Drag-Reducing Flows in Laminar-Turbulent Transition Region

2014 ◽  
Vol 136 (10) ◽  
Author(s):  
Shu-Qing Yang ◽  
Donghong Ding
Keyword(s):  
Transition Region ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Physical Nature ◽  
Weighting Factor ◽  
Intermittent Flow ◽  
Mean Velocity ◽  
Turbulent Transition ◽  
Intermittency Factor ◽  
Laminar Turbulent Transition

This study makes an attempt to investigate Newtonian/non-Newtonian pipe flows in a laminar-turbulent transition region, which is an extraordinarily complicated process and is not fully understood. The key characteristic of this region is its intermittent nature, i.e., the flow alternates in time between being laminar or turbulent in a certain range of Reynolds numbers. The physical nature of this intermittent flow can be aptly described with the aid of the intermittency factor γ, which is defined as that fraction of time during which the flow at a given position remains turbulent. Spriggs postulated that a weighting factor can be used to calculate the friction factor, applying its values in laminar and turbulent states. Based on these, a model is developed to empirically express the mean velocity and Reynolds shear stress in the transition region. It is found that the intermittency factor can be used as a weighting factor for calculating the flow structures in the transition region. Good agreements can be achieved between the calculations and experimental data available in the literature, indicating that the present model is acceptable to express the flow characteristics in the transition region.

Simulation of Cavitating Jet Through a Poppet Valve with Special Emphasis on Laminar-Turbulent Transition

2020 ◽  
Author(s):  
Cong Yuan ◽  
Yan Cai ◽  
Shiqi Liu ◽  
Zunling Du
Keyword(s):  
Coherent Structure ◽  
Large Scale ◽  
Flow Characteristics ◽  
Numerical Results ◽  
Primary Concern ◽  
Potential Core ◽  
Poppet Valve ◽  
Turbulent Transition ◽  
Cavitating Jet ◽  
Laminar Turbulent Transition

One of the major problems in oil-hydraulic poppet valve is the deteriorated performance accompanied by occurrence of cavitation. This is mainly a consequence of lack in understanding of the cavitating jet, which has inhibited the development of sufficiently general and accurate models for prediction of its performance. In the current paper, a two-phase volume of fluid (VOF) methodology combined with Schnerr-Sauer cavitation model is employed to perform quasi-direct transient fully three-dimensional calculations of the cavitating jet inside a poppet valve, with special concern on the laminar-turbulent transition. The numerical results allow separate examination of several distinctive flow characteristics, which show agreeable consistency with experimental observation. The periodic evolution of cavitation structure is related to temporal development of large-scale structure. The potential core indicated by velocity distribution, however, assumes a similar flow pattern regardless of temporal evolution of large-scale eddy. According to the different flow characteristics, the transitional process is divided into several parts, including laminar part, waving fluctuation, cross-linked vortex segments and cloud of cavitating vortexes. A comprehensive discussion on the transition is performed based on the numerical results, with primary concern on the governing mechanisms, including the formation of coherent structure organized as paired vortex, development of instability together with its effects on the coherent structure, and interaction between the vortexes. The streamwise vorticity strength accounts for less than 10% of the total vorticity in the cross-link region. It reveals that the breakdown of paired coherent structure is a result of the successive pairing process generated from combination of longitudinal and circumferential perturbation, instead of the growth of streamwise vortices as in the case of submerged circular jet.

scholarly journals Prediction of By-Pass Transition by Means of a Turbulence Weighting Factor: Part I — Theory and Validation

1999 ◽  
Author(s):  
J. Steelant ◽  
E. Dick
Keyword(s):  
Transport Equation ◽  
Stokes Equations ◽  
Flow Characteristics ◽  
Weighting Factor ◽  
Navier Stokes ◽  
Freestream Turbulence ◽  
Transitional Layer ◽  
Turbulent Spots ◽  
Navier Stokes Equations ◽  
Intermittency Factor

The classical intermittency factor γ for quantifying transition is redefined and extended to enable the modelling of by-pass transition. Therefore, a new parameter, the turbulence weighting factor τ, is introduced to cover both the physics of freestream turbulence diffusion and of turbulent spots. A transport equation is presented for the τ-factor including convective, diffusive, production and sink terms. In combination with the conditioned Navier-Stokes equations, this leads to improvements in the calculation of flow characteristics in both the transitional layer and the freestream.

The influence of Chine nose shapes on a slender body flight vehicle at high angles of attack

2011 ◽  
Vol 226 (2) ◽  
pp. 182-196 ◽  
Author(s):  
S Lim ◽  
S D Kim ◽  
D J Song
Keyword(s):  
Three Dimensional ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Slender Body ◽  
Flight Vehicle ◽  
Side Force ◽  
Turbulent Transition ◽  
Flow Vortex ◽  
High Angles Of Attack ◽  
Laminar Turbulent Transition

The flow characteristics of asymmetric vortices and the side force of a slender body flight vehicle with chine nose at high angles of attack have been studied using a three-dimensional upwind Navier–Stokes method with the k– ω turbulence model and a simple laminar–turbulent transition model. Asymmetrically changing turbulent viscosities that arise from asymmetric laminar–turbulent transition conditions cause asymmetric cross-flow vortex structures and side forces at higher angles of attack. However, certain type of edges may cause fixed flow separations on these edges. In this study, the chine nose shape with chine edge on its both sides is considered for the method to reduce side force. The asymmetric flow control capacity of chine nose shapes at high angles of attack is studied.

Sign in / Sign up

Export Citation Format

Share Document