Favorable Pressure Gradient Research Articles

Effects of compound angle on film cooling effectiveness with different streamwise pressure gradient and convex curvature

Streamwise pressure gradient and surface curvature are two important characteristics of turbine flow and compound angle film cooling is an efficient way to improve cooling performance. Compound angle film cooling effectiveness with those two effects was investigated using PSP technology. The corresponding numerical studies were validated and carried out to investigate the physical mechanism of the interaction between the coolant jet and the main flow. Compound angle increases film cooling effectiveness for high blowing ratios in the near hole region, but the stronger interaction between the jet and the main flow makes the coolant dissipate faster. On the flat surface, the favorable pressure gradient leads to a maximum of 50% increase of film cooling effectiveness compared with the adverse pressure gradient case. On the convex surface, the cooling jet is pushed to the wall by the radial pressure gradient in the free stream which makes film cooling effectiveness 20–50% higher than the flat wall case. The compound angle film cooling performance under strong favorable pressure gradient for the convex surface is quite different from the flat wall case and thus streamwise pressure gradient and surface curvature should be taken into consideration in gas turbine film cooling design.

Shear‐rate‐dependent rheology effects on mass transport and surface reactions in biomicrofluidic devices

Consideration is given to shear‐rate‐dependent rheology effects on mass transport in a heterogeneous microreactor of rectangular cross section, utilizing both numerical and analytical approaches. The carrier liquid obeys the power‐law viscosity model and is actuated primarily by an electrokinetic pumping mechanism. It is discovered that, considering the shear‐thinning biofluids to be Newtonian fluids gives rise to an overestimation of the saturation time. The degree of overestimation is higher in the presence of large Damkohler numbers and electric double layer thicknesses. It is also increased by the application of a favorable pressure gradient, whereas the opposite is true when an opposed pressure gradient is applied. In addition, a channel of square cross section corresponds to the maximum fluid rheology effects. Finally, the numerical results indicate the existence of a concentration wave when using long channels. This is confirmed by analytical solutions, providing a closed form solution for wave propagation speed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1912–1924, 2015

Supercritical natural laminar flow airfoil optimization for regional aircraft wing design

An optimization design method of supercritical natural laminar flow airfoil based on Genetic Algorithm and Computational Fluid Dynamics is tested in this paper. Class Shape Transformation method is adopted as geometry parameterization method. Constraints on pressure distribution are applied to gain appropriate flow field in addition to the L/D performance. A fixed transition computation method is used in the optimization process to save computation time while giving the reasonable friction drag estimation and predicting the influence of the laminar boundary layer on airfoil performances. Specified favorable pressure gradient constraints are used to guarantee the expected laminar length. Objective of optimization is set to weaken the shock wave and minimize the pressure drag. Such a simplified NLF optimization process is verified by natural transition computation. The optimal setting of the favorable pressure gradient constraint, which is important for the trade-off between drag reduction and laminar stability, is then studied via numerical investigation. Results show that the airfoil optimized by constraining a favorable pressure gradient larger than 0.2 is good for both cruise efficiency and robustness. A natural laminar wing is then designed based on the optimized airfoil. Numerical verifications show that the wing has good natural laminar performance and low speed behavior.

Boundary Layer Effect on Jet Control Effectiveness

The mixing characteristics of a sonic jet issuing from a circular orifice and nozzle of same exit area, controlled by U-shaped tabs were studied experimentally, in the presence of a favorable pressure gradients corresponding to nozzle pressure ratios (NPRs) of 3, 4 and 5. Mixing of controlled and uncontrolled jets from orifice and nozzle, with the surrounding environment has been analyzed, using the jet centerline pressure distribution. The waves prevailing in the jet field were visualized using shadowgraph technique.

Interaction of inner and outer layers in plane and radial wall jets

Large eddy simulations of turbulent radial and plane wall jets were performed at different Reynolds numbers using the Lagrangian dynamic eddy viscosity subgrid-scale model. The results were validated with experimental data available in the literature. Compared to the plane ones, the radial wall jets have an extra direction for expansion, which causes faster decay rates. Thus, the resulting pressure gradient distributions are different. However, the comparison of the results with the turbulent boundary layers under adverse and favourable pressure gradients reveals that these pressure gradients are not strong enough to cause any fundamental physical difference between plane and radial wall jets. In both cases, the local Reynolds number is an important determining factor in characterisation of the flow. The joint probability density function analysis shows that the local Reynolds number determines the level of intrusion of the outer layer into the inner layer: the lower the local Reynolds number, the stronger is the interaction of the inner and outer layers. These results can be used to clarify the scatter of the reported log-law constants in the literature.

Flow and heat transfer characteristics around egg-shaped tube

This paper studies the flow and heat transfer characteristics around a new type of egg-shaped tubes made up of a semicircle upstream and a semi-ellipse downstream, numerically and experimentally, for Reynolds numbers varying from 4 000 to 5 0000 based on the hydraulic diameter. These tubes have axis ratios, ɛ = 1, 1.5, 2, 3, 4, and 5 with the major axis parallel to the main-stream. When ɛ = 1, the tube is a circular one and when 1/ɛ = 0, it is a flat plate. Numerical results show that the minimum value of Cp occurs at an angular position, which decreases as decreases, and the maximum value of Cf gradually increases with the increase of Simulated results agree very well with those available in the existing literature. The egg-shaped tube has a higher favorable pressure gradient at its front and a lower adverse pressure gradient at its back, that helps the flow delay a separation from the tubes' surface. Empirical correlations for each tube are obtained by numerical simulations relating the dimensionless heat transfer coefficient with the Reynolds Number and the Prandtl Number. The performance evaluation criterion (PEC) is used to analyze the level of the heat transfer enhancement for egg-shaped tubes. It is found that an egg-shaped tube with ɛ = 2 has the best overall heat transfer performance when Re > 11952.

Mixing Characteristics of Underexpanded Elliptic Sonic Jets from Orifice and Nozzle

The mixing characteristics of elliptical sonic jets issuing from an orifice and a convergent nozzle with an identical elliptical exit of aspect ratio of two were studied experimentally in the presence of favorable pressure gradients corresponding to nozzle pressure ratios of two to five, in steps of one. To quantify the aerodynamic mixing of the free jets with the fluid entrained from the surrounding environment, the jet centerline pitot pressure distribution and radial pitot pressure profiles along the major and minor axes directions of the jet were measured. The centerline pitot pressure variation of the orifice and nozzle jets shows that the decay of the jet from the nozzle is faster than the jet from the orifice at nozzle pressure ratios more than two. But at a nozzle pressure ratio of two, the decay of both the jets are close to each other. The pressure profiles demonstrate that the axis-switching location shifts upstream with an increase of nozzle pressure ratio for both the jets, implying that the near-field mixing is enhanced by the increase in the favorable pressure gradient. The waves prevailing in the jet field were visualized with the shadowgraph technique.

A new wind tunnel for the study of pressure-induced separating and reattaching flows

AbstractThe design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-induced, turbulent separation bubble on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness just upstream of separation, is Reθ≃ 5,000 at a free-stream velocity ofUref= 25ms−1. The length of the separation bubble is estimated at 0°42 ± 0°02m by three different methods. Results of a numerical simulation demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0·05% in the test section. Oil-film visualisation experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualisation.

Experimental and Numerical Study of the Effect of Gaps on Laminar Turbulent Transition of Incompressible Boundary Layers

In this paper, the effects of gaps on the boundary layer transition are investigated on an ONERA-D airfoil both experimentally and numerically. The experiment has been conducted in a subsonic wind tunnel for several gap geometries and aerodynamic conditions. Hot-wire anemometry was used to quantify the transition modification induced by the imperfection compared to the smooth case in favorable and adverse pressure gradients. At the same time, the gap effects are numerically estimated with the help of a ΔN model previously developed at ONERA and compared to the experiment.

Development of Aerodynamic Design of Hypersonic Quiet Nozzles

The hypersonic quiet nozzle has to be carefully designed by the aerodynamic techniques to obtain the quiet-flow test region with low noise levels comparable to flight. Two conceptual aerodynamic techniques tolaminarizethe nozzle wall boundary layer in the past quiet nozzle design, such as the use of the slower expansion nozzle and the bleed slot in the subsonic region upstream of the throat, are summarized. For the design of the expansion contour of quiet nozzles, the analysis based on the favorable pressure gradient effect is performed and relative results seem to be helpful to theoretically understand the quiet nozzle's evolution from rapid expansion to slow expansion. On the other hand, the bleed slot for the suction of the subsonic boundary layer in the contraction is also required to be analyzed in details. The distance from lip to throat and the maximum width of bleed slot could affect the suction of the slot to the boundary layer by changing the separation location and would be also analyzed.

Elliptic jet control with cross-wire

The effect of cross-wire on the aerodynamic mixing of circular and elliptic sonic jet flow fields is studied experimentally in the presence of favourable pressure gradient at the orifice exit. The cross-wire is found to reduce the amplitude of pitot pressure oscillation drastically, and thus weakens the shock cell structure in the jet core. In the case of elliptic jets, the effectiveness of cross-wire is found to be dependent on its location at the orifice exit. For lower aspect ratio jets (AR 2), cross-wire along the major axis proves to be more effective in enhancing mixing, but as the aspect ratio increases to higher values (AR 6), cross-wire along the minor axis becomes more effective in promoting the jet mixing. The pressure profile results indicate that the cross-wire located on the major axis advances the axis-switching phenomenon, while the cross-wire along the minor axis shifts the axis-switching location further downstream compared to uncontrolled jet. The jet bifurcation caused by the cross-wire has been captured in the shadowgraph pictures.

Evolution of accelerated laminar boundary layer subjected to isothermal wall heating

The paper reports on numerical simulation of a laminar near-wall flow subjected simultaneously to a favorable pressure gradient and wall heating at a constant temperature. An important feature of such a flow is an overshoot of the streamwise velocity profile when the velocity within the boundary layer exceeds the velocity in the external main flow. It is shown that the overshoot appears due to the simultaneous action of wall heating and acceleration. The investigation shows that the maximum velocity at a finite Reynolds number arises when the acceleration parameter is positive and the ratio of the wall temperature and the free stream temperature is greater than unity. The overshoot causes an increase of the skin-friction coefficient and the Stanton number. In addition, it leads to a negative momentum thickness. The article demonstrates the applicability of the approximate formula by Volchkov (2006) for the evaluation of the maximum velocity. Based on the own recent research and on an overview of the published literature on similar and related flows, it is concluded that the main prerequisite for a velocity overshoot is an input of energy from a bounding wall.

The Role of Two-Dimensional Roughness Element in the Laminar-Turbulent Process in the Favorable Pressure Gradient of the Swept Wing

In the article using a liquid crystal thermography investigated the development of stationary and secondary disturbances, which were excited by cylindrical and two-dimensional roughness elements. It was shown, that two-dimensional roughness element has a destabilizing effect on disturbances, induced by cylindrical roughness element. Also the twodimensional roughness element is able to excite the stationary structures, and then the secondary disturbances the frequency interval of which is lower than in the case of stationary vortices excitation by cylindrical roughness element

Triangular tabs for supersonic jet mixing enhancement

AbstractThe mixing promoting capability of right-angled triangular tab with sharp and truncated vertex has been investigated by placing two identical tabs at the exit of a Mach 2 axi-symmetric nozzle. The mixing promoting efficiency of these tabs have been quantified in the presence of adverse and marginally favourable pressure gradients at the nozzle exit. It was found that, at all levels of expansion of the present study though the core length reduction caused by both the tabs are appreciable, but the mixing caused by the truncated tab is superior. The mixing promoting efficiency of the truncated tab is found to increase with increase of nozzle pressure ratio (that is, decrease of adverse pressure gradient). For all the nozzle pressure ratios of the present study, the core length reduction caused by the truncated vertex tab is more than that of sharp vertex tab. As high as 84% reduction in core length is achieved with truncated vertex right-angled triangular tabs at moderately overexpanded level, corresponding to expansion levelpe/pa= 0·90. The corresponding core length reduction for right-angled triangular tabs with sharp vertex and rectangular tabs are 65% and 31%, respectively. The present results clearly show that the mixing promoting capability of the triangular tab is best than that of rectangular tabs at identical blockage and flow conditions.

Skin Friction Coefficient and Boundary Layer Trend on UKM Aster i-Bond

This study concerns with aerodynamic drag on a passenger car. By using computational fluid dynamics (CFD) method, we found that values of skin friction coefficients for three different parts of the car: front, top and rear parts, are different. This study addresses three different basic possible flows around a car: favourable, zero and adverse pressure gradients. Generally, cars use approximately 20% of their engine power to overcome aerodynamic drag, which is generally proportional to the frontal area. The boundary layer at each position has been analyzed to ascertain the effect of wall shear stress on the car surface. It is found that the value of wall shear stress velocity is highest at the rear part, followed by front and top parts. Subsequently, it is shown that the front part has the thinnest viscous region despite not being the part with the highest local ambient velocity compared with the top and rear parts. Despite its supposed aerodynamic shape, the rear part of the car sees separation of flow and the total drag per unit area here is the largest, twice as large as front part and more than seven times larger than the top part.

Coupled Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation for Outlet Guide Vane and Prediffuser Flows

The objective of the present study was the development/testing of a novel hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation methodology for predicting the flow at the compressor/combustor interface of a gas turbine engine. The coupling between the flow through the outlet guide vanes and the downstream combustor prediffuser represents the first key component of this interface. Important flow characteristics include three-dimensional turbulent outlet guide-vane wakes, end-wall boundary layers, and both favorable and adverse pressure gradient effects. There is a risk of flow separation if diffuser length and area ratio are not appropriately chosen; alternatively, a significant opportunity exists to reduce system flow losses if this risk is managed properly. Outlet guide-vane/prediffuser flow thus represents a significant modeling challenge. In the present approach to Reynolds-averaged Navier–Stokes/large-eddy simulation computational fluid dynamics, the Reynolds-averaged Navier–Stokes solver is applied to the (attached) outlet guide-vane flow and the large-eddy simulation solver to the (close to separation) prediffuser. Interfacing a large-eddy simulation prediction with an upstream Reynolds-averaged Navier–Stokes prediction introduces the problem of generating a spatially and temporally correlated unsteady velocity field for large-eddy simulation inlet conditions, which is also consistent with the time-averaged Reynolds-averaged Navier–Stokes solution. A new Reynolds-averaged Navier–Stokes/large-eddy simulation coupling procedure has been developed. Experimental data for conventional and advanced outlet guide vane/prediffuser geometries are used to assess the success of the new coupled approach. The results show that the Reynolds-averaged Navier–Stokes/large-eddy simulation approach is able to provide a good balance of predictive capability, improving prediction of velocity profiles at prediffuser exit compared to Reynolds-averaged Navier–Stokes only solution, and computational cost; it offers an attractive option for outlet guide-vane/prediffuser flow prediction.

Pressure Gradient Effects on Smooth and Rough Surface Turbulent Boundary Layers—Part I: Favorable Pressure Gradient

The effects of the pressure gradient and surface roughness on turbulent boundary layers have been experimentally investigated. In Part I, smooth- and rough-surface turbulent boundary layers with and without favorable pressure gradients (FPG) are presented. All of the tests have been conducted at the same Reynolds number (based on the length of the flat plate) of 900,000. Streamwise time-mean and fluctuating velocities have been measured using a single-sensor hot-wire probe. For smooth surfaces, the FPG decreases the mean velocity defect and increases the wall shear stress; however, the friction coefficient hardly changes due to the increased freestream velocity. The FPG effect on the streamwise normal Reynolds stress has been examined. The FPG increases the streamwise normal Reynolds stress for y less than 0.6 times the boundary layer thickness. With the zero pressure gradient (ZPG), the roughness increases the mean velocity defect throughout the boundary layer and increases the normal Reynolds stress for y greater than twice the average roughness height. The roughness effect on the mean velocity defect is stronger under the FPG than under the ZPG for y less than 30 times the average roughness height. For y less than 25 times the average roughness height, the roughness effect of increasing normal Reynolds stress is also stronger under the FPG than under the ZPG. Consequently, for a rough surface, the FPG increases the integrated streamwise turbulent kinetic energy, friction coefficient, roughness Reynolds number, and roughness shift. Furthermore, the FPG increases the roughness effects on the integral boundary layer parameters—the boundary layer thickness, momentum thickness, ratio of the displacement thickness to the boundary layer thickness, and shape factor. Thus, the FPG augments the roughness effects on turbulent boundary layers.

Pressure Gradient Effects on Smooth- and Rough-Surface Turbulent Boundary Layers—Part II: Adverse Pressure Gradient

An experimental investigation has been conducted to identify the effects of pressure gradient and surface roughness on turbulent boundary layers. In Part II, smooth- and rough-surface turbulent boundary layers with and without adverse pressure gradient (APG) are presented at a fixed Reynolds number (based on the length of flat plate) of 900,000. Flat-plate boundary layer measurements have been conducted using a single-sensor, hot-wire probe. For smooth surfaces, compared to the zero pressure gradient (ZPG) boundary layer, the APG boundary layer has a higher mean velocity defect throughout the boundary layer and lower friction coefficient. APG decreases the streamwise normal Reynolds stress for y less than 0.4 times the boundary layer thickness and increases it slightly in the outer region. For rough surfaces, APG reduces the roughness effects of increasing the mean velocity defect and normal Reynolds stress for y less than 23 and 28 times the average roughness height, respectively. Consistently, for the same roughness, APG decreases the integrated streamwise turbulent kinetic energy. APG also decreases the roughness effect on the friction coefficient, roughness Reynolds number, and roughness shift. Compared to the ZPG boundary layers, the roughness effects on integral boundary layer parameters—boundary layer thickness and momentum thickness—are weaker under APG. Thus, contrary to the favorable pressure gradient (FPG) in part I, APG reduces the roughness effects on turbulent boundary layers.

Effect of mean pressure gradient on the turbulent wall pressure–velocity correlations

Two-point correlations of the wall fluctuating pressure and turbulent pressure and velocities in the flow field are computed in three dimensions using direct numerical simulation (DNS) databases. The mean pressure gradient in the flow has strong influence on these correlations. The DNS () in a converging/diverging channel with streamwise curvature near the lower wall is used to compute the statistics. The correlations in favourable pressure gradient and adverse pressure gradient are compared with the correlation in flat channel flow DNS (). The results show that these pressure–pressure and pressure–velocity correlations are not only sensitive to the local pressure gradient and wall curvature, but also to the pressure gradient history.

Influence of pressure gradient on streamwise skewness factor in turbulent boundary layer

The paper shows an effect of favourable and adverse pressure gradients on turbulent boundary layer. The skewness factor of streamwise velocity component was chosen as a measure of the pressure gradient impact. It appears that skewness factor is an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the skewness factor decreases in the flow region subjected to FPG and increases in the APG conditions. The changes of convection velocity and skewness factor are caused by influence of large-scale motion through the mechanism called amplitude modulation. The large-scale motion is less active in FPG and more active in APG, therefore in FPG the production of vortices is random (there are no high and low speed regions), while in the APG the large-scale motion drives the production of vortices. Namely, the vortices appear only in the high-speed regions, therefore have convection velocity higher than local mean velocity. The convection velocity affects directly the turbulent sweep and ejection events. The more flow is dominated by large-scale motion the higher values takes both the convection velocity of small-scale structures and sweep events induced by them.