the stiffness of the springs used in the apparatus. Other parameters such as surface roughness, turbulence intensity, temperature and initial conditions are also briefly discussed. In conjunction with the pitch oscillation measurements, preliminary results reveal vortices to be present in the wake. In an attempt to determine the frequency and character of these flow structures, as well as to understand the relationship between the airfoil motion and wake dynamics, hot-wire anemometry measurements have been performed.Copyright

Non-stationary signal analysis of the von Kármán vortex shedding in the wake of a fluttering airfoil

Stationary data lend themselves well to the Fourier decomposition into harmonic components. Conversely, spectral characteristics of non-stationary data vary with time, and hence do not generally admit the application of Fourier transform. In order to investigate the localized time-frequency characteristics of non-stationary data, the notions of instantaneous frequency and amplitude are invoked. These concepts are applied to the von Karman vortex shedding observed in the wake of a self-sustained pitching airfoil. For this range of Reynolds numbers (104 – 105 ), it has been reported that at any given airspeed the shedding frequency of the vortex street varies with angle of attack (AOA), ranging from the Strouhal number St ≈ 0.6 at zero AOA and tending to St ≈ 0.1 for high AOA. For the pitching motion, which originates from a positive energy transfer from the flow to the airfoil due to negative aerodynamic damping, the von Karman vortex shedding frequency varies with pitch angle hence with time. Hilbert transform provides a robust estimate of instantaneous frequency through the definition of analytic signals. However, Hilbert transform provides meaningful instantaneous frequency for only monocomponent signals. To overcome this difficulty, the Hilbert-Huang transform is commonly exploited. In this paper, both the Hilbert and Hilbert-Huang transforms are applied in order to capture the instantaneous vortex shedding frequency. For multicomponent signals Empirical Mode Decomposition (EMD) splits the signal to monocomponent signals, namely Intrinsic Mode Functions, through a so-called sifting process. Application of Hilbert transform to these functions produces instantaneous frequencies and amplitudes. Therefore the time-frequency-amplitude representation of the signal appears to be a promising tool for obtaining more physical insight into the time-varying vortex shedding frequency in the wake of a pitching airfoil.Copyright