Srinivasan Arunajatesan

Control of Pressure Loads in Geometrically Complex Cavities

The need to reduce the fluctuating surface pressure loads in realistic three-dimensional cavity configurations is clear for many applications. In this paper, we describe the results of an experimental study that examined the properties of flow over a highly three-dimensional cavity, which included angled side walls and a sloped floor. The unsteady pressure measurements revealed that the primary spectral properties, such as the frequencies of the cavity tones, are very similar to those of simpler, rectangular cavities. The study also explored the effects of different active fluidic injection methods, at the cavity leading edge, on the unsteady loads generated in the cavity. Specifically, the two active suppression concepts examined were microjets and rectangular slots at the leading edge. Both concepts showed significant reductions in the fluctuating surface pressures, upwards of 50% on the cavity aft wall, with very modest amounts of mass flowing through the injectors. When appropriately scaled for full-scale applications, the actuator mass flux required falls well within the practical range for most aircraft. Different angles for the fluidic Injection were also examined and maximum reductions were observed when injection was perpendicular to the approaching freestream flow. Additionally, the best blowing configurations were found when the injectors did not fully span the leading edge of the cavity. The reductions observed in the fluctuating surface pressure levels resulted from decreases in both the broadband and resonant features of the surface pressures. By conducting these experiments at two different facilities and over a range offreestream dynamic pressures and temperatures, this study also demonstrated that appropriate scaling of the spectral features can be achieved. This allows for the expansion of the results presented here to larger (and different) scale studies and ultimately to full-scale applications.

Suppression of Cavity Loads Using Leading-Edge Blowing

We present hybrid Reynolds-averaged Navier-Stokes/large eddy simulation-based analysis of the suppression of fluctuating pressure loads on the walls of a complex nonrectangular cavity using leading-edge mass blowing. The unique aspect of the concepts discussed here is the very low mass flow rates used to achieve significant suppression. The simulation results are used to gain insight into the mechanism governing the effectiveness of these jets. The jets are applied to an L/D = 5.6 cavity at supersonic conditions of Mach 1.5. The simulation results show excellent agreement with experiments demonstrating an overall reduction in fluctuating pressure levels on the order of 50% with the control concepts. The primary mechanism of reduction is the break up of the spanwise coherence in the shear layer into smaller vortical structures thus reducing the shear layer flapping and leading to a smaller imprint on the wall pressures.

Hybrid RANS-LES modeling for cavity aeroacoutics predictions

A new hybrid RANS-LES turbulence modeling approach suited for modeling cavity aeroacoustics is presented. The model combines a one-equation LES subgrid model and a two equation RANS turbulence model in a physically consistent manner. The model takes into account the extent of resolution of the flow features by the mesh and computes an eddy viscosity for the turbulence model to reflect this resolution. It is shown that the model replicates the behavior of the underlying RANS and LES models in the coarse and fine mesh limits, respectively. Predictions of a surface pressures due to a supersonic flow over a cavity are compared with experimental measurements to demonstrate the model capabilities.

Three-Dimensional Hybrid RANS/LES Simulations of a Supercritical Liquid Nitrogen Jet

Simulations of an uni-element, shear coaxial injector configuration with liquid nitrogen in the inner tube mixing with a co-flowing warm nitrogen gas stream are presented; the chamber has a supercritical pressure of 4.95 MPa with the liquid nitrogen jet temperature mildly supercritical at 129 K. Unsteady solutions on a complete three-dimensional configuration were computed using a hybrid RANS/LES framework and compared with experimental data. Our results reveal a highly unsteady flow field with strong fluctuations in the post region between the inner jet and co-flowing outer jet. This unsteadiness is caused by strong thermodynamics gradients near the critical point, which cause the inner jet to expand out radially as it mixes with the warmer fluid from the outer jet. Analysis of the turbulence field indicates that nominal temperature fluctuations generate large amplitude density fluctuations which in turn increases the Reynolds stress in the liquid jet shear layer. A key finding of this study is that the unsteady mixing is dominated by three-dimensional helical instabilities on the interface of the liquid jet shear layer. In this important respect, a trans-critical/supercritical jet is very different from a gas jet where the jet nearfield is unstable only to axisymmetric instabilities. A comparison of the mean radial temperature distribution indicates reasonable comparison with experimental measurement; the dramatic mixing of the liquid shear layer is captured well but the mixing in the outer gas shear layer is underpredicted. A comparison of the jet mixing core length prediction along the centerline with the dark core length measurements from flow visualization indicates that the numerical results overpredict the mixing and this may be due to a combination of uncertainty in the temperature measurement values as well as numerical errors.

Large Eddy Simulation of a Circulation Control Airfoil

A detailed study of the compressible, turbulent flow around a circulation control airfoil has been conducted using a large eddy simulation (LES). The circulation control airfoil investigated is the NCCR 1510-7067N, which has a circular arc shaped trailing edge. The Reynolds number for the problem is 5.45x10, based on chord, with a free stream Mach number of 0.12. The non-dimensional blowing rate (Cμ) for the Coanda jet is 0.093. The filtered Navier-Stokes equations are discretized using a fifth-order spatially accurate, upwind scheme with no limiting. The sub-grid scale turbulence model combines a transport equation for the sub-grid scale turbulent kinetic energy with a turbulent length scale, based on local grid dimensions, to form a expression for the sub-grid scale eddy viscosity. The resulting mean surface pressure distribution compares fairly well to experimentally measured values for the airfoil. Resulting mean velocity and Reynolds stress profiles at selected locations around the airfoil are examined.

Multi-element unstructured methodology for analysis of turbomachinery systems

In recent years unstructured mesh techniques have become popular for computational e uid dynamics analysis of external aerodynamic-type problems. The main advantages of such an approach include mesh generation over complex domains, grid adaptation in localized areas, and accuracy in efe ciently identifying complexities in local e ow physics. A hybrid unstructured methodology is used to carry out simulations for predominantly internal e ow turbomachinery applications. Issues related to skewness and other constraints of tetrahedral meshes are addressed in the context of turbomachinery-based propulsive e ows that exhibit a rich variety of length scales and timescales, as well asinteresting e ow physics. The unstructured framework permits the generation of a contiguous grid without internal boundaries between different components of a turbomachinery system and provides good local resolution in regions where the e ow physics becomes important. The increased numerical stability resulting from these factors coupled with the parallel solution framework yields an efe cient solution procedure for complex turbomachinery e ows. Numerical results are presented and compared against experimental measurements for a transonic diffuser‐ volute cone guration and a high Reynolds number pump.

Joint Experimental/Computational Investigation Into the Effects of Finite Width on Transonic Cavity Flow.

Recently acquired experimental data on pressure fluctuations in cavities of equal length (L) to depth (D) ratio but varying length to width (L/W) ratio have shown substantial variations in the dominant modes in the cavity. These observations have been carried out at subsonic and transonic Mach numbers at cavity L/D=5, which puts the cavity flow in the “open” category. This paper presents results from a joint computational and experimental investigation undertaken at Sandia to explain these observations. To this end, simulations of L/D=5.0 cavity at L/W=1.0,1.67 and 5.0 have been carried out and analyzed. The results show strong differences in the mean flow structure between the three widths. The widest cavity shows significantly higher turbulence intensities across the cavity. The unsteady wall pressures reveal that in this case, significant tunnel wall interactions are present, intensifying the pressure fluctuations and the shear layer oscillations. The differences in the wall pressures and turbulent flow field are smaller for the L/W=1.0 and 1.67 cavities. The L/W=1.67 cavity is strongly influenced by the streamwise vortices at the spanwise edges of the cavity, resulting in strong three dimensional variations in mean flow across the width of the cavity. In the case of the narrowest cavity, this effect is minimal, with the resultant flow field showing predominantly two-dimensional character.

Control of Pressure Loads in Complex Cavity Configurations

The need to reduce the fluctuating surface pressure loads in realistic three dimensional cavity configurations is clear for many applications. Here an experimental study was conducted to examine the effects of different leading edge blowing concepts for a highly three dimensional cavity. In addition to the suppression studies the properties of the complex cavity was studied through the use of several simpler cavities with limited features of the full three dimensional cavity. The two leading edge blowing suppression concepts examined were micro jets and segmented slots. Both of the concepts showed significant reductions in the fluctuating surface pressures with modest amounts of mass flowing through the injectors. The reductions observed in the fluctuating surface pressure levels resulted from decreases in both the broad band and resonant features of the surface pressures. Velocity field measurements showed that the controlled cavities had significantly reduced fluctuating velocities in the shear layer and a smaller amplitude reverse flow along the bottom of the cavity. The observations reported in this study have also served as a basis for designing actuators for larger scale tests where consistent results were found.

Simulation of Shear Driven Cavity Flows Using Unstructured Hybrid RANS/LES

** �� *** Two separate, unstructured hybrid RANS/LES methods are used to simulate both resonant, and non -resonant, shear driven cavity flows. The first method uses an upwind -biased discretization for the inviscid flux calculations in the governing equations, along with a non -linear k-� turbulence closure for RANS regions, and the Smagorinsky sub -grid scale closure for LES regions. The second method uses an upwind -biased discre tization for the inviscid flux terms which can be modified to reduce the inherently high dissipation in the associated Riemann solver when applied to cell faces not orthogonal to the flow direction. The second method uses a k-� closure for RANS regions. In LES regions, the second method solves a transport equation for sub -grid turbulent kinetic energy, relating this energy to a spectrum for the energy -inertial -dissipation range, which allows calculation of a less dissipative eddy viscosity. Both methods are applied to a three -dimensional, deep cavity problem, at resonant and non -resonant flow conditions. Resulting pressure time series are compared to experimental measurements.

Suppression of Cavity Loads Using Leading Edge Blowing Concepts

We present Large Eddy Simulations based analysis of the suppression of dynamic loads on the walls of a complex non-rectangular cavity using leading edge mass blowing. The unique aspect of the concepts discussed here is the very low mass flow rates used to achieve significant suppression. The simulation results are used to gain insight into the mechanism governing the effectiveness of these jets. The jets are applied to a deep (L/D=5.6) cavity at supersonic conditions of Mach 1.5. The simulation results show excellent agreement with experiments showing an overall reduction of the noise levels of the order of 5-7 dBs with the control concepts. The primary mechanism of reduction is the break-up of the spanwise coherence in the shear layer into smaller vortical structures thus reducing the shear layer flapping and leading to a smaller imprint on the wall pressures.