Non-equilibrium Boundary Layer Flow Research Articles

Formula neiterativnog modela zida za neravnotežne tokove graničnog sloja

A novel non-iterative (explicit) formulation of generalized wall functions that applies to equilibrium and non-equilibrium boundary layer flows was proposed. The proposed formulation uses a set of variables that are more useful for computational fluid dynamics codes as they allow for calculating wall shear stress without an iterative procedure. In addition, an explicit form of the formulation was provided that applies to wall models with and without pressure gradients. The new variable transformation casts the generalizedwall function into a new form that simplifies the implementation and evaluation of wall-shear stress in computational codes.

The Design and Validation of a Thermal Boundary Layer Wall Plate

A feedback controlled thermal wall plate designed to investigate thermal boundary layer flows is described and validated. The unique capabilities of the design are the ability to modify the thermal boundary conditions in a variety of ways or to hold the wall-temperature fixed even when the flow above the wall is unsteady and strongly three-dimensional. These capabilities allow for the generation and study of thermal transport in nonequilibrium boundary layer flows driven by different perturbations and of varying complexity. The thermal wall plate and the experimental facility in which the thermal wall plate is installed are first described. The wall-plate is then validated in a zero-pressure-gradient (ZPG) boundary layer flow for conditions of a uniform wall temperature and a temperature step. It is then shown that the wall temperature can be held constant even when a hemisphere body is placed on the wall that produces large localized variations in the convective heat transfer coefficient. Last, since the thermal wall plate is intended to support the study of thermal transport in a variety of nonequilibrium boundary layer flow, several possible experimental configurations are presented and described.

A non-equilibrium surface reservoir approach for hybrid DSMC/Navier–Stokes particle generation

An approach for the generation of particles at a hybrid Navier–Stokes/DSMC interface is presented for simple gases and gas mixtures with internal degrees of freedom. DSMC particles generated at a hybrid boundary are assigned thermal velocities using a non-equilibrium surface reservoir approach, in which the fluxes of mass, momentum and energy determined from the Navier–Stokes solution are used to prescribe the appropriate velocity distribution function used in the DSMC particle generation. The non-equilibrium surface reservoir approach is first outlined for a simple (single-species, monatomic) gas, and is then extended to gas mixtures with internal degrees of freedom, in which additional diffusion and internal heat flux terms are included in the Generalized Chapman–Enskog formulation of the perturbation. The significance of the diffusion, shear stress and heat flux breakdown parameters used to compute the perturbation are examined at a hybrid interface within non-equilibrium boundary layer flow, as well as within the breakdown region near a normal shock, in a five-species air gas mixture. The validity of the Chapman–Enskog perturbation at each of these hybrid interfaces is assessed by comparison with the Generalized Chapman–Enskog perturbations. Although a hybrid flowfield solution is not presented, this work provides a rigorous approach for non-equilibrium particle generation involving general hybrid particle/continuum studies of hypersonic flows.

Investigation of Turbulent Boundary Layer Flow Over 2D Bump Using Highly Resolved Large Eddy Simulation

A large eddy simulation (LES) study of turbulent non-equilibrium boundary layer flow over 2D Bump, at comparatively low Reynolds number Reh=U∞h/ν=1950, was conducted. A well-known LES issue of obtaining and sustaining turbulent flow inside the computational domain at such low Re, is addressed by conducting a precursor calculation of the spatially developing boundary layer flow. Those results were subsequently used as turbulent inflow database for the main non-equilibrium boundary layer flow computation. The Sagaut (Rech. Aero., pp. 51–63, 1996) sub grid scale (SGS) turbulence model, based on a local estimate of the subgrid scale turbulent kinetic energy ksgs and implicit damping of turbulent SGS viscosity νt(sgs) in the near-wall region, was selected as a suitable basis for the present LES computations due to the fact that block structured MPI parallelized CFD code used in the current computations did not provide a direct possibility for wall-damping of, e.g., the Smagorinsky constant in the near-wall region. The grid utilized in the main calculation consisted of approximately 9.4 × 106 grid points and the boundary layer flow results obtained, regarding both mean flow profiles and turbulence quantities, showed a good agreement with the available laser Doppler anemometry (LDA) measurements. Analysis of the flow was directly able to identify and confirm the existence of internal layers at positions related to the vicinity of the upstream and downstream discontinuities in the surface curvature and also partially confirm a close interdependency between generation and evolution of internal layers and the abrupt changes in the skin friction, previously reported in the literature.

Improved turbulence generation techniques for hybrid RANS/LES calculations

Hybrid turbulence simulations that use a combination of Reynolds-averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) are becoming increasingly popular to increase predictive accuracy in complex flow situations without the cost of full large-eddy simulations. In a typical application, the RANS equations are solved in the equilibrium or near-equilibrium regions of the flow, while LES equations are used in regions where the flow is away from equilibrium. A difficulty in performing hybrid simulations stems from the mismatch between resolved turbulent scales between the RANS and LES regions: in the RANS region all turbulent scales are modeled, while in the LES region the energy containing scales are resolved. Interfacing these two regions requires the generation of turbulent eddies capable of supporting the Reynolds stresses in the LES field. Previous work by the authors proposed a combination of synthetic turbulence and a controlled forcing method that was capable of quickly generating these eddies. In this paper, we extend this earlier work by investigating the physical meaning of the model parameters to understand their effect on the flow development. We find that by matching the time-averaging window to the local integral time scale of the flow improved results can be obtained compared with previous work. We also determined the range of the controller parameters that give robust results. The method was applied to non-equilibrium boundary-layer flows far from the calibration case and was found to perform well. This technique gives best results if an outer-flow perturbation drives the flow; when the inner-layer dynamics are critical, less rapid establishment of the turbulence is achieved. Even in the worst result, however, the controller was able to establish a realistic flow within five boundary layer thicknesses of the end of the controlled region.