The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Ulrich Rist - One of the best experts on this subject based on the ideXlab platform.
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Detection of Lambda- and Omega-vortices with the temperature-sensitive paint method in the late stage of controlled laminar–turbulent Transition
Experiments in Fluids, 2019Co-Authors: Jonathan Lemarechal, Dominik K Puckert, Christian Klein, Ulrich Henne, Ulrich RistAbstract:An experiment investigating the laminar–turbulent Transition of a Blasius boundary-layer-like flow was set up in the laminar water channel at the Institute of Aerodynamics and Gas Dynamics, University of Stuttgart. The late stage of controlled Transition with K-type breakdown was investigated with the temperature-sensitive paint (TSP) method on the flat-plate surface. Additional velocity measurements in the boundary layer were performed with the hot-film anemometry for better interpretation of the TSP results. The test conditions enable the TSP method to resolve the complete Transition process temporally and spatially. Therefore, it was possible to detect the coherent structures occurring in the late stage of laminar–turbulent Transition from the visualizations on the flat-plate surface: namely, $$\varLambda$$ Λ - and $$\varOmega$$ Ω -vortices. The Transition location is derived from the TSP visualizations with a gradient-based method and with the Turbulence Energy Recognition Algorithm (TERA) from the velocity measurements. The derived average Transition location shows good agreement between the two techniques, but the TSP method detected a later beginning and earlier end of Transition. Graphical abstract
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experiments on critical reynolds number and global instability in roughness induced laminar turbulent Transition
Journal of Fluid Mechanics, 2018Co-Authors: Dominik K Puckert, Ulrich RistAbstract:The effects of isolated, cylindrical roughness elements on laminar–turbulent Transition in a flat-plate boundary layer are investigated in a laminar water channel. Our experiments aim at providing a comparison to global linear stability theory (LST) by means of hot-film anemometry and particle image velocimetry. Although the critical Reynolds number from theory does not match the Transition Reynolds number observed in experiments, there are distinct experimental observations indicating a changeover from purely convective to absolute/global instability very close to the critical Reynolds number predicted by theory. Forcing with a vibrating wire reveals the evolution of the system dynamics from an amplifier to a wavemaker when the critical Reynolds number is exceeded. The mode symmetry is varicose for thick roughness elements and a changeover from varicose to sinuous modes is observed at the critical Reynolds number for thin roughness elements. Therefore, most predictions by global LST can be confirmed, but additional observations in the physical flow demonstrate that not all features can be captured adequately by global LST.
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impact of deep gaps on laminar turbulent Transition in compressible boundary layer flow
AIAA Journal, 2016Co-Authors: Johannes Zahn, Ulrich RistAbstract:Two-dimensional direct numerical simulations are used to study the impact of deep gaps on laminar–turbulent Transition in compressible boundary-layer flow. For these, the gap depth-to-width ratio is always larger than five. They are located on a flat plate without pressure gradient. A steady base flow is used with a Mach number of 0.6, free-stream temperature of 288 K, and free-stream pressure of 1 bar. Subsequently, Tollmien–Schlichting waves are introduced by suction and blowing at the wall, and their growth over the gap is evaluated by N factors. The influence of the gap on laminar–turbulent Transition is quantified by the difference ΔN compared with the N factor obtained for a flat plate without gap. A periodic influence of the gap depth on ΔN is observed. In the direct numerical simulations, acoustic waves enter the gap and form a standing wave due to reflections, similar as occurring in organ pipes. The feedback of the standing wave on the boundary-layer flow above is essential for the observed ΔN v...
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a combined experimental numerical study of unsteady phenomena in a laminar separation bubble
Flow Turbulence and Combustion, 2003Co-Authors: Olaf Marxen, Ulrich Rist, M Lang, Siegfried WagnerAbstract:A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes Transition. Finally the turbulent boundary layer reattaches, forming a laminar separation bubble (LSB). Laminar-Turbulent Transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (PIV), and direct numerical simulation (DNS). The Transition mechanism occurring in the flow-field under consideration is discussed in detail. Observations for the development of small disturbances are compared to predictions from viscous linear instability theory (Tollmien–Schlichting instability). Non-linear development of these disturbances and their role in final breakdown to turbulence is analyzed.
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dns of laminar turbulent Transition in separation bubbles
2000Co-Authors: Ulrich Maucher, Ulrich Rist, Markus J Kloker, Siegfried WagnerAbstract:A Laminar Separation Bubble (LSB) is created under the influence of an adverse (positive) pressure gradient along a wall by laminar separation, Laminar-Turbulent Transition and turbulent re-attachment of the flow to the wall. Direct Numerical Simulations (DNS) of the flow in an airfoil boundary layer which are based on solving the full Navier-Stokes equations exhibit good quantitative agreement of the mean-flow data with wind-tunnel experiments, but only examinations of the unsteady results are able to reveal the underlying physics. Thus, a hitherto unknown temporal amplification of three-dimensional small-amplitude disturbances is observed and explained by the entrainment of three-dimensional fluctuations by the roll-up of the detached boundary layer. Once the three-dimensional disturbances become saturated this mechanism leads to a rapid breakdown of the laminar flow into regions of small-scale turbulence which are organized in a quasi two-dimensional coherent manner.
D M Martinez - One of the best experts on this subject based on the ideXlab platform.
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observation of laminar turbulent Transition of a yield stress fluid in hagen poiseuille flow
Journal of Fluid Mechanics, 2009Co-Authors: Bulent Guzel, I A Frigaard, Teodor Burghelea, D M MartinezAbstract:We investigate experimentally the Transition to turbulence of a yield stress shear-thinning fluid in Hagen–Poiseuille flow. By combining direct high-speed imaging of the flow structures with Laser Doppler Velocimetry (LDV), we provide a systematic description of the different flow regimes from laminar to fully turbulent. Each flow regime is characterized by measurements of the radial velocity, velocity fluctuations and turbulence intensity profiles. In addition we estimate the autocorrelation, the probability distribution and the structure functions in an attempt to further characterize Transition. For all cases tested, our results indicate that Transition occurs only when the Reynolds stresses of the flow equal or exceed the yield stress of the fluid, i.e. the plug is broken before Transition commences. Once in Transition and when turbulent, the behaviour of the yield stress fluid is somewhat similar to a (simpler) shear-thinning fluid. Finally, we have observed the shape of slugs during Transition and found their leading edges to be highly elongated and located off the central axis of the pipe, for the non-Newtonian fluids examined.
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predicting laminar turbulent Transition in poiseuille pipe flow for non newtonian fluids
Chemical Engineering Science, 2009Co-Authors: Bulent Guzel, I A Frigaard, D M MartinezAbstract:We present a new phenomenological approach for quantifying laminar–turbulent Transition in pipe flow. This criterion is based on averaging a local Reynolds number to give ReG. Our localised parameter shows strong radial variations that are maximal at approximately the radial positions where puffs first appear during the first stages of turbulent Transition. We present comparative data for experiments conducted with Glycerin, Xanthan and Carbopol solutions, each of which serves to demonstrate the validity of our approach.
A. A. Maslov - One of the best experts on this subject based on the ideXlab platform.
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structure of subsonic plane microjets
Microfluidics and Nanofluidics, 2019Co-Authors: V. M. Aniskin, A. A. Maslov, K. A. MukhinAbstract:Results of experiments aimed at studying subsonic microjets escaping from a plane nozzle are reported. The Reynolds numbers based on the nozzle height and mean flow velocity at the nozzle exit are varied from 27 to 139, whereas the nozzle size is fixed at 83.3 × 3823 µm. The test gas is air at room temperature. The distributions of velocity and velocity fluctuations along the jet axis and in the lateral and transverse directions are determined. The fact of the laminar–turbulent Transition in the jet is detected. The data obtained are compared with theoretical predictions for laminar plane jets. The experimental and theoretical data are found to be in good agreement at the laminar segment of the microjet.
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influence of roughness of cone nose tip on laminar turbulent Transition at hypersonic speed
AIP Conference Proceedings, 2018Co-Authors: D A Bountin, Yu V Gromyko, P A Polivanov, A. A. MaslovAbstract:Different kinds of roughness appear on the nose of the hypersonic vehicle during the process of thermal destruction of thermal protection coating, which leads to earlier Transition. Another important factor affecting the position of the Laminar-Turbulent Transition is the bluntness of the nose or the leading edge of the vehicle. This factor shifts the Transition downstream. Thus, the presence of a roughness on the blunted nose-tip of the vehicle creates an ambiguous situation: the stabilizing effect of bluntness competes with the destabilizing effect of roughness. Present work is a continuation of the work [1] and focused on the investigation of evolution of turbulent spots in the boundary layer.Different kinds of roughness appear on the nose of the hypersonic vehicle during the process of thermal destruction of thermal protection coating, which leads to earlier Transition. Another important factor affecting the position of the Laminar-Turbulent Transition is the bluntness of the nose or the leading edge of the vehicle. This factor shifts the Transition downstream. Thus, the presence of a roughness on the blunted nose-tip of the vehicle creates an ambiguous situation: the stabilizing effect of bluntness competes with the destabilizing effect of roughness. Present work is a continuation of the work [1] and focused on the investigation of evolution of turbulent spots in the boundary layer.
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effect of the surface roughness of blunt cone forebody on the position of laminar turbulent Transition
Thermophysics and Aeromechanics, 2016Co-Authors: D A Bountin, A. A. Maslov, Yu V Gromyko, P A Polivanov, A A SidorenkoAbstract:In the present paper, data on the effect of the surface roughness of blunt cone forebody on the position of Laminar-Turbulent Transition are reported. The study was carried out under freestream Mach 5.95. It was found that the roughness position plays a substantial role in the Transition process. Critical Reynolds numbers at which the Laminar-Turbulent Transition occurs on the nose-tip of the model were identified. For the first time, hysteresis in Transition position was observed.
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the influence of the laminar turbulent Transition on the interaction between the shock wave and boundary layer at a low supersonic mach number
Technical Physics Letters, 2015Co-Authors: A. A. Maslov, P A Polivanov, A A SidorenkoAbstract:An experimental study is devoted to the problem of the interaction between the shock wave and boundary layer. The influence of the approaching boundary layer state on parameters of the interaction region is studied by use of measurements of velocity fields. It is shown that the case in which the interaction occurs near the zone of the laminar–turbulent Transition in the boundary layer is optimum. The possibility of controlling for the interaction by means of turbulizers is studied.
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on the determination of the position of laminar turbulent Transition in boundary layer by optical methods
Thermophysics and Aeromechanics, 2015Co-Authors: D A Bountin, A. A. Maslov, Yu V Gromyko, P A Polivanov, A A SidorenkoAbstract:As a rule, aerodynamic studies at hypersonic flow velocities are carried out in short-duration wind-tunnel facilities. For such facilities, optical diagnostic methods are most preferable. In the present study, we give for the first time a comparison of two methods for determining the end of Laminar-Turbulent Transition: from the distribution of heat fluxes and from schlieren visualization data for the boundary-layer flow. Parametric data on the position of the Transition are obtained. These data can be used in the future as reference ones while calibrating semi-empirical calculation models for the Transition.
Bulent Guzel - One of the best experts on this subject based on the ideXlab platform.
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observation of laminar turbulent Transition of a yield stress fluid in hagen poiseuille flow
Journal of Fluid Mechanics, 2009Co-Authors: Bulent Guzel, I A Frigaard, Teodor Burghelea, D M MartinezAbstract:We investigate experimentally the Transition to turbulence of a yield stress shear-thinning fluid in Hagen–Poiseuille flow. By combining direct high-speed imaging of the flow structures with Laser Doppler Velocimetry (LDV), we provide a systematic description of the different flow regimes from laminar to fully turbulent. Each flow regime is characterized by measurements of the radial velocity, velocity fluctuations and turbulence intensity profiles. In addition we estimate the autocorrelation, the probability distribution and the structure functions in an attempt to further characterize Transition. For all cases tested, our results indicate that Transition occurs only when the Reynolds stresses of the flow equal or exceed the yield stress of the fluid, i.e. the plug is broken before Transition commences. Once in Transition and when turbulent, the behaviour of the yield stress fluid is somewhat similar to a (simpler) shear-thinning fluid. Finally, we have observed the shape of slugs during Transition and found their leading edges to be highly elongated and located off the central axis of the pipe, for the non-Newtonian fluids examined.
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predicting laminar turbulent Transition in poiseuille pipe flow for non newtonian fluids
Chemical Engineering Science, 2009Co-Authors: Bulent Guzel, I A Frigaard, D M MartinezAbstract:We present a new phenomenological approach for quantifying laminar–turbulent Transition in pipe flow. This criterion is based on averaging a local Reynolds number to give ReG. Our localised parameter shows strong radial variations that are maximal at approximately the radial positions where puffs first appear during the first stages of turbulent Transition. We present comparative data for experiments conducted with Glycerin, Xanthan and Carbopol solutions, each of which serves to demonstrate the validity of our approach.
Christian Poelma - One of the best experts on this subject based on the ideXlab platform.
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Particle-Laden Pipe Flows at High Volume Fractions Show Transition Without Puffs.
Physical review letters, 2018Co-Authors: Willian Hogendoorn, Christian PoelmaAbstract:Using ultrasound imaging velocimetry, we are able to present unique insight in Transitional particle-laden flows. Together with a Moody diagram of time-averaged properties, we demonstrate that the Laminar-Turbulent Transition behavior at high volume fractions is distinct from the single-phase case and cases with low volume fractions. For low volume fractions, a sharp Transition is found with the presence of turbulent puffs, similar to the single-phase case. Seemingly, particles in this regime trigger subcritical Transition. For high volume fractions a smooth Transition is discovered without turbulent puffs in the Transition regime. For this regime, particles cause a supercritical Transition.
