Pressure Coefficient Cp Research Articles

Study of board bending degree on hydrodynamic performances of a single-layer cambered otter-board

The effect of board bending degree on hydrodynamic performances of a single-layer cambered otter-board was investigated using engineering models in a wind tunnel. Three different bending degree boards were evaluated at a wind speed of 28 m/s. Parameters measured included: drag coefficient Cx, lift coefficient Cy, pitch moment coefficient Cm, center of pressure coefficient Cp , over a range of angle of attack (0° to 70°). These coefficients were used in analyzing the differences in the performance among the three otter-board models. Results showed that the bending of the board(No. 2, No. 3) increased the water resistance of the otter-board, and improved the lift coefficient of the otter-board in the small angle of attack (0°<α≤20 °) ; the maximum lift coefficients Cy of otter-board model (No. 1) was higher (1.680, α = 25°). the maximum lift–drag ratios of models (No. 1, No. 2 and No. 3) are 6.822 (α = 7.5 °), 6.533 (α = 2.5 °) and 6.384 (α = 5.0°), which showed that the board bending reduces the lift-to-drag ratio of the otter-board.The stability of the No. 3 model was better than those two models (No. 1, No. 2) in most range of attack angle, but No. 1 otter-board model had a better stability in roll of otter-board. The findings of this study can offer useful reference data for the structural optimization of otter-boards for trawling.

Study of the turbulence models over an aircraft wing

Aerodynamics is defined as the science of handling a fluid that is often the air interacting with a structure. When simulating the flow over airfoils, transition from laminar to turbulent flow plays an important role in determining the flow features and in quantifying the airfoil performance such as lift and drag. These fluidic flows are subjected to viscous stresses and inertia which produces disordered fluctuations, so turbulence affects the behavior of the aerodynamic flow as well as the structure interacting with the fluid in a range of high Reynolds, indeed, it is obliged to control these turbulent flows in this area in order to give a good design of the structure. Several models of turbulence have been developed to facilitate the calculation of characteristic quantities to optimize the simulation of turbulent flows in aerodynamics. In this paper, we carried out a validation of a numerical simulation of a 3D transonic flow over the ONERA M6 wing for which the numerical results, performed using ANSYS/FLUENT©, will be compared with experimental data and NASA CFD results consisting on the pressure coefficient (Cp) along the upper and lower wing surfaces. The flow was obtained by solving the steady-state governing equations of continuity and momentum conservation combined with one of five turbulence models (Spalart-Allmaras (S-A), standard k-e, k-e RNG, standard k-ω and k-ω SST) aiming to the validation of these models through the comparison of the predictions and the free field experimental measurements for the selected wing.

Investigation of a downburst loading event on a full-scale low-rise building

This paper details a downburst event that occurred on June 19, 2003, where both wind and wind-induced pressure data were recorded at the Texas Tech University Wind Engineering Research Field Laboratory (WERFL). This event displayed rapid changes in wind speed (i.e. ramp-up and ramp-down) and wind direction over a period of approximately 30 s. The event also exhibited an atypical vertical wind profile with a peak measured wind speed occurring at 4 m and wind direction changes up to 30 deg over the 50 m anemometer tower height. Wind loading on the WERFL building also displayed ramp-up and ramp-down behavior. When converting these data to pressure coefficients (Cp,3), non-stationary pressures were normalized by a 3-s wind speed, which was shifted by a time lag to account for the spatial separation between building and tower. Differences were discovered in Cp,3 between ramp-up and ramp-down periods. To ascertain any differences between downburst loading and that induced by atmospheric boundary layer (ABL) wind fields, loading characteristics were compared in the same fashion with 128 ABL events at WERFL. Analysis of the comparisons concluded that downburst Cp,3 values mostly stayed within the bounds of the ABL, however fluctuate between extrema of ABL distributions. A significant difference between ramp-up and ramp-down periods was further solidified as ramp-up (ramp-down) Cp,3 values were lower (higher) than expected in the ABL. A physical explanation for these differences is unclear, and further study is needed to ascertain its validity.

Separation bubble characteristics on an axial flow over a cylinder with different noses

Experiments were conducted to study the characteristics of the leading edge separation bubble on an axial cylinder with different nose shapes at different angles of attack and Reynolds numbers. The blunt, conical and hemispherical cylinder noses were examined. The angle of attack α and Reynolds number ReD, based on cylinder diameter D, were varied from 0° (axial) to 3.5° and from 1.5 × 103 to 4.2 × 104, respectively. The time-mean pressure coefficient (Cp) and fluctuating (rms) pressure coefficient Cp′ were measured on the cylinder surface at x/D = 0.15, 1.0 and 2.5. The shear layer reattachment length xR, shear layer transition length xTr, and bubble height W were determined by particle image velocimetry (PIV) and flow visualization techniques. The xR, xTr, and W all shrink substantially with increasing ReD upto ReD = 104. For ReD > 104, variations in xR and W are insignificant but xTr keeps shrinking. At a given ReD, xR and W retreat progressively from the blunt nose to the conical and then to the hemispherical; meanwhile, xTr is prolonged. Following xR, W and xTr, both time-mean pressure coefficient (Cp) and fluctuating (rms) pressure coefficient Cp′ in the bubble are highly sensitive to ReD for ReD < 104, but less so for ReD > 104. The magnitude of Cp is large near the separation and much smaller around and behind the reattachment. On the other hand, Cp′ is small near the separation but large around the reattachment. In the case of the hemispherical nose, the Cp magnitude is smallest of all. The Cp′ is highest for the blunt nose but lowest for the hemispherical. An increase in α leads to an elongation of xR and W and a reduction of xTr for all three noses.

Enhanced wind-driven infiltration model by incorporating wind direction for design purpose

Research has been conducted to improve and enhance existing LBNL infiltration model in order to incorporate wind direction effect on flow rate. Existing wind driven infiltration of LBNL model only consider terrain and shielding parameter as correction factor. Supplemental analytical model considered in this research was Swami-Chandra model, which the calculation was based on average wind pressure coefficient (Cp). It was evident that for same wind direction, Cp value will not change regardless of wind velocity. Process of simulation shows that infiltration modelling has to be performed in three-dimensional environment with transient calculation in order to achieve convergent result. Wind direction strongly affects infiltration flow rate. However, as the infiltration hole has relatively small size compared to wind velocity, flow rate variation is marginal for energy-based design. Nevertheless, for pollutant and particulate matter analysis, wind direction will give significant change in term of concentration and flow direction.

Numerical analysis of underwater flow past columnar projectile with different cross-sections at high Reynolds numbers

Based on Detached Eddy Simulation (DES) technique, the flow around a columnar projectile with different cross-section shapes in the supercritical and extremely supercritical region is simulated by the Fluent. The cross-section of the projectile is regular polygon, which number of edges is 4, 6, 8, 10, 12, 24 and ∞, where ∞ means a circle. The vortex shedding pattern and flow field characteristics are analyzed at Reynolds number 2.5×105 to 2×107. Regarding circular cylinder projectile, when the flow velocity changes from 25 m/s to 200 m/s, the average drag coefficient decreases, and the St Number increases. Regarding regular polygon, when the number of edges for polygon changes from 4 to ∞ at flow velocity 50 m/s, the average drag coefficient decreases, and the St Number increases. The average lift coefficient is almost equal to zero and does not change with the flow velocity and the cross-section. The pressure coefficient Cp of 4-prism, 6-prism, 8-prism, 12-prism and 24-prism has multiple local minimum values at the polygon vertices of the cross section. According to the spectrum analysis, the vortex shedding frequency of 4-prism, 24-prism and cylindrical is single and fixed, so the projectile may cause resonance and deviates from a predetermined trajectory. But for the 6-prism and 8-prism and 12-prism, the cl and cd is multi-periodic vibration. So, considering the flow induced structural vibrations, drag, the power of shrapnel and manufacturing cost, the 8-prism are better choices for cluster warhead underwater in engineering design.

New Surrogate Model for Wind Pressure Coefficients in a Schematic Urban Environment with a Regular Pattern

Natural ventilation and the use of fans are recognized as sustainable design strategies to reduce energy use while reaching thermal comfort. A big challenge for designers is to predict ventilation rates of buildings in dense urban areas. One significant factor for calculating the ventilation rate is the wind pressure coefficient (Cp). Cp values can be obtained at a high cost, via real measurements, wind tunnel experiments, or high computational effort via computational fluid dynamic (CFD) simulation. A fast surrogate model to predict Cp for a schematic urban environment is required for the integration in building performance simulations. There are well-known surrogate models for Cp. The average surface pressure coefficient model integrated in EnergyPlus considers only a box-shaped building, without surrounding buildings. CpCalc, a surrogate model for Cp, considers only one height of neighbouring buildings. The Toegepast Natuurwetenschappelijk Onderzoek (TNO) Cp Generator model was available via web interface, and could include several box-shaped buildings in the surrounding area. These models are complex for fast integration in a natural ventilation simulation. For optimization processes, with thousands of simulation runs, speed is even more essential. Our study proposes a new surrogate model for Cp estimation based on data obtained from the TNO CP Generator model. The new model considers the effect of different neighbouring buildings in a simplified urban configuration, with an orthogonal street pattern, box-shaped buildings, and repetitive dimensions. The developed surrogate model is fast, and can easily be integrated in a dynamic energy simulation tool like EnergyPlus for optimization of natural ventilation in the urban areas.

Wind pressure coefficients for roof ventilation purposes

Wind pressure coefficients (cp) are important inputs for analytical calculations of wind load. The aim of this research is to investigate wind pressure coefficients on a test house located in Norway in order to pave the way for improved analysis of wind-driven roofing ventilation. The large-scale test measurements show that the wind pressure coefficient along the eaves of the house varies with different wind approach angles. Assuming wind-driven air flow through the air cavity beneath the roofing, an average Δcp¯ value of 0.7 is derived for practical engineering purposes. The results from the study are applicable for single or two-storey houses with pitched roofs at different roof angles.

Effect of panel shape on hydrodynamic performances of vertical v-shaped double- slotted cambered otter-board

The effect of panel shape on hydrodynamic performances of a vertical v-shaped double-slotted cambered otter-board was investigated using engineering models in a wind tunnel. Three different shape panels (rhomboid, left trapezoid and isosceles trapezoid) were evaluated at a wind speed of 28 m/s. Parameters measured included: drag coefficient Cx, lift coefficient Cy, pitch moment coefficient Cm, center of pressure coefficient Cp , over a range of angle of attack (0° to 70°). These coefficients were used in analyzing the differences in the performance among the three otter-board models. Results showed that the maximum lift coefficient Cy of the otter-board model with the isosceles trapezoid shape panels was highest (2.103 at α=45°). The maximum Cy/Cx of the otter-board with the rhomboid shape panels was highest (3.976 at α=15°). A comparative analysis of Cm and Cp showed that the stability of otter-board model with the isosceles trapezoid shape panels is better in pitch, and the stability of otter-board model with the left trapezoid shape panels is better in roll. The findings of this study can offer useful reference data for the structural optimization of otter-boards for trawling.

Effect of deflector curvature on hydrodynamic performances of a double-slotted cambered otter-board

The effect of deflector curvature on hydrodynamic performances of a double-slotted cambered otter-board was investigated using engineering models in a wind tunnel. Four different curvature (0.06,0.09, 0.12 and 0.15) were evaluated at a wind speed of 28 m/s. Parameters measured included: drag coefficient Cx, lift coefficient Cy, pitch moment coefficient Cm, center of pressure coefficient Cp , over a range of angle of attack (0° to 70°). These coefficients were used in analyzing the differences in the performance among the four otter-board models. Results showed that the maximum lift coefficient Cy of the otter-board model with the curvature (0.06) of two deflectors was highest (2.020 at °=55°). The maximum Cy/Cx of the otter-board with the curvature (0.12) of two deflectors was highest (3.655 at °=22.5°). A comparative analysis of Cm and Cp showed that the stability of otter-board model with the curvature (0.12) of two deflectors is better in pitch, and the stability of otter-board model with the curvature (0.06) of two deflectors is better in roll. The findings of this study can offer useful reference data for the structural optimization of otter-boards for trawling.

Evaluation of wind pressure acting on multi-span greenhouses using CFD technique, Part 2: Application of the CFD model

Revision of greenhouse design standards is required to ensure the structural safety of greenhouses in strong wind environments. The wind pressure coefficients (Cp) of various greenhouses must be evaluated to revise the newly modified greenhouse design standards. In this study, the Cp values of multi-span greenhouses that are typical in South Korea, e.g., wide-span, Venlo, and 1-2W type greenhouses, were estimated using the developed computational fluid dynamics (CFD) model from the first paper. In particular, the CFD-computed Cp values were analysed with consideration of the wind directions, number of spans, and greenhouse design factors, such as the roof slope and roof curvature radius. Based on the analysed results, the CFD-computed Cp values of multi-span greenhouses were suggested for use in structural and cladding designs based on the defined sections of the greenhouse surface, various wind directions, number of spans and design factors. The Cp values for structural design were analysed for wind blowing in the direction perpendicular to the ridge (0°) and in the direction of the ridge (90°). In addition, the maximum Cp values were examined for cladding design with consideration of every wind direction.

Investigation of the pressure coefficient impact on the air infiltration in buildings with respect to microclimate

Wind pressure coefficients (Cp) are important for calculating the air infiltration rate of a building. Cp depend on a wide range of parameters, such as wind direction and speed, position on the building surface and façade exposure. Most Building Energy Simulation (BES) programs generally calculate air infiltration using simplified pressure coefficients. The Norwegian technical software (SIMIEN) do not consider them. The effect of the pressure coefficients on the air infiltration of the building, taking into consideration the microclimate of the building area, is studied. The wind pressure coefficients are defined through computational fluid dynamics (CFD) simulations. The air infiltration in the building is determined based on the calculated pressure coefficients. Tracer gas measurements were used for validation of the results. Two cases of fully exposed and partially sheltered building were examined. The results indicate that wind pressure coefficients derived from CFD simulations calculate more realistic values for the air infiltration of the building.

Influence of main-panel angle on the hydrodynamic performance of a single-slotted cambered otter-board

The effect of the main-panel angle of a single-slotted cambered otter-board was investigated using engineering models in a wind tunnel. Three different angles (0°, 6°, and 12°) were evaluated at a wind speed of 28 m/s. Parameters measured included: drag coefficient Cx, lift coefficient Cy, pitch moment coefficient Cm, center of pressure coefficient Cp, and the lift–drag ratio Cy/Cx, over a range of angle of attack (0°–70°). These coefficients were used in analyzing the differences in the performance among the three otter-board models. Results showed that the maximum lift coefficient Cy of the otter-board model with a main-panel angle of 0° was highest (1.875 at α = 25°). However, when the angle of attack was smaller (0 < α < 22.5°), the lift coefficient of the otter-board increased as the angle of the main-panel increased. The maximum Cy/Cx of the otter-board with a main-panel angle of 12° was highest (7.417 at α = 2.5°), and the lift–drag ratio increased when the angle of the main-panel increased within the angle of attack at small angles (0 < α < 12.5°). A comparative analysis of Cm and Cp showed that the stability of the otter-board with a main-panel angle of 0° is better than those of the other models. Therefore, the comparative analysis of Cm and Cp, shows that a larger angle of the main-panel can reduce the stability of single-slotted otter-board. The findings of this study offer useful reference data for the structural optimization of otter-boards for trawling.

Wind effect on grooved and scallop domes

This paper numerically studies the wind effect on grooved and scallop domes. The introduction of a groove on a spherical dome causes abrupt change on its wind pressure coefficient (Cp) in the vicinity of the groove. The sharpness of indentation varies with the position angle of the axis of the groove to wind direction and obtains its highest effect at 90°. This paper develops equations for the distribution of Cp on the surface of spherical, grooved and scallop domes with rise to span ratios [0, 0.7]. The results of the equations agree reasonably well with that of CFD.

Naturally ventilated double-skin façade in modeling and experiments

The modeling activity presented in this work aims at the assessment of a simplified model, named BS model, which was specifically developed for integration of DSF in Building Simulation. The BS model is based on a pressure loop and on an integral approach to the heat transfer along the vertical channel. It considers buoyancy as a function of the average temperature in the channel. The wind action is taken into account by means of wind pressure coefficients (Cp) on the façade openings.The focus of this study is the experimental validation of the modeling “core”: the natural ventilation through the DSF. The validation is based on the dataset of the experimental campaign conducted on a DSF test facility, the “Cube”, in Denmark, under IEA ECBCS ANNEX 43/SHC Task 34. Hourly simulations were performed with the BS model for the 15days of the experimental campaign.A CFD modeling activity was also carried out on a selection of four cases, extracted from the experimental benchmark and representative of different temperature and pressure boundary conditions.The results show that the BS model presents a good level of agreement with the experimental data in predicting the mass flow rate and the heat removed by ventilation. Although the two experimental methods used to determine the airflow rate in the DSF cavity produce in many cases divergent results, it was possible to distinguish valid experimental results for comparison with the BS model. This was possible thanks to a thorough analysis of the experimental procedure together with the insight provided by the model into the determination of the driving wind and thermal differential pressures. In particular, by selecting only the measurements associated to sufficiently low wind fluctuations in the hourly averaged data, a good degree of correlation was found between the predicted total driving pressure and the flow measurements.Concerning the four cases investigated also by means of CFD, the agreement between the BS and CFD models is remarkable in terms of outlet temperatures and in the prediction of flow reversal.

Effect of bevel angle of three-layer parallel plate on hydrodynamic performances of otter-board

The effect of bevel angle of three-layer parallel plate of otter-board on hydrodynamic performances is investigated by model wind tunnel test. Four otter-board models are designed with four different bevel angles (8°, 10°, 12° and 14°) , and tested under the wind speed 28 m / s in wind tunnel. Model experiment is conducted to obtain the drag coefficients Cx, the lift coefficient Cy, the pitch moment coefficient Cm, the center of pressure coefficient Cp and calculated the lift-drag ratio Cy/Cx. The result shows that the maximum lift coefficient and the maximum lift-drag ratio of the otter-board model with a bevel angle 12 degree is higher, there is 2.598(a=65°) and 2.607(a=37.5°); For comparison in stability of otter-board, the stability of the otter-board model with the bevel angle 8 degree is better by the comparative analysis of Cm and Cp, the absolute value of Cm is 0.174 and the minimum variation coefficient of Cp is 4.37%. The results can offer reference for the structural optimization design of trawl otter-board.

Wind and buoyancy driven natural ventilation in double skin façades

ABSTRACTThe computational fluid dynamics (CFD) modelling activity presented in this work aims at investigating the reliability of the assumptions employed in a double skin façades (DSFs) simplified model, developed for the integration of naturally ventilated DSFs in building simulation (BS) tools. The simplified fluid-dynamic model considers both the wind action, by means of pressure coefficients (Cp) on the openings, and the buoyancy inside the ventilated channel. Both the BS and the CFD models have been assessed using the database of an experimental campaign carried out in a full scale test facility, named ‘The Cube’, and made available by the Department of Civil Engineering of Aalborg University. The results show a good agreement between the CFD and BS models in terms of predicted temperature increase, with a maximum deviation of 15%. Both models exhibit a high sensitivity to the imposed differential wind pressure, which depends on the Cp source employed. The determination of proper Cp values for the specific case is, hence, a crucial aspect. Moreover, the CFD analysis offers a deeper insight on surface heat transfer and suggests the need to take into account the interplay between thermal and wind-driven convection, which ultimately gives rise to a mixed convection phenomenon.

Experimental study of wind loads on unique buildings and structures in Russia

Design and construction of unique buildings and structures (sports arenas, airport complexes, business centres, etc.) from an engineering point of view is a very difficult task as in most cases these facilities have an original architectural form. Therefore, consideration of wind loads is an important part of the design. The paper presents the definition of wind load for two complex of airport. Researches was applied the combined calculation an experimental method. During the experimental study a wind tunnel architectural and construction type NRU MSUCE was used. Numerical simulations were performed using the software package ANSYS. The result of research on each object are integral aerodynamic loads on the object (coefficients Cx, Cy, Cmz) and picture of the distribution of aerodynamic pressure coefficient Cp obtained in the numerical simulation. In conclusion, we discuss the possible formation of deposits of snow and recommendations to eliminate them from the roof of researched objects.

Analysis of the Fluid Flow in 3D Symmetric Enlarged Channel

A 3-Dimensional fluid flow over the sudden expansion region of a horizontal duct for various Reynolds numbers have been studied by using the CFD Software package ANSYS Workbench Fluent v 13.0. The expansion ratio and aspect ratio for the sudden expansion are taken as 2.5 and 4 respectively. This work deals with the finding of critical Reynolds number for a fluid and also the length of re-attachments on stepped walls at various Reynolds numbers for the same fluid. The simulation is carried out in sudden expansion for Reynolds number ranging from 200 to 4000. The variations of local Nusselt number along the stepped walls of the sudden expansion are presented with the heat flux of 35 W/m2 on the stepped walls. Also, the plots of pressure coefficient (Cp) along the stepped walls for different Reynolds numbers are presented in this work.

The effect of low Reynolds number flows on pitot tube measurements

An investigation on the low Reynolds number effect on hemispherical-tipped Pitot tube measurements was performed by measuring the center-line velocity during the laminar flow of a Newtonian fluid in a 25mm (1in.) diameter vertical recirculating pipe loop. The primary objective of the study was to reconsider the available low Reynolds number Pitot tube data in the literature with modern instrumentation.Using the results of this experimental study, a correlation that accurately predicts the low Reynolds number Pitot tube behavior has been developed. The correlation accounts for an additional viscous term in the relationship for the pressure coefficient (Cp) which is not accounted for in Bernoulli's Equation. The correlation is semi-empirical and accurately fits experimental data gathered in this study, as well as a significant body of experimental data available in the literature. The correlation, which is based on a Pitot tube Reynolds number calculated using the opening diameter (d), has been shown to be provide more accurate predictions of Cp for a wide range of opening diameter to outer diameter ratios (0.22≤d/D≤0.6) than available correlations based on outer diameter.The transition Pitot tube Reynolds number, below which Bernoulli's Equation is no longer appropriate, was predicted to be approximately 35, compared to a value of 79 obtained from fitting data collected by Barker. The correlation developed in this study provides smoother transitions at both ends of the low Reynolds range. At the low end (Re<10) it converges with a Stokes Law’ analogy, while at the critical transition (Re~35) it converges asymptotically with Bernoulli's Equation. The correlation also accurately predicts the behavior of the pressure coefficient with Reynolds numbers between these ranges.