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Jin Lee
Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Korea

Seo Yoon Jung
Mechanical Engineering, Imperial College London London, SW7 2AZ, U.K.; Advanced Reactor Development Institute Korea Atomic Energy Research Institute 898-111, Daedeok-daero, Daejeon 305-353, Korea

Hyung Jin Sung
Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea

Tamer A. Zaki
Dept. of Mechanical Engineering Imperial College London Exhibition Road, London SW7 2AZ, UK; Department of Mechanical Engineering, Johns Hopkins University, Baltimore, USA


Direct numerical simulations (DNS) of turbulent boundary layers over isothermally-heated walls were performed to investigate the effect of viscosity stratification on the turbulent and thermal boundary layer flows. An empirical relation of temperature-dependent viscosity for water was adopted. Based on the free-stream temperature (30°C), two wall temperatures (70°C and 99°C) were selected. In the heated flows, the turbulence energy diminishes in the buffer layer, but increases near the wall. The reduction in turbulence kinetic energy in the buffer layer is accompanied by smaller levels of Reynolds shear stresses and, hence, weaker turbulence production. The enhanced turbulence energy near the wall is attributed to enhanced transfer of energy via additional diffusion-like terms due to the viscosity stratification. Wall heating also results in increased scalar flux in the sublayer. Large wall-normal gradients of U and Θ lead to increased production in the scalar flux budget.