Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A21: Instability: Boundary Layers I |
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Chair: Carlos Pantano, University of Illinois - Urbana Room: 2010 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A21.00001: ABSTRACT WITHDRAWN |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A21.00002: Laminar-to-Turbulence Transition in Hypersonic Boundary Layers: Role of Dilatational Waves Cunbiao Lee, Chuanhong Zhang, Yiding Zhu, Qing Tang, Huijing Yuan, Jiezhi Wu, Shiyi Chen, Mohamed Gad-el-Hak A Mach 6 quiet wind tunnel experiment is carried out to study the turbulent transition in hypersonic boundary layer. It is found that the second instability acoustic mode is the key modulator of the transitional process which experiences a rapid growth and a very fast annihilation due to the effect of bulk viscosity. The second mode interacts strongly with the first vorticity mode to directly promote a fast growth of the latter and lead to immediate transition to turbulence. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A21.00003: Receptivity to thermal noise in real airfoil configurations Paolo Luchini Thermal noise, the macroscopic manifestation of microscopic particle agitation, is present in fluid flow just as in electron flow in conductors or in other physical transport phenomena. When the flow acts as an amplifier, typically during transition to turbulence, the transition position can be influenced by the amplitude of external disturbances through the so called receptivity of the flow instabilities; internally generated thermal noise represents a thermodynamically enforced lower bound to how much disturbances can be reduced. In a previous paper (Seventh IUTAM Symposium on Laminar-Turbulent Transition, IUTAM Bookseries Volume 18, Springer, 2010, pp 11-18), the present author showed that the maximum transition distance in a Blasius boundary layer corresponds to a Reynolds number little above $6\cdot 10^6$ and to an $N$-factor of the order of $13$. Results to be exhibited at this conference show that in a real airfoil configuration the maximum transition Reynolds number imposed by thermal noise is even lower than on a flat wall, and not far from the actually observed transition position. It follows that thermal noise might actually have a role in natural transition; and that even a perfectly silenced laboratory environment cannot push the transition position much farther. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A21.00004: Linear stability of the flow in a toroidal pipe Philipp Schlatter, Jacopo Canton, Ramis Oerlue While hydrodynamic stability and transition to turbulence in straight pipes has been studied extensively, the mechanisms leading to instability in curved pipes are less documented. Here, the first (linear) instability of the flow inside a toroidal pipe is investigated as an initial step in the study of the related laminar-turbulent transition process. In the toroidally bent pipe, the flow is governed by two parameters: the Reynolds number and the curvature of the torus, given as the ratio between the radii of the pipe and of the torus, and is maintained in motion by fixed axial flux. We use classical modal stability analysis, which includes computing nonlinear steady states for each parameter pair, and then studying the stability by solving an eigenproblem of the linearised Navier--Stokes operator. Results show that the flow is indeed modally unstable for all the studied curvatures in the range 0.01--1, with the Reynolds number about 3000. The frequency, wavenumber and mode shapes are strongly dependent on the curvature: The corresponding critical modes are mainly located in the region of the Dean vortices, and represent in general travelling waves. Also time-dependent nonlinear simulations highlight the importance of the linear modes in the transition process in the bent pipe. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A21.00005: Investigation of wall temperature and angle of attack effects on boundary layer stability over a blunt cone Liang Xian, Li Xinliang A new PSE (parabolized stability equations) method based on the general orthogonal curvilinear system of coordinates is developed. Combined with DNS data, the PSE method is used to investigate the effects of wall temperature and angle of attack (AOA) on stability of the boundary layer over a blunt cone. Results indicate that cooling the surface leads to higher wave number appearing in streamwise for a given frequency disturbance wave. Cooling the surface induces stronger harmonic and 3D disturbances comparing to the adiabatic wall case, which further accelerates the growth of multi disturbance modes in blunt cone boundary layer. Thus finally decreases the transition Reynolds number. Although the non-parallelism is markedly in conical flow, the non-parallel effects on the evolution characteristic of disturbance is not so obviously. So the PSE approach is a useful method in analysis of the hypersonic boundary layer stability over a blunt cone. The combined effects of the wall temperature and the AOA on the transition over the blunt cone are further to be studied in our present work. Many new nonmonotonic changes of transition position versus above variables have been found. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A21.00006: Velocity streaks in a Blasius boundary layer induced by external streamwise vortices: numerical simulation and linear stability analysis Lorenzo Siconolfi, Simone Camarri, Jens H.M. Fransson This work investigates numerically the streamwise velocity streaks generated in a Blasius boundary layer (BL) by an array of counter-rotating vortices. The array is positioned outside the BL and generates the streaks by velocity induction. This investigation is motivated by previous studies demonstrating that stable streamwise streaks can lead to a stabilization of the Tollmien-Schlichting (TS) waves and to a subsequent delay of the transition between laminar and turbulent regime. In most of the previous studies streamwise vortices generating the streaks lie inside the BL. Conversely, the conceptual configuration considered here, with vortices outside the BL, has potential advantages due to the lower dissipation rate of the vortices in the streamwise direction. Direct numerical simulations (DNSs) are carried out to study the flow, where the streamwise vortices are introduced in an idealized form. Interesting configurations are identified by DNS and a reference one is selected and investigated in details. Bi-global stability analysis shows benefic effects on the evolution of TS waves and allows the construction of a modified stability curve for the controlled flow. The resulting transition delay is also demonstrated by DNS. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A21.00007: Transition to turbulence by interaction of free-stream and discrete mode perturbations Rikhi Bose, Paul Durbin In this work based on DNS, boundary layer streaks have been induced by free-stream turbulence (FST). The FST is numerically generated following the work of Jacobs \& Durbin (J. Fluid Mech., 428, 185, 2001). Modal interaction of FST induced streaks and a 2D TS wave have been noted by invoking the FST and TS wave disturbances at the inlet of the computational domain. For higher turbulence intensity of FST (3.5\%), the flow undergoes bypass transition to turbulence as in Durbin \& Wu (Ann. Rev. Fluid Mech., 39, 107, 2007). When low intensity FST (1\%) and TS wave are specified at inlet, transition is triggered via secondary instability. Secondary instability is instigated by interaction of FST induced streaks and TS wave. The pattern of $\Lambda$ structures observed in these studies is neither of H or K type transition and depends upon inlet frequency spectrum of FST. Frequencies smaller than the TS wave frequency grow and determine the instantaneous pattern of the secondary instability. The span-wise length scale of the $\Lambda$ structures have approximately half the size seen in Herbert (Ann. Rev. Fluid Mech., 20, 487, 1988). The evolution of the secondary instability is spontaneous rather than forced by the inlet FST or TS wave disturbances. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A21.00008: ABSTRACT WITHDRAWN |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A21.00009: Wavelet-based identification of localized turbulent regions in a transitional boundary layer Joe Yoshikawa, Yu Nishio, Seiichiro Izawa, Yu Fukunishi A numerical study in order to develop a method to identify localized turbulent regions in a transitional boundary layer is carried out using a wavelet transformation. Finding the onset of turbulence is quite difficult because it is not easy to distinguish the localized turbulent regions from ``non-active'' groups of vortices. The base flow with low-speed streaks is generated by placing an array of obstacles. Then a short duration jet is ejected from the wall into the low-speed streak. First, a hairpin vortex appears in the laminar boundary layer which travels downstream growing up. Downstream, localized turbulent regions appear in the boundary layer, where a lot of vortices are entangled with each other. A wavelet analysis is applied to the spatial waveforms of streamwise velocity fluctuations obtained from these two flow fields. It is shown that the hairpin vortex appears as a high amplitude spot in the wavelet spectrum, which is small in both wavenumber-wise and streamwise scales. On the other hand, the isolated turbulent region appears more wide spread in the wavenumber-wise scale. So, using this method, localized turbulent regions can be identified. [Preview Abstract] |
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