Mean Surface Pressure Distributions Research Articles

Global Surface Pressure Pattern for a Compressible Elliptical Cavity Flow Using Pressure-Sensitive Paint

The flow field in a cavity depends on the properties of the upstream boundary layer and the cavity geometry. Comprehensive studies for rectangular cavities have been conducted. This experimental study determines the global surface pressure pattern for elliptical cavities (eccentricities of 0, 0.66 and 0.87) in a naturally developed turbulent boundary layer using pressure-sensitive paint. The ratio between the length (major axis) and the depth is 4.43–21.5, and the freestream Mach number is 0.83. The mean surface pressure distribution of an elliptical cavity resembles that of a rectangular cavity. A change in the value of eccentricity (wall curvature) affects the region for an adverse pressure gradient in an open cavity, an extension of the plateau in a transitional–closed cavity and flow expansion near the front and rear edges. The boundaries between an open, transitional and closed cavities vary.

The Effect of Vortex Generators on Shock-Induced Boundary Layer Separation in a Transonic Convex-Corner Flow

Deflected control surfaces can be used as variable camber control in different flight conditions, and a convex corner resembles a simplified configuration for the upper surface. This experimental study determines the presence of passive vortex generators, VGs (counter-rotating vane type), on shock-induced boundary layer separation for transonic convex-corner flow. The mean surface pressure distributions in the presence of VGs for h/δ = 0.2 and 0.5 are similar to those for no flow control. If h/δ = 1.0 and 1.5, there is an increase in the amplitude of the mean surface pressure upstream of the corner’s apex, which corresponds to greater device drag and less downstream expansion. There is a decrease in peak pressure fluctuations as the value of h/δ increases, because there is a decrease in separation length and the frequency of shock oscillation. The effectiveness of VGs also depends on the freestream Mach number. For M = 0.89, there is an extension in the low-pressure region downstream of a convex corner, because there is greater convection and induced streamwise vorticity. VGs with h/δ ≤ 0.5 are preferred if deflected control surfaces are used to produce lift.

Experimental investigation of the angle of attack dependence of the flow past a cactus-shaped cylinder with four ribs

The aerodynamic properties of a two-dimensional cactus-shaped cylinder with four ribs are studied experimentally. The cross section of the cylinder corresponds to a modified square cylinder with rounded corners and concave sides inspired by tall succulents with four ribs. The mean aerodynamic coefficients, fluctuating lift coefficient, Strouhal number, and mean surface pressure distribution are measured as a function of the angular orientation for Reynolds numbers ranging from 50,000 to 150,000. Hot-wire measurements are conducted at two locations in the wake for several angles of attack to provide insight into the vortex shedding frequencies. The results show that the studied shape exhibits a strong angle of attack dependence while no Reynolds number dependence is found within the tested range. As for the square cylinder, a critical angle of attack of approximately 13° is observed at which drag and fluctuating lift forces are minimised while mean lift and Strouhal number reach their highest values. Reattachment of the shear layer at and above the critical angle of attack is confirmed using PIV. Overall, succulents with four ribs retain some of the aerodynamic benefits that have been observed for cactus-shaped cylinders with many ribs, albeit over a limited range of angles of attack.

Large eddy simulation of airfoil self-noise using OpenFOAM

PurposeThe purpose of this paper is to investigate airfoil self-noise generation and propagation by using a hybrid method based on the large-eddy simulation (LES) approach and Curle’s acoustic analogy as implemented in OpenFOAM.Design/methodology/approachLarge-eddy simulation of near-field flow over a NACA6512-63 airfoil at zero angle of attack with a boundary layer trip at Rec = 1.9 × 105 has been carried out using the OpenFOAM® computational fluid dynamics (CFD) code. Calculated flow results are compared with published experimental data. The LES includes the wind tunnel installation effects by using appropriate inflow boundary conditions obtained from a RANS κ – ω SST model computation of the whole wind tunnel domain. Far-field noise prediction was achieved by an integral method based on Curle’s acoustic analogy. The predicted sound pressure levels are validated against the experimental data at various frequency ranges.FindingsThe numerical results presented in this paper show that the flow features around a NACA6512-63 airfoil have been correctly captured in OpenFOAM LES calculations. The mean surface pressure distributions and the local pressure peaks for the step trip setup agree very well with the experimental measurements. Aeroacoustic prediction using Curle’s analogy shows an overall agreement with the experimental data. The sound pressure level-frequency spectral analysis produces very similar data at low to medium frequency, whereas the experimentally observed levels are slightly over predicted at a higher frequency range.Practical implicationsThis study has achieved and evaluated an alternative aeroacoustic simulation method based on the combination of LES with a simple Smagorinsky SGS model and Curle’s analogy, as implemented in the OpenFOAM CFD code. The unsteady velocity/pressure source data produced can be used for any simpler analytically based far-field noise prediction scheme.Originality/valueA complete integration of the LES and Curle’s acoustic analogy for aeroacoustic simulations has been achieved in OpenFOAM. The capability and accuracy of the hybrid method are fully evaluated for high-camber airfoil self-noise predictions. Wind tunnel installation effects have been incorporated properly into the LES.

Effects of turbulence on the mean pressure field in the separated-reattaching flow above a low-rise building

The effects of upstream turbulence in the atmospheric boundary layer flow on the mean surface pressure distribution within the separated flow above a typical low-rise building roof are investigated experimentally. Time-averaged Navier-Stokes equations are used to evaluate the pressure gradients from planar particle image velocimetry data. The pressure fields are reconstructed by integrating the pressure gradients using an analytic interpolation approach. This reconstruction approach is validated by successfully matching the reconstructed pressure to Bernoulli's equation along a streamline far from the body and with pressure measurements on the surface of the body. Through this process, the mean pressure field can be directly explained from the mean velocity and turbulence fields near the roof. For high turbulence intensity levels, the maximum suction coefficient on the roof surface was found to be increased. Such increased magnitudes are directly related to the reduced size of mean separation bubble in higher turbulence, more rapid variation of the velocity magnitude near the leading edge, and enhanced variation of the turbulence stresses. On the other hand, a higher rate of surface pressure recovery is found in the leeward portion of the separation bubble, which is mainly due to the more rapid variation of the turbulence stresses.

Reynolds stress modelling of wake interference of two cylinders in tandem: Conventional vs. eddy-resolving closure

A differential, near-wall Reynolds stress model (RSM) employed in Unsteady RANS (Reynolds-Averaged Navier–Stokes) framework, coupled with the equation governing the inverse turbulent time scale relying on the so-called 'homogeneous dissipation' rate (ωh=ɛh/k), was applied to two in-line tandem-cylinder flow configurations. The configurations differ with respect to the distance between cylinders, with the consequence of a differently characterized wake structure. The reference experimental database has been provided by Jenkins et al. (2005, 2006) and Neuhart et al. (2009). Complementary to the ‘conventional’ Reynolds stress model based on the Jakirlic and Hanjalic's (2002) formulation, its version sensitized appropriately to account for the turbulent unsteadiness by a selective enhancement of its production in line with the SAS (Scale-adaptive Simulation) proposal (Menter and Egorov, 2010), formulated recently by Jakirlic and Maduta (2015), was also applied. This eddy-resolving RSM formulation has been shown to be capable of returning all important mean flow and turbulence features of the configurations considered, including also the intermittent shedding behaviour of the flow in the gap between cylinders related to the long-distance case. The model's inherent capability of capturing the turbulence unsteadiness resulted consequently in the correctly predicted mean flow and turbulence fields, unlike the conventional URANS RSM model which could reasonably well return only the mean surface pressure distribution.

Tunnel Background Noise on Compressible Convex-Corner Flows

The perforated test section walls of a transonic wind tunnel are responsible for generating edgetones, thus contributing significantly to the overall noise level and possibly invalidating the measurements of surface pressure fluctuations. In the present study, the wind tunnel was assembled with perforated top/bottom walls, solid side walls, and four perforated walls. The focus was to investigate the effects of tunnel background noise on compressible convex-corner flows, which correspond to the upper control surface of an aircraft wing. External acoustic disturbance has a minor effect on the mean surface pressure distributions, including flow expansion and recompression near the corner. Higher peak pressure fluctuations, which are associated with shock excursion phenomena, are observed with more intense tunnel background noise, particularly for the flow with initial separated boundary layers.

The influence of model surface roughness on wind loads of the RC chimney by comparing the full-scale measurements and wind tunnel simulations

A wind tunnel test of a scaled-down model and field measurement were effective methods for elucidating the aerodynamic behavior of a chimney under a wind load. Therefore, the relationship between the results of the wind tunnel test and the field measurement had to be determined. Accordingly, the set-up and testing method in the wind tunnel had to be modified from the field measurement to simulate the real behavior of a chimney under the wind flow with a larger Reynolds number. It enabled the results of the wind tunnel tests to be correlated with the field measurement. The model surface roughness and different turbulence intensity flows were added to the test. The simulated results of the wind tunnel test agreed with the full-scale measurements in the mean surface pressure distribution behavior.

Computational and experimental investigation of the unsteady flow structures around automotive outside rear-view mirrors

The unsteady flow field around two automotive outside rear-view mirrors is investigated. This study includes comprehensive experimental and computational approaches in order to characterize the complex flow structures in the wake of the mirrors. The experiments were carried out in a wind tunnel which included the measurements of the instantaneous and averaged velocity fields as well as mean and unsteady surface pressure distributions. The simulations were performed using Large Eddy Simulation (LES). The LES approach (particularly with the dynamic subgrid viscosity model) provided good agreements with the experiments for the velocity and the surface pressure distributions. The experimental and the computational results of this study will be used as a benchmark to validate the current and the future CFD development and the subsequent aero-acoustic computations.

Turbulent and unsteady flow characteristics of delta wing vortex systems

This paper presents an overview of experimental investigations on a 65 deg swept delta wing as part of the International Vortex Flow Experiment 2 (VFE-2). Results obtained in low-speed wind tunnel facilities include oil flow and laser light sheet flow visualization, mean and unsteady surface pressure distributions as well as mean and turbulent velocity components of the flow field and close to the wing surface. Thus, field and near wall distributions of all components of the Reynolds stress tensor are available. Details of the delta wing vortex structure and breakdown phenomenon are discussed and analyzed. Vortex bursting leads to specific spectral densities of velocity and surface pressure fluctuations characterized by narrow band distributions associated with the helical mode instability of the vortex breakdown flowfield. Further, special emphasis is on the occurrence of an inner vortex detected for the low Reynolds number and Mach number regime. This inboard vortex results from a laminar separation close to the apex due to the spanwise pressure gradient in the area of relatively large thickness while the classical leading-edge vortex progressing from the rear part to the apex is fed from the turbulent shear layers shed at the wing upper and lower side.

Wind-induced mean interference effects between two closed spaced buildings

The mean interference effects between two rectangular buildings located in close proximity in a geometrical configuration of ‘L’ and ‘T’ plan shape are studied through wind tunnel test on 1:300 scale rigid models. The mean surface pressure distributions on all the walls of two buildings located in close proximity as well as in an isolated position are measured over an extended range of wind directions. The mean responses of pair of buildings namely, block-1 and block-2 subjected to interference effects are evaluated using experimentally obtained wind loads and, subsequently compared it, with the responses of a similar building in an isolated position. Effectiveness of upstream building location and wind orientation in changing the mean wind pressure distributions and responses of upstream and downstream building are also assessed. At wind incidence angle of 0°, presence of upstream block-1 reduces the mean along-wind displacement of block-2 of ‘L’ and ‘T’ shape arrangements up to 25% and 71% respectively as compared to that of corresponding block in an isolated position. However, the presence of upstream block-1 increases the maximum mean torque on block-2 of ‘L’ and ‘T’ shape arrangement up to 28%, and up to 88% respectively as compared to maximum mean torque developed on a similar block in an isolated position.

Large-eddy simulations of a turbulent Coanda jet on a circulation control airfoil

Large-eddy simulations are performed of a turbulent Coanda jet separating from a rounded trailing edge of a simplified circulation control airfoil model. The freestream Reynolds number based on the airfoil chord is 0.49×106, the jet Reynolds number based on the jet slot height is 4470, and the ratio of the peak jet velocity to the freestream velocity is 3.96. Three different grid resolutions are used to show that their effect is very small on the mean surface pressure distribution, which agrees very well with experiments, as well as on the mean velocity profiles over the Coanda surface. It is observed that the Coanda jet becomes fully turbulent just downstream of the jet exit, accompanied by asymmetric alternating vortex shedding behind a thin (but blunt) jet blade splitting the jet and the external flow. A number of “backward-tilted” hairpin vortices (i.e., the head of each hairpin being located upstream of the legs) are observed around the outer edge of the jet over the Coanda surface. These hairpins create strong upwash between the legs and weak downwash around them, contributing to turbulent mixing of the high-momentum jet below the hairpins and the low-momentum external flow above them. The probability density distribution of velocity fluctuations is shown to be highly asymmetric in this region, consistent with the observation that the hairpin vortices create strong upwash and weak downwash. Turbulent structures inside the jet, its spreading rate, and self-similarity are also discussed.

Reduction of wind uplift of a solar collector model

Due to the geographical location of Taiwan, solar water heaters on the roof of buildings are frequently subjected to the impact of severe typhoons. An experimental study of a scaled commercial system was conducted to investigate the mean surface pressure distributions and uplift force at different speeds. To reduce the possible damage due to strong wind uplift, one approach is to lift the model and the other is to install a guide plate. It is found that the wind uplift is reduced effectively with a guide plate normal to the wind direction. Effects of the incremental height of model are less significant in reducing the uplift force. Reynolds number independence for the present test configuration is also observed.

Structure of flow separation and reattachment behind an axisymmetric hill

Large-eddy simulation of a high Reynolds number, turbulent flow over an axisymmetric hill of height H = 2δ, where δ is the incoming turbulent boundary layer thickness, is simulated and compared with the experimental data [1]. The flow is highly three dimensional and is characterized by unsteady separations and re-attachments on the lee-side of the hill. Comparisons with measurements indicate reasonable agreement for the mean surface pressure distribution, the mean streamwise and spanwise flow profiles, and the velocity fluctuation profiles. The boundary layer separates on the lee-side of the hill and the separated shear flow is observed to merge and form a counter-rotating vortex pair (CRVP) downstream. Flow pattern over the hill surface is observed to have the same sense of rotation as the respective vortex in CRVP on either side. The separation and re-attachment process is shown to be strongly controlled by the three-dimensional pressure gradient caused by body shape and size.

GCI 를 이용한 수직분사제트 수치모사의 검증 및 확인

초음속 주 유동으로의 수직분사에 의해 생성되는 2차원 정상상태 유동장에 대한 수치모사를 수행하였다. 난류효과를 위해서 무차원 벽면거리(<TEX>$y^+$</TEX>)를 고려한 2방정식 k-<TEX>${\omega}$</TEX> SST 모델을 사용하였다. 또한 격자계에 따른 오차범위를 나타내는 방법으로 GCI(Gird Convergence Index)를 사용하여 해의 수렴성을 측정하였다. 표면 압력분포, 박리거리, 침투높이 등에 대해 실험결과 및 다른 난류모델을 이용한 결과들과 비교하였다. k-<TEX>${\omega}$</TEX> SST 난류모델은 낮은 압력비에 대해서 표면 압력분포 및 박리거리 등을 정확하게 예측하였다. 그러나 압력비가 증가함에 따라 수치적 예측이 실험결과와 차이를 보이고 있다. 상기한 모든 결과는 격자계에 따른 해의 수렴성의 오차범위 1% 이내에서 측정되었다. Two-dimensional steady flowfields generated by transverse injection jets into a supersonic mainstream are numerically simulated. Fine-scale turbulence effects are represented by a k-<TEX>${\omega}$</TEX> SST two-equation closure model which includes <TEX>$y^+$</TEX> effects on the turbulence model. Solution convergence is evaluated by using Grid Convergence Index(GCI), a measure of uncertainty of the grid convergence. Comparison is made with experimental data and other turbulence models in term of surface static pressure distributions, the length of the upstream separation region, and the penetration height. Results indicate that the k-<TEX>${\omega}$</TEX> SST model correctly predicts the mean surface pressure distribution and the upstream separation length for low static pressure ratios. However, the numerical predictions become less consistent with experimental results as the static pressure ratio increases. All these results are taken within 1% error band of grid convergence.

Use of High-Speed Microjets for Active Separation Control in Diffusers

Inlets to aircraft propulsion systems must supply flow to the compressor with minimal pressure loss, flow distortion, or unsteadiness. Flow separation in internal flows such as inlets and ducts in aircraft propulsion systems and external flows such as over aircraft wings is undesirable because it reduces the overall system performance. An experimental investigation is described that was carried out to study the feasibility of using high-speed microjets, supersonic for most cases, to control boundary-layer separation in an adverse pressure gradient. The geometry used is a simple diverging Stratford ramp equipped with arrays of 400-μm-diam microjets. Measurements include detailed surface flow visualizations, mean surface pressure distributions, and velocity field measurements using particle image velocimetry. The results clearly indicate that by activating these microjets the separated flow regions were eliminated. This led to a significant increase in the momentum of the flow near the surface where the gain in momentum was at least an order of magnitude higher than the momentum injected by the microjets. Given the simplicity of the system and its low mass flow requirements, combined with the benefits achieved by this approach, microjets appear to be promising actuators for efficient separation control for internal and external flow applications.

Control of Vortical Flow over a Rounded Leading-Edge Delta Wing

An attempt is made for fluidic control of the vortical flow and its related vortex breakdown phenomenon over a delta wing with a particular geometry. The model has a sweep angle of A = 60 deg and a rounded leading edge. The control system is based on four rectangular slots along the portside leading edge, and it provides continuous or pulsed jets normal to the leading-edge surface and parallel to the leeward side plane. The mass flow rate and the frequency of injection can be varied independently. The results are compared to a reference case defined by a microflap fixed along the leading edge and set perpendicular to it. Qualitative measurements are obtained with surface flow visualizations and boundary-layer transition detections with acenaphten. Quantitative data consist in mean and instantaneous surface pressure distributions measurements under the primary vortex axis and in mean three-dimensional velocity held measurements. The control effect is analyzed by means of laser sheet visualizations to detect the vortex breakdown position and by normal force and rolling moment measurements with a five-component balance.

Wind flow over sinusoidal hilly obstacles located in a uniform flow

The wind flow over two-dimensional sinusoidal hilly obstacles with slope (the ratio of height to half width) of 0.5 has been investigated experimentally and numerically. Experiments for single and double sinusoidal hill models were carried out in a subsonic wind tunnel. The mean velocity profiles, turbulence statistics, and surface pressure distributions were measured at the Reynolds number based on the obstacle height(h=40 mm) of <TEX>$2.6{\times}10^4$</TEX>. The reattachment points behind the obstacles were determined using the oil-ink dot and tuft methods. The smoke-wire method was employed to visualize the flow structure qualitatively. The finite-volume-method and the SIMPLE-C algorithm with an orthogonal body-fitted grid were used for numerical simulation. Comparison of mean velocity profiles between the experiments and the numerical simulation shows a good agreement except for the separation region, however, the surface pressure data show almost similar distributions.

Geometric Effect on Compressible Rectangular Cavity Flows.

Experiments were performed to study the geometric effects on the compressible rectangular cavity flows at Mach 0.33, 0.62, and 0.82. Typical characteristics of mean surface pressure distributions show a slight pressure variation near the front face, a local peak surface pressure ahead of the rear corner, and a low pressure immediately downstream of the cavity. The length-to-width ratio of a cavity has a strong effect on the recompression and downstream expansion near the rear face, and the Mach number effect is minimized. Surface pressure fluctuation distributions show a damping near the leading edge at a higher freestream Mach number, a minor peak near the middle of the cavity floor, an increase toward the cavity rear face, and a peak value immediately downstream of the cavity. The amplitude of the pressure fluctuations ahead of the rear face increases with the length-to-width ratio for transitional- and closed-type cavity flows, and the maximum of peak pressure fluctuations rises rapidly for the transitional cavity flows at higher length-to-width ratio.

Flow in Two-Dimensional Rectangular Cavities.

In this paper we describe the flow associated with two-dimensional surface-mounted rectangular cavities. The experiment was carried out in two types wind tunnel and water channel. A significant aspect of the study has been an examination of the effects of changing the ratio of width/depth in the sectional geometries of two-dimensional cavities from 1 to ∞ to be backward facing step. The time mean surface pressure distributions, mean velocity, fluctuating velocities, and Reynolds stresses inside cavities were measured as the ratio of width/depth was varied. The flow pattern inside cavities was observed by flow-visualization. Consequently it was found that (i) the transition from the open cavities to the closed cavities occurs at W/D=8.8∼9 in wind tunnel experiment, (ii) this is reflected in the mixing phenomenon associated with an increase in cavity drag, and (iii) the similarity is established in the velocity profiles of the shear layer around the cavity flow and the reverse flow in the recirculation region.