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  • 1.
    Alfredsson, P. Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The viscous sublayer revisited-exploiting self-similarity to determine the wall position and friction velocity2011In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 51, no 1, p. 271-280Article in journal (Refereed)
    Abstract [en]

    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.

  • 2.
    Angele, Kristian P.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Muhammad-Klingmann, Barbro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A simple model for the effect of peak-locking on the accuracy of boundary layer turbulence statistics in digital PIV2005In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 38, no 3, p. 341-347Article in journal (Refereed)
    Abstract [en]

    A simple model was constructed to study the effect of peak-locking on the accuracy of particle image velocimetry (PIV) turbulence statistics. A crucial parameter is the ratio between the root-mean-square (rms) velocity and the discretization velocity, which reflects the number of peaks distributed over the velocity probability density functions. When the ratio of the discretization velocity, which is set by the PIV setup parameters, to the rms, given by the flow, is larger than two, the maximum errors introduced in the mean and rms values become significant ( larger than 1%). The errors introduced also depend on the amplitude, or severity, of the peak-locking, and whether the mean displacement corresponds to an integer or a fractional number of pixels. The peak-locking affects the statistical moments of different order in such a way that the errors are phase shifted. The proposed model can be used to predict errors in the turbulence statistics in a laboratory PIV experiment. According to our model predictions, the most significant influence of peak-locking in a boundary layer type of flow is an overall underestimation of the wall-normal rms. Our predictions are in good agreement with our experimental results from turbulent boundary layers and the recent experimental results from a turbulent channel flow by Christensen (Exp Fluids 36: 484 - 497, 2004) for a case of moderate peak-locking.

  • 3. Borg, A.
    et al.
    Bolinder, J.
    Fuchs, Laszlo
    Simultaneous velocity and concentration measurements in the near field of a turbulent low-pressure jet by digital particle image velocimetry-planar laser-induced fluorescence2001In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 31, no 2, p. 140-152Article in journal (Refereed)
    Abstract [en]

    The main purpose of this work is to develop a method for simultaneous measurement of velocity and passive scalar concentration by means of digital particle image velocimetry and planar laser-induced fluorescence. Details of the implementation of the method are given, and the technique is applied to measurements of concentration and velocity in the centre-plane of a liquid jet with a Reynolds number of 6,000. The measurements are compared with large eddy simulations. Mean velocities and concentrations, fluctuating velocities and concentrations, and correlation between fluctuating velocities and concentrations. are analysed for the first six diameters downstream of the jet exit. The general agreement between measured and simulated results was found to be good, in particular for mean quantities. Mean profiles are also found to be in good agreement with other experimental work on jets reported in the literature. The whole-plane measurement method was found to be very useful for detailed comparisons of turbulent statistics with simulated data. The inadequacy of models for turbulent mass transport based on the standard gradient diffusion concept is demonstrated through the experimental data.

  • 4.
    Brosse, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Finmo, Carl
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental study of a three-dimensional cylinder–filament system2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 6, article id 130Article in journal (Refereed)
    Abstract [en]

    This experimental study reports on the behavior of a filament attached to the rear of a three-dimensional cylinder. The axis of the cylinder is placed normal to a uniform incoming flow, and the filament is free to move in the cylinder wake. The mean position of the filament is studied as a function of the filament length L. It is found that for long (L/D > 6.5, where D is the cylinder diameter) and short (L/D < 2) filaments, the mean position of the filament tends to align with the incoming flow, whereas for intermediate filament lengths (2 < L/D < 6.5), the filament lies down on the cylinder and tends to align with the cylinder axis. The underlying mechanism of the bifurcations is discussed and related to buckling and inverted-pendulum-like instabilities.

  • 5.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Håkansson, Karl
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Kvick, Mathias
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Evaluation of steerable filter for detection of fibres in flowing suspensions2011In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 51, no 4, p. 987-996Article in journal (Refereed)
    Abstract [en]

    Steerable filters are concluded to be useful in order to determine the orientation of fibers captured in digital images. The fiber orientation is a key variable in the study of flowing fiber suspensions. Here, digital image analysis based on a filter within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibers suspended in flowing suspensions. In sharp images with small noise levels, the steerable filter succeeds in determining the orientation of artificially generated fibers with well-defined angles. The influence of reduced image quality on the orientation has been quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than 1° for moderate levels. Images from two flow cases, one laminar shear flow and one turbulent, are also analyzed. The fiber orientation distribution is determined in the flow-vorticity plane. For the laminar case a comparison is made to a robust, but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found when comparing the resulting fiber orientation distributions obtained with the two methods. For the turbulent case, it is demonstrated that correct results are obtained and that the method can handle overlapping fibers. 

  • 6.
    Fransson, Jens H.M.
    KTH, Superseded Departments, Mechanics.
    Leading edge design process using a commercial flow solver2004In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 37, no 6, p. 929-932Article in journal (Refereed)
    Abstract [en]

    A design process of an asymmetric leading edge, for laminar stability measurements in a flat plate boundary layer, is reported. The purpose is to minimize the leading edge pressure gradient region, which affects the stability characteristics of the flow. Finally, the design success is verified by wind tunnel testing.

  • 7.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Westin, K. Johan A.
    KTH, Superseded Departments, Mechanics.
    Errors in hot-wire X-probe measurements induced by unsteady velocity gradients2002In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 32, no 3, p. 413-415Article in journal (Refereed)
    Abstract [en]

    Errors in hot-wire X-probe measurements due to unsteady velocity gradients are investigated by a comparison of hot-wire and laser Doppler velocimetry (LDV) measurements. The studied flow case is a laminar boundary layer subjected to high levels of free-stream turbulence, and the hot-wire data shows a local maximum in the wall-normal fluctuation velocity inside the boundary layer. The observed maximum is in agreement with existing hot-wire data, but in conflict with the present LDV measurements as well as existing results from numerical simulations. An explanation for the measurement error is suggested in the paper.

  • 8. Guillard, F.
    et al.
    Tragardh, C.
    Fuchs, Laszlo
    A study of turbulent mixing in a turbine-agitated tank using a fluorescence technique2000In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 28, no 3, p. 225-235Article in journal (Refereed)
    Abstract [en]

    Two-dimensional images of (Plane) Laser Induced Fluorescence (PLIF) have been used to study the turbulent mixing process in a model stirred tank. A calibration procedure is presented and discussed in terms of its accuracy. Data from the literature are used for comparison. A pattern-recognition algorithm has been designed to identify and quantitatively describe large-scale structures in the flow. This methodology, called structural analysis, is based on a conditional analysis of the PLIF data and requires the definition of an appropriate structure-detector function which is calculated locally. The mathematical tools developed have been used to study the mixing in a Rushton turbine-agitated reactor. Particular attention is paid to two specific regions of the tank; namely the bulk and the impeller stream regions, at two measured power input (0.3 and 0.7 W kg(-1)). The averaged concentration fields show a common two-dimensional steady circulation pattern. Concentration probability density functions reflect well the instability of the flow in the two regions investigated. The data reveal the non-isotropic distribution of these instabilities around a reference point when the feed port is situated in the bulk region only. In this case, the structural analysis quantitatively shows the presence of a folding of the concentration field. It was found that this phenomenon can last several seconds.

  • 9. Haggmark, C. P.
    et al.
    Bakchinov, A. A.
    Alfredsson, P. Henrik
    KTH, Superseded Departments, Mechanics.
    Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble2000In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 28, no 3, p. 236-242Article in journal (Refereed)
    Abstract [en]

    Measurements with a directional sensitive hot-wire probe have been carried out in a two-dimensional laminar separation bubble caused by an adverse pressure gradient. The probe has three parallel, in plane wires and can be traversed in the boundary layer in all spatial directions. The central wire, operated as a conventional hot-wire in CTA mode, and two surrounding resistance wires measure the instantaneous magnitude and direction of the flow, respectively. The probe is calibrated and operated in a similar way as a single hot-wire probe for boundary layer measurements. The frequency response is high enough for measurements of naturally occurring instability waves in the bubble. The flow direction intermittency was measured inside the bubble and regions with reversed flow were mapped out. Prior to reattachment periodical oscillations of the flow direction are found associated with shedding of vortical structures from the bubble.

  • 10. Hoseini, Afshin Abbasi
    et al.
    Zavareh, Zahra
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Anderson, Helge I.
    Rod-like particles matching algorithm based on SOM neural network in dispersed two-phase flow measurements2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 4, p. 1705-Article in journal (Refereed)
    Abstract [en]

    A matching algorithm based on self-organizing map (SOM) neural network is proposed for tracking rodlike particles in 2D optical measurements of dispersed two-phase flows. It is verified by both synthetic images of elongated particles mimicking 2D suspension flows and direct numerical simulations-based results of prolate particles dispersed in a turbulent channel flow. Furthermore, the potential benefit of this algorithm is evaluated by applying it to the experimental data of rod-like fibers tracking in wall turbulence. The study of the behavior of elongated particles suspended in turbulent flows has a practical importance and covers a wide range of applications in engineering and science. In experimental approach, particle tracking velocimetry of the dispersed phase has a key role together with particle image velocimetry of the carrier phase to obtain the velocities of both phases. The essential parts of particle tracking are to identify and match corresponding particles correctly in consecutive images. The present study is focused on the development of an algorithm for pairing non-spherical particles that have one major symmetry axis. The novel idea in the algorithm is to take the orientation of the particles into account for matching in addition to their positions. The method used is based on the SOM neural network that finds the most likely matching link in images on the basis of feature extraction and clustering. The fundamental concept is finding corresponding particles in the images with the nearest characteristics: position and orientation. The most effective aspect of this two-frame matching algorithm is that it does not require any preliminary knowledge of neither the flow field nor the particle behavior. Furthermore, using one additional characteristic of the non-spherical particles, namely their orientation, in addition to its coordinate vector, the pairing is improved both for more reliable matching at higher concentrations of dispersed particles and for higher robustness against loss of particle pairs between image frames.

  • 11.
    Håkansson, Karl M. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kvick, Mathias
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Measurement of width and intensity of particle streaks in turbulent flows2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 6, p. 1555-Article in journal (Refereed)
    Abstract [en]

    Fibre streaks are observed in experiments with fibre suspensions in a turbulent half-channel flow. The preferential concentration methods, most commonly used to quantify preferential particle concentration, are in one dimension found to be concentration dependent. Two different new streak quantification methods are evaluated, one based on Voronoi analysis and the other based on artificial particles with an assigned fixed width. The width of the particle streaks and a measure of the intensity of the streaks, i.e. streakiness, are sought. Both methods are based on the auto-correlation of a signal, generated by summing images in the direction of the streaks. Common for both methods is a severe concentration dependency, verified in experiments keeping the flow conditions constant while the (very dilute) concentration of fibres is altered. The fixed width method is shown to be the most suitable method, being more robust and less computationally expensive. By assuming the concentration dependence to be related to random noise, an expression is derived, which is shown to make the streak width and the streakiness independent of the concentration even at as low concentrations as 0.05 particles per pixel column in an image. The streakiness is obtained by applying an artificial particle width equal to 20 % of the streak width. This artificial particle width is in this study found to be large enough to smoothen the correlation without altering the streakiness nor the streak width. It is concluded that in order to make quantitative comparisons between different experiments or simulations, the evaluation has to be performed with care and be very well documented.

  • 12. Inasawa, Ayumu
    et al.
    Lundell, Fredrik
    KTH, Superseded Departments, Mechanics.
    Matsubara, Masaharu
    Kohama, Yasuaki
    Alfredsson, Henrik
    KTH, Superseded Departments, Mechanics.
    Velocity statistics and flow structures observed in bypass transition using stereo PTV2003In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 34, no 2, p. 242-252Article in journal (Refereed)
    Abstract [en]

    It is known from smoke visualizations that in a transitional boundary layer subjected to free-stream turbulence, streaks appear and eventually break down to turbulence after wavy motions. In order to observe the streaky structures directly, a stereo particle-tracking velocimetry system using hydrogen bubbles in a water channel has been developed and validated against laser Doppler velocimetry. Mean flow statistics show good agreement with previous results. With the developed measurement system, the instantaneous spanwise distribution of the streamwise and wall-normal velocities can be measured fast enough to resolve the time development of the streaky structures. Measurements of instantaneous spanwise distributions of the streamwise and wall-normal velocity disturbances show strong negative correlation between the wall-normal and streamwise velocities in the streaks.

  • 13.
    Kalpakli Vester, Athanasia
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    POD analysis of the turbulent flow downstream a mild and sharp bend2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 3, article id 57Article in journal (Refereed)
    Abstract [en]

    Time-resolved stereoscopic particle image velocimetry measurements have been taken of the turbulent flow at the exit plane of a mild and a sharp pipe bend. Cross-sectional flow fields were obtained 1, 2 and 3 pipe diameters downstream the bend in order to capture the flow evolution. Proper orthogonal decomposition (POD) was applied in order to identify the underlying vortical patterns and revealed the existence of a single cell spanning the whole cross section as the most dominant structure, while the Dean cells appeared in the next most energetic modes. The results from these investigations, which indicate the origin of the oscillatory motion of the Dean vortices, the so-called swirl switching, were found to agree with those presented by Hell-strom et al. (J Fluid Mech 735: R7, 2013). Furthermore, the effect of a honeycomb, mounted at the bend inlet, on the flow field has been studied by means of statistical and POD analysis in order to test the hypothesis by Sakakibara and Machida (Phys Fluids 24: 041702, 2012), viz. whether the unsteady behaviour of the Dean cells is related to large-scale structures existing upstream the bend. As a consequence of the honeycomb, the Dean vortices do not appear in the mean field, nor in the most energetic modes, which opens possibilities to overcome or at least delay the problem of fatigue in piping systems which can be caused by the swirl switching.

  • 14.
    Kurian, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Transient growth in the asymptotic suction boundary layer2011In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 51, no 3, p. 771-784Article in journal (Refereed)
    Abstract [en]

    In the present experimental setup, the transient disturbance growth in a spatially invariant boundary layer flow, i.e., the asymptotic suction boundary layer (ASBL), has been investigated. The choice of the ASBL brings along several advantages compared with an ordinary spatially growing boundary layer. A unique feature of the ASBL is that the Reynolds number (Re) can be varied without changing the boundary layer thickness, which in turn allows for parameter variations not possible to carry out in traditional boundary layer flows. A spanwise array of discrete surface roughness elements was mounted on the surface to trigger modes with different spanwise wavenumbers (beta). It is concluded that for each mode there exists a threshold roughness Reynolds number (Re (k) ), below which no significant transient growth is present. The experimental data suggests that this threshold Re (k) is both a function of beta and Re. An interesting result is that the energy growth curves respond differently to a change in Re (k) when caused by a change in roughness height k, implying that Re remains constant, compared with a change in the free-stream velocity U(infinity), which also affects the Re. The scaling of the energy rowth curves both in level and the downstream direction is treated and appropriate scalings are found. The result shows a complex non-linear receptivity mechanism. Optimal perturbation theory, which has failed to predict the energy evolution in growing boundary layers, is tested for the ASBL and shows that it may satisfactorily predict the evolution of all transiently growing modes that are triggered by the roughness elements.

  • 15.
    Lacagnina, Giovanni
    et al.
    Sapienza University of Rome.
    Grizzi, Silvano
    CNR-INSEAN.
    Falchi, Massimo
    CNR-INSEAN.
    Di Felice, Fabio
    CNR-INSEAN.
    Romano, Giovanni Paolo
    Sapienza University of Rome.
    Simultaneous size and velocity measurements of cavitating microbubbles using interferometric laser imaging2011In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 50, p. 1153-1167Article in journal (Refereed)
    Abstract [en]

    In this paper, interferometric laser imaging droplet sizing—ILIDS—is applied to incipient cavitation in the wake of a marine propeller model with the aim to evaluate simultaneously bubbles velocity and diameter. Until now, the feasibility of this technique has been demonstrated especially in sprays of water droplets in air where an optimal light scattering is obtained thanks to the spherical shape and to the given relative refractive index. In the present setup, to allow simultaneous size–velocity measurements, a single camera is used and the object distance over lens diameter ratio is kept as small as possible, thus increasing the size measurement resolution. These details, together with the algorithms used for image analysis at each single frame and in two consecutive frames, allow deriving cavitation bubble size and velocity distributions in the propeller wake.

  • 16.
    Lacagnina, Giovanni
    et al.
    Sapienza University of Rome.
    Romano, Giovanni Paolo
    Sapienza University of Rome.
    PIV investigations on optical magnification and small scales in the near-field of an orifice jet2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56Article in journal (Refereed)
    Abstract [en]

    Among turbulent jets, the orifice jet is still far from being fully understood especially due to the phenomenon of “vena contracta”, which takes place in the near field. In this work, the attention is focused to large-scale statistics (mean field and higher statistical moments of the velocity) and to small-scale statistics (velocity derivative statistical moments) of such a jet, with a special focus onto the effect of optical magnification on particle image velocimetry measurements. In order to investigate this effect on the measurements, framed areas of different size have been considered by changing the magnification factor through the object plane distance. The obtained results show that a limit on the choice of this parameter exists for a proper description of both small and large scales.

  • 17.
    Lacagnina, Giovanni
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Institute of Sound and Vibration Research (ISVR), University of Southampton, United Kingdom.
    Szász, Robert-Zoltán
    Department of Energy Sciences, LTH Faculty of Engineering, Lund University.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental study on the forcing design for an intermittent injection2018In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 59, no 8, article id 123Article in journal (Refereed)
    Abstract [en]

    In this paper we consider the effects of acceleration and deceleration on the forcing of an intermittent jet. This experimental study specifically focuses on the effect of the acceleration and deceleration on the mixing of an intermittent jet with the ambient fluid and on the growth of disturbances that may lead to turbulence. The influence of different injection strategies has been evaluated. The results show that the deceleration phase may be able to contribute significantly to enhance the mixing of the jet with the ambient fluid. This effect is manifested primarily around the tail of the jet, towards the end of injection. The acceleration phase on the other hand has mainly impact at the leading part of the jet, where the leading part of the jet forms a mushroom shaped structure with minor mixing effect.

  • 18.
    Lindgren, Björn
    et al.
    KTH, Superseded Departments, Mechanics.
    Johansson, Arne V.
    KTH, Superseded Departments, Mechanics.
    Evaluation of a new wind tunnel with expanding corners2004In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 36, no 1, p. 197-203Article in journal (Refereed)
    Abstract [en]

    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.

  • 19.
    Lindgren, Björn
    et al.
    KTH, Superseded Departments, Mechanics.
    Österlund, Jens, M.
    KTH, Superseded Departments, Mechanics.
    Johansson, Arne, V.
    KTH, Superseded Departments, Mechanics.
    Measurement and calculation of guide vane performance in expanding bends for wind-tunnels1998In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 24, p. 265-272Article in journal (Refereed)
    Abstract [en]

    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.

  • 20.
    Lundell, Fredrik
    KTH, Superseded Departments, Mechanics.
    Pulse-width modulated blowing/suction as a flow control actuator2003In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 35, no 5, p. 502-504Article in journal (Refereed)
    Abstract [en]

    The effect of pulse-width modulated (PWM) blowing/suction through a hole in a flat plate, above which a Blasius boundary layer develops, is studied. A hot wire is used to measure the streamwise velocity downstream of the hole. The PWM blowing/suction is found to create amplitude-modulated variations in the streamwise velocity, which are well correlated with the signal generating the disturbance.

  • 21.
    Löfdahl, Lennart
    et al.
    Thermo and Fluid Dynamics, Chalmers University of Technology.
    Chernoray, Valery
    Thermo and Fluid Dynamics Department, Chalmers University of Technology.
    Haasl, Sjoerd
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Stemme, Göran
    KTH, Superseded Departments, Signals, Sensors and Systems.
    Sen, Mihir
    Dept. of Aeros. and Mech. Eng., University of Notre Dame.
    Characteristics of a hot-wire microsensor for time-dependent wall shear stress measurements2003In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 35, no 3, p. 240-251Article in journal (Refereed)
    Abstract [en]

    Hot-wire microsensors for the purpose of measuring the instantaneous velocity gradient close to a wall were designed and their characteristics were evaluated. The sensors were made using MEMS (microelectromechanical systems) technology, which permits the fabrication of various microgeometrical configurations with high precision and good repeatability. The design is based on estimates of the heat rates from the sensor wire to the air, through the supports, and to the wall. Several hot-wire configurations were fabricated with wires positioned in the range 50-250 mum from the wall. Requirements for the design and details of the fabrication methodology are outlined. The hot-wire microsensors were calibrated and tested in a flat-plate boundary layer with and without pressure gradients and were found to have good steady-state characteristics. In addition, the developed sensors were used for preliminary studies of transitional phenomena and turbulence, and the sensors were found to have a good time-dependent response as well.

  • 22. MacKenzie, J.
    et al.
    Söderberg, D.
    Swerin, Agne
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Lundell, F.
    Turbulent stress measurements with phase-contrast magnetic resonance through tilted slices2017In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 58, no 5, article id 51Article in journal (Refereed)
    Abstract [en]

    Aiming at turbulent measurements in opaque suspensions, a simplistic methodology for measuring the turbulent stresses with phase-contrast magnetic resonance velocimetry is described. The method relies on flow-compensated and flow-encoding protocols with the flow encoding gradient normal to the slice. The experimental data is compared with direct numerical simulations (DNS), both directly but also, more importantly, after spatial averaging of the DNS data that resembles the measurement and data treatment of the experimental data. The results show that the most important MRI data (streamwise velocity, streamwise variance and Reynolds shear stress) is reliable up to at least r¯ = 0.75 without any correction, paving the way for dearly needed turbulence and stress measurements in opaque suspensions.

  • 23. Miles, Paul C.
    et al.
    Hildingsson, Leif
    Hultqvist, Anders
    Lund Institute of Technology, Sweden.
    The influence of fuel injection and heat release on bulk flow structures in a direct-injection, swirl-supported diesel engine2007In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 43, no 2-3Article in journal (Refereed)
    Abstract [en]

    Particle image velocimetry is applied to measure the vertical (r-z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume.

  • 24.
    Mårtensson, Gustaf E.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Gunnarsson, Jonas
    Bombardier Transportation, Västerås, Sweden.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Moberg, Hans
    Alfa Laval, 14780 Tumba, Sweden.
    Experimental investigation of a rapidly rotating turbulent duct flow2002In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 33, p. 482-487Article in journal (Refereed)
    Abstract [en]

    Abstract Rapidly rotating duct flow is studied experi- mentally with Rotation numbers in the interval [0, 1]. To achieve this, in combination with relatively high Reynolds numbers (5,000–30,000 based on the hydraulic radius), water was used as the working medium. Square and rect- angular duct cross-sections were used and the angle be- tween the rotation vector and the main axis of the duct was varied. The influence of the rotation on the pressure drop in the duct was investigated and suitable scalings of this quantity were studied. 

  • 25.
    Mårtensson, Gustaf
    et al.
    KTH, Superseded Departments, Mechanics.
    Gunnarsson, J.
    Johansson, Arne V.
    KTH, Superseded Departments, Mechanics.
    Moberg, Hans O.
    KTH, Superseded Departments, Mechanics.
    Experimental investigation of rapidly rotating turbulent duct flow2002In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 33, no 3, p. 482-487Article in journal (Refereed)
    Abstract [en]

    Rapidly rotating duct flow is studied experimentally with Rotation numbers in the interval [0, 1]. To achieve this, in combination with relatively high Reynolds numbers (5,000-30,000 based on the hydraulic radius), water was used as the working medium. Square and rectangular duct cross-sections were used and the angle between the rotation vector and the main axis of the duct was varied. The influence of the rotation on the pressure drop in the duct was investigated and suitable scalings of this quantity were studied.

  • 26.
    Odemark, Ylva
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The stability and development of tip and root vortices behind a model wind turbine2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 9, p. 1591-Article in journal (Refereed)
    Abstract [en]

    When designing new wind farms, one has to rely on models describing the flow field around and inside the farm, since direct numerical simulation is far too computationally expensive. In order to develop better models for power prediction of wind farms, knowledge about the flow field around turbines, the stability of the wakes and the interaction between them is essential. Since the conditions during field measurements are difficult to control, wind tunnel measurements play an important role when studying wakes behind wind turbines. Within the present work, an experimental methodology has been developed to study the evolution and stability of the tip vortices shed from the rotor blades of a small-scale turbine model. The stability of the tip vortices was studied by introducing a periodic disturbance to the flow, which is shown to have a clear effect on the development of the vortices. Prior to the vortex breakdown, clear signs of vortex pairing were also observed. A parameter study was performed by varying the amplitude and frequency of the forced disturbance, and the effect on the tip vortices was evaluated. This experiment is one of the first where the influence of a periodic disturbance on a wind turbine wake is studied, something that previously has been performed in a number of numerical studies.

  • 27.
    Otendal, Mikael
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Hemberg, Oskar
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Tuohimaa, Tomi
    KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Hertz, Hans M.
    KTH, School of Engineering Sciences (SCI), Physics. KTH, School of Engineering Sciences (SCI), Applied Physics, Biomedical and X-ray Physics.
    Microscopic High-Speed Liquid-Metal Jets in Vacuum2005In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 39, p. 799-804Article in journal (Refereed)
    Abstract [en]

    We describe a novel electron-impact x-ray source based on a high-speed liquid-metal-jet anode. Thermal power load calculations indicate that this new anode concept potentially could increase the achievable brightness in compact electron-impact x-ray sources by more than a factor 100 compared to current state-of-the-art rotating-anode or microfocus sources. A first, successful, low-power proof-of-principle experiment is described and the feasibility of scaling to high-brightness and high-power operation is discussed. Some possible applications that would benefit from such an increase in brightness are also briefly

  • 28.
    Sattarzadeh, Sohrab S.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the scaling of streamwise streaks and their efficiency to attenuate Tollmien-Schlichting waves2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 3, article id 58Article in journal (Refereed)
    Abstract [en]

    Streaky boundary layers generated by an array of miniature vortex generators (MVGs) mounted on a flat plate have recently shown to have a stabilizing effect on both two-and three-dimensional disturbances. An experimental study on the effect of the geometrical parameters of MVGs on the generated streamwise streaks in the flat plate boundary layer is carried out, and the corresponding stabilizing effect on Tollmien-Schlichting (TS) wave disturbances is quantified. The new experimental configurations have led to an improved empirical scaling law, which includes additional geometrical parameters of the MVGs compared to the previously reported relation. It is found that the MVG configuration can be optimized with respect to the attenuation of disturbances. In addition, the streamwise location of branch I of the neutral stability curve, with regard to the location of the MVG array, is found to be correlated with the initial receptivity of TS waves on the MVG array and the attenuation of the TS wave amplitude in the unstable region.

  • 29.
    Sattarzadeh, Sohrab S.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Spanwise boundary layer modulations using finite discrete suction for transition delay2017In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 58, no 3, article id 14Article in journal (Refereed)
    Abstract [en]

    Discrete suction is deployed in a flat plate boundary layer to create spanwise mean velocity gradients (SVG) with the goal of delaying the onset of laminar-to-turbulent transition. It is shown that finite boundary layer suction through a set of holes in a spanwise oriented array in the flat plate is successful in setting up steady and robust streamwise streaks in the boundary layer. Today, the SVG method for transition control is known to attenuate the growth of Tollmien-Schlichting (TS) waves and delay the transition to turbulence. In this investigation, low-amplitude forced TS waves are attenuated with the implication of extending the laminar flow by at least 120% for a discrete suction of 0.8% of the free-stream velocity. The control technique is also tested successfully for natural transition, with a resulting transition delay of 30%.

  • 30.
    Segalini, Antonio
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bellani, Gabriele
    Sardina, Gaetano
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Variano, Evan A.
    Corrections for one- and two-point statistics measured with coarse-resolution particle image velocimetry2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 6, p. 1739-Article in journal (Refereed)
    Abstract [en]

    A theoretical model to determine the effect of the size of the interrogation window in particle image velocimetry measurements of turbulent flows is presented. The error introduced by the window size in two-point velocity statistics, including velocity autocovariance and structure functions, is derived for flows that are homogeneous within a 2D plane or 3D volume. This error model is more general than those previously discussed in the literature and provides a more direct method of correcting biases in experimental data. Within this model framework, simple polynomial approximations are proposed to provide a quick estimation of the effect of the averaging on these statistics. The error model and its polynomial approximation are validated using statistics of homogeneous isotropic turbulence obtained in a physical experiment and in a direct numerical simulation. The results demonstrate that the present formulation is able to correctly estimate the turbulence statistics, even in the case of strong smoothing due to a large interrogation window. We discuss how to use these results to correct experimental data and to aid the comparison of numerical results with laboratory data.

  • 31.
    Segalini, Antonio
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Uncertainty analysis of the von Kármán constant2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 2, p. 1460-Article in journal (Refereed)
    Abstract [en]

    In 1930, von Karman presented an expression for the mean velocity distribution in channel and pipe flows that can be transformed into the today well-known logarithmic velocity distribution. At the same time, he also formulated the logarithmic skin friction law and obtained a value of 0.38 for the constant named after him through pipe flow pressure drop measurements. Different approaches to determine the von Karman constant from mean velocity measurements have been proposed over the last decades, sometimes giving different results even when employed on the same data, partly because the range over which the logarithmic law should be fitted is also under debate. Up to today, the research community has not been able to converge toward a single value and the favored values range between 0.36 and 0.44 for different research groups and canonical flow cases. The present paper discusses some pitfalls and error sources of commonly employed estimation methods and shows, through the use of boundary layer data from Osterlund (1999) that von Karman's original suggestion of 0.38 seems still to be valid for zero pressure gradient turbulent boundary layer flows. More importantly, it is shown that the uncertainty in the determination of the von Karman constant can never be less than the uncertainty in the friction velocity, thereby yielding a realistic uncertainty for the most debated constant in wall turbulence.

  • 32.
    Semeraro, Onofrio
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Analysis of time-resolved PIV measurements of a confined turbulent jet using POD and Koopman modes2012In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 53, no 5, p. 1203-1220Article in journal (Refereed)
    Abstract [en]

    We present a comparative analysis of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) computed from experimental data of a turbulent, quasi 2-D, confined jet with co-flow (Re = 11,500, co-flow ratio inner-to-outer flow approximate to 2:1). The experimental data come from high-speed 2-D particle image velocimetry. The flow is fully turbulent, and it contains geometry-dependent large-scale coherent structures; thus, it provides an interesting benchmark case for the comparison between POD and DMD. In this work, we address issues related to snapshot selections (1), convergence (2) and the physical interpretation (3) of both POD and DMD modes. We found that the convergence of POD modes follows the criteria of statistical convergence of the autocovariance matrix. For the computation of DMD modes, we suggest a methodology based on two criteria: the analysis of the residuals to optimize the sampling parameters of the snapshots, and a time-shifting procedure that allows us to identify the spurious modes and retain the modes that consistently appear in the spectrum. These modes are found to be the ones with nearly null growth rate. We then present the selected modes, and we discuss the way POD and DMD rank them. POD analysis reveals that the most energetic spatial structures are related to the large-scale oscillation of the inner jet (flapping); from the temporal analysis emerges that these modes are associated with a low-frequency peak at St = 0.02. At this frequency, DMD identifies a similar mode, where oblique structures from the walls appear together with the flapping mode. The second most energetic group of modes identified is associated with shear-layer oscillations, and to a recirculation zone near the inner jet. Temporal analysis of these modes shows that the flapping of the inner jet might be sustained by the recirculation. In the DMD, the shear-layer modes are separated from the recirculation modes. These have large amplitudes in the DMD. In conclusion, the DMD modes with eigenvalues on the unit circle are found to be similar to the most energetic POD modes, although differences appear due to the fact that DMD isolates structures associated with one frequency only.

  • 33. Simon, B.
    et al.
    Fabbiane, Nicolo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nemitz, T.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Grundmann, S.
    In-flight active wave cancelation with delayed-x-LMS control algorithm in a laminar boundary layer2016In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 57, no 10, article id 160Article in journal (Refereed)
    Abstract [en]

    This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broadband Tollmien–Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a ‘black-box’ system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere.

  • 34. Simon, Bernhard
    et al.
    Fabbiane, Nicolo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nemitz, Timotheus
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    In-flight active-wave-cancelation via delayed-x-LMS control algorithm in a laminar boundary layerIn: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114Article in journal (Refereed)
    Abstract [en]

    This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broad-band Tollmien-Schlichting (TS) disturbances is per- formed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with

    experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a “black box” system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS control algorithm allows to operate the controller without a model and an adaption based on varying conditions that may occur during flight in atmosphere. 

  • 35.
    Suryadi, Alexandre
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Velocity measurements of streamwise roll cells in rotating plane Couette flow2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 11, p. 1617-Article in journal (Refereed)
    Abstract [en]

    For the first time, quantitative velocity measurements in rotating plane Couette flow are demonstrated. Particle image velocimetry is used at a low Reynolds number with anti-cyclonic, i.e. destabilising rotation, where the instability is expected to give rise to steady streamwise-oriented roll cells. The streamwise and span-wise velocities of the roll cells were measured on the centreplane of the flow and at two planes on either side. The streamwise velocity is spanwise periodic with an amplitude variation approximately +/- 42 % of half the velocity difference between the moving walls. The wall-normal velocity was estimated by assuming steady, laminar and streamwise-independent flow. Despite the large amplitude of the disturbance, both the spanwise wave length and amplitude ratio between the streamwise and wall-normal components were close to what is obtained from linear theory. A splitting event of a roll cell was also captured by the velocity measurements and its development followed in time.

  • 36. Talamelli, A.
    et al.
    Westin, K. J. A.
    Alfredsson, P. Henrik
    KTH, Superseded Departments, Mechanics.
    An experimental investigation of the response of hot-wire X-probes in shear flows2000In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 28, no 5, p. 425-435Article in journal (Refereed)
    Abstract [en]

    The response of her-wire X-probes in regions of strong velocity gradients (such as in the near wall region of boundary layer flows) is investigated experimentally. Although the wall-normal velocity component should be close to zero near the wall, one usually encounters an increased absolute value of this component when the wall is approached. Moreover some physically inconsistent behaviour in other measured quantities, for instance the Reynolds stresses, may be found. These effects can be due to a physical displacement of the wires (e.g. a wall-normal displacement of the two wires so that they do not cross at their mid-points), but also due to the influence of the probe on the local flow field. The latter might be an effect of blockage or wall interference and can be treated as a virtual displacement. The response equations of an X-probe with different wall-normal displacement of the wires are derived and applied in order to correct the measured data. A systematic experimental investigation of the effect of varying the physical displacement of the wires is also made, and it is shown that both the first and second order correction terms of the probe response equations can be estimated from this experiment. A correction procedure for measurements close to the wall is proposed and used to correct Reynolds stress profiles in a flat plate boundary-layer. It is also shown that the present experimental set-up can be used to estimate some turbulence correlations which otherwise are unaccessible with standard measurement techniques.

  • 37.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Buresti, Guido
    A note on the effect of the separation wall in the initial mixing of coaxial jets2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 3, p. 1483-Article in journal (Refereed)
    Abstract [en]

    Experiments are carried out to characterize the mixing field of two coaxial jet configurations, having, respectively, a thick and a sharp inner duct wall. The influence of the separation wall thickness on the initial development of the "mixing transition'' of the two streams is analysed as a function of the jet velocity ratio through measurements with hot-and cold-wire anemometry and by using temperature as a passive scalar. To study the mixing at the near-exit region, a new thickness measure based on the mean scalar concentration is introduced. It is shown that the presence of a sufficiently thick wall significantly increases the interpenetration between the two streams, effectively enhancing the mixing process. However, this is observed only for nearly unitary velocity ratios, which correspond to the existence of a regular alternate vortex shedding from the two sides of the inner duct wall. Conversely, for small and large velocity ratios, the difference in mixing between the two geometries greatly decreases and becomes of the order of the experimental accuracy.

  • 38.
    Talamelli, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Bologna, Italy.
    Correcting hot-wire spatial resolution effects in third- and fourth-order velocity moments in wall-bounded turbulence2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 4, p. 1496-Article in journal (Refereed)
    Abstract [en]

    Spatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22: 104508, 2011) quantifying the effect of a linear spatial filter of hot-wire probes on the mean and the variance of the streamwise velocity in turbulent wall-bounded flows, to describe the effect of the spatial filtering on the third-and fourth-order moments of the same velocity component. The model, based on the three-(four) point velocity-correlation function for the third-(fourth-) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the second-order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the third-order moment in the region where a zero-crossing of the third-order function is observed and where the equations become ill-conditioned. An "a posteriori" correction procedure, based on the developed model, to correct the measured third-and fourth-order velocity moments is also presented. This procedure, based on combining the measured data by two single hot-wire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693-700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the third-order moment in the region where the latter exhibits a zero-crossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments.

  • 39.
    Todde, Valentino
    et al.
    Högskolan i Gävle, KTHs-forskarskola för inomhusmiljö.
    Spazzini, Pier Giorgio
    Sandberg, Mats
    Högskolan i Gävle, KTHs-forskarskola för inomhusmiljö.
    Experimental analysis of low-Reynolds number free jets2009In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 47, no 2, p. 279-294Article in journal (Refereed)
    Abstract [en]

    The present paper analyzes the features of a low-Reynolds number free submerged jet with special regard to statistical quantities on the jet centerline. Measurements in an environment with very low disturbances allowed to observe details of turbulence and higher-order moments. Some peculiar features of the measured (natural) jet are shown to be in correspondence to observations referring to forced higher-Reynolds number jets. In particular, it is shown that, at low Reynolds numbers, the initial region of the jet is dominated by well-defined vortices in the shear layer. This result is substantiated by both the statistical moments and the spectral analysis. The presence of two distinct regimes is evidenced and discussed from a physical standpoint, also in relation to the mathematical analysis of the jet structure from the bibliography.

  • 40.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nagib, Hassan M.
    Role of data uncertainties in identifying the logarithmic region of turbulent boundary layers2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 6, p. 1751-Article in journal (Refereed)
    Abstract [en]

    Composite expansions based on the log-law and the power-law were used to generate synthetic velocity profiles of zero pressure gradient turbulent boundary layers (TBLs) in the range of Reynolds number 800 <= Re-theta <= 860; 000; based on displacement thickness and free-stream velocity. Several artificial errors were added to the velocity profiles to simulate typical measurement uncertainties. The effects of the simulated errors were studied by extracting log-law and power-law parameters from all these pseudo-experimental profiles. Various techniques were used to establish a measure of the deviations in the overlap region. When parameters extracted for the log-law and the power-law are associated with similar levels of deviations with respect to their expected values, we consider that the profile leads to ambiguous conclusions. This ambiguity was observed up to Re-theta 16; 000 for a 4 % dispersion in the velocity measurements, up to Re-theta 8.6 x 10(5) for a 400 mu m uncertainty in probe position (in air flow at atmospheric pressure), and up to Re-theta 32; 000 for 3 % uncertainty in the determination of u(tau): In addition, a new method for the determination of the log-law limits is proposed. The results clearly serve as a further note for caution when identifying either a log or a power-law in TBLs. Together with a number of available studies in the literature, the present results can be seen as a additional reconfirmation of the log-law.

  • 41.
    Winroth, Marcus
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Ford, Christopher L.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    On discharge from poppet valves: effects of pressure and system dynamics2018In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 59, no 2, article id 24Article in journal (Refereed)
    Abstract [en]

    Simplified flow models are commonly used to design and optimize internal combustion engine systems. The exhaust valves and ports are modelled as straight pipe flows with a corresponding discharge coefficient. The discharge coefficient is usually determined from steady-flow experiments at low pressure ratios and at fixed valve lifts. The inherent assumptions are that the flow through the valve is insensitive to the pressure ratio and may be considered as quasi-steady. The present study challenges these two assumptions through experiments at varying pressure ratios and by comparing measurements of the discharge coefficient obtained under steady and dynamic conditions. Steady flow experiments were performed in a flow bench, whereas the dynamic measurements were performed on a pressurized, 2 l, fixed volume cylinder with one or two moving valves. In the latter experiments an initial pressure (in the range 300–500 kPa) was established whereafter the valve(s) was opened with a lift profile corresponding to different equivalent engine speeds (in the range 800–1350 rpm). The experiments were only concerned with the blowdown phase, i.e. the initial part of the exhaustion process since no piston was simulated. The results show that the process is neither pressure-ratio independent nor quasi-steady. A measure of the “steadiness” has been defined, relating the relative change in the open flow area of the valve to the relative change of flow conditions in the cylinder, a measure that indicates if the process can be regarded as quasi-steady or not.

  • 42.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Comment on the scaling of the near-wall streamwise variance peak in turbulent pipe flows2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 1, p. 1431-Article in journal (Refereed)
    Abstract [en]

    Apparent contradictory results regarding the Reynolds number (Re) scaling of the near-wall peak of the streamwise velocity variance distribution in turbulent pipe flows (Ng et al. Exp Fluids 51, 1261 and Vallikivi et al. Exp. Fluids 51, 1521) are discussed. Conclusions from the Princeton University/ONR Superpipe published between 2010 and 2012 are examined, doubts are raised, and recent numerical and experimental results are supplemented. The compiled results support the view that the inner-scaled peak amplitude increases with Re (at least to a Kármán number (R+) of 3,000) in contrast to the view put forward by several publications based on results from the Superpipe. Consequently, there is a need for further experimental results for R+ &gt; 3,000 to ascertain the trend at higher Re.

  • 43.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On spatial resolution issues related to time-averaged quantities using hot-wire anemometry2010In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 49, no 1, p. 101-110Article in journal (Refereed)
    Abstract [en]

    The effect of spatial resolution on streamwise velocity measurements with single hot-wires is targeted in the present study, where efforts have been made to distinguish between spatial resolution and Reynolds number effects. The basis for measurements to accurately determine the mean velocity and higher order moments is that the probability density distribution is measured correctly. It is well known that the turbulence intensity is increasingly attenuated with increasing wire length. Here, it is also shown (probably for the first time) that besides the probability density distribution and hence the higher order moments, even the mean velocity is affected, albeit to subtle extent, but with important consequences in studies of concurrent wall-bounded turbulence.

  • 44.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Malizia, Fabio
    Cimarelli, Andrea
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Univ Bologna, Italy.
    The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence2014In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 7, p. 1781-Article in journal (Refereed)
    Abstract [en]

    There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations-as they are, e. g. encountered in non-isothermal flows-on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces-despite a local mean temperature compensation of the velocity reading-significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.

  • 45.
    Örlü, Ramis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Comparison of experiments and simulations for zero pressure gradient turbulent boundary layers at moderate Reynolds numbers2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 6, p. 1547-Article in journal (Refereed)
    Abstract [en]

    A detailed comparison between recent direct numerical simulation (DNS) and experiments of a turbulent boundary layer under zero pressure gradient at Re-theta = 2,500 and 4,000 (based on the free-stream velocity and momentum-loss thickness) is presented. The well-resolved DNS is computed in a long spatial domain (Schlatter and Orlu in J Fluid Mech 659: 116, 2010a), including the disturbance strip, while the experiments consist of single hot-wire probe and oil-film interferometry measurements. Remarkably, good agreement is obtained for integral quantities such as skin friction and shape factor, as well as mean and fluctuating streamwise velocity profiles, higher-order moments and probability density distributions. The agreement also extends to spectral/structural quantities such as the amplitude modulation of the small scales by the large-scale motion and temporal spectral maps throughout the boundary layer. Differences within the inner layer observed for statistical and spectral quantities could entirely be removed by spatially averaging the DNS to match the viscous-scaled length of the hot-wire sensor, thereby explaining observed differences solely by insufficient spatial resolution of the hot-wire sensor. For the highest Reynolds number, Re-theta = 4,000, the experimental data exhibit a more pronounced secondary spectral peak in the outer region (y/delta(99) = 0.1) related to structures with length on the order of 5-7 boundary layer thicknesses, which is weaker and slightly moved towards lower temporal periods in the DNS. The cause is thought to be related to the limited spanwise box size which constrains the growth of the very large structures. In the light of the difficulty to obtain "canonical" flow conditions, both in DNS and the wind tunnel where effects such as boundary treatment, pressure gradient and turbulence tripping need to be considered, the present cross-validation of the data sets, at least for the present Re-theta-range, provides important reference data for future studies and highlights the importance of taking spatial resolution effects into account when comparing experiment and DNS. For the considered flow, the present data also provide quantitative guidelines on what level of accuracy can be expected for the agreement between DNS and experiments.

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