Engineering topics

Kirk, Lindsay C.

Publications and source records attributed to Kirk, Lindsay C..

4 records

NASA NTRS2018

Experimental Investigation of Roughness Effects on Transition on Blunt Spherical Capsule Shapes

The present contribution reviews recent experimental results of roughness effects on boundary layer transition on capsule geometries with spherical windward geometries. Experiments in three wind tunnel facilities are considered. The ACE Tunnel of Texas AM University, USA, provided Mach 6 experiments with distributed roughness at relatively low Reynolds numbers, 2.5 · 10(exp 5) < Re(sub d) < 5 · 10(exp 5), with d denoting the capsule diameter. The observed boundary layer transition compared well with correlations based on transient growth theory, even though the roughness heights were in the order of boundary layer thickness. Larger Reynolds numbers, 1 · 10(exp 6) < Re(sub d) < 3·10(exp 6), could be assessed in the hypersonic Ludwieg tube, HLB, of TU Braunschweig, Germany. Transition is observed at rather low, subcritical roughness values in the order of 20 μm for a roughness patch placed about the geometric center of the capsule model. These experiments varied fluctuation levels of the freestream. The authors assume that the observed transitions that occur downstream of the subcritical roughness patch are due to freestream disturbances in the tunnel, which interact with small roughness heights. Additional experiments in the HLB facility with patches of larger roughness height support the relevance of transient growth theory for low-to-medium roughness heights, relative to boundary layer thickness. The effects of Reynolds numbers and total flow enthalpy on transition with isolated roughness were investigated in the HIEST facility of JAXA, Japan. Here, a model insert with roughness elements of varying height for tripping transition to turbulence was employed. The results are compared to known trip effectiveness correlations for isolated roughness. Overall, the transient growth correlation seems to represent roughness-induced transition behavior on the ACE and HLB entry capsule shapes with roughness over the entire capsule surface. These experiment are however for perfect gases. Comparable experiments on roughness induced transition in a high-enthalpy facility are still needed to confirm the validity of transient-growth correlation for vehicle design.

Radespiel, Rolf↗

NASA NTRS2016

Boundary Layer Transition During the Orion Exploration Flight Test 1 (EFT-1)

Boundary layer transition was observed in the thermocouple data on the windside backshell of the Orion reentry capsule. Sensors along the windside centerline, as well as off-centerline, indicated transition late in the flight at approximately Mach 4 conditions. Transition progressed as expected, beginning at the sensors closest to the forward bay cover (FBC) and moving towards the heatshield. Sensors placed in off-centerline locations did not follow streamlines, so the progression of transition observed in these sensors is less intuitive. Future analysis will include comparisons to pre-flight predictions and expected transitional behavior will be investigated. Sensors located within the centerline and off-centerline launch abort system (LAS) attach well cavities on the FBC also showed indications of boundary layer transition. The transition within the centerline cavity was observed in the temperature traces prior to transition onset on the sensors upstream of the cavity. Transition behavior within the off centerline LAS attach well cavity will also be investigated. Heatshield thermocouples were placed within Avcoat plugs to attempt to capture transitional behavior as well as better understand the aerothermal environments. Thermocouples were placed in stacks of two or five vertically within the plugs, but the temperature data obtained at the sensors closest to the surface did not immediately indicate transitional behavior. Efforts to use the in depth thermocouple temperatures to reconstruct the surface heat flux are ongoing and any results showing the onset of boundary layer transition obtained from those reconstructions will also be included in this paper. Transition on additional features of interest, including compression pad ramps, will be included if it becomes available.

Kirk, Lindsay C.↗

NASA NTRS2015

Boundary Layer Transition and Trip Effectiveness on an Apollo Capsule in the JAXA High Enthalpy Shock Tunnel (HIEST) Facility

Computational assessments were performed to size boundary layer trips for a scaled Apollo capsule model in the High Enthalpy Shock Tunnel (HIEST) facility at the JAXA Kakuda Space Center in Japan. For stagnation conditions between 2 MJ/kg and 20 MJ/kg and between 10 MPa and 60 MPa, the appropriate trips were determined to be between 0.2 mm and 1.3 mm high, which provided kappa/delta values on the heatshield from 0.15 to 2.25. The tripped configuration consisted of an insert with a series of diamond shaped trips along the heatshield downstream of the stagnation point. Surface heat flux measurements were obtained on a capsule with a 250 mm diameter, 6.4% scale model, and pressure measurements were taken at axial stations along the nozzle walls. At low enthalpy conditions, the computational predictions agree favorably to the test data along the heatshield centerline. However, agreement becomes less favorable as the enthalpy increases conditions. The measured surface heat flux on the heatshield from the HIEST facility was under-predicted by the computations in these cases. Both smooth and tripped configurations were tested for comparison, and a post-test computational analysis showed that kappa/delta values based on the as-measured stagnation conditions ranged between 0.5 and 1.2. Tripped configurations for both 0.6 mm and 0.8 mm trip heights were able to effectively trip the flow to fully turbulent for a range of freestream conditions.

Kirk, Lindsay C.↗

NASA NTRS2013

Geometric Effects on the Amplification of First Mode Instability Waves

The effects of geometric changes on the amplification of first mode instability waves in an external supersonic boundary layer were investigated using numerical techniques. Boundary layer stability was analyzed at Mach 6 conditions similar to freestream conditions obtained in quiet ground test facilities so that results obtained in this study may be applied to future test article design to measure first mode instability waves. The DAKOTA optimization software package was used to optimize an axisymmetric geometry to maximize the amplification of the waves at first mode frequencies as computed by the 2D STABL hypersonic boundary layer stability analysis tool. First, geometric parameters such as nose radius, cone half angle, vehicle length, and surface curvature were examined separately to determine the individual effects on the first mode amplification. Finally, all geometric parameters were allowed to vary to produce a shape optimized to maximize the amplification of first mode instability waves while minimizing the amplification of second mode instability waves. Since first mode waves are known to be most unstable in the form of oblique wave, the geometries were optimized using a broad range of wave frequencies as well as a wide range of oblique wave angles to determine the geometry that most amplifies the first mode waves. Since first mode waves are seen most often in flows with low Mach numbers at the edge of the boundary layer, the edge Mach number for each geometry was recorded to determine any relationship between edge Mach number and the stability of first mode waves. Results indicate that an axisymmetric cone with a sharp nose and a slight flare at the aft end under the Mach 6 freestream conditions used here will lower the Mach number at the edge of the boundary layer to less than 4, and the corresponding stability analysis showed maximum first mode N factors of 3.

Kirk, Lindsay C.↗