Showing papers by "Nikolaus A. Adams published in 2011"

Scale separation for implicit large eddy simulation

Abstract: With implicit large eddy simulation (ILES) the truncation error of the discretization scheme acts as subgrid-scale (SGS) model for the computation of turbulent flows Although ILES is comparably simple, numerically robust and easy to implement, a considerable challenge is the design of numerical discretization schemes resulting in a physically consistent SGS model In this work, we consider the implicit SGS modeling capacity of the adaptive central-upwind weighted-essentially-non-oscillatory scheme (WENO-CU6) [XY Hu, Q Wang, NA Adams, An adaptive central-upwind weighted essentially non-oscillatory scheme, J Comput Phys 229 (2010) 8952-8965] by incorporating a physically-motivated scale-separation formulation Scale separation is accomplished by a simple modification of the WENO weights The resulting modified scheme maintains the shock-capturing capabilities of the original WENO-CU6 scheme while it is also able to reproduce the Kolmogorov range of the kinetic-energy spectrum for turbulence at the limit of infinite Reynolds number independently of grid resolution For isentropic compressible turbulence the pseudo-sound regime of the dilatational kinetic-energy spectrum and the non-Gaussian probability-density function of the longitudinal velocity derivative are reproduced

SPH simulations of flow around a periodic array of cylinders confined in a channel

Abstract: In this paper, we present a numerical study on the performance of SPH in the case of a very viscous flow of a Newtonian liquid around a linear array of cylinders confined in a channel. This specific flow problem, being characterized by a complex mixing of both shear and extensional behaviour, allows to quantify systematically the accuracy of the standard SPH in, up to now, rarely considered complex geometries. Global accuracy tests based on the estimation of the dimensionless drag force acting on the cylinder as well as the inspection of the local velocity profiles are considered and compared with reference solutions. In agreement with previous findings, the impact of two numerical parameters, namely the smoothing length h and particle spacing Δx, is discussed and found to be crucial for the overall order of convergence of the method. In particular, accurate results can be obtained which are in very good agreement with standard mesh-based methods provided that the number of neighbours is chosen properly. The present results, being based on a detailed convergence analysis in a complex flow problem, justify the applicability of the SPH method to more complex wall-bounded flows upon critical choice of the model parameters and at the same time can serve as a useful benchmark test for further modelling improvement in the field. Copyright © 2010 John Wiley & Sons, Ltd.

Numerical simulation of collapse induced shock dynamics for the prediction of the geometry, pressure and temperature impact on the cavitation erosion in micro channels

Abstract: A compressible 2D Euler CFD code is utilised for the numerical simulation of compressible cavitating liquids with a density-based numerical scheme which has been proved to capture shock-induced dynamics that are the origin of erosion damages [22]. A barotropic cavitation model is applied on planar micro throttles where the qualitative impact of the following parameters on the location and strength of erosion is evaluated and compared to experimental data:  Channel inlet geometry, round versus sharp inlet shape  Channel inlet and outlet pressure  Fluid inlet temperature While the erosion strength and location in the experiment is evaluated by temporal sequences of the material erosion, an erosion probability based on pressure thresholds is extracted from the simulation results which quantifies the magnitude of the stress on the wall. By comparing the wall stress magnitude (simulation) and the material erosion (experiment), the qualitative influence of the channel inlet geometry as well as the inlet and outlet pressure can be well captured. The temperature impact on erosion can not be predicted by the barotropic model. NOMENCLATURE

Numerical simulation of tethered DNA in shear flow.

Abstract: The behavior of tethered DNA in shear flow is investigated numerically by the smoothed dissipative particle dynamics (SDPD) method. Unlike numerical methods used in previous studies, SDPD models the solvent explicitly, takes into account the fully coupled hydrodynamic interactions and is free of the numerical artifact of wall sticking. Based on numerical simulations the static and dynamic properties of a tethered DNA is studied both qualitatively and quantitatively. The observed properties are in general agreement with previous experimental, numerical and theoretical work. Furthermore, the cyclic-motion phenomenon is studied by power spectrum density and cross-correlation function analysis, which suggest that there is only a very weak coherent motion of tethered DNA for a characteristic timescale larger than the relaxation time. Cyclic motion is more likely relevant as an isolated event than a typical mode of DNA motion.

A stochastic extension of the approximate deconvolution model

Abstract: The approximate deconvolution model (ADM) for large-eddy simulation exploits a range of represented but non-resolved scales as buffer region for emulating the subgrid-scale energy transfer. ADM can be related to Langevin models for turbulence when filter operators are interpreted as stochastic kernel estimators. The main conceptual difference between ADM and Langevin models for turbulence is that the former is formulated with respect to an Eulerian reference frame whereas the latter are formulated with respect to a Lagrangian reference frame. This difference can be resolved by transforming the Langevin models to the Eulerian reference frame. However, the presence of a stochastic force prevents the classical convective transformation from being applicable. It is shown that for the transformation a stochastic number-density field can be introduced that essentially represents the Lagrangian particle distribution of the original model. Unlike previous derivations, the number-density field is derived by invoki...

Wavelet-based adaptive multi-resolution solver on heterogeneous parallel architecture for computational fluid dynamics

Abstract: For the efficient simulation of fluid flows governed by a wide range of scales a wavelet-based adaptive multi-resolution solver on heterogeneous parallel architectures is proposed for computational fluid dynamics. Both data- and task-based parallelisms are used for multi-core and multi-GPU architectures to optimize the efficiency of a high-order wavelet-based multi-resolution adaptative scheme with a 6th-order adaptive central-upwind weighted essentially non-oscillatory scheme for discretization of the governing equations. A modified grid-block data structure and a new boundary reconstruction method are introduced. A new approach for detecting small scales without using buffer levels is introduced to obtain additional speed-up by minimizing the number of required blocks. Validation simulations are performed for a double-Mach reflection with different refinement criteria. The simulations demonstrate accuracy and computational performance of the solver.

Numerical Investigation of Inlet distortion on a Wing-Embedded Lift Fan

Abstract: The inlet distortion arising on a generic fan-in-wing wind-tunnel model is investigated by means of unsteady Reynolds-averaged Navier–Stokes simulations. Flow separation occurs on the inlet lip and generates a separation bubble above the rotor blades. A distortion of the total pressure distribution at the fan inlet significantly influences the blade loading. The strong flow-velocity gradient over the inlet lip induces an increase of the axial-velocity magnitude, resulting in a reduction of incidence angle on the rotor blade. In the bubble-core region, the low axialvelocity magnitude enhances the blade incidence angle and provokes blade tip stall. Steady-state calculations, using an actuator disk approach, are also conducted to identify the key parameters affecting lip boundary-layer separation. The ratio of freestream velocity to fan jet velocity and the inlet-lip radius affect the total pressure distribution bymodifying the location of the flow separation. The angle of attack on the wing has a negligible impact on the inlet distortion. An inlet lip with a large radius at its front part suppresses the lip separation and considerably reduces the total pressure distortion. Injecting a jet over the inlet lip can be a solution to actively control the lip flow.

Investigation of Unsteady Vehicle Aerodynamics under Time-Dependent Flow Conditions

Abstract: ow conditions aecting driving stability. Based on experimental and numerical simulations it is shown that the wake ow of a vehicle being subjected to constant crosswind is characterized by a region of high velocity at the leeward side and two counter-rotating vortices establishing above and below that region. This structure reacts with a time delay to a change of the oncoming ow. The delayed response of the wake ow causes phase shifted pressure uctuations at the rear side of the vehicle and thus causes an unsteady increase of the yaw moment and decrease of side force and roll moment. Numerical and experimental results both capture the unsteady phenomena occurring at transient oncoming ow conditions. However, the quantitative estimation of the pressure amplitude at the rear side diers, which alters the ratio of unsteady to quasi-steady loads as well as their phase lag.

Integrated Experimental-Numerical Analysis of High-Agility Aircraft Wake Vortex Evolution

Abstract: The presented investigation includes a combined experimental–numerical approach to quantify the wake vortex system of a high-agility aircraft from the near field up to the far field. Detailed near-field data are obtained by lowspeed wind-tunnel tests on a delta-canard configuration of 1:15 scale. The measurements are performed at several angles of attack applying advanced hot-wire anemometry. For a wake distance of up to 16 wing spans, mean and turbulentvelocity fieldsaremeasured.Theupstreamdataareusedtoinitializeimplicitlarge-eddysimulationsaimed tocomputethevelocity fieldsofthewakevortexsystemoverawakedistanceofupto50spans.Here,avalidationcase is shown comparing measured and calculated wake data over a distance from 4 to 16 spans, with the implicit largeeddy simulations initialized by the measured quantities at a position of two wing spans. Compared with the experimental data,the numericalresults showthe expectedlateral andvertical movementof thewakevortexsystem due to the interaction of the single vortices. The distributions of axial vorticity, crossflow velocities, and turbulence intensities match well with the experimental data. Inaddition, the dissipation process can be observed, resulting in a reduction of circulation. In the context of this study, the measured and computed velocity fields will be used to determine unsteady aerodynamic loads acting on a fighter aircraft encountering the wake. This is of great importance as wake induction may result in critical structural dynamic loads.

Large Eddy Simulations of Turbulent Enhancement due to Forced Shock Motion in Shock-Boundary Layer Interaction

Abstract: We present Implicit Large-Eddy Simulations of a shockwave-turbulent boundary layer interaction with and without localized heat addition. The flow is complex and involves boundary layer separation under the adverse pressure gradient imposed by the shock, turbulence amplification across the interaction and low frequency oscillation of the reflected shock. For an entropy spot generated ahead of the shock, baroclinic vorticity production occurs when the resulting density peak passes the shock. The objective of the present study is to analyze the shock-separation interaction and turbulence structure of such a configuration. The effect of the addition of an entropy spot to the flow field is assessed in terms of turbulence amplification.

CFD Aided Development of an Experimental Setup to Investigate Internal Supersonic Combustion

Abstract: The work presented here summarizes our efforts on the development of a simplified model scramjet combustor to investigate supersonic mixing and combustion in the Stanford Expansion Tube Facility. The system is designed to replicate the conditions and the flow features found in a generic scramjet engine of a hypersonic vehicle. The initial conceptual design was aided by two-dimensional numerical simulations which were targeted to identify possible working configurations, limits of operation and to isolate selected cases of interest. A configurable prototype was then developed, built and tested in our flow facility under realistic aerothermodynamic conditions of hypersonic flight. Finally, more detailed RANS numerical simulations of two representative scramjet model combustors have been undertaken. Besides identifying a suitable RANS solver for further investigations, the goal was to investigate the flowfield within the combustor and to identify the effective conditions of operations for future work.

Interference Effects of Cooling Air-Flows with External Aerodynamics

Abstract: State-of-the-art vehicles already show an aerodynamically well improved bodywork with high-level efficiency. To further improve the aerodynamic drag, one potential area is the interaction between the underhood and the external flow. This study presents an experimental and numerical investigation of the interference effects of the cooling-air flow with the external-aerodynamics. Extensive measurements, like forces and total-pressure were accomplished. A simple dependency between drag and cooling-air mass-flow is derived, which will help for daily wind-tunnel work.

Aerodynamic Investigations on a Helicopter Fuselage

Abstract: The flow around a helicopter fuselage is investigated experimentally and numerically in forward flight condition. The focus is set on the flow phenomena occurring on the lower backside of the fuselage, where massive flow separation and vortex formation appear. The experimental part of the investigation includes force measurements, surface pressure measurements and the capturing of the velocity field in the wake of the fuselage. Two model mount systems are used to investigate the influence of the model suspension system on the flow around the fuselage. The numerical investigations cover a grid study and the comparison of the simulation with experimental data. The experiments showed that the model mount has a significant influence on the flow around the fuselage, especially on the separation and vortex formation at the backdoor section. The comparison of the numerical and experimental data exhibits a good agreement of the aerodynamic loads as well as of the flow structures in the aft body. The analysis of the complementary experimental and numerical data provides a detailed view of the flow characteristics at the fuselage afterbody.

Wall Modelling for Implicit Large Eddy Simulation of Favourable and Adverse Pressure Gradient Flows

Abstract: In order to perform Implicit Large Eddy Simulation (ILES) on complex geometries at high Reynolds numbers, a wall model based on the simplified Thin Boundary Layer Equations (TBLE) is designed in the framework of ILES with a cut-cell finite-volume immersed boundary method. This wall model is validated for turbulent channel flow at friction Reynolds number up to Re τ =2,000 on very coarse grids. The results compared with DNS and LES without wall model show that the wall model has the potential to improve the mean velocity in the outer flow region well at high Reynolds number. The wall model is applied to a complex converging diverging channel flow at Reynolds number Re=7,900 on very coarse meshes. Improved mean velocities and Reynolds stresses are obtained, which shows that the wall model has the ability to perform ILES on complex geometries at high Reynolds numbers.

Large Eddy Simulation of Supercritical Mixing Layers

Abstract: Within the development of a validated and reliable numerical tool for the simulation of a whole rocket combustion chamber, real gas thermodynamics has been implemented into two CFD codes, an in-house code and OpenFOAM. The presented work is part of the validation process, where di erent LES simulations were conducted with these codes. The implicit LES method ALDM and two explicit one equation models - dynamic and non-dynamic - were validated against DNS simulations of a non-reacting transient mixing layer. Both codes reproduced the DNS data very well. A future task will be the optimization of the LES models for real gas flows and subsequent combustion simulations.

CFD Aided Development of an Experimental Setup to Investigate Internal Supersonic Combustion

Abstract: The work presented here summarizes our efforts on the development of a simplified model scramjet combustor to investigate supersonic mixing and combustion in the Stanford Expansion Tube Facility. The system is designed to replicate the conditions and the flow features found in a generic scramjet engine of a hypersonic vehicle. The initial conceptual design was aided by two-dimensional numerical simulations which were targeted to identify possible working configurations, limits of operation and to isolate selected cases of interest. A configurable prototype was then developed, built and tested in our flow facility under realistic aerothermodynamic conditions of hypersonic flight. Finally, more detailed RANS numerical simulations of two representative scramjet model combustors have been undertaken. Besides identifying a suitable RANS solver for further investigations, the goal was to investigate the flowfield within the combustor and to identify the effective conditions of operations for future work.

Experimental and Numerical Investigation on the Flow-Induced Stresses on the Alveolar-Epithelial-Surfactant-Air Interface

Abstract: To develop new protective artificial respiration strategies a profound knowledge of the lung functionality is required. Still unknown are the dominating effects on the alveolar level and the exact geometry of the alveolar structure itself. We try to fill this gap with our multi-disciplinary research, where on the one hand we are interested in visualization techniques to reconstruct the three-dimensional structure of the alveoli and on the other hand develop a numerical tool to simulate the complex physics at the surfactant enriched liquid-lining layer interface.

Investigation of an elastoflexible morphing wing configuration

Abstract: This paper considers the experimental investigation of a morphing wing using an elastic membrane for the lifting surface to allow large variations of the planform. Measurements of the membrane deflection of two different wing configurations at various flow conditions (i.e. dynamic pressure and angle of attack) are presented to provide insight into the complex flow-structure interaction mechanisms governing the behavior of the wing and help optimizing its aerodynamic performances. The results allow identifying the relative influence of the aerodynamic, geometric and structural parameters of the wing on the membrane deflection. In particular, the non-linearity of the interaction between the aerodynamic load and the membrane deflection is pointed out.

Turbulence enhancement by forced shock motion in shock-wave/turbulent boundary layer interaction

Abstract: We present Implicit Large-Eddy Simulations of a shockwave/turbulent boundary layer interaction (SWTBLI) at Ma = 2.25 andReδ = 51,552 with and without localized heat addition. The flow is complex and involves boundary layer separation under the adverse pressure gradient imposed by the shock, turbulence amplification across the interaction and low frequency oscillation of the reflected shock. For an entropy spot generated ahead of the shock, baroclinic vorticity pro duction occurs when the resulting density peak passes the shock . The objective of the present study is to analyze the shockseparation interaction and turbulence structure of such a c onfiguration. The effect of the addition of an entropy spot to th e flow field is assessed in terms of turbulence amplification and turbulence mass flux. INTRODUCTION The SCRamjet is a flight propulsion engine meant to operate at hypersonic speeds. At flight Mach numbers Ma > 5, the flow goes initially through forebody compression and, as it progresses through the isolator and entries into the combustion chamber, is further decelerated through a shock tra in. Combustion usually takes place at around Ma = 2. Such speeds result in a flow residence time on the order of millisec onds. In order to achieve the desired combustion efficiency, a very good fuel/air mixing rate is required. Classically, t he fuel-injector type (strut, ramp or wall injector) is primar ily determining the mixing process. Another, less-investigated way to improve the fuel-to-air mixing is to exploit the presence of the aforementioned shock train. In the SCRamjet isolator an d combustion chamber, oblique shocks propagate by reflection at the wall, where they impinge on turbulent boundary layers. Several reports on reflected shock unsteadiness in shoc kwave/turbulent boundary layer interaction (SWTBLI) point to a low-frequency mechanism of the reflected shockwave (Dussauge et al., 2006; Pirozzoli et al., 2008; Priebe et al., 200 9; Touber and Sandham, 2008; Wu and Martin, 2008). In addition to this, studies showed (Fabre et al., 2001; Hussaini an d Erlebacher, 1999) that convecting local variations of temp rature through a shock generates an acoustic wave and vorticity. In the SCRamjet frame local variations of temperature can be interpreted as hotter regions, where fuel has burned, or colder regions rich in unburnt fuel. These spots are convect ed through oblique shocks with the freestream velocity. Our in vestigation evolves around the benefit fuel/air mixing can o btain from reflected shock oscillation mechanism and convection of entropy spots through an oblique shock. This paper is organized as follows: introduction; description of the num erical method; boundary conditions and computational setup; validation of the turbulence model we used in our simulation s; in the fifth part we focus on SWTBLI simulations; the last part is reserved for discussing the influence of entropy spots (ES ) convection through the aforementioned flow case.