J. Mitch Wolff

Optically interrogated MEMS pressure sensors for propulsion applications

Pressure sensors suitable for propulsion applications that uti- lize interrogation by fiber optics are described. To be suitable for many propulsion applications, sensors should have fast response, have a con- figuration that can be readily incorporated into sensor arrays, and be able to survive harsh environments. Microelectromechanical systems (MEMS) technology is utilized here for sensor fabrication. Optically inter- rogated MEMS devices are expected to eventually be more suitable than electrically interrogated MEMS devices for many propulsion applications involving harsh environments. Pressure-sensor elements are formed by etching shallow cavities in glass substrates followed by anodic bonding of silicon onto the glass over the cavity. The silicon is subsequently etched to form the pressure-sensitive diaphragm. Light emerging from a fiber is then used to interferometrically detect diaphragm deflection due to external pressure. Experimental results for static and dynamic pres- sure tests carried out in a shock tube demonstrate reasonable linearity, sensitivity, and time response.

Dual-Mode Scramjet Combustor: Numerical Analysis of Two Flowpaths

A PRELIMINARY numerical characterization of a U.S. Air Force Research Laboratory (AFRL) test facility was performed. Research cell 22 (RC22) is a supersonic wind-tunnel facility at AFRL. RC22 is presently simulating dual-mode combustion for circular (axisymmetric) combustors. Previous efforts in RC22 have studied combustion in rectangular flowpaths. The current effort in RC22 is to examine the benefits of axisymmetric flowpaths as compared with rectangular. For the same cross-sectional area in which circular flowpaths eliminate the challenges involved with corner flow effects, they have increased structural efficiency (heat load distribution) and have reducedweight. Circular combustors also pose challenges that involve effective fuel penetration and flame propagation. For more information regarding the rectangular experiments performed in RC22, refer to [1].

Dual Mode Scramjet Combustor: Analysis of Two Configurations

** Two engine configurations were analyzed numerically and experimentally. The first configuration had a constant area backstep and the second configuration had a tapered (divergent) combustor with no backstep. Both combustors burned gaseous ethylene fuel. The analysis used a simulated low enthalpy flight condition corresponding to Mach 3.0 flight. Numerical results showed good agreement with experimental data in terms of performance, isolator heat loss and pressure distribution. For the same fuel equivalence ratio and fuel split the tapered configuration outperformed the backstep configuration by approximately 1% in terms of stream thrust, but the backstep configuration had more isolator margin (before unstart) which allowed for higher total equivalence ratios. The backstep configuration achieved stoichiometric fueling. Model calibration via constant turbulent Schmidt number was performed for the first case in order to better match experimental results. Two different k-epsilon turbulence models were analyzed and yielded differences in pressure distribution. A conceptual design analysis involving a reduction in the number of injectors showed better fuel penetration but less circumferential coverage. Future design concepts could include changing the axial placement of the secondary injector, which could have a positive impact on operability for both configurations. Numerically, future analysis could include a variable turbulent Schmidt number which would be more representative of the flow physics and could alleviate the need for calibration in modeling reacting flows.

Variations in Upstream Vane Loading With Changes in Back Pressure in a Transonic Compressor

Dynamic loading of an inlet guide vane (IGV) in a transonic compressor is characterized by unsteady IGV surface pressures. These pressure data were acquired for two spanwise locations at a 105 percent speed operating condition, which produces supersonic relative Mach numbers over the majority of the rotor blade span. The back pressure of the compressor was varied to determine the effects from such changes. Strong bow shock interaction was evident in both experimental and computational results. Variations in the back pressure have significant influence on the magnitude and phase of the upstream pressure fluctuations. The largest unsteady surface pressure magnitude, 40 kPa, was obtained for the near-stall mass flow condition at 75 percent span and 95 percent chord. Radial variation effects caused by the spanwise variation in relative Mach number were measured. Comparisons to a two-dimensional nonlinear unsteady blade/vane Navier-Stokes analysis show good agreement for the 50 percent span results in terms of IGV unsteady surface pressure. The results of the study indicate that significant nonlinear bow shock influences exist on the IGV trailing edge due to the downstream rotor shock system.

Split-domain harmonic balance solutions to Burger's equation for large-amplitude disturbances

A new split-domain harmonic balance approach is presented. The split-domain approach combines the conventional multidomain harmonic balance approach with a split-operator technique in a unique way to solve periodic unsteady e ow problems efe ciently. The new technique is applied to Burger’ s equation to obtain solutions for two large-amplitude periodic boundary conditions— a single-frequency sine wave and a simulated wake function. Solutions containing strong moving discontinuities are obtained with Fourier series containing up to 48 frequencies for various grid densities. The split-domain harmonic balance solutions are compared with conventional time-accurate solutions. The differences between the two are found to be asymptotic with respect to the number of Fourierfrequenciesincluded.In addition, theharmonicbalanceapproachwasfound to besensitiveto grid density.

Unsteady Aerodynamics and Interactions Between a High-Pressure Turbine Vane and Rotor

A set of experimental data is presented investigating the unsteady aerodynamics associated with a high pressure turbine vane (HPV) and rotor blade (HPB). The data was acquired at the Turbine Research Facility (TRF) of the Air Force Research Laboratory. The TRF is a transient, blowdown facility generating several seconds of experimental data on full scale engine hardware at scaled turbine operating conditions simulating an actual engine environment. The pressure ratio and freestream Reynolds number were varied for this investigation. Surface unsteady pressure measurements on the HPV, total pressure traverse measurements downstream of the vane, and surface unsteady pressure measurements for the rotor blade were obtained. The unsteady content of the HPV sur-face was generated by the rotor potential field. The first harmonic decayed more rapidly than the second harmonic with a movement upstream causing the second harmonic to be most influential at the vane throat. The blade unsteadiness appears to be caused by a combination of shock, potential field, and vane wake interactions between the vane and rotor blade. The revolution averaged data resulted in higher unsteadiness than a passing ensemble average for both vane and rotor indicating a need to understand each passage for high cycle fatigue (HCF) effects.

UNSTEADY BLADE ROW POTENTIAL INTERACTION IN A COMPRESSION STAGE

A set of inlet guide vane (IGV) unsteady surface pressure measurements is presented. The unsteady aerodynamic effects of a highly loaded, high speed downstream compression stage on the upstream inlet guide vane/stator surface pressures at a part speed operation is characterized by experimental and computational analysis methods. The axial spacing between the IGV and rotor was varied between 12%, 26%, and 56% of the rotor chord for a 70% speed near choke operating condition, which is subsonic. Unsteady IGV surface pressures were acquired for two spanwise locations on both blade surfaces. Significant potential interaction is evident up to the 50% chord location. The upstream potential effect is strongly nonlinear and three dimensional in character, even though the IGV flow is largely two dimensional. The higher harmonic characteristics of the upstream potential effect decreases as the spacing between the IGV and rotor is increased. Comparisons to a nonlinear unsteady multi-blade row Navier-Stokes analysis show a good trendwise agreement in the IGV unsteady surface pressure envelope results, however computational capabilities to predict the frequency character requires further study.

CFD Analysis of Unsteady Separated Transonic Oscillation Cascade Aerodynamics

With an improved algebraic mesh-deforming algorithm, STAR-CD, a commercial computational fluid dynamics (CFD) solver is employed for the numerical analysis of a transonic oscillating linear cascade of advanced design blades. The center blade oscillates 0.6-degrees about the middle cord. The numerical simulation is conducted for a frequency range from 200 Hz to 500 Hz. A hybrid grid, which utilizes a structured O-grid around the airfoil and an unstructured grid everywhere else is employed. The Spalart-Allmaras (S-A). one equation turbulence model, along with other two equation k-ε models, are also utilized for the steady state simulation. The S-A turbulence model provided significantly better steady state results in the separated flow region than the k-ε model. The agreement between the experimental data and CFD prediction was better for the M=0.8 unsteady results than the M=0.5 on the suction surface. This could be attributed to a larger unsteady variation in the reattachment location for the M=0.5 results than the M=0.8. Finally, the time average of the unsteady skin friction coefficient is significantly different than the steady state value indicating nonlinear unsteady aerodynamics are significant for this analysis.

A CFD Investigation of IGV Flow Vectoring by Counter Flow Blowing

This paper describes a CFD analysis conducted to examine the impact of counter-flow blowing (CFB) on a row of uncambered, circulation-controlled (CC) inlet guide vanes (IGVs). In addition to a Coanda jet blown over the rounded trailing edge of the IGV blade, a second slot was placed on the pressure surface near the trailing edge to inject air upstream, i.e., in a counter-flow direction. Two blade shapes were modeled, each with four basic combinations of Coanda blowing and CFB, and the results were compared to flow turning and loss as modeled for a typical flapped IGV. The Advanced Ducted Propfan Analysis Code (ADPAC) was used to perform the 2-D simulations. While neither of the modified IGVs matched the 39-degree flow turning capability of the flapped IGV, CFB was predicted to augment flow turning by up to 5.3 degrees on the first geometry and up to 2.6 degrees on the second geometry. Cascade losses for the modified IGVs were more than twice as high as the flapped IGV, with the only exception being equivalent losses generated for Coanda-only blowing.Copyright

Experimental and Theoretical Frequency Response of Pressure Transducers For High Speed Turbomachinery

Recent advances in experimental capabilities have led to unsteady pressure measurements in high speed transonic turbomachinery facilities. Unsteady pressures are part of the forcing functions and blade response which contribute to high cycle fatigue failures in turbomachinery. Unsteady pressure measurements are used to validate unsteady CFD design codes, therefore, it is critical that the pressure measurement configuration frequency response limitations can be accurately determined. The pressure frequency capabilities of a typical high speed turbomachinery pressure measurement system are investigated both experimentally and theoretically. The research facility consists of an air to air shock tube, which generates a pressure step response for which various connecting tubes and termination volume configurations are investigated. Data is acquired and analyzed to determine the pressure measuring systems frequency performance. In addition, several classical theoretical models are utilized to compare with the experimental results. For current typical high speed turbomachinery surface pressure measurement applications, a lumped-parameter analysis provides excellent agreement with the obtained experimental results. Therefore, it is highly recommended that the lumpedparameter model be utilized in designing high speed turbomachinery surface pressure instrumentation. Currently, rotating high speed surface pressure measurements are extremely difficult and expensive to obtain, but there is a strong need for this data in the turbomachinery research community. So, new innovative high speed surface pressure measurement methods, like MEMS, fiber-optics, and piezoelectric crystal sensors, which hold the potential for avoiding the current transducer mounting limitations, should be explored.