Witold Elsner

LES modeling of converging-diverging turbulent channel flow

The paper presents the results of the application of large-eddy simulation (LES) to turbulent channel flow with a varying pressure gradient obtained by an appropriately specified shape of one of the walls. The main objective of the paper is to assess various subgrid scale (SGS) models implemented in two different codes as well as to assess the sensitivity of the predictive accuracy to grid resolution. Additionally, the role of SGS viscosity, controlled by a constant parameter of the SGS model, was investigated. The simulations were performed with inlet conditions corresponding to two Reynolds numbers: and . The consistency and the accuracy of simulations are evaluated using direct numerical simulation (DNS) results. It is demonstrated that all SGS models require a comparable minimum grid refinement in order to capture accurately the recirculation region. Such a test case with a reversal flow, where the turbulence transport is dictated by the dynamics of the large-scale eddies, is well suited to demonstrat...

Effect of finite spatial resolution of hot-wire anemometry on measurements of turbulence energy dissipation

The authors deal with the effect of finite hot-wire resolution which reveals itself when both the one-dimensional energy spectrum in a high wavenumber range and turbulence energy dissipation are measured with a hot sensor of a finite wire length. The proposed correction functions derived for isotropic and homogeneous turbulence structures are based on the assumption that the hot-wire sensitivity to velocity fluctuations follows the local excess of the hot-wire temperature Theta w( xi 2) over the ambient temperature Theta a of the flowing medium. The theoretical predictions compared with experimental data show that the measured quantities may effectively be corrected by means of the correction functions proposed.

The effect of wake parameters on the transitional boundary layer on turbine blade

Abstract This paper presents an experimental analysis of the interaction between wakes and boundary layers on an aerodynamic blade profile. The main objective of this research was to investigate the simultaneous influence of the freestream conditions and wake parameters on the wake-induced transition process. The investigations were performed for two levels of inlet freestream turbulence Tu = 0.4 and 3 per cent. The wakes were generated by cylindrical bars mounted in the rotating wheel. It was shown that wake parameters and especially the turbulence in the wake and wake width have a great influence on the position and extent of the induced transition. The role of freestream turbulence on the wake structure passing over the blade surface was also emphasized.

Detection of coherent structures in a turbulent boundary layer with zero, favourable and adverse pressure gradients

The paper presents the analysis of turbulent coherent structures found in a turbulent boundary layer subjected to zero, favourable and adverse pressure gradient. The analysis of the shape of conditionally averaged traces of u and v velocity components obtained by VITA (Variable Interval Time Averaging) technique based on signals recorded by X-wire probe allows to detect four types of coherent structures. The paper documents a presence of coherent vortical structures of positive or negative vorticity moving upward and downward. It is shown also that the acceleration and deceleration of the mean streamwise velocity, due to the pressure gradient, modifies the vortex convection velocity magnitude and the angle. This effect is clearly seen especially for the adverse pressure gradient flow, where Q events in first and fourth quadrants are enhanced near the wall.

Experimental Analysis and Prediction of Wake-Induced Transition in Turbomachinery

The paper presents an experimental and numerical analysis of the interaction between wakes and boundary layers on aerodynamic blade profiles. The experiment revealed that incoming wakes interact with boundary layers and cause the significant increase of velocity fluctuations in the boundary layer and in consequence shift the transition zone towards the leading edge. The full time evolution of periodic wake induced transition was reproduced from measurements. The numerical simulation of the flow around the blade profile has been performed with the use of the adaptive grid viscous flow unNEWT PUIM solver with a prescribed unsteady intermittency method (PUIM) developed at Cambridge University, UK. The results obtained give evidence that the turbulence transported within the wake is mainly responsible for the transition process. The applied CFD solver was able to reproduce some essential flow features related to the bypass and wake-induced transitions and the simulations reveal good agreement with the experimental results in terms of localisation and extent of wake-induced transition.Copyright

Numerical Study of Transitional Rough Wall Boundary Layer

This paper presents the verification of the boundary layer modeling approach, which relies on a γ-Reθt model proposed by Menter et al. (2006, “A Correlation-Based Transition Model using Local Variables—Part I: Model Formation,” J. Turbomach., 128(3), pp. 413–422). This model was extended by laminar-turbulent transition correlations proposed by Piotrowski et al. (2008, “Transition Prediction on Turbine Blade Profile with Intermittency Transport Equation,” Proceedings of the ASME Turbo Expo, Paper No. GT2008-50796) as well as Stripf et al.s (2009, “Extended Models for Transitional Rough Wall Boundary Layers with Heat Transfer—Part I: Model Formulation,” J. Turbomach., 131(3), 031016) correlations, which take into account the effects of surface roughness. To blend between the laminar and fully turbulent boundary layer over rough wall, the modified intermittency equation is used. To verify the model, a flat plate with zero and nonzero pressure gradient test cases as well as the high pressure turbine blade case were chosen. Furthermore, the model was applied for unsteady calculations of the turbine blade profile as well as the Lou and Hourmouziadis (2000, “Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions,” J. Turbomach., 122(4), pp. 634–643) flat plate test case, with an induced pressure profile typical for a suction side of highly-loaded turbine airfoil. The combined effect of roughness and wake passing were studied. The studies proved that the proposed modeling approach (ITMR hereinafter) appeared to be sufficiently precise and enabled for a qualitatively correct prediction of the boundary layer development for the tested simple flow configurations. The results of unsteady calculations indicated that the combined impact of wakes and the surface roughness could be beneficial for the efficiency of the blade rows, but mainly in the case of strong separation occurring on highly-loaded blade profiles. It was also demonstrated that the roughness hardly influences the location of wake induced transition, but has an impact on the flow in between the wakes.

Experimental Analysis of Turbulent Boundary Layer with Adverse Pressure Gradient Corresponding to Turbomachinery Conditions

The paper deals with the experimental analysis of turbulent boundary layer at the flat plate for Reynolds number Re θ ≈3000. The adverse pressure gradient generated by curvature of the upper wall corresponded to the case of pressure distribution in axial compressor. The fully developed turbulence structure was achieved by proper triggering of the boundary layer. The mean and turbulent flow-fields were investigated with the use of hot-wire technique. A substantial effort has been devoted to the precise definition of inlet boundary conditions as well as consistency of measurements obtained with different HWA sensors. The analysis in the paper was concentrated on the problem of scaling of turbulent boundary layer and on the physical background behind scaling laws being compared.

Transition Prediction on Turbine Blade Profile With Intermittency Transport Equation

The paper presents the results of tests and validations of γ-Reθ model proposed by Menter at al [10], which was extended by in-house correlations on onset location and transition length. The tests performed were based on experimental data on the flat plate Test Cases available at ERCOFTAC Data Base as well as on experimental data of turbine blade profile investigated at Czestochowa University of Technology. Further on, the model was applied for unsteady calculations of the blade profile test case, where chosen inlet conditions (turbulent intensity, wake parameters) were applied. For the selected case numerical results were compared not only with the experimental data, but also with the results obtained with other transition models. It was shown that the applied model was able to reproduce some essential flow features related to the bypass and wake-induced transitions and the simulations reveal good agreement with the experimental results in terms of localisation and extent of wake-induced transition.Copyright

Convection Velocity Variation as a Result of Amplitude Modulation Phenomena

The paper discusses the method of convection velocity estimation in turbulent boundary layer using its relationship with amplitude modulation mechanism. To verify this method the two-point correlation measurements using hot-wire technique was applied in strong adverse pressure gradient flow for two Reynolds numbers. Additionally, streamwise velocity profiles were measured in the same locations. It was shown that the changes in the convection velocity due to Reynolds number and pressure gradient results from amplitude modulation mechanism. The convection velocity in the strong adverse pressure gradient region can be two times higher than the mean velocity in the buffer layer.

Investigation of Wake Transport in a Turbine Blade Channel and Its Effect on the Boundary Layer Development

The paper presents the results of experimental investigations of the wake migration within the blade passage of a steam turbine blade cascade. The upstream wakes were generated by the wheel with cylindrical bars rotating at the plane perpendicular to the flow direction. The bars diameter was chosen to match the loss of a representative turbine blade as well as the flow pattern at the inlet to the blade cascade. The measurements were performed with the use of hot-wire technique and double X-wire probe. The application of phase averaging allowed one to reproduce the wake convection. The wake movement was visualized by the perturbation velocity vectors and fluctuating components. Dense spatial resolution of measuring points allows for calculation and analysis of selected terms of turbulent kinetic energy transport equation. The results confirm experimental and numerical observations done already for high-loaded blade profiles, which reveal that as the wake passes through high spatial velocity gradient area the turbulent production appears. The turbulent production causes the increase of turbulent kinetic energy (TKE). The analysis confirms also that the convection is mainly responsible for the wake deformation and that the distortion of shape and wake width change especially at the edges of channel is caused by “jet effect”. It also was proved that the role and share of turbulent diffusion is of minor importance and only a slight increase of diffusion is observed in the rear part of the blade channel close to the suction side, where TKE production appears. It was shown also that transition starts not when the wake touches the boundary layer edge, but earlier under the high energy core of the impacting wake. The earlier start of the transition could be due to pressure coupling caused by high energetic small scale structures.Copyright