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Turbulent boundary layers subject to severe acceleration or strong favorable pressure gradient (FPG) are of fundamental and technological importance. Scientifically, they elicit great interest from the points of view of scaling laws, the complex interaction between the outer and inner regions, and the quasi-laminarization phenomenon. Many flows of industrial and technological applications are subject to strong acceleration such as convergent ducts, turbines blades and nozzles. Our recent numerical predictions (J. Fluid Mech., vol. 775, pp. 189-200, 2015) of turbulent boundary layers subject to very strong FPG with high spatial/temporal resolution, i.e. Direct Numerical Simulation (DNS), have shown a meaningful weakening of the Reynolds shear stresses with an evident logarithmic behavior. In the present study, assessment of three different turbulence models (Shear Stress Transport, k-w and Spalart-Allmaras, henceforth SST, k-w and SA, respectively) in Reynolds-averaged Navier-Stokes (RANS) simulations is performed. The main objective is to evaluate the ability of popular turbulence models in capturing the characteristic features present during the quasi-laminarization phenomenon in highly accelerating turbulent boundary layers. Favorable pressure gradient is prescribed by a top converging surface (sink flow) with an approximately constant acceleration parameter of K = 4.0 x 10^(-6). Furthermore, the quasi-laminarization effect on the temperature field is also examined by solving the energy equation and assuming the temperature as a passive scalar. Validation of RANS results is carried out by means of a large DNS dataset.
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Assessment of Turbulence Models over a Curved Hill Flow with Passive Scalar Transport
An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses ( > 0) inside the recirculating zone. Very strong APG induced outer secondary peaks in and turbulence production as well as an evident negative slope on the constant shear layer.
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- PAR ID:
- 10407177
- Date Published:
- Journal Name:
- Energies
- Volume:
- 15
- Issue:
- 16
- ISSN:
- 1996-1073
- Page Range / eLocation ID:
- 6013
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/en15166013
