Notes on numerical fluid mechanics

应用DES(Detached-Eddy Simulation)方法数值模拟了3种不同失速类型的翼型的升力特性.DES方法结合了RANS(Reynolds-averaged Navier-Stokes)和LES(Large Eddy Simulation approaches)的优点.基于Spalart-Allmaras湍流模型,在近壁面DES体现为RANS模型的特点而在远离物面处又具有LES的亚格子模型的特性.对此模型使用了LU-SGS隐式格式求解.通过和实验结果对比,显示这种方法可以有效地预测翼型的失速特性.

Detached—Eddy Simulation方法模拟不同类型翼型的失速特性

We present a comprehensive framework for augmenting turbulence models with physics-informed machine learning, illustrating a complete workflow from identification of input/output to prediction of mean velocities. The learned model has Galilean invariance and coordinate rotational invariance.

Physics-informed machine learning approach for augmenting turbulence models: A comprehensive framework

Towards In-Flight Applications? A Review on Dielectric Barrier Discharge-Based Boundary-Layer Control

Reynolds-averaged Navier–Stokes (RANS) simulations with turbulence closure models continue to play important roles in industrial flow simulations. However, the commonly used linear eddy-viscosity models are intrinsically unable to handle flows with non-equilibrium turbulence (e.g. flows with massive separation). Reynolds stress models, on the other hand, are plagued by their lack of robustness. Recent studies in plane channel flows found that even substituting Reynolds stresses with errors below 0.5 % from direct numerical simulation databases into RANS equations leads to velocities with large errors (up to 35 %). While such an observation may have only marginal relevance to traditional Reynolds stress models, it is disturbing for the recently emerging data-driven models that treat the Reynolds stress as an explicit source term in the RANS equations, as it suggests that the RANS equations with such models can be ill-conditioned. So far, a rigorous analysis of the condition of such models is still lacking. As such, in this work we propose a metric based on local condition number function for a priori evaluation of the conditioning of the RANS equations. We further show that the ill-conditioning cannot be explained by the global matrix condition number of the discretized RANS equations. Comprehensive numerical tests are performed on turbulent channel flows at various Reynolds numbers and additionally on two complex flows, i.e. flow over periodic hills, and flow in a square duct. Results suggest that the proposed metric can adequately explain observations in previous studies, i.e. deteriorated model conditioning with increasing Reynolds number and better conditioning of the implicit treatment of the Reynolds stress compared to the explicit treatment. This metric can play critical roles in the future development of data-driven turbulence models by enforcing the conditioning as a requirement on these models.

Reynolds-averaged Navier-Stokes equations with explicit data-driven Reynolds stress closure can be ill-conditioned

Experimental and computational study of the flow induced by a plasma actuator

Extending the bounds of 'steady' RANS closures: Toward an instability-sensitive Reynolds stress model

The present work is concerned with development of a new model for the momentum transfer due to plasmaactuator in aerodynamic flow-control applications. A comparative analysis of different volume-force estimation strategies is provided, focussing particularly on the phenomenological and experiment-based approaches. The main objective of the present study is a comprehensive evaluation of these models aiming at derivation of a new model formulation of improved accuracy. All models are implemented into the OpenFOAM code. The models are validated within the RANS framework applied under the quiescent-air conditions by solving the Reynolds equations, closed by a near-wall second-moment closure model based on the homogeneous dissipation rate of the kinetic energy of turbulence. The computationally obtained velocity distribution induced by the wall-mounted plasma actuator is analyzed within the wall-jet flow region along with the experimental PIV data of identical actuator operating conditions. The results demonstrate the improved accuracy of the new empirical model as a first step towards a universal plasma-actuator model for flow-control simulations by means of CFD. 1 BACKGROUND AND MOTIVATION Plasma actuator for the aerodynamic flow control describes a DBD-based actuator (DBD: dielectric barrier discharge) generating an electric discharge between two parallel electrodes separated by an insulating dielectric material (barrier): a grounded electrode and the radio-frequency high-voltage one the upper surface of the latter being coincident with the wall surface. Consequently an electric field originating from the exposed high-voltage electrode will ionize weakly the surrounding air (surface plasma generation) and inHV (a) quiescent air HV

Derivation of a Plasma-ActuatorModel Utilizing Quiescent-Air PIV Data

It is well-known that the separation process is inherently a highly unsteady phenomenon. To capture it correctly LES-relevant models – conventional LES and hybrid LES/RANS models (DES schemes, PITM, PANS) have to be applied. Because of their high spatial and temporal requirements the application of these methods is not straightforwardly affordable for the flow configurations of industrial relevance. On the other hand, apart of the backward-facing step flow geometry characterized by the sharpe-edge separation of a flat plate boundary layer which can be reasonably well solved by an advanced steady RANS model, the flows involving separation are in general beyond the reach of the conventional RANS method independent of the modeling level. Typical outcome is a low level of turbulence activity in the separated shear layer and a correspondingly long recirculation zone. The latter issues motivated the present work demonstrating the possibility to appropriately improve the computational results pertinent to the flow configurations featured by wall-bounded separation in the “Steady RANS” framework. An appropriately designed term modeled in terms of the von Karman length scale (adopted from the SAS modeling strategy for ”unsteady” flow computations, Menter and Egorov, 2010) was introduced into the scale-supplying equation governing the homogeneous part of the inverse time scale ( = ⁄ ). This term (denoted by ) being active only in the narrow area of the separation region acts towards an appropriate enhancement of the (fully-modeled) turbulence in the separated shear layer resulting in a correct mean velocity development and proper size of the recirculation zone. Predictive performances of the proposed model equation solved in conjunction with the Jakirlic and Hanjalic’s Reynolds stress model equation (2002) were illustrated by computing several configurations featured by boundary layer separation including the flow over a periodical arrangement of smoothly contoured 2D hills in a range of Reynolds numbers, flow over a wall-mounted fence and in a 3D diffuser.

R. Maduta

Papers

Experimental and computational study of the flow induced by a plasma actuator

Extending the bounds of 'steady' RANS closures: Toward an instability-sensitive Reynolds stress model

Derivation of a Plasma-ActuatorModel Utilizing Quiescent-Air PIV Data

On “Steady” RANS Modeling for improved Prediction of Wall-bounded Separation

“Steady” RANS Modeling for Improved Prediction of Wall-Bounded Separation