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Olaf Marxen


Lecturer

Biography

Biography

Lecturer, University of Surrey, Mechanical Engineering Sciences, 2014-Research Associate, Imperial College London, Department of Mechanical Engineering, 2013-2014Postdoctoral Researcher, von Karman Institute for Fluid Dynamics, 2011-2013Engineering Research Associate / Postdoctoral Fellow, Stanford University, Center for Turbulence Research, 2006-2011Postdoctoral Fellow, KTH Stockholm, 2005-2006Research Assistant, Universität Stuttgart, Institut für Aerodynamik und Gasdynamik, 1999-2005

Teaching

ENG 3165 Numerical Methods & CFD

My publications

Publications

Kotapati RB, Mittal R, Marxen O, Ham F, You D, Cattafesta LN (2010) Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow, Journal of Fluid Mechanics 654 pp. 65-97
A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing on the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier-Stokes simulations of this flow at a chord Reynolds number of 6 104 have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of lock-on states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 105. © 2010 Cambridge University Press.
Marxen O, Lang M, Rist U, Wagner S (2003) A Combined Experimental/Numerical Study of Unsteady Phenomena in a Laminar Separation Bubble, Flow, Turbulence and Combustion 71 (1-4) pp. 133-146
A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition. Finally the turbulent boundary layer reattaches, forming a laminar separation bubble (LSB). Laminar-turbulent transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (PIV), and direct numerical simulation (DNS). The transition mechanism occurring in the flow-field under consideration is discussed in detail. Observations for the development of small disturbances are compared to predictions from viscous linear instability theory (Tollmien-Schlichting instability). Non-linear development of these disturbances and their role in final breakdown to turbulence is analyzed.
Marxen O, Iaccarino G, Shaqfeh ESG (2014) Nonlinear instability of a supersonic boundary layer with two-dimensional roughness, Journal of Fluid Mechanics 752 (4) pp. 497-520
© © 2014 Cambridge University Press.Nonlinear instability in a supersonic boundary layer at Mach 4.8 with two-dimensional roughness is investigated by means of spatial direct numerical simulations (DNS). It was previously found that an important effect of a two-dimensional roughness is to increase significantly the amplitude of two-dimensional waves downstream of the roughness in a certain frequency band through enhanced instability and transient growth, while waves outside this band are damped. Here, we investigate the nonlinear secondary instability induced by a large-amplitude two-dimensional wave, which has received a significant boost in amplitude from this additional roughness-induced amplification. Both subharmonic and fundamental secondary excitation of the oblique secondary waves are considered. We found that even though the growth rate of the secondary perturbations increases compared to their linear amplification, only in some of the cases was a fully resonant state attained by the streamwise end of the domain. A parametric investigation of the amplitude of the primary wave, the phase difference between the primary and the secondary waves, and the spanwise wavenumber has also been performed. The transient growth experienced by the primary wave was found to not influence the secondary instability for most parameter combinations. For unfavourable phase relations between the primary and the secondary waves, the phase speed of the secondary wave decreases significantly, and this hampers its growth. Finally, we also investigated the strongly nonlinear stage, for which both the primary and the subharmonic secondary waves had a comparable, finite amplitude. In this case, the growth of the primary waves was found to vanish downstream of the transient growth region, resulting in a lower amplitude than in the absence of the large-amplitude secondary wave. This feedback also decreases the amplification rate of the secondary wave.
Marxen O, Kotapati RB, Mittal R, Zaki T (2015) Stability analysis of separated flows subject to control by zero-net-mass-flux jet, Physics of Fluids 27 (2)
© 2015 AIP Publishing LLC.The control of flow around a canonical airfoil-like geometry with laminar separation bubble is analyzed using linear stability theory. The theoretical predictions are compared to data from Navier-Stokes simulations [Kotapati et al., "Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow," J. Fluid Mech. 654, 65-97 (2010)], in which the flow was controlled through a zero-net-mass-flux actuator. Very good agreement between the two approaches is found for a range of frequencies from low to high relative to the most dominant frequency for convective instability. The uncontrolled case exhibits periodic vortex shedding from the separation bubble due to an absolute instability. Linear modes with intermediate frequencies are found to exhibit strongest convective amplification, and forcing at these frequencies is most effective in order to reduce the size and extent of the separation bubble. The corresponding physical mechanism relies on a Kelvin-Helmholtz instability of the separated shear layer in conjunction with the non-linear effect of the mean flow deformation. For low frequencies, the front part of the bubble still diminishes due to the interaction of a vortex that starts from the actuator with the wall. This vortex transiently amplifies downstream due to the Orr mechanism. Actuation at high frequencies leads to visible, amplified instability waves in the shear layer, but is not effective in reducing the size of the bubble.
Marxen O, Henningson DS (2011) The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble, Journal of Fluid Mechanics 671 pp. 1-33
Short laminar separation bubbles can develop on a flat plate due to an externally imposed pressure gradient. Here, these bubbles are computed by means of direct numerical simulations. Laminar-turbulent transition occurs in the bubble, triggered by small disturbance input with fixed frequency, but varying amplitude, to keep the bubbles short. The forcing amplitudes span a range of two orders of magnitude. All resulting bubbles differ with respect to their mean flow, linear-stability characteristics and distance between transition and mean reattachment locations. Mechanisms responsible for these differences are analysed in detail. Switching off the disturbance input or reducing it below a certain, very small threshold causes the short bubble to grow continuously. Eventually, it no longer exhibits typical characteristics of a short laminar separation bubble. Instead, it is argued that bursting has occurred and the bubble displays characteristics of a long-bubble state, even though this state was not a statistically steady state. This hypothesis is backed by a comparison of numerical results with measurements. For long bubbles, the transition to turbulence is not able to reattach the flow immediately. This effect can lead to the bursting of a short bubble, which remains short only when sufficiently large disturbances are convected into the bubble. Large-scale spanwise-oriented vortices at transition are observed for short but not for long bubbles. The failure of the transition process to reattach the flow in the long-bubble case is ascribed to this difference in transitional vortical structures. © 2011 Cambridge University Press.
Marxen O, Rist U (2010) Mean flow deformation in a laminar separation bubble: Separation and stability characteristics, Journal of Fluid Mechanics 660 pp. 37-54
The mutual interaction of laminar-turbulent transition and mean flow evolution is studied in a pressure-induced laminar separation bubble on a flat plate. The flat-plate boundary layer is subjected to a sufficiently strong adverse pressure gradient that a separation bubble develops. Upstream of the bubble a small-amplitude disturbance is introduced which causes transition. Downstream of transition, the mean flow strongly changes and, due to viscous-inviscid interaction, the overall pressure distribution is changed as well. As a consequence, the mean flow also changes upstream of the transition location. The difference in the mean flow between the forced and the unforced flows is denoted the mean flow deformation. Two different effects are caused by the mean flow deformation in the upstream, laminar part: a reduction of the size of the separation region and a stabilization of the flow with respect to small, linear perturbations. By carrying out numerical simulations based on the original base flow and the time-averaged deformed base flow, we are able to distinguish between direct and indirect nonlinear effects. Direct effects are caused by the quadratic nonlinearity of the Navier-Stokes equations, are associated with the generation of higher harmonics and are predominantly local. In contrast, the stabilization of the flow is an indirect effect, because it is independent of the Reynolds stress terms in the laminar region and is solely governed by the non-local alteration of the mean flow via the pressure. © 2010 Cambridge University Press.
Tiyyagura SR, Adamidis PA, Rabenseifner R, Lammers P, Borowski S, Lippold F, Svensson F, Marxen O, Haberhauer S, Seitsonen AP, Furthmüller J, Benkert K, Galle M, Bönisch T, Küster U, Resch MM (2008) Teraflops Sustained Performance With Real World Applications., IJHPCA 22 2 pp. 131-148
Marxen O, Magin TE, Shaqfeh ESG, Iaccarino G (2013) A method for the direct numerical simulation of hypersonic boundary-layer instability with finite-rate chemistry, Journal of Computational Physics 255 pp. 572-589
A new numerical method is presented here that allows to consider chemically reacting gases during the direct numerical simulation of a hypersonic fluid flow. The method comprises the direct coupling of a solver for the fluid mechanical model and a library providing the physio-chemical model. The numerical method for the fluid mechanical model integrates the compressible Navier-Stokes equations using an explicit time advancement scheme and high-order finite differences. This Navier-Stokes code can be applied to the investigation of laminar-turbulent transition and boundary-layer instability. The numerical method for the physio-chemical model provides thermodynamic and transport properties for different gases as well as chemical production rates, while here we exclusively consider a five species air mixture. The new method is verified for a number of test cases at Mach 10, including the one-dimensional high-temperature flow downstream of a normal shock, a hypersonic chemical reacting boundary layer in local thermodynamic equilibrium and a hypersonic reacting boundary layer with finite-rate chemistry. We are able to confirm that the diffusion flux plays an important role for a high-temperature boundary layer in local thermodynamic equilibrium. Moreover, we demonstrate that the flow for a case previously considered as a benchmark for the investigation of non-equilibrium chemistry can be regarded as frozen. Finally, the new method is applied to investigate the effect of finite-rate chemistry on boundary layer instability by considering the downstream evolution of a small-amplitude wave and comparing results with those obtained for a frozen gas as well as a gas in local thermodynamic equilibrium. © 2013 Elsevier Inc.
Marxen O, Lang M, Rist U (2013) Vortex formation and vortex breakup in a laminar separation bubble, Journal of Fluid Mechanics 728 pp. 58-90
The convective primary amplification of a forced two-dimensional perturbation initiates the formation of essentially two-dimensional large-scale vortices in a laminar separation bubble. These vortices are then shed from the bubble with the forcing frequency. Immediately downstream of their formation, the vortices get distorted in the spanwise direction and quickly disintegrate into small-scale turbulence. The laminar-turbulent transition in a forced laminar separation bubble is dominated by this vortex formation and breakup process. Using numerical and experimental data, we give an in-depth characterization of this process in physical space as well as in Fourier space, exploiting the largely periodic character of the flow in time as well as in the spanwise direction. We present evidence that a combination of more than one secondary instability mechanism is active during this process. The first instability mechanism is the elliptic instability of vortex cores, leading to a spanwise deformation of the cores with a spanwise wavelength of the order of the size of the vortex. Another mechanism, potentially an instability of flow in between two consecutive vortices, is responsible for three-dimensionality in the braid region. The corresponding disturbances possess a much smaller spanwise wavelength as compared to those amplified through elliptic instability. The secondary instability mechanisms occur for both fundamental and subharmonic frequency, respectively, even in the absence of continuous forcing, indicative of temporal amplification in the region of vortex formation. © 2013 Cambridge University Press.
Marxen O, Rist U, Wagner S (2004) Effect of spanwise-modulated disturbances on transition in a separated boundary layer, AIAA Journal 42 (5) pp. 937-944
A laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition. The detached shear layer rolls up into spanwise vortices that rapidly break down into small-scale turbulence. Finally, the turbulent boundary layer reattaches, forming a laminar separation bubble. Development and role of three-dimensional disturbances for transition in such a separation bubble are studied by means of direct numerical simulation with controlled disturbance input. In the present case, the level of incoming three-dimensional perturbations is not relevant due to an absolute secondary instability of these disturbances in the region of convective two-dimensional shear layer rollup. In particular, this is true for steady perturbations up to moderate amplitudes. Following their generation by nonlinear interaction of disturbance waves in the region of favorable pressure gradient, these steady disturbances develop as streaks. Their downstream evolution can be first attributed to transient behavior, depending on initial excitation, followed by a universal state with characteristics of a modal instability. Numerical results are confirmed by a comparison with experimental data.
Marxen O, Lang M, Rist U, Levin O, Henningson DS (2009) Mechanisms for spatial steady three-dimensional disturbance growth in a non-parallel and separating boundary layer, Journal of Fluid Mechanics 634 pp. 165-189
Steady linear three-dimensional disturbances are investigated in a two-dimensional laminar boundary layer. The boundary layer is subject to a streamwise favourable-to-adverse pressure gradient and eventually undergoes separation. The separating flow corresponds to the first part of a pressure-induced laminar-separation bubble on a flat plate. Streamwise disturbance development in such a flow is studied by means of direct numerical simulation, a water-tunnel experiment and an adjoint-based parabolic theory suited to study spatial optimal growth. A complete overview of the disturbance evolution in various areas of the favourable-to-adverse pressure gradient laminar boundary layer is given. Results from all investigation methods show overall good agreement with respect to disturbance growth and shape within the entire domain. In the favourable pressure-gradient region and, again, slightly downstream of separation, transient growth caused by the lift-up effect dominates disturbance behaviour. In the adverse pressure-gradient region, a modal instability is observed. Evidence is presented that this instability is of Grtler type. © 2009 Cambridge University Press.
Pitz DB, Chew J, Marxen O, Hills NJ (2016) Direct Numerical Simulation of Rotating Cavity Flows Using a Spectral Element-Fourier Method, Journal of Engineering for Gas Turbines and Power: Transactions of the ASME
in a rotor/stator cavity without heat transfer and buoyant
flow in a rotor/rotor cavity. The numerical tool used employs a
spectral element discretisation in two dimensions and a Fourier
expansion in the remaining direction, which is periodic and corresponds
to the azimuthal coordinate in cylindrical coordinates.
The spectral element approximation uses a Galerkin method to
discretise the governing equations, but employs high-order polynomials
within each element to obtain spectral accuracy. A
second-order, semi-implicit, stiffly stable algorithm is used for
the time discretisation. Numerical results obtained for the rotor/
stator cavity compare favourably with experimental results
for Reynolds numbers up to Re1 = 106 in terms of velocities and
Reynolds stresses. The buoyancy-driven flow is simulated using
the Boussinesq approximation. Predictions are compared with
previous computational and experimental results. Analysis of
the present results shows close correspondence to natural convection
in a gravitational field and consistency with experimentally
observed flow structures in a water-filled rotating annulus.
Predicted mean heat transfer levels are higher than the available
measurements for an air-filled rotating annulus, but in agreement
with correlations for natural convection under gravity.
Åkervik E, Brandt L, Henningson DS, HSpffner J, Marxen O, Schlatter P (2006) Steady solutions of the Navier-Stokes equations by selective frequency damping, Physics of Fluids 18 (6)
A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient. © 2006 American Institute of Physics.
Marxen O, Iaccarino G, Magin TE (2014) Direct numerical simulations of hypersonic boundary-layer transition with finite-rate chemistry, Journal of Fluid Mechanics 755 pp. 35-49
The paper describes a numerical investigation of linear and nonlinear instability in high-speed boundary layers. Both a frozen gas and a finite-rate chemically reacting gas are considered. The weakly nonlinear instability in the presence of a large-amplitude two-dimensional wave is investigated for the case of fundamental resonance. Depending on the amplitude of this two-dimensional primary wave, strong growth of oblique secondary perturbations occurs for favourable relative phase differences between the two. For essentially the same primary amplitude, secondary amplification is almost identical for a reacting and a frozen gas. Therefore, chemical reactions do not directly affect the growth of secondary perturbations, but only indirectly through the change of linear instability and hence amplitude of the primary wave. When the secondary disturbances reach a sufficiently large amplitude, strongly nonlinear effects stabilize both primary and secondary perturbations. © Cambridge University Press 2014.
Marxen O, Iaccarino G, Shaqfeh ESG (2010) Disturbance evolution in a mach 4.8 boundary layer with two-dimensional roughness-induced separation and shock, Journal of Fluid Mechanics 648 pp. 435-469
A numerical investigation of the disturbance amplification in a Mach 4.8 flat-plate boundary layer with a localized two-dimensional roughness element is presented. The height of the roughness is varied and reaches up to approximately 70% of the boundary-layer thickness. Simulations are based on a time-accurate integration of the compressible Navier-Stokes equations, with a small disturbance of fixed frequency being triggered via blowing and suction upstream of the roughness element. The roughness element considerably alters the instability of the boundary layer, leading to increased amplification or damping of a modal wave depending on the frequency range. The roughness is also the source of an additional perturbation. Even though this additional mode is stable, the interaction with the unstable mode in the form of constructive and destructive interference behind the roughness element leads to a beating and therefore transiently increased disturbance amplitude. Far downstream of the roughness, the amplification rate of a flat-plate boundary layer is recovered. Overall, the two-dimensional roughness element behaves as disturbance amplifier with a limited bandwidth capable of filtering a range of frequencies and strongly amplifying only a selected range. © 2010 Cambridge University Press.
Marxen O, Lang M, Rist U (2012) Discrete linear local eigenmodes in a separating laminar boundary layer, Journal of Fluid Mechanics 711 pp. 1-26
The evolution of two- and three-dimensional small-amplitude disturbances in the laminar part of a laminar separation bubble is investigated in detail. We apply a combination of local linear stability theory, results from different experimental measurement campaigns and direct numerical simulations to identify two different discrete eigenmodes in the laminar part of the bubble. A stable eigenmode, the outer mode, governs unsteady oscillations in the upstream part of the bubble. However, this perturbation is quickly overtaken by an unstable eigenmode, the inner mode, which eventually leads to transition of the detached shear layer. Such a behaviour is observed due to an acceleration region with a favourable pressure gradient preceding the adverse-pressure-gradient region. The flow is stable in the acceleration region, in which the outer mode is only moderately damped, while the inner mode is strongly damped. At the onset of instability for the unstable eigenmode upstream of separation, both viscous Tollmien-Schlichting and inviscid Kelvin-Helmholtz instability mechanisms contribute to amplification, while deeper inside the bubble only the inviscid mechanism is active. If the explicit forcing is moved to a region downstream of the favourable pressure gradient, only the unstable eigenmode appears. The same behaviour is found for two-dimensional and weakly oblique waves. © 2012 Cambridge University Press.
Marxen O, Magin T, Iaccarino G, Shaqfeh ESG (2011) A high-order numerical method to study hypersonic boundary-layer instability including high-temperature gas effects, Physics of Fluids 23 (8)
Prediction of laminar-turbulent transition is a key factor in the design of the heat shield of vehicles (re-)entering a planetary atmosphere. To investigate the transition by means of numerical simulation, accurate and efficient computational methods are necessary. Here, the compressible Navier-Stokes equations are solved for a gas where properties such as specific heat, thermal conductivity, viscosity, and specific gas constant depend on one or two thermodynamic variables. Our approach models a mixture of perfect gases in local thermodynamic equilibrium. The gas properties are provided either by means of direct calls to a library based on statistical mechanics and kinetic theory or indirectly in the form of look-up tables. In the first part of the paper, our method of handling a high-temperature gas in thermochemical equilibrium is described and verified. In the second part, the method is applied to the investigation of linear and non-linear boundary-layer instability. We carry out numerical simulations for a laminar flat-plate boundary layer at Mach 10 with a small, convectively amplified perturbation for both Earth and Martian atmospheres. Amplification of the perturbations shows favorable agreement with results obtained from linear theory. The secondary instability of the boundary layer in the presence of a large-amplitude two-dimensional wave is investigated. We observe that the non-linear mechanism of fundamental resonance becomes active and leads to a strong increase in amplification of three-dimensional disturbance waves. © 2011 American Institute of Physics.
Ryu S, Marxen O, Iaccarino G (2015) A comparison of laminar-turbulent boundary-layer transitions induced by deterministic and random oblique waves at Mach 3, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW 56 pp. 218-232 ELSEVIER SCIENCE INC
Pitz D, Marxen O, Chew J (2017) Onset of convection induced by centrifugal buoyancy in a rotating cavity, Journal of Fluid Mechanics 826 pp. 484-502 Cambridge University Press
Flows induced by centrifugal buoyancy occur in rotating systems in which the centrifugal force is large when compared to other body forces and are of interest for geophysicists and also in engineering problems involving rapid rotation and unstable temperature gradients. In this numerical study we analyse the onset of centrifugal buoyancy in a rotating cylindrical cavity bounded by two plane, insulated disks, adopting a geometrical configuration relevant to fundamental studies of buoyancy-induced flows occurring in gas turbine?s internal air systems. Using linear stability analysis, we obtain critical values of the centrifugal Rayleigh number and corresponding critical azimuthal wavenumbers for the onset of convection for different radius ratios. Using direct numerical simulation, we integrate the solutions starting from a motionless state to which small sinusoidal perturbations are added, and show that nonlinear triadic interactions occur before energy saturation takes place. At the lowest Rayleigh number considered, the final state is a limit-cycle oscillation affected by the presence of the disks, having a spectrum dominated by a certain mode and its harmonics. We show that, for this case, the limit-cycle oscillations only develop when no-slip end walls are present. For the largest considered chaotic motion occurs, but the critical wavenumber obtained from the linear analysis eventually becomes the most energetic even in the turbulent regime.
Pitz D, Chew John, Marxen Olaf (2018) Large-eddy simulation of buoyancy-induced flow in a sealed rotating cavity, Proceedings of ASME Turbo Expo 2018 ASME
Buoyancy-induced flows occur in the rotating cavities of gas
turbine internal air systems, and are particularly challenging to
model due to the inherently unsteadiness of these flows. While
the global features of such flows are well documented, detailed
analyses of the unsteady structure and turbulent quantities have
not been reported. In this work we use a high-order numerical
method to perform large-eddy simulation (LES) of buoyancyinduced
flow in a sealed rotating cavity with either adiabatic or
heated disks. New insight is given into long-standing questions
regarding the flow characteristics and nature of the boundary
layers. The analyses focus on showing time-averaged quantities,
including temperature and velocity fluctuations, as well as on
the effect of the centrifugal Rayleigh number on the flow structure.
Using velocity and temperature data collected over several
revolutions of the system, the shroud and disk boundary layers
are analysed in detail. The instantaneous flow structure contains
pairs of large, counter-rotating convection rolls, and it is shown
that unsteady laminar Ekman boundary layers near the disks are
driven by the interior flow structure. The shroud thermal boundary
layer scales as approximately Ra
In this research a spectral element method is used to perform direct numerical simulation (DNS) and implicit large-eddy simulation (LES) of flows induced by centrifugal buoyancy in rotating cavities. These flows occur, for instance, in the compressor cavities of gas turbines internal air systems, in which cooling air is used to extract heat from compressor disks. Buoyancy-induced flows are inherently challenging to study using computational fluid dynamics (CFD), since turbulence models based on the Reynolds-averaged Navier-Stokes (RANS) equations are not able to provide an accurate description of the phenomena induced by the interplay between buoyancy and rotation. For this reason, model-free approaches are desirable, since they can provide an accurate description of the flow physics. First, the method is applied to a rotor/stator configuration, in which regions of laminar, transitional and fully turbulent flow coexist, and the results are compared with experimental data from the literature. Subsequently, flow induced by centrifugal buoyancy in a sealed rotating annulus is investigated using linear stability analysis, DNS and LES. It is shown that the onset of convection for a rotating cavity is similar to that for the problem of Rayleigh-Bénard convection. Analysing flow statistics for different values of the Rayleigh number, it is shown that the disk boundary layer behaves as a laminar Ekman layer, both in terms of its thickness and of its velocity profiles. This is observed even when instantaneous profiles are considered, despite the unsteadiness of the solution. The results also show that the shroud thermal boundary layer scaling is consistent with that of natural convection under gravity. Introducing an axial throughflow of cooling air, some features observed in the sealed cavity are maintained, however a strong reduction in the core temperature and a corresponding increase in the shroud heat transfer occur. The axial throughflow also promotes a significant increase in the range of frequencies observed inside the cavity.
Pitz Diogo B, Chew John, Marxen Olaf (2108) Large-eddy simulation of buoyancy-induced flow in a sealed rotating cavity, Journal of Engineering for Gas Turbines and Power American Society of Mechanical Engineers
Buoyancy-induced flows occur in the rotating cavities of gas
turbine internal air systems, and are particularly challenging to
model due to the inherently unsteadiness of these flows. While
the global features of such flows are well documented, detailed
analyses of the unsteady structure and turbulent quantities have
not been reported. In this work we use a high-order numerical
method to perform large-eddy simulation (LES) of buoyancyinduced
flow in a sealed rotating cavity with either adiabatic or
heated disks. New insight is given into long-standing questions
regarding the flow characteristics and nature of the boundary
layers. The analyses focus on showing time-averaged quantities,
including temperature and velocity fluctuations, as well as on
the effect of the centrifugal Rayleigh number on the flow structure.
Using velocity and temperature data collected over several
revolutions of the system, the shroud and disk boundary layers
are analysed in detail. The instantaneous flow structure contains
pairs of large, counter-rotating convection rolls, and it is shown
that unsteady laminar Ekman boundary layers near the disks are
driven by the interior flow structure. The shroud thermal boundary
layer scales as approximately Ra?1/3
, in agreement with observations
for natural convection under gravity.