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- Gullman-Strand, Johan, et al.
(författare)
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Numerical and experimental study of separated flow in a plane asymmetric diffuser
- 2004
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 25:3, s. 451-460
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Tidskriftsartikel (refereegranskat)abstract
- Computations of the turbulent flow through plane asymmetric diffusers for opening angles from 8degrees to 10degrees have been carried out with the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johansson [J. Fluid Mech. 403 (2000) 89]. It is based on a two-equation platform in the form of a low-Re K - omega formulation, see e.g. Wilcox [Turbulence Modeling for CFD, DCW Industries Inc., 1993]. The flow has also been studied experimentally for the 8.5degrees opening angle using PIV and LDV. The models under-predict the size and magnitude of the recirculation zone. This is, at least partially, attributed to an over-estimation of the wall normal turbulence component in a region close to the diffuser inlet and to the use of damping functions in the near-wall region. By analyzing the balance between the production and dissipation of the turbulence kinetic energy we find that the predicted dissipation is too large. Hence, we can identify a need for improvement of the modeling the transport equation for the turbulence length-scale related quantity.
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| 2. |
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| 3. |
- Törnblom, Olle, et al.
(författare)
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A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:11, s. 115108-
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Tidskriftsartikel (refereegranskat)abstract
- A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator flow control and that the flow case in question is susceptible to separation control. The model results show that the pressure recovery of the diffuser could be increased by almost 10% by applying vortex generators and that, if keeping the shape of the vortex generators fixed, their optimal position is close to the diffuser inlet. Computations also indicated that the time to re-establish the separation zone when the control suddenly is turned off is substantially longer than the time it takes to remove the separation after the control is turned on again. Some work on adapting a differential Reynolds stress turbulence model was necessary in order to make it capable of realistic predictions of the asymmetric diffuser flow in which the vortex generator model is tested. However, the main focus of the article is on the modelling of vortex generator effects.
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| 4. |
- Törnblom, Olle
(författare)
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Experimental and computational studies of turbulent separating internal flows
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The separating turbulent flow in a plane asymmetric diffuser with 8.5 degrees opening angle is investigated experimentally and computationally. The considered flow case is suitable for fundamental studies of separation, separation control and turbulence modelling. The flow case has been studied in a specially designed wind-tunnel under well controlled conditions. The average velocity and fluctuation fields have been mapped out with stereoscopic particle image velocimetry (PIV). Knowledge of all velocity components allows the study of several quantities of interest in turbulence modelling such as the turbulence kinetic energy, the turbulence anisotropy tensor and the turbulence production rate tensor. Pressures are measured through the diffuser. The measured data will form a reference database which can be used for evaluation of turbulence models and other computational investigations. Time-resolved stereoscopic PIV is used in an investigation of turbulence structures in the flow and their temporal evolution. A comparative study is made where the measured turbulence data are used to evaluate an explicit algebraic Reynolds stress turbulence model (EARSM). A discussion regarding the underlying reasons for the discrepancies found between the experimental and the model results is made. A model for investigations of separation suppression by means of vortex generating devices is presented together with results from the model in the plane asymmetric diffuser geometry. A short article on the importance of negative production-rates of turbulent kinetic energy for the reverse flow region in separated flows is presented. A detailed description of the experimental setup and PIV measurement procedures is given in a technical report.
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| 5. |
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| 6. |
- Törnblom, Olle
(författare)
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Experimental study of the turbulent flow in a plane asymmetric diffuser
- 2003
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The separating turbulent flow in a plane asymmetric diffuseris investigated experimentally. The considered flow case issuitable for fundamental studies of separation, separationcontrol and turbulence modelling. The flow case has beenstudied in a specially designed wind-tunnel under wellcontrolled conditions. The velocity field has been mapped outwith PIV and LDV techniques in order to determine all threevelocity components. Knowledge of all velocity componentsallows the formation of several quantities of interest inturbulence modelling such as the turbulencekinetic energy, theturbulence anisotropy tensor and the turbulence production ratetensor. Pressures are measured through the diffuser. Themeasured data will form a reference database, which can be usedfor evaluation of turbulence models and other computationalinvestigations. A comparative study is made where the measuredturbulence data are used to evaluate an explicit algebraicReynolds stress turbulence model (EARSM). A discussionregarding the underlying reasons for the discrepancies foundbetween the experimental and the model results is made. Asimple form of separation control using vortex generators istested with positive result.Descriptors:Fluid mechanics, wind-tunnel, turbulence,turbulence modelling, EARSM, asymmetric diffuser, boundarylayer, PIV, LDV, vortex generator, control.
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| 8. |
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| 9. |
- Törnblom, Olle, et al.
(författare)
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The separating flow in a plane asymmetric diffuser with 8.5 degrees opening angle : mean flow and turbulence statistics, temporal behaviour and flow structures
- 2009
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 636, s. 337-370
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Tidskriftsartikel (refereegranskat)abstract
- The flow in a plane asymmetric diffuser with an opening angle of 8.5 degrees has been studied experimentally using time-resolving stereoscopic particle image velocimetry. The inlet condition is fully developed turbulent channel flow at a Reynolds number based on the inlet channel height and bulk velocity of Re = 38 000. All mean velocity and Reynolds stress components have been measured. A separated region is found on the inclined wall with a mean separation point at 7.4 and a mean reattachment point at 30.5 inlet channel heights downstream the diffuser inlet (the inclined wall ends 24.8 channel heights downstream the inlet). Instantaneous flow reversal never occurs upstream of five inlet channel heights but may occur far downstream the point of reattachment. A strong shear layer in which high rates of turbulence production are found is located in a region outside the separation. The static wall pressure through the diffuser is presented and used in an analysis of the balance between pressure forces and momentum change. It is demonstrated that production of turbulence causes a major part of the losses of mean flow kinetic energy. The character of the large turbulence structures is investigated by means of time-resolved sequences of velocity fields and spatial auto-correlation functions. Pronounced inclined structures are observed in the spanwise velocity and it is suggested that these are due to the legs of hairpin-like vortices.
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