Effects Of Turbulence Length Scale Research Articles

Predicting Separation and Transitional Flow in Turbine Blades at Low Reynolds Numbers—Part I: Development of Prediction Methodology

There is an increasing interest in design methods and performance prediction for aircraft engine turbines operating at low Reynolds numbers. In this regime, boundary layer separation may be more likely to occur in the turbine flow passages. For accurate computational fluid dynamics (CFD) predictions of the flow, correct modeling of laminar-turbulent boundary layer transition is essential to capture the details of the flow. To investigate possible improvements in model fidelity, CFD models were created for the flow over two low pressure turbine blade designs. A new three-equation eddy-viscosity type turbulent transitional flow model, originally developed by Walters and Leylek (2004, “A New Model for Boundary Layer Transition Using a Single Point RANS Approach,” ASME J. Turbomach., 126(1), pp. 193–202), was employed for the current Reynolds averaged Navier–Stokes (RANS) CFD calculations. Previous studies demonstrated the ability of this model to accurately predict separation and boundary layer transition characteristics of low Reynolds number flows. The present research tested the capability of CFD with the Walters and Leylek turbulent transitional flow model to predict the boundary layer behavior and performance of two different turbine cascade configurations. Flows over low pressure turbine (LPT) blade airfoils with different blade loading characteristics were simulated over a Reynolds number range of 15,000–100,000 and predictions were compared with experimental cascade results. Part I of this paper discusses the prediction methodology that was developed and its validation using a lightly loaded LPT blade airfoil design. The turbulent transitional flow model sensitivity to turbulent flow parameters was investigated and showed a strong dependence on freestream turbulence intensity with a second-order effect of turbulent length scale. Focusing on the calculation of the total pressure loss coefficients to judge performance, the CFD simulation incorporating Walters and Leylek’s turbulent transitional flow model produced adequate prediction of the Reynolds number performance for the lightly loaded LPT blade cascade geometry. Significant improvements in performance were shown over predictions of conventional RANS turbulence models. Historically, these models cannot adequately predict boundary layer transition.

Effect of turbulent integral length scale on heat transfer around a circular cylinder placed cross to an air flow

The effect of air turbulence on heat transfer around a cylinder is known to be dramatic. Relations based on the intensity of turbulence have been developed to take it into account. However the range of validity of these relations is under debate since they do not take into consideration the scale of turbulence which should also have an effect. This debate is not only theoretical because if the turbulent length scale is important most experiments usually performed at the laboratory scale would probably be disputable in practical conditions. This paper aims at providing some experimental arguments on the effect of the turbulent length scale on the average transfer coefficient value. The effect of a twofold multiplication of this scale from 0.05 to 0.10 m on the heat transfer coefficient has been measured around a circular cylinder placed in the cross-flow of air whose free stream turbulence intensity was 14%. In this range, the effect of the turbulent integral length scale on the average transfer coefficient value is very small and cannot be compared with the dramatic influence of the turbulence intensity.

The effect of turbulence intensity and length scale on low-pressure turbine blade aerodynamics

Abstract Unpredicted losses have been observed in low-pressure gas turbine stages during high altitude operation. These losses have been attributed to aerodynamic separation on the turbine blade suction surfaces. To gain insight into boundary layer transition and separation for these low Reynolds number conditions, the heat transfer distribution on a Langston turbine blade shape was measured in a linear cascade wind tunnel for turbulence levels of 0.8% and 10% and Reynolds numbers of 40–80 k . Turbulence levels of 10% were generated using three passive biplanar lattice grids with square-bar widths of 1.27, 2.54 and 6.03 cm to investigate the effect of turbulence length scale. The heat transfer was measured using a uniform heat flux (UHF) liquid crystal technique. As turbulence levels increased, stagnation heat transfer increased and the location of the suction-side boundary layer transition moved upstream toward the blade leading edge. For this turbine blade shape the transition location did not depend on turbulence length scale, the location is more dependent on pressure distribution, Reynolds number and turbulence intensity. For the 10% turbulence cases, the smaller length scales had a larger affect on heat transfer at the stagnation point. A laser tuft method was used to differentiate between boundary layer transition and separation on the suction surface of the blade. Separation was observed for all of the low turbulence (clean tunnel) cases while transition was observed for all of the 10% turbulence cases. Separation and transition locations corresponded to local minimums in heat transfer. Reattachment points did not correspond to local maximums in heat transfer, but instead, the heat transfer coefficient continued to rise downstream of the reattachment point. For the clean tunnel cases, streamwise streaks of varying heat transfer were recorded on the concave pressure side of the turbine blade. These streaks are characteristic of either Gortler vortices or a three-dimensional transition process. For the 10% turbulence cases, these streaks were not present. The results presented in this paper show that turbulence length scale, in addition to intensity have an important contribution to turbine blade aerodynamics and are important to CFD modelers who seek to predict boundary layer behavior in support of turbine blade design optimization efforts.

509 高強度乱流における水素・空気予混合火炎の構造(熱工学)

Direct numerical simulations of hydrogen-air turbulent premixed flames propagating in two-dimensional homogeneous isotropic turbulence are conducted to investigate the effects of turbulence length scale on the structure of turbulent premixed flames. Detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to simulate H_2-O_2-N_2 reaction in turbulence. DNS are conducted for the cases of turbulence length scale of 1.00, 2.42, 4.84 and 7.26 times of the laminar flame thickness under the constant turbulence intensity. With the decrease of the turbulence length scale, particular flame structure can be observed in which distributions of heat release rate and major radicals such as H, O and OH show relatively smooth profiles whereas those of temperature, density and minor radicals show complicated patterns in the flame front. These local flame structures seem to represent the regime of well stirred reaction zone in the turbulent combustion diagram. The local flame elements in turbulence were identified by using local maximum temperature gradients and statistics of turbulent premixed flames are investigated.

On the receptivity of the by-pass transition to the length scale of the outer stream turbulence

The effect of length scale on flat-plate by-pass boundary layer transition under free stream turbulence conditions has been investigated. Plane grids generated a constant value of 3 percent intensity turbulent fluctuations and five values of the dissipation length parameter in the range of 2.2 mm to 33.3 mm at 5 m/s flow velocity in the plane of the leading edge of the flat plate. The investigated boundary layer corresponds to the ERCOFTAC Test Case T3A+. Distributions of integral parameters as well as the statistical characteristics of the turbulent fluctuations were measured. They document a significant effect of turbulence length scale on the onset and the end of by-pass transition. It was found that the onset of the final stage of transition comes on later in `fine-grained' turbulence than in the case of large free stream turbulence length scale. At the same time, the extent of the transition region shortens with decreasing length scale. Nevertheless, the transition process terminates sooner in a flow with a large turbulence length scale than in a flow with a small one. An attempt has been made to describe these observations quantitatively.

A Bypass Transition Model for Boundary Layers

Experimental data for laminar boundary layers developing below a turbulent free stream show that the fluctuation velocities within the boundary layer increase in amplitude until some critical level is reached, which initiates transition. In the near-wall region, a simple model, containing a single empirical parameter, which depends only on the turbulence level and length scale, is derived to predict the development of the velocity fluctuations in laminar boundary layers with favorable, zero, or adverse pressure gradients. A simple bypass transition model, which considers the streamline distortion in the near-wall region brought about by the velocity fluctuations, suggests that transition will commence when the local turbulence level reaches approximately 23 percent. This value is consistent with experimental findings. This critical local turbulence level is used to derive a bypass transition prediction formula, which compares reasonably with start of transition experimental data for a range of pressure gradients (λ θ = −0.01 to 0.01) and turbulence levels (Tu = 0.2 to 5 percent). Further improvement to the model is proposed through prediction of the boundary layer distortion, which occurs due to Reynolds stresses generated within the boundary layer at high free-stream turbulence levels and also through inclusion of the effect of turbulent length scale as well as turbulence level.

Response of static pressure probes in turbulent streams

A theory of the effect of turbulence on the response of static pressure probes is given. To apply it to a given probe, the response in a laminar stream must be known as a function of orientation. Results are reported for Prandtl probes. For these the effect of turbulence is negligible at the levels encountered in pipe flows, but is highly significant at those prevailing in jets and other strongly turbulent systems. Experimental verification is given and the effect of turbulence length scale is demonstrated.