Turbulent Heat and Fluid Flow over a Two-Dimensional Hill

Abstract

Experimental investigation has been made on the flow and thermal fields over a heated two-dimensional hill with a cosine-squared shape. The detailed turbulent characteristics are measured by a backscatter-type two-component LDV, a PIV system, a fine thermocouple and a cold-wire probe. In the reverse-flow region on the leeward side of the hill, the turbulence intensities and the Reynolds shear stress show much larger values than in a canonical wall-bounded shear flow. The mean temperature maintains a relatively high value below the location where the horizontal mean velocity rapidly decreases. At the outer edge of the reverse-flow region, there exists a second maximum intensity of temperature fluctuations. The instantaneous temperature waveforms near the heated surface show very large amplitude consisting of high-frequency fluctuations superimposed on the low-frequency motions. Simultaneous measurement of velocity and temperature is also done using a combination of a two-component LDV and a fine-wire thermocouple together with digital response compensation. In the downstream region of the hill, the horizontal and vertical turbulent heat fluxes become maximum at the hill-top height, and tend to diffuse in the vertical direction. On the other hand, in the reverse-flow region formed behind the hill, both of the heat fluxes decrease remarkably. In particular, it is worth noting that the horizontal turbulent heat flux near the surface becomes opposite in sign to that in the forward flow region. This is mainly due to the reversal of the mean flow direction. Nomenclature C pwall static pressure coefficient,height of a two-dimensional hill L half width of a two-dimensional hill P probability density function P mean pressure P 0 reference inlet pressure R θ Reynolds number based on momentum thickness T, T instantaneous and mean temperatures T w , T ∞ wall and ambient temperatures t fluctuating temperature, t = T − T t fluctuating temperature normalized by rms value,t = t/t rms

Direct simulation of turbulent heat transfer in swept flow over a wire in a channel

“…In the primary recirculation region, there is less efficient distribution of heat flux coming from the wall with the rest of the flow in the channel and this results in a net rise in the temperature values in the bubble. This is similar to the temperature rise observed in the separated region in the flow over a backward-facing step [8,10] and in the lee-ward side of a two dimensional hill [37]. The size of the primary recirculation bubble grows with increasing crossflow and this leads to an increase in the size of the region having higher temperatures.…”

“…The shift of minimum occurs toward the lower wall due to an increased level of temperature fluctuations in the separated shear layer with increasing crossflow. Such a rise in the intensity of temperature fluctuation at the outer edge of the separated bubble has also been observed in flow over a two-dimensional hill [37]. After reattachment the near-wall regime recovers quickly towards equilibrium level whereas the outer part of the thermal boundary layer recovers slowly, thus even at location x c , / rms shows an asymmetric variation.…”

“…In this region, the heat transfer coefficient is suppressed below the values observed in the attached region while it is much larger near reattachment when compared with the values in the attached boundary layer zone. A channel with an undulating wall or a two-dimensional hill has also been used for heat transfer enhancement and for studying complex convective and diffusive processes associated with the thermal field [35][36][37]. In this case, the average Nusselt number increases due to increased intensity and elongation of the near-wall streamwise vortices, which causes larger turbulence production and thus favors convection transport.…”

1114 Structures of Turbulent Heat Transfer Having Thermal Stratification in Separated Flow Caused by 2-Dimensional Hill

“…Thus, effects of wall shape and thermal stratification are overlapped in this region. In the near wall region of rear of the 2DH, since the thermal boundary layer again develops, the double thermal boundary layers [1,13] are formed in all cases as shown in Fig. 4(c), in which the wall-normal turbulent heat fluxes are not In order to investigate the detailed distribution of turbulence energy at the separation and reattachment points, local distributions of turbulence energy are shown in Fig.…”