Kursat Kara

Effects of Nose Bluntness on Hypersonic Boundary-Layer Receptivity and Stability over Cones

The receptivity to freestream acoustic disturbances and the stability properties of hypersonic boundary layers are numerically investigated for boundary-layer flows over a 5 straight cone at a freestream Mach number of 6.0. To compute the shock and the interaction of the shock with the instability waves, the Navier-Stokes equations in axisymmetric coordinates were solved. In the governing equations, inviscid and viscous flux vectors are discretized using a fifth-order accurate weighted-essentially-non-oscillatory scheme. A third-order accurate total-variation-diminishing Runge-Kutta scheme is employed for time integration. After the mean flow field is computed, disturbances are introduced at the upstream end of the computational domain. The appearance of instability waves near the nose region and the receptivity of the boundary layer with respect to slow mode acoustic waves are investigated. Computations confirm the stabilizing effect of nose bluntness and the role of the entropy layer in the delay of boundary-layer transition. The current solutions, compared with experimental observations and other computational results, exhibit good agreement.

Receptivity of Hypersonic Boundary Layers Due to Acoustic Disturbances Over Blunt Cone

The transition process induced by the interaction of acoustic disturbances in the freestream with boundary layers over a 5-degree straight cone and a wedge with blunt tips is numerically investigated at a free-stream Mach number of 6.0. To compute the shock and the interaction of shock with the instability waves the Navier-Stokes equations are solved in axisymmetric coordinates. The governing equations are solved using the 5 –order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. After the mean flow field is computed, acoustic disturbances are introduced at the outer boundary of the computational domain and unsteady simulations are performed. Generation and evolution of instability waves and the receptivity of boundary layer to slow and fast acoustic waves are investigated. The mean flow data are compared with the experimental results. The results show that the instability waves are generated near the leading edge and the non-parallel effects are stronger near the nose region for the flow over the cone than that over a wedge. It is also found that the boundary layer is much more receptive to slow acoustic wave (by almost a factor of 67) as compared to the fast wave.

Numerical Study of Internal Flow Structures in a Sweeping Jet Actuator

This study focuses on the generation and interaction of internal flow structures, jet oscillation process, and pressure drop mechanism of a Sweeping Jet Actuator. Timedependent numerical analysis was performed over a range of inlet mass flow rates. The effect of varying inlet mass flow rate on the sweeping jet oscillation frequency was calculated and a strong agreement was found with the experimental measurements. The velocity, temperature and pressure fields are provided. The complex flow field inside the Sweeping Jet Actuator for half an oscillation cycle are presented by velocity magnitude and total pressure contours. Formation of vortices from sharp corners in the actuator core surfaces were observed, and their role in jet oscillation is shown.

Effects of Nose Bluntness on Stability of Hypersonic Boundary Layers over a Blunt Cone

Receptivity and stability of hypersonic boundary layers are numerically investigated for boundary layer flows over a 5-degree straight cone at a free-stream Mach number of 6.0. To compute the shock and the interaction of shock with the instability waves, we solve the Navier- Stokes equations in axisymmetric coordinates. The governing equations are solved using the 5 th - order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. After the mean flow field is computed, disturbances are introduced at the upstream end of the computational domain. Generation of instability waves from leading edge region and receptivity of boundary layer to slow acoustic waves are investigated. Computations are performed for a cone with nose radii of 0.001, 0.05 and 0.10 inches that give Reynolds numbers based on the nose radii ranging from 650 to 130,000. The linear stability results showed that the bluntness has a strong stabilizing effect on the stability of axisymmetric boundary layers. The transition Reynolds number for a cone with the nose Reynolds number of 65,000 is increased by a factor of 1.82 compared to that for a sharp cone. The receptivity coefficient for a sharp cone is about 4.23 and they are very small, ~10 -3 , for large bluntness.

Effects of Wall Cooling on Hypersonic Boundary Layer Receptivity Over a Cone

Effects of wall cooling on the receptivity process induced by the interaction of slow acoustic disturbances in the free-stream are numerically investigated for a boundary layer flow over a 5-degrees straight cone. The free-stream Mach number is 6.0 and the Reynolds number is 7.8x10(exp 6)/ft. Both the steady and unsteady solutions are obtained by solving the full Navier-Stokes equations using 5th-order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using 3rd-order total variation diminishing (T VD) Runge-K utta scheme for time integration. Computations are performed for a cone with nose radius of 0.001 inch for adiabatic wall temperature (T(sub aw)), 0.75*T(sub aw), 0.5*T(sub aw), 0.40*T(sub aw), 0.30*T(sub aw), and 0.20*T(sub aw). Once the mean flow field is computed, disturbances are introduced at the upstream end of the computational domain. Generation of instability waves from leading edge region and receptivity of boundary layer to slow acoustic waves are investigated. Computations showed that wall cooling has strong stabilization effect on the first mode disturbances as was observed in the experiments. T ransition location moved to upstream when wall cooling was applied It is also found that the boundary layer is much more receptive to fast acoustic wave (by almost a factor of 50). When simulations performed using the same forcing frequency growth of the second mode disturbances are delayed with wall cooling and they attained values two times higher than that of adiabatic case. In 0.20*T(sub aw) case the transition Reynolds number is doubled compared to adiabatic conditions. The receptivity coefficient for adiabatic wall case (804 R) is 1.5225 and for highly cooled cones (241, and 161 R); they are in the order of 10(exp -3).

Aerodynamics and Flow Control of Flapping Wings

The objective of the present study is to explore the aerodynamics of flapping wings. The study is done in two main phases. A finite volume CFD code is used to perform a 2D study to compare the performance of the fixed (no flapping) and flapping airfoils. A single element high-lift airfoil section is used in the study. Then, three flow control techniques are applied near to the leading edge to strengthen the leading edge vortices formed during the downstroke namely; blowing with angles 90 and 45° and an alternative blowing suction synthetic jet at 45°. The study indicated that the flapping might increase the generated lift up to two times. This amount can be further increased with the use of the appropriate flow control techniques. The choice of this technique depends on many parameters including the flapping frequency and amplitude and the flying conditions.

Effects of a porous coating on hypersonic boundary layer receptivity over a Cone

The effects of ultrasonically absorptive porous coating on receptivity and stability properties of hypersonic boundary layers are numerically investigated for boundary layer flows over a 5-degree straight cone at a freestream Mach number of 6.0. To compute the shock and the interaction of the shock with the instability waves, we solve the Navier-Stokes equations in axisymmetric coordinates. In the governing equations, inviscid and viscous flux vectors are discretized using a fifth-order accurate weighted-essentially-non-oscillatory (WENO) scheme. A third-order accurate total-variation-diminishing (TVD) Runge-Kutta scheme is employed for time integration. After the mean flow field is computed, acoustic disturbances are introduced at the upstream end of the computational domain. The appearance of instability waves near the nose region on solid and porous wall, and the receptivity of the boundary layer with respect to slow mode acoustic waves are investigated. Preliminary results confirm the stabilizing effect and the role of porous coating in the delay of boundary layer transition.