Present condition of unsteady turbulent flows study

Аuthors

Kraev V. M.

RaiffaizenBank, 15А, Leninskii prospect, Moscow, 119071, Russia

e-mail: kraevvm@mail.ru

Abstract

Heat and hydrodynamic processes are becoming determinant while creating new types of engines for space, aviation and nuclear power systems [1 – 9]. Unsteady hydrodynamic and heat transfer processes study is an extremely important problem of engine building.

Only the combination of fundamental and engineering studies provides most effective way to design precise unsteady process model for practical computation. Experimental studies carried out in Moscow Aviation Institute (MAI) hold a prominent place in this field [10 – 17].

The turbulent flow structure studies carried out in MAI reveal non-stationary conditions fundamental effect on turbulent flow structure.

Axial and radial velocity and temperature pulsations, average parameters and their correlations were measured as a part of the study. Generalized experimental data reveals significant impact of flow acceleration and deceleration on turbulent structure. Three specific zones in turbulent flow were identified: near wall area y/R = = 0...0.02 (y — distance from the wall, R — radius of the channel); maximal turbulent parameters modification area y/R = 0.02...0.4 and flow core. Significant difference of turbulent viscosity between steady and unsteady approaches up to three times was identified. Comparison of quasi-steady and unsteady approach to heat transfer and hydraulic resistance coefficients revealed the two-times difference. Undoubtedly, such huge difference is unacceptable for space, aviation and nuclear energetics. This result agrees well with experimental data obtained by other authors [18, 19].

Based on non-stationary conditions significant impact on turbulent structure a computation model was developed. With flow acceleration, hydraulic resistance coefficient exceeds relative quasi-steady value by 2 times and more. During flow deceleration, it is 35% less.

Experimental study results present reliable base for further theoretical studies to be carried out in MAI [17]. The existing high-Reynolds turbulent models are not able, in principal, to consider non-stationary effect. From turbulence models analyzed in [18], only Menters SST model, which is low-Reynolds model, gives the results close to the experimental. Generalized equations for non-stationary friction and heat transfer coefficients at flow acceleration and deceleration in a tube for engineering design were obtained. The advantage of such models consists in the possibility of their employing for any monotonous flow variation curve, as well as satisfactory convergence with experimental data on hydrodynamic non-stationary gas flow in through channels [20].

Among the works of theoretical character, the studies of Professor Igor Derevich should be noted in the first place. In reference [21] the author considers the gas flow with monotonous consumption decrease/increase, and reveals the causes of computation and experimental data mismatch.

For practice, we recommend to analyze the effect of non-stationary processes on a certain jet engine control system. In case, when the processes are principally non-stationary and the required accuracy must be high, a non-stationary model and/or other approaches, considering non-stationaries, should be used.

Keywords:

turbulent flow structure, hydrodynamic non-stationarity, non-isothermal conditions, heat transfer under unsteady conditions, of non-stationary flows computation models

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