Actuation of the flow field around a frontstep with a rounded leading edge

Large Eddy Simulations (LES) are conducted to study the actuated flow field around a bluff body. The model is a simplification of a section of a truck cabin. The aim is to model the separation of the flow acting at the front part, the so called A-pillar. LES data show the connection between orientation and frequency of the actuation in comparison with drag reduction and separation mechanism. The flow is post processed using modal and frequency decompositions. Relevant results in terms of drag coefficient reduction were observed for the actuated flow. An optimal actuation in terms of induced frequency and drag reduction is also found

A Flow Control Study of a Simplified, Oscillating Truck Cabin Using PANS

This work presents an application of the partially averaged Navier–Stokes (PANS) equations for an external vehicle flow. In particular, the flow around a generic truck cabin is simulated. The PANS method is first validated against experiments and resolved large eddy simulation (LES) on two static cases. As a consequence, PANS is used to study the effect of an active flow control (AFC) on a dynamic oscillating configuration. The oscillation of the model represents a more realistic ground vehicle flow, where gusts (of different natures) define the unsteadiness of the incoming flow. In the numerical study, the model is forced to oscillate with a yaw angle 10 deg > β > –10 deg and a nondimensional frequency St = fW/Uinf = 0.1. The effect of the periodic motion of the model is compared with the quasi-static flow condition. At a later stage, the dynamic configuration is actuated by means of a synthetic jet boundary condition. Overall, the effect of the actuation is beneficial. The actuation of the AFC decreases drag, stabilizes the flow, and reduces the size of the side recirculation bubble

PARTIALLY-AVERAGED NAVIER- STOKES SIMULATIONS IN ENGINEERING FLOWS

This paper presents the most recent applications of the Partially-Averaged Navier-Stokes equations for engineering flows together with the review of the previous work in the field. Partially-Averaged Navier\ua0Stokes (PANS) simulation has been successfully used for several different applications of flows around\ua0ground vehicles. Examples of flows studied using PANS are that of the flow around square-back Ahmed body, flow around simplified passenger vehicle influenced by crosswinds, flow around simplified intercity trains, to the influence of passive and active flow control on the reduction of the aerodynamic drag on simplified vehicles. The idea of the application of hybrid methods such as PANS is to decrease the resolution requirements that are needed in turbulence resolving simulations such as LES. The resolution requirements of LES are normally very high in the near-wall regions, and this is where the PANS method is expected to activate more turbulence modelling, and thereby decrease the computational effort. The PANS method used by the authors is based on the variable switching coefficient that regulates the amount of the turbulence\ua0 modelling in the simulation. Previous studies have shown that such implementation of PANS is in line with the requirements that PANS should adapt to the computational grid. The most recent predictions range from simplified ground vehicle flow, flow around a freight train locomotive to the investigation of active flow control for trucks and ships. The new predictions show good agreement with the experimental observations

Development of Active Flow Control for Trucks

The possibility to actively control the external aerodynamic of vehicles is an attractive yet challenging solution to decrease the aerodynamic drag and the fuel consumption. The work flow that describes the implementation of an Active Flow Control (AFC), for the suppression of the separated flow at the A-pillar of a truck, is summarised in this paper. The presented work spans from a theoretical verification of the method to a preliminary implementation of an AFC on a real full-scale truck cabin. The study involves numerical (CFD) and experimental work, including aerodynamic test in a full scale wind tunnel. The initial CFD simulations of a simplified A-pillar were performed using turbulence resolving numerical method large-eddy simulations (LES). A second step consisted in simulation of a simplified truck cabin using hybrid Partially-Averaged Navier-Stokes simulations (PANS). The AFC was created using synthetic jets produced by the use of loudspeakers mounted in the A-pillars of the model. The numerical and experimental investigations were used to optimise the actuation parameters leading to maximum drag reduction. The final step of the validation of the AFC concept was achieved with a full scale test experimental campaign of a Volvo Truck cabin equipped with the studied AFC device

A novel SIMPLE-based pressure-enthalpy coupling scheme for engine flow problems

A novel method in CFD derived from the SIMPLE algorithm is presented. Instead of solving the linear equations for each variable and the pressurecorrection equation separately in a so-called segregated manner, it relies on the solution of a linear system that comprises the discretisation of enthalpy and pressurecorrection equation which are linked through physical coupling terms. These coupling terms reflect a more accurate approximation of the density update with respect to thermodynamics (compared to standard SIMPLE method). We show that the novel method is a reasonable extension of existing CFD techniques for variable density flows based on SIMPLE. The novel method leads to a reduction of the number of iterations of SIMPLE which translates in many – but not in all – cases to a reduction in computing time. We will therefore demonstrate practical advantages and restrictions in terms of computational efficiency for industrial CFD applications in the field of piston engine simulations

Investigation of the near-wake flow topology of a simplified heavy vehicle using PANS simulations

The near-wake flow topology of a ground transportation system (GTS) is investigated using partially-averaged Navier–Stokes (PANS) simulations at\ua0Re=2.7 7104. Recent numerical investigations for the GTS model using large eddy simulations (LES) showed an anti-symmetric flow topology (flow state II) in the vertical midplane compared to that observed in previous experimental studies (flow state I). The geometrical configuration of the GTS permits bi-stable behaviour, and the realisation of each of the two flow states, which are characterised by an asymmetrical flow topology, is achieved by varying the differencing scheme for the convective flux in the PANS simulations; AVL SMART schemes predict flow state I, while central differencing scheme (CDS) predicts flow state II. When the GTS model was placed away from the ground plane, the AVL SMART scheme fails to predict the flow asymmetry resulting in a pair of symmetrical vortices in the vertical midplane, while flow state II topology is observed when CDS is used. The switch from flow state I (II) to flow state II (I) is achieved by changing the numerical scheme from AVL SMART (CDS) to CDS (AVL SMART), with an intermediate transient-symmetric (TS) state being observed during the switching process. The numerical scheme in the PANS simulations thus plays a critical role in determining the initial flow topology in the near wake of the GTS

An LES investigation of the near-wake flow topology of a simplified heavy vehicle

Recent experimental investigations of McArthur et al (2016) in the wake of a simplified heavy vehicle or commonly known as the ground transportationsystem (GTS) model has shown that the flow topology is invariant over a large range of Reynolds numbers [2.7 7 104 − 2 7 106]. Numerical simulations are performed to investigate the initial flow topology at a Reynolds number of 2.7 7 104, using well-resolved large eddy simulations (LES). In the vertical midplane behind the GTS, a flow state which is anti-symmetric to that reported in McArthur et al (2016) is observed here, thereby, confirming the possibility of occurrence of the complementary bi-stable flow state. The occurrence of this bi-stable state does not depend on the ground clearance between the GTS and the ground plane, as a similar flow topology is observed at both small and large gap heights. Furthermore, the flow topology in the vertical midplane is also found to be insensitive to the incoming flow for small yaw angles. However, complex flow behaviour is observed in the wake for larger yaw angles, where the flow topology in the vertical midplane becomes nearly symmetric, while an asymmetric flow topology is now observed in the lateral midplane in the near wake. Furthermore, the corner vortices which originate from either side at the front of the model merge in the far wake, leading to a large vortex structure nearly equal to the height of the model. The near-wake topology of the GTS is analysed and compared with previous studies for a range of scenarios, and the forces on the GTS are computed

Qualitative assessment of the bi-stable states in the wake of a finite-width double backward facing step

The flow past a simplified frigate shape model which is analogous to a finite-width double backward\ua0facing step\ua0is investigated numerically using well-resolved\ua0large eddy simulations\ua0at\ua0Re≃8 7104. The\ua0geometric configuration\ua0of this model permits the occurrence of bi-stable\ua0flow states, with an asymmetrical flow topology being observed in the lateral midplane behind each step. In each flow state, the flow at the top step is anti-symmetrical to that observed on the bottom step, and the two flow states are anti-symmetrical to each other. Furthermore, incorporating a base\ua0cavity\ua0on the top step leads to the suppression of the asymmetrical flow topology on both the steps. The\ua0recirculation\ua0bubble formed by the\ua0downwash\ua0at each step is elongated when the base cavity is used, as compared to that observed in either of the two flow states, resulting in a decrease of the\ua0drag coefficient. In each of the two flow states, the unequal strength of the\ua0streamwise\ua0vortices close to the\ua0lateral edgeson the top step has been identified as the likely cause of the asymmetrical flow on the bottom step

BI-STABLE STATES IN THE WAKE OF A SIMPLIFIED GROUND TRANSPORTATION SYSTEM (GTS) MODEL

Recent experimental investigations of [1] in thewake of a Ground Transportation System (GTS)model has shown that the near wake topology isinvariant over a large range of Reynolds numbers.A large eddy simulation (LES) is thus undertakento investigate the flow topology in the near wake.In the vertical midplane behind the model, a flowstate which is anti-symmetric to that reportedin [1] is observed here, confirming the possibilityof occurrence of bi-stable flow. Interestingly, theflow topology remains invariant to any increase inthe gap height from the standard case, indicatingthat the underbody flow does not influence the bistablebehaviour. Furthermore, the flow was alsofound to be insensitive to the incoming flow at asmall yaw angle