Base Pressure Distributions Research Articles

A characterization of unsteady effects for transonic turbine airfoil limit loading

To better understand the effect of vortex shedding on the nature of the trailing edge shock system during airfoil limit loading a computational fluid dynamic investigation was performed for a transonic turbine airfoil at sublimit, limit and supercritical conditions. Four modeling strategies were employed: steady state RANS, unsteady RANS, DDES, and turbulence model free. The influence of transient modeling approaches on the predicted mass-flow averaged total pressure loss coefficient, mass-flow averaged flow angles, and on the limit loading pressure ratio were found to be insignificant with the exception of the URANS model during critical loading. It was found that the URANS modeling approach failed to predict the transition from near trailing edge dominated vortex formation to base pressure vortex formation resulting in a drastic rise in predicted total pressure loss. Surface isentropic Mach distributions were predicted similarly for all modeling strategies, with the exception of the trailing edge base pressure region and points of shock impingement along the suction surface. A detailed review of the boundary layer states at the trailing edge was performed. It was found that all of the modeling approaches predicted laminar boundary layer profiles along the pressure surface trailing edge and turbulent profiles along the suction surface. The predicted base pressure distributions were also reviewed, showing the base pressure to decrease with increasing pressure ratio. The unsteady simulation approaches consistently predicted lower average surface pressures than the steady state RANS simulations. Qualitative images of the numerical Schlieren contours were presented and reviewed showing large differences in the prediction of vortex shape, size, and subsequent shock influence.

Effect of von Karman length scale in scale adaptive simulation approach on the prediction of supersonic turbulent flow

Numerical investigations of the supersonic axisymmetric base flow at Mach number 2.46 and Reynolds number 2.858×106 are carried out using the scale-adaptive simulation (SAS) approach. To achieve SAS computation, three mathematical forms of von Karman length scale are utilized. The first one is calculated from the strain rate tensor and second derivative of the velocity field (this SAS can be marked as “SAS-1”). Computation of the second one is employed using the vorticity and its first derivative (this SAS can be denoted as “SAS-2”). The third one is computed from the vorticity and the second derivative of the velocity field (this SAS can be named as “SAS-3”). Their effects on the quantitative predictions are assessed. Two inflow conditions are also investigated, i.e., inflow condition with/without turbulent-boundary-layer profiles. Results show that the inflow conditions have the significant effects on the base-pressure and streamwise velocity distributions, and the predicted results at the inflow condition with profiles show better agreement with experimental data than that at the inflow condition without profiles. The inflow conditions and von Karman length scales only have the relatively slight effects on the radial velocity distributions. Similarly, at the inflow condition with profiles, the base-pressure distributions are also not sensitive to mathematical forms of von Karman length scale. More comparisons reveal that the best predictions can be obtained by SAS-2 and SAS-3. It may be associated with the vortex stretching effects explicitly considered in the von Karman length scales of SAS-2 and SAS-3. Although the mean flow field predicted by SAS-2 is slightly better than that obtained by SAS-3. However, SAS-3 can give the slightly better predictions for the Reynolds stresses than SAS-2.

Characterisation of the wake of the DrivAer estate vehicle

To date, little information has been published on the time-averaged wake of the estate variant of the DrivAer vehicle type. In recent years, this generic automobile geometry has been put forward as a more realistic alternative to investigate vehicle aerodynamics than considerably more idealised geometries, such as the Ahmed body. In this paper, the time-averaged wake is experimentally investigated through wind-tunnel tests. Velocities maps and profiles, drag-force measurements and base pressure distributions are used to characterise and quantify the flow behind and around the vehicle. In particular, the combination of wake-velocity and base-pressure measurements provide insight into the wake behind an estate vehicle. The results indicate the near wake and base-pressure distribution is dominated by up-wash caused by flow exiting the smooth underbody diffuser.

Direct Numerical Simulation of a Supersonic Base Flow Behind a Circular Cylinder

A supersonic flow in the near wake behind a cylinder is considered. Base pressure distributions behind a circular cylinder for various Mach numbers M∞ are obtained and analyzed by means of direct numerical simulation based on high-order approximation algorithms. For M∞ = 2.46, the results obtained in the present study are compared with available experimental and numerical data. Generation of turbulent kinetic energy is calculated for various Mach numbers.

Research on Aeroheating of Hypersonic Reentry Vehicle Base Flow Fields

The structure of the base flow of a hypersonic reentry vehicle and the resulting base pressure and heat transfer have been studied by numerical study. The compressible Navier-Stokes equations are solved by the finite-volume method. SST k-ω turbulence model is used, and comparisons are made with flight test. Attention was focused on assessing the effects of angle of attack and Mach number. It was found that angle of attack can significantly alter the wake flow structure and reentry vehicle base pressure and heating distributions. The results of the simulation may provide a theoretical basis for the design of the thermal protection system of hypersonic reentry vehicles.

Characterisations of force and pressure fluctuations of real vehicles

Both the unsteady aerodynamics force and base pressure distributions are investigated for four model cars in real flow conditions. The low pass cutoff frequency of the measurements is about 5 Hz which is sufficient to consider the global fluctuations of drag, lift and side forces. The drag fluctuations are very low, they never exceed 1% of the mean drag. A clear correlation is found with the base pressure distribution fluctuations. It is found that the regions of smallest pressure fluctuations on the vehicle base are the most correlated to the drag fluctuations or in other words, the regions of largest pressure fluctuations on the base are not associated with the drag fluctuations. Sideslip effect is studied, and one of the model presented a clear bistable behavior on the lift creating huge fluctuations as investigated in the academic experiment of Grandemange, Gohlke and Cadot, Physics of Fluids 25 (2013) 095103.

Experimental investigation of aerodynamic flow over a bluff body in ground proximity with drag reduction devices

An experimental investigation of several drag reduction devices installed in the rear of a bluff body with square back geometry in ground proximity has been conducted. The experiments include drag, base pressure measurements and velocity measurements using particle image velocimetry (PIV). The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved. The results also indicated that models with base cavity have lower drag than their counterparts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The devices were found to have a strong effect on the underbody flow. For the drag reduction devices a rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent intensities in the near wake region.

821 高レイノルズ数 Base 流れ解析における LES/RANS hybrid 手法の信頼性検証

LES/RANS hybrid, LES and MILES simulations are applied to an axi-symmetric base flow for the supersonic flows at high Reynolds number. The idea of LES/RANS hybrid methodology is that RANS model is used in the boundary layer because of its low computational costs than LES, and LES is used in massively separated regions where unsteady flow structures are correctly captured. The subgrid-scale stresses are computed using the compressible form of Smagorinsky's model. Baldwin-Lomax model is applied to the RANS region. The LES/RANS hybrid simulations accurately capture the physics of turbulent flows. The reverse flow behind the base shows satisfactory agreement with the experimental data. The base pressure distributions, which is the primary parameter from the engineering interest, is in good agreement with the experimental results.

Flow Field and Performance Analysis of an Annular-Type Aerospike Nozzle with Base Bleeding

Aerospike nozzle flow fields are computationally analyzed by Navier-Stokes simulations. The secondary flow from the base surface is induced for the purpose of increasing total thrust. The computed result shows that base pressure efficiently increases when base bleeding is induced from the tip region of the base surface and injected radially inward towards the nozzle axis. The base bleeding induced at the tip of the base and injected horizontally with the axis produces maximum base thrust because the momentum thrust does not produce any divergence loss and the base pressure is kept high. Base bleeding induced against the existing recirculation flow results in lower base pressure distributions. Base bleeding that produces maximum thrust interacts with the exhaust flow and creates shock waves that emanate from the tip of the base. The shear layer is squeezed more towards the nozzle axis when base bleeding is induced and forces the stagnation point to move closer to the base surface.

Computations of Axisymmetric Plug-Nozzle Flowfields: Flow Structures and Thrust Performance

Plug-nozzle flowfields are analyzed by the numerical simulations. The method of characteristics is used to design the plug contour, and several types of plug nozzles are considered by truncating the length of the nozzle at different positions. Plug-nozzles' altitude-compensating features are confirmed by the computed results, and the base pressure turns out to play an important role to maintain the thrust performance of the nozzle for high-truncation configurations. The altitude clearly influences the base-pressure distributions under the assumption that the chamber pressure is constant during the ascent. The computed thrust difference between the contoured and the conical nozzle linearly increased as a function of the pressure ratio. The thrust performance of the contoured plug nozzle is estimated to be about 5-6% higher than the conical plug nozzle. Various conditions of external flows over the plug nozzle are also imposed, and the result shows that the external flow does not influence the pressure distribution on the nozzle surface for the pressure ratios higher than the designed point.

Grid-Resolved Analysis of Base Flowfield for Four-Engine Clustered Nozzle Configuration

The objective of this study is to propose a computational methodology that can effectively anchor the base flowfield of a four-engine clustered nozzle configuration. This computational methodology is based on a three-dimensional, viscous flow, pressure-based computational fluid dynamics (CFD) formulation. For efficient CFD calculation, a Prandtl-Meyer solution treatment is applied to the algebraic grid lines for initial plume expansion resolution. As the solution evolves, the computational grid is adapted to the pertinent flow gradients. The CFD model employs an upwind scheme in which second- and fourth-order central differencing schemes with artificial dissipation are used. The computed quantitative base flow properties such as the radial base pressure distributions, model centerline static pressure, Mach number and impact pressure variations, and base pressure characteristic curve agreed reasonably well with those of the measurement.

The effect of member spacing upon the base pressure distributions recorded on perpendicular flat plates

The experimental results presented in this note have provided a comparison between the flow induced pressure distributions associated with non-connecting perpendicular flat plates and similar configurations composed of circular cylinders and square section bars.

地面の不整がＡＣＶの揚力特性に及ぼす影響 ＩＩ

Base pressure distributions of an ACV over irregular ground are measured on two and three dimensional models.Furthermore, jet configurations of two dimensional model placed in water are observed by the aid of hydrogen bubble method. Remarkable results are as follows;Two dimensional model:(1) When both leading and trailing jets are Over higher portions or lower portions of the irregular ground, configurations of jets are as same as those on the plane ground in spite of unevenness in the cavity.(2) In case that one jet is on higher portion and the other jet is on lower portion of the irregular ground, this is underfed condition and that is overfed condition.Crossflow induced from overfed jet to underfed jet is depend upon the ratio of hover height to step height of the ground.(3) Overfed jet is accompanied with vortex attached to the base and underfed jet to the ground.Three dimensional model:(1) Though jet conditions are not so districtly corresponded to the geometrical shape of the ground, the parts of peripheral jet over higher portions are regarded as overfed conditions and other parts over lower portions are regarded as underfed conditions.(2) The intensities of overfed and underfed are affected by the distances from the boundary of the unevenness.

Base Flow Characteristics of a Linear Aerospike Nozzle Segment

The separated base flow region within a linear aerospike nozzle segment is investigated experimentally in an ejector-diffuser system. The nozzle-diffuser system and base pressure characteristics are described over the pressure ratio range from the “open wake” to the “closed wake” operation. Schlieren photographs and static pressure distributions along the test section centerline and top contour describe the transition, from “open wake” to “closed wake” flow fields. Base pressure and static pressure distributions are utilized to present the effects of a ramp diffuser. The effects of base bleed on the base pressure ratio and structure of the nozzle flow field are also presented.