Showing papers by "Nikolaus A. Adams published in 2012"

Analysis of unsteady behaviour in shockwave turbulent boundary layer interaction

Abstract: The unsteady behavior in shockwave turbulent boundary layer interaction is investi- gated by analyzing results from a LES of a supersonic turbulent boundary layer over a compression-expansion ramp. The interaction leads to a very-low-frequency motion near the foot of the shock, with a characteristic frequency that is three orders of magnitude lower than the typical frequency of the incoming boundary layer. Wall pressure data are first analyzed by means of Fourier analysis, highlighting the low-frequency phenomenon in the interaction region. Furthermore, the flow dynamics are analyzed by a dynamic mode decomposition which shows the presence of a low-frequency mode associated with the pulsation of the separation bubble and accompanied by a forward-backward motion of the shock.

Multiscale modeling of particle in suspension with smoothed dissipative particle dynamics

Abstract: We apply smoothed dissipative particle dynamics (SDPD) [Espanol and Revenga, Phys. Rev. E 67, 026705 (2003)] to model solid particles in suspension. SDPD is a thermodynamically consistent version of smoothed particle hydrodynamics (SPH) and can be interpreted as a multiscale particle framework linking the macroscopic SPH to the mesoscopic dissipative particle dynamics (DPD) method. Rigid structures of arbitrary shape embedded in the fluid are modeled by frozen particles on which artificial velocities are assigned in order to satisfy exactly the no-slip boundary condition on the solid-liquid interface. The dynamics of the rigid structures is decoupled from the solvent by solving extra equations for the rigid body translational/angular velocities derived from the total drag/torque exerted by the surrounding liquid. The correct scaling of the SDPD thermal fluctuations with the fluid-particle size allows us to describe the behavior of the particle suspension on spatial scales ranging continuously from the dif...

Numerical investigation of collapsing cavity arrays

Abstract: In most technical applications involving cavitation, vapor bubbles occur in clouds, and their collapse is affected by the interaction with neighboring bubbles. One approach to study the influence of these interactions is the investigation of the collapse of cavity arrays in water under shock wave loading. We describe in detail the collapse mechanisms during the collapse of a horizontal cavity array, with particular consideration of maximum pressures. As general trend, we find a pressure amplification in consecutive cavity collapses. However, by increasing the number of cavities, we are able to demonstrate that the amplification is not monotonic. A parameter study of the bubble separation distance in horizontal arrays shows that a smaller distance generally, but not necessarily, results in larger collapse pressure. Exceptions from the general trend are due to the very complex shock and expansion-wave interactions and demonstrate the importance of using state-of-the-art numerical methods. By varying boundary conditions, we illustrate the significance of large test sections in experimental investigations, as the expansion wave emitted at a free surface has a large effect on the collapse dynamics.

Experimental and Numerical Investigation of the DrivAer Model

Abstract: Automotive aerodynamic research often focuses on strongly simplified car models, such as the Ahmed body and the SAE model. Due to their high degree of abstraction, however, interference effects are often neglected which leads to an unrealistic representation of the flow field. Consequently, these results cannot be directly used for the aerodynamic optimization of production vehicles. On the other hand, aerodynamic investigations of real production vehicles are often limited due to the restricted availability of the geometric data. Therefore, a new realistic generic car model for aerodynamic research — the DrivAer body — is proposed. This paper focuses on the development of the model, summarizes first experimental results of the different configurations of the fastback geometry and compares them to numerical simulations performed using the open source software OpenFOAM®.Copyright © 2012 by ASME

Numerical Analysis of a Rotating Cylinder with Spanwise Disks

Abstract: The aerodynamic characteristics of a rotating cylinder in crossflow are investigated by means of unsteady Reynolds-averaged Navier–Stokes simulations. For a cylinder configuration with endplates, the numerical simulationsmatch the experimental trend for the force coefficients and the Strouhal number. Design parameters are studied including the ratio of circumferential cylinder velocity to freestreamvelocity ( ), the endplate diameter ratio, and the cylinder aspect ratio. The incoming flow separates on each endplate edge and rolls up into two tip vortices thatmerge downstream. They impact considerably on the configuration performance, particularly at high . The tip vortices influence the cylinder flow topology, especially for low-aspect-ratio cylinders with small endplates. Finally, a cylinder configuration with spanwise disks is investigated for < 3:4. The streamwise velocity component increases between the boundary layers of two facing disks, thereby decreasing the effective spinning ratio. At the corner, the cylinder boundary-layer thickness is reduced due to the radial flow component occurring on the disk. Furthermore, adding spanwise disks decreases the strength of the tip vortices. The combination of these three effects leads to a drag reduction at high compared with a cylinder configuration without spanwise disks.

Aerodynamic Investigations of a Morphing Membrane Wing

Abstract: This paper presents the aerodynamic analysis of a morphing wing configuration using a compliant membrane as lifting surface to enable large variations of the planform geometry (aspect ratio and sweep angle) and airfoil shape. Comprehensive tests of a specifically designed wind-tunnel model including force measurements, deformation measurements, surface flow visualizations, and wake flow measurements are used to assess the aerodynamic behavior and the performances of this morphing wing. The results of the force measurements performed on five different wing configurations ranging from a high-aspect-ratio, straight wing to a highly swept back, low-aspectratio configuration show that the active variation of the wing planform effectively alters the lift and drag characteristics in such a way that relatively high lift-to-drag ratios can be maintained over a broad range of flight conditions.Inaddition,duetoitsmaterialproperties,thewingsurfacepassivelydeflectsunderaerodynamicloading, whichaffectstheairfoilshapeaccordingly.Itresultsin anadditional dependencyoftheaerodynamiccharacteristics on the flow conditions, which can be used for passive flow control.

The Interior Design of a 40% Scaled DrivAer Body and First Experimental Results

Abstract: This report addresses the interior design of a 40% scaled wind tunnel model of a generic medium-sized car geometry — the so-called DrivAer body. The model was designed for being investigated inside the wind tunnel facility at the Technische Universitat Munchen, which was recently upgraded by a single-belt ground simulation system. The wind tunnel model is very modular: it features several exchangeable parts, such as three exchangeable rear ends, three different underbody configurations, and different wheel rim geometries. In addition to this, the engine compartment is equipped with a model heat exchanger to adjust the mass flow rate through the underhood area. Apart from the model itself, we would also like to introduce some of the measurement equipment that we used during our wind tunnel tests, for example a set of five independent force balances. Furthermore, a method to account for the falsifying rolling resistance of the wheels is shown. Finally, results of experiments to determine the aerodynamic drag generated at the front and the rear axes of the vehicle will be discussed and a small data base of drag values for various vehicle configurations will be provided.© 2012 ASME

The Interior Design of a 40% Scaled DrivAer Body and First Experimental Results

Abstract: This report addresses the interior design of a 40% scaled wind tunnel model of a generic medium-sized car geometry — the so-called DrivAer body. The model was designed for being investigated inside the wind tunnel facility at the Technische Universitat Munchen, which was recently upgraded by a single-belt ground simulation system. The wind tunnel model is very modular: it features several exchangeable parts, such as three exchangeable rear ends, three different underbody configurations, and different wheel rim geometries. In addition to this, the engine compartment is equipped with a model heat exchanger to adjust the mass flow rate through the underhood area. Apart from the model itself, we would also like to introduce some of the measurement equipment that we used during our wind tunnel tests, for example a set of five independent force balances. Furthermore, a method to account for the falsifying rolling resistance of the wheels is shown. Finally, results of experiments to determine the aerodynamic drag generated at the front and the rear axes of the vehicle will be discussed and a small data base of drag values for various vehicle configurations will be provided.© 2012 ASME

Flow Characteristics of a Helicopter Fuselage Configuration Including a Rotating Rotor Head

Abstract: The reduction of emissions in air transport is clearly a main goal of the aeronautical industry today, addressing both fixed wing aircraft and rotorcraft. The first phase of the ADHeRo (Aerodynamic Design Optimization of a Helicopter Fuselage including a Rotating Rotor Head) project contributed to achieving this goal by providing detailed flow characteristics and drag analysis of a state-of-the-art Twin Engine Light class utility helicopter. This was achieved by means of wind tunnel experiments and numerical simulations. It has been shown that optimizing the parasite drag of such a configuration is a vital approach for achieving efficiency gains. In particular, this can be obtained by reducing the drag of the rotor head, the landing skids and the fuselage. The analysis revealed that reducing the interference drag of the landing skids and the rotor head on the fuselage also provides significant potential for efficiency gains. On the other hand, the analysis showed the importance of considering changes in the lift characteristics when optimizing components. Otherwise, efficiency gains could be lost due to higher power requirements for the main rotor.

A SPH model for incompressible turbulence

Abstract: A coarse-grained particle model for incompressible Navier-Stokes (NS) equation is proposed based on spatial filtering by utilizing smoothed particle hydrodynamics (SPH) approximations. This model is similar to our previous developed SPH discretization of NS equation (Hu X.Y. & N.A. Adams, J. Comput. Physics, 227:264-278, 2007 and 228:2082-2091, 2009) and the Lagrangian averaged NS (LANS-α) turbulence model. Other than using smoothing approaches, this model obtains particle transport velocity by imposing constant σ which is associated with the particle density, and is called SPH-σ model. Numerical tests on two-dimensional decay and forced turbulences with high Reynolds number suggest that the model is able to reproduce both the inverse energy cascade and direct enstrophy cascade of the kinetic energy spectrum, the time scaling of enstrophy decay and the non-Guassian probability density function (PDF) of particle acceleration.

A Study of Expansion Tube Gas Flow Conditions for Scramjet Combustor Model Testing

Abstract: ow process inside an expansion tube facility. The work is a combined numerical/experimental eort. Axisymmetric, viscous, unsteady RANS simulations of the operation of an expansion tube have been performed. Additionally, measurements of observable quantities have been carried out to validate the simulations and to gain detailed physical insight into the operation of the facility. Here, a comparison of transient wall static pressure, shock Mach numbers and stagnation point pressures at the tube outlet have been selected for comparison. The transient test gas ow is then analyzed on grounds of the results of the numerical simulation.

Analysis of intermittency in under-resolved smoothed-particle-hydrodynamics direct numerical simulations of forced compressible turbulence.

Abstract: We perform three-dimensional under-resolved direct numerical simulations of forced compressible turbulence using the smoothed particle hydrodynamics (SPH) method and investigate the Lagrangian intermittency of the resulting hydrodynamic fields. The analysis presented here is motivated by the presence of typical stretched tails in the probability density function (PDF) of the particle accelerations previously observed in two-dimensional SPH simulations of uniform shear flow [Ellero et al., Phys. Rev. E 82, 046702 (2010)]. In order to produce a stationary isotropic compressible turbulent state, the real-space stochastic forcing method proposed by Kida and Orszag is applied, and the statistics of particle quantities are evaluated. We validate our scheme by checking the behavior of the energy spectrum in the supersonic case where the expected Burgers-like scaling is obtained. By discretizing the continuum equations along fluid particle trajectories, the SPH method allows us to extract Lagrangian statistics in a straightforward fashion without the need for extra tracer particles. In particular, Lagrangian PDF of the density, particle accelerations as well as their Lagrangian structure functions and local scaling exponents are analyzed. The results for low-order statistics of Lagrangian intermittency in compressible turbulence demonstrate the implicit subparticle-scale modeling of the SPH discretization scheme.

A SPH model for incompressible turbulence

Abstract: A coarse-grained particle model for incompressible Navier-Stokes (NS) equation is proposed based on spatial filtering by utilizing smoothed particle hydrodynamics (SPH) approximations This model is similar to our previous developed SPH discretization of NS equation ({\it Hu XY & NA Adams, J Comput Physics}, 227: 264-278, 2007 and 228: 2082-2091, 2009) and the Lagrangian averaged NS (LANS-$\alpha$) turbulence model Other than using smoothing approaches, this model obtains particle transport velocity by imposing constant $\sigma$ which is associated with the particle density, and is called SPH-$\sigma$ model Numerical tests on two-dimensional decay and forced turbulences with high Reynolds number suggest that the model is able to reproduce both the inverse energy cascade and direct enstrophy cascade of the kinetic energy spectrum, the time scaling of enstrophy decay and the non-Guassian probability density function (PDF) of particle acceleration

An innovative approach to thermo-fluid-structure interaction based on an immersed interface method and a monolithic thermo-structure interaction algorithm

Abstract: A coupled thermo-fluid-structure interaction approach, consisting of a Cartesian grid finite volume scheme with conservative interface method for the fluid and a finite element scheme for the thermo-structure interaction problem, is proposed. We present a loosely coupled approach for the solution of the thermo-fluid-structure interaction problem based on a Dirichlet-Neumann partitioning. The structural surface is represented by a level set function in the fluid code. The velocity and temperature field required for the coupling are interpolated from structural values on the zero-contour level set surface. Data transfer between the two codes is performed via message passing interface. The proposed method is tested for a cooling-process of a heated metal bar by means of an external laminar boundary layer flow. Results show that the presented approach is able to handle the complexity of the three-field problem.

Simulating three-dimensional turbulence with SPH

Abstract: In this paper, we have investigated the ability of smoothed particle hydrodynamics (SPH) to simulate turbulent flows. It is well known that the standard method without corrections cannot predict the energy cascade of a turbulent flow. In the absence of viscosity, standard SPH simulations produce purely noisy particle motion and at finite viscosities the method overpredicts dissipation. As a remedy, we have introduced a modified transport velocity to advect particles that homogenizes the particle distribution, thus stabilizing the numerical scheme. In addition, artificial dissipation is strongly reduced, and we successfully applied the new method to transitional flows. Here, we present twoand three-dimensional simulation results of the Taylor-Green vortex flow. We analyzed the energy spectra and dissipation rates and found good agreement with DNS data from the literature.

Contact line hydrodynamics with SPH

Abstract: Surface tension effects can dominate multi-phase flows when the length scales of the problem are small. The resulting Capillary forces at a phase interface between two immiscible fluids are proportional to the local curvature of the flow and try to minimize the interfacial area. A more complex situation occurs when three phases are in contact or when two phases are in contact with a wall. The simulation of the contact line at a wall is still a challenging task since the motion of the contact line is contradictory to the no-slip assumption at walls. In this work we present a multi-phase SPH method considering surface tension effects that is capable of simulating contact line problems. Based on previous works [4] we revisit our finite-width interface model and introduce a new stress boundary condition at the wall.

A thermo-fluid-structure interaction approach based on an immersed interface method and on a monolithic thermo-structure interaction approach

Abstract: A coupled thermo-fluid-structure interaction approach, consisting of a finite volume scheme for the fluid and a finite element scheme for the thermo-structure interaction (TSI) problem, is proposed. Both schemes enable a fast, efficient and robust solution of the respective numerical problems. The compressible Navier-Stokes equations are solved on a Cartesian grid and a conservative immersed interface method is used to describe the flow boundaries. The fluid is solved by adopting a 5th order weighted essentially non-oscillatory (WENO) scheme for the discretisation of the convective fluxes. A 2nd order central difference scheme is used for the diffusive fluxes, while a 3rd order Runge-Kutta scheme is adopted for the integration in time. The TSI problem is based on separate discretisations of the structural and thermal fields, both using finite element technology. For the monolithic TSI problem, an iterative solver (GMRES) and a block Gauss-Seidel preconditioner with algebraic multigrid methods is used. A one-step-µ time-integration scheme is used for temporal discretisation. We present a loosely-coupled approach for the solution of the thermo-fluid-structure interaction problem, based on Dirichlet- Neumann partitioning. Special attention is given to the transfer of forces, temperatures and to the structural positions. The structural surface is represented by a level set function in the fluid code. The velocity and temperature field required for the coupling are interpolated from structural values on the zero-contour level set surface. Data transfer between the two codes is performed via message passing interface. The proposed method is tested for a cooling-process of a heated metal bar by mean of an external laminar boundary layer flow. Results show that the presented approach is able to handle the complexity of the three-field problem.