Unsteady Pressure Field Research Articles

Investigation of the unsteady surface pressure field under a Mach 2 compression-ramp shock/boundary-layer interaction

The shock/boundary-layer interaction induced by a 20° compression ramp in a Mach 2 flow was investigated using fast pressure-sensitive paint with a bandwidth of about 10 kHz. The mean separated flow length-scale is about two upstream boundary layer thicknesses, which indicates the interaction is weak. The primary analysis consists of cross-correlations, coherence, and time-domain filtering. Two different frequency bands were investigated: low-frequency (f < 2000 Hz; StL < 0.1) and mid-frequency (0.1 < StL < 0.26). The low-frequency band time sequences and coherence reveal the shock-foot motion is mainly correlated with the reattachment region, which is indicative of the well-established breathing motion of the separation bubble. The breathing motion is observed to occur locally and globally (spanwise-averaged). Furthermore, in the low-frequency band, fluctuations in the upstream boundary layer are moderately correlated with the reattachment region fluctuations, but show no correlation with the intermittent region fluctuations. In the mid-frequency band, the intermittent region, separation bubble and reattachment region all exhibit significant correlation with the upstream boundary layer fluctuations, with the upstream fluctuations leading. The time-sequences in this frequency band reveal broad regions of pressure fluctuations that sweep through the interaction and affect the entire interaction. There is no known turbulent source for such large-scale fluctuations and they are believed to be due to a wind tunnel phenomenon. It is concluded that the dominant low-frequency breathing motion follows an oscillator model, but there remain significant correlations to upstream fluctuations that are not tied to the dominant breathing motion and seem to follow an amplifier model.

Dynamics and spectral character of unsteady pressure field on afterbody of generic space launcher: Transonic flows

The unsteady pressure field over an axisymmetric backward-facing step was investigated experimentally at transonic freestream Mach numbers of 1.05, 1.2, and 1.4. The study was aimed at examining the influence of transonic freestream Mach numbers on the spatio-temporal character of the unsteady pressure field and on the dominant modes/mechanisms driving it. Surface flow visualization, schlieren, and unsteady pressure measurements were carried out as part of the experimental investigation. From oil flow visualization and schlieren, the reattachment region was identified, and consequently, the mean reattachment length was estimated. The mean reattachment length shows an increase with the increase in freestream Mach number. The coefficient of mean pressure along the rearbody imitates a classical backward-facing step flow profile and can be divided into three distinct regions. The peak values of the coefficient of mean pressure and the coefficient of root mean square of the fluctuation are seen to decrease with an increase in the freestream Mach number. Conventional spectral analysis reveals that as the freestream Mach number increases, the dominant peak in the spectra shifts to lower frequencies. From the spectra, three dominant fluid dynamic mechanisms depending on the freestream Mach number have been identified. Proper Orthogonal Decomposition (POD) analysis shows that 79–84 % of the total energy contribution comes from the first six modes. The temporal dynamics of the POD modes indicate three prominent mechanisms are responsible for the unsteady pressure field. Spectral analysis of POD modes indicates that the spectra are primarily driven by the first three POD modes for freestream Mach number of 1.05 and the first two modes for freestream Mach numbers of 1.2 and 1.4. Moreover, it reveals the presence of three dominant modes, and the freestream Mach number strongly dictates the dominant mode that is driving the pressure field.

Unsteady Pressure Field in a Porous Bed with a Weakly Compressible Framework in Nonlinear Radial Filtration of Fluid in It

Unsteady Pressure Field in a Porous Bed with a Weakly Compressible Framework in Nonlinear Radial Filtration of Fluid in It

Fluid–structure interaction of a marine rudder at incidence in the wake of a propeller

The structural response of a rudder in the wake of a marine propeller is investigated by one-way fluid–structure interaction approach. The unsteady pressure field gathered by detached eddy simulations is provided to a structural solver for the computation of deformations and stresses of the rudder. The study compares the structural response of the rudder at neutral and two equal and opposite rotations, which are representative of design conditions in straight motion and maneuvering conditions that are experienced under the action of the autopilot for course control or weak maneuvering. The analysis sheds light on the different structural behavior at the two opposite rotation angles, caused by asymmetrical variation along the span of the rudder of the angle of attack induced by the propeller slipstream, by considering the different role played by the tip and hub vortical systems. The test case consists of a rudder with a rectangular plane area and National Advisory Committee for Aeronautics 0015 sectional profile located past the E779A propeller. The propeller operates at low loading conditions, and the rudder is set at incidence δ=0∘,±4∘. The study shows that the response of the rudder is driven by flap and torsion and is asymmetric for the two and opposite rotations. The mean deformation and vibratory response are magnified for δ=−4∘ by at least 70% and 20% for the lateral and edgewise deflections, respectively, with respect to the opposite rudder incidence. In general, the excitation generated by the tip vortex is stronger than that of the hub vortex. In the most critical condition, at δ=−4∘, the excitation associated with the tip vortex is nearly double that of the hub vortex.

Transonic compressor Darmstadt Open Test Case – unsteady aerodynamics and stall inception

Predicting unsteady aerodynamics and related phenomena in the vicinity of the stability limit of transonic compressors has been a challenging topic within the turbomachinery community since decades. To improve numerical tools and prediction capabilities the Transonic Compressor Darmstadt Open Test Case has been introduced to provide a reliable validation test data set for steady and unsteady compressor aerodynamics. This work investigates the unsteady aerodynamics before the occurrence of rotating stall within the TU Darmstadt OpenStage compressor rotor and introduces unsteady wall pressure data to the Darmstadt Open Test Case. Exemplary pre-stall aerodynamics and stall inception are investigated for nominal (transonic) and part speed (subsonic) operating conditions, comprising the analysis of the unsteady static pressure field at the rotor tip as well as spectral analysis of the corresponding pressure signals during transient throttling maneuvers. To capture the rotor tip flow field, the rotor casing is instrumented with axially arranged flush mounted time-resolving wall pressure transducers. To determine propagation speeds of occurring phenomena, additional circumferentially distributed sensors are considered. Vibration monitoring shows no significant flutter or non-synchronous blade vibration, prior to the occurrence of rotating stall. Thus, this work focuses on aerodynamics, such as the interaction of secondary flow and shocks.

Two-dimensional pressure field imaging of an elastic panel executing post-flutter oscillations

Two dimensional pressure field was acquired at 10 kHz to study the post flutter oscillations of a thin elastic panel placed beneath a Mach 2.5 turbulent boundary layer. The panel was made of aluminum and is secured to the mounting fixture using a collection of rivets, which resulted in a boundary condition that was between both ideally clamped and pinned boundaries. Direct comparison of the mean and unsteady pressure fields were made for the panel executing post flutter oscillations and oscillations away from the flutter boundary. Whereas the mean pressure fields were largely similar during and away from post-flutter oscillations, the unsteady pressure fields showed a significant increase in the pr.m.s. during post flutter oscillations. The spectral content of the pressure oscillations and panel oscillations revealed that the tonal aeroelastic frequency dominate the post flutter oscillations. This tonal frequency was determined to lie at the close vicinity of the (2,1) panel elastic mode. The r.m.s. panel deflection field during post flutter oscillations were also reconstructed from the unsteady pressure fields and the reconstructed panel deflection also corresponded to the (2,1) elastic mode.Further coherence and cross-correlation analyses provided insights into possible mechanisms that control the transition of the panel oscillations away from the flutter boundary. The analyses suggest that the transition away from the flutter boundary is possibly initiated by the decoherence of the organized pressure field during the post flutter region by the turbulent boundary layer.

On the preferred flapping motion of round twin jets

Linear stability theory (LST) is often used to model the large-scale flow structures in the turbulent mixing region and near pressure field of high-speed jets. For perfectly expanded single round jets, these models predict the dominance of azimuthal wavenumbers $m=0$ and $m = 1$ helical modes for the lower frequency range, in agreement with empirical data. When LST is applied to twin-jet systems, four solution families appear following the odd/even behaviour of the pressure field about the symmetry planes. The interaction between the unsteady pressure fields of the two jets also results in their coupling. The individual modes of the different solution families no longer correspond to helical motions, but to flapping oscillations of the jet plumes. In the limit of large jet separations, when the jet coupling vanishes, the eigenvalues corresponding to the $m=1$ mode in each family are identical, and a linear combination of them recovers the helical motion. Conversely, as the jet separation decreases, the eigenvalues for the $m=1$ modes of each family diverge, thus favouring a particular flapping oscillation over the others and preventing the appearance of helical motions. The dominant mode of oscillation for a given jet Mach number $M_j$ and temperature ratio $T_R$ depends on the Strouhal number $St$ and jet separation $s$ . Increasing both $M_j$ and $T_R$ independently is found to augment the jet coupling and modify the $(St,s)$ map of the preferred oscillation mode. Present results predict the preference of two modes when the jet interaction is relevant, namely varicose and especially sinuous flapping oscillations on the nozzles’ plane.

LES application to wind pressure prediction for tall building on complex terrain

This study conducted numerical simulations based on the large eddy simulation (LES) model, which employs an unstructured grid system and exhibits high performance in parallel computing, to investigate the effects of a circumferential environment, such as real terrain and surrounding buildings, on the unsteady pressure fields acting on a tall building with a rectangular section. There are three numerical examples using an irregularly shaped high-rise building (aspect ratios: height/length = 7/4 and length/width = 24/11), the same building on a complex terrain, and the same building with surrounding buildings on a complex terrain. To elucidate the statistical pressure characteristics, we focused on the mean, fluctuating, and peak values on the surfaces of the target building. The pressures for the present models were validated through a comparison with experimental data obtained under the same conditions. Subsequently, the velocity and pressure fields directly influenced by the complex local terrain and surrounding buildings were studied. A concave terrain located on the leeward side of the target building could cause a large negative pressure on the side surfaces as a result of access to the deformed separating flow. In addition, the surrounding buildings led to the movement of the stagnation point to a higher location on the windward surface and intensification of the conical vortex attached to the side surface.

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.

Study of Unsteady Surface Flowfields on and Around Turrets with Different Protrusions

Wind-tunnel experiments were conducted to measure the unsteady surface pressure field on and around a hemisphere-on-cylinder turret of varying protrusion in subsonic flow. These measurements were obtained using fast-response pressure-sensitive paint coupled with pressure transducers. The surface pressure field data resulting from Mach 0.5 flow () over a partially submerged hemisphere, a hemisphere, and a hemisphere on a cylinder were analyzed using proper orthogonal decomposition modal analysis, as well as a variant of this approach referred to as joint proper orthogonal decomposition. The results showed that decreased turret protrusion into the freestream flow reduced the prevalence of antisymmetric surface pressure field fluctuations caused by antisymmetrical vortex shedding. The frequency associated with this fluctuation was found to be around . When a partial hemispherical turret geometry was used, it was shown that the antisymmetric mode was greatly suppressed; and the wake was dominated by a symmetric mode with a broadband spectral peak at a higher frequency of . This suggests that there is a “mode switching” as the protrusion is changed from the hemisphere to the partial hemisphere configuration. An optical flow approach was used to find the convective velocity field in the wake, from which topological flow features could be identified. The size of the wake separation region was found to grow smaller with the decreased protrusion while keeping a similar shape.

Aerodynamics and Sealing Performance of the Downstream Hub Rim Seal in a High-Pressure Turbine Stage

The purpose of the paper is to characterize the aerodynamic behavior of a rotor-downstream hub cavity rim seal in a high-pressure turbine (HPT) stage. The experimental data are acquired in the Transonic Test Turbine Facility at the Graz University of Technology: the test setup includes two engine-representative turbine stages (the last HPT stage and first LPT stage), with the intermediate turbine duct in between. All stator-rotor cavities are supplied with purge flows by a secondary air system, which simulates the bleeding air from the compressor stages of the real engine. The HPT downstream hub cavity is provided with wall taps and pitot tubes at different radial and circumferential locations, which allows the performance of steady pressure and seed gas concentration measurements for different purge mass flows and HPT vanes clocking positions. Moreover, miniaturized pressure transducers are adopted to evaluate the unsteady pressure distribution, and an oil flow visualization is performed to retrieve additional information on the wheel space structures. The annulus pressure asymmetry depends on the HPT vane clocking, but this is shown to have negligible impact on the minimum purge mass flow required to seal the cavity. However, the hub pressure profile drives the distribution of the cavity egress in the turbine channel. The unsteady pressure field is dominated by blade-synchronous oscillations. No non-synchronous components with comparable intensity are detected.

On the pressure field, nuclei dynamics and their relation to cavitation inception in a turbulent shear layer

Cavitation inception in the turbulent shear layer developing behind a backward-facing step occurs at multiple points along quasi-streamwise vortices (QSVs), at a rate that increases with the Reynolds number (Re). This study investigates the evolution of the unsteady pressure field and the distribution of nuclei within and around the QSVs. The time-resolved volumetric velocity in the non-cavitating flow is measured using tomographic particle tracking, and the pressure is determined by spatial integration of the material acceleration. Analysis in Eulerian and Lagrangian reference frames reveals that the pressure is lower, and its minima last longer within the QSVs compared with the surrounding flow. The intermittent low pressure regions, whose sizes and shapes are consistent with those of the cavities, are likely to be preceded by axial vortex stretching and followed by contraction. Such phenomena have been observed before in simulations of stretched vortex elements. For the same axial straining, the pressure minima last longer with increasing Re, a trend elucidated in terms of viscous diffusion of the stretched vortex core. The impact of nuclei availability is studied under ‘natural’ and controlled seeding. Owing to differences in the saturation level of non-condensable gas, the microbubble concentration in the shear layer decreases with increasing Re, in contrast to the rate of cavitation events. Minor differences in entrainment rate into the shear layer also do not explain the substantial Re effects on cavitation inception. Hence, the Re scaling of inception appears to be dominated by trends of the pressure field.

Effect of the radial gap size on the deterministic flow in a centrifugal pump due to impeller-tongue interactions

The research on the flow in centrifugal pumps is quite broad and covers most of the relevant issues. However, most of the studies are focused on the steady behaviors and less literature is available for the dynamic interactions. Unsteady mechanisms for energy transfer and sources of impeller-tongue interaction can be identified in centrifugal pumps using the deterministic decomposition of the flow. Probably, the most important effect to be considered is the radial gap or spacing between the impeller exit diameter and the volute tongue radial location. The numerical dataset of a full 3D URANS model in a centrifugal pump has been employed to study in detail the effect of the radial gap size in the unsteady fluctuations of the velocity field within the pump, using the deterministic analysis as the main novelty. The numerical model developed by the authors has been already tested towards the prediction of the unsteady pressure field inside the volute, at the impeller exit. The new results presented here allow to see the impact of both geometrical and operating parameters on the flow discharge and momentum exchange. In particular, four different radial gaps (23.2%, 17.0%, 11.4% and 8.8% of the impeller diameter) operated at five different flow rates (from 20% to 160% of the nominal rate) have been numerically resolved and analyzed. The deterministic analysis reveals the major impact of the radial clearance on the blade-to-blade flow patterns within the impeller, especially at low flow rates. Unsteady viscous interaction induces radial velocity fluctuations that can be as high as a 40% of the time-averaged value. Moreover, this non-linear term can be perceived up to 1.5 times higher in the case of radial gap reductions from 23.2% to 8.8% of the impeller diameter. There is not a prevailing effect of the radial gap on that velocity component, and a pure temporal term dependence is found. On the contrary, the influence of the gap on the tangential component, responsible for the unsteady evolution of the impeller torque, is found to be the key parameter. Therefore, the fluctuations of the flow blockage are found to be the consequence of the impeller flow patterns and the fluctuations of the torque are to be assigned to the radial gap influence. All the post-processing routines have provided a precise picture of all the classic unsteady mechanisms involved: jet-wake pattern interaction, acoustic wave propagations and recirculating cells at off-design conditions. Hence, deterministic analysis is a useful tool to analyze the flow inside centrifugal pumps and it envisages the introduction of deterministic flow variables as objective functions for design optimization algorithms.

Forced Response Analysis of an Embedded Compressor Rotor Induced by Stator Disturbances and Rotor–Stator Interactions

Accurate predictions of the blade response in a multi-row compressor is one of the most important tasks within the design process of compressor blades. Some recent studies have shown that the decoupled method considering only the stator disturbances cannot obtain accurate results for cases with strong rotor–stator interactions, especially for the interaction between the rotor and downstream stator, and the coupled method with multi-row configurations is necessary. Factors determine what computational domains to model need to be clarified to find a balance between accuracy requirements and computational costs. To this end, this study conducted full-annulus unsteady calculations with decoupled and coupled configurations to investigate the forced response of an embedded compressor rotor induced by upstream and downstream stator disturbances and rotor–stator interactions, respectively. The results show that the upstream IGV disturbances were dominated by the wake, and the IGV and S1 potential fields had little effect on the R1 response. Meanwhile, the IGV–R1 interactions and S1–R1 interactions were dominated by one cut-on mode, respectively. The comparisons of the blade vibration amplitude and the unsteady pressure field calculated by decoupled and coupled methods revealed the mechanism of the forced response, namely, for the R1 response induced by upstream aerodynamic disturbances, the dominant excitation source was the IGV wake, and the blade vibration amplitude can be predicted by the decoupled method. In terms of the response induced by downstream disturbances, the cut-on S1-R1-interaction mode was dominant and the use of the decoupled method without considering its influence will lead to an inaccurate prediction. This study concluded that the formation process of rotor–stator interactions was the key factor that determines whether the decoupled method or coupled method should be used, and analogized a process independent of the downstream stator disturbance. The results can provide a preliminary configuration for accurate and efficient blade response predictions and explain the reason why including downstream stator vanes is very important.

Experimental characterisation and data-driven modelling of unsteady wall pressure fields induced by a supersonic jet over a tangential flat plate

This work deals with the investigation and modelling of wall pressure fluctuations induced by a supersonic jet over a tangential flat plate. The analysis is performed at several nozzle pressure ratios around the nozzle design Mach number, including slightly over-expanded and under-expanded conditions, and for different radial positions of the rigid plate. Pitot measurements and flow visualizations through the background oriented schlieren technique provided a general overview of the aerodynamic interactions between the jet flow and the plate at the different regimes and configurations. Wall pressure fluctuations were measured using a couple of piezoelectric pressure transducers flush mounted over the plate surface. The spectral analysis has been carried out to clarify the effect of the plate position on the single and multivariate wall pressure statistics, including the screech tone amplitude. The experimental dataset is used to assess and validate a surrogate model based on artificial neural networks. Sound pressure levels and coherence functions are modelled by means of a single fully connected network, built on the basis of a recently implemented fully deterministic topology optimization algorithm. The metamodel uncertainty is also quantified using the spatial correlation function. It is shown that the flow behaviour as well as the screech and broadband noise signatures are significantly influenced by the presence of the plate, and the effects on spectral quantities are correctly reproduced by the proposed data-driven model that provides predictions in agreement with the available data.

Acoustic Feedback and Its Impact on Fan Flutter in Short Aeroengine Intakes

The authors have investigated pressure waves being emitted by the structural motion of a rotor blade and their impact on fan flutter sensitivity. This impact results from an acoustic feedback loop caused by reflection phenomena at the intake lip and may lead to a specific form of aeroelastic instability. Harmonic balance simulations of the fan/intake system over a wide operating range and blade eigenfrequencies were carried out. A significant impact on the rotor aerodynamic damping was observed as compared with the isolated rotor analysis due to the presence of the intake. This influence was either stabilizing or destabilizing, depending mainly on the phase relation between the emitted and reflected unsteady pressure fields. Computational fluid dynamics (CFD) data were analyzed accordingly, taking advantage of two-dimensional wave split as well as three-dimensional in-duct modal decomposition techniques. By deriving a simple analytical model based on eigenvalue analysis, a landscape showing regions of aerodynamic damping and excitation could be drawn over the investigated frequency–mass flow range. This analytical model and its verification with CFD data can help in identifying destabilizing areas at an early design stage, and hence support design decisions regarding flutter stability or give indications where analysis with higher fidelity is required.

Experimental Investigation of Unsteady Static Pressure Field Behind a Circular Cylinder

In this paper a measurement of a static pressure in the wake behind a circular cylinder for the different Reynolds numbers is presented. For this purpose, a static pressure probe with built-in pressure transducer was used. The main goal is to describe the unsteady pressure field behind the cylinder and determine a frequency of the generated Von Kármán vortex street. The measurement was conducted in a low speed wind tunnel for a cylinder with 110 mm in diameter. The results were compared to an unsteady RANS simulation.

Wake response downstream of a spanwise-oscillating hemispherical turret

An experimental Pressure Sensitive Paint (PSP) study of the unsteady pressure fields downstream of and on an oscillating hemispherical turret at subsonic freestream Mach numbers between 0.3 and 0.6 and Reynolds numbers of about 2 million is presented. The oscillating turret consisted of a hemispheric shell, mounted on an aluminum rectangular plate and is designed to oscillate in the spanwise direction at a single frequency, which lies within the frequency range of the dominant wake motion. The resonant-based aero-elastic response of the turret to the flow resulted in intermittent turret oscillations. Pressure fluctuations in the wake were found to be less energetic for the oscillating turret and the size of the recirculating region was increased, compared to the stationary turret. POD-based modal analysis of the pressure fields revealed that the wake dynamics is characterized by global anti-symmetric and symmetric shedding and showed that the oscillating turret primarily suppressed the global anti-symmetric shedding. The analysis of the unsteady forces acting on the turret demonstrated that the oscillating turret reduces the spanwise component of the unsteady force, while keeping other force components mostly unchanged. Intermittent nature of the spanwise force, which is responsible for the intermittent motion of the oscillating turret, was observed and discussed.

Numerical investigations on non-synchronous vibration and frequency lock-in of low-pressure steam turbine last stage

The flow phenomenon of rotating instability (RI) and its induced non-synchronous vibrations (NSV) in the last stage have gradually become a security problem that restricts the long-term flexible operations of modern large-scaled low-pressure steam turbines. Especially, if one structural mode of the last stage moving blade (LSMB) is excited, significant blade vibrations may potentially lead to high-cycle fatigue failure. A loosely coupled computational fluid dynamics reduced model with prescribed blade vibrations has been established to investigate NSV of the LSMB and the potential lock-in phenomenon under low-load conditions. Firstly, calculations with reduced multi-passage domain have been verified by comparing with the results of the full-annulus one, and an appropriate reduced domain is determined. Secondly, a set of calculations by controlling blade vibration parameters indicate that lock-in phenomenon between RI frequency and blade vibration frequency may occur when nodal diameters of cascade vibrations is coincident with the wave number of RI. Furthermore, dynamic modal decomposition technology has been employed to identify the unsteady pressure field around the blade surface and to reveal the interaction relationship between the flow modes of RI and vibration-induced pressure disturbance. Finally, the blade response evaluation based on harmonic analysis shows that in NSV, the global maximum dynamic response level of locked-in case is nearly 20 times than that of unlocked one.

Acoustic-roughness receptivity in subsonic boundary-layer flows over aerofoils

The generation of a viscous–inviscid instability through scattering of an acoustic wave by localised and distributed roughness on the upper surface of a NACA 0012 aerofoil is studied with a time-harmonic compressible adjoint linearised Navier–Stokes approach. This extends previous work by the authors dedicated to flat plate geometries. The key advancement lies in the modelling of the inviscid acoustic field external to the aerofoil boundary layer, requiring a numerical solution of the convected Helmholtz equation in a non-uniform inviscid field to determine the unsteady pressure field on the curved aerofoil surface. This externally imposed acoustic pressure field subsequently drives the acoustic boundary layer, which fundamentally determines the amplitudes of acoustic-roughness receptivity. A study of receptivity in the presence of Gaussian-shaped roughness and sinusoidally distributed roughness at Mach number $M_\infty =0.4$ and Strouhal numbers $\mathcal {S} \approx \{46,69,115\}$ shows the effects of various parameters, most notably angle of attack, angle of incidence of the externally imposed plane acoustic wave and geometry of surface roughness; the latter is varied from viewpoint of its placement on the aerofoil surface and its wavelength. The parametric study suggests that non-parallel effects are quite substantial and that considerable differences arise when using parallel flow theory to estimate the optimal width of Gaussian-shaped roughness elements to provoke the greatest response. Furthermore, receptivity amplitudes for distributed roughness are observed to be generally higher for lower angles of attack, i.e. for less adverse pressure gradients. It is also shown that the boundary layer is more receptive to upstream-travelling acoustic waves.