Large-Eddy Simulation for Flows of Engineering Interest

Large-eddy simulations (LES) of engineering application are being carried out by the research group, Turbomachinery Lab, IIT Kanpur. Few examples are unsteady transitional flows within the blade passage, blade- wake interactions, Film Cooling, flow past a circular cylinder in the vicinity of a flat plate, laminar separation bubble, self-sustained cavity oscillation, aero-acoustics and control. The purpose is to resolve flow physics illustrating instability, transition mechanism, turbulence generations including visualization of flow structures.

Our work has contributed to increase the understanding and hopefully directly/indirectly to the advancement of HP/LP turbine design. Our recent work depicting the effect of coherent and semi-coherent eddy structure on transitional boundary layer are of importance in a number of other fields including external aerodynamics, drag reduction, wind engineering and flow-structure interactions. Our work is sufficiently broad to be relevant at the theoretical level to many applications, and to be of fundamental interest in its own right, independently of the valuable potential applications.

LES of wake passing over highly cambered blades are performed using wake data extracted from precursor simulations. The wake-data are interpolated at the inlet plane of cascade considering the kinematics of flow. An attempt is made to describe the effects of wake kinematics and wake turbulence on the boundary layer developing over the suction surface of a LP turbine blade. An insight into the underlying physical mechanism including the flow transition over a blade surface is presented. The present results are compared with experiment and DNS.

The laminar separation bubble is commonly found in low Reynolds Number flows, which is encountered in many engineering applications such as flow over LPT blades, or in airfoils. The transition mechanism of a laminar separation bubble is discussed through LES. The separated layer is highly sensitive to the external disturbances, which cause the flow to undergo transition.

               Iso-surfaces of spanwise vorticity depicting Λ-structures and breakdown to turbulence.

  Iso-surface of at Re = 3900 illustrating shear layer                                                     Flow past Staggered Cylinders

  transition and breakdown

LES of flow past a circular cylinder in the vicinity of a flat plate for different gap-to-diameter ratios and Re has been carried out to understand mutual interaction of wake and boundary layer. This kind of study can directly be related to many engineering problems, such as, flow past a suspension bridge, pipelines near the ground, flow through heat exchanger tubes near the wall and so on. The dynamics of vortex shedding in presence of a wall and also the evolution of boundary layer, its transition under the excitation of migrating wake have been resolved by LES.

   Instantaneous spanwise vorticity: (a) and (b) experiment Price et al., 2001; (c) and (d) present LES.

The perturbations created by the turbulent wake are amplified near the wall with appearance of longitudinal streaky structures: the features of flow transition.

                 Iso-surface of –λ2 depicting evolution of hairpins: the coherent eddies of jets in crossflow.

The flow past an open cavity is very common in aerospace applications and can be a cause of acoustic source due to hydrodynamic instability of the shear layer and its interactions with the downstream edge. The principle objective is to use large-eddy simulation to resolve and control the large-scale structures, which are largely responsible for flow oscillations in a cavity.

 Synthetic jet has been used as an active device to suppress the cavity oscillations: (a) no control;

 (b) amp=1.2, fr =1100 Hz.

Density gradient illustrating shock, expansion wave and feed-back acoustic waves reflecting from end walls.

The numerical simulations of flow and heat transfer over turbine blades involving laminar-turbulent transition apart from film cooling are presented. The predicted results are compared with the experimental surface heat transfer and pressure distributions for transonic turbine blades over a wide range of flow conditions. Different turbulence models are modified for transitional flow.

Neurologists while treating soldiers who had survived explosions in the warfront came across a medical paradox. Evidences of memory deficits, speech problems and difficulties with decision-making were seen in these patients. The paradox was that most of these patients had not suffered a direct head injury even though their scan reports suggested enlargement of brain ventricles with minor bleeding in certain cases. Such medical conditions are called Blast Induced Traumatic Brain Injury (TBI). Analysis through CFD indicates that the pressure dynamics in Cerebrospinal fluid (CSF) may lead to blast induced TBI.

Pressure contours at 2.0, 3.0 and 4.0ms depicting unsteady pressure dynamics within CFS pathways due to exposure to a blast wave. Red denotes a high pressure region, while blue indicates a low pressure zone.