High-speed Pressure Transducers Research Articles

The unsteadiness of tip leakage vortex breakdown and its role in rotating instability

The unsteadiness due to tip leakage vortex (TLV) breakdown was studied using a special experimental test campaign in parallel with numerical simulations. The back flow vortex (BFV), an isolated vortex caused by TLV spiral-type breakdown, was found to play a key role in rotating instability (RI). High-speed pressure transducers were used to measure the unsteady pressure field at the casing end wall of the blade in an isolated subsonic compressor rotor, which identified a low-frequency fluctuation at the near stall condition. A single-passage unsteady Reynolds-averaged Navier–Stokes simulation was used to study the evolution of unsteady flow structures, validated by the experimental measurements. Two distinct kinds of periodically unsteady flow were revealed by the simulations. A high-frequency fluctuation corresponding to 1.0 blade pass frequency (BPF) was caused by the spiral-type breakdown of the TLV. The other low-frequency fluctuation corresponding to 0.5BPF was caused by the feedback interaction between the BFV and the blade loading. The BFV was generated by the TLV breakdown, which was separated from the twisted vortex core of the TLV, and it moved downstream along the pressure side of the adjacent blade. A larger sized BFV reduced the local loading of the adjacent blade. The TLV was weakened as a consequence of the reduced loading, resulting in a smaller sized BFV. The blade tip loading was relatively less affected by the small sized BFV rather than the larger sized BFV. Therefore, the blade loading recovered and the size of the BFV increased, repeating the cycle. This feedback mechanism produced a pressure fluctuation with a frequency equal to 0.5BPF, which was closely related to RI.

High-speed imaging of wave modes in an RDC

Various operating modes have been observed in the rotating detonation combustor (RDC) studied here, as well as by others. These modes can be generally categorized into two groups: one or more co-rotating waves and counter-rotating waves. The combustor is able to stabilize both types of operation, however the stabilization mechanism remains unclear. In order to better understand these cases, these operational modes are investigated with high-speed pressure transducers installed in the combustor annulus combined with simultaneous high-speed video imaging of the natural luminosity of the detonation wave from the aft end of the RDC. These results confirm the operation in both the steady single wave mode as well as the counter-rotating waves mode. The presence of this mode further demonstrates the complex dynamics inherent in the stabilization of the detonation wave. As each wave propagates, they initially begin to weaken followed by re-strengthening after collision which serves to stabilize the mode. Differences in the velocities of the waves was also observed to result in a beating phenomenon expressed as an amplitude modulation of the pressure traces. High-speed video imaging of the high-temperature emission from H2O in the RDC annulus was used in conjunction with the pressure traces for a range of flow rates and equivalence ratios. Processing of the images into the wave frame of reference allowed for the identification of the average luminosity profile, characterized by a steep increase in natural luminosity near the wave front, followed by a trailing tail extending approximately half the annulus perimeter.

T63 Turbine Response to Rotating Detonation Combustor Exhaust Flow

This paper describes testing an axial turbine response when driven by a rotating detonation combustor (RDC). A T63 (C20-250) gas turbine is modified by replacing the combustor with a RDC. The stator vanes of the T63 are heavily instrumented for the measurement of flow enthalpy and pressure. The engine is run at multiple power levels with the stock combustor using JetA and hydrogen fuel. The engine is then modified to have an open loop configuration and is run with both the RDC and the stock combustor hardware with hydrogen fuel. Temperature pattern factor, flow unsteadiness, and turbine component efficiency are measured for all setups. High-speed pressure transducers show substantially higher unsteadiness generated by the RDC than the conventional combustor. RDC turbine component efficiencies are compared to the conventional combustor. Results suggest that RDC unsteadiness does not significantly impact turbine efficiency.

Experiments on the Interaction of a Fast-Moving Shock with an Elastic Panel

Experiments on the interaction of a fast-moving oblique shock with an elastic panel in turbulent supersonic flow at Mach numbers of 3 and 4 were conducted to enhance the understanding of coupled flow–structure phenomena and for the validation of coupled simulations. A complex and challenging experimental setup allowed achieving remarkable shock motion, changing the impingement point within a few milliseconds, to excite high-amplitude oscillations of an elastic panel. The flowfield was analyzed by using high-speed pressure transducers and high-speed schlieren photography. The panel deflection was measured by nonintrusive high-speed capacitive and laser distance sensors. Measurements were also conducted on a rigid panel to be used as reference for the elastic case, and to allow a more detailed analysis of the shock-wave/boundary-layer interaction on the panel, yielding insights into the spatial and temporal distribution of frequencies occurring in the interaction area. Oscillations of the elastic panel with an amplitude of about 1 mm were obtained.

Experimental results on unsteady shock-wave/boundary-layer interaction induced by an impinging shock

Experiments on shock-wave/boundary-layer interaction of an impinging shock on a flat panel at Mach numbers of 3 and 4 were conducted in the trisonic wind tunnel (TMK) of the Supersonic and Hypersonic Technologies Department at DLR, Cologne. To obtain high-frequency data, the model was equipped with 12 high-speed pressure transducers for measurements at 100 kHz and high-speed schlieren photography was used. The experimental setup is designed for quick rotation of the shock generator allowing testing at different ramp angles during one wind tunnel run. The static pressure distribution and high-speed pressure fluctuations in the interaction area were analysed with regard to the spatial and temporal distribution of occurring frequencies. At the beginning of the separation and near the reattachment, a strong increase of low-frequency fluctuations of up to 1 kHz was observed. In the separation area, higher frequencies were also excited. These results were compared to the frequencies and flow topology found in the high-speed schlieren videos.

Supersonic Retropropulsion Dynamic Data Analysis from NASA Langley Unitary Plan Wind Tunnel

A supersonic retropropulsion experiment was recently conducted at NASA Langley Research Center’s Unitary Plan Wind Tunnel, test section 2, for a range of Mach numbers from 2.4 to 4.6. A 5-in.-diam 70 deg sphere-cone forebody with a 9.55 in. cylindrical aftbody section was the experimental model used, which is capable of multiple retrorocket configurations. These configurations include a single central nozzle on the center point of the forebody, three nozzles at the forebody half-radius, and a combination of the first two configurations. A series of measurements were achieved through various instrumentation, including forebody and aftbody surface pressures, internal pressures and temperatures, and high-speed schlieren visualization. Several high-speed pressure transducers on the forebody and in the plenum were implemented to look at unsteady flow effects. The following work focuses on analyzing frequency traits due to the unsteady flow for a range of thrust coefficients for single-, tri-, and quadnozzle test cases at a freestream Mach number of 4.6 and an angle of attack ranging from 0 to . This analysis uses MATLAB®’s fast Fourier transform, Welch’s method (modified average of a periodogram), to create a power spectral density and analyze any high-speed pressure transducer frequency traits due to the unsteady flow.

Fluid bed characterization using Electrical Capacitance Volume Tomography (ECVT), compared to CPFD Software's Barracuda

Being able to accurately predict the performance and operation of multiphase flow systems continues to be a significant challenge. In order to continue the advancement of knowledge and to develop better models, a 10 cm diameter fluidized bed of 185 μm glass beads was used along with an Electrical Capacitance Volume Tomography (ECVT) sensor and high speed pressure measurements. Three dimensional images of the gas–solid flow structure were obtained and analyzed as well as frequency information from the high speed pressure transducers. The experimental data was then compared to four computational models performed with CPFD Software's Barracuda code exploring different techniques to handle the perforated distributor plate. • Barracuda CFD model results were compared to ECVT and pressure measurements. • Four different methods were used to model the distributor plate. • The results indicate that modeling the plenum produces the most accurate model as compared to experiments.

Dispersion and burning behavior of hydrogen released in a full-scale residential garage in the presence and absence of conventional automobiles

Abstract Experiments are described in which hydrogen was released at the center of the floor of a real-scale enclosure having dimensions of a typical two-car residential garage. Real-time hydrogen concentrations were monitored at a number of locations. The hydrogen/air mixtures were ignited at pre-determined local volume fractions ranging from 8% to 29%. The combustion behavior and structural effects were monitored using combinations of high-speed pressure transducers and ionization gauges, standard thermocouples, hydrogen sensors, and digital, infrared, and high-speed video cameras. Experiments were performed both for empty garages and garages with conventional automobiles parked above the hydrogen release location.

Acoustic Characterization of an Ethylene-Fueled Scramjet Combustor with a Cavity Flameholder

*† ‡ § ** †† ‡‡ The occurrence of combustion oscillations has recently raised serious concerns about the development of scramjet engines. Previous studies on supersonic combustion for high-speed airbreathing propulsion applications indicated that combustion may take place in subsonic regions, such as boundary layers and recirculation zones in flame-holding cavities. During this process, a longitudinal mode of thermoacoustic instability may develop in a spatial domain reaching from the shock train to the flame zone. The present work experimentally and analytically investigates such thermoacoustic instabilities inside an ethylene-fueled scramjet combustor with a recessed cavity flameholder. High-speed pressure transducers are utilized to record acoustic signals. The effects of fuel/air equivalence ratio, fueling scheme, and simulated flight conditions on the stability characteristics of the combustor are examined systematically. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. The measured oscillation frequencies agree well with the characteristic frequencies related to the acoustic feedback loop between the shock and flame and the acoustic-convective feedback loop between the fuel injection and flame.

Thermoacoustic limit cycles in a premixed laboratory combustor with open and choked exits

This paper presents an experimental and theoretical investigation of the response of a turbulent premixed flame during thermoacoustic limit cycle in a simple, laboratory combustor. The flame dynamics are examined using high-speed pressure transducers and CH∗ chemiluminescence. The so-called ‘interaction index’ and time delay between the acoustic velocity fluctuations at the flame holder and the flame’s overall heat release fluctuations are then determined. A wide range of operating conditions, traversing the combustor’s flammability limits in Mach number and equivalence ratio, are studied for four different combustor exits, including one where the exit is choked. In all cases the time delay correlates very well with the amplitude of the velocity fluctuations. There is also some correlation between the interaction index and these velocity fluctuations, but this is less clear. These results suggest a novel, nonlinear flame model, derived entirely empirically. An existing low-order thermoacoustic model is then extended to include convection and dispersion of entropy fluctuations downstream of the flame, enabling the effect of the choked nozzle to be examined. The novel nonlinear flame model is integrated into the low-order thermoacoustic model, and used to investigate the experimentally observed thermoacoustic limit cycles. The model correctly simulates the observed switch to a low-frequency, entropically driven instability observed when the combustor exit is choked.

The hydrodynamic ram pressure generated by spherical projectiles

An experimental study was made of the pressures generated by a single hydrodynamic ram event. A generic box type fuel tank simulator was fabricated to study the phenomenon and was capable of containing 3785.4 l of liquid thereby isolating the phases of the event from tank wall reflections, and enable separation of the effects of each phase. Tungsten, steel, and aluminum spherical projectiles were fired into the tank and the result was measured using high-speed pressure transducers, which were located throughout the tank, and compared to the pressure waves and events recorded inside the tank simulator with a high-speed video. It was determined that a high-pressure initial wave is generated at impact and is followed by the more gradual drag pressure around the projectile. The angular distribution of the initial wave and the variation with distance were investigated along with the effect of projectile mass on the drag pressure. This region of pressure decreases in the projectile wake, forming a trailing cavity that remains at a low pressure until the cavity begins to collapse. This results in the largest recorded pressure inside the simulator.

The application of PIV to the study of unsteady gas dynamic flow within pipes and at pipe discontinuities

This paper describes research using a pulse generator that provides discrete waves. The wave motion was recorded using a series of high speed pressure transducers and the ensuing movement of the gas particles within the pipes was recorded using digital PIV. Gas particle velocities of up to 250m/s were a regular occurrence and vortices of high angular velocity were a common problem with associated spin out of seed particles. Although flows were predominantly two dimensional, stereoscopic techniques were used to give optical access to otherwise inaccessible areas of flow. So far these techniques have been applied to the unsteady gas dynamic flow within plain pipes, at sudden expansions and contractions within pipes and at the end of a pipe open to the environment, both plain ended and when fitted with a bellmouth. Examples of all of these are presented in this paper.

Modelling of Unsteady Gas-Dynamic Flow in a Pipe and at Its Exit using CFD

Traditionally the simulation of the thermodynamic aspects of the internal combustion engine has been undertaken using one-dimensional gas-dynamic models to represent the intake and exhaust systems. Computational fluid dynamics (CFD) analysis of engines has been restricted to modelling of in-cylinder flow structures. With the increasing accessibility of CFD software it is now worth considering its use for complete gas-dynamic engine simulation. This paper appraises the accuracy of various CFD models in comparison with a one-dimensional gas-dynamic simulation. All of the models are compared to experimental data acquired on an apparatus that generates a single gas-dynamic pressure wave. The progress of the wave along a constant area pipe and its subsequent reflection from the open pipe end are recorded with a number of high-speed pressure transducers. It was found that there was little to choose between the accuracy of the one-dimensional model and the best CFD model. The CFD model did not require experimentally derived loss coefficients to represent accurately the open pipe end; however, it took several hundred times longer to complete its analysis. The best congruency between the CFD models and the experimental data was achieved using the RNG k-∊ turbulence model. The open end of the pipe was most effectively represented by surrounding it with a relatively small volume of cells connected to the rest of the environment using a pressure boundary.

Particle image velocimetry investigation of a finite amplitude pressure wave

Particle image velocimetry is used to study the motion of gas within a duct subject to the passage of a finite amplitude pressure wave. The wave is representative of the pressure waves found in the exhaust systems of internal combustion engines. Gas particles are accelerated from stationary to 150 m/s and then back to stationary in 8 ms. It is demonstrated that gas particles at the head of the wave travel at the same velocity across the duct cross section at a given point in time. Towards the tail of the wave viscous effects are plainly evident causing the flow profile to tend towards parabolic. However, the instantaneous mean particle velocity across the section is shown to match well with the velocity calculated from a corresponding measured pressure history using 1D gas dynamic theory. The measured pressure history at a point in the duct was acquired using a high speed pressure transducer of the type typically used for engine research in intake and exhaust systems. It is demonstrated that these are unable to follow the rapid changes in pressure accurately and that they are prone to resonate under certain circumstances.

Time-Resolved Fuel-Grain Port Diameter Measurementin Hybrid Rockets

A novel technique is presented for determining the instantaneous spatially averaged port diameter of solid-fuel grains in hybrid rocket motors. This technique requires measurement of the frequency of the Helmholtz oscillation of the motor and is based on the principle that this frequency is inversely proportional to the square root of the chamber volume. This technique was applied to a hybrid rocket motor burning paraffin wax with gaseous oxygen. The calculated variation of port diameter agreed well with the correlation for average regression rate, determined from mass loss during operation. A major advantage is that the only instrumentation required for implementing this technique is a high-speed pressure transducer or a photomultiplier tube.

Sound-Pressure-Level Variations in a Supersonic Rectangular Cavity at Yaw

An experimental investigation was undertaken to examine the effect of yaw angles on  ow oscillations that occur within an open cavity at M = 2. A rectangular cavity with a length-to-depth ratio of 2 and a planar aspect ratio of 11.83 was placed within a thick turbulent boundary layer with a corresponding Re = 7.3 10. Pressure– time histories were acquired at 25 separate yaw angles using high-speed pressure transducers. These signals were analyzed and the sound pressure levels and dominant frequencies determined. This study indicates that large changes in the sound pressure level occur as the yaw angle is changed from 0 to 70 deg. Further, evidence suggesting a mode switch from a transverse acoustic mode at yaw angles less than approximately 40 deg to a  uid dynamic mode for yaw angles >45 deg is presented.

Laser-induced shock waves in liquids

The interaction of TEA-CO2-laser pulses with liquids has been investigated using shadowgraphy and high-speed pressure transducers. It is shown that the absorption coefficient of the liquid at the laser wavelength defines the geometry of the shock wave. Peak pressures in the kbar range have been observed.