Kelvin-Helmholtz instability in the presence of variable viscosity for mudflow resuspension in estuaries

The temporal stability of a parallel shear flow of miscible fluid layers of dif- ferent density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newto- nian fluid mixing can be viewed as a simplified model of the behavior of mud- flow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control pa- rameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin-Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity strat- ification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influ- ence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflec- tion point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities

Peroxisome proliferator-activated receptor alpha (PPAR alpha) activators induce hepatic farnesyl diphosphate synthase gene expression in rodents

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Jet-flat plate interaction: Wall pressure coherence modeling

In this paper the wall pressure fluctuations induced by a compressible jet overflowing a flat plate were analysed with the scope of providing an analytical prediction of the two-points coherence. The reported approach follows the idea formulated by Efimtsov to model the coherence at relatively high Mach numbers and with a good accuracy in the low wave-numbers region. The database used for the model assessment refers to an experimental campaign carried out into the semi-anechoic chamber of the Roma Tre University using pressure transducers flush mounted at the wall of a rigid flat plate. The jet Mach number was varied within the compressible subsonic regime up to 0.9 and two different nozzle exhaust diameters were considered to analyse the effect of the Reynolds number. The tangential flat plate was installed at 0.75 diameters from the jet axis, a position that reproduces well a realistic jet-wing configuration. The ability of the model to predict the coherence function is verified for the whole range of conditions investigated and the achieved results are discussed with reference to the available literature

The Method of the Pseudo Equivalent Deterministic Excitations (PEDE M ) to Bound the Random Response

The analysis of the response of a stochastic system, through a discrete coordinate set, can become computationally challenging, even when using a full modal representation. Nevertheless, many dynamic load cases, in engineering applications, have stochastic behaviour as the wall pressure fluctuations due to the turbulent boundary layer. In this work, a new method is proposed: it is named as frequency Modulated Pseudo Equivalent Deterministic Excitation, PEDEM, and it is based on the Pseudo Excitation Method, PEM. This latter can be considered as an exact representation since it uses a modal decomposition of the cross-spectral density matrix of the excitation; the extraction of the eigensolutions of the load matrix, at each frequency step, is a computational disadvantage. PEDEM overcomes this issue by introducing some approximations based on the analysis of the eigensolutions of the dynamic load matrix versus frequency. Mainly, two different approximations are proposed with reference to extreme frequency ranges (low and high) wherein the dynamic matrix of a random and convective load has different characteristics. A criterion to identify these frequency ranges is proposed versus a dimensionless representation of the frequency. Moreover, it is shown that the proposed approximations represent the bounding curves of the response for the whole frequency range. Fruitful comparisons with a full stochastic approach is discussed. The proposed approximations combine a good accuracy and representation of the stochastic system together with a significant reduction of the computational costs if compared to a full stochastic response or a PEM solution. The method is applied over two simple configurations (a chain of oscillators and two flexural plates) but the possible extensions to more complex cases are motivated by the quality of these preliminary results

Exact Geometric Similitude Laws for Flat Plate Vibrations Induced by a Turbulent Boundary Layer

Similitude laws for the vibration response of simply supported plates under random excitations are derived and tested numerically and experimentally for the case of a turbulent boundary layer. Analytical calculations show that under the assumption of proportional sides, perfect similitude in terms of vibration response scaling can be achieved between plates of variable thicknesses. It is also highlighted that even if the similitude conditions are not all satisfied (i.e., a complete scaling of all the involved parameters, from panel dimensions to flowspeed), an approximation can be made in the mid-high-frequency domain that leads to satisfactorily scaled results. Based on the analytical study, a series of tests is performed in an anechoic wind tunnel on three-scaled simply supported panels at different flow velocities. Applying the proposed procedure to this set of vibration measurements leads to satisfactory scaling of results between each other