165 research outputs found
Dataset for The instantaneous structure of secondary flows in turbulent boundary layers
No full textDataset supports:
Vanderwel, Stroh, Kriegseis, Frohnapfel and Ganapathisubramani (2018). The instantaneous structure of secondary flows in turbulent boundary layers. Journal of Fluid Mechanics.
Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is not very sensitive to the Reynolds number range investigated, and direct numerical simulations and experiments produce similar results in the mean flow as well as the dispersive and turbulent stress distributions. The numerical methods capture time resolved features of the instantaneous flow and provide insight into the near-wall flow structures, that were previously obscured in the experimental measurements. Proper orthogonal decomposition was shown to capture the essence of the secondary flows in relatively few modes and is useful as a filter to analyse the instantaneous flow patterns. The secondary flows are found to create extended regions of high Reynolds stress away from the wall that comprise predominantly sweeps similar to what one would expect to see near the wall and which are comparable in magnitude to the near-wall stress. Analysis of the instantaneous flow patterns reveals that the secondary flows are the result of a non-homogeneous distribution of mid-size vortices.</span
Analytical Study of the Velocity in a Pressure Chamber at High Oscillatory Pressures
No full textThe temporal and spatial evolution in the velocity at high oscillatory pressures due to periodic changes in the boundary conditions in chambers and ducts has been investi- gated analytically. The new models incorporate primary and secondary losses (friction, Carnot-shock effects and stream diversion/division) as well as the theory of characteristics. Therefore the models allow for further insight into the experimental results from Batarseh1 and provide a comprehensive explanation of the spray instabilities. This paper builds upon the pressure-chamber model developed by Kriegseis et al.
Vortex force decomposition in the tip region of impulsively-started flat plates
No full textAn investigation into the influence of seemingly analogous kinematics (plunge versus tow) for rapidly accelerating, low-aspect-ratio plates has been performed. The instantaneous forces and velocity fields were obtained simultaneously using a six-component force/moment sensor together with a three-dimensional particle tracking velocimetry (3D-PTV) system. Despite identical effective shear-layer velocities and effective angles of attack, the force histories are found to vary between the two aforementioned cases (plunge versus tow) once the impulsive motion is complete, as originally reported on by Kriegseis et al. (J. Fluid Mech., vol. 736, 2013, pp. 91–106). In order to uncover the cause for this curious discrepancy between the two analogous cases a vortex force decomposition is implemented. It is shown that the interplay between growth and orientation of the vortical structures significantly affects vortical hydrodynamic impulse and vortex force, and thus the net lift on the body
Enhanced three-dimensional particle detection in microcirculation experiments with defocus particle tracking and ghost red blood cells
No full textExperimental investigations on the motion of rigid particles in microcirculation environments are still scarce owing to the three-dimensional (3D) motion of the particles and to the particle image masking due to the presence of the red blood cells (RBCs). Despite the recent progress on the 3D tracking of rigid particles in RBC flows with defocus particle tracking (DPT) methods, the problem of particle image masking remains to be solved. Here, we propose, test, and evaluate the use hemoglobin-free RBCs, also known as ghost RBCs, as a replacement for normal RBCs in experiments with rigid particles in microcirculation environments. We performed DPT measurements of a pressure-driven flow of normal and ghost RBC suspensions seeded with rigid particles at three different flow rates. We show that the quasi-transparent appearance of ghost RBCs, as a result of the lack of hemoglobin, eliminates the RBC-induced masking of the defocused particle images and allows to achieve the particle matching standards found in cell-free experiments. In fact, ghost RBC suspensions enable the tracking of the rigid particles across the entire height of the microchannel, which was not possible in normal RBC flows. On a fluid dynamic level, we show that ghost RBC suspensions provide similar conditions to normal RBCs in terms of the velocity of the rigid particles and the rigid particles exhibit similar lateral dynamics in both types of cell suspensions. In essence, the findings from this work demonstrate that ghost RBCs are a well-suited replacement for normal RBCs in experiments aiming at deciphering the motion of rigid particles in microcirculation environments
Volumetric reconstruction of drop impact dynamics by means of color-coded glare points and deep learning
No full textThe present work introduces a method for the volumetric reconstruction of the gas-liquid interface of a droplet impinging on a solid surface from experimental image data by means of deep learning.
Differently colored glare points are used in order to encode three-dimensional information of the phase boundary on the image in addition to a standard shadowgraph.
It is demonstrated that this volumetric information can be successfully reconstructed into a three-dimensional representation of the droplet phase boundary by the proposed neural network
Density measurements in cold plasma discharges by Schlieren method
No full textLAUREA MAGISTRALEL’obiettivo della tesi è investigare il comportamento della densità come variabile ter-
modinamica nell’applicazione dell’attuatore dieletric barrier discharge (DBD) mediante
il metodo sperimentale Schlieren. Lo scopo è quello di verificare, localizzare e quantifi-
care il cambiamento nell’attuazione stazionaria del DBD in condizioni quiescenti. La
densità è richiesta per dedurre modelli empirici e fenomenologici accurati per la forza
esercitata dell’attuatore sul flusso, usato poi come dato d’immissione in simulazioni di
fluidodinamica computazionale sugli attuatori. Gli esperimenti Schlieren sono stati effet-
tuati con due orientazioni diverse di blocco (cutoff) della luce nel punto focale: Il metodo
Schlieren è in grado di rilevare la derivata parziale della densità in direzione normale
all’orientamento del blocco. Coniugando due immagini, realizzate con taglio orizzontale
e verticale, è stato possibile ottenere il gradiente della densità. Il gradiente di densità
è un campo vettoriale potenziale, la densità stessa è uno scalare potenziale. Attraverso
un metodo d’integrazione parziale è stato ricavato il campo potenziale dello scalare den-
sità. Le immagini sono state registrati con una risoluzione di 1392 × 1280 pixel, con due
ingrandimenti: uno di 9.7 × 5.5 mm, per catturare l’origine del cambiamento di densità
e uno di 19.4 × 11 mm per catturare la causa più alcuni degli effetti immediati a valle
del plasma. I risultati rivelano che la densità cambia intorno al 30 ± 10% in prossimità
immediata del plasma. L’area con densità diminuita si allunga ed estende a valle; ciò è
dovuto al trasferimento di moto nel plasma che genera il flusso. Come causa principale
per l’alterazione di densità è stato identificato il riscaldamento del fluido nella zona del
plasma, dovuto alla temperatura elevata degli elettroni presenti nello stesso.This thesis presents the investigation into the behaviour of the thermodynamic state vari-
able density in the application of a dielectric barrier discharge (DBD) actuator by means
of Schlieren experiments. The aim is to verify, to localize and quantify changes in den-
sity for steady actuation of the DBD under quiescent conditions. Density distribution
is required to derive accurate empirical and phenomenological models for the body force
term exerted by the actuator on the flow, used as input in CFD simulations about plasma
actuators. Schlieren experiments are carried out with two different orientations of light-
cutoff in the focal point: The Schlieren method detects the partial derivative of density
in the direction normal to the orientation of cutoff. By combining two Schlieren images,
realized with horizontal and vertical cutoff, the gradient vector field could be obtained.
The density gradient is a potential vector field, and the density is the scalar potential. The
density distribution was retrieved from the gradient by the method of partial integration.
Pictures were recorded with a resolution of 1392 × 1280 pixel, with two different magni-
fications, 9.7 × 5.5 mm to capture the actual cause of density change, and 19.4 × 11 mm
to capture the cause plus some effects downstream. The results show that the density
diminishes for about 30 ± 10% in the immediate plasma zone. The diminished density
zone gets elongated, spread out downstream by the momentum transfer causing the flow.
The main cause for density change was identified as the heating of air, caused by the
elevated temperature of the electrons present in the plasma
Dataset to "Flow stages during vortex decay in an impulsively stopped rotating cylinder"
No full textAn infinite cylinder containing a fluid in solid body rotation (SBR) is impulsively stopped. The decay and self-organisation of the initial SBR due to the forming boundary layer and multiple transitions is addressed by means of direct numerical simulations for a wide range of Reynolds numbers. The flow undergoes five distinct flow stages during its decay, which are characterised by means of statistical evaluation. This database features statistics of four different spin-down simulations at different .------------------------------------------------------------------------------------------------------------------------
Statistics on spin down for different Re
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this database features:
* statistics of four different spin-down simulations at different Re=\Omega R^2/\nu;
* global volume-averaged data for turbulent kinetic engery (k), mean kinetic energy (K) and their budget equations;
* temporal evolution of 1D quantities such as wall-shear stress, boundary-layer thickness;
* temporal evolution of spatially (over z and \phi) averaged bulk data such as mean velocity, Reynolds-stresses etc.;
* terms of the k and K budget equations as function of t and r.
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Acknowledgment:
The financial support by the German Research Foundation (DFG) under Priority Programme SPP-1881 is greatly acknowledged.
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References:
F. Kaiser, B. Frohnapfel, R. Ostilla-Monico, J. Kriegseis, D. E. Rival and D. Gatti (2019)
"Flow stages during vortex decay in an impulsively stopped rotating cylinder",
Journal of Fluid Mechanics, (to be submitted)
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Content:
folders for different Re: Re3000, Re6000, Re12000, Re28000
each folder contains:
tot.dat --> 1D data such as wall-velocity, boundary-layer thickness,
volume average K, volume average k etc.
t.dat --> time steps (spacing dt increases during late stages of decay)
r.dat --> radial positions of the grid
bulk --> folder: contains dat files for non-zero bulk quantities as functions of r and t.
mean velocity, mean vorticity, velocity in wall units, Reynolds stresses
mke --> folder: contains terms of the K budget equation as functions of r and t:
K, production, dissipation, turbulent transport, viscous transport
tke --> folder: contains terms of the TKE budget equation as functions of r and t:
k, production, dissipation, turbulent transport, viscous transport, pressure diffusion
Refer to the readme files in each subdirectory for further information on each database.
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Changelog:
May-08-2019: database uploaded
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High Reynolds Number Measurements of Vorticity Generation and Annihilation with Rapidly Changing Boundary Conditions
No full textInternational audienceThe project aims at understanding how rapid vorticity annihilation takes place across a sharp vortical interface over a wide range of Reynolds numbers. In order to exclude any influences of (convective) length scales from the flow scenarios under consideration, all investigations center around boundary layer development at the outer wall of rotating facilities. In continuation of the original thought experiment by Kriegseis et al. [1] on the impact of initial conditions, two distinct yet similar cases are contrasted to characterize the formation process. Particularly, development and annihilation of vorticity is directly compared between one case with an irrotational core and its corollary with opposite-signed vorticity across this interface. As such, the possible impact of initial conditions on the effect of rapidly changing boundary conditions can be uncovered
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