Showing papers by "Terrence W. Simon published in 1995"

Bypass Transition in Boundary Layers Including Curvature and Favorable Pressure Gradient Effects

Abstract: Recent studies of 2-D boundary layers undergoing bypass transition were reviewed. Bypass transition is characterized by the sudden appearance of turbulent spots in boundary layer without first the regular, observable growth of disturbances predicted by linear stability theory. There are no standard criteria or parameters for defining bypass transition, but it is known to be the mode of transition when the flow is disturbed by perturbations of sufficient amplitude.

Effects of Free-Stream Turbulence Intensity on a Boundary Layer Recovering From Concave Curvature Effects

Abstract: Experiments are conducted on a flat recovery wall downstream of sustained concave curvature in the presence of high free-stream turbulence (TI∼8%). This flow simulates some of the features of the flow on the latter parts of the pressure surface of a gas turbine airfoil. The combined effects of concave curvature and TI, both present in the flow over a turbine airfoil, have so far been little studied. Computation of such flows with standard turbulence closure models has not been particularly successful. This experiment attempts to characterize the turbulence characteristics of this flow. In the present study, a turbulent boundary layer grows from the leading edge of a concave wall, then passes onto a downstream flat wall. Results show that turbulence intensities increase profoundly in the outer region of the boundary layer over the recovery wall. Near-wall turbulent eddies appear to lift off the recovery wall and a «stabilized» region forms near the wall. In contrast to a low-free-stream turbulence intensity flow, turbulent eddies penetrate the outer parts of the «stabilized» region where sharp velocity and temperature gradients exist. These eddies can more readily transfer momentum and heat. As a result, skin friction coefficients and Stanton numbers on the recovery wall are 20 and 10 percent, respectively, above their values in the low-free-stream turbulence intensity case. Stanton numbers do not undershoot flat-wall expectations at the same Re Δ2 values as seen in the low-TI case. Remarkably, the velocity distribution in the core of the flow over the recovery wall exhibits a negative gradient normal to the wall under high-free-stream turbulence intensity conditions. This velocity distribution appears to be the result of two effects: (1) cross transport of kinetic energy by boundary work in the upstream curved flow and (2) readjustment of static pressure profiles in response to the removal of concave curvature

Evaluation of local wall temperature, heat flux, and convective heat transfer coefficient from the near-wall temperature profile

Abstract: A technique for determining local wall temperature, local surface heat flux and local convective heat transfer coefficient from a carefully-taken, near-wall temperature profile was evaluated. With the present technique, profile measurements taken with very accurate near-wall values are substituted for these separate instruments. The profiles are extrapolated to the wall to determine the local surface temperatures and their slopes are used to calculate the local surface heat flux. This technique depends on accurate positioning of a traversing temperature sensor with respect to the wall. In the present paper, the technique is evaluated in two simple flows, a fully-developed tube flow and a simple boundary layer flow. In the tube flow, local wall temperatures determined from profile measurements agreed with those obtained with an embedded wall thermocouple to within 5.3% and 12% of the wall-to-bulk temperature difference for two cases of different Reynolds numbers. Nusselt numbers determined using the near-wall profile data were compared to values obtained from two standard pipe flow correlations. Agreement with the average of two correlations was within 7.2%. For the simple, unaccelerated boundary layer flow on a flat plate, Stanton numbers determined from temperature profile measurements agreed with standard flat plate correlation values to within 8%.more » To demonstrate the utility of the technique, it as also applied to an accelerated, transitional boundary layer on a concave wall under high free-stream turbulence conditions and to an oscillatory flow within a pipe. The temperature profiles from the curved, accelerated boundary layer case, when cast in terms of wall coordinates using the profile-determined wall temperature and heat flux, agreed well with analytically-predicted profiles.« less

Measurements in Transitional Boundary Layers Under High Free-Stream Turbulence and Strong Acceleration Conditions

Abstract: Measurements from transitional, heated boundary layers along a concave-curved test wall are presented and discussed. A boundary layer subject to low free-stream turbulence intensity (FSTI), which contains stationary streamwise (Gortler) vortices, is documented. The low FSTI measurements are followed by measurements in boundary layers subject to high (initially 8%) free-stream turbulence intensity and moderate to strong streamwise acceleration. Conditions were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Mean flow characteristics as well as turbulence statistics, including the turbulent shear stress, turbulent heat flux, and turbulent Prandtl number, are documented. A technique called "octant analysis" is introduced and applied to several cases from the literature as well as to data from the present study. Spectral analysis was applied to describe the effects of turbulence scales of different sizes during transition. To the authors'knowledge, this is the first detailed documentation of boundary layer transition under such high free-stream turbulence conditions.

Turbulent Transport Measurements in a Heated Boundary Layer: Combined Effects of Free-Stream Turbulence and Removal of Concave Curvature

Abstract: Turbulence measurements for both momentum and heat transport are taken in a boundary layer over a flat recovery wall downstream of a concave wall (R = 0.97 m). The boundary layer appears turbulent from the beginning of the upstream, concave wall and grows over the flat test wall downstream of the curved wall with negligible streamwise acceleration. The strength of curvature at the bend exit, δ99.5 /R , is 0.04. The free-stream turbulence intensity (FSTI) is ~8 percent at the beginning of the curve and is nearly uniform at ~4.5 percent throughout the recovery wall. Comparisons are made with data taken in an earlier study, in the same test facility, but with a low FSTI (~0.6 percent). Results show that on the recovery wall, elevated FSTI enhances turbulent transport quantities such as −uν and νt in most of the outer part of the boundary layer, but near-wall values of νt remain unaffected. This is in contrast to near-wall νt values within the curve which decrease when FSTI is increased. At the bend exit, decreases of −uν and νt due to removal of curvature become more profound when FSTI is elevated, compared to low-FSTI behavior. Measurements in the core of the flow indicate that the high levels of cross transport of momentum over the upstream concave wall cease when curvature is removed. Other results show that turbulent Prandtl numbers over the recovery wall are reduced to ~0.9 when FSTI is elevated, consistent with the rise in Stanton numbers over the recovery wall.