ANALYSIS OF TURBULENT FLOW STRUCTURES ON DIFFERENTIALLY HEATED CHANNEL WALLS BASED ON DIRECT NUMERICAL SIMULATIONS

Kurzfassung

Direct numerical simulations of asymmetrically heated turbulent vertical channel flow at Reτ = 300, Gr = 9.6 · 105 and Pr 0.71 are performed by solving the Navier-Stokes equations together with the Boussinesq approximation for the bouyancy as well as the thermal energy equation for the temperature field with a finite volume method. The latter is based on a fourth order accurate interpolation scheme [1] in space and a second order accurate Euler-Leapfrog scheme in time. The differentially heated walls pose as sources of aiding and opposing buoyant forces altering the amplification and suppression of turbulent motions. In comparison with isothermal channel flow as published by Kim, Moin & Moser in 1987 [2], this offers the possibility for analysing amplification and surpression of turbulent fluctuations close to the heated and cooled walls, respectively. In 1997, Kasagi & Nishimura (KN) [3] published some statistical results they obtained with DNS of vertical channel flow including stress balances and budgets of the turbulent kinetic energy. They found that the isothermal heating and cooling lead to an asymmetric flow structure. Towards the heated channel wall the buoyancy aids and augments the mean flow rate while it opposes it near the cooled wall. The locally higher Reynolds number in the aiding flow should lead to increased turbulence intensities. Instead, the turbulence is suppressed there and augmented in the opposing flow side. They also presented coarsely resolved depictions of streaky structures in the streamwise velocity component near the walls but did not elucidate their sizes or specific properties in detail.At the ETC16, we will present a detailed analysis of the structure sizes close to the walls and their respective differences in comparison with plane isothermal channel flow. Figure 1 shows color contours of the fluctuating streamwise velocity component, which is organized in streaky structures. The upper panel depicts the streaks in the aiding flow showing their smooth appearance as opposed to the rougher streaks in the opposing flow on the lower panel. Figure 2 compares the energy spectra of the three velocity components and the temperature in the aiding and opposing flow. It is clearly visible that the small flow scales carry more energy in the opposing flow than in the aiding flow. Also shown is reference data from an isothermal channel flow at Reτ = 360 (cf. [2]), which exhibit energy levels in the small scales similar to the opposing flow for the mixed convection flow. The more dispersed flow structures in the opposing flow are connected with the higher energy content of the small scales, apparently leading to a more turbulent flow region. At the conference we will also show two-dimensional two-point-correlations as well as premultiplied spectra. With the help of these, we will present a quantitative analysis of the flow structure sizes over the channel height. Also, their impact on the near-wall statistics will be evaluated.