Method for designing a cyclic symmetric structure

• US 6,542,859 B1
• Filed: 08/21/2000
• Issued: 04/01/2003
• Est. Priority Date: 05/13/1999
• Status: Expired due to Term

• Current Assignee: Rolls-Royce Corporation (Rolls-Royce Holdings Plc)
• Original Assignee: Rolls-Royce Corporation (Rolls-Royce Holdings Plc)
• Inventors: Louie, John R., Burns, Donald W.
• Primary Examiner(s): Jones, Hugh
• Assistant Examiner(s): Ferris, III, Fred O
• Application Number:
  US09/643,904
• Time in Patent Office: 953 Days
• Field of Search: 700/301, 700/287, 703/18, 703/7, 703/8, 703/2, 703/1, 416/190
• US Class Current: 703/7
• CPC Class Codes: G06F 30/23 using finite element methods [FEM] or finite difference methods [FDM]

Abstract

A method for analyzing and designing structures with structural aliasing. The concept of structural aliasing applies in one embodiment of the present invention to structural systems which are cyclically symmetric. In analyzing and designing structural systems such as wheel assemblies of gas turbine engines, it is possible to group the discrete components so as to aliasingly couple particular ordered excitations and harmonic families. With structural aliasing, it is possible to couple the ordered excitations and harmonic families such that resonant response of the wheel assembly does not occur within the operating range of the engine, and also to have the resonant vibratory mode excited in such a manner that the drive coupling of the wheel dampens the wheel assembly.

Claims

1. A method for designing a gas turbine engine component, comprising:

preparing a finite element model of a wheel assembly coupled to another rotating member within a gas turbine engine, the model including a wheel with a plurality of blades and a coupling for coupling the wheel to the other rotating member;

constraining the model at the coupling;

grouping the blades in a first pattern;

computing a vibratory modal response of the model, the response including a first predictor proximate to the coupling;

regrouping the blades in a second pattern;

recomputing a vibratory modal response of the model, the response including a second predictor proximate to the drive coupling;

comparing the first predictor to the second predictor; and

selecting the pattern with the higher predictor.

2. The method according to claim 1 wherein said first predictor, defines a first coupling stress and said second predictor defines a second coupling stress, and wherein said selecting the pattern with the higher predictor defines selecting the pattern with the higher coupling stress.

3. The method according to claim 2, wherein said first coupling stress is indicative of a first coupling interface damping and said second coupling stress is indicative of a second coupling interface damping, and said selecting the pattern with the higher coupling stress is indicative of selecting the pattern with the higher damping.

4. The method according to claim 1 wherein said first predictor defines a first deflection said second predictor defines a second deflection, and wherein said selecting the pattern with the higher predictor defines selecting the pattern with the greater deflection.

5. The method according to claim 2, wherein said first deflection is indicative of a first coupling interface damping and said second deflection is indicative of a second coupling interface damping, and said selecting the pattern with the greater deflection is indicative of selecting the pattern with the higher damping.

6. The method according to claim 1, wherein the model includes a shroud connecting the blades, and said grouping is by connecting a first number of blades to a section of the shroud.

7. The method according to claim 6, wherein the model includes a shroud connecting the blades, and said regrouping is by connecting a second number of blades to the shroud, the first number being different than the second number.

8. The method according to claim 1, wherein the coupling is a drive coupling for coupling the wheel assembly to one of another wheel assembly or a shaft.

9. The method according to claim 1, wherein the coupling couples the wheel assembly to a spacer.

10. The method according to claim 8, wherein the coupling is a curvic coupling.

11. The method according to claim 1, wherein the coupling is a curvic coupling and said constraining simulates coupling the curvic coupling to the other rotating member.

12. The method according to claim 1 which further comprises imposing a predetermined vibratory displacement upon a blade during said computing.

13. The method according to claim 12 which further comprises reimposing a predetermined vibratory displacement upon a blade during said recomputing.

14. The method according to claim 13, wherein the model includes a shroud connecting the blades, and said grouping is by connecting a first number of blades to a section of the shroud.

15. The method according to claim 14, wherein the model includes a shroud connecting the blades, and said regrouping is by connecting a second number of blades to a section of the shroud, the first number being different than the second number.

19. The method according to claim 12, wherein the second harmonic family aliasingly couples into the 1D harmonic family.

16. A method for designing a wheel assembly for a gas turbine engine, comprising:

providing a gas turbine engine with a wheel assembly, the wheel assembly including a wheel and a plurality of blades, the wheel assembly being characterized by a plurality of harmonic families, each family being characterized by an integral number of diametral node lines, each family including a plurality of natural frequencies, the wheel assembly capable of vibrating at the natural frequencies, the gas turbine engine capable of rotating in a range of rotational speeds;

determining an integral multiple of rotational speed that resonates one or more natural frequencies of a first harmonic family, the first harmonic family being characterized by a first integral number of diametral node lines, the first integral number being different than the integral multiple; and

grouping the blades such that the integral multiple of rotational speed does not resonate the one or more natural frequencies of the first harmonic family, the integral multiple of rotational speed resonating one or more natural frequencies of a second harmonic family, the second harmonic family being characterized by a second integral number of diametral node lines, the second integral number being different than the first integral number.

17. The method according to claim 16, wherein the second integral number is less than the first integral number.

18. The method according to claim 17, wherein the second harmonic family aliasingly couples into the 0D harmonic family.

20. The method according to claim 16, wherein the gas turbine includes a predetermined number of gaspath disturbances, and the predetermined number is the same as the integral multiple.

21. The method according to claim 20, wherein the gaspath disturbances are vanes upstream of the wheel assembly.

22. The method according to claim 20, wherein the gaspath disturbances are struts upstream of the wheel assembly.

23. The method according to claim 16, wherein the wheel assembly includes a curvic coupling for coupling a second rotating member to the wheel assembly and said regrouping increases stress proximate to the curvic coupling.

24. The method according to claim 16, wherein said providing includes a second rotatable member coupled to the wheel and the second rotatable member and the wheel are coupled by a curvic coupling.

25. The method according to claim 24, wherein the second rotatable member is a second wheel assembly.

26. The method according to claim 24, wherein the second rotatable ember is a shaft.

27. The method according to claim 24, wherein the second rotatable member is a spacer.

28. The method according to claim 16, which further comprises preparing a finite element model of the wheel assembly to predict at least one of the plurality of natural frequencies.

29. The method according to claim 16, wherein the wheel assembly includes a plurality of shrouds, and each shroud connects an equal number of blades.

30. The method according to claim 16, wherein the wheel assembly is a cast wheel assembly with a single shroud connecting the blades, and said grouping is by slotting the shroud.

31. The method according the claim 16, wherein said grouping is by creating a plurality discontinuities in the wheel.

32. The method according to claim 16, wherein the wheel assembly includes a shroud connecting the blades, and said grouping including forming discontinuities in the shroud and the wheel.

33. The method according to claim 32, wherein the discontinuities are formed by slotting the shroud and the wheel.

34. A method for designing a cyclically symmetric device, comprising:

providing a cyclically symmetric device characterized by a plurality of harmonic families and a device capable of rotating in a range of rotational speeds, each of the harmonic families being characterized by an integral number of diametral node lines, each family including a plurality of natural frequencies, the cyclically symmetric device capable of vibrating at the natural frequencies;

determining an integral multiple of rotational speed that resonates one or more natural frequencies of a first harmonic family, the first harmonic family being characterized by a first integral number of diametral node lines, the first integral number being different than the integral multiple; and

configuring the cyclically symmetric device such that the integral multiple of rotational speed does not resonate the one or more natural frequencies of the first harmonic family, the integral multiple of rotational speed resonating one or more natural frequencies of a second harmonic family, the second harmonic family being characterized by a second integral number of diametral node lines, the second integral number being different than the first integral number.