Flutter of Low Pressure Turbine Blades With Cyclic Symmetric Modes: A Preliminary Design Method

A current preliminary design method for flutter of low pressure turbine blades and vanes only requires knowledge of the reduced frequency and mode shape (real). However, many low pressure turbine (LPT) blade designs include a tip shroud, that mechanically connects the blades together in a structure exhibiting cyclic symmetry. A proper vibration analysis produces a frequency and complex mode shape that represents two real modes phase shifted by 90 degrees. This paper describes an extension to the current design method to consider these complex mode shapes. As in the current method, baseline unsteady aerodynamic analyses must be performed for the 3 fundamental motions, two translations and a rotation. Unlike the current method work matrices must be saved for a range of reduced frequencies and interblade phase angles. These work matrices are used to generate the total work for the complex mode shape. Since it still only requires knowledge of the reduced frequency and mode shape (complex), this new method is still very quick and easy to use. Theory and an example application are presented.

Download Full-text

A Design Method to Prevent Low Pressure Turbine Blade Flutter

Volume 5: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls, Diagnostics and Instrumentation; Education ◽

10.1115/98-gt-575 ◽

1998 ◽

Cited By ~ 6

Author(s):

Josef Panovsky ◽

Robert E. Kielb

Keyword(s):

Mode Shape ◽

Design Method ◽

Turbine Blades ◽

Low Pressure ◽

Stability Parameter ◽

Influence Coefficient ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

Blade Flutter ◽

The Stability

A design approach to avoid flutter of low pressure turbine blades in aircraft engines is described. A linearized Euler analysis, previously validated using experimental data, is used for a series of parameter studies. The influence of mode shape and reduced frequency are investigated. Mode shape is identified as the most important contributor to determining the stability of a blade design. A new stability parameter is introduced to gain additional insight into the key contributors to flutter. This stability parameter is derived from the influence coefficient representation of the cascade, and includes only contributions from the reference blade and its immediate neighbors. This has the effect of retaining the most important contributions to aerodynamic damping while filtering out terms of less significance. This parameter is utilized to develop a stability map, which provides the critical reduced frequency as a function of torsion axis location. Rules for preliminary design and procedures for detailed design analysis are defined.

Download Full-text

A Design Method to Prevent Low Pressure Turbine Blade Flutter

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.483180 ◽

1999 ◽

Vol 122 (1) ◽

pp. 89-98 ◽

Cited By ~ 27

Author(s):

J. Panovsky ◽

R. E. Kielb

Keyword(s):

Mode Shape ◽

Design Method ◽

Turbine Blades ◽

Low Pressure ◽

Stability Parameter ◽

Influence Coefficient ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

Blade Flutter ◽

The Stability

A design approach to avoid flutter of low pressure turbine blades in aircraft engines is described. A linearized Euler analysis, previously validated using experimental data, is used for a series of parameter studies. The influence of mode shape and reduced frequency are investigated. Mode shape is identified as the most important contributor to determining the stability of a blade design. A new stability parameter is introduced to gain additional insight into the key contributors to flutter. This stability parameter is derived from the influence coefficient representation of the cascade, and includes only contributions from the reference blade and its immediate neighbors. This has the effect of retaining the most important contributions to aerodynamic damping while filtering out terms of less significance. This parameter is utilized to develop a stability map, which provides the critical reduced frequency as a function of torsion axis location. Rules for preliminary design and procedures for detailed design analysis are defined. [S0742-4795(00)01401-0]

Download Full-text

Effect of Sector Mode Shape Variation on the Aerodynamic Stability of a Low-Pressure Turbine Sectored Vane

Volume 4: Turbo Expo 2003 ◽

10.1115/gt2003-38632 ◽

2003 ◽

Cited By ~ 6

Author(s):

Olga V. Chernysheva ◽

Torsten H. Fransson ◽

Robert E. Kielb ◽

John Barter

Keyword(s):

Turbine Blades ◽

Low Pressure ◽

Mode Shapes ◽

Two Dimensional ◽

Shape Variation ◽

Aerodynamic Stability ◽

Flow Solver ◽

Low Pressure Turbine ◽

Influence Coefficients

The paper presents a method to investigate the flutter appearance in a cascade, where blades are connected together in a number of identical sectors. The key parameters of the method are vibration amplitudes and mode shapes of the blades belonging to the same sector. The aerodynamic response from a sectored vane cascade is calculated based on the aerodynamic work influence coefficients of freestanding blades performed with two-dimensional inviscid linearized flow solver. A case study based upon the presented methodology shows that, despite stabilizing effect of tying blades together into sectors, a sectored vane consisting of six low-pressure turbine blades vibrating with real single modes, and identical amplitudes can be unstable at realistic design conditions.

Download Full-text

The Impact of Blade Loading and Unsteady Pressure Bifurcations on Low-Pressure Turbine Flutter Boundaries

Volume 7B: Structures and Dynamics ◽

10.1115/gt2015-42857 ◽

2015 ◽

Author(s):

Joshua J. Waite ◽

Robert E. Kielb

Keyword(s):

Mode Shape ◽

Pressure Ratio ◽

Low Pressure ◽

Forced Response ◽

Blade Loading ◽

Unsteady Pressure ◽

Aerodynamic Damping ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

The Impact

The three major aeroelastic issues in the turbomachinery blades of jet engines and power turbines are forced response, non-synchronous vibrations, and flutter. Flutter primarily affects high-aspect ratio blades found in the fan, fore high-pressure compressor stages, and aft low-pressure turbine (LPT) stages as low natural frequencies and high axial velocities create smaller reduced frequencies. Often with LPT flutter analyses, physical insights are lost in the exhaustive quest for determining whether the aerodynamic damping is positive or negative. This paper underlines some well known causes of low-pressure turbine flutter in addition to one novel catalyst. In particular, an emphasis is placed on revealing how local aerodynamic damping contributions change as a function of unsteady (e.g. mode shape, reduced frequency) and steady (e.g. blade torque, pressure ratio) parameters. To this end, frequency domain RANS CFD analyses are used as computational wind tunnels to investigate how aerodynamic loading variations affect flutter boundaries. Preliminary results show clear trends between the aerodynamic work influence coefficients and variations in exit Mach number and back pressure, especially for torsional mode shapes affecting the passage throat. Additionally, visualizations of qualitative bifurcations in the unsteady pressure phases around the airfoil shed light on how local damping contributions evolve with steady loading. Final results indicate a sharp drop in aeroelastic stability near specific regions of the pressure ratio indicating a strong correlation between blade loading and flutter. Passage throat shock behavior is shown to be a controlling factor near the trailing edge, and like critical reduced frequency, this phenomenon is shown to be highly dependent on the vibratory mode shape.

Download Full-text

Preliminary Flutter Design Method for Supersonic Low Pressure Turbines

Volume 6: Structures and Dynamics, Parts A and B ◽

10.1115/gt2009-59177 ◽

2009 ◽

Cited By ~ 5

Author(s):

Markus Meingast ◽

Robert E. Kielb ◽

Jeffrey P. Thomas

Keyword(s):

Harmonic Balance ◽

Design Method ◽

Low Pressure ◽

Shape Sensitivity ◽

Pure Bending ◽

Characteristic Appearance ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

Bending Modes ◽

Cfd Method

The “Tie-Dye” (TD) method is a well-known preliminary flutter design method for subsonic low pressure turbine (LPT) blades. In this paper, a study of 2D mode shape sensitivity using the TD-method for supersonic exit Mach numbers is presented. Using a harmonic balance CFD method, TD maps displaying the critical reduced frequency for a range of pitching axis locations were created. The TD method was run on two geometrically different blades. Subsonically, the characteristic appearance does not change much over airfoil types. An even lesser amount of morphing can be observed between the different profiles in the supersonic range, than for the subsonic cases. Pure bending modes show a high sensitiviy to the actual bending direction. Therefore the single critical reduced frequency value criteria does not hold up for all cases. The method is applicable for supersonic exit flows, and is even more predictable and universal than for the subsonic cases.

Download Full-text

The Impact of Blade Loading and Unsteady Pressure Bifurcations on Low-Pressure Turbine Flutter Boundaries

Journal of Turbomachinery ◽

10.1115/1.4032043 ◽

2015 ◽

Vol 138 (4) ◽

Cited By ~ 2

Author(s):

Joshua J. Waite ◽

Robert E. Kielb

Keyword(s):

Mode Shape ◽

Pressure Ratio ◽

Low Pressure ◽

Forced Response ◽

Blade Loading ◽

Unsteady Pressure ◽

Aerodynamic Damping ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

The Impact

The three major aeroelastic issues in the turbomachinery blades of jet engines and power turbines are forced response, nonsynchronous vibrations, and flutter. Flutter primarily affects high-aspect ratio blades found in the fan, fore high-pressure compressor stages, and aft low-pressure turbine (LPT) stages as low natural frequencies and high axial velocities create smaller reduced frequencies. Often with LPT flutter analyses, physical insights are lost in the exhaustive quest for determining whether the aerodynamic damping is positive or negative. This paper underlines some well-known causes of the LPT flutter in addition to one novel catalyst. In particular, an emphasis is placed on revealing how local aerodynamic damping contributions change as a function of unsteady (e.g., mode shape, reduced frequency) and steady (e.g., blade torque, pressure ratio) parameters. To this end, frequency domain Reynolds-averaged Navier–Stokes (RANS) CFD analyses are used as computational wind tunnels to investigate how aerodynamic loading variations affect flutter boundaries. Preliminary results show clear trends between the aerodynamic work influence coefficients and variations in exit Mach number and back pressure, especially for torsional mode shapes affecting the passage throat. Additionally, visualizations of qualitative bifurcations in the unsteady pressure phases around the airfoil shed light on how local damping contributions evolve with steady loading. Final results indicate a sharp drop in aeroelastic stability near specific regions of the pressure ratio, indicating a strong correlation between blade loading and flutter. Passage throat shock behavior is shown to be a controlling factor near the trailing edge, and as with critical reduced frequency, this phenomenon is shown to be highly dependent on the vibratory mode shape.

Download Full-text

Experimental Investigation of Mode Shape Sensitivity of an Oscillating Low-Pressure Turbine Cascade at Design and Off-Design Conditions

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.2436567 ◽

2006 ◽

Vol 129 (2) ◽

pp. 530-541 ◽

Cited By ~ 9

Author(s):

Damian M. Vogt ◽

Torsten H. Fransson

Keyword(s):

Mode Shape ◽

Three Dimensional ◽

High Sensitivity ◽

Low Pressure ◽

Turbine Rotor ◽

Shape Sensitivity ◽

Aeroelastic Stability ◽

Low Pressure Turbine ◽

Reduced Frequency ◽

Bending Modes

The effect of negative incidence operation on mode shape sensitivity of an oscillating low-pressure turbine rotor blade row has been studied experimentally. An annular sector cascade has been employed in which the middle blade has been made oscillating in controlled three-dimensional rigid-body modes. Unsteady blade surface pressure data were acquired at midspan on the oscillating blade and two pairs of nonoscillating neighbor blades and reduced to aeroelastic stability data. The test program covered variations in reduced frequency, flow velocity, and inflow incidence; at each operating point, a set of three orthogonal modes was tested such as to allow for generation of stability plots by mode recombination. At nominal incidence, it has been found that increasing reduced frequency has a stabilizing effect on all modes. The analysis of mode shape sensitivity yielded that the most stable modes are of bending type with axial to chordwise character, whereas high sensitivity has been found for torsion-dominated modes. Negative incidence operation caused the flow to separate on the fore pressure side. This separation was found to have a destabilizing effect on bending modes of chordwise character, whereas an increase in stability could be noted for bending modes of edgewise character. Variations of stability parameter with inflow incidence have hereby found being largely linear within the range of conditions tested. For torsion-dominated modes, the influence on aeroelastic stability was close to neutral.

Download Full-text