| 1. |
- Gullman-Strand, Johan, et al.
(författare)
-
Numerical and experimental study of separated flow in a plane asymmetric diffuser
- 2004
-
Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 25:3, s. 451-460
-
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.
|
|
| 2. |
- Johansson, Arne V., et al.
(författare)
-
Experimental tests of mean velocity distribution laws derived by lie group symmetry methods in turbulent boundary layers
- 2004
-
Ingår i: IUTAM SYMPOSIUM ON REYNOLDS NUMBER SCALING IN TURBULENT FLOW. - 1402017758 ; , s. 257-264
-
Konferensbidrag (refereegranskat)abstract
- New scaling laws for turbulent boundary layers recently derived (see Oberlack, 2001) using Lie group symmetry methods have been tested against experimental data from the KTH data-base for zero-pressure-gradient turbulent boundary layers. The most significant new law gives an exponential variation of the mean velocity defect in the outer (wake) region. It was shown to fit very well the experimental data over a large part of the boundary layer, from the outer part of the overlap region to about half the boundary layer thickness (delta(99)). A small modification of the innermost part of the log-layer is predicted by the same method, in the form of an additive constant within the log-function, and was confirmed by the experimental data.
|
|
| 3. |
- Lindgren, Björn, et al.
(författare)
-
Evaluation of a new wind tunnel with expanding corners
- 2004
-
Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 36:1, s. 197-203
-
Tidskriftsartikel (refereegranskat)abstract
- A new low-speed, closed-circuit, closed-test-section wind tunnel, called BLT, has been designed and built at KTH. The turbulence intensity in the test section is <0.04%, the total pressure variation is <+/-0.1% and the temperature variation is <+/-0.07degreesC over the cross-sectional area. The concept of expanding corners with an expansion ratio of 1.32 first investigated by Lindgren et al. in 1998, has been implemented successfully with a two-dimensional total pressure loss coefficient of 0.047 at a chord Reynolds number of 200,000. It is comparable to or even better than the values found in most wind tunnels using nonexpanding corners. The findings in this study prove the usefulness of expanding corners to achieve a compact wind tunnel circuit design without compromising the flow quality.
|
|
| 4. |
- Lindgren, Björn, et al.
(författare)
-
Evaluation of scaling laws derived from Lie group symmetry methods in zero-pressure-gradient turbulent boundary layers
- 2004
-
Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 502, s. 127-152
-
Tidskriftsartikel (refereegranskat)abstract
- New scaling laws for turbulent boundary layers recently derived (see Oberlack 2000) using Lie group symmetry methods have been tested against experimental data from the KTH database for zero-pressure-gradient turbulent boundary layers. The most significant new law predicts an exponential variation of the mean velocity defect in the outer (wake) region. It was shown to fit the experimental data very well over a large part of the boundary layer, from the outer part of the overlap region to about half the boundary layer thickness (699). In the outermost part of the boundary layer the velocity defect falls more rapidly than predicted by the exponential law. This can partly be attributed to intermittency in that region but the main cause stems from non-parallel effects that are not accounted for in the derivation of the exponential law. The two-point correlation function behaviour in the outer region, where an exponential velocity defect law is observed, was found to be very different from that derived under the assumption of parallel flow. It is found to be plausible that this indeed can be attributed to non-parallel effects. A small modification of the innermost part of the log-layer in the form of an additive constant within the log-function is predicted by the Lie group symmetry method. A qualitative agreement with such a behaviour just below the overlap region was found. The derived scaling law behaviour in the overlap region for the two-point correlation functions was also verified by the experimental data.
|
|
| 5. |
- Lindgren, Björn, et al.
(författare)
-
Measurement and calculation of guide vane performance in expanding bends for wind-tunnels
- 1998
-
Ingår i: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 24, s. 265-272
-
Tidskriftsartikel (refereegranskat)abstract
- The design of guide vanes for use in expanding bends was investigated both experimentally and numerically. The primary application in mind is the use of expanding corners in wind-tunnels for the purpose of constructing compact circuits with low losses. To investigate the performance of guide vanes in realistic situations expansion ratios between 1 and 3 were tested in the experiments. These were carried out in an open wind-tunnel specially built for the present purpose. The experimental results demonstrated that suitably designed guide vanes give very low losses and retained flow quality even for quite substantial expansion ratios. For wind-tunnel applications expansion ratios around 1.3 seem appropriate, Optimization of a guide vane design was done using a two-dimensional cascade code, Mises. A new vane optimized for an expansion ratio of 4/3 gave a two-dimensional total pressure-loss coefficient as low as 0.041 for a chord Reynolds number of 200,000.
|
|
| 6. |
- Lindgren, Björn, et al.
(författare)
-
Universality of probability density distributions in the overlap region in high Reynolds number turbulent boundary layers
- 2004
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 16:7, s. 2587-2591
-
Tidskriftsartikel (refereegranskat)abstract
- The probability density functions (PDFs) of the streamwise mean velocity in high Reynolds number turbulent boundary layers have been studied. The hypothesis of self-similar, normalized with the root mean square velocity, PDFs has been tested using the KTH database of high Reynolds number zero pressure-gradient turbulent boundary layer flow. The self-similarity was tested using the Kullback-Leibler divergence measure and it was found that the region of self-similar PDFs extends from about 160 viscous wall units to about 0.3 boundary layer thicknesses (in delta(95)). This region is somewhat larger than the universal overlap region. The PDF shape in the universal overlap region is close to Gaussian allowing for a Gram-Charlier expansion approximation of the measured PDFs. A remarkable collapse was found for 57 normalized PDF distributions for different positions within the universal overlap region and Reynolds numbers based on the momentum-loss thickness between 4300 and 12 600, strongly indicating a high degree of flow universality within the universal overlap region. Within the range studied, the Gram-Charlier expansion coefficients (related to the PDF moments) show no Reynolds number trend further supporting the self-similarity hypothesis.
|
|
| 7. |
- Tsuji, Y., et al.
(författare)
-
Self-similar profile of probability density functions in zero-pressure gradient turbulent boundary layers
- 2005
-
Ingår i: Fluid Dynamics Research. - : IOP Publishing. - 0169-5983 .- 1873-7005. ; 37:5, s. 293-316
-
Tidskriftsartikel (refereegranskat)abstract
- The probability density function (pdf) of a streamwise velocity component is studied in zero-pressure gradient boundary layers. From analyzing the data up to R-0 similar or equal to-- 13, 000, it is found that pdfs have self-similar profiles ranging from y(+) +/- 180 to 0.049 Delta(+), where Delta is Rotta-Clauser boundary layer thickness. Pdf profiles asymptote to the universal shape very close to the Gaussian, but are positively skewed at the core region, indicating smaller values in the tail parts. Based on this experimental fact, the mean velocity profile is reconsidered from the standpoint of pdf equation. The log-law profile is expected as the mean velocity distribution. The Karman constant is evaluated to be 0.38, and the log-region starts at y(s)(+) = 180 and ends at y(+) = 0.021 Delta(+) + 96.5 for R-0 > 4000. The end point locates in 0.15 delta(+) <= y(r)(+) <= 0.26(+). The relation to the new scaling law derived from Lie-group theory is also discussed.
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
- Törnblom, Olle, et al.
(författare)
-
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
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 636, s. 337-370
-
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.
|
|
