• A comparative study of separated boundary layer transition on a flat plate with a blunt/semi-circular leading edge

      Yang, Zhiyin; University of Derby (World Scientific and Engineering Academy and Society (WSEAS), 2011-07)
      Boundary layer may separate due to an adverse pressure gradient or due to flow geometry and when a laminar boundary layer separates the free shear layer formed is very unstable even at low Reynolds number, undergoing a transition process to turbulence. This paper presents a comparative numerical study of the transition process in a separated boundary layer induced by changes of curvature of the surface. The geometry is a flat plate with two different leading edges: a blunt and a semi-circular. One of the main purpose of the study is to identify how similar or how different the transition process is with two different leading edges. It is evident that for both cases (blunt and semi-circular leading edges) the primary two-dimensional instability originates from the free shear layer of the separation bubble via the Kelvin-Helmholtz mechanism. Three-dimensional motions develop under any small spanwise disturbances and similar coherent structures have been observed in both cases, strongly indicating that the whole transition process is very similar.
    • Large-eddy simulation of transition process in separated-reattached flows

      Yang, Zhiyin; University of Derby (WIT Press, 2011)
      It is important that scientists who perform experiments, researchers who develop computer codes, and those who carry out measurements on prototypes all communicate effectively. While computer models are now more reliable and better able to represent more realistic problems, experimental measurements need to be conditioned to the requirements of the computational models. Progress of engineering science depends on the orderly and progressive concurrent development of all three fields. This book contains the results of the latest in a biennial series of meetings to facilitate that communication and development that began in 1984. The papers presented at the conference included topics such as: Computational and Experimental Methods; Fluid Flow; Heat Transfer and Thermal Processes; Stress Analysis; Composite Materials; Detection and Signal Processing; Advances in Measurements and Data Acquisition; Multiscale Modelling; Ballistics; Railway Transport.
    • Large-eddy simulation: Past, present and the future

      Yang, Zhiyin; University of Derby, Derby, UK (Elsevier, 2015-02)
      Large-eddy simulation (LES) was originally proposed for simulating atmospheric flows in the 1960s and has become one of the most promising and successful methodology for simulating turbulent flows with the improvement of computing power. It is now feasible to simulate complex engineering flows using LES. However, apart from the computing power, significant challenges still remain for LES to reach a level of maturity that brings this approach to the mainstream of engineering and industrial computations. This paper will describe briefly LES formalism first, present a quick glance at its history, review its current state focusing mainly on its applications in transitional flows and gas turbine combustor flows, discuss some major modelling and numerical challenges/issues that we are facing now and in the near future, and finish with the concluding remarks.
    • Numerical study of transition process in a separated boundary layer on a flat plate with two different leading edges

      Yang, Zhiyin; University of Derby (World Scientific and Engineering Academy and Society, 2012-01)
      Transition from laminar flow to turbulent flow occurs very often and plays a crucial role in many practical engineering flows. There are many different kinds of transition and broadly speaking they can be classified into three categories: classical transition in attached boundary; bypass transition in attached boundary layer and separated boundary layer transition. This paper presents a comparative study of separated boundary layer transition on a flat plate with a blunt/semi-circular leading edge. Boundary layer may separate due to an adverse pressure gradient or due to flow geometry. In the current study the geometry is a flat plate with two different leading edges: a blunt one and a semi-circular one. The main purpose of the study is to identify how similar or how different the transition process is with two different leading edges. This study shows that for both cases (blunt and semi-circular leading edges) the primary two-dimensional instability originates from the free shear layer of the separation bubble via the Kelvin-Helmholtz mechanism. Three-dimensional motions develop under any small spanwise disturbances and similar coherent structures have been observed from flow visualization in both cases, strongly indicating that the transition process is very similar.
    • On transition process in separated-reattached flows

      Langari, Mostafa; Yang, Zhiyin; University of Derby (Pushpa Publishing House, 2010-10)
      Laminar-to-turbulent transition in shear layer of separated flows is of practical importance. A thorough understanding of transition process is crucial for its prediction and control; to delay the turbulent phase where laminar flow characteristics are desirable or to accelerate it where high mixing and heat transfer rates of turbulent flow are of interest. This paper presents a review of transition process and unsteady behavior of shear layer in separated flows. Despite decades of intensive research in this area the transition process is still not fully understood. However, significant progress has been made with the simulation tools such as Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). This paper will discuss several important issues such as instability, vortex shedding, coherent structures and so on in separated shear layer transition, trying to provide the current status of understanding and an appraisal of possible future developments.