Numerical Simulation Of Turbulent Heat Transfer And Turbulence Control

Direct numerical simulation (DNS) and large eddy simulation (LES) are used to study some typical turbulent flow, heat transfer and control problems, including oscillating turbulent channel flows with passive heat transfer, thermally stably stratified turbulent open channel flows, turbulent flows with heat transfer controlled by transverse traveling wave Lorentz force excitation and turbulence over spanwise oscillating wavy wall. We have found that various forced actions, such as the oscillating pressure gradient, the dynamic coupling effect of buoyancy and shear force, the transverse traveling wave excitation, and the wavy wall oscillation, there exist significant influences on the statistical quantities of velocity and temperature, the characteristics of turbulent flow and heat transfer. The results and conclusions are given as follows.In oscillating turbulent channel flows, different turbulent characteristics between the accelerating and decelerating phases occur. Turbulence in the accelerating phases is suppressed. High-speed and low-speed streaks develop and burst into a localized turbulent spot. The turbulent statistical quantities vary correspondingly. At different frequencies, the oscillating pressure gradient shows different effect on the flow. At low frequency, the difference of velocity and temperature quantities for different phases is obvious while small difference is identified with respect to the statistically steady case at high driving frequency. In the mean and phase-averaged temperature profiles, there exists a buffer layer followed by a logarithmic region. The von Karman constant for the temperature profiles is almost independent of the Prandtl number and the phase. The mean and phase-averaged turbulent heat transfer coefficients behave like Pr^-1/3 or Pr^-0.7. At high Prandtl number, the heat transfer is mainly controlled by small-scale turbulent structures very close to the wall.For stably stratified turbulent open channel flows, it is found that under stably stratification, the tendency of relaminarization appears within the turbulent boundary layer. At different Prandtl numbers, the effect of stratification is different. The stratification effect has small influence on the turbulent statistics in weakly stratified high-Prandtl number turbulent flow. The effect of decreased Prandtl number is to enhance the relaminarizing effect of stable stratification, owing to that the velocity and temperature fields are coupled and affected with each other. The von Karman constant for the temperature profiles varies with the Richardson number but independent of the Prandtl number. It is predicted that the diffusive sublayer thickness behaves like Pr^-1/3 or Pr^-0.3 relation near the wall and like Pr^-1/2 near the free surface. The Nusselt number at the free surface is well consistent with the law Pr^1/2. The effect of the Prandtl and Richardson number change the topology of turbulent structures. These alterations further affect the frequency of surface renewal motions and the process of heat and mass transfer at the free surface. In turbulent flows with heat transfer controlled by a transverse traveling wave Lorentz force excitation, the transverse traveling wave Lorentz force introduces spanwise motion and external streamwise vorticity. As the driving frequency decreases, the spanwise motion is intensified. At high frequency, the weakening of the streak intensity leads to a small amount of shear stress reduction. At an appropriate frequency, a wide ribbon of low-speed velocity is formed instead of wall streaks. The basic structure of streaks is stable and the biggest drag reduction of about 30% is achieved. At lower frequency, the streamwise velocity fluctuation and vorticity fluctuation increase obviously due to the spanwise motion, which ultimately results in drag increase. All these shifts make the structures of temperature field, temperature statistical quantities and heat transfer efficiency change correspondingly. The formation of cold and hot spot at the free surface is further influenced due to surface renewal events at low-Prandtl number.In turbulent flows over spanwise oscillating wavy wall, the spanwise oscillation of wavy wall can bring evident effect of drag reduction. The skin friction depends on the frequency of oscillation and the amplitude of the wavy wall. At an oscillating period T⁺ = 100, the friction decreases because of the introduction of spanwise vorticity and the simultaneous weakening of streamwise vortices and increases with the increase of the amplitude. At T⁺ = 200, as the amplitude of wavy wall increases, the spanwise motion of the wall extracts more energy from streamwise flow and distributes to spanwise direction, which brings the enhancement of the spanwise motion and the drag reduction. As the amplitude increases further, the enhancement of the spanwise motion leads to the increase of streamwise vortices and results in drag increase. The results also show that the flow structures near the wall are closely correlated with the shape of the wall.