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Yu H.,Institute of Fluid Mechanics and Aerodynamics | Xu F.,University of California at Los Angeles | Tropea C.,Institute of Fluid Mechanics and Aerodynamics | Tropea C.,TU Darmstadt
Optics Express | Year: 2013

A vector ray tracing (VRT) model is developed to simulate optical caustic structures in the primary rainbow region of light scattering from oblate droplets. The changes of the optical caustic structures in response to shape deformation of oblate droplets are investigated. Then the curvature calculated from the simulated rainbow fringes is compared with that from the measured rainbow fringes and good agreement is found. Furthermore, according to the generalized rainbow patterns and the relation between aspect ratio and curvature of the rainbow fringe, non-sphericities in terms of aspect ratio of an oblate water droplet is measured with high measurement accuracy. ©2013 Optical Society of America.

Lembach A.N.,Institute of Fluid Mechanics and Aerodynamics | Roisman I.V.,Institute of Fluid Mechanics and Aerodynamics | Gambaryan-Roisman T.,TU Darmstadt | Zhang Y.,University of Illinois at Chicago | And 2 more authors.
Langmuir | Year: 2010

Experiments were conducted to study peculiarities of drop impact onto electrospun polymer nanofiber mats. The nanofiber cross-sectional diameters were of the order of several hundred nanometers, the pore sizes in the mats of about several micrometers, and the mat thicknesses of the order of 200 μm. Polyacrylonitrile (PAN), a polymer which is partially wettable by water, was used to electrospin nanofiber mats. The experiments revealed that drop impact onto nanotextured surfaces of nanofiber mats produce spreading similar to that on the impermeable surfaces. However, at the end of the spreading stage, the contact line is pinned and drop receding is prevented. At higher impact velocities, prompt splashing events with formation of tiny drops were observed. It was shown that the splash parameter Kd=We1/2 Re 1/4 (with We and Re being the Weber and Reynolds numbers, respectively) previously used to characterize the experiments with drop impact onto smooth impermeable dry substrates can be also used to describe the onset of splash on substrates coated by nanofiber mats. However its threshold value Kds (in particular, corresponding to the minimal impact velocity leading to generation of secondary droplets) for the nanotextured surfaces is higher than that for dry flat substrates. In addition, water penetration and spreading inside wettable nanofiber mats after drop impact was elucidated and quantified. The hydrodynamics of drop impact onto nanofiber mats is important for understanding effective spray cooling through nanofiber mats, recently introduced by the same group of authors. © 2010 American Chemical Society.

Yu H.,University of Shanghai for Science and Technology | Yu H.,Institute of Fluid Mechanics and Aerodynamics | Shen J.,University of Shanghai for Science and Technology | Tropea C.,Institute of Fluid Mechanics and Aerodynamics | Tropea C.,TU Darmstadt
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

The vector ray tracing (VRT) model is used to simulate the optical caustic structures near the primary and the secondary rainbow angles of oblate water droplets. The evolution process of the optical caustic structures in response to shape deformation of the water droplet is discussed. The dependence of the caustic structures on equatorial radius, refractive index and aspect ratio of the droplet are studied and the curvatures of the two rainbow fringes are calculated. © 2014 SPIE.

Rival D.,TU Darmstadt | Rival D.,Institute of Fluid Mechanics and Aerodynamics | Rival D.,Massachusetts Institute of Technology | Tropea C.,TU Darmstadt | Tropea C.,Institute of Fluid Mechanics and Aerodynamics
Journal of Aircraft | Year: 2010

An experimental investigation into the dynamic-stall process of a pitching and plunging airfoil at low Reynolds numbers has been carried out using direct force measurements and smoke visualization in an Eiffel-type wind tunnel. The strong influence of reduced frequency (κ = πfC/U∞) on the vortical wake of both pure-plunging and purepitching airfoils is revealed. Here, a transition from a bluff body to a mushroom-type wake has been observed at approximately κ = 0.2. Some associated lift and moment hysteresis curves for combined pitching and plunging motions are then presented with an accompanying discussion on the nature of the dynamic-stall process. For these complex motions, it is observed that both lift and moment phase lags grow with reduced frequency from κ = 0.05 to 0.1. Despite substantial lift augmentation in the light- and deep-stall regimes, strong pitching-down moments are not avoided. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Jakirlic S.,TU Darmstadt | Jakirlic S.,Institute of Fluid Mechanics and Aerodynamics | Kadavelil G.,TU Darmstadt | Kadavelil G.,Institute of Fluid Mechanics and Aerodynamics | And 8 more authors.
International Journal of Heat and Fluid Flow | Year: 2010

An incompressible fully-developed duct flow expanding into a diffuser, whose upper wall and one side wall are appropriately deflected (with the expansion angles of 11.3° and 2.56° respectively), and for which reference experimental and DNS databases were provided by Cherry et al. (2008, 2009) and Ohlsson et al. (2009, 2010), was studied computationally by using a zonal hybrid LES/RANS (HLR) method, proposed recently by Kniesner (2008) and Jakirlić et al. (2009). In addition a complementary Large-Eddy Simulation (LES) method has been applied. The flow Reynolds number based on the height of the inlet channel is Re. h= 10,000. The primary objective of the present investigation was the comparative assessment of the computational models in this flow configuration characterized by a complex 3-D flow separation being the consequence of an adverse-pressure gradient evoked by the duct expansion. The focus of the investigation was on the capability of different modelling approaches to accurately capture the size and shape of the 3-D flow separation pattern and associated mean flow and turbulence features. © 2010 Elsevier Inc.

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