2,415 research outputs found
Direct numerical simulation of turbulent flow past a trailing edge and the associated noise generation
No full textDirect numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge (TE). The objective is to investigate the turbulent flow field in the vicinity of the TE and the associated broadband noise generation. To generate a turbulent boundary layer a short distance from the inflow boundary, high amplitude lifted streaks and disturbances that can be associated with coherent outer layer vortices are introduced at the inflow boundary. A rapid increase in skin friction and a decrease in boundary layer thickness and pressure fluctuations is observed at the trailing edge. It is demonstrated that the behaviour of the hydrodynamic field in the vicinity of the TE can be predicted with reasonable accuracy using triple deck theory if the eddy viscosity is accounted for. Point spectra of surface pressure difference are shown to vary considerably towards the trailing edge, with a significant reduction of amplitude occurring in the low frequency range.The acoustic pressure obtained from the DNS is compared with predictions from two- and three-dimensional acoustic analogies and the classical trailing edge theory of Amiet. For low frequencies, two dimensional theory succeeds in predicting the acoustic pressure in the far field with reasonable accuracy due to a significant spanwise coherence of the surface pressure difference and predominantly two dimensional sound radiation. For higher frequencies, however, the full three dimensional theory is required for an accurate prediction of the acoustic far field. DNS data are used to test some of the key assumptions invoked by Amiet for the derivation of the classical trailing edge theory. Even though most of the approximations are shown to be reasonable, they collectively lead to a deviation from the DNS results, in particular for higher frequencies. Moreover, because the three dimensional acoustic analogy does not provide significantly improved results, it is suggested that some of the discrepancies can be attributed to the approach of evaluating the far field sound using a Kirchhoff-type integration of the surface pressure difference
Numerical investigation of transitional supersonic axisymmetric wakes
No full textTransitional supersonic axisymmetric wakes are investigated by
conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M=2.46 for several Reynolds numbers, and relating these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this research: a spatial Navier-Stokes (N-S) code to conduct Direct Numerical Simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to deliberately exclude physical effects is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain-sizes. With this approach, the impact of structures associated with certain modes on the global wake-behavior can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k>0 for ReD>5,000 and with respect to the axisymmetric mode k=0 for ReD>100,000. It is concluded that azimuthal modes k=2 and k=4 are the dominant modes in the trailing wake, producing a four-lobe wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested
An embedded flow simulation methodology for flow over fence simulations
No full textIn this paper, we report the development of embedding a flow simulation methodology (FSM) (Fasel HF, von Terzi DA, Sandberg RD (2006) A methodology for simulating compressible turbulent flows. J Appl Mech 3:405–412 [1]), (Weinmann M, Sandberg RD, Doolan C (2014) Tandem cylinder flow and noise predictions using a hybrid RANS/LES approach. Int. J. Heat Fluid Flow 50:263–278 [7]) region in a global Reynolds-averaged Navier-Stokes (RANS) region and its application to flow over a fence.</p
Development of Brown-Roshko structures in the mixing layer behind a splitter plate
No full textA direct numerical simulation has been performed of the near field of a mixing layer formed between a high-speed turbulent stream and a low-speed laminar stream. For improved realism, the simulation includes the splitter plate in the computational domain. The shear layer develops initially from the viscous sublayer of the upstream turbulent boundary layer. A trend towards two-dimensional or weakly oblique structures is observed immediately downstream of the splitter plate. The structures do not show very high coherence levels, suggesting that additional two-dimensional forcing may be present in experiments
An axis treatment for flow equations in cylindrical coordinates based on parity conditions
No full textA novel axis treatment using parity conditions is presented for flow equations in cylindrical coordinates that are represented in azimuthal Fourier modes. The correct parity states of scalars and the velocity vector are derived such that symmetry conditions for each Fourier mode of the respective variable can be determined. These symmetries are then used to construct finite-difference and filter stencils at and near the axis, and an interpolation scheme for the computation of terms premultiplied by 1/r. A grid convergence study demonstrates that the axis treatment retains the formal accuracy of the spatial discretization scheme employed. Two further test cases are considered for evaluation of the axis treatment, the propagation of an acoustic pulse and direct numerical simulation of a fully turbulent supersonic axisymmetric wake. The results demonstrate the applicability of the axis treatment for non-axisymmetric flow
Efficient parallel computing with a compact finite difference scheme
No full textThis paper proposes an efficient parallel computing approach based on a high-order accurate compact finite difference scheme in conjunction with a conventional domain decomposition method and MPI libraries. The proposed parallel computing approach consists of two major features: (a) a newly developed compact finite difference scheme with extended stencils containing halo points around subdomain boundaries, and (b) a predictor–corrector type implementation of a compact filter that effectively suppresses spurious errors from the subdomain boundaries. The current work employs three halo cells for the inter-node communication, based on which the coefficients of the new compact scheme at the subdomain boundaries are optimized to achieve as high level of resolution and accuracy as the interior compact scheme provides. Also, an optimal set of cut-off wavenumbers of the compact filter that minimizes spurious errors is suggested. It is shown that the level of errors from the proposed parallel calculations lies within the same order of magnitude of that from the single-domain serial calculations. The overall accuracy and linear stability of the new parallel compact differencing-filtering system are confirmed by grid convergence tests and eigenvalue analyses. The proposed approach shows a substantial improvement with respect to existing methods available
Assessing the sensitivity of turbine cascade flow to inflow disturbances using direct numerical simulation
No full textA primer on direct numerical simulation of turbulence - methods, procedures and guidelines
No full textDirect Numerical Simulation (DNS) is the branch of CFD devoted to high-fidelity solution ofturbulent flows. DNS differs from conventional CFD in that the turbulence is explicitly resolved,rather than modelled by a Reynolds-averaged Navier-Stokes (RANS) closure. It differs fromlarge-eddy simulation (LES) in that all scales, including the very smallest ones, are captured,removing the need for a subgrid-scale model. DNS can thus be viewed as a numericalexperiment producing a series of non-empirical solutions, from first principles, for a virtualturbulent flow (see Figure 1). Its great strength is the ability to provide complete knowledge,unaffected by approximations, at all points within the flow, at all times within the simulationperiod. DNS is therefore ideal for addressing basic research questions regarding turbulencephysics and modelling. This ability, however, comes at a high price, which prevents DNS frombeing used as a general-purpose design tool.The defining characteristics of DNS stem from the distinctive characteristics of turbulence.Because turbulence is inherently unsteady and three-dimensional, DNS requires timedependentcalculations within a three-dimensional domain.? These two features are sharedwith LES (and therefore LES/RANS hybrid strategies such as detached eddy simulation(DES)). The unique feature of DNS is associated with the manner in which turbulence isaffected by viscosity. This is responsible for the two chief drawbacks of DNS – its extremecomputational cost, and severe limitation on the maximum Reynolds number that can beconsidered
Boundary data immersion method for DNS of aero-vibro-acoustic systems
No full textThe enormous growth of computational power in recent years has enabled the consideration of multi-physics phenomena, complicated shaped and moving bodies in the framework of high-fidelity numerical simulations.</p
Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
No full textDirect numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles
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