Authors:Ressa Octavianty; Masahito Asai Abstract: Abstract
Effects of short splitter plates on vortex shedding and sound generation in a low subsonic flow past two side-by-side square cylinders were examined experimentally at Reynolds numbers
\({{{Re}}} = 1.0 - 3.3 \times 10^4\)
. The experiment was mainly conducted with the center-to-center distance between the two cylinders of 3.6d (d is the side length of a square cylinder) where vortex shedding from the two cylinders was synchronized with anti-phase relation, generating a quadrupole-like sound source that radiated in-phase sound in the far field. The results showed that the attachment of short splitter plates whose length (c) was ≤0.5d could reduce the sound pressure level of Aeolian tone significantly. Even with the shortest splitter plates of
\(c{/}d=0.1\)
, SPL was reduced by 6 dB at Mach number
\(M_{\infty }=0.15\)
. This was in contrast to the case of a single square cylinder, for which the attachment of a short splitter plate <0.2d led to no noticeable noise reduction. It was also shown that even when short splitter plates with a spanwise length as long as or less than the correlation length of shed vortices were attached on the two cylinders in a staggered array, the anti-phase vortex shedding and the corresponding in-phase sound radiation were still dominant. PubDate: 2016-08-20 DOI: 10.1007/s00348-016-2227-4 Issue No:Vol. 57, No. 9 (2016)

Authors:M. van Nesselrooij; L. L. M. Veldhuis; B. W. van Oudheusden; F. F. J. Schrijer Abstract: Abstract
Direct force measurements and particle image velocimetry (PIV) were used to investigate the drag and flow structure caused by surfaces with patterns of shallow spherical dimples with rounded edges subject to turbulent boundary layers. Drag reduction of up to 4 % is found compared to a flat surface. The largest drag reduction was found at the highest tested Reynolds number of 40,000 (based on dimple diameter). A favorable trend promises further improvements at higher Reynolds numbers. PIV revealed the absence of significant separation inside the dimples but did show the existence of a converging/diverging flow in the upstream and downstream dimple half, respectively. This leads to the rejection of theories proposed by other authors concerning the mechanism responsible for drag reduction. Instead, a fundamental dependence on pattern orientation is observed. Furthermore, preliminary Reynolds-averaged Navier–Stokes (RANS) simulations have been compared with the PIV data. Although the large-scale mean flows show good agreement, the numerical simulation predicts no drag reduction. As the RANS approach is inherently incapable of resolving effects on the behavior of small-scale turbulence structure, the origin of drag reduction is attributed to effects on the small-scale turbulence, which is not resolved in the simulations. It is argued that dimples, when placed in well-designed patterns to create the necessary large-scale flow structure, lead to drag reduction by affecting the turbulent structures in the boundary layer, possibly in a way similar to spanwise oscillations of the wall. PubDate: 2016-08-20 DOI: 10.1007/s00348-016-2230-9 Issue No:Vol. 57, No. 9 (2016)

Authors:Ron Danon; James W. Gregory; David Greenblatt Abstract: Abstract
The transient flow of a two-dimensional wall-jet over a circular cylinder, following rapid initiation and termination, was investigated experimentally. Unsteady surface pressures and unsteady pressure-sensitive paint were used to gain a basic understanding of the flow physics. Jet initiation produced a starting vortex, upstream of which the Coandă flow developed, producing a large low-pressure peak. Immediately following jet termination, the pressure increased over the first quarter of the circumference, while the downstream separation region remained virtually unaffected. Simplifying analyses and dimensional arguments were used to show that the timescales characterizing the transient development of the integrated loads depend only on the square of the slot height and the kinematic viscosity and are thus independent of the jet velocity. Following jet initiation, the resulting loads varied according to a linear transient model, while small nonlinearities were observed following jet termination. Unsteady pressure-sensitive paint showed that the starting jet emerges from the slot in a two-dimensional manner and that streamwise streaks, identified as Görtler vortices, form well before the flow reaches steady state. During termination, the streamwise structures dissipate downstream initially, with the dissipation propagating upstream. PubDate: 2016-08-19 DOI: 10.1007/s00348-016-2226-5 Issue No:Vol. 57, No. 9 (2016)

Authors:Timothy Ombrello; David L. Blunck; Michael Resor Abstract: Abstract
The utility of quantified infrared radiation imaging was evaluated through interrogating ignition and burning processes within a cavity-based flameholder in supersonic flows. Two ignition techniques, spark discharge and pulse detonation, along with quasi-steady cavity burning were used to assess the sensitivities of measurements of radiation intensities in the infrared. The shedding of ignition kernels from the spark discharge was imaged, showing that sufficient signal-to-noise ratios can be achieved even with weak radiation emission levels. The ignition events using a pulse detonator were captured with time-resolved measurements of the plume evolution, including the barrel shock, Mach disk, and shock diamonds. Radiation emissions from subsequent firings of the pulse detonator increased, indicating that heat loss to the tube walls occurred in the early pulses. Imaging of the quasi-steady burning within the cavity demonstrated that the highest burning flux (visible broadband chemiluminescence) and radiation from hydrocarbons (3.4 µm) do not coincide with each other for the fueling strategy used. Numerical simulations provided insight into the species distributions that caused the infrared emissions. Overall, infrared radiation measurements have been shown to be feasible through combustor windows in the harsh combustion environments that were interrogated, and offer a new avenue for rapid and quantitative measurements of reactive flow. PubDate: 2016-08-18 DOI: 10.1007/s00348-016-2210-0 Issue No:Vol. 57, No. 9 (2016)

Authors:Jan F. G. Schneiders; Fulvio Scarano Abstract: Abstract
A method is proposed to reconstruct the instantaneous velocity field from time-resolved volumetric particle tracking velocimetry (PTV, e.g., 3D-PTV, tomographic PTV and Shake-the-Box), employing both the instantaneous velocity and the velocity material derivative of the sparse tracer particles. The constraint to the measured temporal derivative of the PTV particle tracks improves the consistency of the reconstructed velocity field. The method is christened as pouring time into space, as it leverages temporal information to increase the spatial resolution of volumetric PTV measurements. This approach becomes relevant in cases where the spatial resolution is limited by the seeding concentration. The method solves an optimization problem to find the vorticity and velocity fields that minimize a cost function, which includes next to instantaneous velocity, also the velocity material derivative. The velocity and its material derivative are related through the vorticity transport equation, and the cost function is minimized using the limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) algorithm. The procedure is assessed numerically with a simulated PTV experiment in a turbulent boundary layer from a direct numerical simulation (DNS). The experimental validation considers a tomographic particle image velocimetry (PIV) experiment in a similar turbulent boundary layer and the additional case of a jet flow. The proposed technique (‘vortex-in-cell plus’, VIC+) is compared to tomographic PIV analysis (3D iterative cross-correlation), PTV interpolation methods (linear and adaptive Gaussian windowing) and to vortex-in-cell (VIC) interpolation without the material derivative. A visible increase in resolved details in the turbulent structures is obtained with the VIC+ approach, both in numerical simulations and experiments. This results in a more accurate determination of the turbulent stresses distribution in turbulent boundary layer investigations. Data from a jet experiment, where the vortex topology is retrieved with a small number of tracers indicate the potential utilization of VIC+ in low-concentration experiments as for instance occurring in large-scale volumetric PTV measurements. PubDate: 2016-08-16 DOI: 10.1007/s00348-016-2225-6 Issue No:Vol. 57, No. 9 (2016)

Authors:S. Kordel; T. Nowak; R. Skoda; J. Hussong Abstract: Abstract
In the present study, Long-range Microparticle Image Velocimetry (
\(\upmu\)
PIV) and Differential Interferometry (DI) are combined in a novel manner to enable both velocity and depth-integrated density gradient field measurements using the same laser pulse for both recordings. In the present work, temperature-driven boundary layer flows could be successfully determined to an accuracy of
\(\Updelta T=0.17\,\hbox {K}\)
with a spatial resolution of
\(405\,\upmu \hbox {m}\)
for interference and
\(101\,\upmu \hbox {m}\)
for
\(\upmu\)
PIV measurements. The DI measurements are refraction compensated, and both temperature and velocity fields are compared with results from numerical simulations. PubDate: 2016-08-12 DOI: 10.1007/s00348-016-2224-7 Issue No:Vol. 57, No. 9 (2016)

Authors:J. J. Serrano-Aguilera; J. Hermenegildo García-Ortiz; A. Gallardo-Claros; L. Parras; C. del Pino Abstract: Abstract
In order to predict the axial development of the wingtip vortices strength, an accurate theoretical model is required. Several experimental techniques have been used to that end, e.g. PIV or hot-wire anemometry, but they imply a significant cost and effort. For this reason, we have performed experiments using the smoke-wire technique to visualize smoke streaks in six planes perpendicular to the main stream flow direction. Using this visualization technique, we obtained quantitative information regarding the vortex velocity field by means of Batchelor’s model for two chord-based Reynolds numbers,
\(Re_c=3.33\times 10^4\)
and
\(10^5\)
. Therefore, this theoretical vortex model has been introduced in the integration of ordinary differential equations which describe the temporal evolution of streak lines as function of two parameters: the swirl number, S, and the virtual axial origin,
\(\overline{z_0}\)
. We have applied two different procedures to minimize the distance between experimental and theoretical flow patterns: individual curve fitting at six different control planes in the streamwise direction and the global curve fitting which corresponds to all the control planes simultaneously. Both sets of results have been compared with those provided by del Pino et al. (Phys Fluids 23(013):602, 2011b. doi:10.1063/1.3537791), finding good agreement. Finally, we have observed a weak influence of the Reynolds number on the values S and
\(\overline{z_0}\)
at low-to-moderate
\(Re_c\)
. This experimental technique is proposed as a low cost alternative to characterize wingtip vortices based on flow visualizations. PubDate: 2016-08-08 DOI: 10.1007/s00348-016-2222-9 Issue No:Vol. 57, No. 8 (2016)

Authors:Florian J. Zenger; Gert Herold; Stefan Becker; Ennes Sarradj Abstract: Abstract
A generic fan with unskewed fan blades is investigated using a microphone array method. The relative motion of the fan with respect to the stationary microphone array is compensated by interpolating the microphone data to a virtual rotating array with the same rotational speed as the fan. Hence, beamforming algorithms with deconvolution, in this case CLEAN-SC, could be applied. Sound maps and integrated spectra of sub-components are evaluated for five operating points. At selected frequency bands, the presented method yields sound maps featuring a clear circular source pattern corresponding to the nine fan blades. Depending on the adjusted operating point, sound sources are located on the leading or trailing edges of the fan blades. Integrated spectra show that in most cases leading edge noise is dominant for the low-frequency part and trailing edge noise for the high-frequency part. The shift from leading to trailing edge noise is strongly dependent on the operating point and frequency range considered. PubDate: 2016-08-08 DOI: 10.1007/s00348-016-2223-8 Issue No:Vol. 57, No. 8 (2016)

Authors:Davide Giassi; Marshall B. Long Abstract: Abstract
Two alternative image readout approaches are demonstrated to improve the signal-to-noise ratio (SNR) in temporally resolved laser-based imaging experiments of turbulent phenomena. The first method exploits the temporal decay characteristics of the phosphor screens of image intensifiers when coupled to an interline-transfer CCD camera operated in double-frame mode. Specifically, the light emitted by the phosphor screen, which has a finite decay constant, is equally distributed and recorded over the two sequential frames of the detector so that an averaged image can be reconstructed. The characterization of both detector and image intensifier showed that the technique preserves the correct quantitative information, and its applicability to reactive flows was verified using planar Rayleigh scattering and tested with the acquisition of images of both steady and turbulent partially premixed methane/air flames. The comparison between conventional Rayleigh results and the averaged ones showed that the SNR of the averaged image is higher than the conventional one; with the setup used in this work, the gain in SNR was seen to approach 30 %, for both the steady and turbulent cases. The second technique uses the two-frame readout of an interline-transfer CCD to increase the image SNR based on high dynamic range imaging, and it was tested in an unsteady non-reactive flow of Freon-12 injected in air. The result showed a 15 % increase in the SNR of the low-pixel-count regions of an image, when compared to the pixels of a conventionally averaged one. PubDate: 2016-08-04 DOI: 10.1007/s00348-016-2218-5 Issue No:Vol. 57, No. 8 (2016)

Authors:Giorgio Querzoli; Stefania Fortini; Stefania Espa; Simone Melchionna Abstract: Abstract
Cardiovascular flows have been extensively investigated by means of in vitro models to assess the prosthetic valve performances and to provide insight into the fluid dynamics of the heart and proximal aorta. In particular, the models for the study of the flow past the aortic valve have been continuously improved by including, among other things, the compliance of the vessel and more realistic geometries. The flow within the sinuses of Valsalva is known to play a fundamental role in the dynamics of the aortic valve since they host a recirculation region that interacts with the leaflets. The coronary arteries originate from the ostia located within two of the three sinuses, and their presence may significantly affect the fluid dynamics of the aortic root. In spite of their importance, to the extent of the authors’ knowledge, coronary arteries were not included so far when modeling in vitro the transvalvular aortic flow. We present a pulse duplicator consisting of a passively pulsing ventricle, a compliant proximal aorta, and coronary arteries connected to the sinuses of Valsalva. The coronary flow is modulated by a self-regulating device mimicking the physiological mechanism, which is based on the contraction and relaxation of the heart muscle during the cardiac cycle. Results show that the model reproduces satisfyingly the coronary flow. The analysis of the time evolution of the velocity and vorticity fields within the aortic root reveals the main characteristics of the backflow generated through the aorta in order to feed the coronaries during the diastole. Experiments without coronary flow have been run for comparison. Interestingly, the lifetime of the vortex forming in the sinus of Valsalva during the systole is reduced by the presence of the coronaries. As a matter of fact, at the end of the systole, that vortex is washed out because of the suction generated by the coronary flow. Correspondingly, the valve closure is delayed and faster compared to the case with no coronary flow. PubDate: 2016-08-04 DOI: 10.1007/s00348-016-2221-x Issue No:Vol. 57, No. 8 (2016)

Authors:Roman V. Mukin Abstract: Abstract
A new algorithm is presented for post-processing of void fraction measurements with wire-mesh sensors, particularly for identifying and reconstructing bubble surfaces in a two-phase flow. This method is a combination of the bubble recognition algorithm presented in Prasser (Nuclear Eng Des 237(15):1608, 2007) and Poisson surface reconstruction algorithm developed in Kazhdan et al. (Poisson surface reconstruction. In: Proceedings of the fourth eurographics symposium on geometry processing 7, 2006). To verify the proposed technique, a comparison was done of the reconstructed individual bubble shapes with those obtained numerically in Sato and Ničeno (Int J Numer Methods Fluids 70(4):441, 2012). Using the difference between reconstructed and referenced bubble shapes, the accuracy of the proposed algorithm was estimated. At the next step, the algorithm was applied to void fraction measurements performed in Ylönen (High-resolution flow structure measurements in a rod bundle (Diss., Eidgenössische Technische Hochschule ETH Zürich, Nr. 20961, 2013) by means of wire-mesh sensors in a rod bundle geometry. The reconstructed bubble shape yields bubble surface area and volume, hence its Sauter diameter
\(d_{32}\)
as well. Sauter diameter is proved to be more suitable for bubbles size characterization compared to volumetric diameter
\(d_{30}\)
, proved capable to capture the bi-disperse bubble size distribution in the flow. The effect of a spacer grid was studied as well: For the given spacer grid and considered flow rates, bubble size frequency distribution is obtained almost at the same position for all cases, approximately at
\(d_{32} = 3.5\)
mm. This finding can be related to the specific geometry of the spacer grid or the air injection device applied in the experiments, or even to more fundamental properties of the bubble breakup and coagulation processes. In addition, an application of the new algorithm for reconstruction of a large air–water interface in a tube bundle is presented. PubDate: 2016-08-03 DOI: 10.1007/s00348-016-2220-y Issue No:Vol. 57, No. 8 (2016)

Authors:Y. Dossmann; B. Bourget; C. Brouzet; T. Dauxois; S. Joubaud; P. Odier Abstract: Abstract
We present a novel characterization of mixing events associated with the propagation and overturning of internal waves studied, thanks to the simultaneous use of particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques. This combination of techniques had been developed earlier to provide an access to simultaneous velocity and density fields in two-layer stratified flows with interfacial gravity waves. Here, for the first time, we show how it is possible to implement it quantitatively in the case of a continuously stratified fluid where internal waves propagate in the bulk. We explain in details how the calibration of the PLIF data is performed by an iterative procedure, and we describe the precise spatial and temporal synchronizations of the PIV and PLIF measurements. We then validate the whole procedure by characterizing the triadic resonance instability (TRI) of an internal wave mode. Very interestingly, the combined technique is then applied to a precise measurement of the turbulent diffusivity K
t associated with mixing events induced by an internal wave mode. Values up to K
t = 15 mm2 s−1 are reached when TRI is present (well above the noise of our measurement, typically 1 mm2 s−1), unambiguously confirming that TRI is a potential pathway to turbulent mixing in stratified flows. This work therefore provides a step on the path to new measurements for internal waves. PubDate: 2016-08-01 DOI: 10.1007/s00348-016-2212-y Issue No:Vol. 57, No. 8 (2016)

Authors:Cody Ground; Fabrizio Vergine; Luca Maddalena Abstract: Abstract
A defining feature of the turbulent free shear layer is that its growth is hindered by compressibility effects, thus limiting its potential to sufficiently mix the injected fuel and surrounding airstream at the supersonic Mach numbers intrinsic to the combustor of air-breathing hypersonic vehicles. The introduction of streamwise vorticity is often proposed in an attempt to counteract these undesired effects. This fact makes the strategy of introducing multiple streamwise vortices and imposing upon them certain modes of mutual interaction in order to potentially enhance mixing an intriguing concept. However, many underlying fundamental characteristics of the flowfields in the presence such interactions are not yet well understood; therefore, the fundamental physics of these flowfields should be independently investigated before the explicit mixing performance is characterized. In this work, experimental measurements are taken with the stereoscopic particle image velocimetry technique on two specifically targeted modes of vortex interaction—the merging and non-merging of two corotating vortices. The fluctuating velocity fields are analyzed utilizing the proper orthogonal decomposition (POD) in order to identify the content, organization, and distribution of the modal turbulent kinetic energy content of the fluctuating velocity eigenmodes. The effects of the two modes of vortex interaction are revealed by the POD analysis which shows distinct differences in the modal features of the two cases. When comparing the low-order eigenmodes of the two cases, the size of the structures contained within the first ten modes is seen to increase as the flow progresses downstream for the merging case, whereas the opposite is true for the non-merging case. Additionally, the relative modal energy contribution of the first ten eigenmodes increases as the vortices evolve downstream for the merging case, whereas in the non-merging case the relative modal energy contribution decreases. The POD results show that the vortex merging process reorients and redistributes the relative turbulent kinetic energy content toward the larger-scale structures within the low-order POD eigenmodes. This result suggests that by specifically designing the vortex generation system to impose preselected modes of vortex interaction upon the flow it is possible to exert some form of control over the downstream evolution and distribution of the global and modal turbulent kinetic energy content. PubDate: 2016-07-30 DOI: 10.1007/s00348-016-2219-4 Issue No:Vol. 57, No. 8 (2016)

Authors:Michael A. Kegerise; Shann J. Rufer Abstract: Abstract
In this paper, we report on the application of the atomic layer thermopile (ALTP) heat-flux sensor to the measurement of laminar-to-turbulent transition in a hypersonic flat-plate boundary layer. The centerline of the flat-plate model was instrumented with a streamwise array of ALTP sensors, and the flat-plate model was exposed to a Mach 6 freestream over a range of unit Reynolds numbers. Here, we observed an unstable band of frequencies that are associated with second-mode instability waves in the laminar boundary layer that forms on the flat-plate surface. The measured frequencies, group velocities, phase speeds, and wavelengths of these instability waves are consistent with data previously reported in the literature. Heat flux time series, and the Morlet wavelet transforms of them, revealed the wave-packet nature of the second-mode instability waves. In addition, a laser-based radiative heating system was used to measure the frequency response functions (FRF) of the ALTP sensors used in the wind tunnel test. These measurements were used to assess the stability of the sensor FRFs over time and to correct spectral estimates for any attenuation caused by the finite sensor bandwidth. PubDate: 2016-07-28 DOI: 10.1007/s00348-016-2214-9 Issue No:Vol. 57, No. 8 (2016)

Authors:J. H. Lee; Kevin; J. P. Monty; N. Hutchins Abstract: Abstract
The discrepancy between measured turbulence intensity obtained from experiments in wall-bounded turbulence and the fully resolved reference results (usually from DNS datasets) are often attributed to spatial resolution issues, especially in PIV measurements due to the presence of spatial averaging within the interrogation region/volume.
In many cases, in particular at high Reynolds numbers (where there is a lack of DNS data), there is no attempt to verify that this is the case.
There is a risk that attributing unexpected PIV statistics to spatial resolution, without careful checks, could mask wider problems with the experimental setup or test facility. Here, we propose a robust technique to validate the under-resolved PIV obtained turbulence intensity profiles for canonical wall-bounded turbulence. This validation scheme is independent of Reynolds number and does not rely on empirical functions. It is based on arguments that (1) the viscous-scaled small-scale turbulence energy is invariant with Reynolds number and that (2) the spatially under-resolved measurement is sufficient to capture the large-scale energy. This then suggests that we can estimate the missing energy from volume-filtered DNS data at much lower Reynolds numbers. Good agreement is found between the experimental results and estimation profiles for all three velocity components, demonstrating that the estimation tool successfully computes the missing energy for given spatial resolutions over a wide range of Reynolds numbers. A database for a canonical turbulent boundary layer and associated MATLAB function are provided that enable this missing energy to be calculated across a range of interrogation volume sizes, so that users do not require access to raw DNS data. This methodology and tool will provide PIV practitioners, investigating canonical wall-bounded turbulent flow with a convenient check of the effects of spatial resolution on a given experiment. PubDate: 2016-07-26 DOI: 10.1007/s00348-016-2209-6 Issue No:Vol. 57, No. 8 (2016)

Authors:Matteo Novara; Daniel Schanz; Nico Reuther; Christian J. Kähler; Andreas Schröder Abstract: Abstract
The Shake-The-Box (STB) particle tracking technique, recently introduced for time-resolved 3D particle image velocimetry (PIV) images, is applied here to data from a multi-pulse investigation of a turbulent boundary layer flow with adverse pressure gradient in air at 36 m/s (Re
τ
= 10,650). The multi-pulse acquisition strategy allows for the recording of four-pulse long time-resolved sequences with a time separation of a few microseconds. The experimental setup consists of a dual-imaging system and a dual-double-cavity laser emitting orthogonal polarization directions to separate the four pulses. The STB particle triangulation and tracking strategy is adapted here to cope with the limited amount of realizations available along the time sequence and to take advantage of the ghost track reduction offered by the use of two independent imaging systems. Furthermore, a correction scheme to compensate for camera vibrations is discussed, together with a method to accurately identify the position of the wall within the measurement domain. Results show that approximately 80,000 tracks can be instantaneously reconstructed within the measurement volume, enabling the evaluation of both dense velocity fields, suitable for spatial gradients evaluation, and highly spatially resolved boundary layer profiles. Turbulent boundary layer profiles obtained from ensemble averaging of the STB tracks are compared to results from 2D-PIV and long-range micro particle tracking velocimetry; the comparison shows the capability of the STB approach in delivering accurate results across a wide range of scales. PubDate: 2016-07-23 DOI: 10.1007/s00348-016-2216-7 Issue No:Vol. 57, No. 8 (2016)

Authors:Daniele Ragni; Carlos Ferreira Abstract: Abstract
A 3D stall-cell flow-field has been studied in a 4.8 aspect-ratio wing obtained by linear extrusion of a laminar NACA64-418 airfoil profile. The span-wise change in the velocity and pressure distribution along the wing has been quantified with respect to the development of cellular structures from
\(8^{\circ }\)
to
\(20^{\circ }\)
angle of attack. Oil-flow visualizations help localizing the regular cellular pattern in function of the angle of attack. Multi-plane stereoscopic PIV measurements obtained by traversing the entire setup along the wing span show that the flow separation is not span-wise uniform. The combination of different stereoscopic fields into a 3D volume of velocity data allows studying the global effect of the stall-cell pattern on the wing flow. Integration of the experimentally computed pressure gradient from the Navier–Stokes equation is employed to compute the span-wise distribution of the mean surface pressure. Comparison of the results with the ones obtained from pressure taps installed in the wing evidences a span-wise periodic loading on the wing. The periodic loading has maxima confined in the stream-wise direction between the location of the highest airfoil curvature and the one of the airfoil flow separation. Estimation of the periodic loading is found within 2–6 % of the sectional wing lift. PubDate: 2016-07-23 DOI: 10.1007/s00348-016-2215-8 Issue No:Vol. 57, No. 8 (2016)

Authors:Michael Papageorge; Jeffrey A. Sutton Abstract: Abstract
In this manuscript, we investigate the statistical convergence of turbulent flow statistics from finite-record-length time-series measurements. Analytical solutions of the convergence rate of the mean, variance, and autocorrelation function as a function of record length are presented based on using mean-squared error analysis and the consideration of turbulent flows as random processes. Experimental assessment of the statistical convergence theory is presented using 20-kHz laser Rayleigh scattering measurements of a conserved scalar (ξ) in a turbulent free jet. Excellent agreement between experiments and theory is noted, providing validation of the statistical convergence analysis. To the authors’ knowledge, this is the first reported assessment and verification of statistical convergence theory as applied to turbulent flows. The verified theory provides a practitioner a method for a priori determining the necessary temporal record length for a desired statistical accuracy or conversely, accurately estimating the uncertainty of a measurement for a given temporal record length. Furthermore, we propose a new hybrid “multi-burst” data processing scheme based on combined independent ensemble and time-series statistics targeted for shorter-duration time-series measurements. The new methodology is based on taking the ensemble mean of derived statistical moments from many individual finite-duration time-series measurements. This approach is used to systematically converge toward the “expected” value of any statistical moment at a rate of
\(\sqrt M\)
, where M is the number of individual time-series measurements. The proposed multi-burst methodology is assessed experimentally, and excellent agreement between measurements and theory is observed. A key outcome of the implementation of the multi-burst processing method is noted in the estimation of the autocorrelation function. Specifically, an unbiased estimator of the autocorrelation function can be used with much less uncertainty as compared to the biased estimator, which is not the case for single time-series measurements irrespective of record length. The primary outcome of the multi-burst processing scheme is a methodology for achieving high statistical convergence for turbulent flow time-series measurements characterized by limited acquisition time, whether facility or instrument dependent. PubDate: 2016-07-23 DOI: 10.1007/s00348-016-2211-z Issue No:Vol. 57, No. 8 (2016)

Authors:V. Rodriguez; G. Jourdan; A. Marty; A. Allou; J.-D. Parisse Abstract: Abstract
In this work, we conduct experiments to study the interaction between a horizontal free water layer and a planar shock wave that is sliding over it. Experiments are performed at atmospheric pressure in a shock tube with a square cross section (
\(200\times 200\,\hbox {mm}^2\)
) for depths of 10, 20, and 30 mm; a 1500-mm-long water layer; and two incident planar shock waves having Mach numbers of 1.11 and 1.43. We record the pressure histories and high-speed visualizations to study the flow patterns, surface waves, and spray layers behind the shock wave. We observe two different flow patterns with ripples formed at the air–water interface for the weaker shock wave and the dispersion of a droplet mist for the stronger shock wave.
From the pressure signals, we extract the delay time between the arrival of the compression wave into water and the shock wave in air at the same location. We show that the delay time evolves with the distance traveled over the water layer, the depth of the water layer, and the Mach number of the shock wave. PubDate: 2016-07-21 DOI: 10.1007/s00348-016-2217-6 Issue No:Vol. 57, No. 8 (2016)

Authors:J. Lohse; H. P. Barth; W. Nitsche Abstract: Abstract
The possibility of a pneumatic actuator system for controlling the crossflow vortex-induced laminar breakdown is investigated by means of hot-wire measurements. Steady blowing or suction through a spanwise row of periodically arranged orifices initiates a system of longitudinal vortices which reduces the amplitude of the most amplified stationary crossflow vortices. Thus, the onset of high-frequency secondary instability and the following laminar–turbulent transition was shifted farther downstream. All experiments were conducted at the redesigned DLR swept flat plate experiment in the open test section of the 1 m wind tunnel at the DLR in Göttingen. PubDate: 2016-07-19 DOI: 10.1007/s00348-016-2213-x Issue No:Vol. 57, No. 8 (2016)