The application of pulsed vortex generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude aircraft engines and aft stages of industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets have a 30 deg pitch and a 90 deg skew to the free-stream direction. Jet flow oscillations up to 100 Hz are produced using a high-frequency solenoid feed valve. Results are compared to steady blowing at jet blowing ratios less than 4 and at two chordwise positions upstream of the nominal separation zone. Results show that pulsed vortex generator jets produce a bulk flow effect comparable to that of steady jets with an order of magnitude less massflow. Boundary layer traverses and blade static pressure distributions show that separation is almost completely eliminated with the application of unsteady blowing. Reductions of over 50 percent in the wake loss profile of the controlled blade were measured. Experimental evidence suggests that the mechanism for unsteady control lies in the starting and ending transitions of the pulsing cycle rather than the injected jet stream itself. Boundary layer spectra support this conclusion and highlight significant differences between the steady and unsteady control techniques. The pulsed vortex generator jets are effective at both chordwise injection locations tested (45 and 63 percent axial chord) covering a substantial portion of the blade suction surface. This insensitivity to injection location bodes well for practical application of pulsed VGJ control where the separation location may not be accurately known a priori.

Turbine Separation Control Using Pulsed Vortex Generator Jets

The application of vortex-generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude operation of many aircraft gas-turbine engines, as well as the last stages of industrial ground-based gas turbines. Results are presented for steady blowing at jet blowing ratios from zero to four and at several chordwise positions and two freestream turbulence levels. Findings show that above a minimum blowing ratio, which is dependant on the injection location, the pressure loss in the modified blade's wake is reduced by a factor of between two and three. Boundary-layer traverses show that separation is almost completely eliminated with the application of blowing. No significant deleterious effects of vortex-generator jets are observed at higher (nonseparating) Reynolds numbers. The addition of 4% freestream turbulence to the cascade freestream lowers the separation Reynolds number of the turbine blade studied, but does not eliminate the effectiveness of the control technique. The vortex-generator jet control strategy is demonstrated to be a viable technique for low-pressure turbine separation control.

Control of Low-Pressure Turbine Separation Using Vortex-Generator Jets

The effects of pulsed vortex generator jets on a naturally separating low-pressure turbine boundary layer have been investigated experimentally. Blade Reynolds numbers in the linear turbine cascade match those for high-altitude aircraft engines and industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets (30 deg pitch and 90 deg skew angle) are pulsed over a wide range of frequency at constant amplitude and selected duty cycles. The resulting wake loss coefficient versus pulsing frequency data add to previously presented work by the authors documenting the loss dependency on amplitude and duty cycle. As in the previous studies, vortex generator jets are shown to be highly effective in controlling laminar boundary layer separation. This is found to be true at dimensionless forcing frequencies (F + ) well below unity and with low (10 percent) duty cycles. This unexpected low-frequency effectiveness is due to the relatively long relaxation time of the boundary layer as it resumes its separated state. Extensive phase-locked velocity measurements taken in the blade wake at an F of 0.01 with 50 percent duty cycle (a condition at which the flow is essentially quasi-steady) document the ejection of bound vorticity associated with a low-momentum fluid packet at the beginning of each jet pulse. Once this initial fluid event has swept down the suction surface of the blade, a reduced wake signature indicates the presence of an attached boundary layer until just after the jet termination. The boundary layer subsequently relaxes back to its naturally separated state. This relaxation occurs on a timescale which is five to six times longer than the original attachment due to the starting vortex. Phase-locked boundary layer measurements taken at various stations along the blade chord illustrate this slow relaxation phenomenon. This behavior suggests that some economy of jet flow may be possible by optimizing the pulse duty cycle and frequency for a particular application. At higher pulsing frequencies, for which the flow is fully dynamic, the boundary layer is dominated by periodic shedding and separation bubble migration, never recovering its fully separated (uncontrolled) state.

The Fluid Dynamics of LPT Blade Separation Control Using Pulsed Jets

A system of Reynolds equations closed by adding a low-Reynolds-number Menter’s dissipative model of turbulence are solved with the use of a factorized finite-volume method. The generation of vortexlike structures in the case of stalled turbulent flow streaming around a deep well on a plane is analyzed. The effect of vortex intensification due to asymmetry of the well is discussed.

Identification of self-organized vortexlike structures in numerically simulated turbulent flow of a viscous incompressible liquid streaming around a well on a plane

This first segment of the two-part paper systematically examines several turbulence models in the context of three flows, namely a simple flat-plate turbulent boundary layer, an axisymmetric separating flow, and a swirling flow. The test cases are chosen on the basis of availability of high-quality and detailed experimental data. The tested turbulence models are integrated to solid surfaces and consist of: Rodi's two-layer k–e model, Chien's low-Reynolds number k–e model, Wilcox's k–ω model, Menter's two-equation shear-stress-transport model, and the one-equation model of Spalart and Allmaras. The objective of the study is to establish the prediction accuracy of these turbulence models with respect to axisymmetric separating flows, and flows of high streamline curvature. At the same time, the study establishes the minimum spatial resolution requirements for each of these turbulence closures, and identifies the proper low-Mach-number preconditioning and artificial diffusion settings of a Reynolds-averaged Navier–Stokes algorithm for optimum rate of convergence and minimum adverse impact on prediction accuracy. Copyright © 2003 John Wiley & Sons, Ltd.

Evaluation of one‐ and two‐equation low‐Re turbulence models. Part I—Axisymmetric separating and swirling flows

Numerical calculations of the three-dimensional flowfield generated by pitched and skewed air jets issuing into an otherwise undisturbed turbulent boundary layer are presented. It is demonstrated that each such jet produces a single strong longitudinal vortex. The strength of the vortex, as inferred from its effect on the development of skin friction, is shown to be influenced by pitch and skew angles, exit velocity, and downstream distance in ways which accord with published experimental results. The calculated beneficial effect that the longitudinal vortices have on the development of skin friction in an adverse pressure gradient demonstrates the mechanism by which vortex generators delay boundary-layer separation. It follows that the numerical model could be used to optimize arrays of air-jet vortex generators. Furthermore, the facility to quantify the interaction between the vortex and the boundary layer should also be valuable in the application of vane vortex generators, and possible even more generally. 18 refs.

H. H. Pearcey

Papers

Numerical model of boundary-layer control using air-jet generated vortices