Numerical Investigation Of Turbulence Enhancement And Flow Control With Roughness Elements

Flow control using roughness elements shows many advantages shared by passive control methods,can be used for separation control,turbulence enhancement,and drag reduction in engineering practices.Deploying roughness elements may increase the complexity of flow character and the difficulty in investigating their enhancing effects.On one hand,roughness elements induce periodical vortex shedding,and the direction of vortex shedding may change periodically.On the other hand,vortical structures induced by roughness elements scale with their sizes,and show no correlation with the characteristic scale of the background flow,resulting in multiscale flow features.To investigate the enhancing effects of roughness elements and cope with the complexity,regional spatial correlation is adopted to provide a time-averaged measure for vortical structures induced by roughness elements,so that the enhancing effects from roughness elements and their combinations can be measured quantitatively,and the mechanism of turbulence enhancement can be thereby disclosed.The major research works are as follow.First,turbulent inlet boundary condition is implemented based on the precursor method.Validation is performed using data from previous experiments and simulations.Comparative study of flow character and enhancing effects of various roughness elements,such as spherical dimples,tear-shaped dimples,and spherical protrusions,is carried out.Results show that regions with high turbulent kinetic energy(TKE)does not coincide with wall regions with high Nusselt numbers,showing that the distribution of TKE could not be used to explain the heat-transfer enhancement on wall surfaces,even though it could reflect the turbulence enhancement from roughness elements.Meanwhile,dimples with different shapes show only small differences in flow character and heat-transfer enhancement.Protrusions are more effective in enhancing the heat transfer than all dimples,as long as they are of an equal roughness.Second,the combination of roughness elements is investigated numerically using large-eddy simulation with a subgrid modeling based on the transport equation for SGS turbulent kinetic energy.The enhancing effects of the combination of roughness elements are analyzed using regional spatial correlation method.The comparison between single roughness and their combination is conducted.And the effects of Reynolds number and edge-rounding are also studied.Results show that the distribution of local correlation coefficients shows good correlation with local Nusselt numbers,showing that the heat-transfer enhancement on the wall is carried out directly by the vortical structures from roughness elements.Meanwhile,compared to single roughness elements,their combination shows a significant improvement in the enhancing effect.And the flow character inside the dimple is slightly disturbed by a different inlet boundary condition,but shows severe sensitivity to the effects of Reynolds number and edge-rounding.Third,large-eddy simulation is used to investigate the enhancing effects of protrusion-dimple arrays.Regional correlation analysis is adopted to study the enhancing effects of vortex shedding of different scales from the dimple.Characteristic frequencies are collected,spectral analysis is carried out to investigate the characteristic frequencies related to vortex shedding and the periodical deployment of roughness elements.Then,regional correlation analysis and spectral analysis are used together to understand the asymmetry in the enhancing effect of a shallow dimple at middle Reynolds-number stage.And the effects of Reynolds number and gap ratio are compared,the role difference in flow character and enhancing effects between the roughness elements are analyzed,and the active connection between vortical structures induced separately by the dimple and the protrusion is discussed.Results show that where local spatial correlation reaches their local minimum the spectral density of characteristic frequencies obtains a local maximum.This shows that regional spatial correlation provide a valid measure for roughness-related vortical structures.And the enhancing effects for vortex shedding with different scales can be revealed: vortical structures from roughness elements augment the heat-transfer on the nearby wall surfaces while they are convected,and the shape and size of the region with high Nusselt numbers are decided by the primary transport passages for these vortical structures.And results also show that the asymmetry in the heat-transfer enhancement of the shallow,spherical dimple at middle Reynolds-number stage is rooted in the asymmetry of the primary transport passages and characteristic frequencies there related to vortex shedding.Fourth,various numerical methods,such as DES turbulence models,immersed boundary method,and large-eddy simulation,are explored in simulating the external flow around automobiles.And drag reduction of DrivAer car model using roughness elements is studied.Results show that the drag is reduced by 12.9% after introducing the roughness elements.The effects of drag reduction from roughness elements includes two important factors: the convergence of recirculation region and pressure recovery at the car rear,and the separation delay induced by the small-level output of TKE from roughness elements which leads to a large increase in TKE in downstream and renders regions with high-level TKE located closer to wall surfaces.