Wall Pressure Measurements Research Articles

Investigation of Combustion Characteristics in a Kerosene-Fueled Supersonic Combustor with Air Throttling

The paper presents the results of an experimental and numerical study on the combustion characteristics of a supersonic combustor operation enhanced by pilot hydrogen and air throttling, including ignition and combustion. Wall pressure measurements, schlieren images, and hydroxyl-radical planar-laser-induced-fluorescence (OH-PLIF) are used to obtain information on the flowfield and flame development. With the aid of air throttling, the kerosene is ignited by the pilot flame; and the kerosene flame remains stable even after the pilot hydrogen is switched off. The speed of the shock train generated by the throttling air and combustion is about in the expansion section but only about in the cavity region. The stable flame and flame blowout cases can be separated by a curve in a plot of the coexistence time of pilot hydrogen and throttling air versus the air throttling flux ratio. There is a special case lying on this curve for which the flame is unstable and will be blown out before the throttling air is switched off. It is also found that the position of the thermal throat is the same for all the subsonic combustion cases: about 595 mm downstream of the scramjet entrance.

Flowfield Reconstruction and Shock Train Leading Edge Detection in Scramjet Isolators

Within a scramjet, the location of the shock train will change as the backpressure changes. When the shock train leading edge moves out from the entrance of the isolator, the engine usually goes into unstart. A precise shock train leading edge detection technique is indispensable for controlling the shock train within the isolator. In this Paper, a reconstruction-flowfield-based shock train leading edge detection method is proposed. A flowfield reconstruction model based on a convolutional neural network provides instantaneous flowfield structure, and then image processing is carried out on these flowfield images to obtain the shock train leading edge. A supersonic direct-connect wind tunnel was used to collect all the data, which were then postprocessed to train the convolutional neural network model. The trained model successfully reconstructed the flowfield structure based on the measured wall pressure, and the reconstruction effects under different pressure transducer arrangements were compared. The flowfield reconstruction model provides a richer information source for shock train leading edge detection, and the detection accuracy can be greatly improved. The proposed method was compared with the pressure ratio method and the pressure increase method, and the detection performance under different pressure transducer arrangements was examined.

Interactions between a shock and turbulent features in a Mach 2 compressible boundary layer

Large field-of-view (FOV) particle image velocimetry experiments are conducted in the vicinity of a shock wave boundary layer interaction (SWBLI) at Mach 2. The current FOV covers up to 30 boundary layer thicknesses ( ), comprising of upstream and downstream regions relative to the SWBLI, thereby allowing the turbulent boundary layer and shock to be simultaneously captured. The relationship between the boundary layer features and the instantaneous shock location is directly quantified, with the aim of better understanding the mechanisms responsible for oscillation of the reflected shock. The results show that the reflected shock location is clearly influenced by the instantaneous state of the incoming boundary layer. It is found that passage of low-/high-momentum very-large-scale turbulent features through the SWBLI region causes the reflected shock to move upstream/downstream of the mean location. Moreover, interaction with the shock is found to introduce additional velocity fluctuations across a range of spanwise length scales within the boundary layer. The spanwise scales smaller than one recover within one downstream of the SWBLI region. However, at larger spanwise wavelengths, two persistent modes at approximately one and six are observed, where they remain correlated for a longer streamwise extent ( ) than the other spanwise modes. The wall pressure measurements indicate that the low-frequency fluctuations arising from the oscillating shock foot are due to dampening of high-frequency contents beyond the critical frequency associated with the unstable global mode. Thus, the results suggest that low-frequency pressure oscillations are not necessarily an independent phenomenon from the turbulent features entering the SWBLI region and interacting with the shock. Instead, a large scale separation between their dominant time scales is due to the critical frequency of the unstable global mode occurring at frequencies that are orders of magnitudes slower than the dominant frequency of the very-large-scale features.

Effect of Sensor Mounting and Flow History on Measured Wall Pressure Spectra

An experimental study of wall pressure fluctuations beneath a turbulent boundary layer was carried out on a plate model with a streamwise varying pressure gradient along the plate length. The wall pressure fluctuations were measured with flush-mounted Kulite sensors, and mean velocity measurements across the boundary layer were performed by using a single-wire hot-wire anemometer. A variety of streamwise pressure distributions were imposed by changing the position and the angle of attack of a NACA 0012 airfoil installed above the plate. The effect of the upstream flow history on the wall pressure spectra in the presence of the streamwise varying pressure gradient was studied. Spectral attenuation due to the spatial averaging associated with a finite sensor size was corrected via the Corcos correction. The results show that the Corcos correction to the measured Kulite spectra leads to an overprediction of the spectral intensity at medium frequencies and an underprediction at high frequencies. Hence, a new correction is proposed based on the measured spectral attenuation. Finally, the measurements also indicate that even slight imperfections in sensor mounting and relatively weak upstream flow disturbances can have a significant influence on the measured spectra of the wall pressure fluctuations.

Experimental Study of the Wall Pressure Distribution in a Convergent-Divergent Nozzle with Strut Injection

Wall pressure measurements are made in a convergent-divergent nozzle with strut injection to estimate the internal forces and moments. The strut facing the flow was of rectangular cross section and is placed at two thirds of the diverging length of the nozzle from the throat. The design exit Mach number of the nozzle is 1.84. The strut height varied at constant internal total pressure of 690 kPa. Experiments were conducted to explore the possibility of using a solid strut as an alternative to secondary fluid injection for thrust vector control. The wall pressure distribution of the nozzle is studied to interpret the flow interaction with the strut. The calculations based on the experimental data show that the presence of the strut and a variation in its height produce significant variations in side force and pitching moment which would be useful for thrust vector control.

Experimental and Computational Investigations on Flow Characteristics of Supersonic Ejector

Experimental and computational investigations have been conducted to characterize the flow field in the supersonic ejector, with and without secondary inlets of two sizes, at stagnation pressures of 2, 2.5, 3, 4 and 5 bar. In the first case, four secondary inlet openings each with 5 mm diameter are provided on the suction chamber circumferentially. In the second case, four secondary inlet openings each with 10 mm diameter are provided on the suction chamber circumferentially. These two cases are investigated separately, at all the stagnation pressures. The wall pressure variation is validated based on the k-ω SST turbulent model with experimentally measured wall pressure distribution at 2 bar, without secondary inlet flow. Suction is found to increase with the increase of stagnation pressure in the suction chamber. The inducted mass flow promotes the mixing in the shock-train zone, by triggering the significant momentum exchange between the primary flow and secondary flow. Results show that the pressure recovery takes place as the flow enters the divergent portion of ejector.

Oblique Shock Control with Steady Flexible Panels

Flexible panels deforming under pressure loads have been suggested as a passive form of adaptive oblique shock control. This study investigates oblique shock–boundary-layer interactions on a steady flexible panel in a Mach 2.0 flow. Experiments were performed in the Imperial College supersonic wind tunnel, where shock generators were used to produce an oblique shock followed by a corner expansion. A parametric study was conducted, exploring different shock impingement positions and shock–expansion distances. The steady aerostructural response is studied using schlieren photography, static pressure distributions, photogrammetry measurements, and surface oil flow visualization. Two-dimensional numerical simulations were performed to assess the effects of the flexible panel on downstream total pressure recovery. These were validated against experimental wall pressure distributions and measurements from a Pitot rake. Results show reductions in both separation length (of up to 40%) and stagnation pressure losses (of up to 10%) if the flexible plate is used. These improvements occur for a range of shock positions spanning approximately 50% of the panel length and for all the shock–expansion distances considered. A model that captures the flow physics responsible for these trends is proposed. The results highlight the potential of flexible panels for practical oblique shock control.

Interference Elimination of Wall Fluctuation Pressure Test Signal of High-Speed Train Based on Phase Space Reconstruction and Vibration Interference Model

Because of the complexity of high-speed train operation, the measured wall pressure of high-speed train will be affected by many factors, such as train vibration and environmental changes, which make the test signal inaccurate. The main interferences are the electromagnetic interference caused by the complicated electromagnetic environment and the sensor output interference caused by the train body vibration. In this paper, ensemble empirical mode decomposition (EEMD) combined with singular value decomposition (SVD) and the local projection de-noise algorithm with better effect on non-linear time series noise reduction are used to eliminate electromagnetic interference in test signal. Because of the frequency band aliasing of test signal and interference cause by vibration, it is difficult to eliminate vibration interference by the signal processing method. In this paper, a vibration interference model is established by model-train experiment and computational fluid dynamics (CFD) software to eliminate the vibration interference in the test signal. Moreover, the theoretical guidance for accurately extracting the wall pressure of high-speed train has been put forward.

Joint acoustic and wall-pressure measurements on a model A-pillar vortex

This study investigates the noise generation and radiation of a three-dimensional A-pillar vortex, which is a three-dimensional unsteady structure found on most automotive models. A small model is used to generate the vortex and is tested in an anechoic wind tunnel. The flow is validated against the literature using wall-pressure measurements: the maxima in the fluctuating pressure coefficient are found under the vortical structure. A 31-channel acoustic beamforming array is deployed to identify the noise sources on this model in 2 perpendicular planes. The A-pillar is identified to be the main noise mechanism over a wide range of frequency (between 1 and 8 kHz), with a monopolar radiation in a plane perpendicular to the flow. Some coherence between the fluctuating wall-pressure and the far-field radiation is found around 1 kHz, but only for wall-pressure sensors located in the area outside of the vortical structure, where the fluctuating pressure coefficient is low. An additional experiment with an external noise source is conducted and confirms that the wall-pressure fluctuations convected by the vortex structure mask the acoustic pressure fluctuations which result in a low coherence level in the area where the fluctuating pressure coefficient is the highest. This experiment shows that wall-pressure measurements need to be associated to microphone array measurements to carry out a thorough experimental aeroacoustic study of a complex 3D system such as the A-pillar vortex.

Aerodynamic Characteristics of the Blended-Wing-Body VTOL UAV

The aim of the present study was to investigate the effects of the forebody geometry on the asymmetric side forces, separations and vortex shedding behind the vehicles at high angles of attack. It reports wall pressure measurements to describe the asymmetric vortex system developing over an inclined cylindrical body exposed to a low-speed freestream.

Effects of Forebody Geometry on Side Forces on a Cylindrical Afterbody at High Angles of Attack

The aim of the present study was to investigate the effects of the forebody geometry on the asymmetric side forces, separations and vortex shedding behind the vehicles at high angles of attack. It reports wall pressure measurements to describe the asymmetric vortex system developing over an inclined cylindrical body exposed to a low-speed freestream.

Experimental study of hypersonic boundary layer transition on a flat plate delta wing

The boundary layer transition on a flat plate delta wing was investigated in the Mach 6 quiet wind tunnel via global temperature and wall pressure measurements as well as flow visualization. The results show that the boundary layer transition on side line is earlier than that on center line, and larger angle of attack promotes the boundary layer transition. As the Reynolds number increases, the transition position of boundary layer moves upstream. The hot streaks on the center line of delta wing at 0° angle of attack which are symmetrically distributed with respect to the center line at 10° angle of attack are suspected to be arised from the vortical flow rather than the boundary layer transition. High frequency disturbances occurring nonlinear phase coupling transfer the energy to low frequency structures, and finally the low frequency large scale structures induce to boundary layer transition. The intensity of nonlinear interaction prior to boundary layer transition is stronger than that in turbulent boundary layer. In addition, the unstable region of disturbance waves depends on the Reynolds number.

Applying phase averaging technique to analysis of unsteady twin vortex structure observed in tangential vortex chamber

This paper is devoted to experimental investigation of formation of precessing vortices pair in a vortex chamber with a cylindrical working area. Flow structure was registered by means of a high-speed visualization technique. Quantitative measurements characterizing the vortex properties were carried out using a 2D particle image velocimetry (PIV) technique synchronized with wall pressure measurements. The results show that two symmetrical precessing helical vortices are present downstream of the tangential nozzles.The phase- averaged analysis confirms the presence of large-scale twin vortex structure that visualized by small air bubbles dissolved in water.

Measurement of turbulent boundary layer unsteady wall pressures beneath elastomer layers of various thicknesses on a plate

The attenuation of turbulence induced wall pressure fluctuations through elastomer layers was studied experimentally and analytically. Wall pressure statistics were measured downstream from a backward facing step, with no elastomer present and beneath 2, 3, and 4 mm thick elastomers in a water tunnel facility. The step height, h, was 0.635 cm, and the wall pressures were measured at non-dimensional distances of x/h = 10, 24, 36 and 54 downstream from the step. The attenuation of the wall pressure spectra beneath the elastomer layers that was measured experimentally was then compared to analytical model predictions. An analytical elastomer transfer function, which models the transfer of turbulent boundary layer wall pressures on the surface of an elastomer to the normal stresses through the elastomer, was applied to the turbulent boundary layer wall pressure spectra measured in the absence of an elastomer layer and compared to spectra measured beneath the 2, 3, and 4 mm thick elastomers. The attenuation of the turbulent boundary layer wall pressure spectra through the elastomer layer using the analytical elastomer transfer function was in excellent agreement with the attenuation measured experimentally through all thicknesses of elastomer and at all free stream velocities at which the experiments were performed.The attenuation of turbulence induced wall pressure fluctuations through elastomer layers was studied experimentally and analytically. Wall pressure statistics were measured downstream from a backward facing step, with no elastomer present and beneath 2, 3, and 4 mm thick elastomers in a water tunnel facility. The step height, h, was 0.635 cm, and the wall pressures were measured at non-dimensional distances of x/h = 10, 24, 36 and 54 downstream from the step. The attenuation of the wall pressure spectra beneath the elastomer layers that was measured experimentally was then compared to analytical model predictions. An analytical elastomer transfer function, which models the transfer of turbulent boundary layer wall pressures on the surface of an elastomer to the normal stresses through the elastomer, was applied to the turbulent boundary layer wall pressure spectra measured in the absence of an elastomer layer and compared to spectra measured beneath the 2, 3, and 4 mm thick elastomers. The attenuation of...

Experimental Research on Wall Pressure Distribution in C-D Nozzle at Mach number 1.1 for Area Ratio 3.24

In this experimental investigation the work reported is about the influence of control on the flow field in the suddenly expanded duct at low supersonic Mach number. A Convergent-divergent (CD) nozzle was designed and fabricated out of brass material assembled with the suddenly expanded duct which was also made of brass material. At the re-circulation zone, the flow field was controlled by using the micro jets of 1 mm diameter as an orifice and the control was arranged at an interval of 90 degrees at 6.5 mm from the central axis of the main jet. The measured wall pressure distribution was presented for Mach number 1.1 for the duct diameter of 18 mm leading to the area ratio 3.24. The L/D ratio of the duct was varied from 1 to 10, and the nozzle pressure ratio (NPR) considered for the experiments was from 3, 5, 7, 9 and 11. The present results have demonstrated that the micro jets do not influence the flow field in the duct adversely and the flow field remained identical in the presence of control or absence of control

Outdoor measurement of wall pressure on cubical scale model affected by atmospheric turbulent flow

Most studies on indoor ventilation have utilized wind-tunnel experiments (WTEs) or computational fluid dynamics based approaches under well-controlled flow conditions. However, the effects of urban boundary layer flow with variable wind directions and various turbulence scales on the ventilation driven by the pressure differences between the upwind and downwind sides of a building within a block array is still under discussion. Therefore, we conducted outdoor experiments at comprehensive outdoor scale model (COSMO) experiment sites in an urban climate to clarify the relationships between the building wall pressure differences and the approaching flow. The pressure coefficients for the outdoor site were comparable with those obtained during previous WTEs. Accordingly, temporal variations in the wind speed and pressure coefficient on the target block were investigated in detail using low-pass filtering operations. The relationships between the filtered wind speed and the pressure differences indicate that the slower temporal variations in the wall pressure showed good agreement with the filtered approaching flow. In addition, the correlation coefficient between the filtered wind speed and the wall pressure differences quantified the apparent coherence between the turbulent flow and the ventilation rate. Furthermore, the statistics of the ventilation rate were determined based on the conventional model to clarify the effects of the turbulent flow on the natural ventilation rate. The ratios between the mean and short-term ventilation rates imply that the short-term ventilation rate presented dramatic temporal fluctuations owing to the various scale turbulence generated by the atmospheric flow.

Unsteady Flow Numerical Simulations on Internal Energy Dissipation for a Low-Head Centrifugal Pump at Part-Load Operating Conditions

Dredge pumps are usually operated at part-load conditions, in which the low-solidity centrifugal impeller could experience large internal energy dissipation, related to flow separation and vortices. In this study, SST k-ω and SAS-SST turbulence models were used, in steady and unsteady simulations, for a low-head centrifugal pump with a three-bladed impeller. The main focus of the present work was to investigate the internal energy dissipation in rotating an impeller at part-load operating conditions, related to flow separation and stall. The unsteady nature of these operating conditions was investigated. Performance experiments and transient wall pressure measurements were conducted for validation. A methodology for internal energy dissipation analysis has been proposed; and the unsteady pressure fluctuations were analyzed in the rotating impeller. The internal power losses in the volute and the impeller were mostly found in the centrifugal pump. The rotating stall phenomenon occurred with flow separation and detachment at the part-load operating condition, leading to a dissipation of the internal energy in the impeller. The rotating impeller experienced pressure fluctuations with low frequencies, at part-load operating conditions, while in the design operating condition only experienced rotating frequency.

Investigation of Unsteady Pressure Fluctuations in Flow over Backward-Facing Step

Flow over a backward-facing step at , based on the step height, is simulated using the Divergence Free Synthetic Eddy Method of Poletto et al. (“A New Divergence Free Synthetic Eddy Method for the Reproduction of Inlet Flow Conditions for LES,” Flow, Turbulence and Combustion, Vol. 91, No. 3, 2013, pp. 519–539.), implemented in OpenFOAM® 1606, which preserves the instantaneous fluctuating pressure field in the domain, which is normally lost. Examination of the pressure spectra at various positions normal to the wall is well represented by wall-pressure measurements at all but the highest frequencies examined here. Various spectral peaks corresponding to shear-layer movement, discrete vortex shedding from the step, and numerous shear-layer instabilities are investigated using power and cross-spectral measurements of the fluctuating wall pressures. Convective velocities obtained from the cross-spectral phase indicate that the lowest frequencies convect slowly, whereas the higher frequencies near the measured spectral peaks convect at . There is an increase in convective velocity as the measurement locations are moved farther downstream. Additionally, an increase in peak frequency was found in this examination; however, the increase in convective velocity (approximately equal to 33%) and frequency (approximately equal to 7–11%) calculated at different streamwise positions were not found to be directly proportional, indicating that the increase in peak frequency cannot be solely explained by flow acceleration but that the structures are likely undergoing streamwise or spanwise deformation as well.

Assessment of Presumed/Transported Probability Density Function Methods for Rocket Combustion Simulations

Combustion models of different fidelity are applied to a seven-element gaseous methane/oxygen subscale rocket combustion chamber. The covered region of thermo-chemical states is analyzed for two non-adiabatic flamelet-based methods, from which one is well established and the other one has just recently been proposed for hydrogen/oxygen combustion. Of particular interest is their applicability in situations with strong heat losses. Consequently, the tabulated combustion models are linked with a presumed probability density function approach, and the results are assessed using a transported probability density function method. An analytically reduced chemical reaction mechanism with 13 species and 73 reactions is used for all models in order to allow for direct comparison of turbulence–chemistry interaction. The results are evaluated with respect to temperature, gas composition, and wall heat transfer and they are brought in context with their respective computational costs. Finally, the results are compared to available experimental data in terms of wall heat flux and pressure measurements. Locally and temporally resolved results provide additional insights to the experiment.

Cavitation bubble wall pressure measurement by an electromagnetic surface wave enhanced pump-probe configuration

We report on the measurement of the pressure associated with a shock wave within a very thin layer (100 nm) in proximity of a boundary surface. In the experiments, the shock wave was emitted by a cavitation bubble generated by a pulsed pump laser in water. We developed a pump-probe setup based on the detection of the light scattered at the surface of a one-dimensional photonic crystal, which was purposely designed to sustain a surface electromagnetic wave in the visible range and to enhance the optical response. In order to better understand the phenomenon, we implemented numerical simulations to describe the light scattering intensity distributions through a modified Rayleigh's method. We report, with a LoD of ∼0.1 MPa, the measurements of the pressure at a surface in the presence of a laser-induced cavitation bubble generated at different distances from the surface and for different pulse energies.