15:00
15 mins
|
NONLINEAR INTERACTIONS DURING EARLY STAGES OF BOUNDARY LAYER TRANSITION INDUCED BY FREE-STREAM TURBULENCE
Dmitry Sboev
Abstract: The experimental study of a disturbed flat plate boundary layer subjected to moderate free-stream turbulence (FST) is presented. All measurements were conducted in a flow region with zero intermittency. By means of bispectral analysis it was found that after initial linear growth of low-frequency streaks two distinct nonlinear processes arises in a boundary layer. The first one is represented by interactions between low frequencies in upper third of a boundary layer and in immediate vicinity of it and the second one is an interaction of streaks with high-frequency disturbances across whole layer. In present experimental setup the region of nonlinear development had taken length about two-thirds of the measurements domain. Inside boundary layer the critical r.m.s.-amplitude of disturbances needed to initiate nonlinear development was found to be about 2 per cent of free-stream velocity.
|
15:15
15 mins
|
NEUTRAL STABILITY OF THE FLOW IN A TOROIDAL PIPE
Jacopo Canton, Ramis Orlu, Philipp Schlatter
Abstract: This work is concerned with the numerical investigation of the linear stability properties of the viscous, incompressible flow inside a toroidal pipe. A Hopf bifurcation is found and tracked in phase space, showing that the flow is modally unstable even at extremely low curvatures. The bifurcation and the eigenfunctions associated with it are analysed as a function of the two parameters governing the flow, i.e. the Reynolds number, Re, and the curvature, δ. For all curvatures, the critical Reynolds number is found to be about 3000.
|
15:30
15 mins
|
Boundary layer flow control using the method of spanwise mean velocity gradient
Jens H. M. Fransson
Abstract: Over the last decade wind tunnel experiments and numerical simulations have shown that steady spanwise mean velocity gradients are able to attenuate the growth of different types of boundary layer disturbances if introduced in a controlled way. In this paper different techniques to setup the spanwise mean velocity variations are reviewed and their stabilizing effect leading to transition delay are quantified. This control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.
|
15:45
15 mins
|
Tertiary patterns in inclined layer convection
Priya Subramanian, Werner Pesch, Tobias M Schneider
Abstract: Convection in an inclined layer generates various types of spatio-temporal patterns due to interaction of buoyancy and shear.
At small angles of incline, the secondary instability of the uniform base state occurs in the form of buoyancy dominated longitudinal rolls. Above a critical angle of incline marking a co-dimension 2 point, shear driven transverse roll instabilities take over as the secondary instabilities.
Computing the location of the co-dimension 2 point for varying thermal driving and inclination angle and determining all secondary bifurcations together with the resulting tertiary states allows to characterize the nonlinear phase diagram of inclined layer convection system. The semi-analytically computed phase diagram quantitatively matches experimental observations by Daniels et al. Close to the co-dimension 2 point, a subcritical secondary bifurcation leading to bistability is identified. In the bistable region, heteroclinic cycles generate bursting behavior.
|
16:00
15 mins
|
ON A MECHANISM OF DELAYING LAMINAR-TURBULENT TRANSITION
Hui Xu, Spencer J Sherwin
Abstract: In this paper we investigate the boundary layer flows over a flat plate on which smooth localized imperfections are located. The localized imperfections have the width scale (d) comparable with the wavelength of the Tollmien-Schlichting (T-S) waves and the height scale (h) less than the boundary layer thickness . The existence of the localized imperfection gives rise to the change of the instability property of the boundary layer. The investigations are focused on the interaction between the T-S waves and the base flows distorted by smooth forward-facing steps and aim to forge links between the localized imperfections and the mechanisms of stabilizing the T-S waves. Numerical investigations show that isolated smooth forward-facing steps can perform as a robust strategy of delaying laminar-turbulent transition. Finally, direct numerical simulations are implemented to validate the strategy.
|
16:15
15 mins
|
EFFECTS OF FREESTREAM TURBULENCE ON CROSSFLOW INSTABILTIY
Areshir Hanifi, Seyed Mohammad Hosseini, Dan Henningson
Abstract: Direct numerical simulations (DNS) have been performed in order to investigate the interaction of freestream turbulence and crossflow generated instability on a swept wing. The experiments by [3] and [1] are selected as the reference cases. In those experiments the authors explore the interaction between different freestream turbulence characteristics and different roughness element characteristics. In the current study, isotropic homogenous freestream turbulence are generated following experimental parameters and then fed as the inflow boundary condition for DNS of flow over the wing. A spanwise array of roughness elements corresponding to the most unstable stationary modes are used to generate the crossflow vortices. The effects of the freestream turbulence on the crossflow instability and transition to turbulence are later studied.
|
16:30
15 mins
|
Hamiltonian formalism for weak turbulence of inertial waves in rotating fluids
Andrey A Gelash, Vladimir E Zakharov
Abstract: We present the Hamiltonian description of incompressible rotating fluid by using the Clebsch variables. We find the transformation which allows us to present the three-wave interaction Hamiltonian in normal variables in simple explicit form. We analyze the three-wave interaction amplitude and find new anisotropic spectra. Finally we study the convergence of integrals and present the kinetic equation.
|
16:45
15 mins
|
Slow Dynamics in turbulent Helium flows
Javier Burguete, Philippe Roche, Bernard Rousset
Abstract: The presence of slow dynamics is a recurrent feature of many turbulent flows. This behaviour can be created by instabilities of the mean flow or by other mechanisms. In this work we analyze the behavior of a highly turbulent Helium flow (maximum Reynolds number Re=10^8, with a Reynolds based on the Taylor microscale Re_\lambda=2000). We have performed velocity measurements using home-made Pitot tubes. The analysis of the data series reveals that below the injection frequencies there are different dynamical regimes with time scales two orders of magnitude below the injection scale
|