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Puckert D.K.,Institute For Aerodynamik Und Gasdynamik | Dieterle M.,Institute For Aerodynamik Und Gasdynamik | Rist U.,Institute For Aerodynamik Und Gasdynamik
Experiments in Fluids | Year: 2017

Controlling freestream turbulence (FST) in low-turbulence wind or water channels is a common challenge and often difficult to achieve. Particularly at low velocities, design guidelines from literature may not fulfill their purpose and thus require alternative strategies. In this study, we propose the installation of a fine-meshed screen downstream of the contraction and not in the settling chamber as typically advised in literature. With this strategy, the lower operational limitation of our facility could be extended below U∞=0.06ms-1 and the turbulence intensity reduced by 60 % at 0.04 ms- 1. This is not only an improvement of freestream conditions but also the key to experiments in the laminar boundary layer, which is highly sensitive to FST. In fact, two fundamentally different transition mechanism can be distinguished with this approach. © 2017, Springer-Verlag Berlin Heidelberg.

Cadirci S.,Technical University of Istanbul | Gunes H.,Technical University of Istanbul | Rist U.,University of Stuttgart | Rist U.,Institute For Aerodynamik Und Gasdynamik
5th Flow Control Conference | Year: 2010

An oscillatory, zero-net-mass flux Jet and Vortex Actuator (JaVA) was modelled as part of a flat plate and simulated in a boundary layer flow. JaVA is an active flow control device that can be used for flow separation control and thus can delay boundary layer transition. It has been already shown experimentally that JaVA induced flow types in still water include angled and vertical jets, wall jets and vortex flows and that they highly depend on governing parameters such as the frequency and amplitude of the actuator and the mean position of the actuator plate. In this study, a commercial unsteady, incompressible Navier-Stokes solver (Fluent) has been used to study the flow fields generated by JaVA in a water channel. The detailed quantitative information about the performance of JaVA on a flat plate boundary layer is obtained numerically. The results are validated by visualization experiments with the similar CFD set up. The numerical results show vortex like structures emerging from actuator's wide gap with a size that matches the experimentally observed vortex. It has been found that the emerging vortices move along the flat plate surface usually merging with each other downstream of the boundary layer. In addition to governing JaVA parameters in still water (the jet Reynolds number, the scaled amplitude, the mean position of the actuator), the characteristics of boundary layer flow are important for the JaVA performance in channel flow. These include the magnitude of the free stream velocity and the boundary layer profile (e.g. laminar or turbulent). In this study, we consider an approximation to the Blasius profile (a 4th-order polynomial profile). To account the free stream velocity, a new dimensionless parameter (r) is introduced which is the ratio of average jet velocity "Vj= 2abf/ww" to free stream velocity "U∞" (r = Vj/ U∞). Our numerical results clearly show that JaVA, when mounted in a flat plate laminar boundary layer, affects boundary layer profile considerably. That is, JaVA induced boundary layer profiles are clearly more resistant to the flow separation. The effects of JaVA with different operation regimes on the various boundary layer flow characteristics such as the displacement thickness, the momentum thickness, the energy thickness and the friction coefficient are reported. This computational study can be utilized to steer the governing parameters effectively for an improved actuator design. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Dumbser M.,University of Trento | Dumbser M.,Institute For Aerodynamik Und Gasdynamik | Iben U.,Robert Bosch GmbH | Munz C.-D.,Institute For Aerodynamik Und Gasdynamik
Computers and Fluids | Year: 2013

In this article we present a new efficient implementation of high order accurate Godunov-type WENO finite volume schemes on unstructured triangular and tetrahedral meshes for the simulation of real fluids with complex equations of state. The main focus of this paper is the efficient computation of flows in compressible mixtures of liquid and vapor in multiple space-dimensions, including the onset of cavitation. In the present article we use the full equation of state (EOS) for water, vapor and wet steam in thermal equilibrium based on the industrial and scientific standard IAPWS-IF97, as well as the real equation of state for n-heptane provided by Span and Wagner. Since the direct evaluation of these very complex EOS is computationally very expensive, we propose a new robust, accurate and particularly efficient approach for their evaluation in high order Godunov type finite volume schemes. It is based on the L2 projection of the EOS (ρ, e)→(p, T) onto the space of piecewise polynomials of degree q using an adaptive mesh refinement (AMR) approach together with Cartesian cut-cells, which adjusts the grid of the phase space (ρ, e) so that the L∞ error of the L2-projection of the EOS is less than a given error threshold. For a thorough validation of our numerical approach we construct several quasi-exact solutions to the Riemann problem for water and n-heptane using the corresponding detailed equations of state. We furthermore present numerical convergence results of third and fourth order schemes for an unsteady two-dimensional test problem for water with real EOS. We furthermore show the onset of cavitation on a simple model problem with an initially purely liquid fluid at ambient conditions at rest using a strong heat source term in the energy equation as well as the development of a cavitation bubble due to strong rarefaction. Finally, some numerical test problems are solved with the new approach on unstructured triangular and tetrahedral meshes in multiple space dimensions. © 2013 Elsevier Ltd.

Linn J.,Institute For Aerodynamik Und Gasdynamik | Kloke M.J.,Institute For Aerodynamik Und Gasdynamik
AIAA Journal | Year: 2011

The influence of effusion cooling through discrete holes on a laminar Mach 2.67 boundary layer is investigated using direct numerical simulation. It is found that the wall condition (isothermal, adiabatic, or radiative-adiabatic, with neglected heat conduction within the wall) has a very strong influence on the vortex systems that are generated by the holes and thus also on the cooling features. The transfer of results from short-duration isothermal shocktunnel experiments to the more realistic radiative-adiabatic situation is difficult. In addition to trying to establish a coolant film at the wall, a significant increase of the wall shear should be avoided in order to not increase the heat flux into the wall. Copyright © 2010.

Zengl M.,Institute For Aerodynamik Und Gasdynamik | Rist U.,Institute For Aerodynamik Und Gasdynamik
Notes on Numerical Fluid Mechanics and Multidisciplinary Design | Year: 2012

Results of linear stability calculations and direct numerical simulations for flow-control experiments are presented. Good agreements between measurements and simulations are shown. Furthermore, the linear stability of the flow over the experimental wing section is investigated. Hereby, also the use of isotropic and anisotropic compliant materials is assessed. The prevailing surface-based compliantwall model of Carpenter was extended to yaw angles, pressure gradients and oblique-traveling disturbances. The influence of the yaw angle is demonstrated for an anisotropy angle of 75°. Also transient-growth of instabilities over the compliant wall was investigated, since the eigenvalue spectrum of the compliant-wall problem turned out to be sensitive to truncation errors. For the parameters investigated, the maximum transient growth of the compliant-wall case is in the same order as the growth of the rigid-wall case. © 2012 Springer Berlin Heidelberg.

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