| 1. |
- Alfredsson, Henrik, et al.
(författare)
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The Diagnostic Plot—A Tutorial with a Ten Year Perspective
- 2021
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Ingår i: Progress in Turbulence IX. - Cham : Springer Nature. ; , s. 125-135
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Konferensbidrag (refereegranskat)abstract
- The diagnostic plot was introduced in 2010 (Eur. J. Mech. B/Fluids 29: 403–406) but was used already in 2008 during a large measurement campaign as a litmus test to determine if tripped zero-pressure gradient turbulent boundary layers fulfilled basic criteria of being canonical. It used the rms-level of streamwise velocity (urms ) in the outer part of the boundary layer, a region where urms can give clear indications if insufficient or too tough tripping has been used. In standard plots one needs both the friction velocity and measurement of the full velocity and turbulence profiles. By instead plotting urms/ U∞ as a function of U/ U∞, it was found that this gives rise to a well-defined distribution that could be used as a canonical measure. It was later discovered that it is possible to extend the description to the near wall region. It has also been extended to boundary layers over rough surfaces and with pressure gradients, and some further uses. This paper aims to be both a review of the development of the method during the last 10+ years and a tutorial for those who want to employ it in their research and maybe also find new uses of the methodology.
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| 2. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new formulation for the streamwise turbulence intensity distribution
- 2011
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Ingår i: 13th European Turbulence Conference (ETC13). - : Institute of Physics Publishing (IOPP). ; , s. 022002-
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Konferensbidrag (refereegranskat)abstract
- Numerical and experimental data from zero pressure-gradient turbulent boundary layers over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & Orlu [Eur. J. Fluid Mech. B/Fluids, 4 2, 403 (2010)]. In the diagnostic plot the local turbulence intensity is shown as a function of the local mean velocity normalized with a reference velocity scale. In the outer region of the boundary layer a universal linear decay of the turbulence intensity is observed independent of Reynolds number. The deviation from this linear region appears in the buffer region and seems to be universal when normalized with the friction velocity. Therefore, a new empirical fit for the streamwise velocity turbulence intensity distribution is proposed and the results are compared with up to date reliable high-Reynolds number experiments and extrapolated towards Reynolds numbers relevant to atmospherical boundary layers.
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| 3. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new formulation for the streamwise turbulence intensity distribution in wall-bounded turbulent flows
- 2012
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 36, s. 167-175
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of the streamwise velocity turbulence intensity has recently been discussed in several papers both from the viewpoint of new experimental results as well as attempts to model its behavior. In the present paper numerical and experimental data from zero pressure-gradient turbulent boundary layers, channel and pipe flows over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & ÖrlÌ [P.H. Alfredsson, R. ÖrlÌ, The diagnostic plot-a litmus test for wall bounded turbulence data, Eur. J. Mech. B Fluids 29 (2010) 403-406]. In the diagnostic plot the local turbulence intensity is plotted as function of the local mean velocity normalized with a reference velocity scale. Alfredsson et al. [P.H. Alfredsson, A. Segalini, R. ÖrlÌ, A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the outer peak, Phys. Fluids 23 (2011) 041702] observed that in the outer region of the boundary layer a universal linear decay of the turbulence intensity independent of the Reynolds number exists. This approach has been generalized for channel and pipe flows as well, and it has been found that the deviation from the previously established linear region appears at a given wall distance in viscous units (around 120) for all three canonical flows. Based on these results, new empirical fits for the streamwise velocity turbulence intensity distribution of each canonical flow are proposed. Coupled with a mean streamwise velocity profile description the model provides a composite profile for the streamwise variance profile that agrees nicely with existing numerical and experimental data. Extrapolation of the proposed scaling to high Reynolds numbers predicts the emergence of a second peak of the streamwise variance profile that at even higher Reynolds numbers overtakes the inner one.
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| 4. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the "outer" peak
- 2011
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 23:4, s. 041702-
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Tidskriftsartikel (refereegranskat)abstract
- One recent focus of experimental studies of turbulence in high Reynolds number wall-bounded flows has been the scaling of the root mean square of the fluctuating streamwise velocity, but progress has largely been impaired by spatial resolution effects of hot-wire sensors. For the near-wall peak, recent results seem to have clarified the controversy; however, one of the remaining issues in this respect is the emergence of a second (so-called outer) peak at high Reynolds numbers. The present letter introduces a new scaling of the local turbulence intensity profile, based on the diagnostic plot by Alfredsson and Orlu [Eur. J. Mech. B/Fluids 42, 403 (2010)], which predicts the location and amplitude of the "outer" peak and suggests its presence as a question of sufficiently large scale separation.
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| 5. |
- Alfredsson, P. Henrik, et al.
(författare)
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A New Way to Determine the Wall Position and Friction Velocity in Wall-Bounded Turbulent Flows
- 2012
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Ingår i: PROGRESS IN TURBULENCE AND WIND ENERGY IV. - Berlin, Heidelberg : SPRINGER-VERLAG BERLIN. ; , s. 181-185
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Konferensbidrag (refereegranskat)abstract
- Studies of turbulent boundary layers are often focussed on the near wall region. In experiments an accurate determination of the wall (say within 10 μm) with respect to the measurement probe is not easily achieved through optical or mechanical measurements (see e.g. Ref. [1]). Also the friction velocity (uτ ) is hard to determine with high accuracy. One possibility is to utilize the linear velocity profile (U+ = y+ ) to determine one or both of the mentioned quantities, but is usually impeded by the fact that the hot-wire readings are affected by heat transfer to the wall. The motivation here is to utilize the cumulative distribution function (CDF) of the streamwise velocity in a way so that the influence of the heat transfer can be avoided and thereby obtain accurate estimates of both the wall position and friction velocity.
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| 6. |
- Alfredsson, P. Henrik, et al.
(författare)
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Large-Eddy BreakUp Devices - a 40 Years Perspective from a Stockholm Horizon
- 2018
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Ingår i: Flow Turbulence and Combustion. - : Springer. - 1386-6184 .- 1573-1987. ; 100:4, s. 877-888
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Tidskriftsartikel (refereegranskat)abstract
- In the beginning of the 1980's Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.
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| 7. |
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| 8. |
- Alfredsson, P. Henrik, et al.
(författare)
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The diagnostic plot - a litmus test for wall bounded turbulence data
- 2010
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 29:6, s. 403-406
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Tidskriftsartikel (refereegranskat)abstract
- A diagnostic plot is suggested that can be used to judge wall bounded turbulence data of the mean and the rms of the streamwise velocity in terms of reliability both near the wall, around the maximum in the rms as well as in the outer region. The important feature of the diagnostic plot is that neither the wall position nor the friction velocity needs to be known, since it shows the rms value as a function of the streamwise mean velocity, both normalized with the free stream velocity. One must remember, however, that passing the test is a necessary, but not sufficient condition to prove good data quality.
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| 9. |
- Alfredsson, P. Henrik, et al.
(författare)
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The viscous sublayer revisited-exploiting self-similarity to determine the wall position and friction velocity
- 2011
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Ingår i: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 51:1, s. 271-280
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Tidskriftsartikel (refereegranskat)abstract
- In experiments using hot wires near the wall, it is well known that wall interference effects between the hot wire and the wall give rise to errors, and mean velocity data from the viscous sublayer can usually not be used to determine the wall position, nor the friction velocity from the linear velocity distribution. Here, we introduce a new method that takes advantage of the similarity of the probability density distributions (PDF) or rather the cumulative distribution functions (CDF) in the near-wall region. By using the velocity data in the CDF in a novel way, it is possible to circumvent the problem associated with heat transfer to the wall and to accurately determine both the wall position and the friction velocity. Prior to its exploitation, the self-similarity of the distribution functions of the streamwise velocity fluctuations within the viscous sublayer is established, and it is shown that they can accurately be described by a lognormal distribution.
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| 10. |
- Alfredsson, P. Henrik, et al.
(författare)
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Turbulent boundary layers over flat plates and rotating disks-The legacy of von Karman : A Stockholm perspective
- 2013
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 40, s. 17-29
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Tidskriftsartikel (refereegranskat)abstract
- Many of the findings and ideas of von Karman are still of interest to the fluid dynamics community. For instance, his result that the mean velocity distribution in turbulent flows has a logarithmic behavior with respect to the distance from the centreline is still a cornerstone for everybody working in wall-bounded turbulence and was first presented to an international audience in Stockholm at the Third International Congress for Applied Mechanics in 1930. In this paper we discuss this result and also how the so-called von Karman constant can be determined in a new simple way. We also discuss the possibility of a second (outer) maximum of the streamwise velocity fluctuations, a result that was implicit in some of the assumptions proposed by von Karman.
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