Sinisa Krajnovic

Flow Around a Simplified Car, Part 1: Large Eddy Simulation

Large eddy simulations (LES) were made of flows around a generic ground vehicle with sharp edges at the rear end (an Ahmed body with a 25° angle of the rear slanted surface). Separation of the flow at the rear results in large regions with recirculating flow. As the separation is determined by the geometry, the Reynolds number effects are minimized. Resolution requirements of this recirculating flow are smaller than those in LES of wall attached flows. These two consequences of the geometry of the body are used to predict the experimental flow at relatively high Reynolds number. Recommendations are pre-sented for the preparation and realization of LES for vehicle flows. Comparison of the LES results with the experimental data shows good agreement. Copyright

Numerical Study of the Flow Around a Bus-Shaped Body

Flow around a simplified bus is analyzed using large-eddy simulation. At the Reynolds number of 0.21 × 10 6 , based on the model height and the incoming velocity. the flow produces features and aerodynamic forces relevant for the higher (interesting in engineering) Reynolds number. A detailed survey of both instantaneous and time-averaged flows is made and a comparison with previous knowledge on similar flows is presented. Besides the coherent structures observed in experimental and previous numerical studies, new smaller-scale structures were registered here. The mechanisms of formation of flow structures are explained and the difference between instantaneous and time-averaged flow features found in the experimental observations is confirmed. Aerodynamic forces are computed and their time history is used to reveal the characteristic frequencies of the flow motion around the body

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body

We investigate a hierarchy of eddy-viscosity terms in proper orthogonal decomposition (POD) Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to large eddy simulation (LES) data for a flow around a vehicle-like bluff body called an Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (J. Fluid Mech., vol. 260, 1994a, pp. 351–375; J. Fluid Mech. vol. 275, 1994b, pp. 257–283). Robustness of the model solution with respect to initial conditions, eddy-viscosity values and model order is achieved only for state-dependent eddy viscosities as proposed by Noack, Morzynski & Tadmor (Reduced-Order Modelling for Flow Control, CISM Courses and Lectures, vol. 528, 2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier–Stokes-based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.

Large-Eddy Simulation of the Flow Around Simplified Car Model

A hypothesis of using lower Reynolds number large eddy simulation (LES) to simulate the flow around ground vehicle at higher Reynolds number was tested. The simulation was made of the flow around a simplified car model and the results were compared with the experimental data. It has been found that the level of Reynolds number, if sufficiently high, has small influence on the results in highly separated flows around cars. This raises hope that the LES of the external car flow is feasible.

Cluster-based reduced-order modelling of a mixing layer

We propose a novel cluster-based reduced-order modelling (CROM) strategy of unsteady flows. CROM combines the cluster analysis pioneered in Gunzburgers group (Burkardt et al. 2006) and and transition matrix models introduced in fluid dynamics in Eckhardts group (Schneider et al. 2007). CROM constitutes a potential alternative to POD models and generalises the Ulam-Galerkin method classically used in dynamical systems to determine a finite-rank approximation of the Perron-Frobenius operator. The proposed strategy processes a time-resolved sequence of flow snapshots in two steps. First, the snapshot data are clustered into a small number of representative states, called centroids, in the state space. These centroids partition the state space in complementary non-overlapping regions (centroidal Voronoi cells). Departing from the standard algorithm, the probabilities of the clusters are determined, and the states are sorted by analysis of the transition matrix. Secondly, the transitions between the states are dynamically modelled using a Markov process. Physical mechanisms are then distilled by a refined analysis of the Markov process, e.g. using finite-time Lyapunov exponent and entropic methods. This CROM framework is applied to the Lorenz attractor (as illustrative example), to velocity fields of the spatially evolving incompressible mixing layer and the three-dimensional turbulent wake of a bluff body. For these examples, CROM is shown to identify non-trivial quasi-attractors and transition processes in an unsupervised manner. CROM has numerous potential applications for the systematic identification of physical mechanisms of complex dynamics, for comparison of flow evolution models, for the identification of precursors to desirable and undesirable events, and for flow control applications exploiting nonlinear actuation dynamics.

Transient Simulation of the Aerodynamic Response of a Double-Deck Bus in Gusty Winds

The purpose of the research reported in this paper was to investigate the aerod ynamic response of a double-deck bus in gusty winds using a Detached-Eddy Simula tion (DES). The bus was subjected to three different scenarios of wind gusts: g ust in a wind tunnel, gust in a natural wind and gust behind the exit of a tunne l. The proposed scenarios of gusts are in the time domain and take into account the dynamic behavior of natural winds. The Reynolds number of the flow, based on the time-averaged speed of the side wind and a reference length of squre root of 0.1 [m], was 1300000. Detailed transient responses of the aerodynamic coe fficients and flow structures were investigated. Good agreement was found betwe en the DES results and the available experimental data. A comparison between the influence of the different gust scenarios on the aerodynamic coefficients shows that the gust behind the exit from a tunnel has a stronger influence on the aer odynamics than the other gust scenarios. Moreover, the influence of the gusts o n the time history of aerodynamics coefficients is found to be limited to the pe riod of the gust.

LES Study of the Influence of a Train-Nose Shape on the Flow Structures Under Cross-Wind Conditions

Cross-wind flows around two simplified high-speed trains with different nose shapes are studied using large-eddy simulation (LES) with the standard Smagorinsky model. The Reynolds number is 300000 based on the height of the train and the free-stream velocity. The cross section and the length of the two train models are identical whilst one model has a nose length twice that of the other. The three-dimensional effects of the nose on the flow structures in the wake and on the aerodynamic quantities such as lift and side force coefficients, flow patterns, local pressure coefficient and wake frequencies are investigated. The short-nose train simulation shows highly unsteady and three-dimensional flow around the nose yielding more vortex structures in the wake. These structures result in a surface flow that differs from that in the long-nose train flow. They also influence the dominating frequencies that arise due to the shear-layer instabilities. Prediction of vortex shedding, flow patterns in the train surface and time-averaged pressure distribution obtained from the long-nose train simulation are in good agreement with the available experimental data.

Large eddy simulation of flows around ground vehicles and other bluff bodies.

A brief review of large eddy simulation (LES) applications for different bluff-body flows performed by the author and his co-workers is presented. Examples of flows range from simple cube flows characterized by sharp edge separation over a three-dimensional hill where LES relies on good near-wall resolution, to complex flows of a tall, finite cylinder that contains several flow regimes that cause different challenges to LES. The second part of the paper is devoted to flows around ground vehicles at moderate Reynolds numbers. Although the present review proves the applicability of LES for various bluff-body flows, an increase of the Reynolds number towards the operational speeds of ground vehicles requires accurate near-wall modelling for a successful LES.

Large-Eddy Simulation of the Flow Around a Simplified High Speed Train Under the Influence of a Cross-Wind

Large-eddy simulation (LES) is made to solve the flow around a simplified high speed train under the influence of a cross wind. The reynolds number of the flow is Re=300000 based on the height of the train and the incoming air velocity. The results are obtained at a yaw angle of 90 degree. Both the instantaneous and the time-averaged flows are explored. Compareson of the LES flow patterns and aerodynamic forces with experimental data is reported. It is found that the LES predicts the flow in agreement with the experimental observations. Aerodynamic forces are computed and ther time histories are used to find the characteristic frequencies of the flow motion around the train body. The results reveal that the dominated flow motion is very small and approches the resonance frquency of the real trains.

A mixed one-equation subgrid model for large-eddy simulation

A new mixed one-equation subgrid-scale (SGS) model for large-eddy simulation is presented. The scale-similarity part of the model is used for the description of the local energy transport (Domaradzki et al., 1993, 1994), i.e. the energy transport between scales very close to the cut-off. The eddy-viscosity part of the model represents the non-local transfer of energy, i.e. the transfer between all scales smaller than grid-filter size Delta and larger than Delta. A priori tests done by Bardina et al. (1980) have shown a high correlation between the scale-similarity model and the exact SGS stress, tau(ij). The magnitude of the scale-similarity part in the mixed one-equation SGS model is either larger than or equal to that of the eddy-viscosity part. The modeled SGS stress is thus expected to correlate well with the exact stress, tau(jj). In the proposed model, the SGS kinetic energy, k(sgs), is used to obtain the velocity scale for the eddy-viscosity part of the model. The modeled k(sgs) equation is derived and contains some additional scale-similarity parts as compared with the k(sgs) equation used in the models of Ghosal et al. (1995) or Davidson (1997). It has been shown that the model is Galilean invariant and realizable. Moreover, the approximately correct near-wall behavior of the model has been proven. The model was tested for both channel flow and the case of a surface-mounted cube (Martinuzzi and Tropea, 1993). It was found that the model gives accurate results in both cases.