FG Aerodynamik und Strömungslehre
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BTU
Angular momentum transport and flow organization in Taylor-Couette flow at radius ratio of η=0.357"
(2019)
Cooling silicon photovoltaic cells using finned heat sinks and the effect of inclination angle
(2021)
Effect of the Initial Conditions on the Growth of Thermoelectric Instabilities During Parabolic
(2019)
Enhanced outer peaks in turbulent boundary layer using uniform blowing at moderate Reynolds number
(2022)
Uniform blowing in wall bounded shear flows is well known for its drag reducing effects and has long been investigated ever since. However, many contemporary and former research on this topic has confirmed the drag reducing effect but very less is known regarding
how blowing is effecting the Reynolds stresses at high Reynolds number. Therefore, effect of uniform blowing has been experimentally investigated using Stereo Particle Image Velocimetry (SPIV) measurements in a zero pressure gradient turbulent boundary layer (TBL). The data presented in this literature covers a large range of high Reynolds number flow e.g. Reθ = 7500∼19763 where Reynolds number is based on the momentum thickness. Upstream blowing
was varied from 1%∼6% of free stream velocity and measurements were taken downstream after a short interval. Logarithmic and outer region of the TBL was given special attention in terms of investigating statistics and turbulence properties.
Flow Pattern and Heat Transfer in a Cylindrical Annulus Under 1 g and Low-g Conditions: Experiments
(2018)
The mechanism of localized inertial wave excitation and its efficiency is investigated for an annular cavity rotating with Ω0 . Meridional symmetry is broken by replacing
the inner cylinder with a truncated cone (frustum). Waves are excited by individual longitudinal libration of the walls. The geometry is non-separable and exhibits wave
focusing and wave attractors. We investigated laboratory and numerical results for the Ekman number E ≈ 10−6. inclination α = 5.71◦ and libration amplitudes ε 0.2 within the inertial wave band 0 < ω < 2Ω0 . Under the assumption that the inertial waves do not essentially affect the boundary-layer structure, we use classical boundary-layer analysis to study oscillating Ekman layers over a librating wall that is at an angle α = 0 to the axis of rotation. The Ekman layer erupts at frequency ω = f∗, where f∗ ≡ 2Ω0 sin α is the effective Coriolis parameter in a plane tangential to the wall. For
the selected inclination this eruption occurs for the forcing frequency ω/Ω0 = 0.2. For the librating lids eruption occurs at ω/Ω0 = 2. The study reveals that the frequency dependence of the total kinetic energy Kω of the excited wave field is strongly connected to the square of the Ekman pumping velocity wE (ω) that, in the linear limit, becomes singular when the boundary layer erupts. This explains the frequency dependence of non-resonantly excited waves. By the localization of the forcing, the two configurations investigated, (i) frustum libration and (ii) lids together with outer cylinder in libration, can be clearly distinguished by their response spectra. Good agreement was found for the spatial structure of low-order wave attractors and periodic orbits (both characterized by a small number of reflections) in the frequency windows predicted by geometric ray tracing. For ‘resonant’ frequencies a significantly increased total bulk energy was found, while the energy in the boundary layer remained nearly constant. Inertial wave energy enters the bulk flow via corner beams, which are parallel to the characteristics of the underlying Poincaré problem. Numerical simulations revealed a mismatch between the wall-parallel mass fluxes
near the corners. This leads to boundary-layer eruption and the generation of inertial waves in the corners.
Influence of the temperature-dependent viscosity on convective flow in the radial force field
(2017)
Linear stability analysis of the convective flow in a spherical gap with rotating inner surface
(2020)
Time periodic variation of the rotation rate of an annulus induces in supercritical regime an unstable Stokes boundary layer over the cylinder side walls, generating
Görtler vortices in a portion of a libration cycle as a discrete event. Numerical results show that these vortices propagate into the fluid bulk and generate an azimuthal
mean flow. Direct numerical simulations of the fluid flow in an annular container with librating outer (inner) cylinder side wall and Reynolds-averaged Navier–Stokes
(RANS) equations as diagnostic equations are used to investigate generation mechanism of the retrograde (prograde) azimuthal mean flow in the bulk. First, we explain, phenomenologically, how absolute angular momentum of the bulk flow is mixed and changed due to the propagation of the Görtler vortices, causing a new vortex
of basin size. Then we investigate the RANS equations for intermediate time scale of the development of the Görtler vortices and for long time scale of the order of
several libration periods. The former exhibits sign selection of the azimuthal mean flow. Investigating the latter, we predict that the azimuthal mean flow is proportional to the libration amplitude squared and to the inverse square root of the Ekman number and libration frequency and then confirms this using the numerical data.
Additionally, presence of an upscale cascade of energy is shown, using the kinetic energy budget of fluctuating flow.
Laboratory experiments with a rotating cylindrical annulus arereported that reveal a prograde jet, which is adjacent to a (longitu-dinally) librating inner straight cylindrical wall. Here, wall libration isrealised as a time-harmonic modulation of the inner cylinder’s rota-tion rate. The outer cylindrical wall and bottom and top lids rotatewith constant angular velocity. The main purpose of our study is tocontribute to a qualitative and quantitative understanding of non-linearities that are present in oscillating, but centrifugally stable,vertical boundary layers frequently encountered in rotating wall-bounded flows. We consider a problem that is in a sense comple-mentary to that of previous works that focused on oscillating Ekmanlayers but neglected the vertical Stokes−Stewartson layers. A sim-ple analytical model is proposed that is able to predict the magni-tude and spatial structure of the emerging prograde near-wall jet interms of nonlinearity inherent in the inner cylinder’s boundary layerdynamics.
The main focus of the project series “Piston Ring Oil Transport” is the investigation of the tribological conditions in the piston assembly. Within the first part of the project, a 0.5-l single-cylinder research engine was built up [1, 2], which is equipped with numerous sensors to measure the tribological circumstances. In the second project part [3] presented here, additional simulation models are developed, which gain accuracy compared to the recorded measurement data and contribute significantly to an understanding of the oil supply at the piston assembly.
Die Projektreihe Kolbenring-Öltransport dient der Untersuchung der tribologischen Zustände in der Kolbengruppe. Dabei wurde im ersten Projektabschnitt ein 0,5-l-Einzylinder-Forschungsmotor aufgebaut, der mit zahlreichen Sensoren ausgerüstet ist, um diese Zustände messtechnisch zu erfassen. Im zweiten, hier ausschnittsweise vorgestellten Projektabschnitt wurden zusätzliche Simulationsmodelle implementiert, die durch Abgleich mit den aufgezeichneten Messdaten an Genauigkeit gewinnen und zum Verständnis des Ölhaushalts in der Kolbengruppe erheblich beitragen.
Mixing and overshooting in surface convection zones of DA white dwarfs: First results from ANTARES
(2018)
The large-scale flows of the oceans and the atmosphere are driven by a non-uniform surface heating over latitude, and rotation. For many years scientists try to understand these flows by doing laboratory experiments. In the present paper we discuss two rather new laboratory experiments designed to study certain aspects of the atmospheric circulation. One of the experiments, the differentially heated rotating annulus at the Brandenburg University of Technology (BTU) Cottbus, has a cooled inner cylinder and a heated outer wall. However, the structure of the atmospheric meridional circulation motivates a variation of this “classical” design. In the second experiment described, operational at the Institute of Continuous Media Mechanics (ICMM) in Perm, heating and cooling is performed at different vertical levels that resembles more the atmospheric situation. Recent results of both experiments are presented and discussed. Differences and consistencies are highlighted. Though many issues are still open we conclude that both setups have their merits. The variation with heating and cooling at different levels might be more suited to study processes in the transition zone between pure rotating convection and the zone of westerly winds. On the other hand, the simpler boundary conditions of the BTU experiment make this experiment easier to control.
PIV- and LDV-measurements of baroclinic wave interactions in a thermally driven rotating annulus
(2011)
Atmospheric westerly jet streams are driven by temperature differences between low and high latitudes and the rotation of the Earth. Meandering jet streams and propagating Rossby waves are responsible for the variable weather in the mid-latitudes. Moreover, extreme weather events such as heat waves and cold spells are part of the jet stream dynamics. For many years, a simple analog in the form of a simplified laboratory experiment, the differentially heated rotating annulus, has provided insight into the dynamics of the meandering jet stream. In the present study, probability density distributions of extreme events from a long-term laboratory experiment are studied and compared to the atmospheric probability density distributions. Empirical distributions of extreme value monthly block data are derived for the experimental and atmospheric cases. Generalized extreme value distributions are adjusted to the empirical distributions, and the distribution parameters are compared. Good agreement was found, but the distributions of the experimental data showed a shift toward larger extreme values, and some explanations for this shift are suggested. The results indicate that the laboratory model might be a useful tool for investigating changes in extreme event distributions due to climate change. In the laboratory context, the change can be modeled by an increase in total temperature accompanied by a reduction in the radial heat contrast.
Rotating annulus laboratory experiments with application to baroclinic channel flows with narrows
(2010)
Rotating spherical gap convection in the GeoFlow International Space Station (ISS) experiment
(2020)
Fluid stratified by gravitation can be subject to a number of instabilities which eventually lead to a flow that causes enhanced mixing and transport of heat. The special case where a destabilizing temperature gradient counteracts the action of a stabilizing gradient in molecular weight is of interest to astrophysics (inside stars and giant planets) and geophysics (lakes, oceans) as well as to some engineering applications. The detailed dynamics of such a system depend on the molecular diffusivities of heat, momentum, and solute as well as system parameters including the ratio of the two gradients to each other. Further important properties are the formation and merging of well-defined layers in the fluid which cannot be derived from linear stability analysis. Moreover, the physical processes operate on a vast range of length and time scales. This has made the case of semi-convection, where a mean temperature gradient destabilizes the stratification while at the same time the mean molecular gradient tends to stabilize it, a challenge to physical modelling and to numerical hydrodynamical simulation. During the MetStröm project the simulation codes ANTARES and MITgcm have been extended such that they can be used for the simulations of such flows. We present a comparison of effective diffusivities derived from direct numerical simulations. For both stars and the oceanic regimes, the Nusselt numbers (scaled diffusivities) follow similar relationships. Semi-convection quickly becomes inefficient, because the formation of layers limits vertical mixing. In contrast to the complementary saltfingering, these layers tend to damp instabilities so that effective diffusivities of salinity (concentration) are up to two orders of magnitudes smaller than in the former case.