Surface Pressure Coefficient Research Articles

Explicit algebraic Reynolds stress model for shock-dominated flows

Shock waves drastically alter the nature of Reynolds stresses in a turbulent flow, and conventional turbulence models cannot reproduce this effect. In the present study, we employ explicit algebraic Reynolds stress model (EARSM) to predict the Reynolds stress anisotropy generated by a shockwave. The model by Wallin and Johansson (2000) is used as the baseline model. It is found to over-predict the post-shock Reynolds stresses in canonical shock turbulence interaction. The budget of the transport equation of Reynolds stresses computed using linear interaction analysis shows that the unsteady shock distortion mechanism and the pressure–velocity correlations are important. We propose improvement to the baseline model using linear interaction analysis results and redistribute the turbulent kinetic energy between the principle Reynolds stresses. The new model matches DNS data for the amplification of Reynolds stresses across the shock and their post-shock evolution, for a range of Mach numbers. It is applied to oblique shock/boundary-layer interaction at Mach 5. Significant improvements are observed in predicting surface pressure and skin friction coefficient, with respect to experimental measurements.

Prognosis of Wind-tempted Mean Pressure Coefficients of Cross-shaped Tall Buildings Using Artificial Neural Network

The present paper focuses on the study of wind-induced responses of cross-plan shaped tall buildings. Initially, three parametric building models are studied for the purpose with a constant plan area 22500 mm2. The length and velocity scales are taken as 1:300 and 1:5, respectively. Wind angle of attack (WAA) is considered from 0° to 330° with an increment of 30°. At first, the external surface pressure coefficients (Cp) at different faces of the models are carried out for different wind occurrence angles employing Computational Fluid Dynamics method of simulated wind flow. Again, Fast Fourier Transform (FFT) fitted expressions as the sine and cosine function of WAA are proposed for attaining mean wind pressure coefficient on the building faces. The accuracy of the Fourier series expansions is justified by presenting histograms of sum square error (SSE), R2 value and root mean square error (RMSE). The results are also compared by training Artificial Neural Networks (ANN). Training is continued till Regression (R) values are more than 0.99 and Mean Squared Error (MSE) tends to 0, ensuring a close relationship among the outputs and targets. The face-wise value of (Cp) obtained using all three methods, are plotted. The error histograms of the ANN models show that the fitting data errors are spread within a reasonably good range. It is observed that the deviation in the result is not more than 5% in any case. Finally, the ANN predictions are presented for nine parametric models to cover a wide range of possible cross-shaped buildings.

Towards a standard CFD setup for wind load assessment of high-rise buildings: Part 2 – Blind test of chamfered and rounded corner high-rise buildings

Wind load assessment of the CAARC high-rise building performed with Large Eddy Simulation has recently shown to be applicable in Part 1 of the research, where deviations between experimental and digital results were in average 10–13% for the peak wind loads. Part 2 performs CFD blind tests of the CAARC building with chamfered and rounded corner modifications, to investigate how accurate CFD simulations are without prior knowledge of experimental wind tunnel results. The high-rise buildings are examined for the surface pressure distribution, the peak predicted structural responses and the Floor-by-Floor loadings. The settings for the numerical simulations of Part 2 are similar to the tests of Part 1. The CFD simulations are performed as replicas of experimental wind tunnel tests, to secure directly comparable test setup and post-processing procedures. The mean surface pressures on both high-rise buildings had high consistency with experimental results. Furthermore, the minimum and maximum peak surface pressure coefficients showed prominent good agreements. The chamfered high-rise building had average deviations of 14.92, 14.08 and 22.07% for the Mx, My and Mz base peak moments, where discrepancies of 12.56, 11.58 and 15.51% for Mx, My and Mz were found for the rounded corner high-rise building.

Research on Energy-Saving Wind Turbine Blade Based on Advanced Fluid Mechanics Technology

The aerodynamic performance of wind turbine blades is of great significance and value to the power output of wind turbines. Correct evaluation of blade performance is beneficial to the design and design of wind turbines. In the operating environment, the aerodynamic load on the blade is a combination of the aerodynamic pressure of the air and the additional force generated by its own rotation. Under different environmental conditions, the aerodynamic load on the wind turbine blades will be significantly different. Using advanced fluid mechanics, the developed program is used to perform aerodynamic simulation on a certain type of wind turbine blade calculation example to obtain the flow field and blade surface pressure coefficient distribution. The calculated results are in good agreement with the experiment, and the developed program can be used for aerodynamic analysis of engineering wind turbine blades, which is conducive to the development of wind turbine design.

Improved dynamical modelling of freely falling underwater cylinder based on CFD

An improved dynamic modelling approach using CFD is proposed to study the underwater motion of a dropped cylinder. Firstly, validation studies on RANS simulated flow past two dimensional (2D) and three dimensional (3D) cylinders (circular and square sectioned) are carried out. The predicted drag coefficients and pressure distribution for 2D and 3D cylinders are both in good agreement with corresponding experiments. URANS simulated results including quantification of mean surface pressure coefficient, fluid velocity and instantaneous drag coefficient and streamlines are compared between 2D circular cylinder, 3D circular cylinder and 3D square prism. Finally, the CFD predicted drag coefficients are applied to study the free falling of a horizontal cylinder under calm water and current respectively. Both 3D effects and cylinder shape effects are studied and found to greatly affect the motion of the cylinder by changing the drag coefficient.

Aerodynamic characteristics of variously modified leading-edge protuberanced (LEP) wind turbine blades under various turbulent intensities

A series of wind tunnel tests were performed to investigate the effect of turbulent inflows on the aerodynamic characteristics of variously modified leading-edge protuberanced (LEP) wing configurations at various turbulence intensities. A self-developed passive grid made of parallel arrays of round bars was placed at different locations of the wind tunnel to generate desired turbulence intensity. The aerodynamic forces acting over the baseline straight leading-edge model and the modified small, medium and high LEP wing configurations were obtained from the surface pressure measurements made over the wing at different turbulence intensities using MPS4264 Scanivalve simultaneous pressure scanner corresponding to a sampling frequency of 700Hz. All the test models were tested for a wide range of angles of attack ranging between −45° ≤ α ​≤ ​45° at turbulence intensities varying between 0.51% ​≤ ​TI ​≤ ​4.92%. Results reveal that the time-averaged mean coefficient of lift (CL) increases with the increase in the turbulence intensity associated with smooth stall characteristics rendering the modified LEP test models advantageous. Furthermore, based on the surface pressure coefficients the underlying dynamics behind the stall delay tendency were discussed. Additionally, attempts were made to statistically quantify the aerodynamic forces using standard deviation at both the pre-stall and the post-stall angles.

Numerical Investigation of Flow Structures For Four Buildings at Different Distances

In the present study, it was aimed to compare the effects of the different distances between four buildings placed in a wind area on the surface pressure distributions and flow structures around the building. For this purpose, ANSYS-FLUENT 14.0 package program was used and analyses were performed with the RNG k-ɛ turbulence model by 3D-modelling the buildings. The distances between the buildings were initially chosen as half of the building height and then the same as the building height. The average wind velocity was determined by taking the meteorological data for Sivas into consideration and the distributions of the flow line, velocity vector, turbulence kinetic energy and surface pressure coefficients were calculated for the medium height and roof level of the buildings. It is observed from the results that the distances between the buildings significantly affected the flow structures and velocity distributions, positive pressures were effective for front surfaces in the first buildings and negative pressure areas were formed in the rear surfaces and roofs.

Numerical Investigation of Aerodynamic Noise Reduction of Nonpneumatic Tire Using Nonsmooth Riblet Surface.

Unlike conventional pneumatic tires, the nonpneumatic tires (NPT) are explosion proof and simple to maintain and provide low rolling resistance. At high vehicle speeds, however, the complex airflow produced by the open flexible-spoke structure of NPT yields high aerodynamic noise, which contributes to sound pollution in the vehicular traffic environment. Inspired by the idea that a nonsmooth riblet structure can affect fluid flow and offer noise reduction, the analyses of the effect of the nonsmooth riblet surface on the aerodynamic noise of an NPT and noise reduction mechanism were presented in this paper. First, computational fluid dynamics (CFD) was used to analyze the surface pressure coefficient characteristics of a smooth flexible-spoke tire rolling at a speed of 80 km/h and subsequently validating the numerical simulation results by comparing them with published test results. Secondly, large eddy simulation (LES) and the Ffowcs Williams–Hawkings (FW-H) method were, respectively, used to determine the transient flow and far-field aerodynamic noise. Then, the mechanism of noise reduction was investigated using a vortex theory. Based on the vortex theory, the positions and strengths of noise sources were determined using the Lamb vector. Finally, according to the fluid boundary layer theory, a nonsmooth riblet surface was arranged on the surface of the spokes, and the influences of the riblet structure parameters, including size, position, and direction, on aerodynamic noise were analyzed. Based on the vortex theory, it was found that the nonsmooth riblet structure can reduce the Lamb vector, suppress the generation of flow vortices, decrease acoustic source strength, and effectively decrease noise up to 5.18 dB using the optimized riblet structure. The study results provide a theoretical basis for the structural design of a new low-noise NPT.

Verification of Anisotropic Mesh Adaptation for Turbulent Simulations over ONERA M6 Wing

Unstructured anisotropic mesh adaptation is known to be an efficient way to control discretization errors in computational fluid dynamics simulations. Method verification is required to provide the confidence for routine use in production analysis. The current work aims at verification of anisotropic mesh adaptation for Reynolds-averaged Navier–Stokes simulations over the ONERA M6 wing. The present verification study is performed using four different flow solvers, three different implementations of the metric field, and three mesh mechanics packages. Two of the flow solvers use stabilized finite element discretizations (FUN3D-SFE and general geometry Navier–Stokes), one uses finite volume discretization (FUN3D-FV), and the last one uses mixed finite volume and finite element discretizations (Wolf). The mesh adaptation is based on an error estimator that aims to control the quadratic error term in the linear interpolation of the Mach number. Two sets of adaptations were performed; the first one controls the interpolation error in the norm, and the second one controls the interpolation error in the norm. Convergence studies were performed on the forces and the pitching moment using all four solvers, and the results are compared with previously verified convergence studies on fixed (nonadapted) meshes. Both the forces and pitching moment on adapted meshes are found to be converging to the fine-mesh values faster than those on fixed meshes. In addition to forces and moments, the convergence of surface pressure and skin-friction coefficients at various measurement locations on the wing are also presented. Adapted-mesh surface pressure distributions agree with the fine fixed mesh pressure distributions. Adapted-mesh skin-friction distributions contain high-frequency noise with mean values approaching the fixed mesh pressure skin-friction distributions.

CFD simulations of wind flow and mean surface pressure for buildings with balconies: Comparison of RANS and LES

Façade geometrical details can substantially influence the near-façade airflow patterns and pressures. This is especially the case for building balconies as their presence can lead to multiple separation and recirculation areas near the façades and hence large changes in surface pressure distribution. Computational fluid dynamics (CFD) has been widely used to investigate the impact of building balconies, mainly based on the steady Reynolds-averaged Navier-Stokes (RANS) approach. The objective of the present study is to evaluate the performance of steady RANS and large-eddy simulations (LES) in predicting the near-façade airflow patterns and mean surface pressure coefficients (Cp) for a building with balconies for three wind directions θ = 0°, 90°, 180°, where 0° is perpendicular to the façade under study. The evaluation is based on validation with wind-tunnel measurements of Cp. The results show that both RANS and LES can accurately predict Cp on the windward façade for θ = 0° with average absolute deviations of 0.113 and 0.091 from the measured data, respectively. For the other two wind directions, LES is clearly superior. For θ = 90°, the average absolute deviations for RANS and LES are 0.302 and 0.096, while these are 0.161 and 0.038 for θ = 180°. Large differences are found in the computed flow fields on the balcony spaces. Because RANS systematically underestimates the absolute values of both Cp and mean wind speed on the balconies, it is suggested that building design based on RANS might result in excessive ventilation and in too high wind nuisance level.

Quantification of Airfoil Aerodynamic Uncertainty due to Pressure-Sensitive Paint Thickness

In this paper, uncertainty quantification is used to investigate the propagation of the uncertainty of the pressure-sensitive paint (PSP) thickness distribution to the uncertainty of aerodynamic force measured. Specifically, airfoil surface pressure coefficient () and aerodynamic lift coefficient () of a natural laminar flow wing are analyzed with uncertain PSP thickness distribution. The airfoil with PSP applied is parameterized by using a novel parameterization method based on radial basis function interpolation. As a characteristic of wing aerodynamic performance, is determined by surface pressure distribution, which can be affected by uncertainties of PSP binder thickness. A kriging response surface method has been used to develop a surrogate model to represent the Reynolds-averaged Navier–Stokes solution of the flow around the airfoil with PSP. This enables the use of Monte Carlo simulations to obtain stochastic output, including the probability of the occurrence of significantly inaccurate output values affected by the inputs subjecting to Gaussian distributions. An error margin and associated probability for its occurrence are recommended to account for uncertainties for the aerodynamic force occurring in the PSP measurement.

Experimental study on cross-ventilation of a generic building in highly-dense urban areas: Impact of planar area density and wind direction

This study presents the experimental results on cross-ventilation in a generic low-rise building placed in highly-dense urban configurations. Flow visualization studies were conducted by utilization of a smoke generator in order to investigate the nature of the flow pattern inside and around the cross-ventilated building. Moreover, distribution of the wind surface pressure coefficients over windward and leeward façades and internal walls of the target building were measured using a pressure tap system. Furthermore, the airflow rate crossing through the openings was measured using a tracer gas method. Different building configurations, representing highly-dense urban areas, as well as different wind angles were investigated in this study.Surprisingly, the experimental results reveal a noticeable difference between the mechanism of cross-ventilation in moderately-dense and highly-dense buildings arrangements. A clear leeward jet with a highly-transient nature can be observed, which is generated due to a leeward vortex formed by the target and downstream buildings. As another novel finding of this study, the cross-ventilation is understood to be highly transient in highly-dense urban areas with a strong periodic fresh air pulsation through the windward and leeward openings. This behavior is fundamentally far from the steady state models considered for such cross-ventilation scenarios in literature.

Data reconstruction at surface in immersed-boundary methods

This work compares interpolation techniques for data reconstruction at the surface in an immersed-boundary method. Three different methods of surface pressure reconstruction based on inverse distances are presented, which are christened as: Inverse Distance Weight (IDW) method, Inverse Distance Weight at Interpolation Point method (IDW-IP) and Inverse Distance Weight based on Upwinding (IDW-Upwind) method. Additionally, shear stress at the immersed surface is determined using two approaches: direct interpolation of velocity gradient at the surface using IDW method, and interpolation of velocity at a point along the surface normal using IDW-IP method. The interpolation methods are verified against analytic solutions of ideal flow past a circular cylinder and subsonic-supersonic inviscid flow in a convergent-divergent nozzle, and validated against laminar flow simulations of Mach 0.5 flow past a NACA0012 airfoil, Mach 2.0 flow past a circular cylinder, and Mach 3.0 flow past a 10∘ ramp. The verification cases show that while the pressure values reconstructed at the surface by the three interpolation methods are very similar for the incompressible flow, the IDW-Upwind method produces the sharpest pressure rise across the normal shock in the convergent-divergent nozzle. Comparisons of the reconstructed surface pressure coefficient (Cp) and skin-friction coefficient (Cf) with values available from literature or ANSYS-Fluent simulations conducted as part of the validation study show good match, but indicate that the reconstructed pressure and shear stress values at the immersed surface has noise, which, however, reduces with grid refinement. Further, the IDW and IDW-Upwind method for pressure reconstruction, and the gradient reconstruction based method for shear stress calculation are shown to produce less noise in computed values. Integrated drag and lift values using the reconstructed surface pressure and shear stress indicate that while the different methods used for pressure reconstruction result in similar values of aerodynamic loads, the gradient-based shear stress calculations result in more accurate load estimation. Finally, one of the interpolation methods (IDW-Upwind) is used to investigate the variation of the surface pressure coefficient with time for a NACA0012 airfoil undergoing non-periodic plunge motion in a Mach 0.2 flow. The computed surface pressure coefficients are correlated with the leading and trailing edge vortices in the flow field.

Study on Pressure Coefficient in Process of Double Slot Nozzle Jet Impingement on Sinusoidal Colliding Strip

Air cushion furnace is widely used in the heat treatment process of aluminum strip, copper strip and silicon steel, and the double slot nozzle is the core part of the air cushion furnace device. Studying the distribution characteristics of the surface pressure coefficient of the strip during the suspension process could provide important theoretical references for the high-efficiency and high-value production of the air-cushion furnace. In this paper, the distribution of pressure coefficients is studied under the conditions of Re=48 194~12 048, high H/D=1~3, spacing L/D=17.2 and sinusoidal amplitude A=0~30 mm. The results show that with the decrease of L/D, the position of the main stop point moves to the center line of the nozzle, and the value of the second stop point increases gradually, and with the increase of the sinusoidal amplitude A, the position of the main stop point moves gradually to x/D=0. The pressure coefficient Cp decreases with the increase of sinusoidal amplitude A, the pressure coefficient Cp decreases with the increase of H/D, and the difference between the pressure coefficient at the stationary point of the double-slot nozzle and the pressure coefficient at the non-stationary point increases with the increase of H/D. This study provides an important theoretical basis for the design of double slot nozzles in air cushion furnace.

RANS and hybrid LES/RANS simulations of flow over a square cylinder

Unsteady RANS (URANS), hybrid LES/RANS and IDDES simulations were conducted to numerically investigate the velocity field around, and pressures distribution and forces over a square cylinder immersed in a uniform, steady oncoming flow with Reynolds number Re = 21,400. The vortex shedding responses in terms of Strouhal number, the pressure distribution, the velocity profile and the velocity fluctuations obtained by numerical simulations are compared with experimental data. Compared with 2D URANS simulation, 3D simulations using hybrid LES/RANS and IDDES models provide more accurate prediction on the responses in the wake, including mean streamwise velocity profile and rms velocity fluctuations. This also results in more accurate prediction of time-averaged surface pressure coefficient on the rear surface obtained by 3D hybrid LES/RANS and IDDES simulations than by URANS simulation. When a hybrid LES/RANS model or IDDES model is used, a more accurate prediction for either pressure coefficient or velocity profile (especially in the far wake region) is not guaranteed by increasing the mesh resolution along the spanwise direction of the square cylinder.

Numerical simulation and experiments on turbulent air flow around the semi-circular profile at zero angle of attack and moderate Reynolds number

Two-dimensional flow around a semi-circular profile at the zero angle of attack and at Re = 50000 on the self-oscillatory period is extensively studied by the URANS method involving the standard semi-empirical SST turbulence models, the SST turbulence model with the correction for streamline curvature modified within the Rodi-Leschziner-Isaev and Smirnov-Menter approaches, as well as involving Hanjalic's four-parameter eddy viscosity elliptic relaxation model and its analog - eddy viscosity elliptic blending model proposed in the present work. This has been done with the use of different-structure grids (multiblock with structured overlapping and unstructured composite). Different numerical approximation methods realized in six codes (VP2/3, SigmaFlow, Fluent, CFX, OpenFOAM, and StarCCM+) are used. An underestimation (up to 30%) of time-averaged integral aerodynamic loads is revealed by means of the standard near-wall SST model. This is explained by the high vortex viscosity production in the profile wake. Wind tunnel tests show that the location of cutoff washers on the semi-circular profile provides a quasi-two-dimensional flow around it and allows applying measurement data to verify two-dimensional turbulent flow. The best agreement of experimental results and numerical predictions when comparing the Strouhal number and time-averaged surface pressure coefficient distributions is achieved using both the modified SST model with the correction for streamline curvature and the modified eddy viscosity elliptic blending model. When the SST model with the correction for streamline curvature, modified within the Rodi-Leschziner-Isaev, Smirnov-Menter and Durbin approaches, is used, all the above codes yield close predictions of a vertical aerodynamic load on the oscillation period.

Transonic static aeroelastic and longitudinal aerodynamic characteristics of a low-aspect-ratio swept wing

Based on the CFD/CSD coupling method, the transonic static aeroelastic characteristics of the low-aspect-ratio swept wing was studied by numerical simulation. The Navier-Stokes equation was used as the fluid computational control equations. The flow field mesh deformation was performed using the Transfinite Interpolation method (TFI) method, and the data exchange on the CFD/CSD coupling interface was realized by the Constant Volume Transform (CVT) method. Based on the above method, the transonic static aeroelastic coupling calculation program was developed. The results of ONERA M6 wing static aeroelastic calculated by the coupling program were compared with wind tunnel test and reference results, and the effectiveness of the program was verified. Through numerical simulation, the influence of different angles of attack and Mach number on the static aeroelastic deformation and torsion angle of the wing are studied. Finally, by comparing the surface pressure cloud and pressure coefficient of the rigid wing and the elastic wing, the influence of static aeroelastic on the longitudinal aerodynamic characteristics and stability of the wing is mainly analyzed.

Aerodynamic Performance of Micro Aerial Wing Structures at Low Reynolds Number

Corrugations are folds on a surface as found on wings of dragon fly insects. Although they fly at relatively lower altitudes its wings are adapted for better aerodynamic and aero-elastic characteristics. In the present work, three airfoil geometries were studied using the 2-D panel method to evaluate the aerodynamic performance for low Reynolds number. The experiments were conducted in wind tunnel for incompressible flow regime to demonstrate the coefficients of lift drag and glide ratio at two Reynolds numbers 1.9x104 and 1.5x105 and for angles of attack ranging between 00 and 160. The panel method results have been validated using the current and existing experiment data as well as with the computational work from cited literature. A good agreement between the experimental and the panel methods were found for low angles of attack. The results showed that till 80 angle of attack higher lift coefficient and lower drag coefficient are obtainable for corrugated airfoils as compared to NACA 0010. The validation of surface pressure coefficients for all three airfoils using the panel method at 40 angles of attack was done. The contours of the non-dimensional pressure and velocity are illustrated from -100 to 200 angles of attack. A good correlation between the experiment data and the computational methods revealed that the corrugated airfoils exhibit better aerodynamic performance than NACA 0010.

Buoyancy-Aided Mixed Convection Between Shear-Thinning Non-Newtonian Nanofluids and Unbounded Elliptic Cylinders in a Vertical Channel

This work presents buoyancy-driven mixed convective flow and heat transfer phenomena of an isothermally heated horizontal elliptic cylinder in vertically upward unbounded flow of power-law type non-Newtonian nanofluids using ANSYS Fluent. The governing continuity, momentum and energy equations for the shear-thinning power-law nanofluids along with suitable boundary conditions are simultaneously solved within the limitations of Boussinesq approximation. The semi implicit method for pressure-linked equations algorithm along with the quadratic upstream interpolation for convective kinematics scheme for discretizing the convective terms in both momentum and energy equations are adopted. The ranges of parameters considered for this study are: volume fraction of nanoparticles, 0.005–0.045; aspect ratio of elliptic cylinder, 0.5–2.5; and Richardson number, 0–40; and a representative Reynolds number of 20. The streamline patterns, surface pressure coefficient distributions, total drag coefficients, isotherm contours, and Nusselt numbers are presented for better understanding of heat transfer and flow phenomena around elliptic cylinders. Briefly results indicate that the total drag coefficient is found to increase with the increasing Richardson number whereas it decreases with the increasing volume fraction of nanoparticles. The average Nusselt numbers are found to increase with increasing Richardson number and increasing volume fraction of nanoparticles.

Improved Hydrodynamic Analysis of 3-D Hydrofoil and Marine Propeller Using the Potential Panel Method Based on B-Spline Scheme

In this paper, the hydrodynamic performance of lift-body marine propellers and hydrofoils is analyzed using a B-spline potential-based panel method. The potential panel method, based on a combination of two singularity elements, is proposed, and a B-spline curve interpolation method is integrated with the interpolation of the corner points and collocation points to ensure accuracy and continuity of the interpolation points. The B-spline interpolation is used for the distribution of the singularity elements on a complex surface to ensure continuity of the results for the intensity of the singular points and to reduce the possibility of abrupt changes in the surface velocity potential to a certain extent. A conventional cubic spline method is also implemented as a comparison of the proposed method. The surface pressure coefficient and lift the performance of 2-D and 3-D hydrofoils of sweepback and dihedral type with different aspect ratios are analyzed to verify the rationality and feasibility of the present method. The surface pressure distribution and coefficients of thrust and torque are calculated for different marine propellers and compared with the experimental data. A parametric study on the propeller wake model was carried out. The validated results show that it is practical to improve the accuracy of hydrodynamic performance prediction using the improved potential panel method proposed.