C. De Langhe

One-equation RG hybrid RANS/LES computation of a turbulent impinging jet

A one-equation variant of a previously developed two-equation, renormalization group based, hybrid RANS/LES model is presented. The model consists of a transport equation for the mean dissipation rate and an algebraically prescribed length scale. The length scale is proportional to the wall-distance in RANS regions of the flow and to the filter width in LES regions. A very simple, but efficient, near-wall model is also presented, in the manner of Yakhot and Orszags RNG modeling. The only free parameter entering the low-Reynolds formulation has been calibrated against channel flow, and the value corresponds well with experimental values for the same parameter obtained for homogeneous isotropic turbulence. The model is then validated for a turbulent impinging jet heat transfer problem. Results are satisfactory and compare very favorably against DES and dynamic LES for the same computation.

Application of a RG hybrid RANS/LES model to swirling confined turbulent jets

A renormalization group (RG) based hybrid RANS/LES model is validated for turbulent swirling confined jets. The results are compared with the experimental data of Dellenback et al. (1988, Measurements in turbulent swirling flow through an abrupt axisymmetric expansion. AIAA Journal, 26(6), 669–681) and results for the same flows of an unsteady second-moment closure RANS simulation. A general quality/cost comparison is made between the hybrid RANS/LES and the second-moment closure simulations. In the final section, the hybrid RANS/LES result is further compared to a detached-eddy simulation, dynamic -equation LES and dynamic Smagorinsky LES for one of the flows, and the overall good quality of the RG hybrid RANS/LES model demonstrated.

Hybrid RANS/LES modelling with an approximate renormalization group. I: Model development

A hybrid RANS/LES model based on renormalization group (RG) calculations is presented. The result of the RG approach is a one-equation LES subgrid model in well-resolved regions of the flow, and a two-equation RANS model otherwise. The two modes of the model are linked by comparing the filter width with a length scale constructed from subgrid quantities. The one-equation subgrid model uses a transport equation for the mean rate of dissipation, in which the inverse time scale is explicitly filter width dependent. The model is free of adjustable parameters (after the truncations of the approximate RG are made) in its high-Reynolds formulation, has a clear physical interpretation and is easy to implement.

Hybrid RANS/LES modelling with an approximate renormalization group. II: Applications

The hybrid RANS/LES model developed previously is extended with near-wall modifications. The model is validated, in its low-Reynolds form, for channel flow and for flow over a periodic hill. The results for the channel flow are (indirectly) compared with DES results for the same flow, and are qualitatively similar. The separated flow over the periodic hill shows good agreement with the reference LES data, although performed with about 20 times less grid points. Finally the model is applied in high-Reynolds form with wall functions to a sudden pipe expansion; good agreement with experimental data is also obtained.

A Quasi-Realizable Cubic Low-Reynolds Eddy-Viscosity Turbulence Model with a New Dissipation Rate Equation

A non-linear relationship of the Reynolds stresses in function of the strain rate and vorticity tensors, with terms up to third order, is developed. Anisotropies in the normal stresses, influence from streamline curvature or rotation of the reference frame, and swirl effects are accounted for. The relationship is linked to ak–ε model with a modified transport equation for the dissipation rate. A new low-Reynolds source term is introduced and a model parameter is written in terms of dimensionless rate-of-strain and vorticity. The model is checked on different realizability constraints. It is shown that practically all constraints are fulfilled. The model is numerically tested on a fully developed channel and pipe flow, both stationary and rotating. The plane jet–round jet anomaly is addressed. Finally, the model is applied to the flow over a backward-facing step. Results are compared with a linear low-Reynolds k–ε model and the shear stress transport model.

One-Equation RG Hybrid RANS/LES Modelling

A one-equation variant of a previously developed two-equation, renormalization group (RG) based, hybrid RANS/LES model is presented. The model has a transport equation for the mean dissipation rate and an algebraically prescribed length scale. The length scale is proportional to the wall distance in RANS regions of the flow and to the filter width in LES regions. A near-wall formulation is used in the manner of Yakhot and Orszag’s RNG modelling. The only free parameter entering the low-Reynolds formulation has been calibrated against channel flow, and the value corresponds well with experimental values for the same parameter obtained for decaying homogeneous isotropic turbulence. Applications to an impinging jet and a plane asymmetric diffuser are presented.

Electronic gate detection for cell or particle counting and sizing in liquids: front-end characteristics, flow-dependent gate impedance, and its remediation

The detection sensitivity and the sizing resolution of electronic gating are inherently limited by fluctuating gate impedance and flow-induced noise. Instabilities of this type, as shown, are due to varying flow patterns of the carrier liquid beyond the gate. Their effects, although largely hidden in dc-operated gating, cause broadening and shift of cell/particle-size distributions under measurement. RF-operated gating, more specifically the demodulation operation, is much more hindered. For an investigation of these effects, a physical model is proposed along with a procedure for the identification of the system parameters. A detector of dedicated concept is used for evaluating the model, and, more specifically, for investigating the impact of configurational and hydrodynamic parameters. Experiments prove that the origin of flow-dependent gate impedance is to be located inside a zone of only a few-micrometer extent at the gate outlet. This is confirmed by the calculated electric field patterns. On such grounds, electrode configurations are proposed that minimize the current density in the zone of hydrodynamic instability and, hence, the flow-induced noise. The same configurations also minimize the impedance of the gate as signal source, facilitating broadband operation, and multifrequency cell impedance measurements

VERY LARGE EDDY SIMULATION AND RNG TURBULENCE MODELS

The possibility of using turbulence models, derived by the renormalization-group technique (RNG), as subgrid models for Very Large Eddy Simulations (VLES) is investigated. The K — emodel as derived by Yakhot and Orszag1 and Yakhot and Smith2 is put in a form dependent on the cut-off wavenumber of the VLES-filter.

A dynamically optimized finite difference scheme for Large-Eddy Simulation

A low-dispersive dynamic finite difference scheme for Large-Eddy Simulation is developed. The dynamic scheme is constructed by combining Taylor series expansions on two different grid resolutions. The scheme is optimized dynamically through the real-time adaption of a dynamic coefficient according to the spectral content of the flow, such that the global dispersion error is minimal. In the case of DNS-resolution, the dynamic scheme reduces to the standard Taylor-based finite difference scheme with formal asymptotic order of accuracy. When going to LES-resolution, the dynamic scheme seamlessly adapts to a dispersion-relation preserving scheme. The scheme is tested for Large-Eddy Simulation of Burgers equation. Very good results are obtained.

Applications of a renormalization group based hybrid RANS/LES model

ABSTRACT A hybrid RANS-LES model has been constructed using a Renormalization Group approach. The resulting model has explicit filter width dependence in the effective viscosity and in the time scale of the transport equation of the mean dissipation rate. A two-equation RANS limit of the model exists for filter widths that are large compared to the integral length scale. In this paper, after a general overview of the model, its performance is illustrated in low-Reynolds mode for flow over periodic hills and in high-Reynolds mode for flow in a sudden pipe expansion.