Base Pressure Fluctuations Research Articles

Base pressure fluctuations on levitated freestream-aligned circular cylinder

Base pressure fluctuations associated with the large-scale wake structures behind a freestream-aligned circular cylinder and aerodynamic force fluctuations related to them are experimentally investigated in the wind tunnel tests. Measurements at ReD=6.97×104 and 1.04×105 were conducted using pressure-sensitive paint (PSP) and a magnetic suspension and balance system (MSBS) for creating a supportless condition. The obtained pressure fields were mainly analyzed by a modal decomposition combining azimuthal Fourier decomposition and proper orthogonal decomposition (POD). The pressure fluctuations caused by large-scale vortex shedding were observed from the results of frequency analysis for mode coefficients. The states of the fluctuations were classified into three patterns, which are anticlockwise/clockwise circular and flapping patterns. These patterns have been observed in the previous studies for velocity fluctuations in the wake of a freestream-aligned circular cylinder. The conditional sampling analysis revealed that the trend in the amplitude of the pressure fluctuations is different by the state, and the flapping pattern causes a large pressure difference across the cylinder axis. Furthermore, the relationship between the antisymmetric pressure fluctuations and lift fluctuations, which act in the lateral direction of the cylinder, is confirmed.

Active control of flow over a backward-facing step at high Reynolds numbers

This work provides new insight into the transient flow effects of forcing a backward-facing step flow at high Reynolds number. The detailed flow structure and surface pressure is investigated over the range 118,000⩽ReH⩽472,000 using time-resolved particle-image velocimetry. Notably, this research considerably extends the Reynolds number ranges of previous studies. The effect of periodic shear-layer forcing on the global mean and fluctuating flow structure is examined over the forcing frequency range 0.036⩽StH⩽1.98. The effect of forcing on the base pressure, which has received relatively little attention, is also detailed. These studies allow us to characterise the relationship between the forcing frequencies, with a particular focus on frequencies near and significantly above the shear-layer instability. Importantly, despite the high Reynolds number, we are able to perturb the shear layer to control both the reattachment length and base pressure. Forcing at frequencies close to the shear-layer instability results in a significant reattachment length reduction, as has been well reported, with a corresponding base pressure reduction of up to 45%. At the highest forcing frequency, an increase in mean base pressure of 9.7% is found, with a corresponding increase in reattachment length of 3.9% over the unforced case. This high-frequency forcing reduced initial growth of the shear-layer instability and stabilised the latter half of the reattachment zone. This enabled more flow entrainment upstream to the step-base, resulting in a mean base-pressure increase. Although, this came at the cost of triple the base-pressure fluctuations, an important insight for practical flow-control applications. The spatial distribution of spectral power for key instabilities in the flow, and the influence of forcing on these distributions, is also examined. To our knowledge, the spatial distribution of these key instabilities, with or without control, has not been previously presented for high Reynolds numbers (ReH>104).

Mitigation of Antisymmetric Mode and Drag With Base Cavities

Abstract Experiments were carried out to examine the impact of base cavities on the base pressure fluctuations and total drag of a cylindrical afterbody for freestream Mach numbers 0.6–1.5. Significant improvement in the base pressure and a substantial reduction in the afterbody drag was noticed in the presence of a base cavity at subsonic Mach numbers. However, on increasing the cavity length beyond a certain value, its performance deteriorates. At supersonic Mach numbers, their effectiveness drops drastically. Tones in the spectra can be classified into two types depending on the dominant azimuthal mode, which is either 0 or 1 and are referred to as symmetric and an antisymmetric mode, respectively. Spectra at subsonic Mach numbers exhibit tones, which are related either to mode 0 or 1. However, at supersonic Mach numbers, only tones related to mode 0 exist. The base cavity either effectively suppresses the antisymmetric mode or modifies it into a symmetric mode resulting in mitigation of the tones related to antisymmetric mode.

Effect of Passive Flow Control Devices on Base Pressure for Mach Numbers Between 0.5 and 1.4

Abstract Experimental studies are carried out on an axisymmetric cylindrical base body for six freestream Mach numbers between 0.54 and 1.41. Unsteady pressure is measured on the base surface using high-frequency response Kulite pressure transducers. The effect of passive flow control devices on the mean base pressure and the unsteady characteristics of base pressure have been studied. A blunt base, a conventional cavity device, and three different ventilated cavity devices have been tested along with four different rounded base lip devices. A total of 20 different base geometric modifications are tested at 6 freestream Mach numbers resulting in 120 test cases. The cavity devices improve the base pressure as compared to the blunt base case, particularly for freestream Mach numbers more than 0.98. Among all the cases considered, a maximum increase of 8.6% in the base pressure coefficient is noticed for the normal ventilated cavity device as compared to the blunt base case for freestream Mach number of 1.22. The power spectral density of base pressure fluctuations revealed the dominant peaks on the base surface. The Strouhal number associated with the coherent structures developing in the shear layer varies between 0.2 and 0.27 for the six freestream Mach numbers considered. In the presence of cavity devices, dominant peaks are observed for Strouhal numbers between 1 and 5. The root-mean-square, skewness, kurtosis of the base pressure fluctuations for all the cases are presented. Maximum reduction in base pressure fluctuation is observed for the normal and inclined ventilated cavity device configuration test cases.

Modes of Base Pressure Fluctuations: Shape, Nature, and Origin

An experimental study was carried out to examine the unsteady pressure field on the base of a cylindrical afterbody at freestream Mach numbers ranging from high subsonic to low supersonic speeds. The objective was to gain insights into spatial modes of base pressure fluctuations by identifying their shape and nature, and the mechanism of their origin using spectral proper orthogonal decomposition. It is observed that 90–95% of the cumulative energy content resides within the first three modes at all Strouhal numbers for all freestream Mach numbers. Further, at subsonic Mach numbers and for Strouhal numbers , the shape of the modes indicates that their nature is symmetric, and the mechanism for the origin of the dominant mode is the pulsing action of the recirculation bubble. However, for Strouhal numbers , the shape of the modes is asymmetric, indicating that their nature is antisymmetric, and the mechanism associated with the dominant mode is the radial displacement of the recirculation bubble coupled with the motion of structures within the recirculation bubble. At supersonic Mach numbers and for all Strouhal numbers, the shape of the modes indicates that they are symmetric, and the dominant mechanism is the pulsing action of the recirculation bubble.

Mach Number Effect on Symmetric and Antisymmetric Modes of Base Pressure Fluctuations

An experimental study aimed at evaluating the influence of Mach number on the base pressure fluctuations of a cylindrical afterbody was performed over a wide range of Mach numbers from subsonic to supersonic speeds. Time-averaged results indicate that the coefficient of base pressure drops with the increase in the freestream Mach number at subsonic speeds and increases at supersonic Mach numbers. The coefficient of root-mean-square of the pressure fluctuations follows a decreasing trend with the increase in the Mach number. Examination of the spectra reveals different mechanisms dominate the pressure fluctuations from the center to the periphery of the base as well as with the change in the Mach number. Analysis of the azimuthal coherence indicates that all the dominant tones in the spectra can be classified either into a symmetric or an antisymmetric mode at subsonic Mach numbers. However, at supersonic Mach numbers, all the dominant tones in the spectra are symmetric in nature. The results from the cross-correlation suggest that two possible mechanisms of recirculation bubble pulsing and convective motions/vortex shedding are driving the dynamics on the base at subsonic Mach numbers. However, at supersonic Mach numbers, only single mechanism of the recirculation bubble pulsing dominates. Moreover, it indicates that the symmetric mode is associated with the dynamics of the recirculation bubble and the antisymmetric mode is related to the convective motions/vortex shedding.

Base Pressure Fluctuations on Typical Missile Configuration in Presence of Base Cavity

Pressure fluctuations in the base region of a typical missile configuration at a freestream Mach number of 0.7 are examined experimentally in the presence and absence of a base cavity. The objective was to characterize the pressure fluctuations and explain the influence of base cavities on their behavior. Experiments include unsteady pressure measurements at six azimuthal locations. Substantial variation in the characteristics of pressure fluctuations is noticed along the azimuthal direction due to the asymmetry of the model. The base cavities are seen to enhance the base pressure and reduce the root mean square of the pressure fluctuations. Higher-order moments show diminishing trend as the length of the base cavity is increased. Spectra in the absence of cavity exhibit three types of narrow-band tones based on the Strouhal number. Type I tones disapper from the spectra for small-length cavities, whereas type II tones get suppressed. On the other hand, for the large-length cavities, type I tones get enhanced, whereas type II tones disappear. Autocorrelation and cross-correlation plots exhibit a large-amplitude, low-frequency oscillation that exists even after large separation times for the larger-length cavities, and virtually no such oscillations are visible for the small-length-cavity models.

Characteristics of base pressure fluctuations of a typical missile configuration with a propulsive jet

Experiments were carried out to investigate the pressure fluctuations in the base region of a typical missile configuration at a freestream Mach number of 0.7 in the presence and absence of a propulsive jet. The objective was to characterize these pressure fluctuations and identify the underlying mechanisms responsible for their origin. Experiments included unsteady pressure measurements at six azimuthal locations and schlieren visualization. Substantial variation in the characteristics of pressure fluctuations is noticed along the azimuthal direction due to the asymmetry of the model. The probability density function (PDF) of pressure fluctuations indicates a significant drop in the peak PDF value and a substantial broadening at the highest nozzle pressure ratio (NPR). With an increase in NPR, the skewness decreases and approaches a Gaussian value while the kurtosis decreases and is seen to go below that of a Gaussian. Spectra of the pressure fluctuations indicate that the narrowband tones can be classified into three types based on the Strouhal number. Two types of tones among the three aforementioned are seen to be suppressed in the presence of the jet. Cross-correlation plots in the absence of the jet exhibit a large amplitude, low frequency oscillation which exists even after large separation times. On the other hand, in the presence of the jet, the oscillations dampen rapidly and vanish as the separation time increases. Finally, an attempt has been made to identify the possible primary mechanisms that are driving the base pressure fluctuations based on the observed characteristics of the acquired unsteady pressure signal.

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.

Control of base pressure for a boat-tailed axisymmetric afterbody via base geometry modifications

An experimental study was conducted to assess the effectiveness of base geometry modifications in controlling the mean and unsteady base pressure development on a circular-arc 12° boat-tailed afterbody at zero angle of incidence under jet-off and jet-on conditions. Tests were carried out for the freestream Mach number range of 0.6 to 1.06 with nozzle pressure ratios for jet-on conditions varying from 1 to 12. Three base geometry configurations, namely, sharp base (or baseline case), rounded base and base cavity were studied. In jet-off condition, relative to the baseline, an increase in coefficient of base pressure by approximately 53% and 42% in the entire Mach number range was observed for the rounded base and base cavity configurations, respectively. Further, in jet-on condition, the rounded base continued to show better performance up to a nozzle pressure ratio of 8 for M∞≤0.9. For M∞=1.06, however, the base cavity is seen to perform much better than the rounded base configuration. With regard to base pressure fluctuations, the base cavity configuration consistently shows maximum reduction in the amplitude that is also accompanied with suppression of high-frequency fluctuations for both jet-on and jet-off conditions.

High-frequency forcing of a turbulent axisymmetric wake

A high-frequency periodic jet, issuing immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body, its axis aligned with the free stream. It is shown that the base pressure may be varied more or less at will: at forcing frequencies several times that of the shear layer frequency, the time-averaged area-weighted base pressure increases by as much as 35 %. An investigation of the effects of forcing is made using random and phase-locked two-component particle image velocimetry (PIV), and modal decomposition of pressure fluctuations on the base of the model. The forcing does not target specific local or global wake instabilities: rather, the high-frequency jet creates a row of closely spaced vortex rings, immediately adjacent to which are regions of large shear on each side. These shear layers are associated with large dissipation and inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer mode approaches unity gain, is roughly five times the shear layer frequency.

Feedback control for form-drag reduction on a bluff body with a blunt trailing edge

AbstractThe objective of this numerical study is to increase the base pressure on a backward-facing step via linear feedback control, to be ultimately translated to a drag reduction on a blunt-based bluff body. Two backward-facing step cases are simulated: a laminar two-dimensional (2D) flow at a Reynolds number of ${\mathit{Re}}_{\theta } = 280$, and a turbulent three-dimensional (3D) flow at ${\mathit{Re}}_{\theta } = 1500$ using large-eddy simulation. The control is effected by a full-span slot jet with zero-net-mass-flux, and two jet locations are examined. Linear system identification is performed to characterize the flow response to actuation, used to synthesize a control law. The control strategy is based on the premise that an attenuation of the instantaneous pressure fluctuations on the base of the step should lead to an increase in the time-averaged base pressure. Open-loop harmonic forcing is examined within a broad frequency range for both the 2D and 3D flows, which are found to respond differently to actuation. The controllers based on disturbance attenuation lead to sensible increases in base pressure (up to 70 % in 2D and 20 % in 3D) with higher efficiency than the best results achieved in open-loop. The results support the conjecture about the link between the base pressure fluctuations and mean, although it is shown that such a black-box model approach is not suitable for optimization without further physical insight.

Linear-Plug Flowfield and Base Pressure Development in Freestream Flow

An experimental investigation has been carried out to study the linear-plug-nozzle flowfield development with and without freestream flow. The effect of truncation (40%), plug contour, and side plates on base pressure development has been also studied. Relative to no freestream condition, the higher outward deflection of the initial portion of the free-jet boundary in the presence of freestreamprevents the overexpansion shock from the upper portion of the inner nozzle to impinge on the plug surface, modifying the flow development downstream of the overexpansion shock. This reduces the altitude-adaptive character of the plug nozzle and also influences the base pressure value, which is seen to decrease with increase in freestreamMach number. The nozzle pressure ratio for base wake closure is also found to be sensitive to the plug configuration. Contouring the plug significantly enhances the base pressure value, but a smaller base initiates an early wake closure. The addition of side plates results in an early base wake closure in each case. The characteristic frequency of base pressure fluctuations is found to be strongly dependent on plug configuration and base region size.

Measurements of Fluctuating Pressures on a Circular Cylinder in Subsonic Cross Flow

A circular cylinder is tested in crossflow over the subsonic speed range. Time-resolved pressure distributions give information on surface pressure fluctuations; the corresponding drag and base drag coefficients are provided. Measured base pressure fluctuations at low Mach numbers are in agreement with the findings of other researchers. Flow changes at higher subsonic velocities and into the transonic range are described. Illustrations are drawn from the observations of other researchers, enabling physical explanations to be given to assist toward a more general modeling of the problem. At Mach numbers above 0.6 the changing strength of the vortices reduces the base drag coefficient up to a Mach number of 0.9, at which the onset of sonic flow increases the drag. Strouhal number variation is compared with the measurements of other authors. The paper concentrates on providing reliable measurements of vortex shedding and base pressure over the subsonic speed range rather than attempting to provide universal correlations.

Time-Series and Time-Averaged Characteristics of Subsonic to Supersonic Base Flows

Physics of cylindrical base flows ranging from subsonic to supersonic speeds at zero angle of attack are computationally investigated. Time-series and time-averaged investigations of base flows show distinctive characteristics at subsonic, transonic, and supersonic regimes. Normalized time-averaged base pressure decreases proportionally with respect to increasing freestream dynamic pressure in the subsonic regime of M ∞ 1.5. Normalized base pressure fluctuations sharply increase at transonic speeds, whereas they decrease with increasing freestream Mach number at subsonic and supersonic speeds. Appearance of unsteady local shock waves change the characteristics of base pressure distinctively at the transonic speeds. Spectra of the base pressure show one clear peak at subsonic speeds (related to the shear layer dynamics), two clear peaks at transonic speeds (related to the shear layer dynamics and its subharmonic), and three major peaks at supersonic speeds (related to the shear layer dynamics, its subharmonic, and an additional mechanism). Instability of the free shear layers has dominant influence on the overall base flowfield over a wide range of Mach numbers ranging from subsonic to supersonic speeds. However, at supersonic speeds, an additional mechanism of instability within the recirculating region is possibly at work and has dominant influence on the flowfield. The dominant mechanisms significantly cause the strong Mach number dependence of the high-pressure region which is strongly related to the base pressure. The spectrum analysis suggests that the substantial base pressure fluctuations are caused by the pulsing of the flow inside the recirculating region.

Sub-Boundary-Layer Disturbance Effects on Supersonic Base-Pressure Fluctuations

Fluctuating base pressures were measured for an axisymmetric blunt afterbody modified with two different sub-boundary-layer disturbance tabs, a triangle-tab configuration, and a strip-tab configuration, in a Mach 2.46 flowfield. Normalized rms levels indicate that base-pressure fluctuations increase with addition of the triangle tabs and decrease with the addition of the strip tab. Power-spectral-density (PSD) estimates recorded at the two outermost radial locations for the triangle-tab configuration demonstrated that the fluctuation energy increased when compared to the no-tab case at nearly all frequencies, particularly at frequencies above 1 kHz. This suggests that the triangular tabs energize turbulent structures in the free shear layer because of the introduction of streamwise vorticity. It was also found that a prominent peak in the PSD estimate at the outer two radial locations, found in the no-tab case, is absent for the triangle-tab configuration. Dual-transducer measurements for both tab configurations suggest that correlated pressure fluctuations are realized at most locations across the base simultaneously, lending credence to the idea that a global mechanism acts as a source of the base-pressure fluctuations.

Time-Series Analysis of Supersonic Base-Pressure Fluctuations

An investigation into the dynamic nature of supersonic base-pressure fluctuations was conducted. Two axisymmetric models, a blunt and a boattailed afterbody, were studied at a freestream Mach number of 2.46 and zero angle of attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers. Blunt afterbody measurements showed rms pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates at the two outermost radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850 Hz (Sr = 0.094). RMS pressure fluctuations on the boattailed afterbody were on the order of 4.3% of the average base pressure. The PSD estimates at the two outermost locations contained a more defined and larger peak centered near 800 Hz (Sr = 0.089) as compared to the blunt base. The two outermost radial locations on both afterbodies are subjected to similar pressure histories and, hence, display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outermost locations, resulting in low values of coherence. Possible mechanisms for the substantial pressure fluctuation energy contained at low frequencies are pulsing or flapping of the recirculation region, whereas higher-frequency energy could be attributed to large-scale turbulent structures in the shear layer.

On the vortex shedding from a circular cylinder in a linear shear flow

The effects of the shear level on the vortex shedding from a circular cylinder with its axis normal to the plane of the shear profile were investigated experimentally in the Reynolds number regime of 1.0×104 to 8.8×104. The experiments were conducted in a computer controlled multiple-fan wind tunnel, in which the velocity gradient of the linear shear flow could be altered easily. The normalized shear parameter, β, was varied from 0 to 0.3. It was found that the Strouhal number was not sensitive to the shear parameter, while the time-mean base pressure increased with the shear parameter and the root-mean-square magnitude of the base pressure fluctuations decreased with the shear parameter significantly.

Subsonic Base Pressure Fluctuations

Subsonic Base Pressure Fluctuations

Base pressure fluctuations

Base pressure fluctuations