Development of a compact alpha and beta camera for dismantlement applications

Journal of Radioanalytical and Nuclear Chemistry - A new method has been developed to image surface contamination on-site by short range radiation emitters (such as alpha or beta particle...     

Active control of the aeroacoustics of cavity flows from the downstream edge

Active control of the aeroacoustics of flow-induced oscillations in a rectangular cavity using a vibrating plate inserted at the downstream edge is demonstrated. Pressure and phase-locked Particle Image Velocimetry measurements show that the oscillations of the mixing layer can be controlled in order to attenuate or amplify the radiated sound. Results obtained from a previously developed analytical model are used to characterize the effects of the control. To cite this article: L. Chatellier et al., C. R. Mecanique 334 (2006).     

Effect of periodic aerodynamic pulsation on flow over a confined butterfly valve

 An experimental investigation of the flowfield characteristics of a butterfly valve under periodic flow has been made. The results concern a valve at large angles corresponding to high area contraction ratios (K>0.3). In steady flow, the results show that the flowfield within the valve is conditioned by the internal jet formed in the trailing edge fluid area. For very high area contraction ratios (K>0.65), the equivalent diameter of that fluid area is the preponderant length scale of the flow. In periodic flow, an increase in the length scale of flow instabilities is observed. The reorganization length of the flow is, thus, shorter, producing a marked reduction in valve head-loss. This phenomen is maximum when the excitation frequency is close to the nominal instabilities frequency (found in steady flow). Received: 14 August 1997/Accepted: 8 April 1998     

Unsteady near wake of a flat disk normal to a wall

The unsteady wake of a flat disk (diameter D) located at a distance of H from a flat plate has been experimentally investigated at a Reynolds number Re _D  = 1.3 × 10⁵. Tests have been performed for a range of gap ratio (H/D), spanning from 0.3 to 1.75. The leading edge of the flat plate is either streamlined (elliptical) or blunt (square). These configurations have been studied with PIV, high speed PIV and multi-arrayed off-set fluctuating pressure measurements. The results show a progressive increase of the complexity of the flow and of the interaction as the gap ratio decreases. For large values of H/D (1.75), the interaction is weak and the power spectral densities (PSD) exhibit a strong peak associated with the vortex shedding events (St = 0.131) – St = fD/U _∞ is the Strouhal number. For lower values of H/D (0.75), the magnitude of the wall fluctuating pressure increases significantly. A large band contribution is associated with the unsteady wake structure and turbulence. A slight increase of the shedding frequency (St = 0.145) is observed. A critical value of the gap ratio (about 0.35) has been determined. Below this critical value, a three-dimensional separated region is observed and the natural vortex shedding process is very strongly altered. These changes induce a great modification of the fluctuating pressure at the wall. Each interaction reacts in a different way to perturbed upstream conditions. In particular, the disk is an overwhelming perturbation for the lowest H/D value studied here and the relative influence of the upstream turbulence on the wall fluctuating pressure below the near wake region is moderate.     

Theoretical and experimental investigations of low Mach number turbulent cavity flows

Theoretical and experimental investigations are conducted for rectangular cavities of varying sizes in low Mach number turbulent flows. Emphasis is put on the characterization of the generation of self-sustained oscillations in order to develop methods of active control applied to the aeroacoustics of cavity flows. A linearized stability analysis for low Mach number flows is proposed in which the interface of the cavity is modeled by a vorticity layer. Subsequently, the cavity flow is investigated experimentally in a subsonic wind tunnel, using pressure measurements and a phase-locked particle image velocimetry system. Experimental results indicate that the oscillation process is governed by convective waves, with no definite influence of convected vortical structures. The good agreement between the experimental data and the predictions given by the model allows the identification of the oscillations of the cavity interface via the parameters issued from the theoretical analysis.List of symbols c speed of sound, m/s - f frequency, Hz - G Greens function - h displacement of the vorticity layer, m - K_R Rayleigh conductivity of the aperture, m - k₀ acoustic wavenumber, rad/m - k,n, Rossiter formula parameters - M Mach number of the freestream - pressure, Pa - Q volume flux, m³/s - Re_L Reynolds number Re_L=UL/ - Re_x1 Reynolds number Re_x1=Ux₁/ - Re Reynolds number Re=U/ - S frequency based Strouhal number - T period, s - t time, s - U,U,U^± freestream velocity, m/s - v velocity, m/s - W,L,D model cavity dimensions, m - w,l,d analytical cavity dimensions, m - x,y Cartesian coordinates, m - boundary-layer thickness, m - vorticity thickness, m - * boundary-layer displacement thickness, m - ,^± velocity potential, m²/s - acoustic wavelength, m - kinematic viscosity, m²/s - boundary-layer momentum thickness, m - ₀ density, kg/m³ - pulsation based Strouhal number - angular frequency, rad/s - vorticity, s ^–1 - , non-dimensional coordinates x₁,y₁ - non-dimensional displacement h